More than you ever wanted to know about BACKPRESSURE...
09-12-2012, 02:49 AM
#101
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Re: More than you ever wanted to know about BACKPRESSURE...
Thank you Skinny Z That was exactely the information I was looking for.
I do still have questions and hopefully you can answer these:
If the primary header length is not optimal do I add the extra length to the secondary length? IE say for example, say my header length is suppose to be 36" and my secondary is suppose to be 39".
Now if the longest pipe on my header is 20" should I add the 16" of header length to secondary?
I do still have questions and hopefully you can answer these:
If the primary header length is not optimal do I add the extra length to the secondary length? IE say for example, say my header length is suppose to be 36" and my secondary is suppose to be 39".
Now if the longest pipe on my header is 20" should I add the 16" of header length to secondary?
09-13-2012, 04:45 PM
#102
Supreme Member
Re: More than you ever wanted to know about BACKPRESSURE...
In a word, no.
Although the primary and secondary are part of the manifold system, what they do and how they do it are more or less independent of one another.
Unless the primary lengths are unreasonably short (as in a block hugger style header where the length could be 12" or less), it matters little if they differ from one another even by a significant amount. This has to do with how the primary works at helping to scavenge a given cylinder as well as the nature of the V8s we're discussing (or V6s for that matter). Apparently this style of engine is somewhat insensitive to differences in length because the range rpm range the primaries effect is so broad due to the arrangement of the firing pulses, etc. A minimum of 18" to 20" seems to be a documented value (although I should probably double check that).
Please keep in mind that we're discussing an engine that isn't tuned to within an inch of it's life and requires a broad power range rather than being tuned to a narrow rpm band (which can be the case in a Pro Stock engine for example).
The secondary length has a far greater effect on the torque curve which is why, with the correct length and combined with other essential elements such as sufficient valve overlap, it's possible to increase volumetric efficiency and produce more power.
That's the point of the exhaust termination boxes. If you can nail down the optimum secondary length, (through theory or experimentation), then a properly designed ETB arrangement can give you very nearly open header power production while still utilizing a muffled exhaust system.
In conclusion, from what I've studied, don't be overly concerned with the primary length but instead concentrate on achieving the secondary length that provides the power curve your looking for.
Although the primary and secondary are part of the manifold system, what they do and how they do it are more or less independent of one another.
Unless the primary lengths are unreasonably short (as in a block hugger style header where the length could be 12" or less), it matters little if they differ from one another even by a significant amount. This has to do with how the primary works at helping to scavenge a given cylinder as well as the nature of the V8s we're discussing (or V6s for that matter). Apparently this style of engine is somewhat insensitive to differences in length because the range rpm range the primaries effect is so broad due to the arrangement of the firing pulses, etc. A minimum of 18" to 20" seems to be a documented value (although I should probably double check that).
Please keep in mind that we're discussing an engine that isn't tuned to within an inch of it's life and requires a broad power range rather than being tuned to a narrow rpm band (which can be the case in a Pro Stock engine for example).
The secondary length has a far greater effect on the torque curve which is why, with the correct length and combined with other essential elements such as sufficient valve overlap, it's possible to increase volumetric efficiency and produce more power.
That's the point of the exhaust termination boxes. If you can nail down the optimum secondary length, (through theory or experimentation), then a properly designed ETB arrangement can give you very nearly open header power production while still utilizing a muffled exhaust system.
In conclusion, from what I've studied, don't be overly concerned with the primary length but instead concentrate on achieving the secondary length that provides the power curve your looking for.
Last edited by skinny z; 09-13-2012 at 04:48 PM.
09-13-2012, 08:35 PM
#103
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Re: More than you ever wanted to know about BACKPRESSURE...
Since most hot rodders want the car on the road and pulling so hard it will yank a fat lady off a tricycle, this has to be put in perspective. Although I am an older person now than when I was doing big time O.E. Motorsports in the late nineties and earlier 2000's for the company I was with don't lose track of this. This discussion (which goes back a long ways) is beginning to mix a good street car info into something that is a full tilt race car. Even in a full racing chassis I can promise you this. All of the above thread is packed with excellent info.
This must be remembered as we read all this good stuff. I hope you all the best but try to remember this. It is something John Linginfelter was very close to and we often discussed this. Wish he was still around. He always bought breakfast!
Single tube no collector straight pipes can be tuned to produce peak torque at RPM levels that are exactly where you intend to HOLD the engine. This is because of the way we drag race rails and the like today. Next the statement about collector length way up in the thread is right on the money. When you look under a championship car you will see long medium or short collector extensions. These tune the torque curve to where you want it in general the peak will move lower as the collector length increases. Don't forget the other comment way up the chain about cooling hot gas. The combustion temperature with racing gasoline had better be really close to 1400 deg. At 1400 deg. things start to melt. An engine running a Brake Specific fuel combustion slightly under or on .5 and 1375 deg. on the exhaust port. Is in perfect tune. As the exhaust moves out it cools fast and in effect becomes a helper in scavenging. Everything you do to isolate the individual cylinders helps a lot. This also has a great effect on efficiency. Put Jeff Gordon in that car at Watkins Glenn and he are going love it if you give him a very broad torque range (long primaries long collectors etc.). A “peaky” torque curve and he is going to drive it right in the trailer if it falls down anywhere pulling on a curve exit. At Daytona (you probably guessed it I am sure) it’s always wide open. So it is tuned to a very narrow band. Equal length everything here.
On a street car (if you go way back in the thread again) I mentioned in a street car you are wasting your time. We are not talking about a race track. The hot rod needs to do a bunch more stuff. As was pointed out above the difference is very minimal on tuning for length. If you are seeking the absolute max power, tune everything you can. At the port a lot of tricks are used as an evacuation method along with the pipes (cam overlap, and primary length) to draw the burned fuel to a cooler place. No cammed up engine runs better with back pressure. Different chassis combinations and racing tracks require a close eye on this. If you are racing then as was noted above, you want every bit of power and a perfect tune will give it to you. On a street car the "R"'s are everywhere from light to light. So in a theoretical perfectly tuned chassis you should leave no stone unturned. If you drive a Daytona car on the street it would run great at 175 to 200mph. It would be off everywhere else. Hope this helps. A big point here… we are not talking about full blown racing cars. Hot Rods are a blast. You need torque bands that are wide to have fun toolin’ around town.
Let’s answer the V6 issue. Use the Hooker Y pipe and mod it to fit your headers and shorties. Cut it where the pipes join and fab a 90 deg. 4” Elbow in its place. Now use the cat back of your choice (assuming you are going to run a “test Pipe” in place of the cat. You will now see what I mean. The hole inside is about 1.5 inches. When you are done you will have a nice single 3” back to the muff and then 3” duels out. I have this exact system on my car. The motor Dyno said 540 HP peak at 6500. The chassis Dyno said 525 at 6300. All the builders agreed before coffee…. Looks like with all the cooling effect the motor ran fine with a slightly restricted exhaust with the restrictions as far from the exhaust port as possible.
Use a crossover always. It keeps the presure equal side to side. A Y-Pipe is a crossover in function.
Bob
This must be remembered as we read all this good stuff. I hope you all the best but try to remember this. It is something John Linginfelter was very close to and we often discussed this. Wish he was still around. He always bought breakfast!
Single tube no collector straight pipes can be tuned to produce peak torque at RPM levels that are exactly where you intend to HOLD the engine. This is because of the way we drag race rails and the like today. Next the statement about collector length way up in the thread is right on the money. When you look under a championship car you will see long medium or short collector extensions. These tune the torque curve to where you want it in general the peak will move lower as the collector length increases. Don't forget the other comment way up the chain about cooling hot gas. The combustion temperature with racing gasoline had better be really close to 1400 deg. At 1400 deg. things start to melt. An engine running a Brake Specific fuel combustion slightly under or on .5 and 1375 deg. on the exhaust port. Is in perfect tune. As the exhaust moves out it cools fast and in effect becomes a helper in scavenging. Everything you do to isolate the individual cylinders helps a lot. This also has a great effect on efficiency. Put Jeff Gordon in that car at Watkins Glenn and he are going love it if you give him a very broad torque range (long primaries long collectors etc.). A “peaky” torque curve and he is going to drive it right in the trailer if it falls down anywhere pulling on a curve exit. At Daytona (you probably guessed it I am sure) it’s always wide open. So it is tuned to a very narrow band. Equal length everything here.
On a street car (if you go way back in the thread again) I mentioned in a street car you are wasting your time. We are not talking about a race track. The hot rod needs to do a bunch more stuff. As was pointed out above the difference is very minimal on tuning for length. If you are seeking the absolute max power, tune everything you can. At the port a lot of tricks are used as an evacuation method along with the pipes (cam overlap, and primary length) to draw the burned fuel to a cooler place. No cammed up engine runs better with back pressure. Different chassis combinations and racing tracks require a close eye on this. If you are racing then as was noted above, you want every bit of power and a perfect tune will give it to you. On a street car the "R"'s are everywhere from light to light. So in a theoretical perfectly tuned chassis you should leave no stone unturned. If you drive a Daytona car on the street it would run great at 175 to 200mph. It would be off everywhere else. Hope this helps. A big point here… we are not talking about full blown racing cars. Hot Rods are a blast. You need torque bands that are wide to have fun toolin’ around town.
Let’s answer the V6 issue. Use the Hooker Y pipe and mod it to fit your headers and shorties. Cut it where the pipes join and fab a 90 deg. 4” Elbow in its place. Now use the cat back of your choice (assuming you are going to run a “test Pipe” in place of the cat. You will now see what I mean. The hole inside is about 1.5 inches. When you are done you will have a nice single 3” back to the muff and then 3” duels out. I have this exact system on my car. The motor Dyno said 540 HP peak at 6500. The chassis Dyno said 525 at 6300. All the builders agreed before coffee…. Looks like with all the cooling effect the motor ran fine with a slightly restricted exhaust with the restrictions as far from the exhaust port as possible.
Use a crossover always. It keeps the presure equal side to side. A Y-Pipe is a crossover in function.
Bob
09-14-2012, 02:36 AM
#104
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Re: More than you ever wanted to know about BACKPRESSURE...
first I have a 305 v8, not a v6.
Second lets get this back into perspective. This is street car, NOT a race car. All I did was ask some basic questions, that I have not found any answers to, which seemed to be slight oversights, or left out on purpose at some point.
Third, All I wanted to do was tune the engines exhaust. I wanted to bring the TQ curve peak closer to the HP peak curve. This would allow the TQ to extend farther before it drops off. Factory is 3200 rpm TQ and 4800rpm HP. I wanted to bring the TQ peak up to at least 4000rpm without effecting the HP peak rpm.
Fourth, another piece of information I can't find straight answers on is...When you tune the exhaust, are you tuning for the peak TQ rpm, or the peak HP rpm. Every site I have encountered says tune for either or, So what is it? You tune for peak hp rpm or peak tq rpm?
Fifth, Bob, please don't tell me not to do something, just because you can't or wouldn't do it yourself. However if you suggest not doing something, then give me facts as to why not, No speculation, no opinions, no brothers-cousins-sisters-grandfathers-uncle had this happen. Just facts.
Second lets get this back into perspective. This is street car, NOT a race car. All I did was ask some basic questions, that I have not found any answers to, which seemed to be slight oversights, or left out on purpose at some point.
Third, All I wanted to do was tune the engines exhaust. I wanted to bring the TQ curve peak closer to the HP peak curve. This would allow the TQ to extend farther before it drops off. Factory is 3200 rpm TQ and 4800rpm HP. I wanted to bring the TQ peak up to at least 4000rpm without effecting the HP peak rpm.
Fourth, another piece of information I can't find straight answers on is...When you tune the exhaust, are you tuning for the peak TQ rpm, or the peak HP rpm. Every site I have encountered says tune for either or, So what is it? You tune for peak hp rpm or peak tq rpm?
Fifth, Bob, please don't tell me not to do something, just because you can't or wouldn't do it yourself. However if you suggest not doing something, then give me facts as to why not, No speculation, no opinions, no brothers-cousins-sisters-grandfathers-uncle had this happen. Just facts.
09-14-2012, 02:59 AM
#105
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Re: More than you ever wanted to know about BACKPRESSURE...
Oh and here are the cam specs as I have tediously figured out.
in/ex dur=207/213 lift=.415/.430 lsa=117 centerline=110atdc/120btdc overlap=-24
ivo=-6.5 btdc (-=atdc ) ivc=33.5 abdc
evo=50.5 bbdc evc= -17.5 atdc (-=btdc)
in/ex dur=207/213 lift=.415/.430 lsa=117 centerline=110atdc/120btdc overlap=-24
ivo=-6.5 btdc (-=atdc ) ivc=33.5 abdc
evo=50.5 bbdc evc= -17.5 atdc (-=btdc)
09-15-2012, 08:18 AM
#106
Supreme Member
Re: More than you ever wanted to know about BACKPRESSURE...
The comment regarding the V6 was to demonstrate the similarities to that of the V8 regarding how they respond to exhaust pulse tuning.
Second lets get this back into perspective. This is street car, NOT a race car. All I did was ask some basic questions, that I have not found any answers to, which seemed to be slight oversights, or left out on purpose at some point.
I'm always assuming a street car however there has to be the inevitable comparison to a racing engine at some point. Even it if it's only to clarify the definition. Some peoples race is other peoples street. For the record, I both drive and race my car and as such have to compromise on both fronts.
Third, All I wanted to do was tune the engines exhaust. I wanted to bring the TQ curve peak closer to the HP peak curve. This would allow the TQ to extend farther before it drops off. Factory is 3200 rpm TQ and 4800rpm HP. I wanted to bring the TQ peak up to at least 4000rpm without effecting the HP peak rpm.
This isn't so much about exhaust as it is an overall engine program. By working with the secondary length you can manipulate where the engine develops peak torque and thus directly affecting the peak HP value and RPM.
I would suggest a program like PipeMax to really nail down what you after. Besides, theoretical engine is a lot cheaper than building and experimenting.
To really alter the engines torque and horsepower characteristics, you have to get into the cam profile (especially) as well as cylinder head size and flow and compression ratio.
That's where a simulation program really comes into it's own. You can build as many virtual engine configurations as you want until you start to see how things are affected by each other.
Fourth, another piece of information I can't find straight answers on is...When you tune the exhaust, are you tuning for the peak TQ rpm, or the peak HP rpm. Every site I have encountered says tune for either or, So what is it? You tune for peak hp rpm or peak tq rpm?
The answer here is yes to both. One directly affects the other however you can tune the exhaust for a specific rpm or rpm range. It depends on what your ultimate goal is.
Fifth, Bob, please don't tell me not to do something, just because you can't or wouldn't do it yourself. However if you suggest not doing something, then give me facts as to why not, No speculation, no opinions, no brothers-cousins-sisters-grandfathers-uncle had this happen. Just facts.
No comment.
09-20-2012, 10:16 AM
#107
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Re: More than you ever wanted to know about BACKPRESSURE...
Sorry about that skinny z! Old guys who used to race (me) just love to talk about this stuff. I am retired and the bench racing is coming to an end. Definitely didn't mean to upset you on that.
This is a great forum and I really enjoy it. I did not mean to offend anyone. I am learning how to engage you guy's (everyone on all forums) and not come busting in over the top. I don't think we disagree on anything. You are right on track and very knowledgeable in my book. Hope we are ok on that. I know it is a v6 you are looking at and understand your question. I definitely didn't mean to sound like I was telling you to do things. Just letting some experience from my past out there. I'll stop on this one. You have it down to what you want. One last thing that is a huge lobe center @ 117! Most Chevys like 104 to 108 for max effort motors as you know I am sure. The lobe center that is ground in is really a lot more important than most people realize. Too bad we can't change that with an oversize and offset cam bushing. For me it was a lot of fun making changes with the dyno and sitting back and watching what happens. Those were the day's Talk to ya later.
Respectfully,
Bob
This is a great forum and I really enjoy it. I did not mean to offend anyone. I am learning how to engage you guy's (everyone on all forums) and not come busting in over the top. I don't think we disagree on anything. You are right on track and very knowledgeable in my book. Hope we are ok on that. I know it is a v6 you are looking at and understand your question. I definitely didn't mean to sound like I was telling you to do things. Just letting some experience from my past out there. I'll stop on this one. You have it down to what you want. One last thing that is a huge lobe center @ 117! Most Chevys like 104 to 108 for max effort motors as you know I am sure. The lobe center that is ground in is really a lot more important than most people realize. Too bad we can't change that with an oversize and offset cam bushing. For me it was a lot of fun making changes with the dyno and sitting back and watching what happens. Those were the day's Talk to ya later.
Respectfully,
Bob
09-27-2012, 02:08 AM
#108
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Re: More than you ever wanted to know about BACKPRESSURE...
Well I do apologize, an my justification is that everyone follows the trodden path.
My goal was to create a wider torque band with semi stock stuff. I figured if I should be able to spread the peak a lil wider. However I did notice "stock" that @ bout 4k this thing pulled like horny mule. What I can't figure out is if that was the tq or the hp?
lobe seperation angle is 117 ILC is 110atdc and ELC is 120btdc
I was considering doing a degree wheel and dial indicator test, just to see what it REALLY spec out to be. Even certain LS1s' have similar specs (Lack the duration/ higher lift.) I just consider my cam the LB9 hot peanut cam.
My goal was to create a wider torque band with semi stock stuff. I figured if I should be able to spread the peak a lil wider. However I did notice "stock" that @ bout 4k this thing pulled like horny mule. What I can't figure out is if that was the tq or the hp?
lobe seperation angle is 117 ILC is 110atdc and ELC is 120btdc
I was considering doing a degree wheel and dial indicator test, just to see what it REALLY spec out to be. Even certain LS1s' have similar specs (Lack the duration/ higher lift.) I just consider my cam the LB9 hot peanut cam.
09-29-2012, 12:14 AM
#109
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Re: More than you ever wanted to know about BACKPRESSURE...
Ya you would really like those timing events. With that wide of a LCA I'll bet there is very little if any overlap. That almost negates the need for much of an exhaust because without the overlap the rush for the exhaust to attempt to enter the nice cool intake air and fuel stream do to back pressure, won't be as severe. A wide centerline like that can make a car feel like it has "torque from idle to the top". The dyno pull would probably show a torque curve that starts immediately and stays flat till way up the rpm axis's. Unfortunately the trade off is a great deal of HP lost on the top but driveablity would sure be good.
I'll give you a way we used to train new engineers on the torque vs. HP thought. I don't know who started it but it goes like this... "It takes Torque to accelerate and HP to maintain speed".
You are dead serious about that exhaust you are really in there. I think you are definitely taking on the tough part first. I have my son's 84 up in the air to tone the trans down from neck break to a little softer shift. I will take a picture and you can see what I did to this one (kind of made an expansion chamber at the 90 on the Y pipe). If your still trying to figure out what you want to do. Linginfelter wrote a couple of books on building small blocks. His latest one talks about exhaust plenum's. He kept bumping into the issue of to much exhaust gas and not a big enough drain! He built a chamber box or really a plenum in the exhaust in any spot he could find. It worked. I did the same with this Bird (1984 TA) at the 90 on the y-pipe. It cost me about 20 horses from Engine to Chassis dyno but, there is 517 HP @ 6000 and a lot of torque. It is a 383 6" long rod motor. I was really surprised because I came down from the head port with the shorties from Hooker modified their Y pipe and hooked it to a 3" Magnaflow complete single 3" system all the way back. I think it has a lot to do with the effect of the cooling gas. It sounds to me as if you stated earlier, you were going after all the power you can get out of what you have. Is that a factory roller cam you have or is it a flat tappet? There is a machine called a "Cam Doctor" that some speed shops have. They came out in the late 70's because a lot of us wanted to know the exact timing events. It would be interesting if you pulled that cam and sent it to Crane or Chet Herbert or any good cam grinder and kept it all the same except cut that centerline down to 108. Your tuned exhaust should definitely respond to that. I think with the LCA @ 108 it would be easier to evacuate with tuned exhaust and the torque should be awfully good. If you're exhaust is in tune right at the RPM's for the average speed you drive.
Too bad it's your driver. It would probably take to long to have your car down unable to drive. Sure would be interesting though. If your exhaust works the way you want, it should run pretty good. I'll bet it Will scavenge like mad and not be to bad on the hydrocarbons. Good luck man, I am always looking for your posts. There is sure a lot of thought in your car.
Bob
I'll give you a way we used to train new engineers on the torque vs. HP thought. I don't know who started it but it goes like this... "It takes Torque to accelerate and HP to maintain speed".
You are dead serious about that exhaust you are really in there. I think you are definitely taking on the tough part first. I have my son's 84 up in the air to tone the trans down from neck break to a little softer shift. I will take a picture and you can see what I did to this one (kind of made an expansion chamber at the 90 on the Y pipe). If your still trying to figure out what you want to do. Linginfelter wrote a couple of books on building small blocks. His latest one talks about exhaust plenum's. He kept bumping into the issue of to much exhaust gas and not a big enough drain! He built a chamber box or really a plenum in the exhaust in any spot he could find. It worked. I did the same with this Bird (1984 TA) at the 90 on the y-pipe. It cost me about 20 horses from Engine to Chassis dyno but, there is 517 HP @ 6000 and a lot of torque. It is a 383 6" long rod motor. I was really surprised because I came down from the head port with the shorties from Hooker modified their Y pipe and hooked it to a 3" Magnaflow complete single 3" system all the way back. I think it has a lot to do with the effect of the cooling gas. It sounds to me as if you stated earlier, you were going after all the power you can get out of what you have. Is that a factory roller cam you have or is it a flat tappet? There is a machine called a "Cam Doctor" that some speed shops have. They came out in the late 70's because a lot of us wanted to know the exact timing events. It would be interesting if you pulled that cam and sent it to Crane or Chet Herbert or any good cam grinder and kept it all the same except cut that centerline down to 108. Your tuned exhaust should definitely respond to that. I think with the LCA @ 108 it would be easier to evacuate with tuned exhaust and the torque should be awfully good. If you're exhaust is in tune right at the RPM's for the average speed you drive.
Too bad it's your driver. It would probably take to long to have your car down unable to drive. Sure would be interesting though. If your exhaust works the way you want, it should run pretty good. I'll bet it Will scavenge like mad and not be to bad on the hydrocarbons. Good luck man, I am always looking for your posts. There is sure a lot of thought in your car.
Bob
Last edited by bmorgil; 09-29-2012 at 12:45 AM. Reason: spelling
10-24-2012, 05:40 AM
#110
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Re: More than you ever wanted to know about BACKPRESSURE...
as far as overlap. every calculator I've plugged the number into says its -24 degs. I'm not sure if I read it right, but to me, that means there is no or a negative amount of overlap. An from what I remember, and engine is not suppose to be able to run with no overlap. But I'm not 100% on that.
I noticed before messing with the exhaust, The car would kick the posi in at around 1700k. 20mph, first OR second gear, above 1700 and I'm nearly drifting a corner. If I'm at or above 2500k and not at least in third, The term straight doesn't apply. lol
The original owner swore up and down that he bounced the car off of the 150mph mark. I know, personally the car will do 130. highway raced an 96 lt1 t/a. Its funny, some guys are amazed at what a person car can do. All he wanted to know was what I did to it. So I had to tell the truth, Not a damn thing. As far as I know its stock.
Now I've come to the conclusion that its no picknik to tune the exhaust. But I figured, if mathematically my tq peak is at 4k, then I'm going to tune the exhaust, exactely for that rpm. This way, I'm not "Borrowing" from either side of the peak. Normally tuning the secondary length, doesn't change the peak, but rather borrow from either side of it. I contemplated plated and figured its best for me to not do that at this time. This way I figure it will reinforce both sides and the peak without ill effect.
To my knowledge, this a factory hyd roller cam. The reason I keep it stock is its a street car. An street cars don't really need allot of High rpm hp. Its not very often you get to blast the tach like that. But what you want is VERY broad torque band. HP doesn't come into play until you reach the point that the torque an hp converge, at that point the tq is on its way down, the HP is on its way up. TQ can no longer push the car, but HP can pull the car. Just like you pointed out. Its proven once tq goes down, then it cannot by the laws of physics maintain velocity. That is when PH kicks in, as it will maintain and/or increase velocity.
I was taught, tq is acceleration hp is top end. Top end, how fast a car can go has always been rated by HP not torque. There is no engine in existance, that at max rpm creates tq, it always hp. Another way I was taught this, TQ is how much work the engine can do, HP is how FAST that engine can do it. You can see examples of this with the 2.5L L4, 4.3L and most 250/292 L6.
Now a street car requires the torque, and its best the lower that tq and the longer it lasts. The hardest I've pushed this car is about 4100k so I haven't been able to noticed anything beyond that. The TQ comes on around 1500-1700, becomes noticeable at 3k and feel like a freight train is pushing you 4k. So the torque is very broad. Just like my trucks, but more viscious.
I do intend to do a cam spec, but I refuse to take anything off the engine just yet. So a cam chick will be performed while its in the engine and still in the car.
I noticed before messing with the exhaust, The car would kick the posi in at around 1700k. 20mph, first OR second gear, above 1700 and I'm nearly drifting a corner. If I'm at or above 2500k and not at least in third, The term straight doesn't apply. lol
The original owner swore up and down that he bounced the car off of the 150mph mark. I know, personally the car will do 130. highway raced an 96 lt1 t/a. Its funny, some guys are amazed at what a person car can do. All he wanted to know was what I did to it. So I had to tell the truth, Not a damn thing. As far as I know its stock.
Now I've come to the conclusion that its no picknik to tune the exhaust. But I figured, if mathematically my tq peak is at 4k, then I'm going to tune the exhaust, exactely for that rpm. This way, I'm not "Borrowing" from either side of the peak. Normally tuning the secondary length, doesn't change the peak, but rather borrow from either side of it. I contemplated plated and figured its best for me to not do that at this time. This way I figure it will reinforce both sides and the peak without ill effect.
To my knowledge, this a factory hyd roller cam. The reason I keep it stock is its a street car. An street cars don't really need allot of High rpm hp. Its not very often you get to blast the tach like that. But what you want is VERY broad torque band. HP doesn't come into play until you reach the point that the torque an hp converge, at that point the tq is on its way down, the HP is on its way up. TQ can no longer push the car, but HP can pull the car. Just like you pointed out. Its proven once tq goes down, then it cannot by the laws of physics maintain velocity. That is when PH kicks in, as it will maintain and/or increase velocity.
I was taught, tq is acceleration hp is top end. Top end, how fast a car can go has always been rated by HP not torque. There is no engine in existance, that at max rpm creates tq, it always hp. Another way I was taught this, TQ is how much work the engine can do, HP is how FAST that engine can do it. You can see examples of this with the 2.5L L4, 4.3L and most 250/292 L6.
Now a street car requires the torque, and its best the lower that tq and the longer it lasts. The hardest I've pushed this car is about 4100k so I haven't been able to noticed anything beyond that. The TQ comes on around 1500-1700, becomes noticeable at 3k and feel like a freight train is pushing you 4k. So the torque is very broad. Just like my trucks, but more viscious.
I do intend to do a cam spec, but I refuse to take anything off the engine just yet. So a cam chick will be performed while its in the engine and still in the car.
10-24-2012, 10:31 AM
#111
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Re: More than you ever wanted to know about BACKPRESSURE...
There is quite a bit of misunderstanding going on.
An engine can run just fine with no overlap - there is no requirement to have overlap. In fact, way back when (100+ years ago) the 4-strokes of the day had NO overlap whatsoever. They were slow speed motors and the people then were just happy that they ran at all.
Overlap is pretty much unavoidable in a naturally aspirated engine if you want it to have "high" power output at "high" RPM (what defines "high" will vary from engine to engine, and overlap in a supercharged or turbocharged motor is an ENTIRELY different animal). Overlap is partially a consequence of increasing cam duration (the duration has to go SOMEWHERE), partially a consequence of physics (at higher rpm, keeping the exhaust valve open longer and opening the intake valve earlier helps the exiting exhaust to more fully scavenge the cylinder and lower the cylinder pressure, atmospheric pressure then pushes more incoming air/fuel into the cylinder), and (believe it or not) partially an emissions decision - overlap at idle reduces cylinder pressure (and thus temperature) and has been used to control NOx emissions (often called self-EGR). The effects of overlap are very different depending on what RPM you are observing the effects at. Very generally speaking, more overlap decreases low RPM efficiency (and thus low rpm torque) while allowing for a POTENTIAL increase in higher rpm efficiency (and thus a potential increase in higher rpm torque).
Keep in mind that cams are measured in many different ways - two common ways are at 0.050" lift and at 0.06" lift (sometimes called tappet lift, and this is the point that advertised duration is often pulled from). It's entirely possible to have no calculated overlap when measured at the 0.050" lift points, but then to have overlap when measured at the 0.06" lift points.
HP and torque are highly related. Torque is the amount of force the engine can generate for one revolution, whereas HP is the amount of times the engine can generate that torque in some specific amount of time - how many times can the engine revolve in one minute generating that torque. Hence horsepower is a product of torque and rpm. One doesn't push the car, one doesn't pull the car, they don't kick out or kick in. There are diesel engines out there with way more torque than a stock v8, but they have way less horsepower at the same time. Guess which one will win 0-60? Torque may technically be the force that accelerates the car, but horsepower tells you HOW FAST you can accelerate the car. Another example: F1 cars have less torque than my lawnmower (ok maybe not quite but..), but they have obnoxious horsepower, and it's the horsepower that gives them their amazing acceleration.
All engines generate torque at their peak (max) horsepower. Some engines even generate maximum torque at or near their maximum horsepower - usually these are engines with great cylinder heads, great intakes, etc. Torque is pretty much a direct measurement of an engines volumetric efficiency at a given RPM point. The reason our low-rpm V8's have such large torque fall-offs with raising RPM is because they have poor volumetric efficiency at high rpm - the cylinder heads can't flow, the TPI intake especially can't flow, etc. etc. It would be many years before the LS1 came along when GM figured out how to make heads that flow and intakes that flowed. THese engines had way better volumetric efficiency at mid and upper RPM's, which gave them their distinctive flat torque curve, and as a result their great HP. They did not lose their torque nearly as fast as the RPMs climbed, so they made more horsepower.
It's true that lower to mid rpm torque can make a car FEEL fast - GM showed us that with the TPI motor. But can you truly call a stock TPI motor fast? Not really...
An engine can run just fine with no overlap - there is no requirement to have overlap. In fact, way back when (100+ years ago) the 4-strokes of the day had NO overlap whatsoever. They were slow speed motors and the people then were just happy that they ran at all.
Overlap is pretty much unavoidable in a naturally aspirated engine if you want it to have "high" power output at "high" RPM (what defines "high" will vary from engine to engine, and overlap in a supercharged or turbocharged motor is an ENTIRELY different animal). Overlap is partially a consequence of increasing cam duration (the duration has to go SOMEWHERE), partially a consequence of physics (at higher rpm, keeping the exhaust valve open longer and opening the intake valve earlier helps the exiting exhaust to more fully scavenge the cylinder and lower the cylinder pressure, atmospheric pressure then pushes more incoming air/fuel into the cylinder), and (believe it or not) partially an emissions decision - overlap at idle reduces cylinder pressure (and thus temperature) and has been used to control NOx emissions (often called self-EGR). The effects of overlap are very different depending on what RPM you are observing the effects at. Very generally speaking, more overlap decreases low RPM efficiency (and thus low rpm torque) while allowing for a POTENTIAL increase in higher rpm efficiency (and thus a potential increase in higher rpm torque).
Keep in mind that cams are measured in many different ways - two common ways are at 0.050" lift and at 0.06" lift (sometimes called tappet lift, and this is the point that advertised duration is often pulled from). It's entirely possible to have no calculated overlap when measured at the 0.050" lift points, but then to have overlap when measured at the 0.06" lift points.
HP and torque are highly related. Torque is the amount of force the engine can generate for one revolution, whereas HP is the amount of times the engine can generate that torque in some specific amount of time - how many times can the engine revolve in one minute generating that torque. Hence horsepower is a product of torque and rpm. One doesn't push the car, one doesn't pull the car, they don't kick out or kick in. There are diesel engines out there with way more torque than a stock v8, but they have way less horsepower at the same time. Guess which one will win 0-60? Torque may technically be the force that accelerates the car, but horsepower tells you HOW FAST you can accelerate the car. Another example: F1 cars have less torque than my lawnmower (ok maybe not quite but..), but they have obnoxious horsepower, and it's the horsepower that gives them their amazing acceleration.
All engines generate torque at their peak (max) horsepower. Some engines even generate maximum torque at or near their maximum horsepower - usually these are engines with great cylinder heads, great intakes, etc. Torque is pretty much a direct measurement of an engines volumetric efficiency at a given RPM point. The reason our low-rpm V8's have such large torque fall-offs with raising RPM is because they have poor volumetric efficiency at high rpm - the cylinder heads can't flow, the TPI intake especially can't flow, etc. etc. It would be many years before the LS1 came along when GM figured out how to make heads that flow and intakes that flowed. THese engines had way better volumetric efficiency at mid and upper RPM's, which gave them their distinctive flat torque curve, and as a result their great HP. They did not lose their torque nearly as fast as the RPMs climbed, so they made more horsepower.
It's true that lower to mid rpm torque can make a car FEEL fast - GM showed us that with the TPI motor. But can you truly call a stock TPI motor fast? Not really...
10-24-2012, 11:10 AM
#112
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11-08-2012, 03:38 PM
#113
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Re: More than you ever wanted to know about BACKPRESSURE...
Well one thing that I noticed about the torque curve especially between the l98/lt/ls isn't so much of a difference. One graph of an engine was the original l98 config and then switched to an lt1. Granted the difference was the l98 had WAY more torque 40ftlb and about 40hp lower than the lt1. Torque came on hard with the l98 but both the torques curves at the same rpm with the same amount.
Lt 1 was down on the torque but had quite the flat line, the l98 had a fat curve but always had more torque all the way around, even when the curve was coming down, l98 it still had more torque. when I looked closer, even when the l98 torque curve had descended about 1/3 of its path, and the t1 had peaked in its torque curve...The L98 STILL had more torque. Now granted this was on a built can't remember either a 375 or 383.
So what is better? a sooner beefy peaky torque curve, or a later more linear torque curve?
Lt 1 was down on the torque but had quite the flat line, the l98 had a fat curve but always had more torque all the way around, even when the curve was coming down, l98 it still had more torque. when I looked closer, even when the l98 torque curve had descended about 1/3 of its path, and the t1 had peaked in its torque curve...The L98 STILL had more torque. Now granted this was on a built can't remember either a 375 or 383.
So what is better? a sooner beefy peaky torque curve, or a later more linear torque curve?
11-08-2012, 07:06 PM
#114
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08-16-2013, 12:23 AM
#115
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Re: More than you ever wanted to know about BACKPRESSURE...
I know this post was written a very long time ago. and correct me if i'm wrong. But I recently was told that in an engine with a camshaft that has very little to no overlap (I don't know if cams can have no overlap), exhaust tuning has little to no effect. Because of the simple fact that if the exhaust valve is closed during the opening of the intake valve there is almost no chance for exhaust scavenging to take place before the next combustion event. Is this true. any thoughts on this one?
08-16-2013, 04:05 PM
#116
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Re: More than you ever wanted to know about BACKPRESSURE...
An interesting point and a valid one I would think. It would be correct in saying that if the overlap event is very small, then the opportunity to initiate an exhaust pulse induced induction event is limited. This showed up in a test one of the forum members did on his TPI Southern California car. He built exhaust termination boxes as per spec (this helps create the negative pressure wave that arrive at the exhaust valve at the appropriate time) however with his wide lobe seperation cam there was very little overlap and the results of his test I think were less than impressive. I know you can't draw a conclusion based on the results of one experiment however it did raise an interesting point. One that you've brought back to the front.
Any succesful testing that I've read about look to have cams with overlap in excess of 60 degrees. Often considerably more and you can imagine the benefits with this have when combined with a properly tuned header secondary length.
Any succesful testing that I've read about look to have cams with overlap in excess of 60 degrees. Often considerably more and you can imagine the benefits with this have when combined with a properly tuned header secondary length.
10-15-2013, 03:45 AM
#117
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Re: More than you ever wanted to know about BACKPRESSURE...
So an exhaust cutout pipe before the cat on a stock 305 tbi camaro with a car-back exhaust is bad idea?
10-19-2013, 02:19 PM
#118
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07-16-2014, 10:33 AM
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Re: More than you ever wanted to know about BACKPRESSURE...
OK so correct me if im wrong but for a 86 trans am 305 TPI I was looking at a catless 3" borla exhaust w/ the adjustable backpressuer plate with a y pipe and not sure what would be the best headers for it but am looking at 3 inch collector... Was kinda thinking of equal length headers... Would this be a good set up?
I don't have a lot of modes... Lvl 2 tune, no emissions, CAI, air foil, msd HEI cap and rotor, msd wires?
Just want to do it right.
I don't have a lot of modes... Lvl 2 tune, no emissions, CAI, air foil, msd HEI cap and rotor, msd wires?
Just want to do it right.
08-07-2014, 12:27 PM
#120
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Re: More than you ever wanted to know about BACKPRESSURE...
Awesome info man! I have a question for you on my exhaust system.
I have a an 87 Iroc with a TPI 383 stroker that's supercharged. I am thinking of going with into a single 3" exhaust into a hooker aero chamber muffler with 2.5 outlets.
Should I use a high flow cat with this system or not use a cat at all? Should I use a muffler with bigger outlets?
Is 3" exhaust enough for this or should I go bigger? What are your suggestions?
I have a an 87 Iroc with a TPI 383 stroker that's supercharged. I am thinking of going with into a single 3" exhaust into a hooker aero chamber muffler with 2.5 outlets.
Should I use a high flow cat with this system or not use a cat at all? Should I use a muffler with bigger outlets?
Is 3" exhaust enough for this or should I go bigger? What are your suggestions?
Last edited by ThirdGenKen; 08-07-2014 at 12:42 PM.
09-04-2014, 02:00 PM
#121
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Re: More than you ever wanted to know about BACKPRESSURE...
Well, you got me thinking!
I am currently gathering all the parts & pieces for my build, and a big part of it will be modding the floor pan (more precisely, the trans /drive shaft tunnel) .
My plan is to create a" C" channel from the rear of the front seats, that opens out to the rear seat bowl (if you will) , then cutting the" bowl" out and replacing it with a flat piece So that I can fit a 3-3.5" pipe w/X & two 3.5" in/out xr1 Borlas, that dump right in front of rearend
Oh and looking at DOUG'S 13/4" primary tube headers 3.5"collectors, for my 11.5:1cr. 409 sbc, solid roller 4/7 swap 261/271@.050 698/655 lift on a 108 center line, Dart 215 pro1's
ported by HVH) with gasket match to 1207's int. And supplied exhaust gaskets with headers , topped of with tall raised plenum single plain & demon 850do.
Any thoughts and recommendations appreciated
I am currently gathering all the parts & pieces for my build, and a big part of it will be modding the floor pan (more precisely, the trans /drive shaft tunnel) .
My plan is to create a" C" channel from the rear of the front seats, that opens out to the rear seat bowl (if you will) , then cutting the" bowl" out and replacing it with a flat piece So that I can fit a 3-3.5" pipe w/X & two 3.5" in/out xr1 Borlas, that dump right in front of rearend
Oh and looking at DOUG'S 13/4" primary tube headers 3.5"collectors, for my 11.5:1cr. 409 sbc, solid roller 4/7 swap 261/271@.050 698/655 lift on a 108 center line, Dart 215 pro1's
ported by HVH) with gasket match to 1207's int. And supplied exhaust gaskets with headers , topped of with tall raised plenum single plain & demon 850do.Any thoughts and recommendations appreciated
02-17-2016, 03:59 AM
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Re: More than you ever wanted to know about BACKPRESSURE...
Hi i have a normal stock tbi 350 engine in a 95 chevrolet cheyenne ext cab truck 2wd 4l60e with believed 3.42 gears on 24x10 wheels with 255 30 24 tires. Operating range is 500-4800 rpm daily. Im a fan of open exhaust. I'm still running factory manifolds and y pipe. Punched cat to 3inch single exhaust. Flowmaster 50series. Hate flowmaster. I wanna run a dynomax race bullet right at the back axle. What are your thoughts on this. Thank you.
04-27-2016, 08:48 PM
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Re: More than you ever wanted to know about BACKPRESSURE...
Idk what I just read...seriously. So I just did the exhaust on my 92 RS. I hollowed the cat and threw on a flowmaster 80 series. So did hollowing the cat improve anything theoretically? Also the tube running rim the cat to the EGR, what exactly is it's purpose? And what would happen if it was capped off? I had to cut it when I pulled the cat to hollow it out. I haven't connected the two back together yet. After I let off the accelerator and slow down a little the engine light comes on. As soon as hit the accelerator the light goes back off. I'm going to hook the two back together tomorrow. I just wanted to know its purpose.
04-28-2016, 05:47 AM
#124
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Re: More than you ever wanted to know about BACKPRESSURE...
Idk what I just read...seriously. So I just did the exhaust on my 92 RS. I hollowed the cat and threw on a flowmaster 80 series. So did hollowing the cat improve anything theoretically? Also the tube running rim the cat to the EGR, what exactly is it's purpose? And what would happen if it was capped off? I had to cut it when I pulled the cat to hollow it out. I haven't connected the two back together yet. After I let off the accelerator and slow down a little the engine light comes on. As soon as hit the accelerator the light goes back off. I'm going to hook the two back together tomorrow. I just wanted to know its purpose.
-- Joe
04-29-2016, 09:23 PM
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11-12-2016, 09:17 AM
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Re: More than you ever wanted to know about BACKPRESSURE...
Good Saturday Morning! This is a hugely interesting topic.
I ran across this thread while searching for a solution. My 2001 Lumina 3.1L V6 was "kissed" in the rear and my exhaust is messed up, hanging with a coat hanger. Seems like an opportunity for an upgrade.
I've seen that several performance vendors offer a "from the cat back" dual system using 2, 2.5" pipes and mufflers for the 3.1L for 1990-92 Camaros and Firebirds. Since my contemplation is to upgrade to a 2.25" high-flow cat, use a single 2.5" pipe connected to a 2.5" turbo (not a glasspack) muffler, seems like a 2.5" would not be "overkill", but I could be wrong. Now my concern is determining optimal pipe and total system length using the concepts urbanhunter has shared with us, and avoiding the dreaded "drone" I've read so much about that makes people go deaf.
The transaxle gearing makes my car "live" in the 2000-2600 RPM range like 90% of the time, but I'd like a pipe diameter and length that will allow aggressive responsiveness for merging, passing or avoiding sudden emergencies. Does anyone know of a formula or "plug & play" I can use to to compute it (I know-I need to change my pipe tobacco), or have any suggestions?
Thanks a bunch.
Dave
I ran across this thread while searching for a solution. My 2001 Lumina 3.1L V6 was "kissed" in the rear and my exhaust is messed up, hanging with a coat hanger. Seems like an opportunity for an upgrade.
I've seen that several performance vendors offer a "from the cat back" dual system using 2, 2.5" pipes and mufflers for the 3.1L for 1990-92 Camaros and Firebirds. Since my contemplation is to upgrade to a 2.25" high-flow cat, use a single 2.5" pipe connected to a 2.5" turbo (not a glasspack) muffler, seems like a 2.5" would not be "overkill", but I could be wrong. Now my concern is determining optimal pipe and total system length using the concepts urbanhunter has shared with us, and avoiding the dreaded "drone" I've read so much about that makes people go deaf.
The transaxle gearing makes my car "live" in the 2000-2600 RPM range like 90% of the time, but I'd like a pipe diameter and length that will allow aggressive responsiveness for merging, passing or avoiding sudden emergencies. Does anyone know of a formula or "plug & play" I can use to to compute it (I know-I need to change my pipe tobacco), or have any suggestions?
Thanks a bunch.
Dave
Last edited by Luminated; 11-12-2016 at 09:28 AM.
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