OK - I've always heard, read, etc. that you need some backpressure. I've just always taken it for fact (as everyone else does). But I got to thinking about it - a friend of mine has a 1969 Camaro with a 502 - super clean and mean machine. And he has electric exhaust cutouts just after the headers (basically under the front floor boards). Given the headers, and the exhaust cutouts, where is the backpressure?

The exhaust dumps right out after the headers - dual exhaust so no Y pipe. When he opens the cutouts, besides being loud enough to shatter glass, the car runs great - you can really tell the power difference between him running with and without the cutouts open (way more accelleration with them open). So why is it a given that some backpressure is needed?

I mean if he has that little backpressure - there's no way any of us with our restrictions (ie ability to not run true duals very well, no room under car for larger pipes, etc) could not have enough backpressure almost regardless of what we do. Maybe the computer controlled setups need to have more backpressure than a straight non-computerized carbed motor?

Help an idiot understand these things. Thx.

 

Who says it is?

 

i find that people often mistake this, "back pressure", with exhaust velocity...

 

You going to run your vehicle wide open all the time?
Too little and you lose low end torque but gain in upper and vise versa.
A tuned exhaust is supposed to give you the best with good drivabilty.

 

#1- please provide a link to an article, not a chat forum where this is being said

#2- I don't so I guess not "everyone does"

#3- again, why do you believe this? Do me a favor, try running 3 miles with your nostrils pinched a little and mouth shut tight.

You'll become fatigued and need to stop because you can't exhale enough nor intake in enough fresh air because your lungs are filled with stale air

 

this is a good read if you want to know some exhaust stuff OP

http://www.pontiacstreetperformance....p/exhaust.html

The Strong Silent Type

"You can build a quiet exhaust system that performs almost like open headers."

By Jim Hand & Tom Hand

 

backpressure is bad. Exhaust velocity is what you want. the velocity actually helps pull more exhaust out. And since engines are giant air pumps the more you pull out the more you can help pull in etc...I tell my father is all the time and he still doesnt understand.

 

Think some of the "myth" of it is just miss place knowledge about 2 stroke engines. That don't function right without some and better with expansion chambers. Use to be an upgrade for old street 2 strokes that had reg exhaust.

 

OK - so maybe backpessure isn't necessary then. That's why I was asking. I mean if engine guru's state that some degree of backpressure is needed, I won't dispute it - I'm certainly no expert with regards to anything. But it did make me wonder why, since I've seen cars that, the less the restriction the better they ran.

In regards to where I read/see it, I guess here at TGO more than anywhere else (since I'm reading and learning more from here than anywhere else). There's a gazillion threads here that mention "not having enough backpressure" as an answer to many questions.

"Everyone else" was a generalization, and a bad one I see now - sorry. Again, basing my questions and assumptions on the many "backpressure" statements I've seen on TGO.

In regards to WOT all the time, I never stated (or meant) that the performance increase was at WOT - I just meant that the 69 I mentioned with the cutouts runs faster/better when the cutouts are opened. That car can't really run at WOT on the street - it just breaks traction too easily and goes out of control. It's a wicked ride, the owner has yet to take it to the track, which I understand. He has so much cash tied up in that car, that even the slightest scratch would break his heart - it's all go, but used for show - absolutely a perfect restore - if you are a 1st gen Camaro fan you'd gladly castrate yourself to have that car it's that nice.

So all this kinda answers my question then. I mean intuitively I understand that an engine is an "air pump", and so the backpressure issue didn't make alot of sense to me "intuitively" - but I guess I never stopped to think about it until recently.

Thanks for the link Gumby - I'll check it out.

So - one step further then - what happens (besides total deafness) if there is no exhaust piping at all - no manifolds or headers - just open exhaust ports on the heads? Under a hood, I assume the motor would choke on it's own exhaust fumes - but say no hood, or on a stand. Any effect (good or bad)? I assume that, funny cars, tractor-pull engines, etc. that just have 2ft of pipes bent out/up are basically not restrictred in anyway at all right? Does an engine need to be 'tuned' differently for changes in exhaust restriction?

Again - no arguement here - just me questioning and trying to learn. Thanks everyone!

 

The output power of the engine will change with the different tuning of the different exhaust systems.

 

There is alot of science to this and too much information to be posted in a forum thread as books have been written on the subject. David Vizard has written a few magazine articles talking about "zero-loss" exhaust systems and does a good job on explaining how everything works.

Another good book is "Scientific designs of intake and exhaust systems"
http://www.amazon.com/Scientific-Exh.../dp/0837603099

Basically you need some length of pipe on the port to help draw the exhaust gas out of the cylinder. A good header design will develop some vacuum in the exhaust (at the collector) which will help pull more exhaust out of the cylinder. gas moves from high pressure to low pressure, and if you have more exhaust backpressure, you will not beable to pull as much gas out of the cylinder.

Depending on how good your exhaust system is, that will change camshaft design and will effect the tune of the motor. Simple example...4th gen LS1 fbodys gain alot of power with exhaust mods but really capitalize on those mods with a good dyno tune.

 

With a 502, he needs to move a huge amount of air, so I'm sure it's strangled with whatever exhaust system he has on there. Most of the people on this board have very limited experience regarding experimenting with different exhaust system configurations and doing back-to-back dyno tests, but too big a header or even exhaust system tubing will loose power, especially mid range. Obviously, to make power you have to move air into the engine and get exhaust out of it, but there is a strong tuning effect with exhaust systems, and bigger is not always better because it trades off mid-range for high-end, and if you go big enough, you even start loosing high-end power because the engine just doesn't need that much flow potential. Supercharged engines (Tractor pulls, etc) can get away with minimal backpressure, naturally aspirated engines are much less forgiving, and in the type of competitons you have mentioned, drivablity and mid range are absolutely not necessary, only peak power is the objective.

It is a common misconception in circumstances like what the original poster describes that you have picked up a huge amount of high-end power, when in fact, you have really just killed off your mid-range, and the power spike from the now dead mid-range causes the car to blow the tires off and scream like a banshee, when in fact you have gained 5 peak HP and lost 30 in the mid range. Only running against another (unchanged) car or on a dyno allows you to really see what is happening.

TA

 

Well you can have optimal collector length for the rpm band the engine operates in. If the exhaust system is designed right and features the right length collector, you will gain more hp and tq everywhere the engine operates in that power band. There are different optimal lengths for different rpm ranges and some lengths promote best hp and others help produce best torque. There is some tradeoff there.

So it is possible to uncap the exhaust at cutouts and pick up more torque and power everywhere as long as its in the right location and the original full length exhaust was not optimal to begin with.

 

OK. At this point, before it gets too technical for my feeble old brain cells to conceive, I'll state that I understand (although I probably really don't as much as I think).

for the responses and info!

The discussion may continue if anyone likes - not that I won't keep reading and learning, but at this point I'm going to state up front that it'll probably be above my level of brain activity.

 

I agree with this entirely, the system needs to be tuned to the application/combo/rpm band of intended usage, and if done properly can add power across the entire powerband. However, despite the opinion of some, bigger is not always better, although with a hot 502, bigger probably is better. Put the same (big) system on a stock 305 2bbl, and it will loose power everywhere.

TA

 

Like putting a duel 4" exhaust on a SOHC civic... that annoying noise is the exhaust/sound waves bouncing around inside the pipe at low velocity rather than exiting optimally

 

The sound is also louder because it echos inside the drivers' empty head!!

TA

 

basically its tricky. Follow certain guidelines and you'll be close. obviously a 502 needs at least a 3.5 inch exhaust. etc.

 

^^We put mandrel bent 3.5in on my buddys GMPP 502 and it turned out o.k. but we put 1 7/8in headers on it and I think it would have been better off with 2in primaries.

 

deffinately should have had 2 inch primaries.

 

I know one thing, when i ran open headers , i had absolutely no low end torque. Maybe because of the TPI? Not sure, but even without a muffler on my car, the low end torque really suffers. I definately think that i have more velocity with a full exhaust.LOL

 

Guys that ride mtorcycles who gut their pipes or the harley rides that remove a restrictor or baffel will say the same thing. Its like a tuned exhaust with the back pressures at the right time help with velocity. I used to read stuff like this a lot in the motorcycle mags.
I think Yamaha started things with their Exup exhaust.
I think you can kind of explain the use of back pressure with a garden hose. Have the water running and step on the hose and then off, you get a sudden rush of water.
But enough of this. I just came here to unsubscibe to this thread and age old debate.

 

I am running Edelbrock TES headers , n10 (stock) dual ypipe, into a 2 1/4 catback and cheap transverse style muffler. Actually i think my catback piping may be 2 1/8.LOL but the car is faster with this setup , than without a muffler or straight pipe.

 

It's funny because my car is silent too. It is faster on the street this way, never tried it at the track.

 

You'd lose low end power with open headers but gain upper rpm power. now since tpi motors are not high rpm engines anyway it is probably not beneficial. I know my last time at the track I ran best with my electric cutout closed. But the car also wasnt as hot and wasnt being hotlapped as much. Now I know my fuel mileage is just a tad better with the electric cutout slightly open.

 

The exhaust pulse is a biggy. Hadnt seen that mentioned yet.

Like mentioned earlier it is not simple at all.

It has been my experience that anyone that mentions the "some backpressure is good" is the same people that think that huge cams work good in stock motors, put a 850 carb on a 283, and claim that race gas will make you faster (on motors that do not require it). It is something they heard back in the 60s, 70s, and they dont understand how engines work, they just go off what they have heard.

 

As many above already stated, anyone who says exhaust backpressure is necessary or good doesn't know wth they are talking about and I would take anything they said with a grain of salt, exhaust backpressure is BAD it HURTS performance, not improves it, also like someone above said, an internal combustion engine at its most basic level is an air pump, the quicker and more air gets in and out the better, all that being said flow velocity DOES matter, basically the exhaust has to exit the pipes quick enough, before it starts to cool off, and what happens if you put too big a pipe on, lets say a 4" exhaust(to match the 4" muffler thats no doubt already on it) on a civic, it is going to absolutely suck, because it is way too much volume for the exhaust to exit quick enough. The best exhaust is one that incorporates minimal amount of bends, as well as not having any sharp bends, whatever bends it has should be mandrel bent(ie. not kinked), it should be of an appropriate diameter, and last but not least there is "tuned exhaust" basically the header pipes are shaped such that it utilizes the engines exhaust pulses, and if memory serves is generally set to a specifc rpm range.

 

Open headers is better for race - which is high rpm, of course your low rpm power suffered - who races in low rpm range? Now removing the muffler just made the car feel slower since its louder - its a psychological thing.

 

I have had the opposite. Most people I have been around think it is quicker because it is louder.

 

That would be my base assumption as well, because usually(unless its a beater with a broken exhaust) louder means better flowing exhaust, my car is quite loud with a full aftermarket exhaust and certainly quicker than a stock 2.8 or 3.1....could probably go door to door with a 305 tpi. Loud doesn't always mean faster, but USUALLY means better flow, which makes more power, I think pretty much any exhaust that flows better is louder.

 

The right exhaust has good velocity and what the velocity does is help pull out more exhaust. when you are too big. you lose that velocity so the exhaust doesnt flow out as well and more or less "stays" in the pipe which hurts performance too. deffinately DO NOT WANT BACKPRESSURE!

 

Sorry, I can't agree with this. To get velocity you need resistance, a smaller area speeds up the flow (this is how a fire hose works). Velocity is pressure vs resistance, with the resistance, or restriction, or back pressure, whatever you want to call it, your firehose becomes a dribble. Just like in an electical system, resistance is part of the equation and the light bulb doesn't work without it, you cannot have one without the other. Yes, using that term "implies" unwanted or nonfunctional restriction to people who see the world in simple black and white, but just like in a carburetor, if you have zero restriction, you have zero back pressure, your velocity falls off, and the carburetor no longer works. Ask Isaac Newton, I don't make the laws.

Therefore, IMHO, anyone who says back pressure is bad is wrong, because you cannot have velocity with zero back pressure. What you want to do is tune for the application at hand with minimum backpressure, since the two are directly connected, you cannot eliminate backpressure without loosing all your power, unless you have a giant supercharger and don't care about a powerband.

TA

 

You're confusing flow and velocity, among other things. Flow is the volume per time and velocity is the distance per time. A 2" firehose with water moving through it at 1 foot per second, has a flow of 38 cubic inches per second. A 6" firehose with water moving 6" per second has a flow of 170 cubic inches per second. The flow in the larger pipe is greater, even though the velocity is slower.

Maximum flow in the firehose occurs when restriction and backpressure are minimal. Maximum flow doesn't necessarily correspond with maximum velocity.

Water-filled hose analogies fall apart very rapidly when comparing with exhaust. The steady flow behavior of an incompressible liquid is very different from the pulsating flow of a compressible gas.

 

I think you are wrong project31camaro. Our cars are usually losing low end torque with an exhuast leak. I had a 3.1 Beretta at one point in time and the thing would be sooo low with the smallest exhaust leak. My 5.7 tpi car is the same way. If i have a small exhaust leak ,especially before the catback then i am losing a ton of power. Maybe you have a clogged up muffler or something ,but i have never heard of gaining power with an exhaust leak.LOL. With the exception of a turbo car, i think you will gain more velocity and therefore power by having a moderate size exhaust system that is consistent in pipe size all the way back.

 

I think the 3" catback exhaust is only good for your high rpm cars, with stealth ram, bigger heads, cam, 3:73 or better gearing , etc. I would think you would produce a negative effect otherwise. Well, you would gain in the upper rpms, but the rest of the motor would still be set up for low end torque like it comes from the factory.

 

IMO, I would get that exhaust leak fixed project31camaro.

 

I think your 6" firehose is only good for a high rpm water pump flowing that water out!

 

No confusion here. You cannot have flow without velocity, the two are related and inseperable. The confusion seems to be that many people are so hung up on maximum flow that they fail to understand that maximum flow doesn't necessarily correspond to maximum power unless the whole system is tuned for that flow, IE: a huge engine with huge compression, etc. If maximum flow is a cure all, AFR would only make a single cylinder head, and it would be huge. But they make many different sizes. If flow=horsepower, please explain why all the different heads?

And my analogy of the firehose is spot-on. Fluid boiled is a gas, gas compressed if a fluid. Apart from being compressable, they both obey the same laws of physics. Always. Isaac Newton says so, and his observations are called LAWS, not theories.

Back to the firehose. Just the hose with no nozzle burbles out on the ground like a big garden hose. Restricted to about 1/4 the area of the hose, the nozzle shoots the water 40 feet, same size hose, same water pressure. I'm sure everyone here has put their thumb over their garden hose to get it to "squirt". The same principal works on a gas. A blow gun for an airhose is a MUCH smaller orifice than the hose, thus it speeds up the air, and the rushing air cleans your workbench, etc. This is not news to anyone, it's all around us. Pop a balloon and the broken balloon falls directly to the ground, but untie it and let blow out through it's small inlet and it jets around the room. Same energy, but much more work done.

Where the firehose works to explain how your engine benefits from a restriction is this: You want to strike a good balance between velocity and flow, in the firehose, you want distance (velocity) as well as volume to put out the fire (flow). Granted, your exhaust system is much more complicated than a simple firehose, but the same physics apply, you want to find a restriction that maximizes velocity while keeping the restriction on flow to a minimum, squirting a tiny amount a long distance is also undesirable. Eliminating the restriction, or back pressure, entirely is a FAIL, because airflow has both mass and velocity: If it's moving, it wants to KEEP moving, and pushes/pulls whatever is in front/behind it along with it. That is a very much simplified concept of exhaust scavenging, but remove all the restrictions beyond what your conbination likes and you loose power.

TA

TA

 

When i first bought my car it had no catback, just a straight pipe. I then added a 2 1/4 catback and average flowing crossflow/transverse style muffler. I gained a boat load of torque!! especially down low. Then, i took off the muffler and added a straight pipe to the catback. I ended up losing power,especially on the low end. Really annonying at traffic lights . Not to mentioned it didn't sound too good. So, i ended up putting my old muffler back on, it was like night and day. LOL. I think the way i have it set up now is about right for a bolt on car. Edelbrock shorties 1 5/8 into a dual cat 2 1/4 ypipe into a 2 1/4 catback and a little better than stock muffler with 2" tailpipes. Really good velocity and i can break the tires loose in D2.

 

on the garden hose theory. if you have the nozzle to make it shoot 40 ft or hold your hand does that create higher pressure in the hose? If yes then is that not considered backpressure? If no then its velocity right? I am all for the velocity side of this argument and that backpressure is no good. Just another thought...

 

do you have the dyno sheets that you would be willing to post?

 

Remember that these cars have sensitive tunes and anything that alters the power can have a negative effect (without PROM tuning). So saying you got "X" result by adding or removing exhaust components is missleading. Lot's of TBI guys complain that their car feels slower after replacing the super restrictive stock exhaust. The main reason is that it shifts the tune off so it is not running properly and has nothing to do with backpressure.

 

Yes, it does create slightly higher pressure in the hose, that is why I am trying to explain why SOME backpressure is necessary for good scavenging, and why all of our "all back pressure is bad" friends just simply cannot explain why a lot of the real world experiences of board members conflict directly with their theory. Because of the complexity of how our exhaust systems work, the best way to find a balance between velocity, flow and back pressure is to talk to people with similar combos to yours that have done a lot of trial-and-error testing. One of the reasons that NASCAR stockers make the power they do with all the restrictions is that each engine is dynoed with an adjustable system, and the system is tuned to the power characterists that they want, then they reproduce those system dimensions on the car.

Dyno Don changed his mid pipe (between the collectors and the cats) from a 3" to a 3.5", and if our esteemed moderators are right, and all backpresssure is bad always, he would have gained power. He lost about 16 HP PEAK if I recall correctly (I was present for both dyno runs), no matter what tuning adjustments were made, on an otherwise exceptionally stout 355 TPI. This is yet another example of something that just doesn't fit the "all backpressure is evil" model. Nothing else changed, same dyno. He has the dyno charts, I do not.

Trust me, I subscribed to this model in the past (4" cat-back!) and was very dismayed to have someone who has designed high performance exhaust systems for 40 years tell me that it was not going to work with my combo, and why. He was right, it is gone.

TA

 

Oh, I agree. I didn't mean to come off as such. I am a firm believer in exhaust tuning which does require some backpressure. Any IC engine book will tell you that. I just wanted to make sure poeple understood that these cars are sensitive to changes in VE and an exhaust size change might allude them to thinking that their poor results were due to too much or too little back pressure.

 

Thats cause your 02 sensor is smart

 

Sorry, I'm a little defensive because I have been attacked on this point on this board several times, often by moderators, and after they chime in, other members who don't know any better just dog pile on. Not pleasant and not good for the growth of knowledge in our hobby.

But you are spot on that if you make a major change to the exhaust without changing the intake and tune, you're often just jacking up your tune which can cause a drop in performance and huge flat spots in the powerband.

If I had a few simple things to hammer into the heads of our younger/newer members, it would be this:

Louder is not faster. Loud is easy, fast is difficult, fast and quiet is a huge accomplishment.

Better flowing is not always more power, choose changes that are in proportion to the rest of your package, and if you cannot do computer tuning, stick with a Magnaflow muffler and a high-flow air filter and call it a day.

And the best lesson of all: Find someone older who has paid a lot of dues and LISTEN TO THEM. Clubs on the regional boards are the best way to meet these people.

Thanks for the space to

TA
383 Super Ram 389 RWHP W/3500 stall Auto

 

Amen brother. It's like the story of an old bull and a young bull looking down upon the cows in the field..........................