More than you ever wanted to know about BACKPRESSURE... - Page 2 - Third Generation F-Body Message Boards

BirdoFireTA84 · 01-09-2007, 02:25 PM

Thanks for the support ownor. I haven't had the time to look further into the idea of a quick and dirty way to setup your exhaust right before buying anything but hopefully somebody else does, hehe. My friend is a physics major so maybe I can talk him into helping me with the idea, I'm sure he'll have some valuable input. Of course, a precise formula for the proper length would be difficult to come up with due to my limited knowledge on the subject but maybe a little research can change that. If I do come up with something then I'll definitely share it.

In response to brodyscamaro, you're right, lol. I know I'm a little crazy but I simply can't afford trial and error when mistakes cost hundreds of dollars so naturally I'm looking for a more reliable way to select parts. A precise formula to tell me exactly what I need is probably not possible to find or at least it's beyond my ability but if I can find something to at least get me close it would help.

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alexandn · 03-09-2007, 05:06 AM

Quote:

Originally Posted by urbanhunter44

Here are a few questions so you can learn this a bit better. If you answer all of these correctly, you will
totally understand the difference between:

- Tuned length vs. Backpressure
- Restriction vs. Flow
- Velocity vs. Pressure

Quiz A
Using the diagram below, answer the 4 questions.

The RED line is POSITIVE PRESSURE (Initial Wave) and the
BLUE line is NEGATIVE PRESSURE (Reflected wave). Also note that this is very simple, it will be explained below.

The exhaust port is at the left side of the pipe.

#3. What is the resulting pressure of both waves combined at the exhaust port for Graph A?

1. 3 psi
2. 0 psi
3. 5 psi
4. 3 psi

With some simple math above we can see that the
blue line is creating 2 PSI of negative pressure.

- 2 PSI (Graph A)

The Red line in Graph A is producing +3 PSI of exhaust pressure on the inital pulse. That means there is a difference of 5 PSI between both waves. The reflected pulse is leaving a negative zone at the port and literally sucking the exhaust out.

Since the initial wave is +3 PSI, it's going to
come out with higher velocity because of the 5 PSI
differential.

Graph B shows a similar scenario, but the reflected
wave comes back and hits the port at 0 PSI.

The intial exhaust pulse is 3 PSI.

Now there is a difference of 3 PSI.

There's a lot of valuable information in this thread! And the explanation posts are almost perfect, except for...

The answer to question #3 is correct, but the comments are not. The blue curve in Graph A is actually at a positive (+) 2 psi, not -2.

Everyone should carefully note the difference between pressure difference & net effect with respect to wave physics:
The pressure difference between +3 psi and -2 psi is [3 - (-2)] = 3 + 2 = 5 psi , while the net effect (a.k.a. superposition principle) is [3 + (-2)] = 3 - 2 = +1 psi at the port.

However, according to graph A the way it is now, the pressure difference is (+3) - (+2) = +1 psi while the net effect is (+3) + (+2) = +5 psi at the port,therefore drawing the fresh intake charge with less velocity.

For everyone:
Remember that fluids (air/fuel mixture in this case) flow by the virtue of a pressure difference (i.e. vacuum in cylinder during intake stroke vs. atmospheric pressure at the air filter), and understand that the lower the pressure in the cylinder with respect to the atmosphere during the intake stroke, the greater the pressure difference and thus higher the velocity of the intake charge.

I hope I made sense...

Alex

BadAzzGTA89 · 03-17-2007, 09:42 PM

Good info but made me feel like i had ADD

----------
Good info but it made me feel like i had ADD

malibuguy · 01-01-2008, 02:29 AM

alot of good info

i like to use the term "velocity tune" & i absolutly hate the term "some cars need backpressure" NO! they don't, they need the right velocity

i have one theory that the use of mufflers can help tune a system for the right amount of flow for the engine.

For example, my one Malibu;

I have Hooker Competition 1 5/8"X3" headers, 12" long 3" diameter header extentions, Flowmaster reducer cones, Pypes 2.5" dual exhaust with an X-pipe (which i feel needs to be a little closer to the headers, but w/e)

when i initially installed my exhaust, i ran SuperTurbo mufflers, & i had an '80 267 in the car (with cheap headers) because i was "between engines" & it was the only thing i had that ran

the exhaust was very quiet (like a stock truck) & out performed my old 2.5" crush bent exhaust with straight pipe & a h-pipe, the car felt almost not horribly slow

i feel that i substantually (spelling?) increased & smoothed out the exhaust flow, & the mufflers tuned the capacity to almost be right for the little 267

finally, i installed a '83 LE9 305, other then new headers & the exhaust, it was box stock out of the wreck truck it came out of, including the stock Quadrajet

the car felt great, but i had these 40series Flowmasters from a friend that i wanted to try, so i swapped them in, & instantly the car picked up power & response across the board, with the majority being mid-top end

i ran the car like this, & ran a best of the 15.3X@83mph (in the middle of July, around 85*)

so, okay, other then for the noise, i liked them, but i felt, how would the car respond to a striaght-thru muffler?

with no mufflers to borrow, i decided, to take scraps of straight pipe, & put them in place of the Flowmasters

the car seemed to run "flat" it just wasn't peppy, & felt like it lost power everywhere, however i did love the noise it made

so i swapped back in the SuperTurbos to be quiet again & got back the low-mid response it used to have, & also installed a used Performer cam, my old Torquer 2 intake with a Carter 625AFB, tuned for my old 350, along with big wide tires, which allowed me to leave with my foot on the floor (yes i can floor it & stay there @around 2600rpm)

the car ran a 14.84@89mph towards the end of the year in cool November weather (50-60* weather)

so anyways, surprised how far into the 14s the car went (actully baffled), i said to myself, the car really liked the Flows, so i put them back on, along with some tuning i did on the ol' Carter (leaned it out a little), & the car felt a "little" better

went to the track in nice cool April weather (around 60*) the car instantly ran 14.5s@93mph (HOLY S#!T), i kicked the timing up a hair, 14.6s, knocked it back, 14.5s, then i changed up my launch, only launching around 1100rpm & nailing it off the light, the car ran several 14.3s@94-95mph, by now i'm like flipping out, calling everyone i know, etc etc (BTW i got 20 runs in that day & left early)

so i think & my theory; is the straight pipes (2.5") have a velocity peak thats way too high for my current set up to get to & use, while the SuperTurbos tuned the velocity to a more streetable rpm & is optimal for a DD since the torque & response is great at low rpms, & the Flowmasters found the happy middle, & work great everywhere, especially at mid-high rpms where my car pretty much uses, my converter flashes to 3400rpm upon launch, so anything below is just more tirespin anyways

so that being said, the exhaust needs to be tuned to the right velocity for the given combo, & when right, can reap huge benifits

plus it seems like i have a good hunch about what a car needs for an exhaust...i've been obsessed with exhausts for most of my life, & it seems like most of the stuff i do to other cars just work very well

BirdoFireTA84 · 01-01-2008, 03:48 PM

This thread has been quiet for some time and it's an odd coincidence that I came back to visit yesterday and then you posted today with some info that just may be helpful to my current situation. I've been planning to buy an exhaust for my mostly stock 84 trans am but have been changing my mind a lot due to lack of money and my theories about which setup would work best. I originally wanted to go with dual 2.25" pipes from the headers back but I scrapped that idea because I'm new to this stuff and it would require some skill to do it right on this car. Besides, I'm still running the stock 305 (LG4 I believe, it's not the HO) so I doubt it really needs that. My next thought was a single 3" setup but it's been hard to find a good y-pipe for the shorty headers I got. The one from hedman that is designed for my headers is 2.5" in/out which seemed like it would pretty much negate the effect of a bigger intermediate pipe. This led me to my current idea which is to just go 2.5" the whole way back. Since I don't even have enough for the full exhaust right now, engine mods are pretty much out of the question for a while so I won't be putting out much more than stock. At first I thought I'd be screwing myself with 2.5" pipes because if I suddenly had the money to start playing with the engine, the exhaust might become a bottleneck again. However, until then, it seems that 2.5" would be adequate for my setup and going bigger may actually be detrimental. With the stock exhaust, low-end power is decent but after that it's kind of sluggish. I'm thinking that the new setup will allow it to breathe better at higher rpm but won't sacrifice too much on the low-end.

To be more specific about parts:

Hedman Street Hedders - http://store.summitracing.com/partdetail.asp?autofilter=1∂=HED%2D68470&N=700+400143+115&autoview=sku (not certain if these are the exact ones but specs are pretty much identical)

Hedman Y-pipe - http://store.summitracing.com/partde...part=HED-17470

Dynomax Intermediate Pipe - http://store.summitracing.com/partdetail.asp?autofilter=1∂=WLK%2D47638&N=700+4294908153+4294907958+4294925024+4294923429+4294820497+4294748490+115&autoview=sku

Flowmaster 80 series - http://store.summitracing.com/partde...part=FLO-42583

Not the greatest of parts but they're somewhat cheap and should be adequate for my car. My friend was trying to talk me into getting a 4" exhaust like the one he's getting for his 84 camaro but his engine has some work done to it and it needs the extra flow capacity, mine might actually be hurt by it. I may punch out the core of my catalytic converter/remove it entirely since I have to buy my inspection anyway, leaving the muffler as the only real source of backpressure. After considering the stuff discussed in the main post though, I may try to get a better converter so I have a chance to pass emissions tests and also because that should help me out in the higher rpm ranges since the pressure wave gets reflected sooner. If I'm not mistaken, the higher quality converters won't hurt the flow that bad and if that will actually help me tune the exhaust then it may end up being beneficial to have it in there. Gotta love the irony there. What do you guys think?

malibuguy · 01-08-2008, 11:40 PM

i feel...a single 3" should be fine, even on a mild/stock 305 motor...a tad big now, but not crazy, especially if you do plan to do some mods, it will later support them

i'd run the flowmaster now...& once you start really using the full capacity of the pipe...upgrade to a Spintech, or full-flow muffler like a MagnaFlow or Dynomax UltraFlo, single in, single out...pretty much all crossflo mufflers flow pretty badly besides the SpinTech, better off going single tip...less noise, & more subtle....depending how you end it

BirdoFireTA84 · 01-09-2008, 12:41 AM

Thanks for the reply. My friend has an 87 camaro that he's parting out and it happens to have a 3" flowmaster system on it so it looks like I'll be using that. I'm not sure what y-pipe he has or if he's using an 80-series muffler but I know his exhaust sounded pretty nice when the car was still running. I'll probably throw that on and get myself a nice 3" cat so I have a shot at getting through inspection legally. If that doesn't work out, I may take a shot at running dual 2.25" pipes. If it comes out alright then it's time to start saving for a new trans as mine seems like it's on its way out. It's ok for now but I give it another year at best before it starts approaching death.

lukn4trbl · 01-16-2008, 07:16 PM

Thanks for the reference and credit Urban! Someone had just linked meto this thread from Fbody.com and I was surprised to see this post. It's been a while since I wrote that, and it's quite basic compared to the dyno tuning, track tuning and reference texts that I learned from. I highly recommend buying, "Scientific Design of Exhaust and Intake Systems", as well as "Engine Technology, 2nd Edition" to get a bigger picture of whatis happening.

As I skimmed through the replies, I noticed a couple of mix ups:

- A muffler slightly changes the tuned length of the secondary system depending on the change in volume between the inlet pipe and mufflerchamber. The baffle placement also effects the timing of the reflected pulse.

Quote:

Originally Posted by alexandn

However, according to graph A the way it is now, the pressure difference is (+3) - (+2) = +1 psi while the net effect is (+3) + (+2) = +5 psi at the port,therefore drawing the fresh intake charge with less velocity.

- The blue line in graph A is actually representing an expansion wave. The net result is negative pressure. Although the graph shows an absolute value of 2 PSI, when the waves are mixed, the pressure difference is effectively 5 PSI which will cause a higher velocity into
the chamber.

One other comment about megaphone, or convergence pipes (also works withsteps/stepped headers), is that as the pipe volume expands, the gas velocity slows down.This will effect the timing of the reflected pulse at different RPM's.

pathology knight · 01-24-2008, 02:07 PM

i got linked this thread from a local 3rd gen forum and wasn't going to post because i thought it was an old thread, but since it's alive again....

i pm'ed the author about my set up, but i figured i'd get others ideas.

i've got an 89 trans am gta with a 5.7 350 TPI. it's mostly stock. i put on a hypertech chip, got rid of the smog stuff (broken or not working), and put on some hooker headers. the car is mostly for cruising, but i don't want a wussy car! the exhaust is stock 2.5" pipe with a 3" high-flow cat. i'm not sure why the cat is a different size...that just what Midas put on when they did it. (i didn't know it at the time.)

i plan on doing the cat-back later, but the headers-to-cat is a must do now.
i'm not really sure what set up to go with. i know i want to keep it a single pipe for cost reasons and i know that i like the look of the dual tips out of the muffler.
any recommendations?

pathology knight · 01-24-2008, 03:10 PM

i'm leaning towards a 3" cat-back.

what about the headers-to-cat? should i make it all 3"?

urbanhunter44 · 01-26-2008, 11:08 PM

Glad to see you're still around! That was so much great info I had to put it somewhere that people could learn from it. This board has alot more traffic than Fbody and typically has alot of younger owners so I figured it would be a great place.

I actually own "Scientific Design of Exhaust and Intake Systems" and yes it's incredibly interesting and very informative, if a little dated (my copy is early 70s IIRC), however scientifically sound.

Quote:

Originally Posted by lukn4trbl

Thanks for the reference and credit Urban! Someone had just linked meto this thread from Fbody.com and I was surprised to see this post. It's been a while since I wrote that, and it's quite basic compared to the dyno tuning, track tuning and reference texts that I learned from. I highly recommend buying, "Scientific Design of Exhaust and Intake Systems", as well as "Engine Technology, 2nd Edition" to get a bigger picture of whatis happening.

As I skimmed through the replies, I noticed a couple of mix ups:

- A muffler slightly changes the tuned length of the secondary system depending on the change in volume between the inlet pipe and mufflerchamber. The baffle placement also effects the timing of the reflected pulse.

- The blue line in graph A is actually representing an expansion wave. The net result is negative pressure. Although the graph shows an absolute value of 2 PSI, when the waves are mixed, the pressure difference is effectively 5 PSI which will cause a higher velocity into
the chamber.

One other comment about megaphone, or convergence pipes (also works withsteps/stepped headers), is that as the pipe volume expands, the gas velocity slows down.This will effect the timing of the reflected pulse at different RPM's.

skinny z · 01-27-2008, 10:01 AM

Lot's of excellent theory, but what of real world applications in a lowered 3rd gen? There's not a lot of room for appropriatly positioned mufflers so as to provide proper tuned length or terminator boxes ( David Vizard ).
I can see cut outs being a useful solution. There are only a couple of examples I've seen though.
Is there specific info available as to what the secondary length should be with respect to a given engine rpm?
I'm looking to pull the torque curve down to better suit my stall speed. As the primaries are fixed and the collector diameter determined, the only tuning left is in the secondary length and the cut out placement.

Orr89RocZ · 02-01-2008, 05:10 PM

i read this article and some other articles and have come up with a few plans for my 383 build.

i'm in a similar boat of not knowing how long i should have my tuned length for a 383 peaking at around 6400 i think. My custom cam should allow it to peak by then. I plan to run true dual 3inch setup with round tube like mufflers so i will need some sort of "tuning effect" present to allowfor best performance at 6000+rpms. i want to run some kind of terminator box like David Vizard discussed, i'm just not sure yet what to do as i dont know if i'll have room

lukn4trbl · 02-13-2008, 11:22 AM

Quote:

Originally Posted by skinny z

Lot's of excellent theory, but what of real world applications in a lowered 3rd gen? There's not a lot of room for appropriatly positioned mufflers so as to provide proper tuned length or terminator boxes ( David Vizard ).
I can see cut outs being a useful solution. There are only a couple of examples I've seen though.
Is there specific info available as to what the secondary length should be with respect to a given engine rpm?
I'm looking to pull the torque curve down to better suit my stall speed. As the primaries are fixed and the collector diameter determined, the only tuning left is in the secondary length and the cut out placement.

Secondary tuned lengths for RPM ranges of 4000-6000 RPM, fall in the area
of 20" to 26" past the primary merge point. Depending on the collector
you have, the length will vary somewhat.

If you PM, or post your cam specs, I can run some numbers in a software
program to give you a better idea of the length you'll need.

Orr89,

What sort of plenum and runner design are you using to peak at 6400 RPM?
That's pretty impressive. The cam must be ~300 seat duration, with about
110-112 lobe centers?

lukn4trbl · 02-13-2008, 11:25 AM

Urban,

I'm always around, but I'm spending a great deal of time as admin on Speedtalk, and busy with the biz! I'm jumping on here and Fbody from
time to time though.

Scientific Design is a great book for getting the science behind the design,
however the current trends are a bit different know that cylinder head
efficiency and port designs have evolved. Still a great starting point nonetheless!

skinny z · 02-28-2008, 05:04 PM

[quote=lukn4trbl;3637528]Secondary tuned lengths for RPM ranges of 4000-6000 RPM, fall in the area
of 20" to 26" past the primary merge point. Depending on the collector
you have, the length will vary somewhat.

If you PM, or post your cam specs, I can run some numbers in a software
program to give you a better idea of the length you'll need.

Hi lukn4trbl,

Here are some specs for you to work with regarding secondary length:
350 cid/standardbore/stroke/rod length
Vortec heads. 3 angle valve job, no porting, 64cc measured.
9.97:1 static CR measured. ( .028 gaskets, .025 below deck, -6cc piston )
Comp 268H flat tappet cam ( old single pattern version)
IVO (BTDC) 4.0
IVC (ABDC) 34.0
EVO (BBDC) 44.0
EVC (ATDC) -6.0 (or 6 BTDC)
110 LSA
102 ICL
218 @ 050"
4 degrees advanced
.488" lift w/ Comp Pro Magnum 1.6 rockers
600 vac sec Holley
34 degrees total advance w/ 12 initial
94 Octane
Currently using Hedman shorty headers with 2.25 y-pipe and 3" cat and 3" flowmaster cat back system
TCI 1000 over stock stall converter (flash @ 2750+/-)
3700 lbs. race weight
3.73 posi w/ 26" drag radials
Best pass. 13.4 @ 102.5. 1.8 60'.
20 mpg hiway.
I use Dyno 2000 and Drag 2000 for analysis.
I'm interested in your insight.
Kevin

Orr89RocZ · 02-28-2008, 09:45 PM

Quote:

What sort of plenum and runner design are you using to peak at 6400 RPM?
That's pretty impressive. The cam must be ~300 seat duration, with about
110-112 lobe centers?

well i ordered a custom grind and i wanted a peak near 6400 rpms on my 383 with a mildly ported HSR manifold... about 6-7 inch runners on the manifold for about 12-14 inches of overall length.. i forget the exact length but its in that range. The manifold is capable of supporting it i do believe, but the cam i got is 286/306 or 230/245 at .050" On a 109 LSA

i think it will peak more like 6200 if i'm lucky, maybe even 6000, but then again with that overlap it may push 6400. I really dont care as long as it makes 400whp or so and runs 11's

urbanhunter44 · 03-10-2008, 11:49 PM

Here's an informative thread I found on speedtalk:

http://speedtalk.com/forum/viewtopic...er=asc&start=0

Manovox · 04-19-2008, 06:53 PM

HELLO
I AM FROM MALTA AND HERE I CANNOT FIND PARTS FOR MY 89 CAMARO RS WITH 350CI AND I SAW THAT YOU KNOW A LOT ABOUT EXHAUST
I LIKE IF YOU CAN HELP ME DOING HOMEWORK TO INSTALL A NEW DUAL EXHAUST TO MY CAR MAY BE YOU CAN SEND SOME PICS OR DIAGRAMS AND WHAT TIME OF MUFFLERS SHOUL I USE AND PIPE THICKNESS?? YOU CAN ALSO EMAIL ME VOX@ONVOL.NET AS I AM NOT PERFECT USING FORUMS!!

THANKS
LORRY

skinny z · 04-19-2008, 09:41 PM

Anybody here familiar with the Pipe Max program?

Creslin89 · 08-07-2008, 08:03 PM

I dont know if anyone bothered to read the link posted for http://www.pontiacstreetperformance..../exhaust3.html but its got some good stuff on the X and H type of crossovers, and some basic guidelines that they came up w/ from tests back in the late 90's...

Know is kinda dated, but it couldnt have changed that much??

1. Always use a crossover of one kind or another. Our tests indicate that the X type gives additional improvements over the H type.
2. Mount the mufflers as far to the rear of the chassis as possible. An important characteristic of the exhaust system (behind the headers) is its ability to dissipate heat energy. Heat loss brings with it, gas volume reduction, enabling smaller mufflers and pipes to be used without penalty.
3. Always us the largest case muffler that you can fit under the chassis possible. The larger internal volume allows additional acoustical energy to be absorbed ,dissipated and eliminated.
4. Unless an engine is in the 500+ horsepower level or run at very high RPM, the maximum tailpipe size required for minimal power loss is probably 2.5" diameter. When the exhaust pipes and mufflers drop the temperature significantly, the volume of the exhaust gas is reduced and tailpipe sizes is not as critical.
5. When you must adapt various pipe sizes, always use long tapered cone reducers, such as, those available from Flowmaster. You can also use a crossover that has reduced pipe sizes built into it.

TheTech91RS · 08-08-2008, 03:47 AM

Ok heres a question?

I just installed Holley's Flowtech "equal" length headers i belive, they are just a tad bit longer then the stock manifolds, but I don't think theyre considered shorties... ANYWAYS...

I installed them, which required taking a bunch of smog stuff out...
O2 censor is still on, and hooked up (weather its working properly or not i have no idea)

I started the motor with straight headers, no pipes at all.
I noticed MAJOR loss in power, and lots of hesitation that did not happen before the headers, so whats going on? I heard straight headers will make lots more power, and it feels lost through the hole power band 1-5500rpm, I dont know whats going on! Any ideas?

skinny z · 08-08-2008, 07:56 AM

Quote:

Originally Posted by TheTech91RS

Ok heres a question?

I just installed Holley's Flowtech "equal" length headers i belive, they are just a tad bit longer then the stock manifolds, but I don't think theyre considered shorties... ANYWAYS...

I installed them, which required taking a bunch of smog stuff out...
O2 censor is still on, and hooked up (weather its working properly or not i have no idea)

I started the motor with straight headers, no pipes at all.
I noticed MAJOR loss in power, and lots of hesitation that did not happen before the headers, so whats going on? I heard straight headers will make lots more power, and it feels lost through the hole power band 1-5500rpm, I dont know whats going on! Any ideas?

Aside from any other issues you may have, headers have to have some sort of collector extention on them besides the short 4-6" that make up the flange. Without an extention of at least 18" or so, the engine will build very little low end torque. That's the 'lost' feeling. With the extentions, and the lengths do vary depending on the application, it will necessary to tune the AF ratio to take advantqge of the increased cylinder scavenging.
That's where the ' lots more power ' comes from.

1989GTATransAm · 08-08-2008, 01:00 PM

Exactly. You need the correct collector length and diameter. Other wise you will have power loss.

TheTech91RS · 08-08-2008, 07:58 PM

Ok so in the event, i put the Y pipe back on, and keep the cat off, and run piping all the way back to the rear into the muffler, i should get atleast my stock power back?

Also, i've heard rumours, and wouldnt doubt if its true, but I heard running straight headers, CAN or WILL cause your heards to warp when you shut the engine off and allow all that cold air to return into the headers, is this true? May i already have damaged my engine?

And since I already have the attention of the users, what smog wise can I take off this engine, without causing it to run like "kaka"? Smog pump is disconnected, and completely out of the car, cat is obviously cut out, the little valve thing (no idea what it is, so fill me in) thats connected at the exaust flanges on the manifolds and the pipes, is out. Anything else I can do to make more performance without making the motor run bad if i got rid of smog stuffs! hit me up

Bowtiebruiser · 11-17-2008, 11:07 PM

Wow my head hurts and that was a lot to take in, I think I stupider now in some other area to be discovered at a later date because of all this info lol

Excellent write up

TA · 05-13-2009, 07:05 PM

Quote:

Originally Posted by TheTech91RS

Ok so in the event, i put the Y pipe back on, and keep the cat off, and run piping all the way back to the rear into the muffler, i should get atleast my stock power back?:

Maybe. What you have done is created an exhaust tuned for high-end power, while having heads, cam and intake tuned for low-end power. By having the systems grossly mismatched, you get little of either. You need to have your RPM band (Velocity?) tuning at least overlap, IE a diameter/length exhaust for midrange and a cam and intake tuned for low end, then you get good power and a wide powerband. Match them all, and you get excellent power over a narrow band. Our cars are almost all tuned for low-end power in stock form, so it is very easy to change parts to "racing" components and loose power.

So, you can replace the stock system from your headers back and likely your power will come back (unless you've snafu'd something else by accident), but if you put a giant 4" diameter short system, you will only get a tiny bit back. I hope this is useful.

TA

Antguy90 · 06-03-2009, 03:29 PM

ok, so i have a question. i have a 91 RS it has a LO3 tbi and i put headers, 2.5" y-pipe, no cat (3" test pipe), and a 3" flowmaster with a 80 series w/ 3" in/out. ok so at idle its not too loud at all, but when i get on it, it just "opens up" and sounds much louder. but like you know at idle you get that deep throaty sound, well my car doesnt have it like that, i mean you hear the throaty sound but its not really noticeable unless standing behind car leaning on bumper. what caues that deep throaty sound? and (dont get me wrong it sounds good) but can that be adjusted? as in, i know a cam will change the exhaust sound a lot, but is there something that can be done with the actuall piping, or the different components that are in line?
I \know that LO3 stands for "low output" and i also know chevy put a not so high perfromance cam, and ,lower comprsion so it only makes about 170 hp stock. like i said cam will change the sound, so with that smaller cam maybe thats why the exahust sounds different? it doesnt have a crazy mean sound, thats what i kinda wanted.
thanks,
-Anthony

K.I.A. · 06-04-2009, 05:31 PM

Great stick!

andetodd · 09-14-2009, 03:04 PM

Hello UrbanHunter, I just came across this article about
exhaust backpressure and I thought I'd ask for your educated opinion on a situation that I think may have to do with this stuff.

I'm wondering if the dent in my exhaust pipe could be contributing to
the very high knock counts I'm experiencing. I'm figuring this dent is messing up the exhaust gas vibration and could be contributing to this. RBob mentioned I should make sure my driveline is tight, which I'm going to do.

The knocks are happening when I accelerate hard. With the tranny in neutral there are hardly any knocks.

In talking to RBob about this he reckons it could be due to the driveline being loose. I bet that is the case but I'm interested in what you think. I'm putting new exhaust on once I've saved up enough $$.

chevytech88 · 10-04-2009, 12:44 PM

good info to know! alot of times its overlooked