RW91B4C

This should be good performance info for everyone!

I make my own exhaust systems, and am curently making a single 4" for my 91, which is good for 450hp, the expectation of my 383.
My 67 has dual 4" with 3.5" tailpipes, which is good for 650hp (650-maybe in the future).

This is a chart used by Dynomax and Hooker for sizing exhaust per horsepower.

HP-------Single----Dual
200-300----3"------2.25"
300-400---3.5"-----2.5"
375-450----4"-------3"
450-550---4.5"-----3.5"
550-650----5"------3.5"
650-800-------------4"
800-1000-----------4.5"
1000+---------------5"

You should also consider CID, a 350 making the same HP as a 502 will not need as big a pipes as the 502.
Another thing is that alot of people thing that you need a little back pressure, which it false. If you read any good books on the subject, by a reputable author like David Vizard or Dave Emanuel each will tell you that backpressure is bad. The only explenation I can see for this is that people get backpressure confued with scavenging effect/velocity.
My question is why do so many 3rd & 4th gens run a single 3" pipe when it is only good for 300hp, if an engine is putting out 450hp it will be getting choked and loosing hp?
I'm making mine single 4" with is good for 450hp which is what I expect the 383 to make.
So why do so many insist on going with a single 3" - you're just giving away free horsepower.
Please give your opinions with some tech to back them up. Thanks
Did I just open up a can of worms


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91 B4C 305 TPI - SOON TO BE 383
TREMEC 5-SP, STOCK 1-BOLT REAREAND w/342 GEARS, K&N, AIRFOIL
EDELBROCK HEADERS, DUAL CAT TO HOMEMADE Y-PIPE & 3.5" SINGLE PIPE W/ FLOWMASTER, CRANK PULLEY, MSD, FUEL PRESS REG, COWL HOOD, WELD WHEELS
14.1@ 98MPH
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OTHER RIDE
67 CAMARO - STREET CAR, BIG BLOCK, PUMP GAS
350TH w/ATI 10", 12-BOLT w/373 GEARS
10.94@124.13 ON MOTOR - ET-STREETs w/MUFFLERS

[This message has been edited by RW91B4C (edited August 07, 2001).]

SSC

I have been saying this ever since Ive been on this message board but only a few people listen. I guess I should have used simple examples like "try farting through a straw, now you know how you engine feels "
Oh well.
Good info RW.
SSC



[This message has been edited by SSC (edited August 07, 2001).]

jcb999

bc getting anything larger than 3" on a third or 4th gen is pretty difficult. You still have to deal with the issues of headpipes.

mtx28

the reason people dont go with enormous single exhausts is that they dont flow as fast as smaller ones.. and in general people think they need huge exhaust pipes or else their engine is choking. if no backpressure was best, then no pipes would be best, right? on AFR's webpage, they compared exhaust port flow with and without pipes.. their heads flowed substantially more with pipes. so, this means that backpressure may not necessarily be the best thing to have, but pipes that are the right size are the best thing.. if you go with an exhaust that is too big, then it wont flow nearly as well as a smaller pipe. just like the toro leaf blower idea that was going around for a long time, a single 4" exhaust idea did also(went aroudn this board i mean). im not neccessarily arguing with you.. sort of agreeing and sort of adding onto what your saying. the thing i cant stand is when people think they have to have dual 3 or 4 inch pipes when their engine is only slightly modded.. i have an article that compares different size exhausts on a nova with something like a 383 or 406 motor.. and they actually lost some torque when they went up to the biggest pipes(something like 4", i think, was a long time ago).

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83 Z28--not a pretty site, sold thank goodness

74 Z28-- 383/400, green on black. pretty clean. FOR SALE!!!

RW91B4C

Not exactly, some pipe is needed to help scavenge the ones upstream of it. For example the header collector pipe is there to help scavenge the primary pipes on a header, and the same gots for a Y-pipe. That is why you see header extentions on race cars, the longer the extention the lower the powerband (RPM) of that engine. The shorter the extention the higher the powerband, thats why some engine dont use collectors at all, like top fuel dragsters ---> they turn 10000rpm+, but a street engine can use this extra pipe length to its advantage by scavenging the rest of the exhaust system.

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91 B4C 305 TPI - SOON TO BE 383
TREMEC 5-SP, STOCK 1-BOLT REAREAND w/342 GEARS, K&N, AIRFOIL
EDELBROCK HEADERS, DUAL CAT TO HOMEMADE Y-PIPE & 3.5" SINGLE PIPE W/ FLOWMASTER, CRANK PULLEY, MSD, FUEL PRESS REG, COWL HOOD, WELD WHEELS
14.1@ 98MPH
-------------
OTHER RIDE
67 CAMARO - STREET CAR, BIG BLOCK, PUMP GAS
350TH w/ATI 10", 12-BOLT w/373 GEARS
10.94@124.13 ON MOTOR - ET-STREETs w/MUFFLERS

chevyguy1969

iTrader: (7)

First I want to say thank you to all of you for the info you have provided. From what you are saying and what I understand, it is all about matching the correct type of exhaust to the type of engine and power you are producing. One question I did have that I did not see on here is the importance of header tube size. I know there are not alot of options when it comes to this topic on our cars but I always wondered how much of an impact it has. Could you shed some light on this one? Thanks.

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Poor college student(donations welcome ;0) with an all stock 1988 5.7L IROC-Z Hardtop, Auto. trans, white ext. w/gold accents (soon to be silver), black custom cloth interior, 4-wheel discs and G92 pkg. Car is dead stock. Has only standard Z28 power options and all the IROC performance stuff you could get. No pwr. seats, mirrors, or rear defrost.

Best run: 1/4 14.90 @ 92.7 MPH

See My Car here: http://zap.to/chevyguy1969

#3 Gone but shall never be forgotten. Long live the legacy of the man in black. RIP Dale Earnhardt "The Intimidator"

86Trans Am

can some one explain how smaller pipes flow faster?

it seems to me that the air against the pipe is flowing slower than the air in the center of the pipe, when looking at a cross-section, due to some type of frictional lose. a larger pipe would allow a greater inner diameter than a smaller pipe therefore allowing more air to flow through the pipe faster. So how does a smaller pipe allow the air to travel faster?

dont get bent out a shape with my comment, i'm just trying to learn more

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1986 Trans Am
305 TPI
"If it's not one thing, it's another!"
http://www2.hanover.edu/hierlmeier

Jester

Backpressure is a mostly a myth. It only extends as far as the headers. It's true you should size header primaries to each engine, not go too large. After the collectors tho..it doesn't matter. The more flow the better.

I think that chart is close, maybe a little bit too generous. I'd shift the pipe sizes a line up in horepower...maybe.



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"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports

RW91B4C

I'll try to dig something up on the primary tube size, but thats a "black art", with step headers and all, you really would have to dyno an engine to know what works best. But, it all comes down to what the car runs so a heavier car may go faster with a 1 5/8" tube and a lighter one - faster with a 1 3/4".
I was just trying to show people what size they should be running and not giving up free HP, like single 3" on a 400+HP engine. Its like why would you run a 1" air filter when you can run a 4" filter instead

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91 B4C 305 TPI - SOON TO BE 383
TREMEC 5-SP, STOCK 1-BOLT REAREAND w/342 GEARS, K&N, AIRFOIL
EDELBROCK HEADERS, DUAL CAT TO HOMEMADE Y-PIPE & 3.5" SINGLE PIPE W/ FLOWMASTER, CRANK PULLEY, MSD, FUEL PRESS REG, COWL HOOD, WELD WHEELS
14.1@ 98MPH
-------------
OTHER RIDE
67 CAMARO - STREET CAR, BIG BLOCK, PUMP GAS
350TH w/ATI 10", 12-BOLT w/373 GEARS
10.94@124.13 ON MOTOR - ET-STREETs w/MUFFLERS

AJ_92RS

Anytime that you decrease the area that the same amount of air has to flow through, then it increases the velocity. Kinda like when you put your thumb on the end of a garden hose. The water flows faster, farther, and with more pressure. That's really how air works. There is a limit though as descibed earlier. If you push your thumb down too hard, then very little water can exit the area between your thumb and the hose. But it would increase the pressure inside the hose.

I have been trying to explain this to a lot of people for a few months now, but nobody seems to listen. You can very easily oversize a pipe. Going smaller is safer because it will be less torque loss than if you were to go with too big of a pipe. Actually, it will usually give a small increase in torque due to the increase in velocity, which increases scavenging. And we all know that torque is what accelerates a car.

I am a sheet metal apprentice and that's what they keep teaching us. Have you ever noticed that duct work in a house has smaller pipes farther from the furnace. That is to cause back pressure (and yes it does exist) in the vents closer to the furnace, and build velocity in the ones farther away. That way you get just as much air in the rooms that are farther away, as you do in the rooms that are closer, and visa versa. That's exactly why step-tube headers work so well.

If they ran the same size pipes through the whole house, then the rooms closer to the furnace would get very little air flow from lack of back pressure. I would assume that the pressure from the exhaust of an engine would work very much the same way. Going too big would cause the air to become more stagnant. It would just sit there and have to rely on the motion of the car to expel the gases from the pipe. I Probably exaggerated there a little, but you get my point, right?

So for a street/strip car, which I'm sure most of us are all trying for, a little smaller pipe would give more desirable results.

AJ

[This message has been edited by AJ_92RS (edited August 09, 2001).]

RW91B4C

Since when did we start posting commercials on the Tech board? Mr. "Casino"
Whats next those annoying, bad accent infomercials!

------------------
91 B4C 305 TPI - SOON TO BE 383
TREMEC 5-SP, STOCK 1-BOLT REAREAND w/342 GEARS, K&N, AIRFOIL
EDELBROCK HEADERS, DUAL CAT TO HOMEMADE Y-PIPE & 3.5" SINGLE PIPE W/ FLOWMASTER, CRANK PULLEY, MSD, FUEL PRESS REG, COWL HOOD, WELD WHEELS
14.1@ 98MPH
-------------
OTHER RIDE
67 CAMARO - STREET CAR, BIG BLOCK, PUMP GAS
350TH w/ATI 10", 12-BOLT w/373 GEARS
10.94@124.13 ON MOTOR - ET-STREETs w/MUFFLERS

RB83L69

The way an exhaust system helps out an engine, is by harnessing the inertia of the moving air, and using that to help scavenge as much as possible out of the cylinders. A number of people in various forms of racing have discovered that a properly designed exhaust system does in fact cause an engine to produce more power than no exhaust system at all. For instance, if a NASCAR car would go faster without that 15 or 20 lb thing that they use, don't you think they'd leave it off in a heartbeat?

Headers in general are nowadays designed the same way. Back years ago, people used the headers to cool the exhaust gas down, which reduces its size and pressure, which should make an engine flow better. What we have found since then is that in fact the faster the air flows through the pipes, up to the point where the pipes become a restriction, the better the cylinders get emptied of spent gases. That's because of the mass of the gas in the pipes and its inertia, i.e. tendency to remain in motion once set in motion.

That's where the idea of an optimum size comes in. You'll get the most power out of an engine when the exhaust is of the correct size for maximum velocity but still minimal restriction. You can slow a car down by putting too large of an exhaust system on it.

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"So many Mustangs, so little time..."
ICON Motorsports

camaro6spd

one place backpressure is use alot is in 2 stroke high-performance dirtbike and atv engines, you think exhausts for our car are high tech take a look at the ones for the motocross bikes, theyare tech, anyway, they use the backpressure to keep the new intake charge from coming out but this happens in 2 stages, the first stage is the hot gasses reach the expansion chamber(the big bulge in the pipe) and expand(hench the name) and suck out the exhaust and then backpressure keeps the new change from comeing out the exhaust port. But seeing our motors are 4 stroke and don't have the exhaust and intake ports open at the same time we need not worry about it, unless you ride like me.

AJ_92RS

Who are you referring to?

If, by chance, it's a comment toward me, then I'm sorry I tried to educate the guy.

It's the same reason YOU posted the chart. You did it to help people, right? That's all I was trying to do. I was answering the guy's question. Just using something that he might be able to relate to because he saw it before.

Take it however you want to. Again, I was just trying to help. Some of us want to know the "why" as well as the "what should I do", or "what should I use", etc.
AJ

EDIT: BTW, Thank you for posting the chart. It is usefull.

[This message has been edited by AJ_92RS (edited August 10, 2001).]

RB83L69

Somebody was posting something about an on-line casino or lottery or game or something.... it hit a whole bunch of the boards as responses to other posts, which is a whole lot harder to stamp out than originating new ones.... the moderators deleted it/them. Nothing personal.

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"So many Mustangs, so little time..."
ICON Motorsports