[92GTA]

I was under my car the other day and to my shock I noticed the previous owner has the 3" header collectors going into a tiny 2.5" exhaust with an H pipe! Plus the mufflers are not high flow.

I then realized that when the owner took out the original 350 and put in the 461, he must have gone cheap and run the new headers into the old exhaust

I'm thinking my next upgrade after a posi is going to be the exhaust.

How much do you think my engine is leaving on the table with this tiny exhaust and regular mufflers?! Gotta be like 50Hp right?!

[Formulas]

461 with A XE274 cam ... I think the 2.5 will be fine as that isn't a very large cam

[Blued and Painted]

Agree.
IIRC Factory was 2 inch with compression bent pipes.
You will probably be all dun at 5800
So i would see how it works and sounds before cutting it out.

[GRX]

Hard to say what "regular" muffler means. This being the street section and not Race, fully agree on 2.5" pipes being fine. Others are surely much better qualified than myself at explaining this ... smaller equals faster velocity and helps draw exhaust gasses from the collectors. Much to be said for a well tuned exhaust instead of simply dumping it into a huge diameter pipe.

[Scott Stoneburg]

Depending on power level 2.5" could be fine. My firebird was deep in the 11s in the 1/4 before I switched to 3". If I remember right 2.5" should be good to about 500 hp. More important is to check the tailpipes, if you have them make sure they are mandrel bent pipes. If they are muffler shop bent the back side of the bends tends to cave in and kill exhaust flow.

[Formulajones]

2 1/2" exhaust will suffice most street cars making pretty decent power and running well into the 11 second zone.

Just have a look at some of the pure stock cars that are in the 11's. They are mandated by rules to run no more than 2 1/2" exhaust, and using stock exhaust manifolds on top of that!!

You are probable going to find that even though your headers have 3" collectors, necking that down to 2 1/2" for the exhaust probably isn't going to hurt anything at all at your power level. As long as you have a mandrel bent system with good mufflers, that's the most important thing.

I found myself that going from a stock Gardner 2 1/4" exhaust system with crush bends, and switching to a 2 1/2" system with mandrel bends, and a better transverse muffler, my car picked up a solid 4 tenths of a second and 3 mph without touching anything else on the car.

[Scarebird]

Another thought is when I went from 2-1/4" to 2.5" mandrel bent the car got a wee bit quicker but a hell of a lot louder using the same type of mufflers. This with the same RA3 manifolds.

I would keep the 2.5" setup in your case if it were me.

[92GTA]

Thanks everyone! Looks like I'll for sure hold off on upgrading then until I get better heads and cam.

[JLMounce]

Nowhere near 50 hp. You're probably talking about maybe 10 hp and it's probably going to be at an rpm where it's not making a huge amount of difference for you.

Your best bet is to probably put a better flowing muffler on the existing system.

[6D9]

Originally Posted by JLMounce

Nowhere near 50 hp. You're probably talking about maybe 10 hp and it's probably going to be at an rpm where it's not making a huge amount of difference for you.

Your best bet is to probably put a better flowing muffler on the existing system.

x2...The pipe size will be fine but a good straight type muffler will help alot

[unruhjonny]

you might be surprised how few people think about, ore even realize that you need some back pressure;
Most cars will make a discernible power difference with an appropriately sized exahust - rather than jumping straight to 3"...

My car has 2-1/4", and I'm happy with it.

[besserspat]

someone just posted in the street section a thread about the difference on a dyno with a 600hp BBC and its was 20hp from 2.5'' to 3''

On a 340 hp SBC it was nil. Zero difference.

and regular muffs all you need is a set of Dynomax Ultraflows if you want to be louder or Dynomax Super turbo which I really like for being quiet but still flow awesome.

I think thats a very expensive 20 hp considering you have a functional exhaust system already. A decent 3'' system will cost you around 1000$ and that for 20 HP ? If you dont have an exhausts system on your car then it might be a different story.

And with that small of a cam of the OP I dont think he needs a 3'' system, if there is 20hp to be found in his set up it would be from optimizing the tune on the carb, timing,degreeing the cam etc...but not the exhaust... a 3'' would only make it louder and lose a bit on the bottom.

just my .02

Peter

[6D9]

Originally Posted by besserspat

someone just posted in the street section a thread about the difference on a dyno with a 600hp BBC and its was 20hp from 2.5'' to 3''

On a 340 hp SBC it was nil. Zero difference.

I think thats a very expensive 20 hp considering you have a functional exhaust system already.

And with that small of a cam of the OP I dont think he needs a 3'' system, if there is 20hp to be found in his set up it would be from optimizing the tune on the carb, timing,degreeing the cam etc...but not the exhaust... a 3'' would only make it louder and lose a bit on the bottom.

just my .02

Peter

Peter yeah that was me. The 2.5" pipe diameter is fine at his power level I agree is not going to be an issue...but the mufflers can be for sure. In that test with the 340hp sbc, they tested the 3" with Magnaflows straight thru type muffs and Thrust Hush mufflers. Both full 3" systems. The Thrust were down 15 hp to the Magnaflows. So shows the muffs were a big restriction. So yeah a 2.5" system with good mufflers show be just fine IMHO.

Also on another note there was 0 torque loss at all rpm's between the 2.5" and 3" system even on the little 350 sbc.

[ta man]

Originally Posted by unruhjonny

you might be surprised how few people think about, ore even realize that you need some back pressure;
Most cars will make a discernible power difference with an appropriately sized exahust - rather than jumping straight to 3"...

My car has 2-1/4", and I'm happy with it.

I keep hearing this all the time " an engine needs a little bit of backpressure".I know that is true for a 2 stroke and some small engines like a Briggs lawnmower engine...I'd like to see actual tests of theory.

[Chris65LeMans]

Originally Posted by JLMounce

Nowhere near 50 hp. You're probably talking about maybe 10 hp and it's probably going to be at an rpm where it's not making a huge amount of difference for you.

Your best bet is to probably put a better flowing muffler on the existing system.

I'm going to go with the "if it ain't broke.." camp on this one. Since you just bought the car, you'll have plenty of stuff to do to it. Might as well keep the functioning exhaust system on.

I went with 3 inch pipes on my car solely because I wanted it loud - and it is; and that's with a drop to 2 1/2 over the axle.

[77 TRASHCAN]

Originally Posted by ta man

I keep hearing this all the time " an engine needs a little bit of backpressure".I know that is true for a 2 stroke and some small engines like a Briggs lawnmower engine...I'd like to see actual tests of theory.

I've heard that for a long time also...Had some geezer the wanted to put a 2" system behind the 396 in my 68 Chevy pickup (yes, it's the original engine), I went else where.

I believe a lot of the thinking has to do with exhaust gas velocity. There's different types of velocity for different types of engines, and their associated vehicle uses. A hot rod deal is completely different form grandma's car putt putting around town.

I've heard many here and elsewhere that smaller I.D. tail pipes an O.K. once the exhaust gases start cooling the don't need to take up as much space inside the pipe. Maybe smaller tail pipes keep the velocity higher back there and help the system??? Would like to see tests on that too...

[JLMounce]

There's a number of scholarly articles out there relating to the subject. Backpressure in the system may or may not be needed/required based on what you're trying to accomplish. To say it's needed or not needed is blanket and may be wrong or right depending on context.

If you are building a race engine that sees only upper RPM usage and sound cancellation isn't a priority, your engine is going to be most competitive when backpressure is reduced. On the other hand if you're designing a street exhaust system where sound control is important, backpressure plays a roll in how well a muffler works at doing it's job.

There's probably a good month worth of reading out there, but here's one specifically on muffler design that discusses the issue.

https://www.researchgate.net/publica...motive_muffler

[ta man]

I made it as far as "Backpressure: Backpressure represents the extra static pressure exerted by the
muffler on the engine through the restriction in flow of exhaust gasses. Generally the
better a muffler is at attenuating sound the more backpressure is generated. In a
reactive muffler where good attenuation is achieved the exhaust gasses are forced to
pass through numerous geometry changes and a fair amount of backpressure may be
generated, which reduces the power output of the engine.Backpressure should be kept
to a minimum to avoid power losses especially for better performance of vehicle."

[JLMounce]

The Pypes RacePro mufflers are a good example of that in action. Those are a near zero loss muffler and it's characteristics match that.

It tends to be quiet at low rpm when exhaust gasses are not being pushed through the system at higher rates. It allows the gasses, but more specifically the sound waves to leave the perforated inner tube and be absorbed by the muffler's packing. One you step on the throttle and start accelerating exhaust gasses through the muffler, the sound waves and gasses don't have much time to spend in the absorbing material and the muffler gets very loud.

In audio terms, there are several ways to attenuate sound to either change it's characteristic or reduce it's amplitude. Mufflers like the the Pypes RacePro uses sound absorbing material only. This material literally absorbs audio waves and reduces their amplitube. It's effectiveness is directly a result of how much material the waves pass through and how long those waves spend passing through the material.

The other main technique used in exhaust is known as diffusion. Instead of absorbing sound, diffusion redirects it to change it's characteristics. In the case of a chambered muffler like a Flowmaster, the cases uses specifically designed baffles to direct sound waves in different directions. THe result is that audio waves start crashing into each other and creating phase variances. Some frequencies increase, others decrease or cancel out entirely if you have a 100% phase variance. The baffles attenuate desired frequencies while using phasing to cancel undesirable frequencies. The net effect is the sound is changed and some of it has been cancelled.

You'll find that absorption based mufflers all tend to be about the same size but can achieve different levels of sound depending on how the inner exhaust tract is designed. Chambered mufflers tend to be smaller with less baffles for louder units and larger with more baffles for quieter units.

So if you have a given volume of exhaust gasses being pushed through a 2.5" pipe vs a 3" pipe you will have a differing level of pressure in the system. It's why going to a 3" system with the same muffler is louder than the 2.5" system. More backpressure which forces the changes inside the muffler.

The engine pumps against that pressure and it acts on the engine in different ways. Changing VE characteristics which may benefit certain rpm's over other rpm ranges. So you can't just say "an engine needs backpressure, or an engine doesn't need backpressure."

[Formulajones]

Originally Posted by 77 TRASHCAN

I've heard many here and elsewhere that smaller I.D. tail pipes an O.K. once the exhaust gases start cooling the don't need to take up as much space inside the pipe. Maybe smaller tail pipes keep the velocity higher back there and help the system??? Would like to see tests on that too...

That's been tested and proven a couple decades ago. I'm pretty sure if my memory isn't failing that it was Jim Hand and his Wagon that performed that testing for a magazine back in the day. He showed how smaller tail pipes didn't slow his car one bit. I believe at that time he had a 2 1/2" system and the tailpipes dropped to 2 1/4". He also tested a bunch of mufflers that day as well as "H" and "X" cross overs.

I do the same thing with one of mine, but not for sound or performance reasons, I do it to keep the stock appearance at the rear of the car with correct bent and sized tailpipes on the back of my 69 Z28. The rest of the system is a 2 1/2" mandrel bent transverse muffler setup, with simple reducers at the muffler for the tailpipes. Looks bone stock when viewing the car. You have to crawl under it to see the rest of the system is larger than stock.