Inliners International Forums General Discussion Engines Exhaust size effect - is bigger always better?

JimW- bear in mind, usually if you just pull the muffler off it will be slower - down low and also sometimes even up on top!

You have to generally kick the timing up a couple degrees, and jet up say 3 numbers richer. These changes will NOT help with the muffler on. But now, when you uncork it, if the open pipes are the right length, you will hear a roar out the headers as the rearend fishtails from the low end, and your friends will hear the car from 4 miles away as it sings at top end. BTDT!!

J78: If you still have the stock 250cid in there (about 140hp gross, 110net) I'm not sure that you need more than a single 2", since 2" duals are plenty up to near 300hp, a 2" single is plenty up to maybe 140-150hp, no need to go bigger.

Its more like BIUB (Bigger Is Usually Better), but BINAB (Bigger Is Not Always Better)!

I have 302 GMC with a mild cam and reworked head -about 200hp.

The inline 6 is really two 3 cyl. engines whish run better with separate exhausts. Exhaust size depends on cylinder size, hp. and rpm range which produces exhaust gas flow volume.

My best running exhaust has been dual headers with 1.5" on the single port and 1.625" on the dual port both merging into 1.75" header output flanges and exhaust pipes. Stole many pieces and ideas from a double OH cam Jaguar XJ6.

A couple of questions, Tom what kind of muffler is that in the picture and is that a 250 or 292 oil pan? Jim W what kind of carburator are you running? Thank both of you in advance, Jay 6155

It is a big Magnaflow unit. Biggest I could fit under the car.
250 oil pan. Same as yours. No need to run a 292 pan for street use.

8007 holley - 390CFM. Offenhauser 4 barrel intake with HEI and Langdon cast iron headers. FWIW, this was the first Holley I ever messed with, and I spent 2-3 months getting it tuned for both mileage and performance. WHAT A BATTLE. We have ran another Holley on a 'V' (double pumper). One sentence - goes like hell and sucked gas like hell at the same time. We changed over multiport and megasquirt. With that said, I will hopefully never mess with another Holley sgain.

Multiport? I was under the impression that multiport did not work well on these engines due to the siamese intake and timing of engine strokes?

Tom and Jim W thanks a lot for the info. I thought that that car had a 250 oil pan because of the gap between the crossmember and the sump. I have very little experience with Holley carbs, I have a 2bbl on the boat that has a custom metering block by the Volvo factory. In a boat half throttle or more is common ,no need to worry about low speed drive ability. Deuce Coupe, the only nonstock things I have is no cat and a open element air filter. I am considering a z28 y pipe, mufflers and tailpipes mostly for sound. The glasspack on there is LOUD. Also when I either get a 2bbl head on or adapt the dual manifold to the head on the car I can run 2 2"pipes to the z28 mufflers. Jay6155

Here is what my 65 Chevelle sounds like. Skip thru the video to the start ups and driving. It is a bit loud. I still want to add a resonator to the tailpipe.
https://www.youtube.com/watch?v=3NU7n3kOWe8

I enjoyed that video! My nova is that loud at idle without the nice. Cam lope or the higher compression. On the road I am a little louder again without the "nice"part. That wagon looks fun to drive with the 5 speed. I have a th350 for now and will stick with an auto trans of some type. Compared to you I have "racing" 2.73 gears and 25inch tall tiresHA HA! Jay6155

exhaust systems are complex circuits. i do not pretend to be able to design one, but i know the physics behind analyzing them.

exhaust energy is a series of timed pulses; it's very much like chirping/blowing into a musical instrument.

the "resistance" to gas flowing through the exhaust varies with the pulse rate -- the frequency, eg. rpm -- of the gas pulses. it is directly analogous to electrical impedance. "impedance" is the same as "resistance" as in Ohm's Law, but dependent on frequency (as in our engines). at D.C. calculating with resistance in electricity is easy. calculating with impedance, using varying frequency electricity, is complex.

if you get it right, the exhaust has a low impedance -- low resistance -- at a range of rpm (frequency), that can even be *negative*, eg. draw (suction) in a narrow frequency range. really good, smart, tuners can calculate and build that, but it's very complex math with added complexity of real-world metal.


that's why what sometimes seems intuitively like a restrictive pipe -- a long straight "skinny" pipe might be series-resonant at a useful RPM range, and actually be optimum for exhaust flow, where increasing pipe diameter will make it worse.

in sloppy terms -- what i am able to actually build for -- overall low loss, low resistance, eg. short, large, unrestrictive exhaust is a hell of a lot better than most old/early factory systems which were designed to make the car quiet and HP wasn't really considered. (my Rambler came with 1.25" pipe with, seriously, 360+ degrees of bends, but wow was it quiet).

there's some wikipedia pages on series-resonance vs. parallel resonance and musical tuning and all that, but the practical physics of bending pipe for automotive purposes seems to remain a black art.

(lol, now if you made a "trombone" that varied in length via software and servomechanism you could to active exhaust scavenging... all you need is a huge shop and a lot of instrumentation to work it up :-)

Tom,
Curious what Rambler you have that came with 1.25" pipe, all the way or just tailpipe?
That's about the smallest Ive ever heard of.
Guessing the 196cid? Which one?

the pre-1964 Americans came with a 1.25" pipe, all the way to the back. looks like a soda straw. there was one at a U-Pull-It in Sun Valley CA a few years back, i almost saved it, as it was the optional porcelain version -- blue porcelain inside and out (outside, at least). i had a 62 Ambassador that had the porcelain option (i sawed it in half to stick a Cherry Bomb on it -- just out of high school, seemed like a good idea (was not). looked like vintage kitchen ware! i forget what the Ambo had but it wasn't that strangled (the Nash-designed 327ci V8).

the pre-1964 Americans came with a 1.25" pipe, all the way to the back. looks like a soda straw. the 195.6 six, usually flathead. there was one at a U-Pull-It in Sun Valley CA a few years back, i almost saved it, as it was the optional porcelain version -- blue porcelain inside and out (outside, at least). i had a 62 Ambassador that had the porcelain option (i sawed it in half to stick a Cherry Bomb on it -- just out of high school, seemed like a good idea (was not). looked like vintage kitchen ware! i forget what the Ambo had but it wasn't that strangled (the Nash-designed 327ci V8).