Why less power with sidepipes?

[Willard M.]

In some recents posts about FI cars it was noted that sidepipes actually caused a loss in power. Why? I would think that exhaust going through the mufflers would loose more power than through sidepipes which are close to no restriction at all. I have only had one sidepipe car a 66 350 horse coupe. Maybe it was the sound but it sure sounded like it had more punch.

[Steve B.]

Re: Why less power with sidepipes?

It has to do with the size of the pipes which I believe are 2 inch pipes or smaller as opposed to 2 1/2 inch for under car set ups. I agree in that it does sound great however.

[Don W.]

Re: Why less power with sidepipes?

It has to do with the flow pattern in the side pipes which is much more restrictive than the dual undercar exhaust. People assume it is like headers that go into a straight pipe which is not correct. There is actually a reverse flow pattern in the side pipes.

[Donald T.]

Re: Why less power with sidepipes?

It has to do with the size of the pipes which I believe are 2 inch pipes or smaller as opposed to 2 1/2 inch for under car set ups. I agree in that it does sound great however.

Side exhaust was available in 2" or 2.5" just the same as undercar exhaust.

[Michael M.]

Re: Why less power with sidepipes?

Don it has to do with back pressure. The side exhaust cars exhaust meets the restriction much sooner then a under car exhaust. The under car exhaust mufflers are past the rear axle there fore less restriction more power. The 2" pipes compaired to the 2 1/2" pipes also make a difference.

[Don W.]

Re: Why less power with sidepipes?

I have a '69 and the EPA jumped in a stopped most of the production due to noise pollution.

[Don W.]

Re: Why less power with sidepipes?

69 Data

The double-walled/perforated/crimped straight section of the factory sidepipes functions as a muffler, although the 1-7/8" diameter inner pipe is highly restrictive (only has half the cross-sectional area for flow vs. the standard under-car exhaust system); the sidepipe option was driven by Styling and Marketing, not by Engineering.

[Duke W.]

Re: Why less power with sidepipes?

It has to do with both the pipe diameter and basic design of the sound attentuation elements. The basic core diameter of side pipes, both big and small block is 1 7/8", and the side pipe design does not attenuate sound as efficiently as a typical reverse flow muffler.

Net result is sidepipes generate more exhaust backpressure, which means more engine power is required to pump the exhaust through the side pipe system, which leaves less at the flywheel/rear wheels.

I can't quantify it, but if anyone wants to test the same engine (preferably SHP) with side pipes and the 2.5" under-the-car-system I will bet five to one that sidepipes will produce noticeably less power beyond the typical 1-2 percent dyno test repeatability.

While you're at it why not test the new off-road mufflers reproductions vs. a set of base mufflers?

Back in 1983 a buddy and I both bought twin (red and white) Honda CB1100Fs. In OE trim Dick was a little faster because, dripping wet, his 140 pound is 40-50 less than my weight. He installed an aftermarket 4-into-1 header in place of the OE 4-into-2 system. It was louder, and he swore it was faster. When we got around to doing some more roll-ons IT WAS SLOWER! Then he installed individual filters on the carbs in place of the OE air box, took fifty pounds off the bike, AND IT WAS EVEN SLOWER!!!

The only change I made to my 1100 was one size larger pilot jets and .020" shims under the needles to get rid of the off-idle leaness/driveline s-n-a-t-c-h for more linear off-idle throttle modulation.

Dick is a talented fabricator, but not an engine system engineer. I turned him into a believer, and now he has the world's fastest 2.8L Fiero - 131 MPH at Bonneville with a little help from my system engineering and his fabrication. The block is OE (never touched 8:1 CR that runs on regular unleaded) including the cam, which we retarded four degrees. All the work was on the heads, inlet, and exhaust system. The work killed the low end torque (which I knew it would), but the 30 percent increase in top end power was good for ten percent more top speed, which was 119 in OE trim. The power requirement inceases with the cube of speed, so he needed about one third more to break 130.

BTW, we both still own our CB1100Fs.

Duke

P. S. The board won't let me write snat..., so I had to do as I did above. I guess in one context it's considered a naughtly word, but I didn't mean it that way.

[Roger P.]

Re: Why less power with sidepipes?

This is all very interesting to me as I always assumed that sidepipes would provide a little more power - certainly not less power. How do the sidepipes affect fuel mileage? My '65 convertible (4-speed & sidepipes) averages about 12 miles per gallon for weekend cruising on the highway and local roads. Not that I am planning on changing anything, but would the MPG's improve with standard under-car exhaust? Is my 12 MPG "normal" compared to others, or does my girl have a drinking problem?

Thank you,
Roger (50141)

[Steven B.]

Re: Why less power with sidepipes?

This is all very interesting to me as I always assumed that sidepipes would provide a little more power - certainly not less power. How do the sidepipes affect fuel mileage? My '65 convertible (4-speed & sidepipes) averages about 12 miles per gallon for weekend cruising on the highway and local roads. Not that I am planning on changing anything, but would the MPG's improve with standard under-car exhaust? Is my 12 MPG "normal" compared to others, or does my girl have a drinking problem?

Thank you,
Roger (50141)

Roger, which rear ratio do you have, are your tires equal the height of the factory delivered 7.75's, where do you drive (city, rural, etc.) and how heavy is your foot? A bunch of variables when comparing, and personally I wouldn't care as long as I was having ! 'Sounds like a great car.

Steve

[Duke W.]

Re: Why less power with sidepipes?

Exhaust pumping power loss increases with the cube of flow, and flow increases somewhat more than linearly with power output. At highway speeds on a level road the engine only needs to produce 30-50 horsepower, so there would probably be no noticeable effect on fuel consumption in most driving situations.

Duke

[Tom D.]

Re: Why less power with sidepipes?

Duke: Keep the details coming. Thanks!

[Clem Z.]

Re: Why less power with sidepipes?

the 90 degree bends are part of the problem.

[Roger P.]

Re: Why less power with sidepipes?

Steve,
I'm not a mechanic, so I'll do my best to answer the questions. Going from memory, I was told that my rear ratio is 3.70; my tires are new Yokohama 215/70 R15 white walls; I drive 50% on the highway and the other half in city streets with stop signs & traffic lights; and my foot isn't light, but I'm sure it's not nearly as heavy as many (I like to cruise, not "race" - a red convertible Vette is begging for a ticket ). The engine used to run rich, so I had the carb professionally tuned and smaller jets installed (it's a 750 Edelbrock). You're right; it's a great ride and I have a blast every time I'm behind the wheel! For my weekend cruising, I guess 12 MPG vs. 15 MPG would make little difference looking at the big picture. I was just curious what others are getting in terms of MPG.

Thanks,
Roger (50141)

[Duke W.]

Re: Why less power with sidepipes?

Okay, so it's a '65 with a 3.70 axle. So it must either be a L-78 or L-79, but if it has a "Edelbrock 750" it's been modified. So...?

Twelve MPG from ann OE L-78 is probably not bad, but an OE L-79 should be in the mid-fifteens.

If it's been modified, you're on your own.

Duke