Why less power with sidepipes?

[Steven B.]

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)

MPG doesn't sound bad for the city driving, carb, etc. The 70 series tires are not as tall as the 7.75's and therefore changes your effective gear ratio to a number greater than 3.70:1 and your MPG. As long as the tach needle moves alot faster than the gas gauge needle you are OK .

[Joe C.]

Re: Why less power with sidepipes?

"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". Last edited by Duke Williams (22045) : Today at 08:43 PM.

I haven't heard that word, (snāch) in a long time.It is "refreshing" to hear it again. So, is your friend's name, DICK, a coincidence?
(snāch) (v.)
c.1225, perhaps from M.Du. snacken "to (snāch), chatter." The noun is attested from c.1300; vulgar slang sense of "vulva" is recorded from 1903; from a much older sense of "sexual intercourse quickly performed" (c.1589). Weight-lifting sense is attested from 1928.

[Joe C.]

Re: Why less power with sidepipes?

In addition to the actual 1 7/8" core diameter of the sidepipe mufflers, you must also consider its effective diameter. The louvered perforations in those cores further intrude into the flow path. To make matters even worse, those louvers have their open ends facing into the direction of flow, so as to direct exhaust gas into the chambered outer annulus of the mufflers.

[Roger P.]

Re: Why less power with sidepipes?

Duke and Steve,
Thank you both for responding to my gas mileage question. Duke, before you leave me on my own, I guess you need some additional information so you don't think that my car is a mechanical mess. My original engine (327/300) is out of the car on an engine stand as one of the previous owners put in a new GM Performance ZZZ crate engine (350/345) because he wanted more HP. I spoke to all previous owners (other than #1), and this guy was a GM mechanic for 6 years and did a body-on in 1989. I will one day put the original motor back in the car, but for now, I have a motor with only 6,000 miles so I'll run & play until it checks out. Perhaps this information saves me from the "modified" engine category?? Does the 12 MPG sound reasonable now, or should I be expecting better?

Thanks,
Roger (50141)

[Mike R.]

Re: Why less power with sidepipes?

Is there any other muffler available that fits under the stock covers and has less restriction with equal sound reduction?

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.

[Clem Z.]

Re: Why less power with sidepipes?

Is there any other muffler available that fits under the stock covers and has less restriction with equal sound reduction?

try www.sweet-thunder.com

[Willard M.]

Re: Why less power with sidepipes?

Now I understand. Thanks folks.

[Joe R.]

Re: Why less power with sidepipes?

try www.sweet-thunder.com

Clem,

There are several versions of side pipes on the web site. Which set of pipes are you recommending? It looks like maybe the high performance version can only be used with headers. Are there any high(er) performance side pipes that will work with the O.E. exhaust manifolds?

Joe

[Clem Z.]

Re: Why less power with sidepipes?

Clem,

There are several versions of side pipes on the web site. Which set of pipes are you recommending? It looks like maybe the high performance version can only be used with headers. Are there any high(er) performance side pipes that will work with the O.E. exhaust manifolds?

Joe

your best bet is to contact them with those questions as i am sure they will have the answers.

[Mike R.]

Re: Why less power with sidepipes?

I was looking at these on their website and listened to some utube clips but it is impossible to know how loud they are. On the thread I started on FI performance issues, there was a poster that had found a 40hp drop with sidepipes. If I could get half of this back, I would be thrilled but only if it is somewhat civilized.

Right now I have a few options to better performance while keeping the car pretty much stock. I can bump compression half a point with the (original type) shim head gasket and I can have the heads ported back in the valve pockets. The problem is that neither of these will do much if I have a lot of exhaust back pressure. All these might net 30-40hp which would really make a difference.

I want the car original but I am fine with easily added and easily removed mods that improve my driving experience.

try www.sweet-thunder.com

[Wayne M.]

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 -- A federal government agency may well have imposed regulations by 1969, but here's an earlier example (non-Corvette, but Chevrolet) where it was decided to eliminate under-car chambered exhaust because of local law enforcement citations.

(to all) Can anyone describe the internal differences of the N14 sidepipes between 1969 and the '65-7 chambered versions

[John H.]

Re: Why less power with sidepipes?

I was looking at these on their website and listened to some utube clips but it is impossible to know how loud they are.

Mike -

All of their offerings are far less restrictive than the OEM sidepipes, but all are also LOUD. The words "sidepipe" and "quiet" are mutually exclusive.

[Don W.]

Re: Why less power with sidepipes?

Yes, they are also HOT. Very easy to burn your leg getting in and out.

[Joe C.]

Re: Why less power with sidepipes?

Don -- A federal government agency may well have imposed regulations by 1969, but here's an earlier example (non-Corvette, but Chevrolet) where it was decided to eliminate under-car chambered exhaust because of local law enforcement citations.

(to all) Can anyone describe the internal differences of the N14 sidepipes between 1969 and the '65-7 chambered versions

Great question Wayne!

1965-67 style chambered mufflers are as described above, using a small, inner core, which is actually louvered, and whose projections intrude into the already small (1 3/4" I.D.)flow path. The inner tube is surrounded by the crimped outer shell, which forms annular chambers around the inner tube. These chambers are not filled with glass wool or any other sound absorbing material. The openings of the louvers in the core are facing the direction of flow, so there is NO streamlining effect (they act as air scoops). The exhaust gas is then diverted by the louvers into the chambers formed between the inner core, and the volumes between the crimped outer muffler shell, where it is cooled, attenuated, and reflected back into the inner core.

The 1969 sidepipe design is a sea-change over the old design............greatly improved, but still not nearly as efficient as a conventional "canister" type flow-through muffler. This later design employed two parallel tubes. The first, which corresponds to the inner core of the old type, has openings into the parallel attenuation tube, which is divided into chambers by means of welded inner divider plates. This design employs the same basic mechanism for attenuation as the older design, but inherently is more free flowing because the primary tube is larger, as are the attenuation chambers. The primary tube still has projections into the flow path, however, which cause turbulence, cooling, and resultant loss of velocity, increased density, and pressure gradient, albeit somewhat less than the older design.

I like to say that chambered muffler restriction factor is approximately inversely proportional to decibel level. Substantial noise attenuation requires large volume chambers, as found in conventional mufflers.

The only way to achieve significant attenuation with 1965-67 style chambered mufflers, is to have larger volume outer chambers. This would require outer cases on the order of 2-3 times the diameter of the inner core. For a free flowing 2 1/2" inner core, you can see that this clearly presents a packaging issue.

Joe

[Roger P.]

Re: Why less power with sidepipes?

Mike,
I was considering something to make my sidepipes a little less loud to appease my wife, but I am going to hold off since I think they sound great (they're supposed to have a mean rumble). However, down the road, I might do something that "firstgear" did to his pipes that I read on the CF (you should be able to search the thread on the CF). As he explained and showed with a clear photo, he went to a local muffler shop and had them "custom make a couple of dual glass packed connected mufflers" (two mufflers connected by a small pipe on each side). They are bigger than the exhaust pipe, but still fit inside the sidepipe cover. He claims that they provide "an awesome low rumble, not a loud clanging or metallic sound". You might want to check it out as it could solve some of the HP and sound issues.

Roger (50141)