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Why Exhaust is problem for HP on 75-81


Well-known member
Jan 7, 2001
385 CI 77 CORVETTE FE-7 Hi-Perf Sus.
Maybe these 2 examples will explain.

350 stock single exhaust w/ cat
Backpressure 16.5 psi ............. 152 hp
Remove cat, & replace stock
muffler w/ Cyclone sonic turbo
Backpressure 3.5 psi .............. 210 hp
Duals & 2 Cyclone sonic turbos
Backpressure -1/2psi ............... 224 HP
That is +45%
Headers not tested.

Example 2
350 81 Z28 stock ex. ............. 214 hp
Headers ............................... 228 hp
Replace cat w/ hi flo cat. ........ 241 HP
That is over +12%
Not tested w/ duals, expect another 15 min., which would be 256 hp.
That would be +20%

The more hp one has the more headers help.

Now do a Cam change & can be over 300 HP.
This may be a stupid question...

I see how changing the backpressure of the exhaust can increase horsepower, but will lowering that pressure affect anything with the motor adversely?
No, the less backpressure the better.

I agree you do need back pressure, you want to lower it but you need the pressure difference between the exhaust and the exit of the exhaust to keep the fluid flowing. I will give you a better explanation just let me get the info.

Believe I'm an excellent example here...Installed my CompCams 260H and Weiand Dual Plane intake with very nice results last winter...then ripped out my 2 Y pipes and went to chambered pipes...with an awesome difference....the thing could finally breathe...It runs like a Vette and sounds like a Vette!!!
your right tyla

yeah it does, you are refering to the H-pipe.

It helps increase low end torque and at the same time helps lower the sould level of the car.

Its kind of an extension of the collector from your exhaust manifold.

I could be mistaking but i think that the x-pipe developed from this to.
Suggest Increase Power. :cool

^ for the new guys.
As a general rule, smaller duals are better for a small block/street motor for torque, and larger size for higher rpm/HP and race oriented motors. There's allot of grey area though. Nothing beats tons of money and a dyno, but it's a good generalazation.
The same applies to header primary tube size.

There are such things as "tuned" headers and such, makeing use of exhaust scavaging, where they arange the primary tubes in such a way that the firing order of the motor helps there be low preasure to help suck the combustion chamber free of exhaust, hence the name scavageing. I dont know how common it is these days, but it was a good setup a few years back.
Great Post Ganey!! Just goes to show, ya gotta let 'em breathe! Hey Rob, put this one into Tech Sect for ref! -Dave

Thanks. Yes, many really like this info.

Ken said:

:nono To a point, yes. But some back pressure IS necessary.

That is true or else we would be running open hedders and monstruos primaries.
An engine with no exhaust manifolds at all, suffers greatly in the power dept. Reducing back pressure helps; eliminating it hurts! An H, X, or any other named crossover pipe effectually cuts the exhaust back pressure in half aft of the crossover. After all, only one cylinder fires at a a time, and the crossover gives it two pipes, mufflers, and cats through which to exit!
2 questions...

What happens to an engine that has run several months with straight headers, no pipes or mufflers? I want to know what the negative effects are as an example of why this is bad.

What is chambered exhaust?
I want dual exhaust put on the 78, and I want a nice sound, but not something overloud at town speeds. This project will have to wait on a back burner for some other safety issues to be addressed first, but I'm thinking about it now, since I saw this post.
Larger exhaust systems and less backpressure hurts torque at low rpm especially when a cam with a lot of overlap is installed. This is one reason why Winston Cup cars typically will not even run below 2000-2500 rpm. As I understand it, a small amount of backpressure helps stablize flow into and out of the cylinders on the exhaust and intake strokes. The same principles of air velocity in the intake ports also applies to the exhaust ports. A more restrictive exhaust forces gases to move more quickly through the system, creating a kind of momentum and a vacuum that pulls gases from the exhaust ports as they open. By modifying the length and size of the header runners and the shape of the collector, you can enhance this phenomenon called "scavenging". In the case where the exhaust system is completely removed, this phenomenon ceases, and if a cam with a lot of overlap is installed, gases could be pushed up the intake port on the exhaust stroke or drawn back in from the exhaust port on the intake stroke, causing a loss of intake manifold vacuum, and a weak combustion.

As engine speed increases and the volume and velocity of gases into and out of the cylinders increases, the negative effects of extremely low backpressure go away and horsepower increases because of the lower restriction.

This is a combination of things I have read and my own theory. Does anybody have any good exhaust tuning resources?

Here in Southern California, as you probably know, we have very strict emissions testing. We have to pass a dyno test, including the use of a tail pipe 'sniffer'. The standards for older cars such as my '75 Vette are pretty liberal. A well tuned engine, as long as you have retained the Cat usually does not have a problem with the tail pipe readings.

Regardless however of the tail pipe readings, the vehicle also has to pass a visual inspection. Anything that can be seen that is not stock must carry a California Air Resources Board (CARB) certification, which 'proves' that this particular part does not increase emissions.

At this point, I am still having to run stock exhaust manifolds in order to pass this visual portion of the test. I have not been able to locate any manufacturers of headers for a '75 vette that are CARB certified. Some of them appear to be certified until you start reading the fine print and footnotes.

So question . . . does anyone out there produce California legal headers for this particular application? I know someone will come back with "well I bought them for my '75 Camaro from so and so". Problem is, here in SoCal, they are model specific. Even if they are legal for an F body of the same year, running the same engine and smog components, they still will not pass. So I need headers that are model and year specific. In lieu of locating correct headers, any suggestions for freeing up the flow would be appreciated.

The current configuration is as follows:

Stock exhaust manifolds
2 1/2" cross over head pipe
1 into 2 free flow cat
Dual 2" out of cat
Dual Flowmasters

I had shorty headers on the car, but had to switch back to stock manifolds for my emissions testing. I really don't want to have to do that every two years. On the other hand, I loved the headers and hate the old rams horns . . .
Tyla said:
The only thing you do is put on a set of hooker sidemounts, stock out of there box, why much HP with you lose?:confused

Thanks, T.

Thanks for the compliments, Tyla. I'm not sure I understand the wording of your question above, but if you're asking if you'll lose any horsepower with those headers and sidepipes, the answer is probably not, as long as they are reasonably sized. Ask somebody who has the setup you have in mind. You can probably find dyno tests on the internet.

Rpounds, I offer my sympathies. Here in SC, we have no such emission worries. However, don't be too grim at the prospect of sticking with the stock manifolds for a while. My engine builder has a good bit of local racing experience. In some race classes, the cars have to use stock exhaust manifolds, and according to him, the manifolds on my Corvette ('72) are the ones everyone in those classes try to run. Assuming that the ram horns on your car are the same as mine, you can take some comfort in knowing that you have some of the best exhaust manifolds (so far as exhaust manifolds go) around. Still, I plan on switching to headers when I get the chance. The main reason I stuck with the manifolds during the rebuild was to get the engine running in top condition again.
inferno-vette said:

That is true or else we would be running open hedders and monstruos primaries.

How much back pressure would be enough for you guys, would 14+ do it.

Open headers would be best! There are loudness issues which is why there are mufflers to make quieter. Having the pri. too big hurts Velocity flow.

Restrictive is relative. Depends on how much power. Since the stock type sidepipes are restrictive, then the more stock the engine is the more likely someone is to be happy w/ them. For example, a single ex. to sidepipes on a stock engine. For a modest 350 hp setup you mentioned then expect you would be happy. For perf., header type sidepipes & better mufflers like Moroso spiral will help a lot.

Have helped several get better perf. from their sidepipes.

Many of the Calif. guys that I advise run Headman headers on Corvettes which are available for the air pump setup.

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