Is there a certain back psi needed?

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Zorm
Posts: 44
Joined: Sat Oct 10, 2020 6:33 am

Is there a certain back psi needed?

Post by Zorm »

Like the subject states, is there a certain back pressure needed for the exhaust? I ask because my son removed his stock ford GT mufflers and they are in real good shape, ashame to toss em and they are free! The diameter is much larger than the Corvair stock pipe, (2 1/2 or 3"). I need to replace the exhaust anyway on my 65 Corsa vert. I was thinking of getting new duel exhaust pipes from Clarks, then modify them for the mustang mufflers. Like the only piece I might use would be the flange to the manifold, then go larger right after that. Fitment is not too bad, just a little larger then the, I guess, turbo mufflers the PO has on there now. I will need to make to make some new hangers since they are not there anyway. Also was thinking of a cross over pipe between the two sides? would it really make any kind of difference though?

thanks for thoughts
66vairguy
Posts: 4531
Joined: Tue Dec 21, 2010 3:44 pm

Re: Is there a certain back psi needed?

Post by 66vairguy »

The old "you need back pressure in the exhaust" to make a car run has been debated forever. IF the engine is old and worn out creating exhaust back pressure will help it run better at idle, I've seen this done on old worn out cars. Other than "tuning" an exhaust to promote better exhaust flow via scavenging there is no demonstrated argument for "exhaust back pressure" that stands up to examination based on what I've read.

About those Mustang mufflers. Keep in mind they are tuned for a V8 firing order and I'm guessing they are AFTER the catalytic converter. So using them on the end of a pipe directly from the Corvair exhaust manifold will produce a different sound. You may like it, or maybe not. After a few loud exhaust systems in my youth I found they get annoying, especially on longer highway trips. Just me.

BTW I found the Clark's dual mufflers WITH the larger tail pipe (muffler outlet) have a nice mellow idle sound, but NO DRONING noise on the highway.
joelsplace
Posts: 1950
Joined: Wed Oct 13, 2010 12:51 pm
Location: Northlake, TX

Re: Is there a certain back psi needed?

Post by joelsplace »

With open pipes you have around 15psi back pressure. A good exhaust system will promote the best velocity to overcome that pressure so if you just have open pipes or the pipes are too large you will actually have too much back pressure. You can also have a system that is too restrictive but that isn't what we have here. If you want better performance then you could try one of those mufflers with a single exhaust if you want duals for the look or sound a crossover will reduce the performance loss.
157 Corvairs, 5 Ultravans and counting
Northlake, TX
Wagon Master
Posts: 474
Joined: Thu Jul 28, 2011 6:36 pm

Re: Is there a certain back psi needed?

Post by Wagon Master »

I'm officially lost. It sounds like you're saying the more free flowing, less restrictive the exhaust, the more back pressure. Am I getting that right?
66vairguy
Posts: 4531
Joined: Tue Dec 21, 2010 3:44 pm

Re: Is there a certain back psi needed?

Post by 66vairguy »

Wagon Master wrote: Fri Dec 04, 2020 3:53 am I'm officially lost. It sounds like you're saying the more free flowing, less restrictive the exhaust, the more back pressure. Am I getting that right?
This reminds me of the ads claiming huge HP gains on Corvettes if you used an aftermarket exhaust system. A magazine took two days to bolt up and test all these different exhaust systems on a dyno and track. The biggest improvement was 5HP - YUP. And when the testers looked at the data they noticed that gain was also there AFTER they put the stock exhaust back on. So they retried the test and found that 5HP gain was due to a weather COLD FRONT. Yes the cooler damper air caused the 5HP gain!

I remember when the Corvair folks started saying the turbo engines were held back by a restrictive exhaust. Not true.

Bottom line - Most of the improvement on a stock Corvair engine when you install a larger exhaust is because the greater sound makes the car seem faster.
joelsplace
Posts: 1950
Joined: Wed Oct 13, 2010 12:51 pm
Location: Northlake, TX

Re: Is there a certain back psi needed?

Post by joelsplace »

That can happen. That is one of the reasons a system that is too large hurts performance. Proper sizing to achieve proper velocity is best to combat the atmospheric pressure that is trying to keep the exhaust in the engine. Too large or too small increases back pressure. The reason single exhaust works better is that you have all 6 exhaust pulses fighting the constant atmospheric pressure instead of just 3 with a dual exhaust. The crossover will allow all 6 and that is why it helps.
157 Corvairs, 5 Ultravans and counting
Northlake, TX
joelsplace
Posts: 1950
Joined: Wed Oct 13, 2010 12:51 pm
Location: Northlake, TX

Re: Is there a certain back psi needed?

Post by joelsplace »

That is true the turbo Corvairs have a very free flowing muffler. That is where the term "turbo" muffler came from and why performance guys started using them on other cars.
157 Corvairs, 5 Ultravans and counting
Northlake, TX
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