Looking for a large diameter exhaust.

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Dec 1, 2004 | 10:15 PM

#26

2dark2Bslow

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Joined: Apr 2003

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From: san antonio

Quote:

Originally Posted by Mahjik

And?

The RX7.com street class drag car put down more power with a 3" exhaust.

Well, the car wont be making power from a rotary anymore. Ls1's dont care for that much backpressure, just pure open spaces...where the buffalo roam

sorry, couldn't resist.

Dec 1, 2004 | 10:20 PM

#27

Jism

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Joined: Oct 2004

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From: Midwest

Quote:

Originally Posted by 2dark2Bslow

ok, but what doesnt make sense is with a 3.74 inch pipe and a 75mm flange, how can it fit flush to a 75 mm dp. (Not that the car is even having one but for explainations sakes.)
The flanged exhaust would need a coupler to to make a transition in size, therefore the pipe can't be 3.75 all the way...right?

Im sorry i didn't go into enough detail. Yes there is a transition, you have to have one for the flange to fit. Thats What your looking at in the pic. There is no reason to complain about the transition because you only have a 3" dp or a 3.5 dp your not losing any hp going from 3"-3.74". Your actually gaining hp going from a smaller pipe to larger. Yes its not as efficient as having a 3.74dp, 3.74 test pipe, and a 3.74 cat back but your not going to be able to find any of that stuff in 3.74 made already. It would have to be a custom job and if you do that you might as well spend the extra 500 and get the hks 4" all the way because a 4" dp/midpipe is going to be more common than teh 3.74

Dec 1, 2004 | 10:30 PM

#28

turbogarrett

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From: wisconsin

I'm in the same boat as you. Right now I am using a 3" pettit exhaust, but will need to go bigger with the long tubes on the way. Two 3" collectors feeding a single 3" sounds like a bottleneck to me. I will probably go the custom route.

Dec 1, 2004 | 10:58 PM

#29

Mahjik

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From: Kansas City, MO

Quote:

Originally Posted by 2dark2Bslow

Well, the car wont be making power from a rotary anymore. Ls1's dont care for that much backpressure, just pure open spaces...where the buffalo roam

sorry, couldn't resist.

Yea, and you might want to do some reading as well:

Quote:

Well, not really that easy. Back pressure doesn't necassarily give or take horsepower. It's all about the exhaust dynamics. ON NA cars with high flow heads and large exhaust valves(anything over 1.75 " or so) back pressure helps keep the exhaust near the port longer and allows it to heat more evenly. When this occurs you can maximize the exhaust gas velocity coming out and it helps scavenge the gasses out of the head. When you maximize this scavenging effect the exhaust flow maintains a nice flow that allows the exhaust valves to move more freely(less residual exhaust gasses in the port following combustion) There by allowing for better torque production. In a fairly large and free flowing head you will gain power by lowering the back pressure and allowing the exhaust to exit quickly. It really depends on the whole combination when it comes to this topic. In a fairly well modified na V8 reduction of backpressure will result in a gain in hp and not suffer much if any negative effect.

There is more to performance than just making your exhaust larger....

Dec 1, 2004 | 11:46 PM

#30

slidingsky

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Joined: Aug 2002

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From: Honolulu

according to corky bells book you cant put down 600 on a 3" exhaust. THats piston. So even less on a rotary. Most big power cars using 3" pipe dump the wastegate seperate.
Im running 4" to dual 3" on my 20b. Getting ready for turbo. 650whp

Dec 2, 2004 | 01:16 AM

#31

2dark2Bslow

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Joined: Apr 2003

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From: san antonio

Quote:

Originally Posted by Mahjik

Yea, and you might want to do some reading as well:

In a fairly well modified na V8 reduction of backpressure will result in a gain in hp and not suffer much if any negative effect.

There is more to performance than just making your exhaust larger....

So are you trying to make a point for or against you because it states in YOUR research youve fond exactly what i was saying.

Now can we just get back on on topic!

I saw the HKS but it really didnt grab my eye with the dull metal finish. I already plan on some of it custom but i wanted the car to still look like a normal rx7 from the back and not with some dumped flows hanging out the back.

Dec 2, 2004 | 10:51 AM

#32

Mahjik

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From: Kansas City, MO

Quote:

Originally Posted by 2dark2Bslow

So are you trying to make a point for or against you because it states in YOUR research youve fond exactly what i was saying.

That's not my research. However, if you read the post, you'll see that it depends on more than just the size of the exhaust for maximizing performance. In a perfect world, simply using the largest exhaust possible would always be the best choice.

However, this is far from a perfect world.

That information came from a discussion about some built Mustangs creating some backpressure to maximize performance. A person had made a post "less backpressure=more horsepower" which is what the quote I posted was responding to.