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Old 05-07-2009, 03:25 AM   #1
iRick.
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Default quick question. real quick. promise.

Does running a 2.25" straight pipe run the risk of damaging the exhaust valves?

I know on turbo cars it doesn't, but I haven't even thought about on N/As....
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Old 05-07-2009, 09:27 AM   #2
Syph3r_
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what is your definition of "2.25" straight pipe"?

Do you mean after the header using a straight pipe to a muffler of some sort?
In that case I have never even herd of exhaust valves and damage in the same sentence until you said it.
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Old 05-07-2009, 12:55 PM   #3
iRick.
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Close.

I'm talking a straight pipe from the headers to the back, no muffler. like this.

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Old 05-07-2009, 01:39 PM   #4
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I've never gotten the valve thing. It just doesn't make sense on a system that's designed to handle explosions.

A straight pipe won't do anything. The only place you can cause problems is with the header itself. A poor design leads to less then ideal operate. The engine is an air pumps, so you want it to pump air as efficiently as possible. With this there are certain ideas of ideal flow velocities and the use of pressure wave tuning to boost volumetric efficiency.

With the exhaust valves specifically, I don't know. It's not a concept that makes sense. I'm curious if it's simply something that was passed down from racing applications to a myth in consumer applications. It's simply that we are not burning as energetic a fuel, and we are not seeing temps like that of race applications. The only problem we end up having on a consumer level is a poor exhaust design that puts exhaust gases back into the combustion chamber and back up into the intake which will yield poor efficiency. The lack of cool air would elevate temps some as well. However, the engine is still liquid cooled, so it really shouldn't be a major issue anyways. I think you're worse off having valve problems from poor combustion where you get a lot of carbon build up and start having problems with the valves completely closing.

Maybe someone else can shed light on the valve thing. It's something I've heard a number of times, but it's never been something that has ever seemed feasible.

There's nothing wrong with your straight pipe other then the fact that it'll be really loud, annoyingly so.

I'll also note that you won't really see much in terms of power differences irregardless of the exhaust parts you use. You toss the car on a dyno and run a straight pipe catless, with a cat, with a glasspack, with a full muffler, etc. You'll see nearly identical dyno plots for all of them. Most of the gains and behavior come from the header design as this is the primary factor in defining flow velocity out of the combustion chamber and any pressure wave tuning used. The rest of the exhaust doesn't matter much at all. The only thing you can do is add too much restriction, but realize the engine flows about 300cfm of air and a 2" pipe can fully support that flow level.

Team Integra did some testing of exhaust pipe size:
http://www.team-integra.net/forum/di...g+Common+Topic

By the way, they actually have a lot of great articles covering exhaust, intake, etc. It's not our engine, but the same principles apply.

Note that pipe diameter increase bump up high rpm gains. However, realize this is after the header which has already defined the main characteristics of flow. All you can do with the rest of the exhaust system is choke it, so bigger is better. The downside is dB level. As you go larger, dB levels rise. There may also be a rise in cost for parts/material. Fitment may also be a concern.

The general rule for pipe size is 115 cfm flow per square inch of pipe area. An engine like ours that flows about 300 cfm, this comes out to a 2" pipe. The 115 cfm rule is considered "lossless" as in you will lose no more then 5% of the theoretical maximum and larger pipe sizes yield diminishing returns. I'll note stock is less then 2".

Pipe size is less important since there are more restrictive parts like the cat and muffler, depending on what you pick. This is sometimes why it is preferable to step to a larger exhaust size. A 2.25" or 2.5" cat flows more then a 2" cat. Although the pipe size doesn't need to be larger, we can compensate for a lower flowing part by stepping to a larger diameter product.

Also note in that article of the dyno graph showing a test pipe and a catted pipe. Notice how little a difference exists. Similar will go for the muffler too.

One aspect that does not show up on a dyno is responsiveness. This is one thing you'll gain with a freer flowing design. For example stepping from a stock header to an aftermarket one yields not only tq/hp gains but also noticeable quicker throttle response simply because the air can flow more readily. It's a matter of inertia, in this case air flow.
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Old 05-07-2009, 01:45 PM   #5
iRick.
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I really appreciate all your info and taking the time to do so.

But I should make a few things clear.

I have borla replica headers, and currently a full 2.5 inch exhaust.

I've heard what my car sounds like with nothing after the headers [just turned it on real quick]

and I liked it annoying yes...gnarly yes...may lose interest quickly...yes.

I understand what you are saying about the cat, but the plan is to run headers to a straight pipe.

So what diameter should this straight pipe be? 2.25" and taper to a 3" so it doesn't look like a little straw?
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Old 05-07-2009, 01:49 PM   #6
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FYI, Burnt exhaust valves due to exhaust pipe changes is an old myth from the days gone bye. It was never actually true. Just a flawed assumption made by some old gear-heads looking for a reason for their engine problems.
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Old 05-07-2009, 03:44 PM   #7
SOHCEJ25
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I have ran open header on many cars and never seen any valve issues ... should be fine !! Verrrrrrry loud ..but fine ...
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Old 05-07-2009, 04:16 PM   #8
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You can run any size 2" and up. Later on when you do toss in the cat, a resonator/glasspack in the midsection and a muffler in the back, you just need to grab the parts off Ebay, drive down to your local exhaust shop and tell them to weld them in. The size of straight pipe might be more of a matter of what size parts you want to install later into the system. I'll note the Borla header exits at 2", but you're not confined to that in any way after the header.
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Old 05-07-2009, 05:02 PM   #9
iRick.
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alright. thanks for all the info. sounds like I got myself a way to run it now.

I never planned to run any cat or resonator... I was going to do a straight pipe until I got tired of it and then run a muffler I have.

thanks.
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