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Re: What size exhaust system for VR4?

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Galant VR-4 org Post #: 17026 posted 07/22/03 09:00 PM     Remind Me!  Send Private Message   Edit Post      
Originally posted by scooter:
i was trying to put it in simple terms. so its because the exhaust can not flow out he wastegate huh? well i wonder if its because its to easy for it to flow out the turbine hole instead. yep, i believe so. when you have backpressure, its easier for it to come out the wastegate hole. meaning that the exhaust going out the turbine hole is getting backed up, so some pushes out the wastegate, which keeps the wheel from spinning as hard. if its real easy for it to all go out the turbine hole, it just bypasses the wastegate and all goes through the wheel, which makes it build more boost, and this is boost creep. also you never said i had a false view, spoulson did. im not trying to be a dick to you and i know its hard to admit when youre wrong, but im not the only one with this view on boost creep. you even say the only thing a 3in will do is add more flow to the turbine. well you think it can add so much that all the exhaust just starts bypassing the wastegate, well i and some others here do.

I understand what you are saying, and I agree that, according to the law of equilibruim (fluid mechanics), a high pressure source will always exhaust to a low pressure source. This is what you are referring to with the larger exhaust. The "back pressure" (which incidentally does not exist, you are referring to a resistance to flow), is reduced because of a larger pipe diameter. I totally agree. However, keep in mind that the pressure on the other side of the wastegate is at atmospheric pressure, (1 atm, or 14.7 psi, standard atmospheric pressure at sea lever, which may vary if you live in the mountains). Taking into relation the pressure in the exhaust vs. the pressure outside the wastegate, your exhaust will always first flow out the wastegate. Now, let's also keep in mind that an outlet can only flow so much volume of air at any given pressure (PV=nRT). While you increase the volume of air through your exhaust, you have not increased the diameter of your wastegate to allow the excess to vent to the atmosphere. That brings me back to the earlier point, which was that increasing your wastegate diameter will prevent "boost creep". The problem arises not because the exhaust "chooses" to flow through the exhaust piping, but rather because the wastegate is unable to vent the proper volume of air.

If anyone thinks this is wrong, please let me know.

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Galant VR-4 org Post #: 17027 posted 07/22/03 09:03 PM     Remind Me!  Send Private Message   Edit Post      
You guys really know how to pick fly-specks from pepper. You really are saying (more or less) the same thing. I'll try a different tack...see if you both don't agree with this. If not, please reply...

Velocity (volume of air per unit of time) through the system (exhaust manifold to exhaust tip) depends on a couple of things...flow rate through the most restrictive component and pressure difference between the manifold and the outside world.

Turbine speed depends on the pressure difference between the manifold and the O2 housing (and the rest of the exhaust system).

A less restrictive exhaust system increases the flow velocity of exhaust gasses, lowering the pressure within the entire system downstream of the turbine. This means a greater difference in the pressure in the exhaust manifold and the pressure in the downstream exhaust system. The result is a higher rate of flow through the turbo and waste gate...up to the flow limits of each. (Any orifice has a practical limit to the VOLUME of air that will flow through it even if pressure difference is further increased.)

So if the turbo housing can flow more volume than the waste gate it is possible that the increased pressure differential that resulted from the less restrictive exhaust will cause flow to reach its limit through the waste gate before it reaches the flow limit of the turbo. Result: boost creep.

Obviously, there are a LOT of variables the volume and temp of exhaust gasses being pumped out of the engine, outside air pressure and air temperature, thermal characteristics of the exhaust system, etc.

Of course a turbo with a too-small waste gate (whether internal or external) can do the same thing if you pump enough hot exhaust gas into it.

And I think the reason you don't hear the term "back-pressure" much in technical discussions is because the term is sort of a misnomer. Backpressure implies that there is some external pressure pushing back up the exhaust system. But the only thing pushing back into your exhaust system is atmospheric pressure and it doesn't care what kind of exhaust you are running. It is probably more accurate to refer to flow limit. A test pipe increases the flow limit of your exhaust system. It doesn't reduce "back-pressure". Driving into a vacuum or sticking a vacuum hose on your tail pipe would reduce atmospheric backpressure.

Boy, do I love to hear myself talk. Sorry to run on.


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Galant VR-4 org Post #: 17028 posted 07/23/03 01:25 AM     Remind Me!  Send Private Message   Edit Post   
Looks like i'll have to get either 2.75inch or 4 inch exhaust system. Otherwise you guys will kill each other.

All i know is i had a full 3inch system in my mr2 turbo when the car was stock standard and when the car had big turbo intercooler boost etc. I never had that creepy boost creep you guys are talking about.

My final decision on my galant vr4 is a 3inch turbo back system with 3inch high flow cat going down to 2.5inch after the cat connecting to 2.5inch remus muffler.

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