What size exhaust system for VR4?

[spoulson]

quote:Originally posted by scooter:
some of you peeps need to research before you make yourself look dumb. You made a good point until I read this part.

 

quote:Originally posted by gvr4ever:
Do a google search. Boost creep is due to not having enough flow out of the wastgate. The only way the exhaust can play a roll is if the exhaust can't flow enough for the wastgate and engine exhaust. That's when a external dump is used. It is also used to scare old people to death. And what happens when enough flow doesn't go out the wastegate? DING DING DING, it goes out the exhaust, because, as I said, the restriction is less than going through the wastegate. At the same time, you're contradicting and validating your own theory.

And yes, I know exactly what an external dump is, because I have one on my tubular O2 housing.

Your theory is headed in the right direction, but incomplete. You state "Boost creeps because the turbo makes more boost than the wastegate can control". A better way of thinking about it is "The wastegate can't bleed off enough boost because it's too small and the exhaust is nice n' big, SO boost rises because not enough exhaust is being bled. You're thinking excessive compressor side boost pressure causes boost creep, but it's more a fault of the turbine/WG side.

quote: So before you call me dumb, do you really understand, or just post links that others wrote? A) Who called you "dumb"
B) I didn't post any links or reference anyone else's thoughts on boost creep

 

Its weird that no matter what board you're on, lots of posts end OT questioning intelligence.

 

[Dan D]

I posted earlier then removed it because I didn't want to get "involved". Try this on for size and see how it fits:

Begin removing (hypothetical here) sources which add backpressure starting at the tailpipe. Peice at a time. Each piece you remove reduces the effective backpressure which the turbine outlet and wastegate passage outlet see.

Once you make it all the way to 02 housing and have everything else off, then you have one possible source adding backpressure at the turbine and wastegate outlets, Agreed?

On large turbo's the turbine has a very high flow capacity and in general any turbo's exhaust outlet is larger than the wastegate passage. At 0 psi effective backpressure at the turbine outlet and wastegate outlet, if the wastegate passage is small enough, it can be a less resitive path for gasses to go through the turbine instead of around.

Backpressure induces a condition where a significant amount more energy must be used to spin the turbine. The exact amount more dependant on the design of the wheel and housing. With enough backpressure introduced to the turbine and wastegate outlets, the turbine is no longer the path of least resistance. This occurs at the point when simply overcoming the backpressure through the smaller wastegate passage requires less work than spining the turbine against backpressure and overcoming the backpressure throught the larger turbine passage.

Scenario:
To hold desired boost level, 1/3 of exhaust flow must go around the turbine. Due to wastegate passage size, only 1/4 of the exhaust gasses actually due because it is easier to move the turbine than squeeze throught the hole.

Solution 1 (good):
Increase the size of the wastegate passage so that it becomes less work to go around than through.

Solution 2 (not as good): Introduce enough backpressure at the turbine and wastegate outlets so that it becomes more work to go through and spin the turbine than to go around.

If the right combination of conditions/variables exist, going to a 3" exhaust can cause boost creep when it did not exist before.

 

You should've posted this earlier to prevent the 2 pg's of nonsense! Good job.

 

quote: If the right combination of conditions/variables exist, going to a 3" exhaust can cause boost creep when it did not exist before. That is exactly right. Boost creep has zero to do with large/small turbos and everything to do with backpressure/turbine variables.

 

[scooter]

ok, before i ever posted the links, i tried to explain what it was. actually my first post was trying to explain. then i was told that was not right. so i try to post some links of someone you might believe. i was trying to explain it had nothing to do with turbo size, like you said. i also did not call you dumb, i said research before you make yourself look dumb. so yes, im pretty sure i knew what creep was before i posted the links. also, i never said not to get 3in cuz of boost creep. i never said anything about what to get. i jumped in on the boost creep part of the thread. if you ask me, i would get 3in all the way back, then if creep is a prob. i would fix it. what is my false view on boost creep though?

 

[gvr4ever]

quote:Originally posted by scooter:
boost creep is caused by the exhaust outflowing the wastegate. like as in its easier for the exhaust gas to flow out the turbine exit hole and into the downpipe, instead of some flowing out the wastegate hole. it all just wants to flow out where its easiest to get out.(3in downpipe would be way easier than the 29mm wastegate hole.) with the stock exhaust it would back the flow up and cause some gas to go out the wastegate hole. not everybody will have boost creep, but some will have it bad. you can port out the wastegate hole and install a 34mm flapper though and you should be fine if you do have it. Scooner, I said you had a false view becuase you said that it is easier to flow out the exhaust then the wastgate. Maybe you do understand, but that is worded pretty badly. It's because exhaust can't flow out the wastgate, not because the 3" is easier the flow. Since the wastgate can't slow down the spinning of the turbo, boost creep starts. A larger exhaust will allow the turbo more flow, thats it.

Out of all the " how does a turbo work " pages I read, they all suggest boost creap happens because the wastegate can't flow enough to slow the turbo down. No one says anything about there not being enough back pressure.

http://www.nwstreetracing.com/tech/turbo_fundamentals.htm

 

[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.

 

[gvr4ever]

quote: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.

 

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 here...like 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.

Mike

 

[gvr4ever]

quote:Originally posted by howard:
quote:Originally posted by gvr4ever:
Do a google search. Boost creep is due to not having enough flow out of the wastgate. The only way the exhaust can play a roll is if the exhaust can't flow enough for the wastgate and engine exhaust. That's when a external dump is used. It is also used to scare old people to death. And what happens when enough flow doesn't go out the wastegate? DING DING DING, it goes out the exhaust, because, as I said, the restriction is less than going through the wastegate. At the same time, you're contradicting and validating your own theory.

And yes, I know exactly what an external dump is, because I have one on my tubular O2 housing.

Your theory is headed in the right direction, but incomplete. You state "Boost creeps because the turbo makes more boost than the wastegate can control". A better way of thinking about it is "The wastegate can't bleed off enough boost because it's too small and the exhaust is nice n' big, SO boost rises because not enough exhaust is being bled. You're thinking excessive compressor side boost pressure causes boost creep, but it's more a fault of the turbine/WG side.

quote: So before you call me dumb, do you really understand, or just post links that others wrote? A) Who called you "dumb"
B) I didn't post any links or reference anyone else's thoughts on boost creep Sorry about that, I think it was scooner that suggested I made myself look dumb and posted links to buschurracing.com. His FAQ says there is a danger is having boost creep when using a 3" exhaust system with smaller turbos that have stock size wastgates.

I apologize for getting carried away. I just though telling someone not to go 3" because of boost creep was lousy advice. There are many things to over come when upgrading our cars. We all had to deal with upgrading the wastgate flapper or going external.

What really bugged me was no one could explain how I was able to run with a open o2 housing without getting boost creep.

Like I said before, I never heard of anyone getting boost creep when installing a exhaust. I'm sure it's happend, but I've been around for awhile and have pretty active in local clubs before.

All I wanted was a why to bigger exhaust = boost creep. What I got was links to other people saying it can happen.

There is one thing I've learned over the years is not to quote someone (no matter who it is) if you don't really understand what you are talking about. We are all guilty of it and I still do it when I get carried away. I got a little pissed off when I asked why and got cause David Buschur says so. That faq is so short and doesn't go into any detail. If we were all lemmings in DSM land we would blow up our engines or something. If I read something, no matter who wrote it, I always try and find the why to it. After many google searches, I can't find the why behind a bigger exhaust = boost creep.

I've read alot on how turbo's work in the past and I read up again since this thread started and on boost creep. Also I didn't read up on any race sites. I read up on howthingswork.com (thats always fun to just read again) and some guys paper for collage or something on everything about turbos. They all talk a little about boost creep. The wastgate flow is really the cause or fix for boost creep. I think we can all agree on that.

However, what really is the cause for larger exhaust = boost creep with a smaller turbo? The only known cause I know the reason behind is a bigger turbo with the stock o2 housing and WG flapper.

 

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.