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Discussion Starter · #1 ·
I'm not really sure if i should post this in this forum but its a lot friendlier than the american one so...

It might sound like a dumb question but how does fitting a bigger exhaust make more power in a turbo car, sure it will flow better and make more boost which increases power. I witnessed this with me and a friends tx-5. When we were both running stock boost he was quicker than me because his "stock boost" was higher than my "stock boost" due to his $800 exhaust. but when we both wound it up to 12psi we were dead even. I can see how a exhaust and filter would make the turbo spool quicker, but thats not really a problem with stock turbos.

I guess this answer really is due to my lack of knowlegde of how a larger turbo running 12psi is better than a stock turbo running 12psi, besides obvious cooling differences. To me if a motor gets 12psi that will increase the compression by the same as if it was getting 12psi from somewhere else?

Thanks for your insight, but don't give me **** thanks, i just want to find out more about my car
 

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Hey, don't worry about asking questions - everyone here is friendly, funny, knowledgeable and willing to share it! I ask weird questions all the time in here, and apart from keeping a regulation 3ft distance from me at any MX6 meet, there's been no other repercussion :)

While I know absolutely nothing about turbos and exhausts, google is your friend for a start - turning up a few interesting articles which may help give you some more ammo while you wait for the people who actually have turbos to give their 10c.

Stuff I understood:
How turbos work
What happens when you put on a bigger exhaust
12 things to know about turbos

Other stuff which may be interesting to you:
Some dyno graphs and stuff (commercial alert) in the Integra world

Biggles
 

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Ooh I love technical questions like these.

Ok well an exhaust increases flow and power on all cars, but a good exhaust is especially important on a turbo car because the turbine stops the gases from flowing as fast, basically because it just gets in the way, and it takes energy to spin it, energy that is normally used to push the gases out the pipe. So a freer flowing exhaust helps reduce the extra backpressure that is created.

With regard to turbo size vs. power when running the same psi, well its about volume, psi just measures the density of the air, how many pounds per square inch, it doesn't tell you how much air in total there is. A 2.2ltr engine (like ours) needs a turbo that is capable of processing 2.2ltrs of air, while a 4.0ltr engine like the XR6T needs a big ass turbo to compress 4ltrs of air. Get it? So a bigger turbo can compress a greater volume of air more easily, while our small turbos find it hard to provide enough compressed air to meet the engine's needs at high revs.

Hope that makes sense.
 

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Discussion Starter · #4 ·
thanks 2bad i guess then if its actually volume thats important intercooler piping would become a major restriction.

Does anyone know how much boost pressure raises compression?, i'm sure if the turbo is making say double atmos pressure at the small outlet this would be a lot less in the motor so it wouldn't actually double the compression would it? other wise wed be running some very high theorically compression motors?
 

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Pretty good explaination 2bad, but technically the turbine spins on pressure differential not flow as such. High pressure on the engine side of the turbo & low pressure on the exhaust pipe side. the greater the pressure differential the faster the turbine spins as the gasses try to equalize across the blades. Of course the engine keeps pumping out the exhaust pressure so it never equalizes. Bigger the dump pipe, the lower the pressure, the quicker the spin. You need a free flowing exhaust to keep the pressure low in the dump pipe, which is what you were saying, I'm just splitting hairs.

Mitch: Compression is a ratio which doesn't change with the amount of air/fuel in the pot. The actual pressure in the bore is greater, but i think it would still need to be lower than the equivilent of 10.5:1 other wise we'd still get pre-ignition, but I'm only guessing????

*edit* I think Trip had a 3" intercooler to TB pipe, he said it created a bit of lag.
 

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Discussion Starter · #6 ·
Fair enough the compression doesn't actually change but if were stuffing more air into the motor, wouldn't it be like similiar to running a heap more compression?, if you can pressurise the bore twice the normal atmos pressure wouldn't that mean the motor is around 4.4L and that the comparable compression is huge?
 

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On what Crusty was saying about the bigger IC pipes...

The thing is, if your smallest part of the intake system is 2" (just using any number) and the piping is all 3" then all you doing is filling up the pipes with air rather than getting it into the engine when the turbo starts to spin. And why bother doing that?

I did notice a bit less lag when I got new pipes made up the same size as the turbo outlet.
 

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yeah Mitch what you're saying is sort of true, if you want to think of it like that. Forced Induction means more air and more fuel in the cylinder, which means a bigger combustion, just like if you made the cylinders bigger, or upped the compression. But its not really talked about in those terms, see atmospheric pressure is about 14psi, so if you run a 7psi turbo it is compressing atmopheric air to 7psi more than 14psi. If you run a 14psi turbo then you are compressing the air to double its normal pressure, so your fitting twice as much air into the cylender. Then the pistons compress the air even further. That's why turbo cars run less piston compression that normal cars, because if you try to compress already compressed air too much it detonates.

So yes you are sort of right, a 4.4ltr atmo engine would produce roughly the same amount of power as a 2.2ltr turbo runnning 14psi, provided the engine tech and design were roughly equal, but there are so many other factors involved, such as efficiency and revs, and their torque curves would still be very different, even though they may produce roughly the same kws.

Hope that clears up something for you.
 
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