bigger exhaust, port wastgate, same boost, still more power?
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Full Member
Joined: Mar 2002
Posts: 75
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From: Melbourne, Australia
bigger exhaust, port wastgate, same boost, still more power?
I have a bit of a stupid question here. If you put on a bigger exhaust the boost goes up. If you port the wastgate, the boost comes down. If FOR EXAMPLE boost was 5.5psi (stock S4) before exhaust takes it up to say 1opsi and then ported wastegate brings it back down to stock 5.5, what happens to performance? I guess the bigger exhaust must still add hp as the engine is still breathing better like it would on a non-turbo. I suppose the turbo is still spinning much faster even though the wastgate is ported too which isn't real good. I have been wondering about this for a while and about why people spend so much time, effort and money getting the boost way high only to bring it back down again. Why not just do enough of the exhaust to get the boost up to where you want it in the first place? Or is it what I said about the engine performing better with a bigger exhaust, even though the boost hasn't changed? Sorry if this post is confusing (or real stupid).
I'm a boost creep...
Joined: Jan 2002
Posts: 15,608
Likes: 8
From: Auckland, New Zealand
Ask and you shall receive! 
Too many people mistakenly think boost = power. This couldn’t be more wrong. Putting on big exhausts and intakes reduces or removes the restrictions flowing gases have to overcome. This does two things: it lowers pumping losses, the work the engine has to do to physically pull and push air into and out of the engine on the intake and exhaust strokes; and it improves volumetric efficiency, meaning more air gets into each chamber (with more fuel) meaning a bigger bang and more power. This can be done with the same boost pressure.
Think of it this way. A water tap in your home has roughly the same pressure behind it all the time. Yet the more you open it, the higher the water flow out of it. This is because opening the tap reduces the restriction to the water’s flow.
Yeah, that’s a real simple example, but you get the idea!
Too many people mistakenly think boost = power. This couldn’t be more wrong. Putting on big exhausts and intakes reduces or removes the restrictions flowing gases have to overcome. This does two things: it lowers pumping losses, the work the engine has to do to physically pull and push air into and out of the engine on the intake and exhaust strokes; and it improves volumetric efficiency, meaning more air gets into each chamber (with more fuel) meaning a bigger bang and more power. This can be done with the same boost pressure.
Think of it this way. A water tap in your home has roughly the same pressure behind it all the time. Yet the more you open it, the higher the water flow out of it. This is because opening the tap reduces the restriction to the water’s flow.
Yeah, that’s a real simple example, but you get the idea!
Yes, same boost w/ less exhaust restriction gives you a bit more power since you are in effect raising the Volumetric Efficiency of the engine. A very nice side affect (and the main benifit, really) is the onset of boost is much sooner as there is a greater pressure differential between the engine and exhaust sides of the turbine. My S4 w/ ported WG, TID mod and RB 2.5 downpipe/presilence (and fuel mods) could achieve full boost at 2000 rpm when under load and boost onset was "no lag" at cruising rpm.
