TelBoy

Can someone explain, in simple terms, why the boost pressure drops off over a certain rpm? Why doesn't it continue to produce it's maximum pressure and just stay there?
I suppose that's the same as asking why a power curve slopes downwards at the end, at very high rpm. What's actually happening in the engine that means that power output actually falls at this point?

Terry

john banks

The exhaust housing/turbine wheel is too restrictive on the small turbo at high revs so the wastegate gets blown open.

The UK cams also limit the power at high revs - unless you have VVT then they are a compromise.

[Edited by john banks - 7/23/2002 1:49:11 PM]

TelBoy

John,

Ok thanks, but what about in a normally-aspirated engine, where there's no wastegate to open? Where's the power disappearing to then?

john banks

The cam timing and lift is not optimised for that engine speed.

Perhaps the exhaust is too restrictive?

[Edited by john banks - 7/23/2002 2:10:39 PM]

TelBoy

I wasn't asking with specific reference to any particular car, it was a generic question.

I thought all power curves were like that. No? Does that mean that with the right set up, you could get a power curve that just keeps going up until you run out of revs?

nom

Umm, no.
At some point, the stresses in the rods, etc. get to such a point that they would crack/explode. So the revs are limited to never (hopefully) reach that point. So, the rest of the system is designed to peak before that point as well.
Making tougher rods etc. requires a higher quality metal/process (so costs more), and a higher stressed engine requires more work to be done to it. So, budget-wise, manufacturers come to a compromise.
Given a fictitious material that could withstand any forces that the internals of an engine could throw at it, the volume of gasses produced at higher rpm would also prove to be a huge restriction. You couldn't get rid of the amount of gas produced, and probably couldn't suck it in in the first place.
Well, that's some angles to t anyway

swan

"The exhaust housing/turbine wheel is too restrictive on the small turbo at high revs so the wastegate gets blown open."

Does an EBC then just force the wastegate closed in order to maintain boost?

john banks

No the EBC only controls the pressure on the diapragm in the actuator. Even if this pressure is low the exhaust gas will still blow the wastegate open if the exhaust housing is too restrictive. So an EBC cannot achieve the physically impossible on a particular turbo. It may well let you drive it hard and make a lot of hot air, but even then you won't get a high flat boost curve right to the red line on a turbo that is too small for the job.

John (big turbo convert )