Just something I was thinking about today...
My car doesn't suffer from this problem being a 98 WRX with a big standard VF22, but why does it happen if the turbo's smaller like in later UK cars? Does boost drop, fueling reduce etc? This also happens in many normally aspirated cars as well - are the reasons similar?

[Edited by scoobysnacks - 3/28/2002 9:10:33 PM]

 

Is'nt it cos the power is shifted futher up the rev range - actually the VF22 does drop off - just a bit higher than the VF28

Surprised your VF22 bearing aint failed yet - they dont tend to last long!

 

Doesn't seem to drop to me... I've hit the limiter somwhere between 7500 and 8000rpm a few times while still accelerating hard.
But about the samller turbo cars - why can't the power continue to the red line even if it does begin lower down?

 

Although the smaller turbo will deliver the same boost, because the internal volume is smaller it cannot pass as much air. The engine becomes strangled at higher revs. and power drops off. You often find smaller turbo engines produce good torque figures at midrange and often result in a more useable power band. The later Escort Cosworths were an example where a smaller turbo produced a nicer car to drive in standard form. I had a Power Engineering turbo`d Mondeo 2ltr. with a tiny Garret T2 turbo. Went very well and developed max. power (206bhp) at only 4785 rpm! torque was 240lbft! Beyond 5500rpm it simply died. The tiny turbo simply could not pass enough air.
JohnD

 

Ok, so the turbo can't pass anymore air. I thought this was the case - but why doesn't the power stay the same as the turbo continues to pass it's maximum flow of air? If this isn't the case and less air is actually flowed by the turbo higher up the rev range, then why? Questions questions...

 

If you look at EcuTek's web site you can check the actual turbo airflow Vs boost pressure from a dyno run. The airflow stays fairly constant but the boost pressure drops away at higher revs. The result is a torque reduction due to reduced cylinder filling.

 

For any particular boost, cylinders consume twice as much air at 8000rpm than at 4000rpm. If your turbo can't supply that kind of flow rate and simply "runs out of pumping capacity", the manifold pressure will drop.

 

Thanks RT. Didn't think of that...

 

On top of this, the harder you drive the turbo, the hotter the air charge density is, and the less air you actually get into the cylinders.

Mark.

 

Good point Mark

The smaller turbo has to work a lot harder to produce the boost higher up the rev range, the harder it has to work the more heat energy it creates and eventually it ends up pushing hot air instead of cold at which point it has a negative effect. It also increases the under bonnet temperatures significantly.

Pete