Hi,

I have an EVO IX with stage 1 tuning incl. an MBC (Dawes device)

I am boosting 1,7 bar (24 psi) up until about 5-5500 rpm when the boost starts to drop and ends up at 1,2-1,3 bar (18,5 psi) at 7000 rpm.

Is this a common problem? How should I solve it? Stronger wg actuator? Other?


Thanks

 

Hmm...I don't think there is a way to completley stop boost taper in high
rpm's on a stock turbo.

Mine is tuned to 23lbs of boost and it also tapers down to about 18-19lbs
by redline.

 

Is it a common problem???? Come on, man, it happens on every Evo. I think that would be considered "common." People worry way too much about "boost taper" as opposed to power delivery.

 

same here..

 

It must be a way to get at least 1,45-1,5 bar (21-22 psi) at 7000 rpm??

 

I run 1.7 in the midrange and drop to around 1.4bar up top, but I am running cosworth cams which should increase boost taper due to improved flow...

 

Ok but what is causing it? Would a better/stiffer wg actuator make it possible to get more boost at higher rpm?

 

^ Agreed... That said you can force the turbo to push more air at the upper RPMs but then it may be operating outside of its efficiency range.

 

You are pushing much more air at 7K @ 18.5 psi than you are at 3.5K rpm and 24 psi...Focus less on the psi and more on vol. of air being pushed into the engine...That is why when you see some folks with 35Rs with boost of 24 psi but 500whp + (airflow )....

Google turbo compressor maps and read about turbo efficiencies/ etc. and you will understand...

 

I know about turbos and compressor maps but didn't think i would be running out of turbo at that low boost.. is that what you are saying?

I still have the stock airbox which is said to be restrictive, could opening up the lid a bit help?

 

You will hold about 1 - 1.5 psi more beyond ~6700 with the modified airbox...Sorry but your original post sounded as if you had little to no understanding of compressor maps, efficiencies, or turbo basics...

 

The only way to eliminate the taper, is with a larger turbocharger. Backpressure through a small turbine housing is the main culprit, and a turbocharger operating beyond its efficiency threshold. The turbocharger is spinning faster, but not pumping enough air to feed your engine at 7000rpm+....so manifold pressure will drop.