firebad

Wondering why boost falls off at higher rpms with most of the dyno graphs Ive seen on here.
There's usually a note on the graph saying, boost tapering from 28psi to 22psi, which relates to tq numbers falling a lot!
Is this a problem with the internal wastegate not holding boost, the turbo falling out of its efficiency, or the head not flowing enough?

Im coming from the boosted honda world (obviously completely different engines) where you have your wastegate set at a number, and it stays there for a very flat tq curve right to redline (typically 8000rpms).

There's considerable power to be made if only the psi would stay constant...

Someone shed some light on this...tanks!

drewdq

iTrader: (9)

Boost taper usually occurs because the turbo can't keep up with the volume of air it needs to compress at those higher RPMs to maintain a desired boost level.

Most Turbo Honda guys I've known have larger turbos running 7-12psi, where the turbo has no problem flowing at higher Mass Flow Rates.

buchnerj

iTrader: (19)

I agree, the turbo just has a difficult time keeping up with that much pressure at the higher RPMs. Some of the tapering can be traced back to the wastegate, which is why some people (myself included) buy i stiffer or adjustable WGA. Another problem that can lead to boost taper is your method of boost control. EBCs are generally the hardest to keep from tapering at higher RPMs. If you want to minimize your boost taper, make sure your turbo is sufficient for the air you want to flow. Make sure it has little backpressure and can easily pull air through the intake, etc. Upgrade your WGA to 18 or 21 psi (depending on your needs), and consider a MBC or the new and popular ECU tuned boost.

Good luck


psshhhh 7-12 psi, silly Hondas.

fre

iTrader: (4)

Boost taper can be related to many different things (boost leak, turbo seal, turbo not large enough). At that boost on a stock turbo, that kind of taper is to be expected simply because of the turbo size.

I have a gt35r and I get boost taper on the order of 5psi from 7400-8000 and I believe it's my turbo just getting old (75k on the turbo). I get a similar drop no matter what boost I run which points towards a leak/bad turbo.

Try turning your boost down to 20psi and see if you get the same drop or can hold it steady. If you can hold boost steady at lower levels, then you are looking at a simple flow limitation.

Having said that, I believe most turbos will spike a little, especially on a manual boost controller, but you shouldn't see more than a few psi fluctuation if all is well on a turbo that is flowing efficiently.