Originally Posted by
PEI330Ci
I posted this on another forum, thought it would interesting info to add here as well:
"The turbos aren't too small, they just don't fit this engine properly from a flow perspective.
The exhaust housing is not a big restriction, there's still more left in it before it chokes. Corrected mass flow choke point is about 15 lb/min @ a PR of 1.8, which I'm guessing is pretty close to what these turbos are running at on pump fuel. That's 300hp worth of exhaust flow per turbo...which they aren't doing yet. So in this case, we are not "turbine" limited.
Looking at the compressor housing, it can flow a bit more than what is being used, but you need a higher pressure ratio to tap into that.
The issue here is that the compressor STARTs in the middle of it's efficiency range at 4000 RPM which is 76% adiabatic, progresses through 78%, then continues to decrease down to about 65% by redline. In layman's terms, the engine is too big for this turbo. The compressor needs a little bit more flow potential to match the airflow requirements of the engine.
If I was designing a twin turbo system, I would have the target airflow at peak torque fall on the left of the compressor map closer to 70% efficiency, so that as RPM increased the airflow would walk "through" the highest efficiency band of the compressor. The way it is, it's starting the middle and dropping off as RPM increases. On the dyno chart, you could gain another 10-15% at peak power just by matching the compressor side better at the same pressure ratio. (Manifold pressure would be the same, the air flow would just be denser)
Now I'm sure every will say "Just get a billet wheel, and all will be good". That isn't the case, as testing has shown that the new billet wheels make power on larger frame turbos, and that the gains in efficiency scale down as you reduce the compressor size and increase compressor speed. Billet wheels aren't going to solve this issue, the size of the compressor wheel will. In this case, a custom wheel (probably billet by design) and cover would be the solution without having to re-design all the supporting hardware that HPF has built.
BTW, adding meth would help the situation a little bit, but it's not going to change the shape of the torque curve dramatically.
Just my opinion."