Dave's Evo X Build/The Life of Dave blog

[Halon]

Originally Posted by Shane@DBPerformance

One thing though, even though it is fully spooled at 4000RPMs, it doesn't look like the motor is able able to due much with the air since the HP and TQ at 4000RPMs are nothing crazy even though it's spooled up.

This may sound like a newb question, but can anyone explain why this is, in layman's terms?

Guess I'm used to seeing the TQ line at least somewhat resemble the boost line, so this doesn't compute in my head. Usually I'm used to seeing when the boost line starts rising rapidly, so does the TQ line. Then when full boost is reached and the boost line tapers/flattens out, the TQ line isn't to far behind.

I think if I was going after a very fast spooling setup, and I managed to get a setup to spool as quickly as that one does, just to find out on the dyno that even though it spools fast it doesn't matter...

Random evo dyno charts as an example of what makes sense in my head:

[Shane@DBPerformance]

Originally Posted by Halon

This may sound like a newb question, but can anyone explain why this is, in layman's terms?

The motor isn't big enough or efficient enough at that specific rpm to use the air.

People always get all hyped about variable vane turbos, spool valves, or other tricks, so they can have the dream 1000whp turbo that spools at 1500rpms. But even if you get that to happen, it doesn't mean the car will make any power. That dyno chart seems to be a great example. It obviously hits full spool well before it starts making good power. That's not a bad thing, since it's probably decently responsive or at respooling at higher RPMs for a turbo that size. Maybe that Evo has cams and and intake manifold that give up low/mid range power for top end.

But anyways... Here is a helpful way to think about it. First off, a turbo is not like a roots style/positive displacement supercharger. It is not designed to make boost by stacking air and holding it it. A turbo makes boost inside the compressor housing and sends the air on it's way to the intercooler, engine, etc. So when you put a big turbo on a small motor and you get that turbo to spool really fast what happens? You get compressor surge. What is compressor surge? It is when the turbo puts out more air than the engine can take in and the air starts backing up and eventually backs all the way up to the turbo itself which causes the comp wheel to drastically slow down or stop(this is bad and very hard on a turbo). Compressor surge goes away as you go up in the RPMs because engine can finally use all that air. So what does an anti-surge/port shroud compressor housing do? It lets that excess air flow back out into the intake pipe in front of the turbo, so you are essentially just wasting some of the boost/air you worked so hard to build. It is also better to run a non-anti surge compressor housing if you don't need one, they usually make a little better power.

In other words, you can't have your cake and eat it too.