Quote:
Originally Posted by tendogy
Indeed! Using a few pieces of information (the C-30 makes 10psi at 93k rpm) we can tell that our little FA20 works through about 0.37 kg/s of air at redline.
You could rig up a C-38 to move 0.37 kg/s of air for your 1.7 pressure ratio (i.e. make 10psi at 7400rpm) but it would only be spinning about 72k rpm. That seems great until you realize that you have no usable boost until 3000 rpm when the blower finally speeds up to its minimum 30k impeller speed. In fact, the car is slower than stock below 3000rpm due to parasitic loss and does not match the compression of the C-30 until 5000rpm.
You could rig up the C-38 to start making usable boost at 2000 rpm like the C-30 does, but then you would be over-spinning the impeller by the time you were at 5800 rpm.
You could rig up the C-38 to redline at the same time as your engine. It would still only make usable boost at 2400rpm but that's not too shabby. Then, when you were at 7400 rpm you would have 26psi blowing you up.
This is probably why Kraftwerks and Jackson have both stated multiple times that the C-38 is only going to be a viable choice for well-built engines.
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Whoa there! We need some log data. I'm pretty sure no one is trying to push that much air through a rotrex into an FA20 (yet
). My rough rule of thumb from tuning awd turbo Scoobies is 1 WHP per g/s of airflow. The twins have rather lower drive train losses, so let's guesstimate (in the absence of dyno logs) 1.1. For 300 RWHP, that suggests airflow around 0.272 kg/sec, so I think the range of interest for street guys at peak power is going to be in the 0.25 - 0.3 kg/sec range.
From this and the known maximum impeller speeds on the current C30 kits, we can infer that the pressure ratio
at the blower is closer to 2.3, due to some combination of intake, intercooler and plumbing losses (or, curse my curmudgeonly thoughts, optimism in the map).
Picking the max pressure / mass flow point on the C38 map at 2.4 / 300 g/s tells us we need 80k rpm at the impeller at redline. So at half of redline, you'll get (ballpark) 1.2 PR at the blower and something a little less than this at the manifold.
Doing the same "analysis" on the C30 shows me that we might expect as much as 1.3 PR at half of redline, but there's enough Kentucky windage on all this handwaving that I don't think we'll know for sure how a C38 system works in the real world until someone builds one. Probably, there's enough difference to show up on a dyno and probably there's enough difference to say "C30 is better for street", but it looks to me as though the difference might be small. I'm certainly interested to find out.