Turbochargers - turbine housing AR selection - how to work out?

**NME308** · 30-05-2011, 11:28 AM

Yeah we cannot get the figures to work on the Garrett calculator either... Makes choosing new turbo's to step up a bit difficult! The current turbo's should be pushing 30+psi boost to achieve the numbers by the garrett formula. Just for more confusion we are measuring boost above the throttle plates as the fuel in the air was fouling the sensor when mounted in the manifold... There are a couple of different fudge factors which can be entered to the calcs which make our numbers fit so we have used the 'toss all available observations in the air' method and decided to buy a pair of borgwarner S300SX3 turbos to attempt chasing down 1600hp. The XR6 turbo brigade up here are talking about matching our figures this year so we cant sit still!

My celica however responds quite well to the garrett calculations i.e. the low compression 3TC is out of puff on a GT2560 at just over 200hp.

Oh and yes the HQ's wastegates are obviously working hard as they changed colour from a nice anodised silver to a blueish colour soon after installation!

Cheers,
Jason

**boxh34d** · 30-05-2011, 07:23 PM

Yeah, once things like serious head work and cams are involved, the usual assumptions made for engine VE are well and truely out the window. I think the Garrett calcs are pretty good in most instances, but in your case (and I imagine others as highly modified) you just need to plug in different VE numbers until the boost and HP add up. At the end of the day, it is nearly universal that 10 engine horsepower needs 1lb/min of air so you can always just work backwards from there.
I've just quickly tried a few different numbers, 1600HP at your 6500RPM limit looks like about 30PSI worth of boost unless you can get the engine breathing even better again. Starting to get pretty serious! (not exactly like you were just fucking around before hand though

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**Supra967** · 31-05-2011, 07:36 AM

I remember reading a quote from someone on the honda turbo f1 team, something along the lines of '... we were turning 32% of the fuel energy to power, and we'll never do that again". Might be a bit outdated now. Pretty sure that was from the 1200+hp qualifying engines running 60 or 70psi. Thing is though that boost is effecting all sorts of parameters, VE, thermal effeciency (burn efficiency) etc. A lot of the calculators would take the usual 20-25% thermal effeciency and leave it at that, and a few other generic factors as well.

We did a fairly indepth spreadsheet for a uni assignment, even went to the point of working out ring friction and piston side wall friction, but in the end I still had to juggle things like VE to get figures for stock motors, which then had to be rejuggled to get figures for the same motors under higher boost or whatever. Just got to see how close, yet so inaccurate the maths is at such a low level.