Hi Guys,
I have some data from a couple of engines that my brother and I have run. When trying to run calculations for turbo sizing etc the end horsepower with the boost used to obtain the said horsepower is rather hard to reconcile. If you care to run some calculations I would love to hear some expert advise/opinions!

Engine A:
350CI SBC with twin GT3540r and .63 turbine housings
6250RPM (Peak HP on chassis dyno at this rpm)
940RWHP (1050HP at crank not too optimistic through powerglide etc??)
20psi boost
VE 0.9 (?)
175 deg F Intake temp? - guessing here as we have no intercooler but we do inject a little meth pre carb (about 70deg farenheit ambient air outside)
BSFC 0.8? - I've seen 0.8 to 1.0 being tossed around for E85... With 7.2:1 comp could it be any better?
E85
A/F ratio of .8 Lambda
Roughly sea level for 14.7psi atmospheric
P2c of 4
P1c of 1? - we run air cleaners so could be worse than this?

Engine B:
110CI 3TC pushrod 2 valve 4 cylinder swinging a BW S200SX turbo with .5 turbine housing
5200RPM peak power
270RWHP (give me 300hp at the flywheel minimum with manual box?)
22psi boost
VE 0.9 (?)
175 deg F Intake temp? - once again a guess with water meth pre turbo
BSFC 0.6 or maybe worse with 7.0:1 compression?
93 octane pump gas
A/F of 11.0 on the gasoline scale
Roughly sea level once again for 14.7psi atmospheric
P2c of 4
P1c of 1 with air cleaner?

The twin turbo small block ends up off the right and sky high above the GT3540 compressor map with 'normal' values if what I have used above for the unmeasured components of the calculation can be considered ballpark. The actual boost used however means something needs to be 'adjusted' by the looks of it.

The 4 banger shows itself to be almost out the top of the compressor map on the S200SX turbo to obtain the HP yet only 22psi was used so yet again an adjustment required? This once again assuming my unmeasured values are within ballpark spec.

So out of all the figures you can change to make the data fit the calculations would it be Volumetric Efficiency which needs to be given a greater than 1 value? This given the BSFC figures can't be great with bugger all compression, and the charge pipe sure is hot after a pull!

Correct me or fire away some theory's please!

Cheers,
Jason

 

When you are too far off the map to the right that usually says that your Pounds per minute air mass/ pressure ratio is not lining up with the sizing of the turbo. A simple assumption is to put the maximum mass flow vertical line no farther to the right than the edge of the THIRD Compressor Efficiency island on the map. The Pressure Ratio for the assumed boost level should also intersect that point.

This example is where the Compressor is too small.

In another example where you are close to the surge line on the left side of the map (and that line is not an absolute line, that means that the engine mass flow is not high enough to keep the compressor in the Highest efficiency island or to the right side of the map in the second or third island at max mass air flow.

Tom V