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Registered User
Join Date: Feb 2002
Location: Atlanta, GA
Posts: 933
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Compression ration question...
Fellow 930'ers,
No doubt documented somewhere, but what is the CR for a Turbo when it is on full boost? For example, say your car is 8:1 CR off boost and you're running a 1 bar spring. Does that mean that at full boost you have doubled the atmospheric pressure and therefore doubled the CR to 16:1? I'd imagine there would be some losses and you wouldn't really double the air pressure going into the motor but what's the final result going to be? ![]()
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C.H. Boost Addiction - honestly, I have it under control 1989 911 Turbo Cabriolet Mods include: Kokeln IC, GHL Headers, Hooligan muffler, Modified K27 7200, BL WUR, LC-1 & XD-16, Bilstein Sports (4), TRG sway bars, Oversize torsion bars, Strut brace |
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Registered User
Join Date: Jul 2006
Location: Valrico Florida
Posts: 200
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Try this link. It has a turbo compression calculator.
http://www.turbofast.com.au/TFcompB.html
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'77 930 turbo Garretson I/C 1 BAR spring, (2) '82 Triumph Bonneville Royal Wedding Edition Past rides: '74 914 1.9 liter twin plugged track car, '83 928S, '87 924S, '75 911S w '78 ROW 3.0, '72 911T, '70 911T and various other insignificant domestic examples. Happiness is a grey tailpipe! Turbo lag......it's worth the wait! |
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Registered User
Join Date: Feb 2002
Location: Atlanta, GA
Posts: 933
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Holy Toledo!
Anybody know these figures for a 3.3 930? Bore: Cylinder diameter in inches. Stroke: Engine stroke in inches. Engine VE: Engine Volumetric Efficiency in percent. VE changes with engine RPM, a engine at wide open throttle at 3000rpm could have a VE of 90% and at 6000rpm 60%. This is a rough guide. At maximum RPM modified engines may have a better VE, eg: average overhead valve engines 45% to 75%, overhead cam twin valve 65% to 80%, four valves per cylinder 75% to 90%. Standard Comp: The Compression Ratio of the engine. Boost Pressure: Boost pressure in pounds per square inch.
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C.H. Boost Addiction - honestly, I have it under control 1989 911 Turbo Cabriolet Mods include: Kokeln IC, GHL Headers, Hooligan muffler, Modified K27 7200, BL WUR, LC-1 & XD-16, Bilstein Sports (4), TRG sway bars, Oversize torsion bars, Strut brace |
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Smart quod bastardus
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97.0mm bore, 74.4mm stroke, stock boost pressure = 0.7bar or about 10psi
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1979 930 Turbo....3.4L, 7.5to1 comp, SC cams, full bay intercooler, Rarlyl8 headers, Garret GTX turbo, 36mm ported intakes, Innovate Auxbox/LM-1, custom Manually Adjustable wastegate housing (0.8-1.1bar),--running 0.95 bar max ---"When you're racing it's life! Anything else either before or after, is just waiting" |
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Registered User
Join Date: Feb 2002
Location: Atlanta, GA
Posts: 933
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So using Fred's figures and assuming a VE of 90% and a stock compression of 8:1 -> one bar (14.7psi) would yield a 13.6:1 compression ration when on full boost.
Assuming a VE of 80% and a stock compression of 8:1 -> one bar (14.7psi) would yield a 12.2:1 compression ration when on full boost. Assuming a VE of 70% and a stock compression of 8:1 -> one bar (14.7psi) would yield a 10.8:1 compression ration when on full boost. I'm not sure what the VE for a 930 is but I suspect we would fall under the category of "maximum RPM modified engines may have a better VE, eg: ... 65% to 80% " which is in line with the 12:1 compression ratio I've heard used when referring to our cars. Sound about right? ![]()
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C.H. Boost Addiction - honestly, I have it under control 1989 911 Turbo Cabriolet Mods include: Kokeln IC, GHL Headers, Hooligan muffler, Modified K27 7200, BL WUR, LC-1 & XD-16, Bilstein Sports (4), TRG sway bars, Oversize torsion bars, Strut brace |
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Registered User
Join Date: Jul 2006
Location: Valrico Florida
Posts: 200
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I backed into a VE of approximately 78% for the 911.
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'77 930 turbo Garretson I/C 1 BAR spring, (2) '82 Triumph Bonneville Royal Wedding Edition Past rides: '74 914 1.9 liter twin plugged track car, '83 928S, '87 924S, '75 911S w '78 ROW 3.0, '72 911T, '70 911T and various other insignificant domestic examples. Happiness is a grey tailpipe! Turbo lag......it's worth the wait! |
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Smart quod bastardus
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Stock compression for a 3.3 is 7.0 to 1.
You know what would be great is for a survey of a few users that have dynoed their cars and see if we could back into the efficiency ratings for each case to get a feeling for what it should be once we average all the results. This is just a guess for now without real data to back it up. I had my completely stock 3.3 dynoed at 249 hp at the wheels at 5500rpm.... -------For example--------- 1) doing the math yields: 249hp * 1/.88(12% loss at drivetrain)= 282hp at the crank. 2) using the chart on the site and assuming intercooler efficiency of 75% and a turbo efficiency of 60%yields: ENGINE EFFICIENCY of 76%......not too bad. Anyone else got some results to post? |
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Registered User
Join Date: Feb 2002
Location: Atlanta, GA
Posts: 933
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Here's an interesting link showing dyno results for a number of Turbos, including my own. By the way, what does a stock 3.3 look like?
![]() Turbo Guys Comment on this Dyno Sheet ![]()
__________________
C.H. Boost Addiction - honestly, I have it under control 1989 911 Turbo Cabriolet Mods include: Kokeln IC, GHL Headers, Hooligan muffler, Modified K27 7200, BL WUR, LC-1 & XD-16, Bilstein Sports (4), TRG sway bars, Oversize torsion bars, Strut brace |
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Smart quod bastardus
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A stock turbo is too fast to capture on film at rest.....needs a high speed camera, though I get your point.
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__________________
1979 930 Turbo....3.4L, 7.5to1 comp, SC cams, full bay intercooler, Rarlyl8 headers, Garret GTX turbo, 36mm ported intakes, Innovate Auxbox/LM-1, custom Manually Adjustable wastegate housing (0.8-1.1bar),--running 0.95 bar max ---"When you're racing it's life! Anything else either before or after, is just waiting" |
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