Turbo boost vs. Compression ratio
07-11-2001, 10:38 AM
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Turbo boost vs. Compression ratio
I read that the old rules of thumb for maximum compression ratio of a turbo car are no longer accurate. The article pointed out that improvements in combustion chamber shape and dynamics have been made in the last 20 years to help eliminate detonation, which have raised the acceptable compression ratio.
After doing a search on the web for a couple of hours, I found a couple of dozen sites that all quoted 8:1 as safe, 8.5:1 as borderline, and anything higher as a blown engine waiting to happen. No one provided any insight into where these magic numbers came from, other than one mention of a 20 year old book.
Does anyone have any idea of what the safe compression ratio would be on a well designed, modern car? Are those old numbers still close to accurate?
Thanks,
Tanq
After doing a search on the web for a couple of hours, I found a couple of dozen sites that all quoted 8:1 as safe, 8.5:1 as borderline, and anything higher as a blown engine waiting to happen. No one provided any insight into where these magic numbers came from, other than one mention of a 20 year old book.
Does anyone have any idea of what the safe compression ratio would be on a well designed, modern car? Are those old numbers still close to accurate?
Thanks,
Tanq
07-11-2001, 02:05 PM
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Yep, they're wrong :-)
If you want to look for evidence of reasonable Fi compression ratios these days, just look at production cars. I posted this in another thread on the main forum, but pay particular attention to the cars from germany (some of these may be off by a tenth of a point, but we're close):
Audi/VW 1.8T (base) - 9.5:1
Audi/VW 1.8T (225) - 9.3:1
Audi 2.7TT (base) - 9.4:1
Audi 2.7TT (RS4) - 9.2:1
Porsche 911 GT2 - 9.4:1
These cars, fom the factory, run anywhere between 8 and 14 psi of boost. They're also designed to last a very long time and run on available premium gasoline. Most of them are also capable of handling boost pressures 30-50% higher than stock without detonation issues.
So, what conclusions can you draw from this information? Well, you could assert that 9.5:1 is a pretty reasonable compression ratio for a well designed turbo car these days, even one that will be running 18-20 psi of boost. I don't think anyone would disagree too strongly with that. Modern engine control, efficient turbos and intercoolers and good chamber design make it possible.
But, if you're really interested, I suggest you start looking into effective comrpession ratios, timing and fueling concerns, etc. assuming you build enough strength into the rods and pistons, what is the issue associated with too much compression and/or too much boost? Hint, effective compression ratio and temperature are key. For a given combustion chamber design and fuel, increase either one of those and you increase detonation risk - oops, I just answered the question for you :-) So then, how do you combat that risk? Cooler intake charge, more fuel and retarded timing (all within limits).
UL
If you want to look for evidence of reasonable Fi compression ratios these days, just look at production cars. I posted this in another thread on the main forum, but pay particular attention to the cars from germany (some of these may be off by a tenth of a point, but we're close):
Audi/VW 1.8T (base) - 9.5:1
Audi/VW 1.8T (225) - 9.3:1
Audi 2.7TT (base) - 9.4:1
Audi 2.7TT (RS4) - 9.2:1
Porsche 911 GT2 - 9.4:1
These cars, fom the factory, run anywhere between 8 and 14 psi of boost. They're also designed to last a very long time and run on available premium gasoline. Most of them are also capable of handling boost pressures 30-50% higher than stock without detonation issues.
So, what conclusions can you draw from this information? Well, you could assert that 9.5:1 is a pretty reasonable compression ratio for a well designed turbo car these days, even one that will be running 18-20 psi of boost. I don't think anyone would disagree too strongly with that. Modern engine control, efficient turbos and intercoolers and good chamber design make it possible.
But, if you're really interested, I suggest you start looking into effective comrpession ratios, timing and fueling concerns, etc. assuming you build enough strength into the rods and pistons, what is the issue associated with too much compression and/or too much boost? Hint, effective compression ratio and temperature are key. For a given combustion chamber design and fuel, increase either one of those and you increase detonation risk - oops, I just answered the question for you :-) So then, how do you combat that risk? Cooler intake charge, more fuel and retarded timing (all within limits).
UL
07-11-2001, 02:09 PM
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compression is not as big of a concern as it use to be. you still have to becareful, but you can still turbo your car with high compression.i also own a integra type r with a drag turbo kit.
compression is stock 10.5 to 1 and i'm boosting 7 lbs. making
328 horsepower to the wheels. i drive the car hard each time i drive it and it's holding up like any honda would. just becareful and make sure the car is tuned properly and you should be ok. also make sure you have proper amount of fuel and spark. hopefully that helps you a little. if you need call dynamic autosports in lakeforest, ca.
take it easy
thanh
compression is stock 10.5 to 1 and i'm boosting 7 lbs. making
328 horsepower to the wheels. i drive the car hard each time i drive it and it's holding up like any honda would. just becareful and make sure the car is tuned properly and you should be ok. also make sure you have proper amount of fuel and spark. hopefully that helps you a little. if you need call dynamic autosports in lakeforest, ca.
take it easy
thanh
03-13-2003, 10:03 AM
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My 91 Civic Si D16 has 10.3:1 and 14-15 psi. No detonation problems on premium.
BTW, I cc'd one of my spare F20 heads/pistons and didn't come up with 11:1. Anyone else?
BTW, I cc'd one of my spare F20 heads/pistons and didn't come up with 11:1. Anyone else?
03-13-2003, 10:53 AM
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my last car, a twinturbo nsx, ran perfectly for 2 years with a 10.5:1 compression ratio on 1 bar of boost. that's with over 100 1/4 runs and many many days if drifting through the canyons. the trcik with high compresion motors and boost, is making sure you have the right fuel and ingnition timing maps. timing retard on boost is a must. make sure you're carefull on cold mornings, where the air is colder and dense. i found out the hard way one morning in my fd rx7. one ping and it was all over.
