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Location:
Hunter Valley
Registered:
February 2003
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Explaination of engine compression ratios??? Sorry, need to learn though.
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Thu, 30 October 2003 05:10
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Hello,
A newby type question, still learning and I dont understand about engine compression ratios and how they apply, in which is best for forced induction....
It is apparently lower is better, but why?
As in 20V have a high number and gze have a lower one.
And how many psi equals a bar?
Sorry but thanx, plz dont blast me, I feel stupid enough asking lol.
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I supported Toymods
Location:
Perth, Western Australia
Registered:
December 2002
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Re: Explaination of engine compression ratios??? Sorry, need to learn though.
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Thu, 30 October 2003 05:37
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It's the ratio as to how many times the air fuel mixture is compressed during the engine process, and is determined by dividing the swept volume (Pi(3.14) x (bore/2)2 x stroke) to the cylinder head plus head gasket chamber. Forced induction engine have lower compression ratios as they can fill the cylinder more than an amtmospheric engine, which result in a higher compression ratio any way. 14.7psi equals 1bar
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Location:
Hunter Valley
Registered:
February 2003
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Re: Explaination of engine compression ratios??? Sorry, need to learn though.
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Thu, 30 October 2003 05:46
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Thankyou heaps Clint GOD lol!
Appreciate, well explained as well.
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Location:
Brisbane
Registered:
May 2002
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Re: Explaination of engine compression ratios??? Sorry, need to learn though.
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Thu, 30 October 2003 06:00
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| clintgodde wrote on Thu, 30 October 2003 15:37 |
Forced induction engine have lower compression ratios as they can fill the cylinder more than an amtmospheric engine, which result in a higher compression ratio any way. 14.7psi equals 1bar
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Forced induction will increase cylinder pressures, but the compression ratio is fixed regardless of the presence of forced induction.
Without getting into the technical details, forced induction engines generally have a lower compression ratio to prevent detonation; it's a reliability thing more than anything. Higher cylinder pressure = greater chance of detonation = internally damaged engine. On the other hand, a high compression ratio is desirable because it increases the engine's thermal efficiency (and thus power), allows the use of more radical cam profiles, and promototes better throttle response. This is true for both forced induction and NA engines.
Back in the bad old days, intercoolers were unheard of, cylinder head design was often less than ideal and engine management was primitive, so turbocharged engines had to be built with a large safety margin. This explains why engines like the 3T-GTE had a very low 7.5:1 compression ratio. Fast-forward to the late 90's and we have technologically advanced engines like the 1JZ-GTE and 2JZ-GTE with a 9.0:1 compression ratio. They can get away with this because of the advances in technology in the last 20 years. This is also why late-model atmo engines are reaching the 11:1 mark; better engine management and cylinder head design allows them to build their engines to run right on the edge of detonation.
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Location:
Hunter Valley
Registered:
February 2003
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Re: Explaination of engine compression ratios??? Sorry, need to learn though.
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Thu, 30 October 2003 06:59
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Gee wow, thanks heaps too Norbie.
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Location:
Brisbane
Registered:
October 2002
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Re: Explaination of engine compression ratios??? Sorry, need to learn though.
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Thu, 30 October 2003 10:32
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thats why top fuelers have a comp ratio of 7.5:1 and somewhere in the vicinity of 80 psi  on full overdrive
nowadays in the world of drag racing, the honda full race engines are turbo'd and have a static comp of like 11:1
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