What Compression Ratio Determines Octane Rating

What compression ratio determines the octane rating needed for a vehicle? Let me site an example with two similar engines:


Mitsubishi Eclipse (http://www.mitsubishicars.com/MMSA/jsp/eclipse/features_specs.jsp?t=mechanical)
V6 3.0L SOHC
CR: 9.0:1 (GT) Req Fuel: Premium Unleaded
CR: 10.0:1 (GTS) Req Fuel: Premium Unleaded

Honda Accord (http://www.hondacars.com/models/specifications.asp?ModelName=Accord+Sedan)
V6 3.0L SOHC
CR: 10.0:1 Req Fuel: Regular Unleaded

Compression Ratio for the Honda Accord is 10.0:1 and only requires Regular Unleaded. But why does the Mitsubishi Eclipse having both 9.0:1 and 10.0:1 compression ratios require premium unleaded?

Well the higher compression will increase the probabily of detonation so often higher octane will be required.

However a less agressive ignition map or better combustion chamber designs can reduce the posibilty of detonation therefore alowing the use of lower octane fuel in higher compression engines.

What compression ratio determines the octane rating needed for a vehicle? Let me site an example with two similar engines:


Mitsubishi Eclipse (http://www.mitsubishicars.com/MMSA/jsp/eclipse/features_specs.jsp?t=mechanical)
V6 3.0L SOHC
CR: 9.0:1 (GT) Req Fuel: Premium Unleaded
CR: 10.0:1 (GTS) Req Fuel: Premium Unleaded

Honda Accord (http://www.hondacars.com/models/specifications.asp?ModelName=Accord+Sedan)
V6 3.0L SOHC
CR: 10.0:1 Req Fuel: Regular Unleaded

Compression Ratio for the Honda Accord is 10.0:1 and only requires Regular Unleaded. But why does the Mitsubishi Eclipse having both 9.0:1 and 10.0:1 compression ratios require premium unleaded?

It doesnt. More than just static compression ratio determines fuel requirement.

Well the higher compression will increase the probabily of detonation so often higher octane will be required.

However a less agressive ignition map or better combustion chamber designs can reduce the posibilty of detonation therefore alowing the use of lower octane fuel in higher compression engines.

What do you mean less aggressive ignition map or better chamber design? Can you elaborate further on this one? Does that mean that the Honda Accord's chamber is better designed than the Mitsubishi Eclipse?

What do you mean less aggressive ignition map or better chamber design? Can you elaborate further on this one? Does that mean that the Honda Accord's chamber is better designed than the Mitsubishi Eclipse?


Well higher compression, leaner a/f ratio, advanced spark timing (what I meant by aggressive) all contribute to more power however they all increase the likelihood of detonation.

But you can compensate for one with another.

For example the mitsu
"V6 3.0L SOHC
CR: 9.0:1 (GT) Req Fuel: Premium Unleaded"
might require premium despite its low compression because it runs lean, has an advanced timing map or both

Or the Honda
”V6 3.0L SOHC
CR: 10.0:1 Req Fuel: Regular Unleaded"
could get away with regular despite its high compression because it runs a richer a/f ratio, has a more retarded timing map, or both.


Beside those factors comes in the combustion chamber design. A superior design will reduce the possibility of detonation trough a better a/f mix or better flame ignition and propagation. An example of a good design would be for example ski slope pistons to create more turbulence and therefore a better mix, pentroof head and centrally located spark plug.

Hope I explained myself better this time.

Well higher compression, leaner a/f ratio, advanced spark timing (what I meant by aggressive) all contribute to more power however they all increase the likelihood of detonation.

But you can compensate for one with another.

For example the mitsu
"V6 3.0L SOHC
CR: 9.0:1 (GT) Req Fuel: Premium Unleaded"
might require premium despite its low compression because it runs lean, has an advanced timing map or both

Or the Honda
”V6 3.0L SOHC
CR: 10.0:1 Req Fuel: Regular Unleaded"
could get away with regular despite its high compression because it runs a richer a/f ratio, has a more retarded timing map, or both.


Beside those factors comes in the combustion chamber design. A superior design will reduce the possibility of detonation trough a better a/f mix or better flame ignition and propagation. An example of a good design would be for example ski slope pistons to create more turbulence and therefore a better mix, pentroof head and centrally located spark plug.

Hope I explained myself better this time.

Thats probably the answer I was just looking for. Yes, you explained it better. Its much clearer to me now. Thanks a lot again. :smokin:

No problem