whats the difference between high compression ratios and low compression ratios in power and economy?
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well the higher the compression ratio the more power you will have
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In the absence of other concerns, the efficiency of a piston engine increases as compression ratio increases. More efficiency means more power for a given fuel consumption, or the same power for less fuel consumed.
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so 11:1 compression would eat less gas than 9:1 compression?
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Yes, but if everything else is equal it will also require a higher octane fuel.

Also, when using supercharging or turbocharging you can get more power with a lower compression ratio since this allow for a higher boost pressure without knocking.
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No, it would just extract more energy (power) from the same amount of gas.

Jon
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It may require a higher octane fuel; depends on how high you go and how low you were to begin with.

You use a lower compression ratio with forced induction to prevent knocking. Adding forced induction to a hi-compression engine will add alot of stress to engine internals; it will also create very high pressure in the combustion chamber, which will increase the likeliness of the fuel self-igniting, causing pre-ignition or worse, detonation.

Jon
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It will always require a higher octane unless you change something else. But if you run the "low compression engine" on a specific fuel you don't always need to increase the octane level when increasing compression ratio, but this depends on that the fuel you are using are more than enough for the "low compression ratio".

A higher compression will also lower the fuel consumption. The fuel consumption will be the same with a specific rpm and full load (since power have increased) but since power have increased we can use a lower rev for the same power need and the part load efficiency also tend to be better.
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Again, changing upping CR does not always mean you need to up octane. It depends on too many factors to make a blanket statement. If I up my ratio from 9.0:1 to 9.1:1, will I need more octane? No.

Since one cannot speak towards another's driving habits, you cannot say a higher compression will lower fuel consumption. It will make the engine more effecient--making more power with the same amount of gas--but it will not affect fuel mapping.

Jon
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I believe that what SJ is trying to say here (and I happen to agree), is that if you assume that you started out with fuel that was at the knock limit initially, then yes, you will need to increase the octane rating when you increase the compression ratio, unless you change something else within the engine (ignition timing, squish area, etc) to avoid knock. The octane rating of a fuel is established in a manner similar to what we're describing here - compression ratio of a test engine is increased until the onset of knock, and "octane rating" is looked up from a table. The table was derived based on similar tests (historical) of mixtures of isooctane and heptane...

Quit trying to "win!" Discussions around here get pretty silly at times, with people making all sorts of oddball statements just to make the other guy wrong. SJ happens to know what he's talking about here. Over a given driving cycle, higher compression ratio will result in lower fuel consumption, if knock and other problems are avoided. SJ's description of the means by which this is accomplished is essentially correct. If you want to make the driving cycle an unknown, then there is no way you can ever compare the fuel consumption of any two vehicle configurations in a sensible manner (hence the "oddball" nature of the above statement). For all we know, after "souping up" his engine, the driver may never start it again, chosing instead to trailer the vehicle around - then fuel consumption for the vehicle would drop to zero! How much fun could we have with arbitrary assumptions, and how little useful information would be gleaned? The gutter is the limit.
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I am not trying to win, I am simply trying to help this person out. I think it is misleading to say that a higher compression ratio will save gas. I understand completely what SJ is saying, and I am not trying to split hairs; my explanations are for camaro, not SJ or you. I know who understands this theory, and I know who doesn't. Do you? I am giving complete facts with no assumptions--just straight answers. I respect your posts IvyMike, but I am no troll or internet argument instigator. As for octane rating, I understand how it was conceived and how it is measured; all fuels go through two different testing methods and then the results are averaged. Again, I could care less who wins, I just want to make sure the person asking the question completely understands the answer. Lord knows there are way too many newbs on this website throwing around opinion and candied assumption like it is engineering fact.

Jon
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wow... u people confused the hell out of me. thanks for the info guys.
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A little simpler answer then; specific fuel consumption drops when CR increases, power increases. When CR gets too high we will se no more gains because of that the pumpinglosses will be too high.

Going from 8:1 to 11:1 decreases specific fuel consumption with about 60 g/kWh and increases power with about 10%.
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I like that answer, SJ. No flames...


Jon
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you can also prevent detonation without upping the octane by adjusting the timing, or cooling the intake/fuel. or both.
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