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Originally Posted by backlash1
Hi
I just want to point out that you're comparing 2 very different engines. You have tha OHV and most people here are talking about the SOHC, which is a more modern engine that should have a noticeably better fuel economy. So don't feel bad about your figures, I think they're OK.
Also, it's not the brand of gas that you use what affects consumption, but rather, the octane of the fuel you put in your car. The lower the octane the worst the fuel economy, because the PCM has to keep retarding the timing to avoid detonation (pinging). Low octane fuel also speeds up the formation of carbon deposits in the intake side of the engine (throttle body, manifold, etc) restricting the amount of air the engine gets at any given time and therefore affecting fuel economy. cheers
Marc
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A couple points....
At normal highway demand (specific RPM and power output), the old pushrod motor actually has a slightly lower BSFC than the overhead cam motor. It also has more torque in the 1500 - 2500 RPM range. Not like it's a big deal, though.... your driving habits will have a MUCH greater influence on fuel mileage than the engine type.
As to the old wives tale about all the benefits of higher octane fuel.... Unless your compression ratio is so high (or your combustion chamber deposits so bad) that you're getting compression ignition, you don't need any higher octane - and backing off the spark timing won't stop the knock - the ECM will realize that it's compression ignition, quit retarding the spark, and light the "check engine" light. In normal operation, the spark will be advanced to the point where the knock detector signals the ECM, and then backed off slightly... this is adjusted constantly - and when incipient knock is determined by spark timing, the fuel grade doesn't matter.
Higher octane fuel does not have any more inherent energy than regular gasoline..... it simply has a higher ignition temperature (and a higher price).
Manifold deposits, combustion chamber deposits, injector fouling, fuel pump wear, etc. are all treated with additives to the fuel, and all grades of a particular brand get the same additives.... and even many different brands use the same stuff.
