What octane should i put in my V8?
#14
12-19-2010, 09:31 AM
By law, any on-road passenger vehicle sold in the United States must be able to run on 87 octane fuel without harming the vehicle. Some vehicles are tuned such that they can run on 87 octane, but will enjoy a modest power increase when using higher octane (up to the limits of the tune) fuel. Some vehicles that come to mind are LSx-powered performance cars (F-body, GTO, Corvette, etc.), the 4.7 H.O powered Dakotas, etc.
A vehicle whose PCM tune wasn't designed for higher octane fuel could actually lose power with higher octane levels. Higher octane supresses knock/ping because it has less energy content (very simplified explanation) than lower octane fuel. Unless the vehicle's PCM is programmed with maps that can advance ignition timing, etc, to take advantage of the higher octane fuel's knock supression characteristics, the vehicle will lose power.
Bottom line, putting high octane fuel in a vehicle not designed for it is not only a waste of money, it could actually reduce power and fuel economy. Any observed drivability improvements are most likely the result of additives and have nothing to do with the octane rating.
I was once helping a friend shop for cars and we were test driving an '02 Buick Regal GS (powered by a supercharged 3.8 liter V6). I commented that it felt slower than most - more like a 16 second car than a mid-14 second car. I had my AutoTap (laptop-based OBDII tool) with me and checked the knock retard. As I had suspected, we were getting a significant amount of KR, probably because the dealer had put regular fuel in the car (they have no idea about performance cars and just put the same old crappy gas in everything). We returned to the lot and the salesman agreed to fill it up with premium. The AutoTap showed 0 KR and there was a very noticable increase in power. Since the Regal was a forced-induction vehicle, this was an extreme case, but some cars really do run stronger on premium.
A vehicle whose PCM tune wasn't designed for higher octane fuel could actually lose power with higher octane levels. Higher octane supresses knock/ping because it has less energy content (very simplified explanation) than lower octane fuel. Unless the vehicle's PCM is programmed with maps that can advance ignition timing, etc, to take advantage of the higher octane fuel's knock supression characteristics, the vehicle will lose power.
Bottom line, putting high octane fuel in a vehicle not designed for it is not only a waste of money, it could actually reduce power and fuel economy. Any observed drivability improvements are most likely the result of additives and have nothing to do with the octane rating.
I was once helping a friend shop for cars and we were test driving an '02 Buick Regal GS (powered by a supercharged 3.8 liter V6). I commented that it felt slower than most - more like a 16 second car than a mid-14 second car. I had my AutoTap (laptop-based OBDII tool) with me and checked the knock retard. As I had suspected, we were getting a significant amount of KR, probably because the dealer had put regular fuel in the car (they have no idea about performance cars and just put the same old crappy gas in everything). We returned to the lot and the salesman agreed to fill it up with premium. The AutoTap showed 0 KR and there was a very noticable increase in power. Since the Regal was a forced-induction vehicle, this was an extreme case, but some cars really do run stronger on premium.
Last edited by RealityCheck; 12-19-2010 at 10:10 AM.
#15
12-19-2010, 10:05 AM
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