87 vs 89 in a Hemi
Thread Starter
|
All Star
Joined: Jan 2009
Posts: 757
Likes: 0
From: Dallas, Texas
Um, I'm glad someone else knew.
Nothin to speed up Bonnie like removing all the extra dirt and grime weight...
Nothin to speed up Bonnie like removing all the extra dirt and grime weight...
Veteran
Joined: Dec 2008
Posts: 321
Likes: 0
From: Alberta, Canada
run the lowest grade fuel you can before you get pre-detonation is how we look at it for bikes....but with trucks the added octane gives you better timing specially if you have a tuner so i dunno
Veteran
Joined: Dec 2008
Posts: 321
Likes: 0
From: Alberta, Canada
There IS a difference in fuels - aside from the octane rating
Different fuel blends burn at different rates under the same temperatures and pressures. Absolute fact.
Peak cylinder pressure, for best power output, MUST be timed to occur just after the top of the stroke.
To tune for maximum power, you would like to use a fuel that burns at the quickest possible rate - without "knock" (there - that covers pre-ignition and detonation) so you can initiate burning at the latest possible time and still produce peak cylinder pressure at around TDC. That way, the rising cylinder pressure pushes back down on the top of the rising piston for the shortest period of time - decreasing power lost there.
A more specific location for peak cylinder pressure would be ~15 degrees after TDC. There is some variance from that figure - but not more than a degree or so in an engine using gasoline. When the ignition timing is correct, power output will be highest for that rpm and throttle position.
(thanks, Al Cline!)
Octane?
If you look at the octane rating as only an indicator of what the fuel was designed for, you'd guess that a 120 octane fuel was probably designed for an engine that was prone to "knock" - like an older American V8 with relatively poor cooling and high compression. That fuel will be blended to burn at controllable rate that matches the high pressures and temperatures present AND resist "knock". A good match for those V8 engine conditions.
Burn Rates- That's the ticket!!
If you take that same fuel that worked well in the above V8, and run it in an engine, like a cbr900, with it's lower cranking compression and lower combustion chamber temps, it will, without a doubt, burn much too slowly at those lower temps and pressures and reach maximum cylinder pressure too far after TDC for best power. Things generally burn slower when they are cooler and vice versa. Peak cylinder pressure will occur much past TDC - decreasing the power produced if you keep the same ignition timing.
You can advance ignition timing to try to recover power, but that will cause the air/fuel mixture to burn earlier in the crank stroke and spend, percentage wise, more of the energy produced by the expanding, burning mixture, pushing back down, trying to prevent the piston from rising up to the power stroke - robbing power.
If you MUST use a slow burning fuel, which USUALLY has a high octane rating, advancing the ignition timing will lessen the power loss, but the best results are usually obtained with the quickest burning fuel obtainable, that, of course, doesn't "knock".
All other factors being the same, except for burn rate - use the quickest burning fuel that doesn't "knock", light the spark in the middle of the combustion chamber, adjust ignition timing to reach peak cylinder pressure ~TDC and keep your mixture correct. When the ignition timing is correct, the engine will make best power for that fuel.
Different fuel blends burn at different rates under the same temperatures and pressures. Absolute fact.
Peak cylinder pressure, for best power output, MUST be timed to occur just after the top of the stroke.
To tune for maximum power, you would like to use a fuel that burns at the quickest possible rate - without "knock" (there - that covers pre-ignition and detonation) so you can initiate burning at the latest possible time and still produce peak cylinder pressure at around TDC. That way, the rising cylinder pressure pushes back down on the top of the rising piston for the shortest period of time - decreasing power lost there.
A more specific location for peak cylinder pressure would be ~15 degrees after TDC. There is some variance from that figure - but not more than a degree or so in an engine using gasoline. When the ignition timing is correct, power output will be highest for that rpm and throttle position.
(thanks, Al Cline!)
Octane?
If you look at the octane rating as only an indicator of what the fuel was designed for, you'd guess that a 120 octane fuel was probably designed for an engine that was prone to "knock" - like an older American V8 with relatively poor cooling and high compression. That fuel will be blended to burn at controllable rate that matches the high pressures and temperatures present AND resist "knock". A good match for those V8 engine conditions.
Burn Rates- That's the ticket!!
If you take that same fuel that worked well in the above V8, and run it in an engine, like a cbr900, with it's lower cranking compression and lower combustion chamber temps, it will, without a doubt, burn much too slowly at those lower temps and pressures and reach maximum cylinder pressure too far after TDC for best power. Things generally burn slower when they are cooler and vice versa. Peak cylinder pressure will occur much past TDC - decreasing the power produced if you keep the same ignition timing.
You can advance ignition timing to try to recover power, but that will cause the air/fuel mixture to burn earlier in the crank stroke and spend, percentage wise, more of the energy produced by the expanding, burning mixture, pushing back down, trying to prevent the piston from rising up to the power stroke - robbing power.
If you MUST use a slow burning fuel, which USUALLY has a high octane rating, advancing the ignition timing will lessen the power loss, but the best results are usually obtained with the quickest burning fuel obtainable, that, of course, doesn't "knock".
All other factors being the same, except for burn rate - use the quickest burning fuel that doesn't "knock", light the spark in the middle of the combustion chamber, adjust ignition timing to reach peak cylinder pressure ~TDC and keep your mixture correct. When the ignition timing is correct, the engine will make best power for that fuel.
