This is something I've always wondered about. Obviously an engine with 10 to 1 pistons (or higher) is going to run better with a higher octane fuel, but realistically, what kind of power is lost from old fuel to new? Would an engine, like, for instance the 425 nailhead in my Riviera, that originally made 340 hp & 465 ft lbs of torque (back in 65 running on leaded gas with higher octane), make near that much power running on newer fuel? How much power has been lost? 20hp? 50 hp? 100hp? Is my 340 hp engine now a 250hp engine? This is truly a subject on which I am completely ignorant. Anyone out there care to enlighten me???
Equal as long as the car is jetted properly. Unless you're literally talking old fuel, like last summer's tankful in a car that's sat all winter.
Noooo, not the dreaded "varnish". Just 60's fuel standards vs new. No difference? Really? Wow. Not the answer I expected.
So you want to compare 103 sunoco to 87 arco ?? That is not very fair. Certainly your engine would know the difference, but alas, will never have the opportunity to taste the 'rich life' again. Now you get to use less compression and less advance. I guess it does help save on parts...all except for that detonation issue. My best guess is at least a 10% loss based on memory of the 60's vs today. But then the 'hot' street engine of the 60's is pretty mild by todays standard so a trip to the dyno may be needed...memories of chevron 'white pump' fill my head...lol .
If you can run the stock spark advance settings on current fuel basicaly no change. If you need to retard the spark any you will lose power ,how much ? Depending on how much retard is needed. 20-30% would be a good guess. Richening up full throttle settings may let you avoid retarding the spark or at least not as much. As far as heat value in the fuel any thing with alcohol in will have less but that also increseases octain . So kinda back to square one, not much change.
Btu's are similar (depending on the amount of ethenol in the blend more E=< Btu) However todays gas is blended to have a much slower flame front, to control expansion. The old gas went Bang, delivering a sharp short hard push. Today's fuel goes whoomph, yielding a less firm push but over a longer period. I read a couple articles that suggest initial timing on older engines needs to be set a few degrees BTC in order to light the mix of earlier to anticipate full expansion at TDC. If you look back at old engine specs you don't see much advance until the mid 60's when compression ratio was increasing from the 7's tot eh 9's and 10's. My factory spec is TDC at idle (450) Rpm. I have mine set at 5 before. This takes into respect the increased CR from millig the head and deck, and the current fuel formulas. I set my timing with the vacuum gauge. Attaining the greated vacuum available at idle. the do some road tuning if it shows any pre ignition.
To put it another way: Changing the octane rating of a given gasoline does not changes its energy value (BTU/ lb.) Higher octane only allows more compression and spark lead which leads to higher power outputs. If you have to detune your engine to make it run well on lower octane fuel it will obviously produce less power but it's not because of fewer BTU/lb. Higher octane ratings were initially achieved with the itroduction of tetraethyl lead. The compound,when added to gasoline, slowed the rate of burn in the cylinder and eliminated knock-a condition which occurred in the primitive (pre 1930) gasolines of the day and which prevented the use of higher compression ratios. Over the years the octane was increased to ever higher levels and so were compression ratios. Many of today's computer controlled engines can run on 87 octane even with C.R.s on the high side of 9:1 because the knock sensors can retard spark instantaneously-like before that cylinder is ready to fire again. ]This is something I've always wondered about. Obviously an engine with 10 to 1 pistons (or higher) is going to run better with a higher octane fuel, but realistically, what kind of power is lost from old fuel to new? Would an engine, like, for instance the 425 nailhead in my Riviera, that originally made 340 hp & 465 ft lbs of torque (back in 65 running on leaded gas with higher octane), make near that much power running on newer fuel? How much power has been lost? 20hp? 50 hp? 100hp? Is my 340 hp engine now a 250hp engine? This is truly a subject on which I am completely ignorant. Anyone out there care to enlighten me???[/quote]
The Jalopy Journal
