Octane

[PanozDuke]

I am trying to figure out the advantage of running 110 in a 10.5 motor with a cam that runs out of steam at about 6500 rpm. My experience is that 100 should be more than sufficient for any Ford with 10.5 and still run an aggressive timing curve and 38 degrees total. Slowing the fuel burn more than necessary with higher octane doesn't increase power as far as I know. In motors with say 12.5, the 110 would be needed to get to the sweet spot while avoiding detonation. I've been running my 10.5 carburated Ford 302 on 93 at the track with 38 degrees all in at 2500 with no issues. Its peak power is 5800 rpm. Compression is 170 lbs.

Am I misinformed that octane relates only to the speed of the burn, not the energy content of the fuel- meaning that it only increases power to the point where it allows optimum timing for a given compression ratio and cam combination (yielding the cylinder pressure of the motor) assuming proper mixture? If not, then 110 in the spec 351 GTS motor should be overkill by a wide margin. It would be interesting to put one on a chassis dyno and perform the comparison test as Kel is planning. Pushing the timing to the margin of detonation would indicate how much additional safety you get from 110 over 100. I'd really like the data for clarification of the issue. I know PAD specifies 110, but that could well be because they want to safe side their recommendations given they can not control how customers will set their total timing or jet their carbs trying to beat those Porsche cup cars

Mike

[NZGTRA17]

Originally Posted by PanozDuke

I am trying to figure out the advantage of running 110 in a 10.5 motor with a cam that runs out of steam at about 6500 rpm. My experience is that 100 should be more than sufficient for any Ford with 10.5 and still run an aggressive timing curve and 38 degrees total. Slowing the fuel burn more than necessary with higher octane doesn't increase power as far as I know. In motors with say 12.5, the 110 would be needed to get to the sweet spot while avoiding detonation. I've been running my 10.5 carburated Ford 302 on 93 at the track with 38 degrees all in at 2500 with no issues. Its peak power is 5800 rpm. Compression is 170 lbs.

Am I misinformed that octane relates only to the speed of the burn, not the energy content of the fuel- meaning that it only increases power to the point where it allows optimum timing for a given compression ratio and cam combination (yielding the cylinder pressure of the motor) assuming proper mixture? If not, then 110 in the spec 351 GTS motor should be overkill by a wide margin. It would be interesting to put one on a chassis dyno and perform the comparison test as Kel is planning. Pushing the timing to the margin of detonation would indicate how much additional safety you get from 110 over 100. I'd really like the data for clarification of the issue. I know PAD specifies 110, but that could well be because they want to safe side their recommendations given they can not control how customers will set their total timing or jet their carbs trying to beat those Porsche cup cars

Mike

Mike, per the post I have added to Uwe's thread we are on the same page on this one. My view is, use only the amount of octane number that gets you detonation free with some window of safety for your combo and not necessarily any more unless particular running conditions require this.

On the dyno I normally set the car up to produce max hp/tq (detonation free) and then play with the timing to see what additional window of safety there is when advancing the timing prior to detonation setting in. I like to have around a 2 degree buffer between the setting I race on Vs the start of detonation. This is how I set up for the recent 6 hour I did and have done this with my previous race engines.

Our fuel octanes for street start at regular being 91 then super is 95
and we have a premium product that is 98. Avgas and pump available race fuel are 100 octane. There are specialty race fuels available (VP etc) but by drum only and I see these primarily as drag race fuels.

Kel.

[PanozDuke]

Kel,
I meant this to be part of Uwe's tread, but must have pressed the wrong button (again).

Mike