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Running on 87 Octane

craig_collins

craig_collins

Regular Contributor
I have older (rebuilt) Crusader 454's with 8:5:1 compression. My fuel dock sells both 87 and 89 octane. Would I notice any difference if I run 87 octane? I have an older Egg Harbor and never run above 3000 rpm's.
 
my mid-80's manual say to use 89 octane for all models....states to reduce timing 4 degrees if using 87 octane....again, I think a very conservative number.
 
I agree with Mark. You'll want to cut back on your spark lead.

The higher octane rating doesn't necessarily render any more BTU's or "burn" or horse power, per 'se. The higher octane rated fuel simply stabilizes combustion.

If I were to run the 87 (in lieu of a higher octane fuel), this is what it would mean to me:

Reduce the Octane rating, and we must reduce spark lead in order to avoid detonation.
I would reduce my ignition TA as to avoid this.

Reduce spark lead, and we avoid detonation, however, we also change the LPCP by delaying it.
LPCP should be at/near 12*-14* ATDC (according to many engineers).
A delayed ignition advance may place LPCP further ATDC (perhaps as late as 17*-20* or so).
When we delay LPCP, we leave Torque and Performance on the table.

BTW, in my opinion, 3,000 rpm is an RPM whereby detonation may be at a greater potential, and in particular if the boat is not fully up on step.
I'd try my best to avoid 3k rpm, unless 3k rpm gives the hull a planing attitude.
 
I guess I will stick with the 89 so I don't have to keep changing the timing. A mechanic told me that 87 would be fine as long as I don't run high rpm's.
 
craig_collins said:
............... A mechanic told me that 87 would be fine as long as I don't run high rpm's.
Click to expand...
I'd be curious as to his definition of "don't run high rpm's."

The higher octane rated fuel is to prevent detonation......., given the same ignition advance and engine load.
I don't quite follow his suggestion to run at the lower RPM range as though somehow this will avoid detonation!
IMO, Marine Load Detonation is at it's greater potential at/near the 2k to 3k RPM range.
Once our typical marine engine reaches 3+ k rpm, our ignition advance is pretty much Full In by then.

I think that staying with the 89 octane is a good choice.
 
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