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View Full Version : Could it be the Fuel? 3-MPH improvement



RickSE
08-26-2002, 04:03 PM
I made some engine changes 2-years ago, 2-barrel to 4-barrel with a new cam and prop. I've always run the boat on 87-octane figuring it really didn't need more (basically a stock 1996 Mercruiser 350). With the engine changes and a new prop my best speed was around 58-MPH, made me happy since the 2-barrel speed was 53-MPH, 18-Classic running at 3700-ft. elevation.

Well since I'd recently read some info on octane levels dropping during winter storage I decided to top off the tank last week with 89-octane instead of 87. Took the boat out this weekend and it ran a best ever speed of 61-MPH. I'm assuming it's the octane since nothing else has changed. The higher octane/slower burn must be allowing me to get more power out of the fuel. I'll try some 92 but I'm assuming it won't make as much of a difference.

Anyone else experience this?

FJB20
08-27-2002, 10:16 AM
I Guess the Only Thing to Do is Go Back to the 87 and See If Ya Lose the MPH's What Prop Did Ya Change to.??? I've got the Same 5.7 Lookin for More Steam. THANX!!

RickSE
08-27-2002, 11:12 AM
FJB20,
I went from a 19P Quicksilver to a 21P Mirage Plus. I believe you have a roller cam in your motor? If so then you may not have to change the cam like I did.

blackhawk
08-27-2002, 12:10 PM
Higher octane = slower burn = lower temps = LOWER HP

However, stale gas could definitely cause power loss.

MOP
08-29-2002, 09:11 PM
I had the very same experiance last time out with one grade higher, it may have been a weather change. I know engines run better on cool air.

Digger
08-30-2002, 09:47 AM
agree with MOP. Temp, Pressure Altitude, and Density Altitude collectively have huge impact on engine performance, more than is generally recognized.