Flapjack

Ok- top of the motor/arrestor says 87 octane but mechanic says at least 91 if not 93. If the motor is made for 87 wouldn't the premium fuel go right out the exhaust (unburnt). I always thought it took higher combustion temps to require premium fuel (supercharged or really high compression).
Thanks for advice

P.S. Doesn't unburnt 93 octane fuel end up in a black transom?

bajarick

It does on my 496 s, I have 500 hp s in my
36 and havent noticed any black soot no matter
what fuel I use, so I just 89 oct but it does
say 87.

firehawkcat

What is the compression ratio

Flapjack

Really not sure on compression ratio, she is bone stock, whatever stock is on a blue 502 carb...with 330 hours. She'll run out at mid 80's but I don't want to risk detonation if 87 octane won't cut the mustard at high RPMs....most of the time I cruise her like an old lady anyway

brians fever

your compretion should be 8.5-1 and you should have knock sensors on the blocks, just in case . i ran 87 in mine for a long time.

ezstriper

you should not see black on transome at all..has nothing to with octane...over rich does that...and if you can I would run 89 , thats what I run in mine...87 will work....but with todays fuels the 89 will give you a bit of insurance that you have a "good" 87 min as fuels today only maintain their octane for 30 days...so 87 that has sat for some time will be less than required...Rob

audacity

knock sensors do not and will not react quick enough to prevent deto.

87 vs premium is a 7 hp loss on a 500hp engine!

play with some VP fuel like U4.4 fuel and u will see 550hp!

audacity

ps...on that engine/carb u can take it to someone that knows what they are doing and machine and flow it on a bench. match port and add a spacer. few minor changes in the carb and 35hp on the dyno. and you can toss the choke assembly in the trash.

for damn near no money you can goto autozone and buy a hego and measure AF ratio. livorsi sells exhaust temp gauges.

don't be afraid of drilling right through the jacked exhaust!

and don't be afraid to place a valve in the exhaust water supply. u only need enough to keep the flex hose from melting. the extra water pump it out the side.

Griff

You are exactly correct if everything is set to stock specs.

Rage

Check out the 'Full Throttle Marine 496 Mag Dyno Testing & Myth Busting' report on Bob Lloyd's Full Throttle Marine web site.
http://www.fullthrottlemarine.com/

He dyno tested 87 vs 93 octane on the same 496Mag motor and found no significant differeance in HP.