Can I get away with 91 Octane with a 525SC
 

Hello All,
I have just Purchased 1995 27 Fountain Fever with a 525SC and I'm in love with it! The motor is Stock (or at least the way it came from Fountain) and I am wondering if I can get away with 91 Octane. The breather cover says 92 octane but I boat in a pretty rural area and all I can get on land is 91 octane. Thanks in advance for your help on this!

I would guess the 1 less octane point wont hurt anything.

:eek:mmmmmmmmmmmmmmmmmmmm

I wouldn't want to test the theory, blue engines cost too much. The best solution would be to add 5-7% race fuel 110 octane or better. I assume your tank is less than 100 gallons so 5 gallons of race fuel will do the trick and you’re safe.

If you must do it, then reduce boost to lower the effective compression ratio. I would not talk myself into 91 octane being ok. Fuel quality is hit and miss anymore and blower motors are more sensitive to detonation.

FWIW, I've been running 91 oct. in my blower motors (pretty much all you can find up here for premium fuel) for the past two seasons. My combo is 8:1 static 671's 8 lbs boost, Superchiller intercoolers, 28 deg total timing. No signs of detonation on the 91.

The 525SC is a 7.5:1 static comp motor, I believe it's less than 5 lbs boost. Merc timing spec is 35 deg total. I'd knock the timing back to 30-32 and you should be fine.

Chillers are the key. You can take timing out of that motor, but, you risk destroying the exhaust valves. My boat's previous owner felt he was more qualified than Merc Racing to determine the timing spec and with 144 hours on it, the engine had about 50% leakdown through the exhaust.

Just don't run the piss out of it and 91 will be fine.

I don't think 91 would hurt it at all. They are setup so rich out of the box anyway.
Run it hard for a few & then shut it down & pull some plugs to take a look if you feel like you need to.
I'd run 91 & not think twice about it if it were mine.

Ditto what biggus says. I would and do set my timing at 30* with 525sc blowers on my engines (454s). No valve probems after two yrs of hard running. I do run 93 oct but my static is 9.5:1 w/5psi. I run hyper pistons as well so you can be sure its not detonating.

gas
 

gas is the cheapst part of boating..burn good gas.i found out the hard way early this year.detionation is a mother.

I have run 89 for 6 seasons when had the 525SC's... Just make sure your timing is set at no more than 35*@4500rpms.
Don't waste your time on an innercooler.

I'm 99% sure you will be fine running the 91 octane on a stock 525SC. I ran 92 octane in mine and had the boost cranked up to almost 8# with no chiller. Never any problems at all and it had over 400hrs on it. Stock boost on a 525SC is 5.5#. Maybe dial the timing back 2 degrees to be 100% safe, but I highly doubt you really need to.

BTW, full advance on the timing does not come in until about 4500rpms.

Dont run the Piss out of it??? NO WAY!!! I'll Hi-jack a Tanker first!

correct they are rich and from my own experience 91 has worked fine with no signs of detonation however i would never keep it pinned for more than about 15 to 20 seconds.

running them rich doesn't really help in the detonation though. you still need sufficient octane to with stand the combination of heat and pressure.

Running a little rich will help keep cylinder temps down and help avoid detonation. They are plenty rich from Merc and have #98 jets squared. I had ran mine up to about 5 mins at WOT, buit mostly only 2-3 mins. The water temp would creep up about 5-10* and I would back off.

Some info...http://www.blowerdriveservice.com/techcharts.php

an extra burst of fuel by cracking the throttles always helps cool the temps. every performance engine i run i always have an exhaust temp probe / gauge. just a little extra insurance.

Mississippi math

say you burn 300 gallons a month , the differance in gas price 91 to 93 at $2.75 is usally 10 cents,thats $30 dollars a month you save.:drink:

But being thats all ya have there, like Biggus says,Ive never liked running timming real low,exhaust temps on pump gas:eek:

thats my $30.00 worth:drink:Goodluck
Rob

Unfortunately we can't even get 93 at the pumps where I boat in SOCAL. 91 is tops. I would gladly spend the extra 30 bucks for the added protection.


Darrell.

Not that $ is the only issue but I can only get 93 at the Marina which is $1.25 more per gallon than 91 I get on land next to the station where I store my boat. So at 300 Gallons per month thats $375 per month difference not $30. I really appreciate all of the input but must admit that I am even more confused now. Both sides make very good cases and back them up with real life experiences and math. I guess bcfountain said it best. Detonation is a mother! Although I really think the 91 would be fine (hell that's all you can get in some states) I'm not gonna risk it. I didn't buy a 75mph boat to take go easy on it. I'll burn the good stuff! Thanks again all!

I put some 104 with my 93 when I know I'm going to run it hard,kinda insurance.

On my Scarab I would get in the engine compartment get a guy to drive it in gear.I would work the throttle at the carbs,get someone to hold the hatch,whale on it see if it pings,yea its a crude way,

Do they still make a lead substitute?


Also had a buddy that had a spapific gravity checker, we would go arong to the gas station see wich one had the best #'s 715-725.The race fuel he was making was 705.
Rob

Add 1 gallon of Toluene or 114+ race fuel for every 24 gallons of 91 octane. That will put you at 92 octane.

That or run about a 50/50 mix of the 91 land fuel and 93 on water fuel.

Old tread. But I did use the search function. What is worse using 93 with ethanol in the 525sc or use non-ethanol 91?

:food-smiley-007: