Best gasoline octane for old Chrysler Outboards

[WinnerCougar74]

This is likely a no brainer but here goes. RECreational gas seems to be best for these old motors primarily because it's non-ethonol. Where I live REC gas octane rating is 90. My Clymer manual says you can run 87 in an old Chrysler. Of course the 87 around here is 10% ethanol so I have some marine Stabil with me in case I can't find the REC gas. I have used 87 octane gas with Stabil added when I know I will be using up the entire tank full of gas so it won't be sitting around condensing water in the tank.

If Chrysler outboards are made to run on 87 octane gas can you increase the timing to take advantage of the 90 octane REC gas. It seems like it would be very difficult to hear detonation with the noisy Chrysler's. Also it seems like a very risky thing to increase timing on these motors since detonation seems to be one of the main things that blows these engines.

I am hoping to improve the performance and gas mileage of my 1975 135hp Chrysler. Most say leave these motors alone. I look forward to your comments.

 

[Redbarron%%]

How much can you afford to save?

 

[WinnerCougar74]

The perfect question Redbarron. So far here's my math. It's a word problem. My favorite kind of math. Of course these are all theoretical figures. If I take one 20 mile boat trip exery week for 6 months (26 trips) and pay $4.50 a gallon for REC gas that would cost $1170 @ 2 mpg $720 @ 3 mpg and $540 @ 4 mpg per year. So then a yearly savings of $390 with a 1 mpg improvement and $585 with a 2 mpg improvement. My guess is improvements in the tune of these outboards would only change the mpg by 1 mpg tops and if that put a motor in danger of blowing up then it's not worth it. I wonder if anyone is a crazy as me and thinks like this. Yes I am cheap. It's ok to call me that. Along time ago it was called a nice name like frugal. If I wasn't "cheap" I would have moved on from this motor long ago.

 

[jerryjerry05]

What I would do: use the 87 with 10% Ethanol.
Add Ethanol stabilizer if the boat "might" set for a while???
Otherwise I just ran pump gas, usually WallyWorld gas, and kept the tank full after using. Try t keep it out of the sun.
The biggest problem was the rubber hoses melted from the Ethanol.
Google "premium vs regular gas" from what I found was the premium had longer burn time and might/could/maybe do damage over the long run.
If you run premium, do a plug check: run premium on a spare tank, go for a ride and run wide open for 5 min. Then turn the key off, don't throttle back just kill the motor and pull the plugs and check the burn.

 

[Redbarron%%]

My point is advancing the timing to save fuel might cost you a piston or two.
If saving fuel is a priority then perhaps a 4 cycle outboard would be indicated, but then they are expensive too.
I operate a Hobie Jet Fisherman with basically a 90 hp Force motor. I rebuilt the motor, twice. The last time to replace a burned piston.
No other choice for the engine for this boat, but then for me fuel on short fishing trips is not a concern, but rather ageing of the fuel between trips.

 

[WinnerCougar74]

Yes all good points Redbarron. There is a really good video on YouTube on the Project Farm channel where he tested gas with ethanol and the non ethanol (REC) gas with and without fuel stabilizer. He found the stabilizer did not eliminate corrosion on aluminum parts with the ethanol gas. Also the longer all of the fuels were stored the less flammable they became. Jump to 7:40 to see a carb that was completely ruined by gas with ethanol in it. This has made me think. Old gas will reduce the performance of our engines and thus lower the fuel economy. The hard part is you can't drain the fuel and put it in your truck or lawn mower because it has 2 stroke oil mixed with it.

From the research I have done 4 stroke engines get much better gas mileage at low RPM's than 2 stroke but almost the same at 3/4-WOT. So a newer 4 stroke might not help much.

I'm gonna stick with standard timing and just try to keep the gas as fresh as possible. I would hate to blow my engine trying to save a buck or 2. I'm also going to stick with REC gas for the same reasons. I'm gonna regularly pull the spark plugs and clean them. I think even marginal fouling significantly lowers the performance of these engines. I'll save a liitle by not needing to use stabilizer with the REC gas.