Older boat, Chevy 350, what grade

[rsn48]

I purchased an older boat, 1969, and with it came the Chevy 350 (270 hp). It is rated to use 93 grade fuel, mildly laughable as the fuel on Vancouver Island is around 91 max. If the engine is using 91 fuel, anything I should or can do to up the premium, or just I just relax and use the 91 grade stuff.

[AKA-None]

May we assume 91 without ethanol? It should run fine. If not then you either reject the carb, reprogram the fuel injection or use an additive. You want to avoid pinging under load.

[rsn48]

Hadn't thought about rejetting the carburetor, thanks.

[a64pilot]

I would not change the Jets, you seek a stochimetric fuel ratio of 14.7 to 1 by weight of fuel and air, that changes by fuel type but not by octane.
http://prius.ecrostech.com/original/...chiometric.htm
I would either not run the snot out of it, or more likely back the timing down a few degrees.
Almost all modern, and I mean cars built in this this Century have knock sensors. They literally listen for “pinging” and if they hear it they back off of the timing until it stops, that allows the max timing advance for that fuel, under those comditions, which means max power and economy.
My Wife’s Cadillac, a CTS-V is a very high HP engine and will run fine on regular fuel, but you can sure tell the performance difference as it turns down the timing, on good Premium, it’s a different car.
Same for our MazdaSpeed turbo Miata.

Backing off the timing will slightly reduce max power and very slightly reduce max power, you’ll likely notice neither, but it will protect the engine from detonation.

[jimbunyard]

Oh good lord, don't mess with the jets or the timing. Put the 91 in it and see what happens. If you have problems, they will be evident and then you may want to make 'adjustments'. 'Octane booster' is available off the shelf; knocking the timing back a couple or three degrees is simple enough; the gas you get from the pump likely varies quite a bit, so the 'adjustments' you make may or may not have strictly repeatable results. If you get your fuel from the same place every time that might help with more consistent operation...

[wallythacker]

Originally Posted by jimbunyard

Oh good lord, don't mess with the jets or the timing. Put the 91 in it and see what happens. If you have problems, they will be evident and then you may want to make 'adjustments'. 'Octane booster' is available off the shelf; knocking the timing back a couple or three degrees is simple enough; the gas you get from the pump likely varies quite a bit, so the 'adjustments' you make may or may not have strictly repeatable results. If you get your fuel from the same place every time that might help with more consistent operation...

Run at least 2-3 batches of fuel from your marina to establish a baseline before you do anything. Watch for pinging under load, keep you eye on temperatures, then retard your timing 1-2 degrees and repeat until you are satisfied the engine will be OK.

I personally don't like octane booster, it's an extra expense, you can forget to add it,you can flub the ratio, your supplier can be out of it, the booster can (not very often) vary in quality.

[Jazzrockskier]

My 1988 has a Chevy 350 and is rated for 89 octane

[oleman]

I doubt that after all these years it is an original compression ration engine.

If it is it probably requires the higher octane. A 270 HP 350 was probably 10:1 compression.

There will be no knock sensor unless someone went to TBI and replaced the carb. You can judge the static compression ration by checking compression. A low compression 350 is around 135 PSI and a high compression 350 is over 150 PSI. With either engine in good shape.
IF it has unacceptable pinging under load the cheap fix is to add "head gasket shims" to reduce compression. The are also called head savers when an aluminum head requires massive machining to stop it from leaking. Any late model SMOG head (any normal 350 head after 1973) will reduce CR and power to run on current fuel also.

IMO a 69 HP 350 was one of the best American engines ever produced.

[Jammer]

If a 1969 smallblock then yes it was probably set up originally for higher octane fuel to get the extra horsepower. Usually accomplished by using a 4 bbl carb and a piston/head combination that provides more compression.


We have 93 octane nonoxy fuel available locally; you may somewhere as well.


Pulling back the throttle to the point where there is no knock or retarding the timing are the best strategies if there is a problem when you run the best fuel you can get. I would leave the carburetor alone.


A fact to keep in mind is that fuel grade mixups are common, usually from a bulk transport delivering fuel into the wrong tank at the retailer, so you may occasionally get a tank that is closer to 87 than to 91 or 93 or whatever you paid for.

[rsn48]

Turns out I lied, Chevron carries 94 octane. My tank is 100 gallons or 380 litres so I take two portable 5.2 gallon tanks and fill up at a station close to the marina. And if I'm going to the boat, though not taking it out, I'll add more gas; much cheaper than marina fuel.

I got the 93 rating from the still existing Merc manual. I agree with others, 1969 was pre-ping computer thingy - so am I.

[Cadence]

Originally Posted by rsn48

I purchased an older boat, 1969, and with it came the Chevy 350 (270 hp). It is rated to use 93 grade fuel, mildly laughable as the fuel on Vancouver Island is around 91 max. If the engine is using 91 fuel, anything I should or can do to up the premium, or just I just relax and use the 91 grade stuff.

You said BC not the US. Just curious, buying the gas at a gas station or a Marina. In the US trailering a boat and filling up at a gas station all you can get is corn oil gas. Maybe ESSO still has real gas?

[a64pilot]

You can get ETH free fuel in almost all States in the US.
Try looking up https://www.pure-gas.org/extensions/map.html

[Dooglas]

Originally Posted by Cadence

In the US trailering a boat and filling up at a gas station all you can get is corn oil gas.

I can't speak for SC, but I do boat in Oregon, Washington, and Florida. In all 3 states, some stations carry what is called Rec Gas. It is ethanol-free gas intended for boats, motorcycles, and other small engines. My local station carries both 87 and 91 octane rec gas. I make a point of using it in any vehicle which is going to be stored over the winter.

[rsn48]

Chevron in Canada claims 94 is ethanol free. Here is a listing of North American ethanol free gas.

https://www.pure-gas.org/index.jsp?stateprov=BC

[Scubaseas]

Rejetting won't do much anything for it unless it's pig rich, like 10:1. While Stoich of 14.7 is fine for chemistry for lower head and exhaust temps run about 13.4:1. You actually have a bit more power and much lower exhaust gas temps with a slightly richer mixture than Stoigh. How high octane you need is a function of your compression ratio (CR). Ping or knock (predetonation/preignition) is what you need to worry about. You can buy programmable ignition boxes that will allow you to use octane below spec if programmed correctly. However it will probably run fine on 91 (depending on how you are measuring that) but if you notice a sound like marbles in a coffee can while accelerating hard then you can either lower the CR (expensive) or retard the timing some, about 2 to 4*. Ideally you would have a knock sensor that would automatically advance your timing to the point of preignition and then automatically retard it until the knock is not there. This is fairly complicated but doable. Or put a programmable ignition control on it if preignition/predetonation is actually an issue. Run it and see what happens.