What Octane?

[Craig]

Was reading an old Trailer Boat Mag. (from around '87). Noticed an article about tow vehicle tests that made some comments about MUCH improved performance with 92 octane vs like 87 or 88 (where do you get 88 octane anyway?!?).

They said it made enough difference in this '87 station wagon type pull vehicle that they went immediately after filling from only being able to pull hills on the interstate at about 55 mph, or less, to (after filling with 92 octane) an improved "easy" 63 mph on similar grades. Hey, and you gotta figure when they filled they gained 150 pounds (+-) in fuel.

Anyway, what's the correlation, if any to aiding top speed in a boat. I know it's apples and oranges, but figured someone could shed some light. I do get an occassional sputter on take off using 87, but then I've never tried higher octane with the new engine.

I ran high octane AND lead additive in the old 1969 351. Made each fill up seem a tad expensive to this "one income, wife and 3 kids" guy. When I put in the new PCM crate motor (also 351) I was so happy it recommended (or at least said it would run on) 87 octane due to the cheaper fuel, I never have given it a thought until now.

Think I should try 89 or 92?

[Last Tango]

New boat motors with microchip ignition systems have a knock sensor. The knock sensor retards the ignition advance to prevent pre-ignition from low octane fuels. There would be very little seat-of-the-pants improvement from switching from 87 to 89 octane. Whether it would be measerable on the speedo or GPS is questionable because of the variables involved.
93 Octane might have a measureable difference. It depends on how far the spark was retarded by the microship in order to avoid "ping" on 87 octane. If you have a high compression engine it may improve the performance - but only to the limit that the chip will advance the spark in the improved combustion environment. If the highest octane rating you have locally is 92, then try it. But remember that there are a ton of daily variables to be considered (altitude above sea level, temperature, humidity, barometric pressure, fuel level on boat, passenger/equipment load, water conditions, etc.)
It will not HURT your engine to try the higher octane (only your wallet). Higher octane fuels are designed to minimze pre-ignition in high compression engines. If your compression ratio is below 9.5:1, going to higher octane will probably have very little noticeable effect. If you are using carburation and standard ignition parts, higher compression ratio is probably the ONLY reason to increase Octane. If your engine runs hot due to cooling limitations or local heat conditions, higher octane will help reduce the onset of pre-ignition, but probably not otherwise improve performance.

[Craig]

Last Tango - That's a lot of information; thanks.

My new motor's CR is (I think) is either 9.5:1 or 9.3:1 - have to check (old one, like most everyone else here's '69 351 was 10:1). Old one had a bit more power when it ran good, but this one isn't too bad.

I changed it to electronic ignition too when I changed engines.

Can't hurt to give higher octane a try, just for fun.

[Rootsy]

dynamic compression ratio and quench distance play a bigger role than static compression ratio when it comes to octane requirements.. this as well as the camshaft timing, shape of the piston crown, combustion chamber shape and ring location will dictate the amount of ignition advance the motor wants for optimal performance and will aslo dictate how much you can run on a certain octane... dynamic compression ratio takes into account camshaft timing events to calculate actual cylinder pressure vs just a volume at tdc vs a volume at bdc...

this is just scratching the surface but are some main factors that really help shape what you can run for fuel...

lastly.. higher octane fuels are more difficult to ignite... and can actually hurt the performance of an engine. you should only run as much octane as you need...

[blackhawk]

lastly.. higher octane fuels are more difficult to ignite... and can actually hurt the performance of an engine. you should only run as much octane as you need...

That summed it up right there!

When I used to race snowmobiles in the grass I would actually run 87 instead of 92 in my stocker cause it would make more hp on 87.

[Craig]

Interesting. Thanks to all.

[Cuda]

lastly.. higher octane fuels are more difficult to ignite... and can actually hurt the performance of an engine. you should only run as much octane as you need...


Dang, you beat me to it! I have 10:1 compression in my 302 Formula, and I run 93 octane. I've never tried a lower octane for fear of dire results.

[Donzigo]

Ditto ------------- what Cuda says.

I used to use 87 octane until I found out that Oneill's Marina sold 93 octane for about the same price as 87. So, I have been using 93 octane for a year or so. It might be my imagination; but, I swear that the boat runs better.

[rickrsbro]

here is a nice little horsepower vs octane dyno session.

octane vs hp (http://www.diabolicalperformance.com/hotrodoctane.html)

[MOP]

here is a nice little horsepower vs octane dyno session.

octane vs hp (http://www.diabolicalperformance.com/hotrodoctane.html)

Nice articale did not digest it yet, but it will make good reading.

MOP