running lower octane

If your engine is 100% stock- meaning the compression ratio has not been raised, the ignition system hasnt been modified and the timing is set per the factory recommendations- then you will not notice a bit of performance improvement by running anything greater than the minimum recommended 89 octane.

The only performance boost would come from advancing your timing, in which case higher octane would prevent pinging.

Unless your engine is modified, just run 89.

Are you familiar with using a timing light? If so you could check to see what your base timing is in the spring and let us know how many degrees before top dead center, btdc, the engine is idling at. If youre running stock timing then TRBenj is 100% correct about using 89 octane fuel!

Here's what the '89 PCM manual says-

89 works fine in my 93.

As the price goes up the higher grades are cheaper percentage wise if your getting something from them. Doesn't make much difference in these old boats, but in newer cars you can save a little from the improve MPG you might get. The reverse example of this works in my truck, I can get E85 for 20% less, but it drops my mileage about that much so its a wash.

One factor regarding gas is the altitude. As the altitude increases the density of the air molecules diminshes, acting like lower compression.

At my running altitude, about 24000 feet above sea level, I considered using 87 octane, and tried some. However the noise and wind did not allow me to hear any knocking, even with an engine stethoscope.    I gave up and continue to run 89 octane in my 1988 Dominique.   

To my mind, this "altitude derating" is a longetivity issue with the engine, allowing less power, but less stress on the engine and longer engine life.

Same thing with the jetting on the carb. Sea level jetting means I run slightly rich, but likely avoid any damage from being over lean.

The difference in gas price is a small part of the cost of running my boat and is cheaper than major engine work.

Must be somewhere outside of Maryland...Maybe somewhere up the side of the Himalaya's.

Maryland Elevation   
- Highest point Backbone Mountain
(Hoye Crest)3,360 ft (1,024 m)

Mount Everest Elevation 8,848 m (29,029 ft)

I've always felt that my Excel it thrifty on the fuel because of the shape. Similar hull to yours with the V, and I also have the SB Ford instead of the BBC. My thought was that it is designed to get out of the water for speed where most of the other boats want to push the nose down to get a smaller wake at skiing speeds. Don't know if that's true or not, and there wasn't a similar slalom boat to mine so no real way to test.

I did run across something related sorta last night. It was test reports from a boat testing site who had tested various boats over the years with different motor options for the same models.   The Tige's were interesting because they switched to PCM motors last year and the fuel economy improved. The other thing I noticed was the fuel economy also improved upgrading from the 343 to the 409. I don't know if it was enough to justify the cost of the motor or not, but interesting since I've heard these claims but never seen actual proof. They were simply looking for the most range at what RPM and MPH, but quite a few were around 24 to 26 MPH which is our usual riding speed. I think the best I saw was a little over 4 MPG and there were some Mastercrafts down in the 2.5 MPG range. Unfortunately there were no Nautiques tested.

SNobsessed wrote: I have an '89, run 87 octane (not ethanol). Timing is at 9 deg BTDC. No knocking problems at all.

Ethanol e10 87 works fine with no knocking as well as regular non ethanol 87.

Never heard of it. I'm assuming you've been there?

horkn wrote: SNobsessed wrote: I have an '89, run 87 octane (not ethanol). Timing is at 9 deg BTDC. No knocking problems at all. Ethanol e10 87 works fine with no knocking as well as regular non ethanol 87.

Yuck. I'm not doubting your results, but I'd either burn the gas quickly or treat it. I'm still a fan of 89 in boats over the 87.