buck183

I was thinking the same thing. My manual has the answer to this question already. Straight from Merc, doesn't get any easier than that.

Buck

Whiteknuckle

When I dyno'd my 461 cu in with 8.5 compression, we started the engine and did the cam break in with the gas that was used on the engine that was previously run on that dyno. It was a NASCAR Busch series small block, so we were running a high octane racing fuel.
We did a few power pulls then we swithed to the gas I brought with me which was 89 octane, that I would be buying at the marina. The engine made more power on the 89 octane. This was a surprise to me, but not to the engine builder.

mcollinstn

I beg forgiveness: there were plenty of hi-perf Merc mills that ran on high octane pump gas and some that took race gas. These of course were Merc racing motors but that was before they went to blue paint.

These days, if it doesn't have a blower on it, and it is stock, and it has been mfd in the last ten years or so then "regular" should be fine in it.

jpclear

Okay, I've stayed out of this thread till it drove me nuts. But now I'm in. Let's say we have messed with the engine a bit and we now have a compression ratio in the neighborhood of 9.25 or 9.75 to one. I certainly want to run "pump gas" but I'd be real nervous running 87 without a booster. Whaddya think?? --- Jer

mcollinstn

I made clear reference to the "blue paint" exclusion earlier in the thread. I wasn't meaning to say that blue motors were safe for 87 as long as they didn't have blowers (even though the current crop IS okay with 87 except the Scorpion). Was meaning to say that black motors in the last 10 yrs stock and blowerless were 87 octane motors.

And the Scorpion is blue (cept for the exhausts).

But when I go back and re-read it, I wasn't very clear on that, so - I guess you got me.

Aaaahhh, I'm hit....

mcollinstn

JP

If you got your "messed with" motor on a dyno, then it's easy to determine the octane requirements.

If it's in the boat, then you can use the poor man's dyno.

Start with a half tank of 87 octane. Have a fresh set of plugs with you. Back the timing down to 26 total.

Get up on plane, set the trim to a moderate top end setting (not fully trimmed, just loosened up fairly well - note the trim gauge). Make a WOT pass and take note of the rpm and GPS speed. Hold WOT for 15 seconds. Slow and stop.

Change plugs out - look closely for speckles of aluminum "dust". Advance the timing 2 degrees. Repeat the test. You SHOULD see additional RPM and speed. Note the rpm and speed. Slow and stop.

Repeat. You will continue to show more rpm and speed with each additional advancing of the ignition. At some point, you will NOT see any additional rpm or speed. This is the saturation point of your motor combo and fuel. You will need to back down 2 degrees fro this point to run 87 octane. If you are above 32 degrees, then it is okay. If you are below 32 degrees, then you are not optimizing the burn (in a BBC) and you should increase the fuel to the next available octane and increase timing to find THAT saturation point.

The above method WILL work and is safe. You cannot, though, let the ambient temperature change much, nor can you get sloppy with your methods. As with anything, sloppiness makes a mess...

Good luck.

cuda

Here's a good article on octane.

octane

jpclear

Thanks for the good input here, Mc & Cuda! In the boat it's quite a process but no doubt well worth it given the consequences of the "too much timing/not enough octane" situation and current pump prices on the water. Only thing bothers me here...if I see the "speckles of aluminum dust", haven't I already done some damage? --- Jer

Tricky919

Nice article Cuda, very interesting reading, thanks for the link.............

mcollinstn

JP

Aluminum speckles DO indicate some damage. But it is from the very top edges of the piston crown (or sometimes from an edge in a valve relief). Erosion, I guess is what you'd call it.

This erosion can take place for a while before anything major happens. Erosion is from the "very beginning" of detonation. Heavy detonation makes noise, and hammers parts. Very light detonation erodes the edges of the piston crown and raises piston temps.

We've torn down many a two stroke that had obviously been running into erosion for quite a while. Edges of the pistons looked sandblasted, but otherwise were fine. Not something you do intentionally, as you are beyond the optimal curve there, but on a roadracer if you isjudge the air density or it doesn't get as hot as you had planned, then it puts you over the edge.

15 seconds of light erosion from "creeping up" on it isn't going to cause any catastrophes. It's a far different set of circumstances than losing fuel pressure on a blower motor - that takes you way past erosion and into heavy detonation.

Chances are, that you will see the performance plateau BEFORE you see any speckles of dust. It's just that some piston/combustion chamber combos will show erosion before others. And you may still be climbing the performance curve in 6 sylinders while 2 are over the peak and are showing dust. This will be a fuel related issue on those 2 cylinders. That's why you gotta keep an eye out for the dust.