What Fuel Octane
 

I'm running a 500Hp what octane fuel is recommended?
I have heard that these engines like the 87 octane and run their best.

heard right.

cheaper is better

mcollinstn,
What is your experience with this octane? Do you run a 500 hp? I'm wondering if anyone has done test on this engine on a dyno with different fuels?

AM,

"Additional octane" over and above the design spec of an engine NEVER gives additional performance, if the engine is in a proper state of tune.

The higher the octane rating is, the slower the fuel burn is, and the less btu per pound the fuel has. This is true for all "pump gases".

(We're not talking about race fuel or horsepower cocktails employing oxidizers and/or high volatiles like Nutec for example).

Now if you choose to increase your compression ratio or advance your ignition timing beyond the factory specs then you *may* be able to increase your engine's octane needs beyond the design spec. In that case, you will indeed obtain additional performance because lower octane fuels will be handicapped by the knock sensor on the motor retarding ignition timing away from optimum.

If it makes you happy, though, run 93 in it. Some HP500EFI's will soot up the transom with 93 fuel, but it can always be wiped off...

If I don't have to run the higher octane that is fine with me specially with the price of fuel this summer.
What octane is recommended the lowest on the pump or stay around the 87?

HP500 EFI - 500 HP
Specifications
HP: 500
Full Throttle RPM Range: 4800-5200
Compression Ratio: 8.75:1
Fuel System: MPI
Fuel Requirements: 87 Posted Octane (R+M)/2 (92 RON) International

As long as you are in the good ole U.S. of A., the yellow 87 sticker on the pump will be just fine.

If you travel outside of the US, you will need to know what method they use to figure their octane...

How about the same question for a 454mag 365hp carb'd engine? What octane?

Iv'e heard the same is true for the 496's, is this also right?

Guys, it is clearly spelled out in your manual.

And I believe the rule of thumb is that all black Mercruiser motors are setup for 87 octane.

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

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.

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.

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

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....

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.

Here's a good article on octane.

octane

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

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

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.

The article Cuda linked to is a good one. It does, however, make a statement that is easy to misconstrue.

It says "But most of all, we discovered that our presumption that higher-octane fuels burn slower than lower-octane fuels (and therefore require more ignition lead) is largely incorrect."

This statement is misleading. The presumption mentioned IS correct, not "largely incorrect". It is only incorrect to apply it to any fuel when comparing only the octane numbers.

What I mean by this (and what they go on to illustrate) is that within a given "category" of fuels, higher octane fuel will give a slower and less powerful burn. But you gotta stay within that "category" of fuels.

Pump gas: when comparing pump gas fuels, you gotta compare the fuels available within a single region, as different climates or local regulations will have a dramatic affect on what additive packages are in that base fuel. This affects the way it runs..

High octane gasoline: avgas, 100LL, etc: These are always going to be "purer" fuels than the crap we pour into the tank of the Accord at Mity-Mart. The purity of the distillates will create a better chemical platform for combustion and better performance can be realized from it.

Race fuels: Race gas is designed for optimal burn characteristics. Period. There are more than one producer of race fuels so their internal competition for marketshare dictates that they produce "better" fuels. You will always get better performance from race gas even at low octane ratings - it's bacause it is made from the more preferable volatiles than the junk ones.

Exotic or "enhanced" fuels: There are fuels that are designed to be "liquid horsepower". Exotic cocktails that add oxidizers and such to the fuel can offer a fairly dramatic performance benefit. For example, a 10:1 Vee Twin 1000cc Suzuki TL1000R showed 6 horsepower improvement (over its 117hp) by doing NOTHING except pouring Nutec into the tank. I mean no timing changes, no fuel changes (okay, so we DID bump the injector timing by a click or two, but earlier tests showed that we DIDN'T HAVE TO), just pour it in.

Hey! why don't we run that in the boats? Cause it's about $7/gallon.

Anyhow, the article does a god job, I simply felt that at the bottom it was a little vague on standing beside the results.

Also, note that timing changes accompanied every fuel change in the test in order to optimize the combustion event...
hope this helps.

Good info...I always thought 93 burns cleaner though. I have 1993 502's carb, mags(stock) and have always put 93 fuel in. Maybe I'll reconsider.

Great article on Octane
 

Great article Cuda - could have used it two weeks ago - got into an interesting discussion at the Marina re: octane and additives. A guy who keeps his boat at a Marina I frequent for petrol made the statement that adding 104+ to the marina 89 was a waste of time and money and that it did nothing. I tried to politely (well as politely as possible after a few beverages) point out that he was incorrect; however, nothing doing....

I have see a few articles re: the effects of booster and none have indicated anything other than positie results - in fact I think that a report came out of USC or UCLA which confirmed the effectiveness...

Dirt,

You didn't read the article very well, did you?

It showed a 1/2% (One HALF percent) increase in hp and torque on a 10.4:1 motor. It wouldn't show squat on a 8.5:1 motor. And that was with 87 octane, not 89.

Oh well, people seem convinced that booster is good stuff. Guess that's why every major brand sells a bunch of it.

your missing the point - while the octane boost may have not produced the HP, it did prevent detonation - I cannot always find premium fuel on the water and need to run 89 from time to time - at least with octane boost I will not risk detonation.

Best quote of the article:


Wusz said, "An engine does not know what the octane rating of the fuel is, unless it is too low"

Yes, dirt, I suppose I am missing your point entirely. And I guess I am still missing it.

If you have a motor that will run on 87 octane at 34-36 degrees ignition lead with no detonation, then there is no reason to run a higher octane fuel and no reason to run octane booster. I thought the article made that very very clear.

If, however, you are running a motor that requires 93 octane fuel to run with no detonation, and all you have available is 89 octane, then YES, you must run octane booster.

Otherwise, your friend was correct. No reason to use it - just a waste of money.

mcollinstn -

I completely agree with you - for the average boater, it will not help. Both my boat and the other boa in question have 525Sc's in them - thus requiring min 91 Octane - since there are only a few marinas that carry 92, we have two choices when we need gas and are not near the right marina - use octane boost or risk detonation. That was my only point.

Otherwise, I agree - no reason to use it.

Yep. You need the good stuff.

You may want to look into buying toluene in bulk. Toluene's octane rating is 114. It's around $2.75 per gallon if you buy in 30 gallon quantities.

2 gallons of toluene to 10 gallons of 87 octane (12 gallons total) will yield 91.5 octane. 3 gallons to 10 gallons yields 13 gallons of 93.2 octane.

1 gallon of toluene to 10 gallons of 89 octane (11 gallons total) will yield 91.3 octane. 2 gallons to 10 gallons yields 12 gallons of 93.1 octane.

Lots cheaper than a bottle of 104+. Takes up a little more room, though.