Compression And Fuel

[Jay Smith]

Mitch , contrary to most belief 87 octane burns much hotter and faster than higher octane fuels , thats the reason pistons light up on the exhaust side and detonate , over octane of a 2 cycle as I’ve posted before WILL kill power because of the interrupted cuts in the bore as the slower burning fuel is “lit” the slower burning fuel if the motor doesn’t have allot of compression it will not be “used” up in its entirety and some of the burn escapes out the exhaust port , I’ve found most of the motors I build make the best safe HP about 1-2 points above detonation , my race people also use the weather ( adjusted altitude) to their tuning advantage , I suggest to my serious racers to tune using a Kestrel 4000 to tell them what the air is doing to get the proper mix of octane to make max HP at each site they are racing at , I’ve had several racers learn that 112 octane will kill a motor with 200 lbs compression and they find after checking the air a mix of 3 gals of 110 to 2 gallons of 91 works great and they win using that base mix , I know this is a long post but running a fast , hot burning fuel ( 87 octane ) with elevated timing in high altitude with low humidity is just a matter of time it’s gonna catch up with whom ever gets comfortable ... JMO

Jay

[Motv18]

Is that a direct injection, that throws normal octane rule out the window. Since the gas can’t knock if it’s not in the cylinder till injected.

[mitch184]

I understand why lower burns faster and hotter and how too much octane is bad. Hence why I'm kind of trying to learn more about why theres blanket guidelines for higher octane.

So say sea level, colder air temp, higher humidity. Air is denser but should lean out the mixture a little. So does the density cause a slower burn or the leaner mixture cause a faster burn?

[Jay Smith]

Mitch ,
The hotter the ambient temps and higher the humidity is makes incoming air dense -thick with lots of water grain numbers so when this occurs trying to mix thick dense air with the same fuel curve makes a motor fat or rich ( Lazy ) in a racing situation if you want to run at peak you need to read the air , confirm adjusted altitude numbers and lean or richer fuel mixture for peak power , when air is cool and humidity low water grain numbers will be low motor will demand and can utilize MORE fuel to run at peak ...

No it not a direct injected motor , and YES it can and will melt the top of a piston clean off if it’s too lean , go to the bank !!!

Just what I’ve learned over 25 years of racing V-6 Mercury’s ,

Jay @ JSRE

[Motv18]

I’ll go out on a limb and say get that compression gauge checked, if 150psi is running 87 with no knock.

Fat rich and retarded timing would keep it from knocking

if you run 91+ with the same setting it should be down on power.

If the 87 is a mtbe fuel it will be fat but burn or piston read correct. Depending on the amount of mtbe sometimes you can be 2 jets up. If you run non e or mtbe fuel through it it will be fat and slow.

Mtbe is rare to find but but it’s still around in some states. And some of the can race fuels use it.

Thats is all speculation on my part for the hypothetical mr, Mitch

[mitch184]

Motv, double check with another gauge is its accurate. Ironically the motor is a tiny bit lean right now, so I'm going up one jet size. Especially since its gonna get colder soon. But plugs show absolutely no sign of detonation at all. I heard an audible knock once before on a hard holeshot.

This motor is an out board jet so I'm thinking maybe between that and the porting, recuded quench, its able to run on the 87.

[Motv18]

Not a 2 stroke but the gm 6.2 is a 10:1 which equals about 140psi in a 4 stoke and their fine if you put pump 87 in it but recommend e85 or 91, being a car though it has all the gizmos knock sensors o2 Ect. The gen 3 di is 13:1 over 160psi and you can still put 87 in it.

You might find it interesting that a lot of blends even some race gas have motor/mon of less then 90 and research/Ron that’s high over 100.

Oh, if it’s oxidized it will run and read perfect or lean, when you use non oxidized is when it would show rich. Provided that it is an oxidized 87. I’m just guessing from epa regulations on road gas that if it’s not alcohol it’s mtbe or the state allows like South Dakota non oxidized 87 in road stations.

It is a neet debate over whats is actually needed vs recommended

im going on a limb and say a dyno might show 89-91 tuned right give more power. Or better egt or whatever reason.

It it could be the temp with compression formula showing theory temp over the average auto ignition temp.

Heh you could be running like a diesel only it’s in time and not knocking to far ahead. Heheh does it run without the spark hooked up.

[mitch184]

Whoa.... that just took it to another level. I'm just buying Chevron shell 76. Not sure if its oxidized or what not???

[Merc 2.5]

Whoa.... that just took it to another level. I'm just buying Chevron shell 76. Not sure if its oxidized or what not???

Scream and fly takes o/b to new levels haha, it's all useful info

[rude tim]

There is a lot of generalizations stated here. There is so much more to compression, timing and octane. Head design will change things big time, squish band area percentage and angle, sharp edge at the end of the squish band or a radius not to mention the angle difference between the piston and the squish band all play a part in compression octane and timing in a two stroke engine. I run 145 pounds of compression on my 4 liter V8 erude with 18 degrees advance on a heavy boat on 91 octane with no detonation issues and this engine makes over 400 hp. On my 4.4 liter V8 I ran 155 pounds of compression with 18 degrees advance on 91 with no issues either. In saying that my head design is far from stock and the V8 seems to be able to handle more compression than a V6 erude.