Engine temperature, detonation, and timing

[Fastmax32168]

I have posted before about my LT1 383 that I built when I had the ability to buy race fuel locally and no longer can. It really has too much compression now for optimum performance on available premium fuel. I have spent a lot of time trying to optimize the timing at this octane level, but I am still down a bunch of HP vs running it on race fuel. One thing I have not tried yet is lowering the operating temp of the engine. It runs around 200 degrees with the factory thermostat, it seems to have plenty of cooling capacity so I think if I put a 180 thermostat in it the normal operating temp would drop down into that range. So my question is, If I lower the operating temp of the motor 20 degrees, is it going to let me get away with more ignition timing before I experience detonation? I can find plenty to read about how timing affects engine temperature, but I have not been able to find anything about how engine temperature affects the timing detonation threshold.
Thanks,
Roy

[In-Tech]

Hiya,
In the high hp boat stuff I do, we generally try for 120 degree cylinder head temp on 91 octane at well over 1200hp and they run at WOT for hours. The car stuff does like ~180 degree cyl head temp better than over 200f. The octane requirement increases with temperature and time. I know that's not a helpful answer, you will have to test and see.

[NomakeWan]

Detonation is caused by uneven combustion of fuel, where pockets of fuel in the combustion chamber ignite far more rapidly and violently than the intended flame front from the spark plug. This can be caused by an increase in cylinder pressure, an increase in cylinder temperature, or both. As you lower the knock threshold of your fuel, the chance of running into knock increases (because lower octane fuels burn faster, meaning there's less headroom for unintended burn rates).

If you remember your high school chemistry, PV = nRT. Volume doesn't change here because your engine is mechanically set in stone, and R doesn't change either since it's a constant. n could potentially change (you could alter your air induction or fuel delivery) but for now we'll keep that constant as a demonstration for your question about engine temperature. That leaves P and T, Pressure and Temperature. If you increase cylinder temperature, then all other things being equal and constant, cylinder pressure must increase. Once cylinder pressure and temperature are over the knock threshold, you'll experience knock.

So to keep knock at bay, you need to reduce cylinder temperature (keep head temps down), and/or reduce cylinder pressure (reduce ignition timing). If however you find that these two things are not adjustable enough to keep knock at bay reliably, then you're left with only two remaining options. First, you can try to alter the compression ratio of your engine to reduce your octane requirements (thicker head gasket?). This, again, is a way to further reduce cylinder pressure. Second, if available, you could switch to a fuel like E85. It provides the same anti-knock properties as your race fuel, but it may be more available near you (I know it is for me; there are tens of stations supplying E85 nearby and only one supplying 100 octane race gas). E85 however has roughly 30% higher fuel consumption, so you would have to account for that with your fuel system (fuel pump, fuel injectors) and your tune.

EDIT: Oh hey, this forum actually quite literally has a breakdown that goes into detail even more. If you're curious: http://www.gearhead-efi.com/Fuel-Inj...d-Pre-Ignition

Very well-written article, highly recommended.

[sanderson231]

Not how short of octane you are but can get 2 number of octane with an MMT containing additive for less than a dollar gallon of treated gas

[LeMarky Dissod]

I have posted before about my LT1 383 that I built when I had the ability to buy race fuel locally and no longer can.
It really has too much compression now for optimum performance on available premium fuel.
I have spent a lot of time trying to optimize the timing at this octane level, but I am still down a bunch of HP vs running it on race fuel.
One thing I have not tried yet is lowering the operating temp of the engine.
It runs around 200 degrees with the factory thermostat, it seems to have plenty of cooling capacity so I think if I put a 180 thermostat in it the normal operating temp would drop down into that range.
So my question is, If I lower the operating temp of the motor 20 degrees, is it going to let me get away with more ignition timing before I experience detonation?
I can find plenty to read about how timing affects engine temperature, but I have not been able to find anything about how engine temperature affects the timing detonation threshold.

No insult intended by any of the following.

More than 11.0to1?

OE LT1 thermostat is 180F. There's a 160F thermostat available.
My LT1 experiences (sedan & wagon) with 160F tstats did not yield much more power, and cost 2-4MpG.

Are your motor oil & ATF coolers integral with the radiator?
Separately external motor oil & ATF coolers will let the engine run cooler, especially when the powertrain is working harder.

What did get a lil more power AND better MpGs was using an adjustable fuel pressure regulator to up the fuel pressure from 3bar / 43.5psi to 4bar / 58psi.
Raised the injector constant, engine felt younger and crisper.

[NomakeWan]

Not how short of octane you are but can get 2 number of octane with an MMT containing additive for less than a dollar gallon of treated gas

Ah, true. I forgot about this. We actually use this in our Skyline GT-R for race days to get headroom. She'll run 91 octane just fine bipping around, but technically the motor was designed to run 93 at an absolute minimum. So we just dump Boostane Premium into the tank after filling up with 91 near the track. Half the bottle gets us to around 95 octane in the 20 gallon tank. Bottle costs $20, so that's $10 added to the cost of the tank, or 50 cents per gallon.

So yeah, that too is a way to do it if you don't mind using additives.

[1project2many]

I use MEK in the Sunbird. Locally cheap. And I mix it with a touch of two stroke oil. It keeps the knock sensor happy and so far it still has the same head gasket as when I bought it in... 2001?

The article referenced earlier is definitely good.

Reducing coolant temp can help in a couple of ways. If detonation is ultimately a function of combustion chamber temperature then reducing the intake air temp can potentially lower the mixture temp at the start of combustion. Cooler engines generally mean cooler intake air. Reducing coolant temp can also decrease the temperature of the thermal boundary layer of gas that forms close to chamber during combustion. Efficency improvements are often found when increasing the boundary layer temperature so expect increased fuel usage by switching to a cooler thermostat.

One of the oldest tricks in the book when facing detonation is to deliver too much fuel to the cylinder. Excess fuel slows combustion and greatly cools the mixture. Tradeoff? You won't generally reach the same power level. But it can keep the engine alive. This is how GM kept their non-intercooled turbo cars alive during the '80s.

If you can stand the thought of pulling the intake then you could use thermal barrier coatings to reduce heat transfer into the manifold. Clean and coat the entire manifold including gasket mating surfaces. Remember, cooler mixture = lower risk of detonation. If you can pull the heads you can add thermal barrier coatings to the combustion chambers and valve combustion faces. TBC's work to significantly reduce heat transfer out of the combustion chamber and at the same time can prevent hot spots from developing. I have used TBC's in a number of applications and usually see consistent results.

Is there an option to machine the piston tops to reduce compression ratio? If you have the time and resources it may be the best way to go.

[Fastmax32168]

So it sounds like the answer is...maybe. The motor is 12.8 to 1 and I have been down the additive road and done extensive tuning on it. Currently have a snow performance methanol injection system on it. It is all with limited success. Nothing restores the allowable advance and power to the level that race gas does. There are obvious solutions if I rip it back out and take it apart but would rather not. Thanks everyone for the input.

[steveo]

you will lose power over race gas no matter what you do, but you'll lose less by doing it properly and bringing compression back to a sane level. there's a reason nobody really likes to build very high compression but very low octane engines

[sanderson231]

I browsed internet concerning unleaded AVGAS and it looks like 100 octane is just starting to come to market. The octane tests are different for AVGAS and MOGAS. One source said 100 octane AVGAS is about 105 R+M/2 rating. R+M/2 is what is displayed at the pump.

Maybe convert to E85? It's about 105 octane.

10% toluene will increase octane about 1.5 numbers. Toluene is already in gasoline. MEK is methyl ethyl ketone. MEK is not found in retail gasoline. It is a strong solvent similar to lacquer thinner. So I'd have some concerns with MEK as an octane booster.

[NomakeWan]

So it sounds like the answer is...maybe. The motor is 12.8 to 1 and I have been down the additive road and done extensive tuning on it. Currently have a snow performance methanol injection system on it. It is all with limited success. Nothing restores the allowable advance and power to the level that race gas does. There are obvious solutions if I rip it back out and take it apart but would rather not. Thanks everyone for the input.

If your goal is having the engine run the same way it does on race gas but not on race gas, you're fighting a losing battle.

At that point, your only real alternative is E85.

[Fast355]

If your goal is having the engine run the same way it does on race gas but not on race gas, you're fighting a losing battle.

At that point, your only real alternative is E85.

I would use E85 myself. My high compression 383 in my Express van loves the stuff. Responds to it like a small shot of nitrous.

[sanderson231]

I checked on availability of E85 here in central Texas and was surprised at how many stations carried it.

[Fast355]

I checked on availability of E85 here in central Texas and was surprised at how many stations carried it.

There are many stations across Texas that sell it. My 97 Van runs on a P59 with a flex fuel composition sensor. Varying alcohol percentage is not an issue for my setup.