E85 timing curve?
 

Something I was wondering is what an e-85 timing curve looks like as compared to a gasoline curve. Would it be basically the same shape, just advanced more or would it be totally different from gasoline, like propane is?

I'm building a motor that would really like the 105 octane rating and even with the economy loss I'm still going to be money ahead of buying premium or race fuel. I already know what needs to happen with the carb and fuel system, timing is the only thing i'm still trying to figure out.

Justin

Well, theoretically, you have a few factors to think about:

1. Compression ratio. Typically the higher the compression, the quicker the burn. Thus, less timing here.

2. "Squish" or "quench". This will accelerate the burn, thus requiring less timing.

3. Fuel burn speed. This is what you're going to want to be concerned about, in your application. I've heard that ethanol burns faster than gasoline, thus requiring less timing.

4. Octane. Ethanol has a high octane rating, so detonation is not an issue. Thus, we can advance the timing to where the motor likes it best.

What we need to consider is that 99.9% of the information on timing curves is based on regular medium to low compression gasoline engines "converted" to use ethanol. The reason these were advanced is that the motor was tuned around detonation on gasoline, and the use of ethanol allowed the timing curve to be brought back to optimal for the engine.

An efficient high compression engine like you're building will probably not like a lot of timing. In fact, I'd say a regular gasoline type curve would probably be optimal. At any rate, put it on a dyno and tune it right. I'm going to guess about 36 degrees total, with all the advance in at about 3000rpm. Of course that's a wild guess; I dont know what you're building!

It's basically a stock rebuild on a '64 390 PI, but with a different cam. It's 10.5:1 and will make about 350hp by 5,000 rpm.

The point about people advancing the curve because the motor was originally desgined for gasoline is a good point that I hadn't considered. Thinking about it stock should get me pretty darn close. I'm not going to put the money into dyno tuning the motor because i don't have it, but i am going to track tune it which will give me a chance to figure it all out.


Justin

Keep in mind, that it probably wont ping even when over-advanced to the point of losing power. So you'll have to play with the timing to get it to where it produces the best power.

Also, 10.5:1 is still fairly "low compression" for E85.

Yeah, but it's on the verge of too high for premium. If i ran a more aggressive cam to bleed some more compression off i could get it safely within the pump gas range, but aggressive isn't what i'm after.

And i was figuring it probably wouldn't ping when over advanced, that's why I'm going to track tune it. get it setup to make the fastest runs, plus it'll let me try a step rich and lean on the jetting.

Justin