Timing Strategies

[twg]

Hey Guys,
I've started to look at my current maps and decided to start tweaking them...

My current tune was originally for 91 octane, but since I have access to 94 octane, I want to take advantage of that (I've been filling up with 94 octane anyway).

As per other posts, I've got essentially a USDM stage 2 STI setup (engine-wise, it's really a FXT, but has all the sti components, except for headers).

Specifics for those that don't know: 2.5L USDM STI block, VF39, mostly catless (except for stock 3rd cat, which isn't too much of a bottleneck as per logs)

Boost wise, I'm boosting up to 17-17.5psi in 3rd, and hitting 18 in 4th...

I think the 91 vs 94 octane really is in the timing... I think the timing I am currently using is very conservative, I've seen other stage 2 with prob about 3-4 more degrees... however, I also understand that timing that provides maximum cylinder pressures isn't necessarily just before knock... ie. if 30 BTDC causes knock, 29 BTDC doesn't necessarily provide the most power, it could be in fact 20 BTDC... some say that there really isn't any formula, it depends on the engine and timing is best tuned on a dyno...

Anyone care to comment on this ? How should I go about changing the ignition timing to accomodate 94 octane vs 91 octane ?

thanks,
Wayne

[swifty]

Wayne
I think you're referring to mbt or minimum best timing. This is the least amount of timing required to give max power, and any more will either not make more power or even start to lose power.

You are unlikely to get close to mbt using pump gas as it's resistance to detonation is not high enough to allow it. Generally you will only be able to run to the edge of detonation then retard a couple of degrees with pump fuel.

The power you will get from you engine will be as a result of a combination of fuelling, timing and boost. You may get more power from running a little less boost and a degree or 2 more advance, maybe a couple of points leaner on the fuelling. Every engine is different though so you have to find the best combination for your engine.

Running higher octane fuel usually allows more timing, or maybe a bit more boost, and maybe a slightly leaner afr depending on what you are running to start with.

I think I'd start by making sure all my fuel was 94 and that the 91 stuff had all been used up. Then I would log to see what you have now. Start tuning by checking the afr, then look at the boost and try adding timing a degree or 2 at a time.

[secretpickle]

You could always add water/meth injection then you can increase your timing a lot more. Maybe even to mbt.