I have a question.
Everyone has a rule of thumb for hp/point of compression and hp/1psi... but what about timing?

Let's say everything is as perfect as you could imagine. How much would each degree of timing be worth? 93 octane? C16? What about something as strong as alcohol?

Would it be safe to say that the heavier the fuel- the more hp/degree you can get, or is there a universal formula, no matter what fuel you use?

Thanks!

 

Engine timing is a funny thing. Too much and you lose hp, too little and you lose hp. And each engine is different in what it likes and dislikes. Kinda like a woman, ain't it?

 

Rich? Injuneer? Lonnie? Anyone?

 

Steve: optimum timing is best determined empirically, as it really does differ from combo to combo and will also depend on the fuel used as well as the test conditions. But timing on a blower car is often a compromise in that the optimum timing for power may lead to detonation.

You sometimes hear rules of thumb like xx hp/degree, but don't believe them. The further you are away from optimal, the more hp/degree you will see. As you approach the optimum, the power is usually pretty insensitive to timing. We were testing mine on Monday. Max hp was at 26 degrees. There was a 15hp difference between 24 and 26 degrees, and 22-24 degrees was worth 25hp. But advancing it to 28 degrees made no difference and going to 30 caused ~10hp loss.

Regardless of detonation issues, blower cars seem to need less timing than a similar NA motor. I am not sure why, but suspect it has to do with more efficient combustion from all the swirl and turbulence induced by the high charge density.

Alcohol burns slowly and requires more advance than gasoline. In general, but not always, higher octane fuel burns more slowly and requires more advance. And in a blower car, higher octane may allow you get to optimum advance due to the higher octane. Note that octane and burn rate are not the same thing, though they are related.

Another fuel related issue for blower cars is density. Density of fuels can vary a lot, by as much as 10% between different blends. A 10% difference is huge - if you tuned with a very dense fuel for a nice, safe AF ratio of 12.5:1 and then ran with a fuel that was 10% less dense, you would now be close to 14:1, way too lean! This is one of the reasons that stock vehicle calibrations are nearly always a bit richer than optimal. Of course, in closed loop the ECU will compensate. So, you should always run the same fuel you have tested with, or have different tunes for different fuels.

Rich Krause

 

You could also run a retard box to turn back the timing under boost if you are having problems with detonation with different fuels since the stock PCM probably won't compensate enough.

 

Due to increased volume in the combustion chamber as well as increased cylinder pressure you should reduce timing approximately 1 deg for every 75 hp over the best power normally aspirated timing. Obviously you need to know the N/A best power timing to do this.

This being the case & also heavily influenced by combustion chamber desgn, only actual testing will give you the correct answer.

The typical LT1 likes approximately 32deg N/A.

Add approx. 400hp with a charger like Rich did & you need to roughly take 5-6deg out of the best N/A numbers.

A general guideline would be to take your N/A motor power, guestimate the power increase (15# boost = 100%, 10# = 66%, 5#=33% etc.). Divide the power increase by 75 to get your approximate amount of retard.

NOTE! only use this as a "maximum" giudeline... do not start with these values.

Start with at least 4 deg less than this value at high boost. Work toward the high timing numbers slowly.
Obviously detonation is the most limiting factor on a street engine w/ pump fuel.

If you see any detonation before you get to the desired numbers, you likely need better fuel for best power. If you exceed these numbers without detonation, power will likely stabilize or drop off. Run the least timing that you need to make best power.

Excessively retarded timing also adds a lot of heat to the exhaust valve so be careful. Generally do not go below 20 deg timing.

I generally start with 32deg at 0 boost & taper off to 22 at max boost for initial tuning. From there I slowly add timing & watch the HP & also watch for any knock.

My car likes 26 as well. Lower compression often wants more & high compression generally less.

The best advice I can give is to start cautiously & SLOWLY add timing. The more power you make, the less margin for error there is. 2 degrees can be the difference between no detonation & changing the head gaskets (or worse).

 

Het Lonnie: very informative post. Thanks.

Rich Krause