I understand that for the fuel curve you want to achieve 128 BLMs across the entire RPM range for max power. But what about the spark curve? Do you want as much advance as possible without retarding? Any help would be greatly appreciated, as I am a newbie at this. Thanks.

 

I will get the "ball rolling" on this one. While virtually EVERY hot rod magazine will say "The most spark advance without having detonation occur", the proper answer is "The most power with the least amount of spark advance and no detonation occurring".

 

I understand that for the fuel curve you want to achieve 128 BLMs across the entire RPM range for max power.

Max power and 128 BLM's are mutually exclusive. You want 128 BLM's to allow for highway mode in open loop and to reduce tip in tuning. The closer the fuel is to right the easier tip in tuning typically is.

But what about the spark curve? Do you want as much advance as possible without retarding?

No you actually want the least advance that makes the most power. For part throttle you want the advance value that allows min TPS for a given load. This is making the most out of the smallest amount of air and fuel to do the job. For WOT you want the value that gives max power and performance. More is not always better.

HTH

John

 

Now where have I heard that before?.
Hehehehe

 

Thanks for the replies guys.
I was wondering if you could explain this a little more. I don't really understand. Anything else I should know about fuel/spark curves before I tinker with them? I don't want to screw something up severely. Any help would be greatly appreciated. Thanks in advance.

 

Ok, I'll give this a shot. What he's saying about the BLM's is that that is for cruise mode and max fuel efficiency, not max power. For max power at WOT you want the car to run a richer than at cruise mode for sure. With EFI you can run pretty close to stoichiometric at cruise (14.7:1) sometimes even a little leaner, but at WOT your gonna want to run closer to 13:1, maybe even a little richer. Most tuners like to go just a little richer as well to be on the safe side and avoid detonation.

As far as the spark curve well, that's a lot of fun as well, at cruise mode and light throttle you'll actually want more advance than at WOT due to less efficient cylinder filling (therefore less cylinder pressure) and less load. Remember detonation sensitivity goes up with cylinder pressure and load. An example of this is you can set the timing as far as 20-30* advanced at idle and the car will idle great, soon as you hit the gas your gonna get detonation even before the advance starts to kick in. Also more advance does not automatically mean more power. The best advance is determined by the time it takes to burn the mixture (and of course detonation). Optimally you want the timing to be set to completely burn teh mixture in just about the time it takes for the cylinder to get through its travel to the bottom of the bore. This isn't exactly right due to the nature of the 4 cycle internal combustion chamber, but gives a good starting point. Leaving out detonation if you set the advance to early you are pushing against the piston as it comes up, and burning out the mixture before it's pushed the piston all the way down, costing power. If you retard it to much your not burning it all before you get to the bottom of the stroke and pushing against it as it starts to come up for the exhaust stroke, once again costing power. This is why fast burn chambers require less advance than others. I saw one test with fast burns that best power was at 30* rather than the accepted 36. There's also an interesting chart I saw that showed a comparison of increased power due to advance and increased heat. After a certain point (different for each combo of course) power increased drop down, but heat keeps climbing, so all the sudden your seeing a 30* engine temp increase for only a 2hp gain, and probably losing the hp back to increased intake temps and detonation.

Don't forget that tuning for cruise mode and light throttle is very different from WOT, and that different cars like different combinations so theory is great but nothing beats test and tune for getting it dialed in just right.

Hope this clears it up for you a little more. You'll have to talk to someone a little more knowledgable than me about the actual programming, I'm just learning that myself.

 

What would be the best way in your guys opinion then to go about testing for max power then...which would you prefer 1/4mile testing or dyno or a mixture of both and why.

 

road testing, dyno time isn't that cheap and dyno's don't accurately simulate road conditions, though they can get you close it won't be right on til you do some drive and tune sessions.

 

Something I'm doing for performance testing is to use CM's software (which records data such as elapsed time, engine rpm, road speed) and making 3rd gear runs. I start ~1700 rpm in 3rd gear which is something like 25 mph. I make sure I hit WOT before the tach reads 2000 rpm and hold it open until the engine revs to ~ 4600 rpm, which is just over 75 mph. Leaving it in a singlegear removes the shifting variable. It's then easy to look at the logged data and calculate time from 30-75 mph. This represents real world "performance" to me. There can be slight variations due to sampling frequency but it's something to think about. I'm playing with timing advance now, trying to eliminate some knock and so far have picked-up .2-.3 sec. improvement.

With an A/T you could probably do a 0-60 run using a G-Tech but then wheelspin is an issue.