I was hunting through the forums for a good amount of time trying to find a thread with this subject, and to my surprise I couldn't find one. I'm pretty bad at finding threads, so if there is one already please point me to it.

I'm just interested in how much the optimal ignition advance for lower compression pistons (8.6:1) differ from motors with a more "normal" (9.4:1) ratio. I understand the dish on the lower compression piston head not only lowers the cylinder pressure, thus causing the flame front to travel slower, but it also puts a greater distance between the ignition spark and the piston surface. To adjust for these things, ignition timing should be more advanced, but what I don't know is how much more advanced it must be.

I'm running low compression pistons in my turbo'd 1990 1.6, and I'm tuning it on the street, so naturally I'm inclined to err on the conservative and safe side, which I'm fine with. I just want to know if I'm missing out on some efficiency. I set up the table based on some I've seen online, and set it to have a linear-ish decline into higher boosts, but the power gains from 0-7 psi feel a lot greater than the gains between 10-15. I feel inclined to pick it up a little more, but I'd like some second and third opinions before I do (or don't do) anything.

 

Forgot to mention I'm running on 93

 

On 100RON pump gas, I'm running ~20* at 220kpa and 16* at 290kPa. Figures at max revs (7500rpm).

 

Generally speaking, I believe you can get about two degrees more timing with 8.6 vs. 9.4 but highly recommend electronic det cans for tuning boost above 10psi. There are so many variables even in the same generation of engine once they have been rebuilt or if they have any miles on them.

 

Another general question that has been on my mind for some time. We know that a larger turbo (or any EFR) makes much more power for a given boost. Wouldn't that mean that the advance a some given boost would need to be lower than the same boost level with a small turbo.

In other words, would not the power level be more the determining factor than the MAP? More Power being due to more Air and Fuel?

Or is that too simplistic?

 

A larger turbo at the same horsepower will generally create less Heat and has a much less restrictive turbine housing Etc. I haven't thought too much about how that would affect timing at a particular pressure.

 

I wonder if the larger less restrictive turbine is offset by the fact the engine is making more power meaning they both cancel each other out in a sense. This is interesting to think about. I would think that with less heat and less backpressure even with more power you could have more timing because the engine is less knock prone, but thats a total guess.

 

1 word: FMIC

 

1 Word: Iliketopoopalot

 

Boost is a measure of backpressure through the engine and therefore inefficiency. Lowering the Restriction of the turbine housing and turbine wheel will usually reduce that back pressure.