www.RBFORUMS.org

Pulled this from the other thread, it's a good read concerning timing and how air flow affects it. It was written by enthaply in a FA post.


"Alright...I am hesitant to post all this in a public forum as it is alot of knowledge learned form hard work and expensive mistakes. these are some of my personal tricks, but since there is apparently a need for the info to sort out confusion i will post it.

timing should pretty much level out over 3500 rpm, like this

for 0 boost:

500 RPM - 20 deg
1000 - 20
1500 - 22
2000 - 25
2500 - 27
3000 - 30
3500 - 32
4000 - 32
4500 - 32
.
.
.
7500 - 32

thats a good baseline to start with...rememeber though thats 0 boost...not full vacuum. the reason for having the timing increase as you get toward 3500 is that the actual time that the combustion has to take place gets relatively long at lower RPMs. so the igntion advane should be less. the burn time of the a/f mix is a relatively fixed time for a given intake pressure. so if the time of the compression stroke is getting longer due to low RPM then you want the ignition advance to be less so that you can reach peak cylinder pressures at the appropriate time (15-20 Deg AFTER TDC).

when i build my timing maps i keep 20 as my minimum timing all the way down to full vacuum but the 3500rpm value goes up to 40 or 42 deg. so it looks like this:

500 RPM - 20 deg
1000 - 20
1500 - 24
2000 - 28
2500 - 32
3000 - 36
3500 - 40
4000 - 40
4500 - 40
.
.
6500 - 40
7000 - 36
7500 - 34

the only weird change is that I like to roll off the timing in the high rpm high vacuum areas...just as a safety measure. when you have timing increasing as vaccum increases you get the equivalent of a vacuum advance distributor. very good for part throttle driving. vacuum advance is necessary for becasue the air / fuel mix is at a much LOWER initial energy state when it is pulled in at a high vaccum. all the molicules are pulled farther away from each other and thus your burn time takes longer. so you need to start the burn sooner to reach peak pressure at the correct time.

as the car comes into boost you will have boost retard...so say for an example of 1.0 kg/cm2 (14.7 psi) with .75 deg/lb retard you will need to take away exactly 11 degrees of timing. so your timing map at 14.7 psi should look like this

500 RPM - 9 deg
1000 - 9
1500 - 11
2000 - 14
2500 - 16
3000 - 19
3500 - 21
4000 - 21
4500 - 21
.
.
.
7500 - 21

the timing retards under boost for the exact opposite reason that it advances under vacuum. witht he af mix coming in at a much HIGHER energy state due to pressure you get a much faster burn rate. so you need to fire the mix later (less advance) to get peak pressure at the correct time. you can see that i would pull the timing out from everywhere. even in places that you might not ever see boost. it's more of a safeguard than anything. also it make s for very linear maps, and ones that are easy to view if you have 3-d map viewing in your EMS software (haltech e-11).

one of the things that people miss most about making basemaps and tuning cars is the linearity of the system. if you have big bumps or changes in your maps you are doing something wrong. the engine is a remarkably linear device. evene when you include turbocharging...values should always be smooth for both fuel and ignition. "