Timing Retard

During the time in first gear prior to clutch lockup the engine is really in a "unloaded" state so the dynamic compression ratio is fairly low and because of this the fuel mixture appears rich. Right at the clutch lockup point the engine is now under extreme load and the dynamic cylinder pressure increases sharply and the fuel mixture will appear to leans out. Because of this change in engine compression ratio and lean out the retard is required to prevent detonation. The ignition advance prior to the clutch lockup is to make the engine more responsive and help compensate for the rich mixture.

 

good post mike. in tafc applications, some are advancing the timing through low, some are taking it out at the top of low to prevent tireshake, some are retarding it in high gear.

i've tried advancing the timing in low, then kicking it back. its not my favorite, and be careful!

 

My understanding is for the basic tuning once that clutch has locked up you need to retard and keep it there to prevent detonation. Like Will posted there are more advanced tuning methods to use the timing but without knowing your car they would be hard to prescribe.

 

Once the clutch stops slipping the motor is put under tremendous load and the force required to push the piston down increases so the expanding gases build up more pressure. Don't confuse this with rpm increase because you are looking at cylinder performance one firing at a time and not over a range of firings. The flame expansion in the chamber actually changes shape from being an eclipse shape in the vertical to being a dougnut shape in the horizontal plane and now pushes up against the hemi shape chamber and then pushes back down evenly from all sides which is what actually gives the hemi head the advantage.

 

If the motor does not suddenly RPM when the clutch locks up I would suggest that you fuel mixture is probably too rich to start with. Normally a engine pretty well flat lines while the clutch is slipping then when it locks up the RPM should suddenly start climbing.

 

Bob, I would have to say yes but that may have a little guess factor in that answer. It would seem to me that the motor beneath that blower doesn't care what is making boost. The big difference between a Roots type blower and a PSI is that the Roots blower heats the incoming charge of air higher than the PSI and because of this the volumetric efficiency of the roots motor is reduced at higher RPM so one has to pull out fuel whereas the PSI has a cool charge so one pours in more fuel at RPM. After all that then theoretically the AFR with a Roots blower and a PSI blower is the same but the PSI has a cooler charge of air so it is less prone to detonation but it makes more boost which would require less timing. Actually in the end I believe the timing requirements just happen to work out close to the same.

 

i dont know what kind of combo you're running, but those timing numbers seem way high!! i don't have a lot of experience with veney stuff, but that's wayyyy too much timing for a brad motor...if you're running brad start at 25 and work your way up watching bearing squash.

it's been my experience static timing works best on a blown clutch car. fuel management seems to work best for managing power. these cars aren't that smart. you can run real good with a main jet and a pump loop. a six shooter could be useful for controlling tireshake at top of low if you don't like changing trans ratios for track conditions.

like i said these cars aren't that smart, but often times they are smarter than us....

 

Well, if the tire shake is caused because the motor is down on power then I find it hard to believe that pulling out timing would be the way to go. If you pull out timing then the horsepower is going to go down even more. Yep, that may stop tire shake but it will slow you down but if that is the way you want to go. I have always considered tireshake in the top of 1st gear to be caused by the fuel mixture being wrong due to the clutch locking up loading the motor, the motor going up in rpm but the fuel mixture is off so it cannot support the power the motor wants to make so here comes tire shake.

Being too tight on the clutch will cause tire shake if it locks up at too low of an RPM so the screw blower is not high enough to make boost so no power. From what I have seen a loose clutch appears to be best for a PSI/Whipple type screw blower. Some tuners never lock their clutches up during the entire run thus keeping themotor ahead of the clutch.

 

the only way pulling timing out at the top of low to cure tireshake would be if you were shaking due to too much wheelspeed. if your car is shaking due to falling off the tire, thats obvious that would be the wrong way to go. typically at divisional events you see a decent to good starting line that goes to hell in a hurry, typically where the top of low is. if it is breaking the tire loose there you could knock some timing out at that point to calm it down through that transition. if the car is indeed falling off the tire, then it would clearly need more power, clutch ratio, something rather than taking power out.

remember there are about a million different reasons for tireshake. if you got something screwed up, tireshake is usually the result. my old man said garlits used to have all the reasons for tireshake on his trailer door...too aggressive, not aggressive enough, good track, bad track, too much clutch, not enough clutch.....

the key to finding out the culprit is watching your wheelspeed on your computer. you can get carried away looking at all the other data, but the bottom line is wheelspeed in the right places and interpreting what the causes of problems are. remember wheelspeed is whats actually happening on the track...