I'm a new bee. Working on a $6E. I'm modifing the main Spark Advance verse LV8 table when I noticed a PE spark advance table as well. Why have both. Why not zero out the PE spark advance table and update the main spark table to reduce timing as load increase. Would this not cover WOT or PE mode via teh high end LV8 numbers?

 

It has to do with the commanded AFR. In closed loop the commanded AFR is around 14.7:1. In PE mode the commanded AFR is richer, for more power. Along with that is the requirement for more SA (typically).

RBob.

 

RBob, I understand the need to richen up the mixture to acheive max performance. But what I don't understand is why add timing on WOT (PE) over and above the main timing table.

I'm an old CARB guy where Initial Timing + mechanical timing+ Vacuum timing = TOTAL timing. When you increase RPM without increasing LOAD, Timing increased via the mechnical distributor. When you increased load, vacuum timing is pulled away. So in my mind as LV8 increases to the 190- 200 range the timing valves in those verticals should be set to support the load. Yo should not hit those LV8 values unless your under heavy load (ie WOT).
Yet GM cut the timing back in those areas and then adds it back using the PE timing table. Confusing to an old carb guy.

 

High LV8 can be reached with part throttle (14.7 closed loop), or with just a little more throttle (12.X PE). That (roughly) same LV8 can have 2 different A/F's based on throttle position, so it has correspondingly 2 different spark advances.

 

so LV8 is stead load, not load under acceleration.

 

LV8 is ANY load. It's very similar to MAP, or VE actually. It's basically how full the cylinder is. At light throttle (load), LV8 is low. At high throttle (load/acceleration/whatever), LV8 is high.

 

Then I'm back to my orginal question. Why does GM take spark timing away as LV8 increases in the main spark table, only to add it back in when in PE mode? The PE spark advance/RPM table adds 9.14 dergees at 3200 and tapers it down to 1.05 at 4800.

 

I think it's used by GM to induce knock and ride on the knock sensor. That's ARAP - for aluminum heads. I'd never use ARAP's spark for a starter bin. Look at AUJM for a good iron head bin. It's actually newer than ARAP.

 

It might be easier to think of it in a non-performance application like a 1-ton pulling a tractor up a steep hill. Lots of throttle but not enough for PE and a high LV8. You must remove spark to try and keep it out of detonation. Step on the gas a bit more and hit PE....more fuel and spark to give the old rig a little kick. I think GM put all kinds of driver's in these things to test them.

I rode with someone once pulling a camper trailer that would ping the heck out the engine on every hill. I told them to floor it.....the truck stopped pinging and the coolant temp. went down. They didn't get it. Thought it should have gotten worse by flooring it. The extra fuel cooled the cyls, was more detonation resistant and could handle the extra spark to make more power.
Does that help?