'87 vette, $32B, ABTC (stock bin) -

I am in the process of modifying the stock ABTC bin. I am using TunerPro and looking at the "Spark advance vs. RPM vs. Load" table. I have a few questions regarding load and spark advance:

If I am understanding what I've read so far, load is a measurement of how hard the engine is working, or the mass of air being burned at a given time. I also understand (I think) that as a general rule of thumb, as load increases, spark advance should decrease.

So why would spark advance in the ABTC bin, at lets say 3200 rpm, start out at 41 degrees at the lightest load of 32, increase to 46 degrees at a higher load of 80, THEN continue to steadily decrease from there to 24 degrees at a load of 208?

This happens across all RPM ranges at a load of 80. What does the load number "80" signify?

I want to understand this relationship. Also, my cam manufacturer recommended no more than 42 degrees total advance, so I'm assuming I need to modify this table. Should I bring down just those cells that are over 42 degrees, or should I globally reduce timing to hold the same relationships across the table?

(Engine is a Mini Ram 383, 58mm throttle body, 30 lbs injectors, AFR 195 heads, 240/240 @.050, .580 lift, 112 lsa).

Thanks very much!
Dan

 

1) Load, is air consumption vs RPM, a search for LV8 will lead you to where RBob explains it in detail, if you want all the info about what it means, and the math behind it.

2) For a given RPM, yes.

3) Might be part if what they had to do to meet some EPA thingy....

4) 80, is an LV8 figure. Over 80 is generally when you start actually loading the motor down.

5) There's a max timing entry somewhere in the code. And 42 is more along the lines of old school or some head with huge combustion chambers. Personally, I like running the LEAST amount of timing consistant with max performance. That's a whole lot different the running the max timing possible.

 

The lowest load region of the table is almost exclusively for decelleration. In manual tranny .bin's, it is common to see the timing retarded slightly from maximum in this area to help slow the vehicle down from engine compression braking without putting too much load on the engines bearings. I could also be way off too.

 

Grumpy / HaulnA$$ -

Thank you very much! I like that approach to timing, and the table makes more sense to me now.

 

It's hard to understand why your cam manufacturer would have given that sort of advice. 42* is a lot of timing, and usually you'd only see that under light load conditions where you have low VE and the mixture is relatively harder to ignite (hence the bigger advance). I can't ever recall seeing a performance cam manufacturer concerned about anything much beyond WOT (ie high LV8) conditions. The other thing to note is the distributor problem where you only have 45* between terminals, and you have a chance of mis-firing to the next terminal when you get up to this sort of number. 42 is almost 45 anyway .

John

 

The "spark advance vs. RPM vs. load" table from the stock bin was showing total timing in the 46-48 range for lighter loads (80, 96, etc) at 4800 RPM. Based on what you are saying below, is this not correct, or am I reading it incorrectly? Why would a stock bin have timing numbers that high (albeit under much lighter loads) and not a performance cam?

I had asked the tech at Howards Cams for some direction with respect to idle and timing settings. His response was:

"The initial timing should be 14-18deg. Looks like you are O.K. at 17. My concern is total timing. That is what will determine performance. Good # is 38-42 deg at about 2800-3000 RPM. Idle I would expect to be about 950-1050 RPM."

I didn't build the engine, and the cam used is a custom grind. On the cam specs / calculations I received, there is a "4" next to "adv or ret". This number was subtracted from the LSA of 112 to result in a 108 intake phase, and added to the 112 LSA to result in a 116 exhaust phase. Overlap is 82 degrees, advertised duration is 306 (other specs are in first post).

Do the timing numbers make more sense based on my cam?