What controls my ignition advance curve?

[vetteboy86]

I know on my older TA, it is done with weights and springs, to get your mechanical advance. I run 12 degrees initial, and another 24 which gives me a total of 36 degrees total advance.

What controls it on my corvette? Is it still done with weights and springs, or is the EST wire somehow in control. Does it have any relationship to a vacuum line on an older distributor?

Here is my problem, I checked my advance curve, and by 2000 RPMS, my car is already 45 degrees advanced. I think this is pretty much the maximum, but the car is pulling timing when I crowd it and it shifts into second. Under normal acceleration, or top gear acceleration it doens't do it as much. I did notice though that the spark retard even happens if I am sitting still and rev it up past 3000 RPMS or so. I know I need this advance to come down, but how do I do it?

 

[MPoore]

The ecm is in complete control of spark advance. Advance is determined by the calibration (prom), and adjusted based on load calculations and knock sensor input. The calibration for my 85 includes max spark advance of 41.8 deg. It would only get to the max advance if I revved the engine in neutral (no load). Bad sensors could alter the advance curve. Otherwise, you need to redo your prom.

 

[vetteboy86]

Where are some areas I can look, because I know my advance is way too much. I can set back the initial timing some, it is at 6 now.

 

[gmjunkie]

Where are some areas I can look, because I know my advance is way too much. I can set back the initial timing some, it is at 6 now.

According to my Book 0 deg. initial timing with single wire unpluged from ECM!! the wire should be around dist. or ECM along cowl junk!!

 

[vetteboy86]

Okay, so when I time the car, the EST wire should be unplugged, and timing set to 6 degrees. It would make sense that if he timed it with the wire on, and set it to 6 that I would be advancing about 6 degrees too much. I will try that.

I also have a dial timing light, so I am going to check it against the software when I get it set.

 

[vetteboy86]

I put the timing light on, and with the EST wire unplugged, it is dead nuts on 6 degrees. Put it on, and the mark advances well past the timing plate.

So I need a new prom I am thinking.

 

[MoeJr]

Vetteboy when you plug it in the advance is supposed to do do that. What is the stock timing spec for your car?

 

[vetteboy86]

Yes, I know that, but didn't know how much it would advance. I believe the stock timing for my car is 6 degrees. The builder set it, and said this was the best he could get the car running at.

I know have a problem with the timing advancing too far. When I get on the car hard, knock retard is actually pulling 10 or so degrees. The advance was reaching around 45 to 47 degrees.

I need a custom prom, that will not advance the timing too far.

 

[gmjunkie]

Yes, I know that, but didn't know how much it would advance. I believe the stock timing for my car is 6 degrees. The builder set it, and said this was the best he could get the car running at.

I know have a problem with the timing advancing too far. When I get on the car hard, knock retard is actually pulling 10 or so degrees. The advance was reaching around 45 to 47 degrees.

I need a custom prom, that will not advance the timing too far.

There Should a Emission's Tag on that Car!! Look at it and see what the initial Timing is suposed to be set at!! My Book on it say's (0 deg.) with wire unpluged!! But I'm a Hillbilly and not May be Reading Right!! Or They May Be Hillbilly's and and did not print it Right!! junk!!

 

[MoeJr]

I am gonna bet you are gonna need somewhere between 37-40 degrees total timing. I think you gonna need to scrape up the cash and get that prom done soon. I have seen what spark knock can do to an engine and sometimes you cannot hear it going on even if the computer is not indicating any. It can be a silent killer.

 

[vetteboy86]

Can anyone shed some light on what Junkie is saying. I was almost sure it was 6 degrees.

Also, can anyone shed some light on why my computer is advancing the timing that far. Is there anything I can do, perhaps my fuel pressure is not right. I have the same hypertech chip my brother has, and his maximum advance was around 41.8 degrees also. Why is mine so different, should I try to swap them?

 

[bossvette]

Can anyone shed some light on what Junkie is saying. I was almost sure it was 6 degrees.

Also, can anyone shed some light on why my computer is advancing the timing that far. Is there anything I can do, perhaps my fuel pressure is not right. I have the same hypertech chip my brother has, and his maximum advance was around 41.8 degrees also. Why is mine so different, should I try to swap them?

there should be a sticker with the proper initial setting on it, his repair manual says "0" degrees so try it at "0".
with computer controlled ignition all the initial timing (where you set it)is a reference point for the computer, the computer controls the advance. If it thinks it is at 0 when it really is at 6 you will be advanced too much and the knock sensor will be taking timing out quite often; try setting it at 0 with the EST disconected and see what happens.

 

[vetteboy86]

Is this normal to set the timing at 0 degrees??? Never really heard of anything like that. I can give it a try and see what happens though. It would make sense, that would put me back at around 40 degrees advance. It still makes me wonder how my brother is getting away with 6 degrees of timing. What about these other guys that run 12 degrees timing like Mad Mic. Is it just because my motor is modded, that I cannot run timing?

 

[bossvette]

the computer takes more into consideration such as temp, MAP, throttle position etc. the only thing setting the initial does is tell the computer where the engine is in its cycle, I will see if I can find a link to the tuning manual for the Holley Commander 850 it might help you understand all the variables.

holley manual

look around page 32 for what a spark map looks like

page 50 tells you how to tune it I'm gonnas have to download it again they have made a few advances since I did mine

 

[vetteboy86]

Okay, bear with me through this. I have the same prom my brother has. Isn't that set up to provide a maximum amount of advance regardless? What is allowing mine to go so high???

I could set the timing to 0 with the EST wire unplugged, but wont the car be sluggish?? I guess I dont understand why the timing is advancing so far.

 

[MoeJr]

I know that some engines are timed at 0. Early TBI 4.3L S10's were timed at 0. You are running 10.5:1 CR which on todays crap gas is alot in an older technology engine. An 86 TPI 350 is light years behind in technology compared with todays engines. Thats why the new Vettes can run huge CR on pump gas and the older ones can't. Of course aluminum heads and blocks dissapate heat better and that helps alot. I do not know why your advance is different than your brothers. One thing that comes to mind is possibly your harmonic balancer has slipped and is not accurate. In any case you need to get that prom done to your engine. IMHO the prom should have been done the same time as the engine. The builder should have required it. Our shop has built alot of engines and if it would have been me I would have refused to do the engine if the customer didn't agree to having a prom burned. Boils down to doing the job the right way versus peacemeal. Just my .02

 

[vetteboy86]

Okay, so where can I get a prom done at? Will they require me to scan the car, or just go by the mods done to the engine?

 

[gmjunkie]

Okay, bear with me through this. I have the same prom my brother has. Isn't that set up to provide a maximum amount of advance regardless? What is allowing mine to go so high???

I could set the timing to 0 with the EST wire unplugged, but wont the car be sluggish?? I guess I dont understand why the timing is advancing so far.

It's purdy simple!! Ok ECM Think's Timeing is at 0 deg.!! But timeing is 6 deg.!! so it run's Timeing up to 45-47 ( it thinks it's only 39-41) Knock sencer pick's up Knock and retard's timing!! Depending on Knock severaty depend's how far it retard's!! The knock sencer has better hearing than You and I will ever have!! The Knock sencer is going to run it back as far as it need to to get rid of Knock!! Take in to acct. ECM is thinking timeing is 0 deg. and send's sig to Injectors on how much fuel to disperse at given time at given RPM!! and then oxy. sencers are sencing what un-burned fuel is in exaust and thay do there job and change fuel mix. Bottom Line if Initial timeing is not right, None of it will be right!! The ECM can't do it's job right either!! Set it to 0 deg. and see, My gess is it will do Just Fine!! junk!!

 

[vetteboy86]

Yeah, I will do whatever. It might actually help. I cannot do any harm by retarding the timing can I???

 

[scorp508]

The timing figures you are seeing are perfectly normal for a C4 chip. Heck, I have 50.6* of total timing in certain low load areas of my chip.