1985 Intotial Timing
Racer
Joined: Mar 2020
Posts: 387
Likes: 148
From: Langton, Ontario, Canada
2025 C4 of the Year Finalist - Unmodified
I believe that the initial timing is 6 degrees BTDC. This has to be set with the tan/black wire disconnected from the distributor. There is nothing to be gained by advancing beyond 6 degrees. The EFI system controls the timing, and if you advance it, you take away the ability of the ECM to retard the timing when it senses knock. It's already advancing the timing to what it needs, and it needs the full range available to it, in order to do its job properly.
Melting Slicks
Joined: Jul 2005
Posts: 2,207
Likes: 302
The ECM adds on to that 6 to command the final timing programmed into the calibration. Example... if the ECM's timing map has a maximum of 32 deg, the ECM is adding 26 to the 6 you put in at the distributor. So if you add 4 more at the distribtutor to get 10, then you'll end up with 36 max (32 + 4).
So if you add to that 6, you'll get the same amount added everywhere in the timing map.
Also, it's not that you completely take away the ability to detect knock, but you reduce the amount of knock retard (degree for degree) that the ECM can command. There's a table in the calibration that tells the ECM the maximum amount of timing its allowed to pull out if it detects knock. So the added timing may increase the propensity for knocking and at the same time the ECM may not have enough knock retard allowance to cover it.
The only correct way to alter the timing is to get into the calibration and do it.
So if you add to that 6, you'll get the same amount added everywhere in the timing map.
Also, it's not that you completely take away the ability to detect knock, but you reduce the amount of knock retard (degree for degree) that the ECM can command. There's a table in the calibration that tells the ECM the maximum amount of timing its allowed to pull out if it detects knock. So the added timing may increase the propensity for knocking and at the same time the ECM may not have enough knock retard allowance to cover it.
The only correct way to alter the timing is to get into the calibration and do it.
Melting Slicks
Joined: Jul 2005
Posts: 2,207
Likes: 302
The earliest factory calibration I have is for an 87 Camaro. They get about 29 deg max at WOT.
The Corvettes tend to run about 2-3 deg more, so you might be around 32-ish at WOT.
What GM also does is reserve about 3-4 for WOT. So if you're not at WOT, but still at high load you may only be getting around 25-ish.
The Corvettes tend to run about 2-3 deg more, so you might be around 32-ish at WOT.
What GM also does is reserve about 3-4 for WOT. So if you're not at WOT, but still at high load you may only be getting around 25-ish.
Thread Starter
Instructor
Joined: Jul 2023
Posts: 172
Likes: 16
The earliest factory calibration I have is for an 87 Camaro. They get about 29 deg max at WOT.
The Corvettes tend to run about 2-3 deg more, so you might be around 32-ish at WOT.
What GM also does is reserve about 3-4 for WOT. So if you're not at WOT, but still at high load you may only be getting around 25-ish.
The Corvettes tend to run about 2-3 deg more, so you might be around 32-ish at WOT.
What GM also does is reserve about 3-4 for WOT. So if you're not at WOT, but still at high load you may only be getting around 25-ish.
Melting Slicks
Joined: Jul 2005
Posts: 2,207
Likes: 302
The 32 includes the 6. The ECM assumes you have the correct setting at the distributor. If you set the 6 to 0, and you could theoretically hold a timing light while driving, you'd only see 26 showing up on the balancer.
There is a setting for base timing in the calibration...it's set to 6 from the factory. Not sure why GM settled on that number. If for some odd reason you wanted to set your distributor to 10, you could change that value to 10 in the calibration and all your timing maps and tables would still be valid.
So the ECM basically takes spark map value minus base timing value and calculates how much timing to add to get to the spark map value.
For example, on my Miniram with large cap HEI, if I turn the distributor until it hits the fuel rail, it's at 10 base timing. So I set my calibration to reflect 10 base timing. So whenever I pull my distributor out and put it back in, I don't have to break out the timing light... lol. Just turn it until it hits the fuel rail.
My guess is they did that for emissions. High spark timing generates more NOx emissions. So they figure unless the driver really wants the power by going WOT, they'll err on the side of emissions.
There is a setting for base timing in the calibration...it's set to 6 from the factory. Not sure why GM settled on that number. If for some odd reason you wanted to set your distributor to 10, you could change that value to 10 in the calibration and all your timing maps and tables would still be valid.
So the ECM basically takes spark map value minus base timing value and calculates how much timing to add to get to the spark map value.
For example, on my Miniram with large cap HEI, if I turn the distributor until it hits the fuel rail, it's at 10 base timing. So I set my calibration to reflect 10 base timing. So whenever I pull my distributor out and put it back in, I don't have to break out the timing light... lol. Just turn it until it hits the fuel rail.
My guess is they did that for emissions. High spark timing generates more NOx emissions. So they figure unless the driver really wants the power by going WOT, they'll err on the side of emissions.
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
[QUOTE= This has to be set with the tan/black wire disconnected from the distributor. [/QUOTE]
This may be different for other years, or maybe both work, but I've always set timing by disconnecting a 1-wire connector near the battery. It isolates the distributor from the ECM to prevent it adding any advance.
I believe my 91 has a max of around 43 degrees or so. I unfortunately don't have access to my computer with my tuning software at the moment but it was near 40 degrees.
This may be different for other years, or maybe both work, but I've always set timing by disconnecting a 1-wire connector near the battery. It isolates the distributor from the ECM to prevent it adding any advance.
I believe my 91 has a max of around 43 degrees or so. I unfortunately don't have access to my computer with my tuning software at the moment but it was near 40 degrees.
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