Technical Timing question

CarMichael Angelo · Nov 20, 2011

Like it says, Does anybody know what the computer adds (if any) to the initial timing at start up? And on a hot restart? All in at whatever RPM total?

I have the ability to build a timing curve w/ the MSD timing computer I use on my carb'd 2V 4.6, and I still get a hot start backfire,... alot.

Currently, I'm running 22* initial at start through idle and a total of 32*, all in by 2000 RPM.

nyuk98gt · Nov 20, 2011

madmike1157:

Well, Sir, there are many, many modifiers to spark. Depending on the model year, there could be 3 tables (MBT, borderline knock, and max allowed). Coolant and inlet air are obvious modifiers. There is a maximum spark allowed for start-up, too. I don't know if it is used just for cranking or for the first little while until the engine goes into closed loop. If you can datalog, you can see the final spark commanded. This doesn't answer your question finally but it gives you something to investigate. Do you have access to a datalogger (XCal and a laptop), or equal?

Chris

wmburns · Nov 20, 2011

One of the causes of back fire is timing too far advanced. 22 degrees seems like a lot of start up timing.

CarMichael Angelo · Nov 20, 2011

Again, I am using an aftermarket MSD computer to handle timing chores. I believe that there isn't enough timing. Every "Race car" I've ever had was set to static timing and the curve was locked. Typically there was anywhere between 32 and 36 degrees at the crank upon start up, and the side effect was a hard crank, NOT a back fire, if there was too much timing on a hot start. ( to which the remedy was to retard it 2-10 degrees to alleviate that)

I just wanted to know if anybody knew what the factory ECU did to the timing as soon as the key was turned, and where the timing typically was on a cold/hot start if different. to include total timing at whatever RPM the computer determined to bring in full advance.

midnightdriver · Nov 20, 2011

madmike1157 said:
Again, I am using an aftermarket MSD computer to handle timing chores. I believe that there isn't enough timing. Every "Race car" I've ever had was set to static timing and the curve was locked. Typically there was anywhere between 32 and 36 degrees at the crank upon start up, and the side effect was a hard crank, NOT a back fire, if there was too much timing on a hot start. ( to which the remedy was to retard it 2-10 degrees to alleviate that)

I just wanted to know if anybody knew what the factory ECU did to the timing as soon as the key was turned, and where the timing typically was on a cold/hot start if different. to include total timing at whatever RPM the computer determined to bring in full advance.

Stock 4.6 ECU base timing is 10* and around 20-24* total IIRC.

wmburns · Nov 20, 2011

Ok, I used my ODB2 scanner on my 2003 GT. Key on, engine not runnning, timing 10 degrees. As soon as it is cranked, the timing goes to zero.

Once started and at idle, the timing is between 20-24 degrees (motor hot).

At 2000 RPM's (motor hot), the timing is 40 degrees. I did not rev any higher as my 2003 is an automatic. I was afraid the PCM would sense the "neutral" and exessive RPM's and begin to pull timing.

The outside temperature at the time was 70 degrees.

I did not run the tests with the motor cold.

nyuk98gt · Nov 20, 2011

madmike1157:

In my calibration (SXA2 for a '98 GT with an automatic), there is "maximum allowed spark table cold starts" and the values in the table range from 25 degrees at minus 30 F to 20 degrees at +30 F to 15 degrees at 196 F. I don't know if these are changed by the coolant temperature and air temperature and open loop A/F modifiers at start-up or if the modifiers only apply when the engine exits 'start-up'.

Anyway, I hope this is useful info for you.

Chris

Technical Timing question

CarMichael Angelo

my rearend will smell so minty fresh,

nyuk98gt

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my rearend will smell so minty fresh,

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