When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I just came across this thread.... after putting in my Accel HEI module. I'm running the Corvette large cap dizzy with the ANHT spark latency table in a 7730 ECM with $8D. Started thinking about the spark latency table being potentially impacted by the change to this Accel HEI module. I did send an email today to Accel asking if they have recommended values, so hopefully they get back to me in the coming week.
But, just running some numbers using RBob's math above and then the ANHT and AUJP tables... I seem to have gotten some shocking numbers...
I got usec per deg by RBob's math. To get deg's added, I divided the table value in ANHT or AUJP by the usec/deg value.
Look how much timing is in play here! I guess I need to probably do the experiments that were described earlier in this thread in order to characterize the Accel module since my guess is the tech guys who get my email will probably say "WTF is this guy talking about.... spark latency??". But does this even pass the smell test in terms of my calculations and how much timing is getting added because this delay?
Set everything in the spark tables to 20... main table, closed throttle table, idle speed correction tables, etc. Base coolant correction vs coolant table was already zeroed out.
Was able to get my laptop in front of the car using some USB extension cords and had TunerproRT up and running.
Grabbed my Craftsman dial back timing light to make the measurements. Also checked my base timing with the EST disconnected to validate my 10deg setting for base timing in the calibration.
Very interesting results. At idle (750 rpm), I was getting 20 deg. Soon as I got up to 1200 rpm and above (up to 4400 rpm), the dial back was only showing 15 deg. It just had this 5 deg retarded offset all the way up the rpm range. I stopped at 4400 since it gets a little hair rasing standing next to the engine doing that high an rpm. It seems like it would just maintain that 5 deg offset though if I kept going...
Could the Accel module have this near perfect 5 deg offset from stock across nearly the entire rpm range? I guess if I compare ANHT against AUJP, the ANHT has up to 3 deg less timing, seemingly attributable just the module design? So maybe for the same large cap module design, Accel's circuits generate a blanket 5 deg retard over the stock GM module.... hmmm...
I think what I may do is rent a dial back from Autozone tomorrow and repeat the measurement to verify it's not the timing light. If I get the same measurement, then I guess I need to modify my ANHT latency table to add in 5 deg.
So I guess it'd be something like this if I had to add 5 deg everywhere?
I just came across this thread.... after putting in my Accel HEI module. I'm running the Corvette large cap dizzy with the ANHT spark latency table in a 7730 ECM with $8D. Started thinking about the spark latency table being potentially impacted by the change to this Accel HEI module. I did send an email today to Accel asking if they have recommended values, so hopefully they get back to me in the coming week.
But, just running some numbers using RBob's math above and then the ANHT and AUJP tables... I seem to have gotten some shocking numbers...
I got usec per deg by RBob's math. To get deg's added, I divided the table value in ANHT or AUJP by the usec/deg value.
Look how much timing is in play here! I guess I need to probably do the experiments that were described earlier in this thread in order to characterize the Accel module since my guess is the tech guys who get my email will probably say "WTF is this guy talking about.... spark latency??". But does this even pass the smell test in terms of my calculations and how much timing is getting added because this delay?
Your numbers make sense. My OEM ATNX was identical to AUJP. That gave a real total SA of 38* at 4800rpm instead of 32* calculated by ECM. I thought I was fine with 32* but heard the motor pinging like crazy on 91 octane. That's what led me to all this testing. Had to lower SA to 23* for the pinging to stop. The performance guys I know were dumbfounded. They Could not believe a SBC 383 on 91 octane could not take more than 23*. Well in fact it was 29*. By going with 94 octane I now have 32* no problem just like recommended by GM Performance on 93 octane. Once we dyno the car I might be able to increase SA.
Cheers!
Set everything in the spark tables to 20... main table, closed throttle table, idle speed correction tables, etc. Base coolant correction vs coolant table was already zeroed out.
Was able to get my laptop in front of the car using some USB extension cords and had TunerproRT up and running.
Grabbed my Craftsman dial back timing light to make the measurements. Also checked my base timing with the EST disconnected to validate my 10deg setting for base timing in the calibration.
Very interesting results. At idle (750 rpm), I was getting 20 deg. Soon as I got up to 1200 rpm and above (up to 4400 rpm), the dial back was only showing 15 deg. It just had this 5 deg retarded offset all the way up the rpm range. I stopped at 4400 since it gets a little hair rasing standing next to the engine doing that high an rpm. It seems like it would just maintain that 5 deg offset though if I kept going...
Could the Accel module have this near perfect 5 deg offset from stock across nearly the entire rpm range? I guess if I compare ANHT against AUJP, the ANHT has up to 3 deg less timing, seemingly attributable just the module design? So maybe for the same large cap module design, Accel's circuits generate a blanket 5 deg retard over the stock GM module.... hmmm...
I think what I may do is rent a dial back from Autozone tomorrow and repeat the measurement to verify it's not the timing light. If I get the same measurement, then I guess I need to modify my ANHT latency table to add in 5 deg.
So I guess it'd be something like this if I had to add 5 deg everywhere?
Bottom line is if you change the ICM you gotta re-tune the SA latency table. I did the timing light up to 5500-5700 rpm. Scary!! Had the same setup with laptop and usb cable. I did mark my balancer well so I could see clearly.
I guess what I'll do is repeat the test tomorrow with the corrected latency table. Since it's low load, I shouldn't have any risk with an additional 5 deg of timing.
I guess what I'll do is repeat the test tomorrow with the corrected latency table. Since it's low load, I shouldn't have any risk with an additional 5 deg of timing.
Post your results and new table data with ICM used.
Translation: "I have no idea what you're talkng about. Just assume its stock and you won't notice the difference"
I'm going to borrow a friend's timing light in another day or two then repeat my test to rule out a false timing light reading.
If i still get the 5 deg offset, I'm not sure what else to attribute it to since I've verified base timing and everything in the calibration will have been set to 20 deg. Unless the damper somehow shifted. Though I guess I can verify that with TDC finder in the spark plug hole.
Ok, I ended up getting the 2nd timing light today.
Confirmed! I'm getting ~5 deg offset all throughout the RPM range above idle speed...
I re-established the 10 deg base timing first. Then confirmed the ~5 deg.
Regarding the possibility of the balancer elastomer having slipped, the fact that the idle timing is still showing 20 deg at the timing mark (matching the calibration), leads me to think that the harmonic balancer elastomer hasn't actually slipped, otherwise I should see the same 5 deg offset at idle as well.
What's more, my spark latency modifications based on RBob's math (that I posted earlier) brought the timing back to exactly 20 deg, matching my test bin.
So I guess that settles that.
Curious what this car is going to feel like once I gain back that 5 deg. So instead of 34 deg at WOT, I was only getting 29. Should be worth a little bit of power I think...
Ok, I ended up getting the 2nd timing light today.
Confirmed! I'm getting ~5 deg offset all throughout the RPM range above idle speed...
I re-established the 10 deg base timing first. Then confirmed the ~5 deg.
Regarding the possibility of the balancer elastomer having slipped, the fact that the idle timing is still showing 20 deg at the timing mark (matching the calibration), leads me to think that the harmonic balancer elastomer hasn't actually slipped, otherwise I should see the same 5 deg offset at idle as well.
What's more, my spark latency modifications based on RBob's math (that I posted earlier) brought the timing back to exactly 20 deg, matching my test bin.
So I guess that settles that.
Curious what this car is going to feel like once I gain back that 5 deg. So instead of 34 deg at WOT, I was only getting 29. Should be worth a little bit of power I think...
Let's spread the word: OEM spark latency tables are inaccurate. They should be re-tuned, especially when installing new engine or modifying one. I had a brand new engine pinging on me because of it. Also, everytime the ICM is changed, it means retuning.
I just can't get over how it was just a blanket 5 deg across the board. If anything I would have expected the inaccuracy to be varying on a row by row basis in the table. Very strange...
The 5 deg definitely got me some power. But the real benefit was that now that I have 100% confidence in what the ignition timing is, I decided to get a little more aggressive with the timing spark and see what happened...
All I have to say is, with the Miniram, spark timing is definitely your friend!! I'm actually now running about 5-6 deg more than the LT4 Vette at mid-range RPMs (2000 to 4000) and man, did the engine like that! I thought the LT4 had an aggressive map. 4000 and above, the LT4 was at 33 and I'm at 35 and there didn't seem to be any noticeable benefit by adding more.
But No knock counts, no spark retard showing up on the TP RT, nor any audible knocking... just silky smooth power delivery. And that's with 9.8:1 on our lousy pump gas here in SoCal.
And because of the added timing, it liked a little more fuel in that region as well.
Any rate, the end result of all of this... I came around to a freeway on-ramp on my way home, and nailed the gas... of course in first gear, the tires didn't stand a chance... Then I hit the Gear Vendors for 1st-over and the tires were still blazing (also not unusual)... But I went into 2nd gear and was literally fish tailing almost to the top of 2nd gear... that was a surprise.
I guess I don't know how much of this is due to the Accel module and coil vs if I still had the factory module with my old Hypertech coil (when it was new anyway).
But holy freaking cow... what a difference in the final outcome...
The 5 deg definitely got me some power. But the real benefit was that now that I have 100% confidence in what the ignition timing is, I decided to get a little more aggressive with the timing spark and see what happened...
All I have to say is, with the Miniram, spark timing is definitely your friend!! I'm actually now running about 5-6 deg more than the LT4 Vette at mid-range RPMs (2000 to 4000) and man, did the engine like that! I thought the LT4 had an aggressive map. 4000 and above, the LT4 was at 33 and I'm at 35 and there didn't seem to be any noticeable benefit by adding more.
But No knock counts, no spark retard showing up on the TP RT, nor any audible knocking... just silky smooth power delivery. And that's with 9.8:1 on our lousy pump gas here in SoCal.
And because of the added timing, it liked a little more fuel in that region as well.
Any rate, the end result of all of this... I came around to a freeway on-ramp on my way home, and nailed the gas... of course in first gear, the tires didn't stand a chance... Then I hit the Gear Vendors for 1st-over and the tires were still blazing (also not unusual)... But I went into 2nd gear and was literally fish tailing almost to the top of 2nd gear... that was a surprise.
I guess I don't know how much of this is due to the Accel module and coil vs if I still had the factory module with my old Hypertech coil (when it was new anyway).
But holy freaking cow... what a difference in the final outcome...
5* will make a huge difference
Good to hear. Enjoy it.
One of the most insidious flaws of the TPI: inaccurate timing!
I just checked my SA using two different timing lights just to be sure. I was 3 degrees under from 2000 through 6000 RPMS. I would have never thought to check and it is so easy to correct. I can feel the difference, especially midrange going up the freeway ramp. Thanks to all to have contributed to this thread.
I just checked my SA using two different timing lights just to be sure. I was 3 degrees under from 2000 through 6000 RPMS. I would have never thought to check and it is so easy to correct. I can feel the difference, especially midrange going up the freeway ramp. Thanks to all to have contributed to this thread.
I just checked my SA using two different timing lights just to be sure. I was 3 degrees under from 2000 through 6000 RPMS. I would have never thought to check and it is so easy to correct. I can feel the difference, especially midrange going up the freeway ramp. Thanks to all to have contributed to this thread.
Oh yeah... it is definitely noticeable!
I'm also glad to see someone else had a constant offset as well... even though the data told me my observation was correct, it still seemed extremely odd...
The info in this thread was huge... many thanks to everyone who contributed to it!!
Only reason I'm posting in here about it too is it gave me the opportunity to check the factory HEI module in terms of the spark latency table.
It was 5 deg too advanced with the latency table developed around the Accel module. I put the factory ANHT/AXCN large cap HEI table back in and viola, right on the money.
Just changed my ICM with the one included with the JEGS 40006 HEI distributor. The ICM has no markings but it is identical to the OEM "369". Sure enough I had to retune the SA Latency table. There was not much difference thought. Just a bit off around 4000rpm by as much as -2*. Here's the table corrected.
11-14-2020, 05:24 PM
