78Chev
11-13-2006, 11:09
I've been playing with the timing on my 6.2/6.5 with a 4911 IP. I have a friend's snap on 1480 and have been using both the lumi probe and a ferret pulse detector for comparison. On this equipment negative numbers are ATDC and positive numbers are BTDC. I used #1 for the lumi with an offset of 9.5 degrees and #3 for the pulse detector with an offset of 99.5 degrees as specified in the snap on materials. (the a/c compressor makes it difficult to use the ferret on #1) Here are my results:
Pump Position A
700 RPM: -0.5 (lumi), +5.5 (pulse)
1400 RPM: -1.5 (lumi), +7.5 (pulse)
Pump Position B (rotated 1/16th toward driver side)
700 RPM: +2.5 (lumi), +8.5 (pulse)
1400 RPM: +1 (lumi), +10.5 (pulse)
The lumi results make sense to me. Rotated to the driver side from position A to B the pump advanced by 3.0 at 700 RPM and 2.5 at 1400 RPM. The pulse results advanced by about the same amount from position A to position B. But if you look at how the timing changed at each position as the RPMs were changed from 700 to 1400 you see that with the lumi probe the timing got more retarded but with the pulse detector it got more advanced. This was true at both positions with repeated measurements. I had both the lumi and the pulse detector hooked up at the same time and just switched leads back and forth to the snap on.
It was my understanding that the timing should retard from idle to 1400 RPM, which the lumi probe indicated, but was surprised to see that the timing advanced as measured by the pulse detector. I interpret this to mean that the difference in timing between the flame and the pulse is variable (from 6 to 9 degrees in this case) and depends on engine RPM. As the RPM gets higher the pump injects sooner even though the flame might occur relatively later. I'd like to hear your thoughts on the numbers I got and whether my interpretation is completely whacko or whether these numbers are to be expected. I was hoping to calibrate my ferret pulse detector using the snap on as compared to the ferret with a timing light, but if the difference between lumi and pulse varies with RPM then that might not be very reliable. Thanks for your thoughts.
Randy
Pump Position A
700 RPM: -0.5 (lumi), +5.5 (pulse)
1400 RPM: -1.5 (lumi), +7.5 (pulse)
Pump Position B (rotated 1/16th toward driver side)
700 RPM: +2.5 (lumi), +8.5 (pulse)
1400 RPM: +1 (lumi), +10.5 (pulse)
The lumi results make sense to me. Rotated to the driver side from position A to B the pump advanced by 3.0 at 700 RPM and 2.5 at 1400 RPM. The pulse results advanced by about the same amount from position A to position B. But if you look at how the timing changed at each position as the RPMs were changed from 700 to 1400 you see that with the lumi probe the timing got more retarded but with the pulse detector it got more advanced. This was true at both positions with repeated measurements. I had both the lumi and the pulse detector hooked up at the same time and just switched leads back and forth to the snap on.
It was my understanding that the timing should retard from idle to 1400 RPM, which the lumi probe indicated, but was surprised to see that the timing advanced as measured by the pulse detector. I interpret this to mean that the difference in timing between the flame and the pulse is variable (from 6 to 9 degrees in this case) and depends on engine RPM. As the RPM gets higher the pump injects sooner even though the flame might occur relatively later. I'd like to hear your thoughts on the numbers I got and whether my interpretation is completely whacko or whether these numbers are to be expected. I was hoping to calibrate my ferret pulse detector using the snap on as compared to the ferret with a timing light, but if the difference between lumi and pulse varies with RPM then that might not be very reliable. Thanks for your thoughts.
Randy