Fine tuning spark advance with dwell angle changes
12-18-2016, 12:43 AM
#21
Drifting
This from the TDB archives:
Sometimes I know what I'm doing, but don't know why I'm doing it 
June 1st, 2010, 10:40 AM
Increasing dwell retards the timing. Dwell and timing should always be checked together. Check and set dwell first, then timing.
To check dwell variation free rev hydraulic lifter engines to 5000, 6000 for mechanical lifters. More than a couple degrees variation indicates a sloppy distributor that needs help. Engines with relines above 5500 should use the 28-32 oz. break arm tension points, like NAPA/Echlin CS89 or BWD A112P - same part made by Standard Motor Products - just a different box.
Duke
Increasing dwell retards the timing. Dwell and timing should always be checked together. Check and set dwell first, then timing.
To check dwell variation free rev hydraulic lifter engines to 5000, 6000 for mechanical lifters. More than a couple degrees variation indicates a sloppy distributor that needs help. Engines with relines above 5500 should use the 28-32 oz. break arm tension points, like NAPA/Echlin CS89 or BWD A112P - same part made by Standard Motor Products - just a different box.
Duke
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65tripleblack (12-18-2016)
12-18-2016, 12:01 PM
#22
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We need a handful of guys to run the test - good Christmas break project for those in mild climates. Get out your timing light and dwell meter. I ran the test three times and reported the results, so we need verification to settle the "polarity controversy".
If you do the test by checking initial timing, make sure revs don't go above the centrifugal advance start point or the results will be contaminated. If you have an aggressive centrifugal curve, run the test a few hundred revs above the max centrifugal point. Of course, the test should be done with the VAC disconnected and signal line plugged.
VAC action should have no effect on dwell on a Delco single point distributor. Dwell may fall off at very high revs, but more than a couple of degrees indicates a sloppy distributor that needs a "blueprint/overhaul".
Changing dwell angle definitely changes timing. That is why dwell should be set first and then timing, but since timing changes two degrees for every one degree change of dwell you can use can use small dwell angle changes to make minor adjustments to timing.
Duke
If you do the test by checking initial timing, make sure revs don't go above the centrifugal advance start point or the results will be contaminated. If you have an aggressive centrifugal curve, run the test a few hundred revs above the max centrifugal point. Of course, the test should be done with the VAC disconnected and signal line plugged.
VAC action should have no effect on dwell on a Delco single point distributor. Dwell may fall off at very high revs, but more than a couple of degrees indicates a sloppy distributor that needs a "blueprint/overhaul".
Changing dwell angle definitely changes timing. That is why dwell should be set first and then timing, but since timing changes two degrees for every one degree change of dwell you can use can use small dwell angle changes to make minor adjustments to timing.
Duke
Last edited by SWCDuke; 12-18-2016 at 12:14 PM.
12-18-2016, 01:06 PM
#23
Racer
12-18-2016, 01:58 PM
#24
Race Director
Well - which is it? Too damn cold here to go experiment and besides I have TI. We need someone down in Florida or out in sunny CA to run the test.
12-18-2016, 04:47 PM
#25
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PS. The snow is flying here so I won't be doing any experimenting. I do know this from years ago, sometimes I'd check my point gap and find out it had lost about half the gap I'd set many miles ago. Gap set around .018. So if .018 equals 30* dwell, then I'd have lost 15* dwell which according to your statement(s), that would equal about 30* ADVANCE on my timing. Since I'd set the timing at 12* that would give me static timing of 42*. Will the starter even crank an engine over at 42*? I don't think so. If it did start, and the timing was advanced like you say, it should have rattled it's heads off. I never needed any more than 97 octane back then. What did I miss that you said?
There was an article years ago printed in Hot Rod Magazine describing the trick of cutting off one set of dual points (on an eight lobe distributor) to pull timing out at high rpm.
So, what are you fishing for here. I think you are going to try and pull a rabbit out of your hat.
Last edited by MikeM; 12-18-2016 at 04:48 PM.
12-19-2016, 12:11 PM
#26
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Just trying to keep this from sliding into oblivion without a rebuttal from the OP.
12-19-2016, 12:23 PM
#27
Race Director
I'm not sure what the ongoing "debate" is all about. It's pretty clear that dwell does affect timing.
How valuable that is in creating a performance enhancement probably depends a great deal on the specific characteristics/mechanical details of the vehicle involved.
How valuable that is in creating a performance enhancement probably depends a great deal on the specific characteristics/mechanical details of the vehicle involved.
12-19-2016, 03:20 PM
#28
Drifting
[/QUOTE]
There's an ignition sticky in C3 Tech that explains everything in more detail.[/QUOTE]
Thanks 69427 very good write up.
There's an ignition sticky in C3 Tech that explains everything in more detail.[/QUOTE]
Thanks 69427 very good write up.
Last edited by DSR; 12-19-2016 at 03:21 PM.
12-19-2016, 03:42 PM
#29
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There's an ignition sticky in C3 Tech that explains everything in more detail.[/QUOTE]
Thanks 69427 very good write up.[/QUOTE]
Thank you.
Thanks 69427 very good write up.[/QUOTE]
Thank you.
12-19-2016, 04:24 PM
#30
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As the dwell increases, the timing retards, period. Easy to see with a dwellmeter and a timing light. I've been seeing this for decades. As the rubbing block wears, the dwell increases, the points stay closed longer, and the timing retards accordingly. It is true that more dwell=more coil saturation and a better potential spark, but the timing must be set accordingly after the dwell is set to whatever setting you deem appropriate. If you have a points set at 30 degrees dwell, and the timing spot on, adjusting the dwell to 35 degrees will require re-adjusting (advancing) the base timing.
12-19-2016, 04:32 PM
#31
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12-19-2016, 04:32 PM
#32
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As the dwell increases, the timing retards, period. Easy to see with a dwellmeter and a timing light. I've been seeing this for decades. As the rubbing block wears, the dwell increases, the points stay closed longer, and the timing retards accordingly. It is true that more dwell=more coil saturation and a better potential spark, but the timing must be set accordingly after the dwell is set to whatever setting you deem appropriate. If you have a points set at 30 degrees dwell, and the timing spot on, adjusting the dwell to 35 degrees will require re-adjusting (advancing) the base timing.
The C3 forum and Duke are disagreeing with you and everyone else here. I'm still waiting on the "rabbit out of the hat" trick.
12-19-2016, 10:18 PM
#34
Drifting
12-20-2016, 01:32 PM
#36
Race Director
It may be correct but I can't help being a little suspicious of the ratio of 1 degree change in dwell results in 2 degrees change in initial timing. That's quite a bit.
As MikeM noted above if the correlation is dwell increase results in timing decrease (retarded), that would explain why a tune up to replace old points (probably worn or gap closed up) would make the car feel peppier again due to restored timing. Of course as teenagers we used to joke that the car felt stronger after changing the oil.
As MikeM noted above if the correlation is dwell increase results in timing decrease (retarded), that would explain why a tune up to replace old points (probably worn or gap closed up) would make the car feel peppier again due to restored timing. Of course as teenagers we used to joke that the car felt stronger after changing the oil.
12-20-2016, 01:52 PM
#37
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I'm still waiting for a few guys to run the same simple test I did and report their results.
Duke
Duke
12-20-2016, 02:23 PM
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I think that diagram you posted was meant to cause eyes to bug out even though it's probably legit!
Last edited by MikeM; 12-20-2016 at 02:27 PM.
12-20-2016, 02:54 PM
#39
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