How to fine tune spark advance with dwell angle changes

[Duke W.]

A couple of weeks ago I evaluated a local SoCal chapter member's '65 coupe with a rare option combination - L-76 with C-60... did the normal spark advance map/idle mixture evaluation, and we took it for a ride on my "test course" behind LAX. It's solid car, and we plan to drive it over to the Laughlin Regional in April. The AC is still R-12, and it works! I used this opportunity to verify the dwell angle - timing relationship that I knew to be 2:1... or is it 1:2? I forgot!

Turns out it's 1:2. A one degree change in dwell changes initial timing by two degrees. Increasing dwell advances timing and decreasing dwell retards timing. Since the 365 HP engine won't idle stably below 700 where the centrifugal curve starts I checked the dwell-timing relationship at over 2500, which is above the 2350 revs that centrifugal is all in with the OE spark advance map. This one actually didn't have the OE weights and springs and total centrifugal was more than the OE 24 degrees, but it was all in by 2500. This is a useful relationship to know. You can easily experiment with finding the detonation limit or retard timing if you get into a situation such as hot weather or too little octane fuel that might get the engine into detonation, like on a road trip. All you need is the allen wrench and a dwell meter... no need to mess with the distributor.

Being as how the recommended dwell angle range is 28-32, if you set it at the nominal 30 degrees and know the intial and/or total WOT advance you can easily advance or retard 4 degrees with just a dwell angle change and know exactly where it is without the need for a timing light. Back in the sixties the Royal Pontiac (Detroit) "Bobcat Tune" included quickening the centrifugal curve and setting the dwell angel at 35 degrees. This keeps the points closed a little longer, which increases the engine speed that the coil remains "saturated" for a little more spark energy. Also, since a higher dwell angle means the points don't open as far it can add a few hundred revs to the point bounce speed, which is good if you're using standard 19-23 ounce breaker arm tension points on SHP engine.

Magazine road tests showed that the Royal Pontiac Bobcats were quicker than the OE tune. Back in the sixties I would increase the dwell angle a couple of degrees when I went out looking for some Saturday night action along with adding a few gallons Chevron Custom Supreme (about 103 RON) to the nearly empty tank. Duke

[Duke W.]

Re: How to fine tune spark advance with dwell angle changes

Sorry about the above mess. The paragraph spacing keeps getting destroyed despite multiple attempts to edit it back in. This happens to me sometimes, but in the past I've been able to fix it... not this time. Maybe a moderator can clean it up by adding some logical paragraph spacing. I'm giving up. Duke

[Paul Y.]

Re: How to fine tune spark advance with dwell angle changes

A couple of weeks ago I evaluated a local SoCal chapter member's '65 coupe with a rare option combination - L-76 with C-60... did the normal spark advance map/idle mixture evaluation, and we took it for a ride on my "test course" behind LAX. It's solid car, and we plan to drive it over to the Laughlin Regional in April. The AC is still R-12, and it works! I used this opportunity to verify the dwell angle - timing relationship that I knew to be 2:1... or is it 1:2? I forgot!

Turns out it's 1:2. A one degree change in dwell changes initial timing by two degrees. Increasing dwell advances timing and decreasing dwell retards timing. Since the 365 HP engine won't idle stably below 700 where the centrifugal curve starts I checked the dwell-timing relationship at over 2500, which is above the 2350 revs that centrifugal is all in with the OE spark advance map. This one actually didn't have the OE weights and springs and total centrifugal was more than the OE 24 degrees, but it was all in by 2500. This is a useful relationship to know. You can easily experiment with finding the detonation limit or retard timing if you get into a situation such as hot weather or too little octane fuel that might get the engine into detonation, like on a road trip. All you need is the allen wrench and a dwell meter... no need to mess with the distributor.

Being as how the recommended dwell angle range is 28-32, if you set it at the nominal 30 degrees and know the intial and/or total WOT advance you can easily advance or retard 4 degrees with just a dwell angle change and know exactly where it is without the need for a timing light. Back in the sixties the Royal Pontiac (Detroit) "Bobcat Tune" included quickening the centrifugal curve and setting the dwell angel at 35 degrees. This keeps the points closed a little longer, which increases the engine speed that the coil remains "saturated" for a little more spark energy. Also, since a higher dwell angle means the points don't open as far it can add a few hundred revs to the point bounce speed, which is good if you're using standard 19-23 ounce breaker arm tension points on SHP engine.

Magazine road tests showed that the Royal Pontiac Bobcats were quicker than the OE tune. Back in the sixties I would increase the dwell angle a couple of degrees when I went out looking for some Saturday night action along with adding a few gallons Chevron Custom Supreme (about 103 RON) to the nearly empty tank. Duke

Thanks for this Duke. I usually carry a dwell meter on a long road trip.

[Douglas L.]

Re: How to fine tune spark advance with dwell angle changes

Interesting thread. I've never used this in V8 engines but use it on my 74 Yamaha RD350. The points plates on that engine are a less than spectacular fit to the engine so making timing changes has a huge effect on dwell which makes for somewhat of a vicious cycle when trying to get timing and dwell identical cylinder to cylinder. To protect my sanity I usually set timing close and then mess with point gap to get it perfect. Although it is a 2 stroke, it has individual coils for each cylinder so I'm not too worried about coil saturation.

[Duke W.]

Re: How to fine tune spark advance with dwell angle changes

...sounds like my erstwhile '73 Honda SL 350. It, too had a separate system for each cylinder. You set the point gaps, then rotated the breaker plate as required to set the timing on #1. Then you checked the timing on #2 and if it needed adjustment you did so by altering the point gap. Of course, the points didn't have the handy-dandy screw adjustment feature like the Delco single point, so you just had to guess, start the engine (kick starter only on this model) and check timing, then repeat until you got it right.

It was a real PiA. Fortunately my CB1100F has an electronic ignition, so. like the TI/HEI system, you set the timing on #1 and forget it. Unless something goes haywire with the system it will never change.

Duke

[Michael H.]

Re: How to fine tune spark advance with dwell angle changes

...sounds like my erstwhile '73 Honda SL 350. It, too had a separate system for each cylinder. You set the point gaps, then rotated the breaker plate as required to set the timing on #1. Then you checked the timing on #2 and if it needed adjustment you did so by altering the point gap. Of course, the points didn't have the handy-dandy screw adjustment feature like the Delco single point, so you just had to guess, start the engine (kick starter only on this model) and check timing, then repeat until you got it right.

It was a real PiA. Fortunately my CB1100F has an electronic ignition, so. like the TI/HEI system, you set the timing on #1 and forget it. Unless something goes haywire with the system it will never change.

Duke

A question for the moderators-- Was this thread truncated? I was following it with interest, trying to learn from the authorities here (who I very much respect) about the interconnection between timing and dwell, found it to be too complicated (at least for me) to absorb on a quick reading, decided to set it aside until I could devote more mental capacity to it, and came back to it only to find that most of the follow-on discussion now seems to be deleted?? I realize there seemed to be some disagreement concerning whether increasing dwell advances, versus retards, timing, but that's why I came back to find out whether there appeared to be some final resolution of the question. I've been reading this forum basically everyday for several years, and I haven't seen threads deleted before due to a good faith debate (although admittedly maybe it's occurred, and I just missed it.). By the way, I don't have a dog in this fight -- I just wanted to know the final answer. Did I miss something (or did I inadvertently return to the wrong thread)??

[Don H.]

Re: How to fine tune spark advance with dwell angle changes

I don't think anyone altered this thread. Is it possible you were reading the same thread in the CF?
It is three pages long there, with several alternative opinions.

[Michael H.]

Re: How to fine tune spark advance with dwell angle changes

Don, You're absolutely correct -- I didn't pick up that the same (or certainly related) thread was also commenced on the "other" Corvette Forum, and the additional postings that I read were over there. (Apparently like you, I follow both forums.) Thanks for straightening me out.

[Duke W.]

Re: How to fine tune spark advance with dwell angle changes

As Don said, and AFAIK this thread had not been changed by any moderators. I copied and pasted it to the CF, and it's generated a lot of controversy with several pages of theories. I've asked that a couple of guys run the test and post results, but so far nobody has, so maybe someone here can do it. Just make sure that you do it at an engine speed that doesn't afect centrifugal advance - either above the engine speed that it's all in or idle speed or lower. A 300 HP engine is a good candidate because you can start out at about 400. As timing is advanced engine speed will increase by maybe 100-200 revs, but as long as you don't exceed 700 with the OE curve, centrifugal will not come into play. Duke

[Frank D.]

Re: How to fine tune spark advance with dwell angle changes

I don't think the CF moderators have touched the thread and the opinions are all over the place... Between the time of year, cold weather or just plain disinterest nobody claims to have done the test. Yes, it certainly would be interesting. My split window has the Breakerless SE so I'm not sure its relevant or anybody would 'buy' the results anyway for that reason.

[Stuart F.]

Re: How to fine tune spark advance with dwell angle changes

FWIW! The promo info on the Pertronics II notes: "The Ignitor II senses current levels in the coil and adjusts the dwell to maintain peak energy throughout the entire RPM range. Dwell time is increased or decreased with changing engine RPM and operating conditions. This provides more energy at high RPM, reducing misfires while improving engine performance." Works for me. My engine hasn't missed a beat in 12 years of use under all conditions over the years including AFB flooding, etc.

Stu Fox

[Duke W.]

Re: How to fine tune spark advance with dwell angle changes

Someone on the CF ran the test on a Sun distributor machine and initially reported a four degree increase in dwell angle advanced timing twp degrees (at the distributor, which is four at the crank), but then reposted a couple of hours later that he misread the machine and timing actually retarded two degrees. It would still be a good idea if someone can run the test on a car to independently validate the results including the indicated 1:1 dwell angle/crank timing relationship.

[William F.]

Re: How to fine tune spark advance with dwell angle changes

Just use a timing light to change timeing . Leave dwell where it should be.

[Steven B.]

Re: How to fine tune spark advance with dwell angle changes

Adjusting the timing with the dwell change is interesting. I remember pulling into NAPA and buying a distributor, swapping it in the parking lot (I always carried a dwell-tach and timing light) and then taking the old distributor into the store for the core charge. Never thought of changing dwell to manipulate the timing after initial setup.
Having looked at the posts on the CF, a couple of posts seem to make the most sense. I believe there is total agreement as to the dwell-timing relationship except for whether there is a retard or advance and to what extent. Since there is a 1:2 relationship between the distributor and the crankshaft, the question becomes how many degrees of change there is with a change in dwell, and whether that is working to advance or retard timing. It would seem to me that as has been suggested, a change of one degree in dwell angle results in .5 degrees change in point opening and .5 degrees in closing. An increase in dwell angle should equate to an increase in the degrees the points are closed meaning that they are going to open later and close sooner. With that in mind an increase in dwell would retard the timing. If you were interested in increasing advance by adjusting dwell, it would seem that you would then want to run a 32 degree dwell so you could back out of it to get the advance while staying in a dwell range that would provide adequate coil saturation. I think the distributor machine was telling in that it showed a 1:1 relationship between changing dwell, and timing at the crank. The only confusion was whether change in dwell retarded or advanced timing. I believe it is the former. I don't generally post on the CF, because I don't like to get jumped. I think we have a little more relaxed and friendlier atmosphere on the TDB. However, I am sure that there are other opinions. I have a 300 hp 327 that I just rebuilt the distributor on and mapped the advance. As soon as things calm down because of the holidays, I intend to give Dukes test a try.

Steve

[Jim D.]

Re: How to fine tune spark advance with dwell angle changes

Nice write up Steve. Let us know what you find out when you test it.