I'm helping a friend with his newly installed GMC 302 in his '38 Chevy pickup. The point gap spec is .020 but I can't find the corresponding dwell setting. I'm thinking there must be a way to compute the dwell value but I can't seem to find it. Does anyone have a formula we can use or have the correct dwell?
Point gap/dwell settings are an either/or situation. Just set the gap to .020", run the engine, and use a dwell meter to check for the corresponding dwell setting. It's impossible to set the points to one gap, and then make a dwell setting without changing the point gap. "Dwell" is the number of degrees of distributor rotation the points are closed ("Dwell Together" is the literal meaning). Point gap and dwell are inversely proportional- - - -as the gap gets narrower, degrees of dwell increase. It's possible to use an analog voltmeter to set dwell, but that's a subject for a PM exchange or a phone call. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
I don't know the year of the engine (though I will have the serial number on Tuesday) but my Motor's Manual shows the 302 as being available from '52-'54, all years with the same tune up specs. My GMC maintenance manual only shows information for the 228, 248 and the 270.
The reason I'm interested in the dwell number is that I thought it was a more precise measurement than the feeler gauge derived gap setting. I have an old Sears Engine Analyzer that gives the dwell reading and with a couple of quick point adjustments I can get it set accurately.
Chevy dwell settings changed from close to 40 degrees to the low 30's when the electrical systems switched from 6 to 12 volts. I have an idea that GMC would have also followed this pattern. It has to do with the current flow in the coil primary and the ability to develop enough magnetic saturation of the coil core to produce a hot spark- - - -more voltage, less coil saturation time needed. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
I have to wonder, what does engine size have to do with setting the points or dwell? As Jerry said above, the more dwell (or the narrower the point gap) the more time available for the coil to produce a hotter or higher voltage spark. I really don't think the spark or points care what engine they're sparking for. Of course to little gap is a bad thing too.
If memory serves (sometimes it doesn't these days) .017 is a pretty good setting for points. Back when I was doing work under a shade tree, a pop top tab was often the precision instrument of choice for points. Didn't matter if it was a dual point distributor in a car that was about to school the local fast car or a Ford 9N tractor.
As a very general rule of thumb, "points closed" time needs to be about 2/3 of the total angle between high points on the distributor cam. For a V8 engine with 45 degrees between cam lobes, the dwell usually maxes out at around 30 degrees. Since a six has 60 degrees between lobes, theoretically, the dwell could be as much as 40 degrees. For some reason, the later stovebolt distributors dropped the dwell from 39 degrees to the low 30's. I believe that corresponded with the switch to 12 volts, but that's just a personal assumption, and all of us know what "assume" means! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
I will have to check my Delco books but I am pretty sure the cam lobe assembly itself changed and with a new cam lobe assembly, for example the 2403 distributors, came a change in dwell time. That occurred coincidentally around the same time the switch from 6 volt to 12 volt occurred. That being said, my Delco books list the 2403 distributor as the service replacement for the earlier distributors which is another reason that leads me to believe there was a difference in the lobes. I will check though in my 59 GM catalog when I get home tonight.
This is a sun tune up card for 59 235. You will note that the dwell is expressed as a range from like 28 degrees to 35 degrees. So you set your points and then check the dwell to see if it is in range. So that’s what you are looking to do. Not really a more precise specific number, but a range that you want to fall into after setting the point gap.
Thanks everyone for the information and discussion. Following Jerry's regular advice to "test not guess" I'm bringing as much information, as well as assorted test equipment, as I can to help figure out why a freshly rebuilt engine has developed running problems. I don't actually expect the dwell angle is the problem but I'll want to get all the basic stuff set properly before drawing a conclusion. If we can't find out what's happening with his engine I'll be starting a new thread---with more information--and asking for your help again! Jim
Don't forget changing the dwell setting will affect timing as well. You will need to retime. As a general rule on a 12 volt engine 30 degrees dwell should get you in the ball park. You will need to find a spec sheet for your engine for the correct dwell
