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Computing Dwell From Point Setting
#1341777 Sun Jan 12 2020 09:17 PM
Joined: Feb 2013
Posts: 221
E
Shop Shark
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?

Re: Computing Dwell From Point Setting
Eureka Jim #1341802 Sun Jan 12 2020 10:53 PM
Joined: Dec 2002
Posts: 3,576
Moderator - The Electrical Bay and Rocky Mountain Bolters
what year is the 302?


Another quality post.


Real Trucks Rattle

HELP! The Paranoids are after me!
Re: Computing Dwell From Point Setting
Eureka Jim #1341803 Sun Jan 12 2020 10:56 PM
Joined: Dec 2002
Posts: 3,576
Moderator - The Electrical Bay and Rocky Mountain Bolters


Another quality post.


Real Trucks Rattle

HELP! The Paranoids are after me!
Re: Computing Dwell From Point Setting
Eureka Jim #1341812 Sun Jan 12 2020 11:50 PM
Joined: Feb 2004
Posts: 20,164
H
Boltergeist
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




The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Computing Dwell From Point Setting
Hotrod Lincoln #1341816 Mon Jan 13 2020 12:15 AM
Joined: Feb 2013
Posts: 221
E
Shop Shark
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.

Re: Computing Dwell From Point Setting
Eureka Jim #1341850 Mon Jan 13 2020 02:55 AM
Joined: Jun 2011
Posts: 3,329
E
Shop Shark
270 would be closest.

Ed


'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
Re: Computing Dwell From Point Setting
Eureka Jim #1341855 Mon Jan 13 2020 03:47 AM
Joined: Feb 2004
Posts: 20,164
H
Boltergeist
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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Computing Dwell From Point Setting
Eureka Jim #1341874 Mon Jan 13 2020 02:11 PM
Joined: Sep 2013
Posts: 1,008
M
Shop Shark
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.

Last edited by moparguy; Mon Jan 13 2020 02:18 PM.

1951 3600 with Clark flatbed, T5, 4.10 rear
1970 340 Duster
1990 5.0 V8 Miata (1990 Mustang Gt Drivetrain)
1951 Farmall Super A



Re: Computing Dwell From Point Setting
Eureka Jim #1341890 Mon Jan 13 2020 03:29 PM
Joined: Feb 2004
Posts: 20,164
H
Boltergeist
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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Computing Dwell From Point Setting
Eureka Jim #1341906 Mon Jan 13 2020 05:12 PM
Joined: Dec 2017
Posts: 709
D
Shop Shark
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.


Mike
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