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GMC V6 and V12 Engines Engine repair and rebuilding

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Old August 22nd, 2017, 01:18 AM
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Question Impact of low dwell

Hello all,
I've just had a tinker with my dwell (don't laugh, but it was an exciting moment for me as I'm a complete novice to this kind of stuff).

It was at 25. I've adjusted it to 32. My question is, would 25 have been low enough to notice issues with performance etc..?

Also, if I understand it correctly by increasing the dwell I've increased the length of time that the points stay closed. This should (I think?) result in a better spark and (hopefully) a better burn?

If someone could point me in the right direction, that would be awesome. I'm a bit of a sucker when it comes to learning new things. So please don't worry about over explaining LOL
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Old August 29th, 2017, 08:01 PM
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Default Re: Impact of low dwell

Dwell angle is the degrees of rotation of the distributor shaft with the ignition points closed. The number should be as large as possible allowing the coil to have the most electricity possible so it creates the biggest hottest spark. By design, a 4 cylinder distributor has the highest dwell angle, a 6 cylinder next largest, and a V-8 the lowest.

So yes, working to get the dwell angle as high as possible in the range specified is worth the effort.

Dad had all our tune-up equipment in a duffel bag, timing light, special wrenches to loosen distributors, tach/dwell meter, the flexible allen wrench screwdriver to adjust Delco distributors on GM engines. I did tune-ups on my car till the late 1970's, '77 to be exact when I got my first HEI car, '77 Firebird. The Pontiac dealer had a really good shop, had a SUN ignition scope the size of a player piano, everything spent time on that scope before the tune-up was done. They caught a defective rotor in the distributor of my Firebird, It was jumping random sparks from the coil to the distributor shaft thru the rotor, had an intermittant stumble or miss. Tough to find unless you swap lots of parts, but crystal clear on that Sun scope.
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Old August 30th, 2017, 03:24 AM
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Default Re: Impact of low dwell

Thanks for the info.... I do wish I could get my hands on the flexible GM tool. They're quite hard to find over here. I had to get a bit creative in order to get my fat sausage fingers into the general area with a standard allen key LOL

Would the change from 25 to 33 be significant enough to notice a difference do you think?
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Old August 30th, 2017, 04:50 AM
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Default Re: Impact of low dwell

I remember back in the 1960's & early '70's every auto parts store had a wall of pegboard or revolving parts tree with those distributor tools hanging on them along with GM door window crank removing tools and other special tools on them. That was before 3-4 companies owned the franchize to every parts store in town.

The change from 25 to 33 degrees should make it run much better.

Delco-Remy made a style of points and condensor that were attached, called "Unipoints" that I always used, still had the hex screw to fine tune dwell. Then came the inevitable, I got a Borg-Warner tune-up kit, Power Brute points & separate condensor, and I looked for most of a day for the screw that held down the condensor!

I read a funny joke in Hot Rod Magazine about 1967 about adjusting dwell on a GM. Had to do with the window in the distributor cap and the distributor mounted way back behind the carb & air filter. Punch line had something to do with You can tell Ford mechanics that work on small blocks with front mounted distributors, they have a smile on their face. Can't remember the specifics of the joke, was 50 years ago. Last vehicle I had that even had a distributor was an 87 F-150.
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