Are distributor vacuum advance units vehicle specific? Car vs truck with same engines? Manual vs auto trans? 2 bbl vs 4 bbl? Or do they all do exactly the same thing?(within the same engine family) Just wondering as distributors get swapped around between rigs over the years. I've got a pile of them(SBC) I bought at various swap meets......no clue what they came out of.
Here is a list of information and spec's for the HEI, http://www.crankshaftcoalition.com/wiki/images/e/e4/Vacuum_Advance_Specs.pdf
I have swapped many vacuum advance units from different distributors. the mechanical portion of the advance is different. Different advance rates.
The GM vacuum advance cans have number stamped into identifying the amount of advance.Ford vacuum cans are adjustable the later stamped ones you put an allen wrench into the vacuum nipple,the earlier cast aluminum ones with the thread in nipple you unthread the nipple to access a spring and travel limit spacer in there.
The Crankshaft Coalition WIKI is a good source of information. Good information here https://www.davessmallbodyheis.com/...ce-stand-alone-degrees-stop-plate-information and here https://www.davessmallbodyheis.com/services/general-motors-vacuum-advance-degree-stop-information on modifications to an aftermarket canister to get the number of degrees of advance back down from the 22°-24° the aftermarkets have to the 10°-12° the stock one has. (or somewhere in between of your choosing). I examined the 3 canisters I had: the no-longer-holds-vacuum stock unit I was replacing, an Accel, and a Crane. The question is not how long the slot is (it is longer on the aftermarkets) but also how far the pin can travel from its starting position in the slot (slightly different position between units) until it hits the leading edge of the slot, making its length of travel different in each of the 3 units. A couple of weeks ago I replaced the nonworking stock unit in my 350 SBC crate motor, HEI ignition, with the expensive Crane unit (which included the Crane Vacuum Timing Limiter Plate #99619-1 with it). I checked the pin movement with a MityVac for stop and start ‘inches of vacuum’ values and decided to limit the top end of the pull, not the start end of the pull, so I had to drill and tap a 6-32 hole for the plate to go at the end of the stroke, not at the beginning. The Crane unit was more robust at that area than the Accel, so I chose the Crane to install. The stock unit’s slot measured 13/64”, the Crane slot measured 21/64”; with the limiter plate I set the Crane up to provide 14/64”. A good starting point and easily adjustable if the car, when dialed in, exhibits surge at light throttle cruise. With the virus lock down now, I haven’t been out in it (or any car) too much since. Summit lists a number of them for Fords.
The articles from 'dave's small body hei's' above we're well written. I especially recommend the first link in post #7. Phil
MOAR, MOAR . . . Apologies for the length: I realize that the OP neither desires this for an HEI unit, nor mentions the mechanical advance weights and attendant curve. But there’s no reason not to inspect and correct the mechanical advance mechanism when the cap and rotor are off. Taking off the cap and rotor in order to get to the canister’s two mounting screws, on my HEI it was instantly noticeable that the little unobtainable bushings that go between the advance weights pivot pins and the advance weights themselves were completely worn out. One was simply a few micro fragments, the other one was very, very badly worn. The pins themselves were starting to show some wear (the new bushings would take care of that). Clearly, the pins aren’t hardened. More on hardening coming right up! I found 6 or 7 cross references for aftermarket bushings, but none have been obtainable for perhaps 8 or 10 years. If you have an aftermarket recurve spring kit (some include new cheezy looking Chineseium weights and base plate) it has two new bushings in it. I have two different brand kits with bushings, but those bushings are for the .250” hole in the new cheezy weights; the actual GM weights have a .210” hole. Simple, drill it out, right? Not exactly; the GM weights are HARDENED (IIRC Bubba on here points that out); my freshly sharpened drill bits did nothing. So I ordered a package of 100 quantity #91145A148 “Electrical Insulating Nylon 6/6 Sleeve Washer for Number 8 Screw Size 0.313 Overall Height” bushings, from McMasterCarr. That’s a mouthful, huh? Grainger may have them also, not sure. All the dimensions are good except that the 0.313 overall length is too long. Simple, put a bushing into the GM weight, using the weight as your guide, gently grind down the excess nylon sticking out of it on your bench grinder. A 30 second project. To complete this repair, you’ll need a #8 small diameter nylon washer to go on the pin between the top of the weight and the bottom of the advance spring to keep excess up and down slop out of the weight movement so that it can’t flop around when it contacts the moving base plate. Obviously, the advance springs holds this all together in addition to controlling the advance curve rpm. Last point was to carefully and very lightly grease the bushings, the pins, the weight edges, the base plate edges, and the little plastic(?) things under the weights that the weights rest on as they open. To make sure that the moderators are fully aware that this is on-topic, pre-1965 stuff, I’d like to conclude by saying that the grease I used was Delco Remy (notice it isn’t ACDelco) Distributor Cam Grease, purchased by me in 1963 or 1964. Thanks, no need to get up, I’ll show myself out . . .
They advance differently or can. many of the better replacement units that you can buy are adjustable. here is an example, a 350 horse 327 is going to want the timing to advance quicker than say a 230 horse 283. both can use the same distributor so the advance would want to act differently.
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