I have a lightly used Mallory Unilite Distributor, Magnetic Breakerless Electronic Ignition System, Part # 5018002H. This model is a mechanical advance only distributor and came on my 1962 261 ci Chevrolet engine. I am at the point of rebuilding this engine where in reading about the advantages of having BOTH forms of advance I took measurements and found that IF .21" is removed from the Mallory shaft, the stock vacuum advance of a stock distributor would serve not only as the hold down for the clocking of the distributor but adding ported vacuum advance as well. Just got off the phone with a tech rep at Mallory who said that I would not see any mpg difference by adding the vacuum advance to the distributor, void the warranty (I'm the second owner of this at least 15yo unit), that it wouldn't fit and wouldn't be worth it. I don't think he understood how a 1950's-60's distributor vacuum advance/hold down looks like, I tried to describe it to him but words were getting in the way. As far as not being worth the effort, the only effort would be to mill .21 off the shaft. At that point I could easily run the distributor with and without the vacuum advance to see the difference comparing, say three months of with/without driving. No, not going back to points after 120k + miles on a big top modified HEI distributor on my present 235 that has only one ignition module failure.
I don't see what the problem would be. The distributor doesn't know that it's being advanced by vacuum...as you said, the vaccum advance setup on a stovebolt 6 is quite different that most other engines, as the advance mechanism works to rotate the entire distributor, not just the point plate. And it only works when the engine is lightly loaded, so the engine can tolerate the extra advance. You might see one or two MPG improvement. I added vacuum advance to the blown 454 I had in my 55, and that one MPG increase saved me a lot of gas over the next 80k miles!
In addition to added mpg, vacuum advance should allow the engine to run a little cooler also. Seems like an obvious choice for a street driven engine.
Top picture, Stock on left, Mallory on right 2nd picture, pencil mark approximately .21”above step, lathe to remove material to accommodate clamp/vac advance 3rd picture, width of clamp/vacuum advance
Might need to eliminate the O ring to let it turn easily? And how does the OD of the new one compare to the original, where it fits into the hole in the block?
Agree on possible elimination of O ring. It's a perfect fit, I've run the engine with the Mallory distributor. I just need a little more room to mount the Clamp/Vacuum Advance.
In my case that would be 1.8 mpg as I roll at 65-75 with my 1969 Saginaw/Borg-Warner 3sp+od, normally get about 18 mpg.
I'd leave room for a greased soft felt washer below the clamp to act as both a bearing surface and an oil seal all in one once the o-ring is out. Maybe I'm overthinking it though and it really won't need to be sealed. Running a PCV valve would really help any leakage path, it would be trying to draw air into the crankcase there rather than push oil out. You've got a great workable idea, Go for it. If you try it without a seal and it leaks, you can always remachine a bit more to get room for a sealing washer. Or maybe this idea....use a split sealing ring like used on a automatic transmission shaft, such as a hooked split ring or a splice cut teflon ring....in place of the stock o-ring.?? Or the type of ring used on accumulator or servo pistons?
From the installation manual... "If your vehicle is not equipped with an ignition ballast resistor, install a Mallory Ignition Ballast Resistor Part No. 700 in the wire between the ignition switch and the coil (+) terminal. Failure to use an ignition ballast resistor will result in the eventual destruction of the Ignition Module." This TMK, is a variable resistance resistor and is expensive and rare from what I have learned searching for one for an hour yesterday. I've got a .8 resistor here in the garage, can I get by without the Part No. 700? I am using the recommended Mallory Coil Part# 29219, Primary Resistance 1.36 Ohms, Secondary Resistance 10.87 K Ohms
I don't think it's a variable resistor, it just has a spec of 0.75 to 1.5 ohms. So your 0.8 ohm resistor should work. So would a more common 1.5 ohm resistor. Also, the unilite is finicky about it's ground, if you don't have it properly and reliably grounded it can blow out instantly.
Setting up the distributor for the shaft reduction. Milling Finish looks as good as original. Great fit, vacuum clamp on distributor turns with ease.
I have been running a mechanical on an off topic car for 50 years and love it. Would have built my current with one if I could have found an affordable one
Neat idea. Try running full vacuum to the chamber. Set timing and idle speed and mixture with line disconnected. Connect line and reset idle speed and mixture. This will allow full vacuum at idle and result in a cooler running engine.
Yes, going manifold vacuum is the right way, when you are after mpg. If you want to step it up, get a tunable vacuum can, and tailor the curve to your engine, since the stock settings "by the book" are just a starting point. Turned out my 261 loves up to 50 deg of total advance at cruising speeds.
123Ignition electronic in my old Mallory dual housing. I can set my advance/vacuum curves as I like via laptop/phone (bluethooth) https://123ignitionusa.com/
The 123 add on (they need a marketer because the 123 site page is wanting, IMHO) does not even enter into a cost/benefit analysis for me. Perhaps it would have for the 15 year period that my 1949 truck was a dd/only vehicle, but fuel prices and 8 hours of driving, at one time a 2-3x a year event are now on the every other year schedule. It looks like you have a distributor that does not have a vacuum advance but has a program to change your mechanical advance. I suggest you start a separate thread with this interesting information. PM me privately if you like to clarify as I don't want this thread to turn into a bluetooth war between phone brands or what laptop is used to change your advance values with your copilot doing the curve manipulating (my active imagination thought up that scenario).
Cosmo… I converted a 194-292 HEI to early Chev/GMC’s by machining off the hold down and making a clamp way before the internet and modern parts were available. I was racing so I didn’t want a vacuum can to move the plate and locked it. The upper weights advanced the rotor as normal.. Many times I thought of doing what you did to upgrade a distributor to work with movable advance.
I bought a used large cap model (194-292 HEI to early Chev/GMC’s) made by Tom Langdon I was told in '01. You could have made yours about the same time he was selling them in the mid nineties or so. Mine had an distributor module go bad once in 20 years, I learned to carry a spare.
After I machined off the flange I sent it to Performance Distributors in Memphis Tn. to get hopped up. I’ve continued to use them over the years. When I removed the oil pump from the pan I decided to do one leaving the flange and moving the gear to center on the cam gear. I cut off the top of an oil pump and installed it in the block for stabilizing the shaft. Neither have failed. 2 yrs ago I bought a complete unit from Tom for his street GMC; nice piece and works great.
Cosmo, have you tested MPG with and without the vacuum advance? Enquiring minds want to know! In that lathe picture it looks like it's really spinning. Did it get dizzy? Haha, sorry!
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