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Vacuum Advance Conversion
I'm considering swapping the MSD billet distributor on my 427 for a Ford Duraspark with vacuum advance.
My question for the group is this: Where would be the best place to tap into a vacuum signal? I have a pretty valuable original 1965 Holley with the cathedral float bowls, and I really don't want to modify it if I don't have to. I'm thinking either the carb spacer, or somewhere on the manifold? Whaddya think, guys? |
is there a port on the primary metering block? that's where it should be pulled from
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If I can't tap vacuum there, would a spot under the carb on the spacer, or into the intake plenum under the carb work? |
Drill and tap the spacer and put a fitting in.
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If you want to do it with a spacer, vetical height is sometines an issue, this will minimize it and give you factory support:
1970 Ford Mustang 428 Cobra Jet 1971 429 Carb Spacer | eBay |
Where Rick has the right idea, the ebay listing has a 7/16 nipple on it, kind large for a dizzy vacuum hose.
1965-68 fords with the autolite 4bbl had a thin 1/4 inch spacer under them. the dizzy pulled vacuum from a a small nipple coming out at a slight angle in the front passenger side. you may find one of these babies at a t-bird or mustang parts house |
here, i found one......
1966 Ford Thunderbird Carburetor Spacer - 390 & 428 V-8 - MAC's Antique Auto Parts |
FWB....Your point is well taken, yes that would be a BIG hose for a distributor, mor like for PCV hose probably. While we are on the subject, what is "timed" vacuum?
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That looks like it will work. I'll order one today. Thanks for the link. Thanks to everyone who replied. Doug |
I'm no expert here, but there seems to be a lot of discussion on the internet (google: ported vacuum advance) about whether to use "ported" or "manifold" vacuum for ignition timing advance. From what I read briefly, you need to determine which is best for the tune of your specific engine. I suggest you do a bit more research to determine from where to access vacuum for your ignition. Also, I don't recall you saying "why" you are swapping out your mechanical-only distributor for one with vacuum advance.
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That spacer is tapered, the carb will be slanted with the front lower than the rear.
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DO NOT, I repeat, DO NOT run ported spark. Connect your advance to a manifold (constant) vacuum source. |
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I'm swapping for a several reasons - including reliability, maintenance, replacement parts availability, and driveability/mileage. I bought the MSD because I didn't think I could afford (or find) an original 427 distributor (I know they weren't vacuum advance) and because I didn't know that the Duraspark would fit. My car's not a race car, and spends way more time cruising on the street than at the track. My research concluded that manifold vacuum is the way to go. Rick That won't be a problem. My engine is slightly lower in the rear. Doug |
There are two different vacuum sources that kick in at two different time(s) I call them manifold and the other carb. - in the case of old engines , part throttle cruising at 2000-2500 RPM gave high carb vacuum "pull" which had your engine timing advance in that "Cruise" mode - it would add say 5-7 degrees in that situation.
Manifold vacuum was used on early 327 stingrays to give a nicer idle ,adding 5-7 degrees at idle to 2000 RPM then dropping off. --example dropping out at WOT to run static and mechanical advance timing when you were "on it-" Starting in 1968 - ford had a vacuum switch in the thermostat housing - that had both ported ( carb ) and manifold vacuum sources and depending on temperature it would either apply carb vacuum (COLD) or manifold ( HOT ) to advance timing to have the engine run cooler in traffic with the AC on.... now if you then take smog into mind, they then started applying manifold vacuum on the Dual ported vacuum advance canisters to either advance timing, or even retard timing -= lowering NOX emissions ( again 1960s smog systems - ewww. ) I would try both and see what works best -- keep an eye on operating temperature, - gas consumption , pinging , - and driveability. |
Here is one of the best definitive articles on vacuum advance. I converted my 66 428FE from a non vacuum MSD back to a Ford dizzy with a Pertronix ignition and vacuum advance. Unless you are running your car at WOT, you should run vacuum as this article so perfectly explains. You will improve cooling at idle, drivability and plug fouling. Take your information from a power train engineer and not from a backyard hobbyist. Rick Distributor Tuning and Theory - Part 1 - Hot Rod Forum : Hotrodders Bulletin Board
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Thanks for posting that article. I've read it before, and I think it's the simplest, best-written primer on vacuum advance that I've seen. I intend to change out my MSD dist for exactly the reasons detailed in the article. I plan to do some testing on my engine to determine what it has for manifold vacuum at idle, and then tune the vacuum can on the Duraspark to work properly. The article states that I should set up the vacuum advance to work at around 2 inches of vacuum less than the engine has at idle, so I'll start there. It is my understanding that there is an adjustment inside the vacuum canister of the Duraspark, so hopefully I can make changes to suit my particular combination. Doug |
Doug, I do believe that the Duraspark vac. can is adjustable. I have an after market Echlin VC 945 can on my distributor. It is adjustable as I believe your is with an allen wrench (5/32 if memory serves me). I adjusted mine to begin to advance at about 6 in. of vac. It is all in at 14 In. With my cam/heads/carb. set up I pull about 10 in. of vac. from the manifold port at idle. This will get for me about 11 1/2 degrees of additional advance at idle. Full advance at 14 in. is 14 degrees. My set up is 12 degrees static, 36 degrees total, all in at 2800 rpm.(no vac) After the vac is engaged my idle is 23 1/2 degrees at a steady 850-900 rpm. You will notice a much quieter exhaust note at idle and at cruise, better throttle response and less heat in the side pipes. Also you will need to adjust your idle speed back down with the additional advance. This will help cure any run on issues you may have had with the MSD set up and no vacuum advance. Keep us posted as you make the conversion. Rick
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Rick
Thanks for the detailed info. This is exactly what I was looking for when I started this thread! I have a buddy with an old Sun distributor machine and he's agreed to help me make the necessary adjustments. I ordered the carb spacer yesterday, so that will provide a great place to tap the vacuum source. I bought the distributor when I was home in September (I'm in Brazil 'till mid-February). I plan to install it all as soon as I get back to the States. I'll update this thread then. Thanks again. Doug |
Doug, here is another article that will be helpful in setting up your Duraspark distributor. I am using the slot market 12. That gives me 24 degrees of mechanical advance. You will be very pleased with your conversion. Good luck, Rick
Duraspark_distributor_recurve_instructions_index |
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