There's only one problem with his information and its credibility on here. GM!

 

Ok, Gary, on my y block there is no extraneous gadgetry to confuse the vacuum advance.

One of the reasons I haven't messed with it, (the swap itself takes about 2 seconds) to manifold vac tho i'd imagine the idle mixture needs adjusting and the initial backed way off? Dunno. No big deal, I guess. Maybe tomorrow.

Need to "bed in" new drums and shoes and make a final adjustment, so in the furtherance of advancing FTE knowledge base... Stay Tuned!

 

Dimora - Try it, try it first by unplugging the vacuum advance entirely and adjust the timing up manually by turning the dizzy to see how it responds. I'll bet that you'll notice a stronger idle that idles lower and smoother, takes less fuel, and if it's an auto drops less RPM when put in gear.


Tedster9 - An auto going in and out of gear can be planned for in tuning, the AC going on an off is an issue, but I find less of an issue with the higher timing provided by manifold.


You're right about it being a package deal, one adjustment effects all others, things must be thought out and done rather methodically in steps. "Curved" dizzies on the other hand are something else, all dizzies are curved but who knows to what goal and to what expertise. "Curving" a distributor is really something best done either by yourself or by a professional who has YOUR vehicle to work with.


The amount of vacuum provided with an open throttle by the two sources can vary greatly. Much depends on the carb in question and how it's ports are done. A Holley for example will give a stronger initial ported vacuum then an Edelbrock. This is cause the port is lower on the Holley. Also higher throttle positions can vary a lot to, there can be a significant vacuum difference between the base of the venturi where the throttle plate is and under the carb or in a runner where the manifold can be sourced.


If you try it, do not automatically back off the initial, it shouldn't need to be backed off at all unless it develops a part throttle ping. The issue will be how much the vacuum advance advances, it can be too much with many stock vacuum cans. I may have mentioned it earlier or in other threads of mine but I suggest finding your ideal idle timing first, then see where it lands with the swap.


Gary - I know your a smart guy and have put a lot into this engine. Still may I suggest, if you have an adjustable vacuum advance back it off all the way so that the lightest amount of vacuum brings it to full advance then change to manifold source. This will produce a stable advance and thus stable idle and can be adjust from that starting point. The issue will then be a mix of how much advance where, initial, vacuum, idle etc. This I can not predict for you, but assuming the vacuum doesn't over advance will likely work with current settings. Risk will be a part throttle ping which may need to be adjusted out.

 

Okay, sounds like a plan. Right now I'm at 14-16 deg initial with a 10 slot, does that sound right? I never looked inside, but total mechanical indicated on the tape is 36 deg all in before 3000, so it's gotta be a ten slot.

I have the vac advance pegged at 12 turns clockwise. It did ping once today in high gear, low RPM, low speed tooling around town, just barely though. That's pretty close to ideal torture test, tho maybe should back off initial a bit.

It runs better than ever, having taken the time to get everything dialed in, ignition, valve lash, timing, then carb adjust, etc.

 

Tedster9 - You're working off the Crane instructions aren't you? http://www.cranecams.com/uploads/ins...s/272e_(1).pdf
FWIW you'll notice it says manifold vacuum.


Anyway, all the way clockwise(full advance with minimal vacuum) is the way to start, and sounds like your right on track with your numbers. I see no need to make any other timing changes then to swap sources.

Once you do you'll have to re-adjust idle settings on the carb. Then get a timing reading and make sure it's rather stable, IDK what dizzy you have but with 15ish initial probably in high 20s at idle. A little on the high side for what I find to be an average ideal(Fords tend to advance a lot), but I also find that a little high at idle has no ill effects, at idle anyway, as long as it drops when throttle is applied so there is no low throttle ping. If this change causes a problem that(low throttle ping) or an unstable idle like Gary has had issues with are it. The problem is they have opposite solutions. If it pings a turn or two counter-clockwise will make the timing advance fall off faster to avoid the ping, but will also tend to lead to a less stable idle. If a balance can't be found the answer is to take out initial but this requires a re-curve to get back total. Most of all I find it's absolutely worth the trouble.


Enough ranting, good luck, looking forward to hearing what you find.

 

Well, sort of. I just tend to read. A lot. I'm sure I've looked at that .pdf before. The dizzy is a stock Motorcraft, rebuilt by somebody. 34-2808, don't quote me on that. Pertronix and Flamethrower coil, two barrel Holley.

A good, tight dizzy curved right really helps make a motor purr. Gummed up, worn out and rusty inside, they get intermittent/erratic timing and sloppy, poor idle.

I don't see how (in theory) switching to manifold vacuum could possibly cause ping (compared to ported) unless there was a fault in the breaker plate (sticky) this would be off idle acceleration, right?

 

Brute - I will give it a try, although not right at the moment as things are really, really busy. But I do understand the concept of turning the adjusting screw on the can such that the least amount of spring preload is on the diaphragm. That won't change the total amount of vacuum advance, but it shifts the advance down in the vacuum curve, meaning that advance starts and ends earlier. The idea would be to get the vacuum advance curve below the level of vacuum at idle, so that any changes in vacuum don't change the advance and therefore don't change the idle speed.

As long as that can be done without the engine pinging under part-throttle load then all will be well. The problem will be if it does ping, but I'll cross that bridge when/if i get there.

Tedster - The way manifold vacuum can cause ping is if there is a difference in the amount or timing of the vacuum vs ported. And that depends on the carb. I don't know about Holleys as I'm an Edelbrock fan, but down close to idle most carbs will give you less vacuum on ported than you will get from the manifold. It isn't on or off but a gradual thing, and if you are driving in that part of the throttle then you can get more vacuum from the manifold than on ported.

 

Well I got it warmed up well after finishing the brake job. Swapped out connections and took it for a spin. Right away it seemed to ping a lot just cruising around. Going to have to experiment a bit and make the ping go away.

Seems to me the main benefit would be cooler idling temps in stop and go in the summer. Other than that, not sure.

 

If you are getting pinging then you have more advance than with ported vacuum. So, your ported source doesn't have as much vacuum as manifold vacuum. Answers that question.

 

I'm not sure it has more suck, just that it's already advanced 100% rather than coming up on a curve.

 

Well, that's a good point. I guess I assumed that he was beyond the point where ported is fully "on", but perhaps not. So, when are you doing your experiment?

 

You guys got it, manifold does have more suck, but with the adjustable can that's easy to deal with. The issue that causes the ping is that the advance starts at full advance and then must fall away as throttle is applied and manifold vacuum drops. If it doesn't fall away enough you get a ping.


Tedster9 - Tuning wise the next step is to adjust the can a turn or two counter-clockwise and try again. I'm assuming that when you made the change you had to adjust down the idle, and what was the timing at idle?


Benefits wise a cooler idle is on there, though if that's an issue you need cooling system upgrades. For me it's a stronger idle and off idle engine with better response, easier starting(mostly hot) and fuel economy.

 

Kind of. I'm thinking there's a threshold level at the part throttle, just off idle point. It's actually not retarding the timing fast enough at those load levels. So it needs the total vacuum backed off a bit

The trick would be to see what the total advance is at highway cruise speeds between both manifold and ported, maybe this would be somewhat different despite an equal vacuum indication, I dunno. A somewhat shorter statured person riding in the engine bay 60 - 70 mph with a timing light would work best to verify this. Hm.

 

No, the cooling system is excellent. Though I try to avoid stop and go traffic jams in the summer on general principles. The idle sped up a bit but nothing to adjust as far as test purposes seemed to me. Didn't wander or anything. I did not check with a light. Need to do that.

 

Exactly, this is a little worse with Fords then Chevys as Fords are MUCH harder to limit the amount of advance the vacuum can apply and tend to over advance. Still it's quite manageable in most cases.


I'm not sure what you mean by "total vacuum backed off a bit." though true anyway that statement can be taken, the solution that can easily be used is turning the adjuster to increase the amount of vacuum needed for full advance and generally move the curve down. This will make the vacuum advance fall off faster reducing/eliminating the ping.


Essentially if you idle at 20in vacuum and it's adjusted to only take 10in to reach full advance then when at part throttle and you have 12in vacuum you still have full advance and it pings. But if adjusted to say take 18in of vacuum to reach full advance then it will have full advance at idle but as soon as you tip into the throttle the advance will fall away.