I recently bought a 1981 F250 with 460cid, Duraspark ignition, and a Carter AFB carb. The carb has both timed and full vacuum ports for the distributor.

All emissions stuff was removed.

Anyone know which vacuum port on the carb - the distributor vacuum advance should connect to? thx!

 

On Fords it should be Ported Vacuum, GM products use full vacuum.

 

I've read a lot about this subject. I think the majority of performance engine builder agree that timing should be controlled by manifold vacuum.


It was said "ported or timed" vacuum only came about due to meeting emissions requirements.


One reason not use manifold vacuum is if you have a "loopy" cam, in which case as the idle surges around that will also be changing timing making getting a stable idle speed more difficult.
Also, another case for using ported vacuum is when you can't get the idle speed low enough when using manifold vacuum (with regards to the throttle plate position on transition slot, and such).

Here's a good article on the subject Ported vs. Manifold Vacuum

 

Full Manifold vacuum is a GM thing Ford has used Ported vacuum since the start and long before emissions were even considered or contemplated.

You will consistently see those for full manifold vacuum are GM guys.

 

GM used both, just like Ford occasionally did. It depends completely on how the engine is setup, carb, distributor, etc.

Real world experience with these trucks, if it's a automatic you had better use ported vacuum. You can try manifold vacuum, but you will find the idle is very high when in park. So you turn the idle screw back. Then you will find when you pull it in drive, the engine wants to stall. So then you find a tree and let the truck rest against it in gear while you adjust the idle so it won't stall. Then you put it in park and the idle goes way high, and then when you put it in drive the whole thing "bangs" into gear.

Then you give up and put it back on ported vacuum. I believe if you retuned the distributor to not have as much vacuum advance, and turned up your initial timing, that may be a start to getting it to run on full manifold vacuum. But it's much easier to just plug it in ported vacuum.

 

This truck is a manual shift 4 speed, not automatic, would one port have a performance and/or fuel economy advantage over the other?

I't's a 1983 460 (don't know what it came out of) with stock exhaust manifolds into dual exhaust & high flow mufflers, straight up timing set, Edelbrock performer intake manifold, Carter AFB 9605S carburator, tall 3.00 gears in the 4x4 axles.

 

There are advantages to either. The thing to keep in mind is that the total vacuum advance doesn't change connected to manifold, except at idle. Other than that it makes no difference. Performance will not change, because under acceleration there is no vacuum advance, when the engine is under load there is no engine vacuum. That's why racing engines don't even have vacuum advance at all - it's just something else to go wrong. The mistake for someone who is going to be driving their truck on the street is to think they don't want vacuum advance. It does add overall smoothness and driveability in part throttle conditions - where most street driving is done - and about 15% to 20% better fuel economy.

If you can achieve a steady idle using manifold vacuum, then that would be the way to go. I didn't have much success, it wasn't bad, but it was just enough to be annoying.

When cruising down the highway on level ground at any speed the vacuum signal is identical or near enough regardless of where it is connected. Also keep in mind that an engine under load can only tolerate a certain amount of ignition advance, but at idle - no load - it will happily purr with 30° or 35° BTDC.

Once the throttle is pressed under load, that advance disappears in an instant, back to the baseline initial timing of 6° or 12° or whatever it's set at. Manifold connection of the vacuum advance tends to keep the engine a whole lot cooler in stop and go driving and extended idling. Heavily cammed engines may need manifold connection. As I understand it a general rule is stock cam - ported vacuum. Hot cam - manifold vacuum.

The Y block in my slick is about as simple as it gets, and have experimented quite a bit with curving and different amounts of intial timing. For example one could set the distributor either to 4° BTDC or 17° and then adjust the total cruise timing with different amounts of vacuum advance on the far end. Can connect to ported, or vacuum advance. So long as the total mechanical timing governed by the weights and springs, and the advance slot is adjusted to around 36° it's the same total advance, except at idle.

Basically any engine runs best both power and economy with the maximum possible ignition advance at all times under all conditions up and down the RPM range and at idle, short of ping or engine knock. The crankshaft or initial timing number really doesn't matter all that much, though when it approaches 20° the engine may become hard to crank, especially in hot weather, due to starter kickback. I like deceleration exhaust rumble, a healthy amount of initial advance helps with this.The key to tuning is make sure the distributor mechanical curve is set just right, as much as it will stand without ping or knock. If you want to experiment with lots of initial timing, generally have to get inside the distributor and limit the total amount of advance that can be brought in on the high side.

All of the experimentation should be done with the vacuum advance disconnected and plugged. When you're satisfied with that, re-connect it and get that dialed in. When you're cruising at a steady speed on level ground in high gear - from 30 to 75 say - the RPM is relatively low, so the mechanical advance won't be at maximum advance. This is where the vacuum advance comes in. It utilizes the high engine vacuum that's only present under those specific conditions and pulls in extra advance, usually around 10° or 15°. At highway cruise fuel mixtures are very lean and need a lot of additional advance to get the fire lighted earlier. Most V8 will cruise on level highway around 50° timing BTDC

 

I tried it both ways on my 460, and couldn't tell a difference in performance.

Mine is currently connected to ported vacuum because that lets me set the idle (throttle blades in the correct position related to the idle transfer slot).

 

Wow, great information from all - many many thanks!

I think this info should be in a sticky so anyone tuning their distributor can get it easily...

Ok, my first order of business will be to get the dizzy unstuck - refuses to turn despite my attempts at persuasion - corroded in place - thankfully the hold down bolt & clamp aren't frozen though. At idle it's sitting at 3 degrees BTDC...then I'll see about a spring recurve kit & an adjustable vacuum pot & play a bit.

Repeated PB Blaster soaks have done nothing so instead of getting Kroil, I'm going to make a small batch of that ATF/Acetone 50/50 mix that's supposed to be even better than Kroil...we'll see...

 

Time to break out or invest in a strap wrench. All the modern OHV fords are bad for having this happen. Once you get it to move pull it out and clean it up then replace the O ring and give it a light coat of NOALOX. Be sure to coat the O-ring with motor oil before reinstall this will keep it from seizing again.
NOALOX is the best low temp (below 300F) anti-seize on the market.

 

I haven't heard of NOALOX before. Thanks for the tip.


It better be the best at $23 for 4oz.
It looks like it is mainly intended for electrical work (aluminum wiring and conduit).


I use regular Never-Seize on bolts and such where needed and just regular motor oil on the distributor base/o-ring. What I find causes the distributor to stick is when the owner doesn't change his oil regularly and it builds up sludge under the mounting boss (so you can turn the distributor and maybe lift a little, but not pull all the way out).

 

I'm running stock intake, carb, and exhaust manifolds with the cam timing set "straight up" on my 460.

Looking at the vacuum diagram the factory vacuum advance was manifold but modified so that there'd be no vacuum at idle, unless the motor was getting hot and then a thermal switch would open to provide manifold vacuum at idle, which advances the timing, to increase idle speed, to promote engine cooling.

But somewhere along the line that system failed and now I'm running full manifold vacuum to the distributor at idle which I'm running through the stock blue "v-rest" (restrictor oraface that buffers sudden changes in the vacuum signal).

Base timing is set at 12* so with another 15* or so of vac advance total advance at idle is around 27*. I thought this much advance at idle would cause problems but it has not. Idle is smooth and in park is about 850 and 650 in gear, and throttle response is nice and crisp off the line.

Good luck freeing up your distributor! Also FYI, you can't always trust the timing marks because sometimes the balancer will slip (because the rubber center of the balancer deteriorates) and throw off the marks.

 

Hadn't thought of the balancer beinf off - thanks for that.

 

Thx - I hadn't heard of NOALOX either - looks like it's around $7/bottle at Home Depot - I'll pick some up!

 

Not sure where you were getting pricing from lol $6 for 4 oz
Ideal NOALOX 4 oz. Anti-Oxidant Compound-30-026 - The Home Depot
Yes is it designed for use with for AL electrical connections but works better then anything as an anti-seize in low temp lower torque applications.
For Hi Temp Hi torque AL joints there is Silver Plus.
Jet-Lube of Canada l Silver Plus - Metallic Anti-Seize

Noalox is the best anti seize for keeping AL or steel rims from seizing up on hubs, nuts and bolts that are exposed the elements or any place where there is a risk of galvanic corrosion between mating surfaces, such as in AL housings, like the water pump/housing, in that application I coat the whole length of the bolt in a light film and it will never corrode in the bore of the housing. I have been using it for those sort of applications for 40 years and have never had a single item that was coated with it seize up or corrode together even after a couple decades of service.

It is far superior to the other anti seize products for lug nuts as it wont wash out/off like Kopr Kote or Nikal tends to do over time plus it resists wash out/off from pressure washing. Just don't use it on high heat applications.