Mechanical advance only? Mallory Dual Point? Oh, and a cool pic of my engine!!!

[James_Douglas]

Other than trial and error with a set-test drive-ping-redo approach...the only way to properly match a distributor curve to an engine is to have a distributor machine AND a chassis dyno.

The idea of course being able to have the maximum advance across all RPM and max loading conditions just short of detonation.

I would attack the problem a little different. I talked with someone a very long time ago that was running those carburetors and ran into the same issue. He said an "old timer" showed him were to drill into the carburetor (90 degrees to the venturi) and installed a brass tube. He soldered it in as I remember. The tube he said was welded closed on the venturi end and a small hole drilled that was the same size as some other 1bbl that had a vacuum port. The tube was ground flush to the venturi wall to match the venturi.

It would be a fair amount of work, but if you can find or identify that location, you can have your cake and eat it as well.

James

[Jerry Roberts]

Crazy Casey , On the Pilot house portion of this site , there is a guy wondering if anyone needs a distributor from a 1954 dodge power wagon , 6 cylinder , without a vacuum advance . Perhaps this would work for your application . Look under todays postings there , look under " can anyone ID this distributor " the distributor is owned by billwillard in N . Carolina .

[grey beard]

Don't know if this will help your situation, but all B series Dodge trucks over one ton in size used distributors without any vacuum advance - just the centrifugal advance alone. Interestingly, when I checked the shop manual specs for these distributors, they all used a 20-22 degree centrifugal advance value - wirhout regard to whether a vacuum advance was present or not. Please understand I'm talking crankshaft degrees when I say 20, and not distributor degrees, which are only half that value. AND all the distributor specs called for max centrifugal advance at 2500 rpm on our L6 engines.

All high performance engines I have known did not use a vacuum advance. They rely on a good centrifugal advance curve and then dial in initial advance for the total they wanted at peak torque rpm. So, if performance is your goal, you really do not need a vacuum advance. My understanding of the vc is that it is primarily an economic fuel saving device for part-throttle on-and-off driving. Tractors, big trucks, industrial engines and most high performance engines do not use vacuum advance units.

[crazycasey]

grey beard,

That is just the info I am looking for. So from what I can tell from your post, just unhooking my vacuum advance will net me the same timing curve as swapping to a non-vacuum distributor? Am I understanding that right?

Thanks,

Casey

[James_Douglas]

With respect...

I still say for a street car that not using a vacuum advance is a bad idea.

http://www.stoveboltengineco.com/howto/vac.htm

Trucks use gearing to accelerate and then run at a constant speed. That is why they had non-vacuum advance distributors.

Saying that because trucks used non-vacuum it is optimum to use them on cars is mixing apples and oranges.

James

[crazycasey]

James, that's a good link! I read your post earlier about running a port into the venturi. My carburetors have a port going into the venturi, right below where it is at it's smallest diameter, just as it's starting to increase in size again. I have driven my car with a vacuum gauge hooked up to this port, and I am failing to get a significant signal (I think the most I ever saw was 3" of vacuum. I don't know if it weld help to make the hole bigger or not, but I am leery of screwing up a set of good carburetors. The local Holley 94 guru over on the HAMB site says that for the Dodge's vacuum advance to work properly it needs to be hooked up to a port right above the throttle blades. He informed me that there are bases for the Holley 94 that have a port in them for this purpose...I just have to find two of them, rebuild them, fit them with extended throttle shafts and new butterflies...hence the reason I was looking for an easier option.

[crazycasey]

You know...I just re-read Tom's bit about vacuum advance over on the stovebolt site. He was talking about how vacuum advance adds timing at idle and light load conditions. That makes sense if you are hooked up to manifold vacuum, which is highest at idle and under light load, but when you're hooked to a ported vacuum source above the throttle blades the vacuum signal is reversed, which would only advance timing at higher engine speeds. I know Tom Langdon is a GM guy mostly, and the GM and Dodge vacuum advance systems are different. Now I'm really confused!

[dezeldoc]

You will only need one base, you don't need to pull the vacuum from both. i may have one let me check.

[grey beard]

Casey,

Fpr what it's worth, I ran my Pilothouse for two months with no vacuum advance until I found a working unit. I found very little difference in performance with or without the unit, and no significant difference in economy either. I have read threads on this forum where people claimed extraordinary things changed in their lives when the vacuum advance kicked in. This is a half ton pickup with a garden variety three speed that tops out around 60 mph, so the comparison is perhaps not too similar to your own unit.

You are absolutely correct in your assumption that manifold vacuum and ported venturi vacuum are diametrically the reverse of each other. Venturi vacuum increases with engine rpm, while manifold vacuum does exactly the opposite. The GM distributors that use manifold vacuum to the advance units all required disconnecting the vacuum line in order to set initial timing. When hooked up at idle, the timing advanced significantly. Then when the throttle was opened, the vacuum advance unit actually retarded the timing.

Our MoPar distributors that run on venturi vacuum are the exact opposite of this. As engine rpm increases, so does venturi vacuum and distributor advance. When coupled with centrifugal advance, the two will give just a little more than 40 crankshaft degrees total advance - in addition to whatever initial timing advance you give it at idle.

All my training in this area comes from Ford Division, when I was a service trainer, and went to school regularly on these issues back in the muscle car days of the late sixties and early seventies. My understanding of timing advance is that no engine can tolerate much more than 45 crankshaft degrees of timing advance without diminishing effects. When you think about it, 45 degrees of total advance at 2500 rpm means the spark event is occurring when the crank throw is half way up on the compression stroke. This means the piston is only half way up its stroke, as well, and here we are, lighting the fire already.

We all know that flame front propogation inside a combustion chamber takes place at the same speed, whether the engine is at idle or at wide open throttle. Gasoline and air mixtures all burn at the same speed under the same compression. As engine rpm increases and flame front speed remains unchanged, the timing event must be made to occur earlier and earlier, in order for combustion expansion to have its maximum effect on the engine's performance, which is what all this is about, anyhow.

To get this, we need to have maximum combustion chamber pressure occur just past top dead center on the crankshaft, when the piston is beginning decent on the power stroke. This gives the full stroke of the engine's downward power stroke to the rear wheels. If the timing event begins too late, the piston will already be part way down the power stroke at maximum torque rpm. We don't want this because it costs power.

If your car was on a chassis dyno and we ran it at rated rpm, we could just dial in the exact amount of total advance it takes to get maximum torque and horsepower, but this amount would stall the engine at idle, and would not allow it to start. The idea of advance curves that occur as engine rpm builds was a good one, and is very helpful for economy and performance. Unfortunately, just maintaining factory spec advance curves today is problematic, without a distributor test bench. I can do it with my digital advance timing light, but it takes a lot of time and figuring.

When you modify an engine as you have, the advance curve requirements for your engine are now unknown without a chassis dyno to help set a new curve. The best you can hope for is total advance of 45 crankshaft degrees at whatever engine rpm you hope to find peak torque and horsepower - I'm guessing mebby 3500 rom in your case. Our stock motors want this at 2500 rpm. How fast this advance kicks in is determined by the combination of centriugal and vacuium advance you are using - a tough recipe to make up without a dyno or lotsa' trial and error.

Hope this is not too confusing . . .

[52cambridge]

beautiful!!!!!