Get the crane cams adjustable vacuum pot.Put the hose on a ported source

Set the lockout to about 10 degrees full advance
Adjust the spring tension to the light side so the vacuum pulls in timing easy
set the base at 14 degrees.
run 1 light and 1 medium spring on the mechanical

Using a manifold source can pull timing at engine startup while cranking and can cause starter kickback . it can make starting hard especially when hot, and can break starter housings at the nose.

 

You don't want to dial back your initial advance, because as soon as you go to WOT the vacuum drops to near zero and all vacuum advance drops away, leaving you with just your ultra low initial timing and turning the engine into a real dog.

What you need is to limit the vacuum advance diaphragm can's travel to limit the amount of advance it adds.
MSD and Crane both make adjustable stops depending on what distributor you have.
You may also want to have an adjustable vacuum advance diaphragm, but don't get confused, the adjustable can only adjusts the inches of vacuum needed to operate the mechanism, but does not limit the amount of degrees the mechanism adds....that's why I said you need a travel limiter.

Crane's version for the HEI and Points style distributors, you can get a kit with the vacuum can ,limit bushings, and the limiter plate, or you can get just the plate if you already have an adjustable can:
http://www.cranecams.com/333.pdf

MSD travel limiter for the HEI type is #84281

 

You don't want to dial back your initial advance, because as soon as you go to WOT the vacuum drops to near zero and all vacuum advance drops away, leaving you with just your ultra low initial timing and turning the engine into a real dog.

Right on the money! If done right, you can still have 32-36 degrees total advance and 10-14 degrees vac advance with NO pinging.

I don't know how the MSD stop plate works, but the Crane limiter is kind of a kludge, because it actually pre-loads the diaphragm spring. In addition to messing up how the diaphragm is supposed to work, it requires resetting the initial advance every time you adjust the stop plate. And if you don't use the limiter, the Crane adjustable canister will get you nearly 30 degrees vacuum advance! Yes, the pin moves over .300". (Insert "WTF" emoji here.)

Instead of pre-loading the spring, I guess you could always drill and tap a hole and install the plate on the other side to limit pin travel. But you can make your own stop easily. I don't have a diagram or pics handy, but what you do is install a stop plate that limits pin movement to around .110", which gets you 10-12 degrees max without a rate change. You might try to google "HEI stop plate".

I used the canister below on GM HEI distributor #93440806 to replace its stock canister that pulled >20 degrees advance. Works well for a moderate performance engine making 16-18" vacuum. I am running it in a 355 with 9.4:1 compression, Vortec heads, and 32 degrees total (initial + centrifugal) advance. Works awesome, and now my Crane kit is sitting on the edge of a workbench, perilously close to the trash can.

ID#: AR12
P/N: Eichlen/NAPA VC1838
Borg-Warner V386
Application: 1975 350 Buick
Starts @: 7-9” hg
Max Adv (Distr. Degrees): 7 @ 10-12” hg (14 crankshaft degrees)

 

I am using a Cardone basically OE distributor atm that I am pretty sure is non-adjustable so I will be looking into getting an MSD. For the meanwhile is 14btdc with timed VA 55* at 2000~2500 rpm too much until I get new distributor? I haven't taken it on the street yet as I still have to finish putting the driveshafts back on and I don't know how it will do under load. A lot of research I have done suggests that 50~* isn't that bad when cruising...

 

I haven't bought the vac can yet but I am going to. What would happen if I just bump initial even more? Is there a difference between having 10* vac adv + 10* mechanical vs just 20* mechanical?

 

Not at idle or cruise...but there will be a big difference once you go to WOT and that vacuum advance drops away.

You want to set things up with the initial being as much as the engine will take without detonation as the mechanical advance adds in more as the rpm goes up until you reach the total mechanical. A curve timing curve that keeps the engine just below the detonation threshold will usually result in the best power.

The vacuum advance is just to improve mileage if it is hooked to a ported source, or for mileage and idle quality if hooked to a full time manifold vacuum source.

 

http://www.crankshaftcoalition.com/wiki/Hot_rodding_the_HEI_distributor

This pretty much sums up how you setup a timing curve. It is easier with a distributor that allows for easy adjustments (MSD, Crane, etc) but can be done what you have right now.

Midway down the linked page you'll find the subheading 'ignition advance' - read it and ask questions about anything you don't understand. Everyone can offer up some advice as to what you ought to do, but it's a great thing when you can dial it in yourself to achieve maximum performance and fuel economy.

FWIW, and don't take this the wrong way, how you've described you're dialing in your timing is bass ackwards. Read the article and you'll see what I mean.

Good luck - Jim

 

The way I've always thought about it since back in the day is this: If you hear pinging when you slightly roll on the throttle, but it goes away when you apply more throttle, then you have a vacuum advance issue. That means either too much vacuum advance (pin traveling too far) and/or vacuum advance not dropping out soon enough as manifold vacuum decreases under moderate load.

The first problem can be fixed by limiting movement of the pin as discussed in above posts. The second requires a canister that starts reducing vacuum advance sooner as throttle is opened. For example, when gradually opening the throttle, the AR12 canister I mentioned will start reducing advance when manifold vacuum drops to 12", and it will be gone by 7". Others may not start reducing vacuum advance until 8", which is pinging waiting happen with an engine that makes 18-20" vacuum.

As someone above mentioned, get mechanical advance (initial + centrifugal) optimized with the carb vacuum advance port plugged. Then connect the vacuum advance hose and work on getting rid of any pinging at throttle roll-on. But DON'T TOUCH mechanical advance again, unless you were too close to the ragged edge and pick up some pinging on hot days or with poor gasoline.

 

You hear a lot of people to say, set it all in at 34-36 at 3000 and let the initial or base be what it may after total is set. While i do subscribe to that logic, I also take a different view on timing. A friend once taught me to utilize a vacuum gauge to set your base and to be mindful of your cam's powerband or 'at the rpm the cam starts to come alive' when setting all in time. Most articles and books you read don't speak to this, rather they always harken to the conventional wisdom of "34-36 all in at 3-3500. As to timing curves, they're just basically guides. If you buy an MSD pro billet distributor, it'll show you timing curves such as use the light silver and light blue with 'this bushing' to get all in at 2700 RPMS. Again, I'd tell people to think about your cam. If you've got a really aggressive one, say "it starts at 3500," then what good does getting all in at 2700 going to do ya? If that car is going to race on the strip 'or elsewhere,' you're going to launch at 3500-4500 RPMS or whatever stall rating you have. So take the timing curves with a grain of salt depending upon your application and cam...,

 

Well I decided to keep my current setup for the moment however.. I started using full manifold vacuum and I have it set up at 32* at 850rpm idle and it hot starts without any issue and it hits 50* at 2k~2.5krpm. It seems to run smooth and idle is much quieter now. I also had a bog and sputter/backfire on my edelbrock carb so I put in new rods, springs, and primary jets and now that's pretty much gone.

 

I prefer full manifold vacuum advance on most any build with an above stock cam spec - it always seems to help the starting and cold idling. From your statement I'd guess that you didn't dig into the link I posted in #12. If it's working out OK for you, then fine. And there is a decent chance that you have it set close to optimally, but there is a better chance that you are leaving both some performance and fuel economy on the table.

 

shouldnt initial timing be set by a stomp test? how dose it take off from 1000 to 2000 rpm depending on wieght,trans,gears,ect.

 

No, unless you're running a stock converter that locks up at 16-1800.

 

If this is testing with no load on the engine the values of advance to RPM insofar as vacuum is concerned is not accurate because it take very little throttle opening to hold 2000 RPM compared to the opening when the engine is tugging the vehicle so the vacuum reading in this case is much higher and the amount of advance follows that.


Ping will occur if the vacuum advance is too far and the centrifugal is starting to come in. This becomes an additive situation that requires again a driving solution or a distributor machine that simulates driving conditions so that an adjustable advance and the centrifugal can be lined up. The factory doesn't use adjustable advances for the most part because they are building to a specification and the assembly line parts are designed to fall inside that specification without adjustment. Hotrodders are outside the factory specification so working out the details of what will or will not work falls on us.



Bogie