66BlueRdster

I’m trying to find a way to limit vacuum advance to 10 degrees crankshaft advance in my 66 roadster. It’s running a ZZ383 with a MSD tach drive distributor (MSD 8572). I’ve read the write-ups by Lars, JohnZ, and Duke, which have been extremely helpful.

I need some help with my application. The ZZ383 has aluminum fast burn heads, which are unique in that they only like/need 32 degrees of total timing advance vs. 36 for traditional small block Chevys. This has made it difficult to run vacuum advance, given most vacuum canisters bring in at least 16 degrees of crankshaft advance. While traditional small block Chevys can deal with up to 52 degrees (36+16) max advance at light cruise, the fast burn heads don't like a lot of advance and ping at 48 degrees (32+16) at light cruise. 91 octane in California may also be limiting the advance tolerated.

I don’t want to reduce the total mechanical advance given the engine runs great at 31-32 degrees with vacuum advance disconnected – never pings. But I’d like to take advantage of vacuum advance for the benefits of throttle response and fuel economy under light load.

My engine set-up:
> ZZ383 stroker
> Idle vacuum: 16 inches at idle
> Initial timing (vacuum advance unplugged) = 10 degrees
> Centrifugal advance = 21 degrees
> Total mechanical advance = 10 + 21 = 31 degrees (GM recommends no more than 32 degrees on Fast Burn heads)
> Vacuum advance (B26 can supplied with MSD distributor) = 16 degrees crankshaft advance
> Max timing advance = 31 + 16 = 47 degrees under light load / steady cruise --> The engine pings at light load, but with vacuum advance disconnected, never pings with 31 degrees mechanical advance.

Net, I've been looking to modify a vacuum can to only add 10 degrees advance via limiters instead of 16 degrees that most cans provide. I’ve thought about the Crane adjustable can but would prefer to use a non-adjustable unit with a limiter plate.

Does anyone know of places to get a limiter plate fabricated? Any other suggestions?

Thanks,

Ed

NEVERL8

Have you considered a variable vacuum advance canister? I have been chasing my vacuum advance settings since rebuilding my engine. I finally just disabled the vacuum advance and set the mechanical advance to 14 deg BTDC. I do have the stock distributor and was able to adjust the total advance curve by putting lighter springs.

Sky65

I've never done it but seems like there should be a way to shorten the notch in the vac can to reduce the amount of advance. Other possibility is to install a bushing on the limiting rod to make it stop sooner. Or, maybe bend the limiting rod closer to the max advance stop shortening it length. Good luck.

Five Window

Drill the rod and insert a small cotter pin to limit travel. I've done this on two cars and it works well.

tangel2

I've put a piece of heatshrink tubing on the end of the vac can where it rides in the channel. Worked for me in the past.

Muttley

Came across this, although I have no idea whether it will work with an MSD distributor:

http://cranecams.com/?show=browsePar...30&showAll=yes

http://www.cranecams.com/pdf/254g.pdf

66since71

Try Kendrick Automotive.

http://www.kendrick-auto.com/ignition.htm

The link is to a section in their on-line catalog for parts just like what you are looking for. Page down pretty far to get to the part you want. (I think the B26 can is a stock chevy part, so this kit should work. Ask them though.)

I have used them before and got great quality parts and personal follow up on my order.

Harry

StrayDog

The MSD distributer accepts GM vac advance units/ cans, there are a variety ...Lars has a print out on the various units & part numbers...once you have selected the unit that works for you, bring part number to Nappa & they will get it for you at a fair price......... my 66 427, with short duration & high lift Comp cam exteme energy cam, alos has the same 17 " of vac at idle ..... i cant recall what vac can i eventually used but i also had to get a unit with approx 10 " vac advance..just a sugestion keep using a vac can as it makes a street engine more throttle responsive & helps with cooling...........better to get a can with the vac advance that will work , rather than try to modify your existing can......................... The Dog

GCD1962

Don't waste your time trying to modify something, buy an adjsutable vac can that can be dialed in exactly as you need it.

olescarb

We limit the travel of the vacuum advance pin to 0.120" but making a bushing that goes on the vacuum advance pin, this limits the advance to 10 degrees from the vac advance. you could also add some metal in via welding and then use a file or a dremel tool to get the desired travel.

henry @ olescarb

NEVERL8

I limited the travel on the OEM vacuum advance by putting a piece of rigid tubing on the the advance rod, the theory being that it would limit total vacuum advance. It did do this but did not solve my problem. The issue was (is) twofold; when does the advance come in (the curve for the advance) and what it is the total advance. I am back to the point of putting in a variable vacuum advance because I (and most likely you) need to control both parameters.

There are two other variables that you may need to look at. The first is the mechanical advance characteristics. I am fighing total advance issues at small throttle openings and light loads (surging). The dance between vacuum and mechanical is well documented and explained by Lars and others. The second thing I need to consider is the idle mixture and how it may also contribute to surging. At very small throttle openings and light loads part of the idle circuit is operating. An overly lean condition could also contribute to surging.

I am still an advocate of a variable vacuum advance but you may also have to look at you total advance and total advance curve.

StrayDog

totally ,,,,,,

AZDoug

I believe those 32 and 36 degree numbers you postulated apply to WOT conditions where the vacuum advance is not in play anyway, as most vac cans don't start to open until say at least 8"-12" hg of manifold vacuum are present.

At WOT, only initial and centrifugal advance are active.

The vac simply advances at high manifold vacuum where the cylinder isn't that full of air/fuel mixture, for better gas mileage.

Now, whether you need to back off your vacuum advance at part throttle conditions may or may not be required, depending on whether you get pinging at true steady state cruise or if it ahppens upon light application of more throttle at steady state cruise. Simply limiting your total vac advance travel will result in worse idle, hotter running and more fuel consumption, if it isn't needed.

You may be better served by getting a vac can that starts to work at an optimum manifld vacuum for your engine and with less total travel.

Driving around with a vacuum gauge in the passenger compartment,a nd having someone note teh manifold vacuum while drive,a nd when pinging occurs will give you a ton of info.

Doug

AZDoug

Also, when you say ping at light load, do you mean steady state cruise, or going from steady state cruise no ping to slightly opening the throttle and then get pinging??

If stead state, no ping, your total vac adavance is fine; what you need to alter is the curve, with a can that starts to work at higher vacuum, and has a shorter travel.

If ping at cruise, yes, the total amount of advance needs to be less.

Doug

66BlueRdster

I’ve been thinking about the adjustable vacuum advance canister but have heard mixed things about it. I like the B26 non adjustable unit’s specs in terms of what vacuum level it starts pulling in advance, but want to figure out how to limit the total crankshaft advance brought in. I’m open to suggestions.

Thanks,

Ed

66BlueRdster

Thanks Harry. Kendrick Auto's web page has the vacuum advance limiter I'm looking for. However, I checked with Kendrick and they no longer sell the vacuum advance limiter plates. Their web site is out of date. They said demand was too low. Bummer.

Thanks,

Ed

66BlueRdster

Doug,

It pings sometimes when accelerating from a steady part-throttle speed. So I think its bringing in too much vacuum advance.

Thanks,

Ed

66BlueRdster

Thanks to all for the great suggestions. I don’t trust myself in fabricating my own limiter lol… so I’d like to go to someone skilled who can. My mechanical advance is pretty dialed in. And the B26 can seems to come in at the right vacuum but needs to be limited in the total that it brings in. So I think I’ll try to add a limiter to the B26 can.

Henry @ olescarb, I’ll follow up with you.

Thanks,

Ed

AZDoug

Then you don't have too much vacuum advance.

What you have is too much advance over too large a vacuum range, in the wrong range.

The B26 starts at about 5" vacuum and is full in at about 12-13" vacuum.

I would try a B1 vac can

the B1 starts at about 10" and is full in at about 16-18".

That will give you good idle and cruise vac advance, then rapidly drop off upon throttle opening.

PS: it is a cheap experiment.

Doug

DansYellow66

I'm interested in this product as my 427 has too much vacuum advance with the NAPA vacuum can that was recommended here (#1813 I think) for cars with low vacuum at idle (7-8 inches). What I experience, and others have confirmed, is a constant light surging - almost a miss) when cruising at light throttle. Disconnect the vacuum advance and it goes away. Of course the engine will barely idle and my temp gage goes up 25 degrees.

Does anyone make an adjustable vacuum can that is functional with 7 - 8 inches vacuum?