63 / 340 HP Vacuum advanced can replacement

[Thomas S.]

Late 63 / 327 / 340HP Original motor.

The VAC is stamped with B1 which I realize is not the correct or at least not the recommended one based on Duke William's very well written article on timing for SHP motors. Plan is to replace it with a B28.

Can the VAC be replaced without removing and disassembling distributor?

[Edward J.]

Re: 63 / 340 HP Vacuum advanced can replacement

Tom, yes it can be replaced, the screws are partly obstructed by manually rotating the advance plate you can remove the screws, The plate will take a little pressure to overcome the spring in the vacuum can.

[Jeffrey S.]

Re: 63 / 340 HP Vacuum advanced can replacement

Use a vacuum pump like a MityVac to draw the plate away from the screws. Makes it really easy. Replace in the same way.
Jeff

[Thomas S.]

Re: 63 / 340 HP Vacuum advanced can replacement

Ed & Jeffrey

Thanks for pointing out to me what should have been obvious. I did try to move the plate by hand and when it would budge, I came to the wrong conclusion that is didn’t move. Of course I should have realized that it had rotate somehow or there would not be any vacuum advance.

Tom

[Duke W.]

Re: 63 / 340 HP Vacuum advanced can replacement

AFAIK the only "brand" that has any B28s left is Airtex, p/n 4V1053.

Duke

[Thomas S.]

Re: 63 / 340 HP Vacuum advanced can replacement

Yes - exactly correct. That's the part number I just received. The car actually runs very well with the current setup so I am looking forward to setting up the vacuum and mechanical advance per your paper which should make a big difference.

Two related questions -
Stock Carter carb 3461S - Measuring the vacuum with a tee into the VAC line is OK, yes or no?
The stock PCV setup on this motor has a few points for potential leakage, the motor block donut, both ends of the tubing leading to the PCV valve. How much of a factor is leakage through the PCV plumbing or is that factored into all of the vacuum measurements?

[Duke W.]

Re: 63 / 340 HP Vacuum advanced can replacement

Since the 3461S provides full manifold vacuum to the VAC, teeing into the VAC signal line is a good and convenient way to measure manifold vacuum.

ANY vacuum leak can cause problems, so if you suspect there is a leak anywhere in the PCV system you should verify and correct, especially on the suction side, which would be from the back of the block grommet to the carb. The inlet side should also be sealed so only clean air from the downstream side of the air filter is drawn into the crankcase.

What number VAC is installed that you will be replacing with the B28.

Duke

[Thomas S.]

by Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto Repair & Restoration
Broomfield, CO Rev. New 4-19-01


..............................1964 Corvette exc. FI
..............................1964 Impala, Chevy II
..............................1965 396 High Perf.
..............................1965-67 283, 409
..............................1966-68 327 exc. Powerglide
..............................1967-68 All 396
..............................1969 Corvette 427 High Perf.
..............................1969 396 Exc. High Perf.
..............................1969 Corvette 350 TI
..............................1969-70 302 Camaro
..............................1970 400 4-bbl
..............................1970 396 High Perf.
..............................1970 Corvette 350 High Perf.
..............................1973-74 454 Exc. HEI

[Duke W.]

Re: 63 / 340 HP Vacuum advanced can replacement

Over the years ill-conceived parts consolidations were made by GM and the aftermarket followed suit, plus they are full of plain old errors. Note that the second list is "1964 Corvette ex. FI". What about L76? It has the same cam and same idle behavior, and the OE VAC was the 236 16 (B28 current replacement) same as FI except (I think) very early L-84s with the 375R systems that were built with '63 distributors.

Parts catalogs, both GM and aftermarket can't be relied on for some items and the VAC is probably at the top of that list. Lars did a great job on his paper but he can't have been expected to correct all the errors for the scores of applications.

Selecting a VAC is very easy. The first task (for original full time vacuum advance or a ported configuration converted to full time) is to get an accurate manifold vacuum reading at normal idle speed (in neutral with a manual trans and Drive with an automatic) with the engine fully warmed up. Then apply the Two-Inch Rule and select the least aggressive of the following three that meets The Rule.

8" B28
12" B26
15" B22

That's it! Very simple.

Duke

[Thomas S.]

Re: 63 / 340 HP Vacuum advanced can replacement

So after installing a B28 can, pre-ignition upon any kind of acceleration. 327/340SHP, 4-speed, 4:11, Carter 3461S full-time vacuum @ 750RPM was 13". 91-Octane fuel. R44 plugs. Advanced set with dial-back to 10-BTDC. Mechanical advance springs unknown but appeared to be a much heavier gauge wire than the Mr Gasket springs. Weights have the number 37 stamped on the bottom. Running a Lectric single wire pointless ignition instead of points.

I am not the original owner so I can't vouch for any internal modifications but with the B1 can, the car ran real strong, with no detonation at any speed including good good top-end. Low end is very good but of course 4:11's will tend to make any engine pull at any speed so that is a subjective statement.

With the B28 installed, I changed the advance springs to the Mr Gasket silver type. Again, with the 4:11's you are getting into high revs relatively soon, so starting at 2,000 RPM seemed Ok. The detonation was still present like before.


So I went back to the B1 can, left the Mr. Gasket silver springs in place and as before, no detonation at any speed or load.

There are literally a lot of moving parts here, not to mention unknown factors such as internal modifications that could have been made.

[Duke W.]

Re: 63 / 340 HP Vacuum advanced can replacement

Based on my experience, including recent, of helping owners develop an optimized spark advance map, I'm surprised to hear this, but the underlying cause may not be obious.

The centrifugal advance curves in the plots looks generic. In fact, they really doesn't make any sense. Springs do not effect the total amount of centrifugal advance, just how fast it comes in. What we need is the specific curve for you car.

The problem with the B1 VAC is that it doesn't start until about 10", which is barely below your reported idle vacuum. When new, my 340 HP SWC had a serious idle stability problem and would stall after a couple of minutes idling unless you blipped the throttle. I finally traced the problem to the OE 15" VAC, which was dithering. That caused a loss of total idle advance, which lost idle speed, which lost more vacuum... it was a positive feedback loop in the wrong direction. The solution was the 8" B28, but at the time (mid sixties) I was able to buy an OE 236 16, which has the same specs. This is how I came up with the Two-Inch Rule.

If you want to pursue this I need the following info.

1. The centrifugal measured with a dial back timing light... RPM at start point and end point with maximum advance supplied. The OE curve is... start @ 700, 24 @ 4600.

2. Vacuum readings for start and stop points for the B1 and B28.

3. Total idle advance, which is the sum of initial and vacuum and maybe a degree or two of centrifugal if it starts at less than idle speed. It should be in the range of high 20s to low 30s.

It would be best if we talked by phone. Email me through the TDB and I'll send you my phone number.

Duke