Selecting The Best VAC for My Engine

[Dan D.]

I could use some help in selecting the best vacuum canister for my engine. I am in the process of trying to set and map the ignition advance. Here is what I have:

1957 Corvette Driver
327 CI + .030
375 (1963) Rochester Fuel Unit.
Melling 22200 Hydraulic Cam
Compression Ratio 9.5:1 (calculated)
Restored fuel unit, engine rebuilt - first run-in
Advance converted to full time manifold vacuum

The engine is making only 11 -12" Hg vacuum at idle. I have the centrifugal weights wired closed until I get the vacuum working right. I have lighter springs for the centrifugal when I get to that point.

When I bought the injection unit in 1970 it had a B1 vacuum canister. I don't have a specification for what canister it was suppose to have, but given that the B1 is a poor choice and being factory configured with port advance that might well have been the factory supplied can. The engine now has a B22 can as supplied by the FI restorer. That wont work as a B22 needs 16" + the 2 inch rule.

In looking over the listing of different vacuum cans GM made I don't see a real good match. The listing I have does not include 1963, for whatever the reason, but that certainly is not what I would use anyhow. Mood point.

The B20/B26 (may be no longer available) would be marginal; 11 - 13" for full in. The only other choice I see is a B28 which may be too aggressive - 5.75 - 8" full in. This is the canister they used with the 30-30 cam. The advance will come in very quickly with this can.

The Melling cam I have is fairly aggressive - 222* duration - same as the 097 cam only hydraulic.

So what do you guys think? All recommendations are welcome.

One other question - how do you set dwell on these FI units? The distributor cap window points straight to the front of the engine, and has about 3/4" clearance to the back of the plenum. It is impossible to get an Allen wrench on the adjuster screw. If I retard the timing enough to access the screw the can hits the coil bracket and is so retarded it wont run. So what I did is retard it all I could with the engine off where I could just barely get a wrench on it and turn it a little bit. Then advance the timing back, start the engine and measure dwell. Several attempts later I hit 30*. There must be an easier way.

Appreciate all help and advice.

-Dan-

[Troy P.]

Re: Slecting The Best VAC for My Engine

Seems to me I cut off a 90 degree allen wrench so it would fit between the plenum and distributor. But eventually I gave up on points and went with solid state.

[Dan D.]

Re: Slecting The Best VAC for My Engine

I did the same thing Troy. I bought a piece of 1/8" hex rod and bent it, but I still could not get on it. But that might have been just me. Anyhow, I glad to see that it is a real world problem and not just me doing something wrong. I hear you on the points/solid state.

-Dan-

[Thomas S.]

Re: Slecting The Best VAC for My Engine

Search this site for Duke Williams. Tons of info about timing and VAC selection.

[Dan D.]

Re: Slecting The Best VAC for My Engine

I have read most of Duke's articles. They are a Godsend. But I don't see one that addresses this specific problem. I'm hoping he will chime in. He is good at making recommendations.

-Dan-

[Duke W.]

Re: Slecting The Best VAC for My Engine

Since the original ported vacuum advance is converted to full time you need a B28 (Airtex 4V1053) VAC in order to pass the Two-Inch Rule.

With you modest CR and relatively long duration, high overlap cam the engine should accept a very aggressive spark advance map. I set up my 340 HP SWC with the 30-30 cam spark advance map back in 1965. Since the centrifugal start point may be below where the engine will idle stably long enough to set initial timing, use the total WOT timing method. With light springs rev the engine a few hundred above the point of max centrifugal (VAC disconnected and hose plugged) and set at 36-40, as high in that range as the engine will tolerate without detonation. Given your modest CR and fairly long duration, high overlap cam, you should be able to achieve the upper end of the range.

Set idle speed "as low as possible, commensurate with acceptable idle quality" and IMO you should target about 900.

As a final check, look at total idle advance (VAC connected) and assuming the distributor still has the OE 24 degrees centrifugal, total idle advance should be low 30s.

The good news is that if the cap window is approximately normal to engine centerline, the dist. is likely correctly installed.

Duke

[Dan D.]

Re: Slecting The Best VAC for My Engine

Hi Duke, thanks for helping me.

I was leaning toward the B28, but I was concerned that it might be too aggressive. I have the centrifugal wired closed right now, but I will install the light springs and map it. I have a dial-back light so I should be able to set the WOT advance okay and will try and get close to 40*. In the meantime I will get a B28. I have a NAPA close by, but I'm thinking they may have to order it.

Right now I have the initial set at 10* and without the vacuum I can get a stable idle at 1000, but 900 is iffy.

The centrifugal has been measured at 11.5* (23 crankshaft) so I consider that acceptable.

The distributor window is dead-on center line with 10* initial and the canister clears the coil bracket by a little bit. So I take it that Melling has indexed the gear correctly. I initially indexed the distributor for #1 pointing forward and with the damper timing mark at 8* I adjusted the points to where they just just broke using an ohmmeter. I pre-primed the FI unit and had gas flowing to it before I attempted to start the engine. The result was the engine started instantly and ran perfect for almost 20 minutes. How good I felt!

Thanks for the advice Duke. I will do everything you said.

-Dan-

[Duke W.]

Re: Slecting The Best VAC for My Engine

NAPA no longer has B28 VACs under their Echlin brand. The only remaining B28s that I know of is the Airtex number I previously mentions. Do a Web search.

An aftermarket dist. gear worries me. Verify per the '63 Shop Manual the plug wires are properly indexed. No. 1 should be on the terminal near the pass. side of the window. If not, reindex them properly, which will likely mean that you need to remove the dist. and reinstall it.

Duke

[Dan D.]

Re: Slecting The Best VAC for My Engine

Duke, I just found that out about the B28; they could not cross it to any other brand, and so far I have not find any on the web. This is getting serious.

Regarding the wire indexing, I don't don't have a 63 shop manual so I indexed them like the older engines are with #1 pointing forward. The only thing I see in doing this is the plug wire fitment. The wires seem to lay okay, none too short or long, so I am tempted to leave them as they are.

If I am missing something here please let me know.

-Dan-

[Jim D.]

Re: Slecting The Best VAC for My Engine

Rock Auto has the B28 - https://www.rockauto.com/en/parts/wv...m+advance,7232

[Dan D.]

Re: Slecting The Best VAC for My Engine

Thanks Jim. I ordered one. Hope it is the correct one - will find out when I get it. Will let you know.

-Dan-

[Jim D.]

Re: Slecting The Best VAC for My Engine

Thanks Jim. I ordered one. Hope it is the correct one - will find out when I get it. Will let you know.

-Dan-

I ordered 4 of them with that part number and they were B28's and I verified their specs. using a Mighty-Vac.

[Duke W.]

Re: Selecting The Best VAC for My Engine

Like I said the only remaining B28 VAC is the Airtex 4V1053. Web search that for available sources.

#1 plug wire is on the tower that skirts the pass. side of the window, which would be the left side of the window when you look at the dist. head on. Then go in the firing order 1 8 4 3 6 5 7 2.

Duke

[Dan D.]

Re: Slecting The Best VAC for My Engine

Jim, your post makes me fell much better. I just don't trust anything. I will measure it with my Mityvac as soon as I get it. Got an e-mail this morning from Rock Auto that said the can had been sent. I will post the results when I get it.

-Dan-

[Dan D.]

Re: Selecting The Best VAC for My Engine

Okay Duke. It sounds like I have it one tooth off. I have #1 pointing right under the window. I have a B28 coming. Will post when I get it and measure it.

-Dan-