another BB vacuum advance canister question

[Keith B.]

For my 66 Chevelle 396 engine board .30 over with a stock GM 454/425 crate replacement roller cam(GMPP 454 cam 211/230), stock 702 heads, stock 396/360 Holley carb and a stock GM 396/325 points distributor. on acceleration its fine. but once around 2K RPM it wants to have a hiccup up or miss. I reset the dwell on the points to 30. the timing is set to the service manual. I am thinking my vacuum advance is not correct or working correctly. I was at a parts store and they can get me a standard motor unit for all 3 engines. the V375 is listed for the 396/375 solid lifter engine, and the V280 is listed for the mid rage 396/360. The vacuum can I have now is stamped 357

when I un-connect and plug the vacuum line from the carb the hiccup goes away and the car accelerates better.

[Joe L.]

Re: another BB vacuum advance canister question

For my 66 Chevelle 396 engine board .30 over with a stock GM 454/425 crate replacement roller cam(GMPP 454 cam 211/230), stock 702 heads, stock 396/360 Holley carb and a stock GM 396/325 points distributor. on acceleration its fine. but once around 2K RPM it wants to have a hiccup up or miss. I reset the dwell on the points to 30. the timing is set to the service manual. I am thinking my vacuum advance is not correct or working correctly. I was at a parts store and they can get me a standard motor unit for all 3 engines. the V375 is listed for the 396/375 solid lifter engine, and the V280 is listed for the mid rage 396/360. The vacuum can I have now is stamped 357

when I un-connect and plug the vacuum line from the carb the hiccup goes away and the car accelerates better.

Keith-------


The GM #1115357 is among the last vacuum controls I would recommend for your application. I would convert to full time vacuum advance and use a B20 or B-26 vacuum advance.

[Keith B.]

Re: another BB vacuum advance canister question

Thanks Joe. Would you have a part number for the B20 or B26?

[Duke W.]

Re: another BB vacuum advance canister question

If it has full time vacuum advance the only way to select a functionally correct VAC is to measure idle manifold vacuum in neutral with manual trans and Drive if an automatic and apply the Two-Inch Rule.

Anything less is just guessing.

Duke

[Joe L.]

Re: another BB vacuum advance canister question

Thanks Joe. Would you have a part number for the B20 or B26?

Keith-------

GM #88924983

Delco # D1310C

Standard Motor Products #VC-181

However, my suggestion was just a general "rule of thumb". I suggest you do as Duke suggests before selecting the appropriate vacuum control.

[Keith B.]

Re: another BB vacuum advance canister question

Thanks Joe. this coming weekend I should be able to get a vacuum gauge on it and test it out. and hopefully get the right one.

[Keith B.]

Re: another BB vacuum advance canister question

I was able to get the vacuum readings on everything. The vacuum advance can holds at 17HG. With the engine running and idle out of gear it reads a steady 12 HG. And engine revved up it reads a 17HG. So knowing all that and the two inch rule what can do I need?

[Duke W.]

Re: another BB vacuum advance canister question

Quoting manifold vacuum without specifying idle speed is MEANINGLESS!!!

So...? Also, what idle speed is required to achieve 14"? Higher idle speed will pull more vacuum.

Duke

[Keith B.]

Re: another BB vacuum advance canister question

idle speed is 700 RPM

[Duke W.]

Re: another BB vacuum advance canister question

If you're satisfied with the idle quality, then ,clearly, a B26 won't pass the Two-Inch Rule at 700. An 8" B28 is required. I would think a 700 @ 12" idle behavior is pretty lumpy, and I wouldn't find it acceptable.

The other alternative is to increase idle speed until it pulls about 14" in which case you can use the B26. I would guess that would be in the range of 850-900.

I recommend 900 on OE SHP big blocks and at that idle speed with full time vacuum advance they pull about 14" with a very mild lope.

Duke

[Keith B.]

Re: another BB vacuum advance canister question

Thanks Duke. The engine idles smooth with very little lumpy behavior. I was advised to get the B28 can. And the adjust the idle mixture. I will get this can and install it this week and hopefully all will be well.

[Duke W.]

Re: another BB vacuum advance canister question

I looked at the CF this morning. You must be "Nowhere Man". I could not believe that thread - four pages and 64 posts. No wonder you are confused... and some of those responses... I won't mention any names... but the guy whose posts look like they were written by a 12-year old dropout... totally clueless!

One of the keys to your problem is likely that "357" VAC. Now this is important! There should be two numbers following the 357. You might have to loosen the distributor cap to see them. What are they?

Part of your problem is also the mix of parts you have. I don't know what the centrifugal and vacuum specs are for a 396/325 HP big block distributor, but you can find them in the appropriate AMA specs from the GM Heritage Web site, and you should measure them with a Mity-Vac and dial back timing light because there's a good chance they've been changed in the past.

Depending on the CR of your engine, the relatively high overlap cam needs an aggressive centrifugal and vacuum advance... say 26-30 centrifugal all in at 2500-3000 and the B28 VAC with initial timing set so the sum of initial and full centrifugal of about 38-40 degrees, limited only by detonation. Total idle timing should be in the range of 25-34, which is the sum of initial and full vacuum advance, and maybe a degree or two of centrifugal is the centrifugal starts below idle speed.

Setting up a proper spark advance map is relatively simple, Usually all that's required is a properly selected VAC (using the Two-Inch Rule) and some lighter centrifugal springs, but a vacuum pump/gage and dial back timing light are essential tools for the job... about 75 bucks for the two, but most guys likey know someone that has these tools.

Duke

[Keith B.]

Re: another BB vacuum advance canister question

Yes I am that handle on the CF. I think I'm slowly learning all this. The number after 357 is "20". I know the weights and springs have been changed. A few years ago I asked on here where to buy the correct ones. But I was told they are no longer available and has been for many of years.

[Joe L.]

Re: another BB vacuum advance canister question

I looked at the CF this morning. You must be "Nowhere Man". I could not believe that thread - four pages and 64 posts. No wonder you are confused... and some of those responses... I won't mention any names... but the guy whose posts look like they were written by a 12-year old dropout... totally clueless!

One of the keys to your problem is likely that "357" VAC. Now this is important! There should be two numbers following the 357. You might have to loosen the distributor cap to see them. What are they?

Part of your problem is also the mix of parts you have. I don't know what the centrifugal and vacuum specs are for a 396/325 HP big block distributor, but you can find them in the appropriate AMA specs from the GM Heritage Web site, and you should measure them with a Mity-Vac and dial back timing light because there's a good chance they've been changed in the past.

Depending on the CR of your engine, the relatively high overlap cam needs an aggressive centrifugal and vacuum advance... say 26-30 centrifugal all in at 2500-3000 and the B28 VAC with initial timing set so the sum of initial and full centrifugal of about 38-40 degrees, limited only by detonation. Total idle timing should be in the range of 25-34, which is the sum of initial and full vacuum advance, and maybe a degree or two of centrifugal is the centrifugal starts below idle speed.

Setting up a proper spark advance map is relatively simple, Usually all that's required is a properly selected VAC (using the Two-Inch Rule) and some lighter centrifugal springs, but a vacuum pump/gage and dial back timing light are essential tools for the job... about 75 bucks for the two, but most guys likey know someone that has these tools.

Duke

Duke------


The GM #1115357 produces 20.5 crankshaft degrees of advance at 16-18" of vacuum.

[Duke W.]

Re: another BB vacuum advance canister question

That's what I thought, but I wasn't sure. That might be the VAC used on the '65 327/300 that caused problems and Chevrolet wrote a TSB that has been posted here on the TDB.

So Keith, in addition to the 357 VAC not meeting the Two-Inch Rule, it has too much advance. The B28/26/22 have nominally 16 degrees at nominallly 8", 12", and 15" respectively. Some report that this is actually too much and should be cut back to 12, but I've never had a problem setting up a vintage Chevy engine with a suitably aggressive centrifugal and initial to bring the total WOT advance to the 38-39 range along with a 16-degree VAC that meets the Two-Inch Rule.

If typical freeway cruise revs are at or above the point of maximum centrifugal, manifold vacuum will usually be above the point of maximum advance, so total cruise advance is the sum of initial, full centrifugal, and full vacuum, which places it in the 54-55 degree range, which is just about right for maximum fuel efficiency.

Cruise surge or lack of response to small throttle increases can be caused by a couple of issues. One is too lean a mixture and the other is too much advance. In most cases it's too lean a mixture, but if the VAC has more than 16 deg. total, it is suspect.

Duke