Vacuum advance for 1963 340 hp

[Thomas B.]

My engine was rebuilt in the mid 80's. I don't know if the cam was changed from OE. I measured idle vacuum at idle as 13 to 14 Hg. Initial timing is set at 10 degrees with vacuum line removed from vacuum advance control. I get no additional timing advance when I plug the vacuum line back into the advance control. So my existing B1 advance control is probably bad or maybe sticking. Engine idle is a very steady 850 rpm.

I have read Duke Williams tuning articles and have questions on which replacement vac unit would be appropriate. In the parts list in one of the articles B1 is listed for a 1963 corvette. B26 or B20 are listed as alternatives and would better meet the 2" rule than B1 but in the parts list in one of the articles, 1963 corvette is not listed as an application for those advance units. Will B20 or B26 fit my stock '63 distributor or do i need to stick with B1?

Thanks

[Leif A.]

Re: Vacuum advance for 1963 340 hp

Thomas,
The B26 vacuum can is the one you want. A correctly functioning B26 can will be "all in" with about 12 Hg and that should be fine for your cars' idle vacuum of 14 Hg. The B1 isn't "all in" until somewhere around 18 Hg, hence the reason you're not seeing any difference in your initial timing when you hook it up.
NAPA sells the VC1765 which is what you're needing. I would bring a MityVac with you when you go to buy and check them before purchasing...they can vary.
And, yes, the B26 will fit your stock '63 distributor.

https://www.napaonline.com/en/p/ECHVC1765?cid=paidsearch_shopping_dcoe_google&campaign=GSC-Ignition-Electrical-Lighting&campaign_id=10947851940&adgroup_id=107773573419&adtype=pla_with_promotion&gclid=CjwKCAjwvsqZBhAlEiwAqAHElUUeBG6pAjBgb1WXuAzeuWBGMFZG8dfFLukL3Da53cO1thLBjKpx4BoCNAMQAvD_BwE&gclsrc=aw.ds&

[Thomas B.]

Re: Vacuum advance for 1963 340 hp

Leif,
Thank you for the explanation. I believe the specs say vacuum advance starts at 8 to 11 Hg with max at 16 to 18 Hg on the B1. Wouldn't that mean that you would get partial advance at 14Hg?

Looks like I can get the B26 tomorrow from Napa.

Tom

[Leif A.]

Re: Vacuum advance for 1963 340 hp

Leif,
Thank you for the explanation. I believe the specs say vacuum advance starts at 8 to 11 Hg with max at 16 to 18 Hg on the B1. Wouldn't that mean that you would get partial advance at 14Hg?

Looks like I can get the B26 tomorrow from Napa.


Tom

Sure you would but you want vacuum advance fully in at idle...anything less is useless if you're wanting a properly running engine. It's all part of a correctly tuned engine. These old small blocks will "run" with all kinds of things out of whack i.e. spark plugs not gapped correctly, dwell not set properly, timing off, A/F mixture to rich or lean etc but, again, if you're wanting to optimize the tune of your engine then all things need to be working in concert/correctly.

[Duke W.]

Re: Vacuum advance for 1963 340 hp

I measured idle vacuum at idle as 13 to 14 Hg.

Unless you report the actual engine speed that you measured "idle vacuum" your data is MEANINGLESS!!!

So?

The B1 is a BOAT ANCHOR! It got into the parts books decades ago as part of an ill-conceived parts consolidation effort on the part of Chevrolet, and the aftermarket followed suit. It takes about 10" to start to pull and is not pulled to the limit until 18-19". It won't even meet the Two-inch Rule for a base cam engine! I don't know what "articles" list a B1... certainly not mine unless I said it's a BOAT ANCHOR... totally unsuitable for ANY Corvette engine.

Typical Duntov cam idle behavior is 12" Hg @ 850-900. For any engine the higher the idle speed the higher the vacuum. That's why idle behavior FULL CONTEXT REQUIRES idle speed to be stated when the vacuum measurement is made.

You say you don't know what cam is in the engine. Do you know if it's mechanical or hydraulic lifters? Do you have any invoices from the rebuild that give a manufacturer and part number for the cam?

Based on vacuum at typical idle speeds there are only THREE VACs to choose from, and I've previously listed which are the best Two-Inch Rule fit for OE engines, but should be verified with an actual vacuum measurement at the lowest idle speed you find satisfactory, in neutral with a manual trans, DRIVE with an automatic, and the AC compressor ENGAGED if so equipped.

8" B28 (Airtex 4V1053, getting harder to find)

12" B26 NAPA VC-1765 or cross reference to another brand.

15" B22 (NAPA VC-1802 or cross reference to another brand)

Duke

[Thomas B.]

Re: Vacuum advance for 1963 340 hp

13 to 14 Hg was read at 850 rpm with 10 degrees initial timing and no vacuum advance. I believe I can get a satisfactory idle at 750 to 800rpm. Will have to re-check after I get the new vac control installed. Currently apart.

The articles I refer to are "Duke Williams 2012 Nationals tuning seminar" and "Vacuum Advance Specs". Both are in the Database of Restoration Documents on NCRS site. I attached a copy of "Vacuum Advance Specs". B1 is mentioned in this article and specs and vehicle applications are shown but B1 is not recommended of course.

I currently have a B1 in my car and it doesn't hold vacuum at this point. I plan to replace it with a B26 in the next day or so. B28 seems a bit radical and B22 locks in too high.

I have no information on my cam and whether or not it was replaced when the engine was rebuilt. It does have mechanical lifters.

Tom

[Duke W.]

Re: Vacuum advance for 1963 340 hp

Lars Grimsrud originally prepared the vacuum advance specs document. I wrote the supplemental "Vacuum Advance Principles and Applications" section that is on the last three pages of the document.

These VACS are inexpensive simple mechanical analog devices and not super accurate. The specs are nominal and the vacuum required to start and pull to the limit is about +/- one inch Hg, and max advance can vary up to two degrees from the nominal specs.

At 850 RPM a B26 is marginal on the Two-Inch Rule. Raising idle speed to 900 or a little more should be okay. Lowering to 750 and a B26 will likely not be locked at full advance that can lose idle speed, which reduces vacuum and vacuum advance and so on in a negative feedback loop that will cause the engine to stall.

When new my 340 HP SWC would not idle more than a minute or two before loosing idle speed and eventually stalling unless I blipped the throttle before it stalled. The dealer and "small block experts" I talked to were clueless. Other than saying disconnect the VAC or it's a racing engine, you have to keep blipping the throttle. They were all clueless.

I finally figured it out myself as a 19-year old ME undergrad. The OE 201 15 VAC (start at 8", 15 deg. @ 15.5") was not locked at full advance at idle. Approaching a stop I always double clutch downshifted and the high vacuum pulled it to the limit, but after being stopped for a few seconds with the engine idling at 850 @ 12", the VAC started to retract and pull timing, which further reduced idle speed and vacuum until I blipped the throttle to keep it from stalling. The briif high vacuum when I lifted pulled the VAC to the limit, then the process started again.

The other weird thing that happened was a "rattling noise" as the engine lost idle revs and vacuum and I traced that to a rapid oscillation of the AFB's metering rods!

1963 was the first year that the Chevrolet calibration engineers and techs added vacuum advance to Duntov cam engines and they just screwed the pooch on this one, plain and simple.

I was aware that beginning in early '64 the 365/375 HP engines got a new spark advance map. Centrifugal stayed at 24 starting at 700, but was all in at 2350 compared to 4600 for '63 (and maybe early '64) and the VAC was start @ 4" and 16 @ 8", which would keep the VAC locked at idle for constant total idle advance. So I bought the centrifugal weights and springs and VAC from Chevrolet and installed them.

The transformation was dramatic. Now the engine would idle all day long and there was a noticeable amount of new found low end power due to the more aggressive centrifugal.

In addition to a VAC that meets the Two Inch Rule, you should also improve the centrifugal rate of advance and add more initial timing. The GM Power Manuals from the seventies recommend 36-40 degrees total WOT advance, but with only 10 initial and 24 centrifugal you only have 34. In fact it could be less because if you set initial at a speed above 700 you are already into the centrifugal curve, plus the timing tab is not accurate... reads about 2 degrees low in the 12 to 16 range.

Use a timing light to determine the start and stop points of your centrifugal. Is it the lazy OE or modified? You want to get as close to the 365/375 HP spec as possible. The black springs in the kit mentioned in the tuning seminar will get you close. Then use a dial back timing light to set total WOT advance in the range of 38-39 a few hundred revs above the speed of max centrifugal. If it doesn't detonate, you're done. If it does detonate try stiffer springs (silver then gold) or back off total WOT advance in two degree increments or some combination of both until the detonation goes away.

Given your vacuum readings at 850 you might have an OE Duntov cam, but do you believe it has mechanical lifters?

Duke

[Thomas B.]

Re: Vacuum advance for 1963 340 hp

Thanks for the info. I have the B26 installed and will start it up shortly to see where I stand. Here is a pic of the centrifugal advance setup. Can you tell if this is OE or not? I believe the springs are silver (they have a shiney finish anyway). “37” stamped on the weights.

[Duke W.]

Re: Vacuum advance for 1963 340 hp

The weights look OE, but I can't remember if the OE '63 weights carry the 37 number. All '63 engines used the same weights, springs, and 201 15 VAC. It didn't cause problems with the FI engine because it had ported vacuum advance... another MISTAKE!

I don't think the springs are OE, but they are probably lighter. Also it looks like they have little preload in the retracted position, so the centrifugal may start well below any possible stable idle speed.

It also appears that the dist. is either improperly assembled or installed because the VAC is nearly butted against the inlet manifold at only 10 degrees advance, so you probably can't add more. This could be because the drive gear is installed 180 degrees out of the correct orientation, which is gear dimple pointing the same direction as the rotor tip.

So like I said you need to use a dial back timing light to determent the centrifugal start and stop points, then set the total WOT advance as I previously described. Of course the VAC hose should be disconnected and plugged with a golf tee or equivalent.

You should also check that the B26 start and stop points are reasonably in spec. Then you can check how much advance it adds on the engine, by comparting advance with it connected at idle speed with advance at the same speed, not connected.

You would really be wise to remove the dist. and disassemble it for overhaul. As long as the shaft bushings are okay it's easy and the procedure is in your 1963 Corvette Shop Manual, except it doesn't say how to install the gear with the dimple properly oriented. I found out about that over 55 years ago. Again the dealer and "experts" were utterly CLUELESS about the dimple.

Before removing the dist. set the notch on the balancer at 10-12 degrees on the timing tab. I've explained this before here and on the Corvette forum (search threads started by SWCDuke).

Also verify that the wires are installed on the cap IAW Fig. 51, page 6Y-28 of the 1963 Corvette Shop Manual. If any bubbas chime in and say you can indexe the wires any way you want, ignore them. I've been fixing their screwups for nearly 60 years.

Duke

[Thomas B.]

Re: Vacuum advance for 1963 340 hp

Duke,

Doesn't look like it in my photo but there is plenty of room to advance the timing. Optical illusion I guess. The previous owner had advanced the timing quite a bit probably to make up for the B1 vac. The vac ended up touching the coil bracket when I set the timing back to 10 degrees.

So I started the car this afternoon with B26 vac installed. 14 Hg vacuum reading at 900 rpm. Idle is a little unsteady but acceptable. No stalling or other problems.

I don't have a digital timing light. Still using the Craftsman one I bought in high school. Still works great. I will have to get a hold of a digital light to get a better reading but it appears I'm getting a good bit if not all of the 16 degree advance from the B26. Just eye-balling the advance from the initial 10 degree setting.

I will also look into the centrifugal once I get a digital light.

Also I was told the engine has solid lifters. Suer sounds like too.

Thanks for all of your help.

Tom

[John D.]

Re: Vacuum advance for 1963 340 hp

I like Dukes answer on the B1 vacuum advance. It's totally worthless..
Now for a story. Many years ago I bought about a ton of supposedly '63 vacuum advances from Chevy parts. The part number on the box was correct. But the boxes contained B1's. I sold a lot of them to my close friend who used them for parts.

There once was a repro 201 '63 vacuum advance. Stamped corretcly etc but it was really a B1. Way out of specs.
My '63 FI car: 900 RPM 16" of vacuum. 3736097 cam in it. NOS service replacement 201. Later version.
An original correct 201 '63 vacuum advance is a two piece affair. Means the front of it that has the tube sticking out for the hose is SOLDERED
onto the cannister. I have a correct one around here someplace. Lost it is a better term. John

[Duke W.]

Re: Vacuum advance for 1963 340 hp

The 201 15 was eliminated from service parts while it was still used in production, I recall on the '71 or '72 LT-1. Go figure! It was replaced by the 163 16 and the aftermarket followed with the B1. If you got a B1 in a Delco box with a the ...201 part number that was after Delco quit manufacturing VACs and bought them from Standard Motor Products.

The B1 on a '63 FI engine won't effect idle behavior because that was a one-year only FI system with ported vacuum advance, so technically the Two-Inch Rule doesn't apply, but I have always recommended converting those engines to full time vacuum advance with the '64-'65 VAC and centrifugal weights and springs.

The closest B-number VAC to the 201 15 is the B22 that I recommend on manual trans base cam engines and manual trans optional engines that use the same cam., which would be 283-220/230/250/275 HP, 327-250/300 HP and ALL base 350s.

Duke

[Thomas B.]

[Duke W.]

Re: Vacuum advance for 1963 340 hp

Get a Mr. Gasket 928G spring kit, install the black springs, which should bring all the centrifugal in by 3000. Then use the dial back light to set total advance at a few hundred revs over 3000 at 40 with the VAC signal line disconnected and plugged.

If it doesn't detonate, your done. If it does detonate back off total advance in 2 degree increments until it stops.

Duke

[Richard G.]

Re: Vacuum advance for 1963 340 hp

For what it's worth here is a picture of the original 340hp Vacuum Advance.
I have replaced it with Duke's recommended vacuum advance.
I have to say my motor will idle all day, perfectly.

I did take a minor hit at the regional as I failed to reinstall the V/A before hand.

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