66 SB vacuum advance question

[Bill Richards]

What exactly is the purpose of the vac. advance. My car seems to run equally as well with or without the advance hooked up. I have my timing set a 6 BTDC at 500 RPM With no vac. Hook it up and it jumps to about 20. It goes to 45 degrees at high RPM. No vac. advance and it tops out around 30 degrees at 2500 plus RPM. Thanks, Bill

[Duke W.]

Re: 66 SB vacuum advance question

In the future when asking an engine related question can you (and everyone else) state the model year, engine configuration, and whether or not you think it is OE or modified? This would make it much easier to answer questions. Just between 1962 and 1967 ther were about two dozen different small block configurations. Even though horsepower ratings may have been unchanged, distributor calibrations often changed and different distributor calibration is a different configuration.

If you disconnect the vacuum advance at idle, revs should drop. If not that tells you something - either the vacuum can is inop or it is ill-suited to the engine's manifold vacuum characteristics, or someone has converted to ported vacuum advance, which is a BIG MISTAKE. Based on your timing measurements it sounds like your vacuum can is operable, but is it properly matched to the engine's manifold vacuum characteristics?

Case in point. A buddy with a '62 wanted me to look at his distributor. An "expert" had set it up. The basic powertrain configuration is not very good as it has a 30-30 cam and 700R4 auto. As you can imagine, it has terrible idle quality and stability, and it was running hot at idle and low speed traffic.

The first thing I noticed was the vacuum can jammed against against the inlet manifold and figured that the drive gear was improperly indexed with the dimple pointing the wrong direction. Then I checked wire indexing and they were indexed one position CCW from the proper OE indexing.

A vacuum check at idle in drive - 700 RPM - showed 7.5" (Yeah, a 30-30 and auto are just a GREAT combination!) yet the "expert" installed a "B1" vacuum can, which doesn't even begin to pull advance until 8", so it effectively had ported advance. The "expert" probably installs the B1 can in all distributors because he is clueless how to match the vacuum can to manifold vacuum characteristics. "Expert". How 'bout "bubba"!

Flame propagation speed varies with mixture quality. Low density mixtures and exhaust gas dilution slow flame propagation speed, which is why engines run better with full time vacuum advance. If you disconnect the can at idle, speed and quality will drop because you have subtracted a substantial amount of timing from what was otherwise probably in the ideal range.

The proper total idle timing is in the range of mid-twenties to low thirties and higher overlap cams cause more exhaust gas dilution at idle, which means slower flame propagation speed, so they need the high end of this range. Proper cruise timing in the range of 3000 revs is 42-54, so you are in that range and total WOT timing at high revs should be 34-38.

If you drop the idle timing below the ideal range, EGT goes up, which throws more heat into the cooling jacket as the exhaust gas traverses the ports, and the engine runs hot/overheats in low speed stop and go traffic. I will also return poorer fuel economy without vacuum advance because spark timing is not optimized to generate peak thermal efficiency over the total range of engine operating conditions, including idle and part load, which is where the engine is 99.99 percent of the time.

The can should provide full vacuum advance at no less than 2" less than typical idle vacuum, so since a 30-30 cam idles at 900@10" (in neutral with a manual trans) it needs an 8" can. Duntov and LT-1 cams idle at 900@12" and also work best with an 8" can. L-79 idles at 750@14" and a 8" can was OE, but a 12" should work okay. Big blocks idle at 900@14" for SHP and 600@15" for L-36/68, and a 12" can is good.

327/300s idle at 500@17" and a 15" can is good, but 12" for PG since idling in Drive drops revs and vacuum.

It turned out the drive gear on the '62 was not on backwards, and a 8" can eliminated the overheating, but idle stability is still poor because idle vacuum can barely lock the can at full advance at idle and 8" is the most aggresive can available. Overall idle quality will never be better than "poor" because a 30-30 cam is just totally unsuited to any auto trans.

It obviously needs a proper cam and the 929 is the way to go. With the 3.06 first gear of the 700R4 and 0.70 fourth through the 3.55 axle it should be both quick and docile with easy low rev highway cruising and a whole lot more fun to drive than having to kick it into neutral every time you come to a stoplight and constantly fretting over the temp gage.

My guess is that you have a 327/300, and because the centrifugal stopped at 30 with 6 initial it is probably a '63 or '64, but it probably has lighter than OE springs because the OE springs didn't fully deploy the centrifugal until about 4600. The '66 and '67 327/300 have 30@5100 centrifugal, so they should keep advancing beyond 2500 and achieve 36 at about 5000 with 6 initial.

The '66 and '67 327/300s used a 12" can - probably to better accomodate PG, and this should work fine on earlier versions, which have different vacuum and centrifugal curves. In fact the 250/300 HP spark advance maps went through quite a change from '62 to '66, but the '66-'67 version is probably the best because it represented several years of development.

The initial timing, vacuum and centrifugal advance specs are in the appropriate year service manual and AMA specs. Since bubba had been screwing up distributors since these cars were built, it would be a good idea if everyone checked to see if their's are OE or modified.

Some modifications are productive. Quicker centrifugal curves improve low end torque, but can cause detontion on today's pump gas and a the '63 327/340 OE vacuum can is not suited to its manifold vacuum characteristics. It needs an 8" can. For a reason incomprehensible to me Chevrolet set up '63 FI with ported advance. They fixed the problem in '64 with full time advance and an 8" can. I recommend this configuration for '63 FI, too.

Duke

[Bill Richards]

Re: 66 SB vacuum advance question

Duke, thanks for the detailed explanation. Sorry I didnt mention my configuration. It is a 66, 327-300 HP, power glide. I guess I need to find a good vacuum gauge to see what I am pulling at idle. Where on the vac can does one find any info. I cannot find any numbers on mine. Thanks, Bill

[Joe C.]

Re: 66 SB vacuum advance question

Bill,

Remove the cap and rotor. The numbers are stamped into the link attached to the breaker plate.

Joe

[Bill Richards]

Re: 66 SB vacuum advance question

Thanks, Joe

[Bill Richards]

Re: 66 SB vacuum advance question

The only thing that I can see stamped into the link is the number B1 ?
Bill

[Gary S.]

Re: 66 SB vacuum advance question

Sears makes an affordable and reliable vacuum gauge.

Gary

[William C.]

Re: 66 SB vacuum advance question

The B-1 can was referenced in Duke's post as being a can which requires 8" of vacuum to start opening, I think it is "all in" at about 12". If you have an engine package that will not hold 12" of vacuum at idle, with the trans in gear, the use of a ported vacuum setup might be in order to allow you to setup the idle without the vacuum advance, while still retaining the vacuum advance advantages in highway cruising. If you can pull 13-14 inches or more at idle, then the vacuum advance can be hooked up to manifold vacuum, and you will see a decrease in idle exhaust temps.

[Duke W.]

Re: 66 SB vacuum advance question

Per my previous discussion, the "B1" can requires about 8" to start and is at full advance at 16-18". I do not recommend it for ANY Corvette engine. The original "355" can on all '66-'67 300 HP starts at 6" and is fully advanced (16 deg.) at 12". I doubt if your engine pulls enough vacuum to pull a B1 to the limit at idle in drive. That's why I figure GM configured a 12" can as OE - so a PG equipped 300 HP engines will pull max vaccum advance at idle.

You can buy a Mighty Vac for about $25 at Harbor Freight, which is handy because the pump allows you to test vacuum cans by pumping them down. I always test vacuum cans BEFORE I install them to verify that the link starts to move and is fully deployed at the proper vacuum values and then test them again once installed to ensure that they are within advance@vacuum spec. Alternatively, you can use a plain vacuum gage and oral vacuum. I made a 1/8" vacuum tubing harness with a tee that allows me to quickly add a vacuum gage to any circuit and golf tees are good for plugging vacuum lines.

A correct OE replacement can is NAPA/Echlin VC1765(B22) or VC1808(B26) Specs for both are 0@6" (start point) and 16@12".

Also, the '66 300 HP max centrifugal is 30@5100, so it should continue to advance beyond 2500, but most of it is probably in by 3500, and 6-8 initial should yield 36-38 total WOT timing at high revs, which is just right.

You should check to see if the distributor still has the band with the number on the base and determine if it is the orig dist. no. or something else.

I would like to know the vacuum@idle speed for your engine idling in Drive with a proper vacuum can.

Duke

[Duke W.]

Re: 66 SB vacuum advance question

The B1 can is not at full advance until 16-18", and I do not recommend it for any Corvette engine. The OE can (355) on '66-'67 300 HP engines only requires 12", and I believe the reason is so the can will be at full advance with PG idling in Drive. A manual trans 300HP is okay with a 15" can.

At 500 in neutral a 300 HP engine should pull about 17-18" Idling in Drive at 450-475 it will be lower, and I would like to find out how much vacuum they pull.

Can someone with a 300HP/PG check vacuum idlling in Drive?

Ported vacuum advance will cause hot running or overheating at idle and low speed stop and go driving, and is not the way to go.

The proper way to set up the spark advance map is to use a OE 12" vacuum can signaled by full time manifold vacuum.

Duke

[Bill Richards]

Re: 66 SB vacuum advance question

Duke, I will get a vucuum reading a little later today. Where do I take the reading from . My vac. can is hooked to the base of the carb at this time. Is this where I take the reading? Bill

[John D.]

Re: 66 SB vacuum advance question

Bill, We have found that the B1 vacuum advance is good for no more than a paper weight on all FI's and carb Corvettes. Problem came about 15 years ago or so when I bought a ton of them from Delco or Chev. The part number on the Delco box is D1328 and #1116163. They were supposed to be a replacement VA for the '63 Corvette, etc. But in fact they are a reboxed Standard part that are just about worthless. I probably bought 50 or so. I did sell some of them to a friend who uses them for parts for rebuilding originals. Buy an original or go to NAPA and get a B22 or a B28, etc. In otherwords get a weaker vacuum advance. Throw the B1 away. If any one of you ever bought one of these things from me at a show let me know and I will give you a refund.
On another note the original '63 vacuum advances are stamped 201 and are a two piece part. Two pieces means that the small tube that the vacuum hose attaches to is soldered onto the can. Later 201's are one piece. I know you have a '66 but this is a little more '63 trivia. I don't know the orig. # of the '66VA.

[Duke W.]

Re: 66 SB vacuum advance question

Get two or three feet of 1/8" rubber or vinyl tubing and a 1/8" tee and make an adapter harness so you can attach the vacuum gage IN PARALELL.

Place a short piece of tubing on one end of the top of the tee. Disconnect the signal hose from the vacuum can and connect the short piece of tubing connected to the tee on the vacuum can nipple. Place the vacuum can signal hose on the other top of the tee leg.

Using a convenient length of tubing so you can lay the vacuum gage in a safe and convenient place, connnect the vacuum gage to the base of the tee.

I you just disconnect the vacuum line from the can and connect it to the vacuum gage your reading is invalid because you have eliminated the vacuum advance function and thus altered the circuit and engine configuration.

Vacuum gages must be connected in parallel so the original vacuum circuit remains undisturbed.

The tee adapter harness I've described can be used to attach a vacuum gage in parallel to any any vacuum circuit without distrubing/altering the actual circuit.

Duke

[Gerard F.]

Re: 66 SB vacuum advance question

This is an original VA for a 67/300HP/manual trans. 1111194 dist:


The PG one is probably different with an 1111117 distributor (67)

Jerry Fuccillo
#42179

[Duke W.]

Re: 66 SB vacuum advance question

The 117 distributor was used ONLY on 300HP/PG WITH AIR (K-19), and K-19 was required on cars shipped to California. The same vacuum can was used as on the 194 distributor, but the signal line was ported. The 117 centrifugal is 25@1500 and 40@5100, and the initial timing spec is 4 ATDC - that's AFTER TDC!

Take a look at your CSM or AMA specs.

This weird ignition map was used to retard the timing map at idle and low revs in order to increase EGT, so the injected air would initiate oxidation of HC and CO. Above about 1500 with the throttle above idle the total timing was about the same as the non-K-19 300HP engines.

The 1115355 vacuum can provides full vacuum advance at 12" and the suitable replacements are as I mentioned in a previous post.

One should be on the lookout for this distributor if you have an original 300 HP/PG K-19 car as setting the initial timing to the conventional BTDC setting is going to cause heavy detonation. My recommendation is to convert the 117 dist. to a more conventional centrifugal curve and change the ported vacuum signal to full time regardless of whether you have the air pump installed or not.

Duke