1965 L78 Vacuum Can

[Joel T.]

Guys;

OK, I put the B28 can in my '65 L78 and now the idle is solid and the low end performance is just super... no stumble, no hesitation, just serious get up and go.

I have noticed a couple of things... First, the basic curb idle is higher now than when it had the 201 can in there... (BTW I adjusted nothing other than installing the B28).. which tells me that this can may provide more by way of total advance or that 201 was never reaching full pull at idle. Second, I see that the high idle is a bit higher as well, again with no other adjustments.

Now I am getting some engine knock at partial throttle which I did not have before. Right now I am the running base timing at 8 degrees. I know that the B28 is probably too aggressive, achieving full advance, or being at full advance almost all the time... Should I be dialing base timing back or should I try and find the correct can.... I know those things are getting hard to find....

[Duke W.]

Re: 1965 L78 Vacuum Can

The 201 needs 15-16" idle vacuum to pull to the limit for full advance.

SOOOOOOOOOO....

After installing a suitable VAC for the engine's idle characteristics, one MUST go through the idle speed/mixture adjustment procedure.

I believe a L-78 with 25-30 degrees of total idle timing should idle at 900 @ 14"

The B28 is more aggressive than necessary, and this can lead to part throttle or transient detonation.

The best VAC for L-78 is the 2" B20/26.

They are commonly available and cost about ten bucks.

Duke

[Tony S.]

Re: 1965 L78 Vacuum Can

Joe. There has been alot of discussion on this board regarding the correct vacuum can for an L78 car. Although the 5th ed. JG shows the 201 can to be correct, I believe that the 6th ed. (now under edit) will change the L78 vacuum can to the 355 can. Having the wrong can on your distributor may explain why you were having trouble with your engine's performance.

You may want to look at Bill Williamson's post from some years ago.

Tony

[Joel T.]

Re: 1965 L78 Vacuum Can

Thanks guys;

I'm still learning about how all this interacts... I do suspect Duke is correct because that is exactly what I am getting; part throttle detonation...

Napa does show that VC1765 part number online, so I ordered one last night.. hopefully they will actually have it. When I went looking for a B28 can for my '63/340 they were really tough to find.. I actually wound up with 2 and squirreled one away, just in case...

Assuming that the B26 shows up, I will get it installed and readjust everything and let you guys know how I make out.

Thanks again! I really appreciate the help!

Joel

[Duke W.]

Re: 1965 L78 Vacuum Can

Corvette News published the tune-up specs when the L-78 came out as a running change, and the specs match the 201 - 15 @ 15.5".

The 355 is 16 @ 12", which is equivalent to the B20/26, which is the best functional fit.

The 355 was pretty common - used on base 327/300s in '66 and '67. The 327/300 only needs a 15" B22 with a manual trans, but a 12" is a better functional fit with Powerglide, so one size fit all.

OE numbered VACs are getting hard to find in functional condition, but having a B-number is an OE replacement, which in my opinion is only a one point deduction from the three points allocated to the VAC - two for originality and one for condition.

The 201 is also a rare bird. It was used on the 327/340, which wasn't a good functional fit, but it was only available in service parts for a few years.

It was also used on L-71 and was an okay fit with L-71's ported vacuum advance, but if a L-71 is coverted to full time vacuum advance, which I highly recommend, the VAC must be changed to the 12" B20/26.

Duke

[Wayne M.]

Re: 1965 L78 Vacuum Can

Joe. There has been alot of discussion on this board regarding the correct vacuum can for an L78 car. Although the 5th ed. JG shows the 201 can to be correct, I believe that the 6th ed. (now under edit) will change the L78 vacuum can to the 355 can. ....

Tony -- and the Delco Remy test specifications book DR-324S-2 (1964-1976 Supplement) agrees that the 1111093 distributor takes the 1115355 can.

[Duke W.]

Re: 1965 L78 Vacuum Can

Corvette News Vol. No. 3 lists the distributor number as 1111093.

Centrifugal advance:

0 @ 1000
15 @ 1800
28 @ 4600

This is probably one of the first non-linear centrifugal curves.

Vacuum advance

0 @ 8"
15 @ 15.5"

which corresponds most closely to the 201, so either Corvette News is in error or the Delco Remy spec book is in error.

Initial timing is specified at 10 degrees and recommended idle speed is 700, which is typical as recommended idle speeds for SHP engines of that era where unrealistically low. I recommend idling at 900.

Also of note is that the hot valve clearance is listed as .020/.024", and as is the case with iron block/head engines the difference between cold and hot idle valve clearance is nil.

Using the exact same camshaft, Chevrolet increased the valve clearance to .024/.028" for the L-72, which IMO is too loose for a .012" high clearance ramp even if you assume the true rocker ratio at the lash point is 1.7, but it's probably somewhat less.

I recommend using the L-78 clearance specs for L-72/71 and the indexing procedure in the Hinckley/Williams paper for SB vavle adjustment.

I think it's even better to run the exhaust vavle clearance at .022".

So does anyone have a known original unmolested L-78 who can read the numbers off the VAC?

Duke

[Wayne M.]

Re: 1965 L78 Vacuum Can

All -- is it possible there were running changes in the 1111093 distributor's vac advance ? After all, it was only used for half a year in '65 and all of '66.

A. Colvin's Chev-by-the-Numbers, 1965-69, pg 245 says he had access to original Delco Remy blueprints, and he shows that the "093" had the "236"-->"201"-->"355"-->"360", to which he says "There were often 2 or 3 vacuum units used on the same distributor over its usage period". On the same page he has photos of an MS 355_15 and a MS 360_12.

So maybe the '65 L78's had (say) two vac advances over a 5 month period, and the '66s had one of the latter '65 numbered units plus maybe another for late production L72's.

The reference to the "355" can, in my post above, was taken from the Delco spec sheet dated 1-1-66.

Just a thought.

[Duke W.]

Re: 1965 L78 Vacuum Can

Without seeing the drawings myself, I don't know, but I would think that if a VAC was replaced with one that was FUCTIONALLY different, the distributor would get a new part number.

If replaced with one that had virtually the same specs, then the dist. part number could remain the same.

The 236, 355, and 201 are definitely different from a functional standpoint - 8", 12", and 15" respectively.

In fact, ONE of the above three will meet the Two-Inch Rule for virtually every OE engine/transmission combination with full time advance, and every ported vacuum advance OE engine/transmission combination that is coverted to full time, and the respective modern replacement B-numbers are B28, B20/26, and B22.

Duke

[Jody B.]

Re: 1965 L78 Vacuum Can

I just went out and looked at my VAC (it appears to be original) and it's a 355.

[Joel T.]

Re: 1965 L78 Vacuum Can

Without seeing the drawings myself, I don't know, but I would think that if a VAC was replaced with one that was FUCTIONALLY different, the distributor would get a new part number.

If replaced with one that had virtually the same specs, then the dist. part number could remain the same.

The 236, 355, and 201 are definitely different from a functional standpoint - 8", 12", and 15" respectively.

In fact, ONE of the above three will meet the Two-Inch Rule for virtually every OE engine/transmission combination with full time advance, and every ported vacuum advance OE engine/transmission combination that is coverted to full time, and the respective modern replacement B-numbers are B28, B20/26, and B22.

Duke

Duke;

Here is some more information....

First, this motor is not a "stock" L-78... When I had it built, I had a few things done to it...

The block was actually bored out to standard 427 displacement.. Custom pistons (with heavier rods) were built to give a CR of roughly 10.5. A Competition Cams unit was used Part# 11-671-4. I have all the spec on that cam if needed.

The basic timing was off.. was 6 degrees..so I adjusted it to 10. I checked the centrifugal advance and it is consistent with the spec; 0 @ 1000 and 15 @ 1800. Vacuum readings ( via MightVac with the VAC not connected) were 11" @ 800 RPM and 10" at 700 RPM (timing @ 10 degrees).

The can which was in there is a 355 15, which would appear to be original. I tested the can with the MightVac.. It will begin to move at 6" and reach full pull (rod touches frame) at 14".

Based upon these numbers, and the "two inch rule", the 355 VAC would not be functionally correct unless I can generate more vacuum at idle... I picked up 2-3" by correcting the basic timing... Given this cam, should I bump the basic timing up a couple of more degrees?

Joel

Joel

[Tony S.]

Re: 1965 L78 Vacuum Can

Thanks Wayne, Jody et al.

I've known Jody's Duntov car for quite some time. Between Jody's car and Bill Williamson's unmolested original, I think they confirm the 355 can.

Tony

[John H.]

Re: 1965 L78 Vacuum Can

I just checked the vacuum at idle, engine warmed... I get about 9" (which fluctuates more than 1" and is not steady, the 9" is about the mid-point) at 700 RPM measured with a MightVac, off the vacuum port from the carb metering block, with basic timing at 8 degrees. Connecting the vacuum back up to my B28 can gives me slightly over 1000 RPM (I know it is high) with a total advance of 15 degrees.

The can which was in there was a 355 15, which would appear to be original. With that can in there I was getting that very low idle symptom from time to time.

Joel

Joel -

With 8* initial timing, the B28 VAC should add another 15* when connected, and you should see 23* advance at the timing tab, not 15*.

[Joel T.]

Re: 1965 L78 Vacuum Can

Joel -

With 8* initial timing, the B28 VAC should add another 15* when connected, and you should see 23* advance at the timing tab, not 15*.

John;

I probably did not explain this properly... I have one of those timing lights which permits me to roll the observed timing back by turning a knob on the light. With the VAC back in, the dial read 15* when I rolled it back to the basic timing point of 8*.... I suspect if I rolled it back to TDC, versus the 8*, it would have been 23*.

Joel

Joel

[Duke W.]

Re: 1965 L78 Vacuum Can

Now you finally tell us it doesn't have an OE cam. Why does it take guys 20 posts before they finally explain what they have.

If you have a non-OE cam use the Two-Inch-Rule to determine what VAC to use.

Duke