67 L71Timing update and thanks.

[Richard S.]

Thanks to the advice of Duke and a number of other NCRS members I am happy to report that the 67 427/435 is now running stronger than ever, on 94 octane pump gas...without detonation or overheating at idle. The distributor was curved to factory spec...a Napa/Echlin VC 1765 Vacuum advance can was installed...and connected directly to the FULL manifold vacuum source below the center carb (not the original "ported" source).....the timing was set at 8 degrees initial...10 degrees at 3600 RPM and 38 degrees total.
I happened to stop by Dave Feidler's booth at Carlisle and he was nice enough to discuss some of my distributor issues with me. In particular, and perhaps of real interest to those who read and post on this board, he said that many of the reproduction vacuum cans he has tested did NOT provide the indicated/required vacuum. He has gone back to recalibrating his own for certainty. We tested my 201 reproduction can(bought from Long Island)and it was way off. The VC1810 and the first VC1765 cans I bought from NAPA were also off. The second VC1765 can was very close to spec and that is what we installed. Dave also said that he had seen some discussion on the NCRS Discussion Board that seemed to say that the 360 can was correct for the 427/435 distributor....and he did not understand why anyone would think that. He said he has seen many original 427/435 distributors with the 201 can and he is sure that is the correct can for that application. Well, I hope others can benefit from what I have learned...and thanks to all who provide advice and guidance with respect to our Corvette issues.

[Clem Z.]

the correct NCRS can may not

be the best for use now with the difference in the gasoline compared to when these cars were new. JUHO

[Duke W.]

Re: 67 L71Timing update and thanks.

Rick - glad to see you got it squared away. Did you find a carb. problem? As I recall you were experiencing some backfiring, which I thought might be a carb. problem.

We need to define the word "correct" in the context of vacuum cans, being as how it can have more than one meaning. It usually refers to having the identical OE part, but that doesnt' necessarily mean that the part is "correct" in an engineering sense.

The primary selection criterion for a vacuum can that is to be connected to a full manifold vacuum source is that if provide FULL advance at not less than 2" less than normal manifold vacuum at idle speed with about 25 degrees total idle timing (the typical sum of initial and full vacuum advance).

The 201 OE L-71 can does not meet this criterion, but it isn't a big issue if the engine is configured with the original ported vacuum advance. However, if you convert to full manifold vacuum, the L-71 won't pull enough vacuum to "lock" maximum vacuum advance in at idle because the 201 can requires 15.5". The 360 can used on L-72 ( 15 deg. at 12") WILL, so it can be used as is when converting to full vacuum advance, but the L-71 can must be changed to a can with full advance at 12". The 360 can is suitable, and the closest aftermarket match is the NAPA/Echlin VC1765 - 16@12".

I think I've got a pretty good handle on both L-72 and L-71. Both have ported vacuum advance, and better operation, including better fuel economy and less tendency to run hot/overheat at idle and low speed driving can be achieved by converting to full time vacuum advance. L-71 will require a vacuum can change as discussed above, and one may also want to slow the centrifugal curve if it detonates being as how L-71 has a quicker centrifugal curve than L-72. In the case of L-72, merely converting to full time vacuum advance with no other changes should achieve the desired results.

Here is a summary of OE SHP BB centrifugal and vacuum advance and initial timing specs (data from Corvette News):

L-78: 10 initial; 0@1000, 15@1800, 28@4600; 0@8", 15@15.5"
L-72: 8 initial (8-14 range); 0@900, 30@5000, 0@6", 15@12"
L-71: 5 initial; 0@900, 30@3800; 15@15.5" (no vacuum start point listed)

I still don't know if the OE L-78 setup is ported or full vacuum advance, and I don't know the OE part number on the vacuum can, but it looks like a 201. Note the signficant year to year differences even though these engines have fundamentally the same manifold vacuum characteristics because they all have the same valve timing. Seems that Chevrolet Engineering could not make up it's mind on the ideal spark advance map.

One other question, Rick. What is your current idle speed and idle vacuum with the new ignition map setup? Also, did you verify the total idle timing (vacuum can connected). With eight degrees initial it should be about 24 degrees.

Interesting point on the vacuum cans being out of spec. I always recommend that vacuum cans be checked against spec. A new one should be within one inch and one degree of spec, and I would replace a used can if it deviates by more than two inches or two degrees.

Duke