1966 L79 with A.I.R. optimum tune up specs

[Robert B.]

Re: 1966 L79 with A.I.R. optimum tune up specs

I decided to run some additional tests to figure out what was going on with ported vacuum, total crank advance, and exhaust manifold temperature, and learned that the advance can , connected to ported vacuum, at 800 RPM idle, is actually producing total crank advance of 25 degrees. I also checked exhaust manifold temperatures again, this time holding the IR gun much closer to the manifold (about 3 inches).

Before I started these tests, I again rechecked initial timing at about 600 RPM which is about the lowest I could get it to idle. It was spot on at 10 degrees BTDC.

Here are the readings: (All measurements were taken at 800 RPM)

With ported vacuum connected to the advance it measured 7", total advance 25 degrees, temperature 495 degrees.
With ported vacuum disconnected and plugged, it measured 12", total advance 14 degrees, temperature 590 degrees.

Then I connected the mityvac to the vacuum advance can in order to control vacuum to the can and got these readings:

At 4" vacuum, ported vac was measuring 12", total advance 14", temperature was 570 degrees.
At 6" vacuum, ported vac was measuring 10", total advance was 17 degrees, temperature was 500 degrees.
At 8" vacuum, ported vac was measuring 5", total advance was 24 degrees, temperature was 500 degrees.
At 10" vacuum, ported vac was measuring 2", total advance was 30 degrees, temperature was 470 degrees.

I think what this means is:

The centrifugal is kicking in about 4 degrees of crank advance at 800 RPM.
The advance can is doing it's job of producing 25 crankshaft degrees at 800 RPM.
Exhaust manifold temperatures are down about 100 degrees F with 25 degrees of advance compared to 10 degrees.
The ported vacuum measurements are a function of throttle plate position - as I had to adjust engine RPM whenever I changed vacuum to the advance can.

I'm going to install the lighter springs and recheck these measurements to see what effect the springs have. But at this point I've got the idle characteristics in good order. After flight judging, I'll convert the car to full vacuum and make sure real world driving performance is OK.

I ordered a Standard Motor VC-171 which is supposed to be an equivalent to a B22 can for the '67. I could not find a Delco D1308C or a NAPA VC-1802. When I get that advance I'm going to work on converting that car to full vacuum.

Bob

[Duke W.]

Re: 1966 L79 with A.I.R. optimum tune up specs

It's possible that a little centrifugal is added at 800. I believe the specs say it starts at 700 (check) but this simple mechanical mechanism is not a precision device. Regadrless of what brand/part number VAC you buy, it's the same part manufactured by Standard Motor Products and the ID number is B22. I hope the '67 300 HP K-66 is a manual transmission. If Powerglide it has a whole different screwball spark advance map.

[Robert B.]

Re: 1966 L79 with A.I.R. optimum tune up specs

Well, the '67 300HP car is (was) a powerglide car - now running a 200-4R transmission. It is also a factory A/C car, and a K-19 car, although I have removed and saved all of the K-19 parts. It runs and drives great, smooth idle - nothing to complain about. It is a great road trip car - in fact we just competed a 5,000 mile trip last fall. It performed perfectly during the entire trip. My only objective in running manifold vacuum on that car would be to get engine temperatures down.

Can you let me know about the advance map for that car, and what should I use for initial advance, etc.?

Bob

[Duke W.]

Re: 1966 L79 with A.I.R. optimum tune up specs

Look in your CSM and AMA specs. Briefly, with AIR and P/G total centrifugal is 40 degrees and the initial timing spec is 4 degreee AFTER TDC, so total WOT advance is 36. I never understood why this was done because automatics usually have lower "engine out" emissions than manuals. Manuals yield a HC spike every time you upshift, which is one reason why Corvettes were not offered with manuals in some model years during the seventies. The OE spark advance map would have caused very high EGT and operating temperatures in stop and go driving. Of course, it may have been modified over the years to something more conventional, so you should measure the spark advance map, and converting to full time vacuum advance should improve performance, cooling, and fuel economy in low speed driving. I'd also like to know vacuum @ idle speed in Drive with the AC compressor both engaged and not engaged. Duuke

[Robert B.]

Re: 1966 L79 with A.I.R. optimum tune up specs

I've started working on the '67 again with the plan to convert to full time vacuum advance. This is a 300HP coupe with factory A/C running R12, 200 4R transmission, PS, PB, PW.

I ordered a Std. Motor Products VC171 advance can which is marked B22, new points and condenser as the original points had some wear and tear.

I checked the baseline settings before changing anything and found the following:

Vac advance was connected to a port on the carb that had no vacuum signal at all. (not my doing)
At 750 RPM idle speed dwell was 33 degrees, timing was 3.5 degrees BTDC, manifold vacuum was 19 inches.
The vacuum advance (#355) was defective - would not hold vacuum.

So basically, the vacuum advance was doing nothing - hard to believe the engine was running as well as it was.

I replaced the advance can with the new B22 can, replaced the points with NAPA CS786SB points, and new condenser.
Changed the mechanical advance springs to the silver springs from the Mr. Gasket kit, and cleaned and lubed the weight contact points.

Started the engine up, set dwell to 30 degrees, and measured the vacuum can - it started at a little over 7 inches, and is all in at 15 inches.
I set the initial timing at 600 RPM at 7 degrees. I noticed that initial timing increases to 10 degrees at 750 RPM, so the silver springs are bringing in advance even at idle.

With full vacuum connected, timing is as follows:

750 RPM - 25 degrees
800 RPM - 28 degrees
1000 RPM - 32 degrees
1500 RPM - 35 degrees
2000 RPM - 44 degrees
2500 RPM - 48 degrees

Duke, I measured engine RPM with A/C off it idles nicely at 800 RPM, which works pretty well with the 200 4R transmission. With A/C on, the RPM dropped to 600 RPM. Forgot to measure manifold vacuum - I'll do that later and report. (I do have an idle compensator installed on the carburetor to avoid stalling when the A/C is running.)

Road testing of the car went well - no pre-ignition under any conditions - no hesitation, etc.

So I think these settings should work well.

Exhaust manifold temps dropped from over 530 degrees before these changes to about 410 degrees after.

Bob

[Duke W.]

Re: 1966 L79 with A.I.R. optimum tune up specs

I'm getting confused because you're talking about two cars with different configurations. Shoot me an email and I've give you my phone number so we can get everything squared away with a phone call. My fingers are getting tired.

...good to see you in Laughlin.

Duke

[Robert B.]

Re: 1966 L79 with A.I.R. optimum tune up specs

Duke, my email is bob@behlman.us -

Thanks,

Bob