72 Distribuot installation issues

[Brian M.]

Re: I now make house calls in my NEW C6

Does that mean that you'll be going with the Ambulance paint job with Dr. Rebuild logo's? Will staff be dressed accordingly?

[Gary S.]

Re: Drs. DO make house calls

Duke, I apologize for not answering your question fully but in my initial response I was thinking, "how the he@# do I know what the factory curve is/was?". However, after searching in the 1972 Service Manual, I found the following:
Centrigual adv: 0 @ 1090 rpm, 2 @ 1310, 21 @ 2350, 28 @ 5000.
Vacuum adv: 0 @ 8" Hg, 15 @ 15.5
Vacuum signal full time, but controlled by the TCS. However, I am now runnig straight, full time vacuum without the TCS although all of the components are there.

Thanks for the explanation of the total timing issue. Your knowledge on this DB is well appreciated.

Gary

[Patrick H.]

Re: I now make house calls in my NEW C6

Yeah, then he could get a tax write off out of it by driving it to shows.

Gotta think outside the box, Geoff.

Patrick

[Joe L.]

Re: I now make house calls in my NEW C6

Geoffrey-----

No, you've got it all wrong. A HOUSE CALL is when the doctor goes to the patient's house. It's not when the doctor brings his house to the patient.

[Geoff C.]

[Duke W.]

Re: Drs. DO make house calls

If you compare your centrifugal curve to a typical pre-emission curve you will find that it is very "lazy" at the low end, with a very slow rate of advance below 2000. This combined with the low 4 degrees initial means the timing is quite retarded at low speed relative to what whould be ideal for peak efficiency and minimum fuel consumption, especially when you consider that TCS prevents vacuum advance except in top gear and maybe third gear. This retarded timing along with AIR is how GM meet the emissions bogeys on the EPA emission certification test.

Your previously stated centrifugal curve of 26@3000 is okay, but it would be interesting to know where it starts and at least one intermediate value at, say, 1500.

I'm surprised vacuum advance is not ported, but in any event, the vacuum advance is also lazy, and I doubt if LT-1 pulls enough vacuum at idle to lock the vacuum can at full advance. That would take at least 16", but the point is moot since TCS blocks the vacuum signal while idling in neutral. Bypassing TCS means full manifold vacuum signal to the can at idle, and the can must provide full vacuum advance at 2" less than normal idle vacuum to keep it "locked" at full advance at idle. If not, the engine can exhibit idle instability. Slight variations in manifold vacuum will cause the vacuum advance to dither or wander, and without constant advance at idle idle quality and stability is likely to be poor.

The "default" vacuum can to use is a NAPA/Echlin VC1810, which provides 16@8", and a possible alternative is the VC1765, which is 16@12".

The way to select is to disconnect the vacuum can and advance the initial timing to about 25-30 degrees, which will simulate initial timing plus full vacuum advacne. As you increase timing engine speed will increase, which will cause the centrifugal to deploy, so you will have to use the idle speed adjustment to reduce speed. Also, tieing up the centrifugal temporarily with a stiff rubber band will take it out of the picture, which is useful. Once you are in the range of 25-30 degrees of advance and the engine is at what you consider to be the best idle speed, which is probably in the range of 750 to 900, go through the idle/speed mixture adjustment procedure to achieve the highest speed and vacuum. Reset the idle speed and go through the procedure again until there is no increase in speed or vacuum.

Record your final vacuum reading. If it is OVER 14" use the VC 1765. If it is 14" or less use the VC 1810. Set the initial timing back to 10-12, untie the centrifugal and install the vacuum can. Check that it functions by looking at total idle timing (vacuum can connected). It should be in the range of 25-30 degrees. If it checks out, do a final idle speed/mixture adjustment and you are done.

The '72 LT-1 was set up to operate on low octane unleaded fuel equivalent to today's regular. Even with the ignition map modified as above it may still operate okay on regular unleaded. If you get any detonation, switch to mid grade. There is no need to run higher octane fuel that what is necessary to keep the engine out of detonation.

Duke

[Patrick H.]

[Gary S.]

Re: Drs. DO make house calls

Thanks, Duke. Great stuff. I will be using your information to tune the car, hopefully to its maximum potential.

Gary