I have the original type tach drive unit that has been converted with a Pertronix HEI. I installed a Crane adjustable vacuum advance kit, and at that time put in two silver spings, which are set for advance to start at 600 rpms and have full advance at 2200 rpms. I had much heavier springs for my small block, my thinking is that I want the advance to come on quicker with the big block? Is this right or should l go back to a heavier spring? The car runs good from take off and seems to be good at wot.

 

Ratman,
You might consider taking your distributor to a pro with a Sun machine and letting him spin it to correctly adjust the curve.
..redvetracr

 

If your springs are too light the engine will let you know by rattling and clacking the valve train. Do a search on it and you should find a lot of info on how to blueprint the distributor.

 

Rat -
I agree with Joe - For max performance, drop in the lightest springs and get the quickest advance curve that the engine will take without knocking. If you have reasonable compression and you're running pump gas, a 2200 rpm full advance curve is usually about as fast of a curve as most engines will take.

 

I dropped the vette off at the garage this am, he checked the timming and at idle it was about 32, then when you hit the gas it actually retarded the timming? He said it was most likely the vacuum advance, and they are going to check it out. Do I need the vacuum advance? I don't really care about gas mileage and everyone has told me the reason for it is economy.

 

It will run better with the vacuum advance if it is set up correctly. It can work like a computer where it will constantly adjust your timing in relation to the manifold vacuum. For example, when cranking there is no vacuum and therefore no vacuum advance. At WOT also no vacuum advance. At idol and cruz condition (if you have it connected to direct manifold vacuum) it will advance. It works independent of the mechanical (spring and weights) system. Your initial timing should be checked with vacuum advance disconnected and plugged. IMO

 

Rat -
What you're seeing on the timing (32 degrees and timing retarding upon throttle application) is correct and normal when you have the vacuum advance hooked up and running to a manifold vacuum source - there is nothing wrong with your vacuum advance, and I highly recommend you retain the feauture for best throttle response, best idle, and overall performance. As noted earlier, you can play around with the hookup, and try connecting it to a ported vacuum source if you'd like to run less timing at idle. You might also want to check to see which advance unit you have - they're all calibrated differently, and getting the correct one installed and working right can make a big difference in your performance.


EDIT: Ooops - I just noted you're runing one of the Crane adjustable vac units. I'd really recommend you yank the adjustable unit and go with a known fixed unit - that way you'll know exactly how and when the thing is set up, and you eliminate an unneccessary variable. Echlin part number VC1810 works really well on a "hot" engine combo. Run part number VC680 if you're close to stock.


[Modified by lars, 12:05 PM 9/6/2002]

 

Picked up the car at lunch time, and seems to be a good blend of low end and strong high end power. They changed the springs to a heavier set, starts at 800rpm and has full advance at 2800rpms. He said my intial timming is at 12 degrees, the vacuum advance is 22 degrees and the total is 53 degrees. I took it for a drive on the highway and seems smooth, no chugging and at wot it really pulls strong! I don't know how those numbers add up but sure runs good. I think the gas gauge dropped a 1\4 after only a few miles!!! :D

 

Rat -
Those numbers look very good for a street car, and should give you very nice performance, providing your total mechanical advance is set right. Did they tell you what the total mechanical (less vacuum) was set to...?

 

Lars, would that number be the total of 53 minus the vacuum of 22 for 31 degrees? I will ask them about it asap. Thanks for your help!