Thanks Herb! Actually, I finally found enough other info on the web to confirm what you explained. Also, Chuck went through the distributor upgrade/curve adjustment on last week's HP TV which helped as well.
What was not totally clear from the first few references I found was how to set the "total advance". I read that the optimum total advance is around 36-38 degrees, in by ~2500 rpm. So I went ahead and set my total advance to 36 degrees, with both the vacuum and mechanical advance in place. The problem is, I am getting 13 degrees of advance from the vacuum, and only 10 degrees from the mechanical system. So at WOT, the total advance is only 23 degrees (no vacuum). What I now understand is that the "total advance" should be 36-38 degrees with the mechanical only. The vacuum advance should add another 13-17 degrees above the 36-38 degrees, for a total of 50-55 advance under light load. I was just surprised to find that the engine could take that much advance. Apparently, this can significantly increase mileage. Of course, one should backoff the advance if detonation occurs. Tom -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Herb Lumpp Sent: Monday, July 28, 2003 6:49 PM To: [EMAIL PROTECTED] Subject: RE: [Chevelle-List] Advance curve adjustment Hi Tom, I noticed there haven't been any replies to your question "Is vacuum advance really necessary in a modified engine?" so I'm going to make an attempt... For a street driven vehicle, yes the vacuum advance is necessary as well as the centrifugal advance. Why? Because in a street driven vehicle, the engine is placed under constantly varying loads. During most normal driving speeds, the engine rpm isn't high enough for the centrifugal advance to increase the timing when needed so the timing is advanced/retarded based on the vacuum signal from the engine, depending of course on the load at the time (high load = low vacuum, low load = high vacuum). Of course at higher rpm when there isn't much vacuum signal the centrifugal advance takes over. On a race only engine where peak power and performance is achieved in a very narrow rpm band, vacuum advance isn't needed bacause the centrifugal advance puts the timing at a specified postition within a small rpm window. I'm sure I oversimplified it but hopefully it will help you understand it better. Herb Lumpp http://users.adelphia.net/~hlump/ -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Tom Tomlinson Sent: Thursday, July 17, 2003 11:24 PM To: [EMAIL PROTECTED] Subject: [Chevelle-List] Advance curve adjustment I've been reading up on how to change the advance curve on my 71 350 Malibu with stock distrubutor. I'm planning to replace the points with a Mallory unit, and I thought I would also play with the weights and springs to see if I can improve the performance, but I have a couple of questions. I've measured the current advance curve using tape to degree the balancer and my timing light. Total vacuum advance is 13 deg, and total mechanical advance is 10 deg, and it is all engaged (23 deg total) by about 2800 rpm. I've read that the optimum total timing advance is about 36 deg on a SBC, so that is where the timing is currently set at 3000 rpm. Timing at idle works out to be about 13 deg (BTDC). My question is, is vacuum advance really necessary in a modified engine? >From what I've read, the vacuum advance is designed advanced the timing between 1000 and 3000 rpm, but retard the advance when you go to WOT to compensate for a leaner mixture. But on a properly tuned carb (I'm currently using an Edelbrock 1405 with vacuum advance on the ported opening), the mixture doesn't go lean, in fact, it goes rich. A rich mixture takes longer to burn, so timing should stay advanced. With the current setup, at WOT the vacuum will go away, so the timing advance will be 13 degrees retarded, for a total advance of 36-13=23 deg, not the optimum 36 deg. Would I be better off to remove the vacuum advance and adjust the mechanical advance to provide the total advanced from 13 deg (idle) to 36 deg (3000 rpm)? Hope this makes sense. Tom Tomlinson '71 350 Malibu _______________________________________________ Chevelle-list mailing list [EMAIL PROTECTED] http://chevelles.net/mailman/listinfo/chevelle-list_chevelles.net _______________________________________________ Chevelle-list mailing list [EMAIL PROTECTED] http://chevelles.net/mailman/listinfo/chevelle-list_chevelles.net
