Proper HEI advance curves

[ddsmith]

There has been a bunch of interesting discussions on advance curves for truck following HEI installation. I just wanted to share something with everyone on this subject. A long time ago I purchased a book called The Doctor's Guide to Automotive Ignition. The book was written by Christopher Jacobs. You may have heard of Jacobs Ignition products. I am by no means making an endorsement for their products but the book was well written and describes how to determine the correct advance curve for your engine. Just as a tidbit of information, the factory advance curve is a compromise. Then we go and change a bunch of parts and then how accurate is it. Wouldn't you rather have the perfect advance curve for your engine or at least as close as you can get? I'm going to try and put together a FAQ on determining the best advance curve for your engine. One thing that Jacobs stressed was that in vehicle determination is by far the most accurate means of determining the correct curve. I'm sure this will create some interesting discussion. That's what this board is all about. I figured the discussion could take place here and the information on determination would be placed in FAQ when the discussion is complete. Anyone else that has read the book feel free to comment on your experience. Thanks,

Don

[1970 CST Short Wide]

I'm interested. Dont have an opinion on it but would like to know more

[72longbed]

ddsmith,
I look forward to your discussion on proper advance curve. I feel this is one of the most important, easily influenced, yet greatly misunderstood aspects of proper engine tuning for performance. Please share with us what you learned. Thank you.

[drink2mny]

I could use some knowledge..I can't seem to tune mine right with the vacuum advance hooked up.With it unhooked it runs great..
Weird

[JimKshortstep4x4]

I would be interested in any conclusions that you come up with.

This is the machine that I use to curve my distributors. The Chevy engines that we run seem to run real well with 34-36 degrees total advance in by 2800 rpm and that is the way all of our distributors are set up.

Now, if I just had a dyno machine to verify that the curve is optimum for the particular engine!



Jim

[crazy longhorn]

I like to run a 34-36 degree total also,(all in @ 2800-3000),as that seems to work good with pump fuel. I try to shoot for about 10-12 on the vac timing(crankshaft,& hooked to manifold vac),but am running closer to 16 on the longhorn. I would also be interested to read any availble info on ign curves.......would like to hear more on the vac advance setup. I think a lot does depend on weight,gearing,& drivinghabits as well. I also agree that the factory curve does not optimize the power that the eng will put out. crazyL

[ddsmith]

The problem with a distributor machine setup is how do you determine the correct advance to use to set the distributor to. Every engine will be somewhat different even within the stock engine category. When you throw in a bunch of different components together thing get even more complicated. I've tried to condense the information down somewhat in the FAQ section at this link http://67-72chevytrucks.com/vboard/s...threadid=62040 . Although this is a little time consuming to accomplish I think the information and the results will be well worth the effort. I need to get my tachometer hooked up on my truck so I can install the correct curve in my HEI. I did previously perform this on my 71 Vette with a 454 and was very happy with the results.

JimKshortstep4x4

Now, if I just had a dyno machine to verify that the curve is optimum for the particular engine!

Strange you should mention that because the book also has a test for a road dynamometer accurate to 2%. Without the use of an elaborate dyno machine. So this stuff is right up the backyard mechanics alley. It's been a while since I've reviewed this book. It's been on my shelf for many years and I forgot how useful it could be.
Have fun,
Don

[hecticblasts]

I think that preety much it is accepted as common knowledge by virtue of all the ink that has been printed that a small block engine likes the 34-36 degrees of timing,seems that every article that it's written supports that after testing at the dyno and the strip.What I allways wonder about is the fact that all these articles are focused on hot street engines destined to be installed on camaros ,novas,etc. and will see duty on both street and strip.Since that's the intended purpose of said engine the builders or tuners tend to want their timing to be all in as soon as possible or just a little late for the sake of street manners.I have yet to see one article on a strictly street driven but yet performance oriented engine that will never see the strip.Taken our trucks as an example we are dealing with a heavy vehicle, some are 3/4 tons with tall tires and towing capabilities.It is MY OPINION that on this truck and following the general knowledge I would want the 34-36 degrees of total timing but I THINK that it will be beneficial to have it all come in a tad later,say at 3200-3600 RPM in order to take advantage of maybe not a quick but steadily strong torque curve.I don't have any data to support this tought process just my thinking on the matter,yet I would like to find out what others think or if this makes sense at all.I think before it is all over this will be a great thread to reference to in the future,thanks for posting Don.HF

[68 Suburban]

http://www.chevyhiperformance.com/ho...38/index1.html

http://www.73-87.com/garage/hei.htm

[crazy longhorn]

Jimfulco posted this link on eng & driveline,& it looks to fit this thread. http://www.corvetteforum.com/techtip...=115&TopicID=3 pretty good reading on vac advances.

[Bill72BB]

I would be interested in any info on the Advance curve for the HEI. I plotted these figures when I installed the HEI and wish I had put in a Adjustable vacuum advance .

No Vacuum .. Vacuum Hooked up ....... Diff

Idle 12% .............. 12% ..... 0
1500 15% .............. 31% ...... 16
2000 22% .............. 38% ...... 16
2500 27% ............. 40% ..... 13
3000 30% ............. 41% ..... 11

any thought on these #
Thanks

[ddsmith]

This particular quote comes from one of the link articles in 68 Suburban's post.

Ideal ignition settings will allow your engine to run the maximum amount of timing at all engine speeds without detonation. Now that you know how distributor timing works, you can manipulate it to improve your torque and horsepower curves.

I think that this is true whether you are driving a highly modified 2000# drag car or a ever dependable chevy truck. So I guess that what I'm saying is that just because your running a truck doesn't mean you need the advance curve to come in slower. The best performance will come when the advance allows flame propagation to reach the point of providing maximum cylinder pressure at just past TDC. The closer this happens to TDC the more torque that it developed during each cylinder firing. When you advance the timing to a point where detonation happens, you are hearing the flame igniting the major portion of the air fuel mixture before TDC and fighting the upward movement of the piston trying to make it to TDC. By using the method identified in the FAQ for finding the correct advance curve you advance to the point of detonation and retard the timing a small amount. This should put you at the best possible advance.

Bill72BB,
I haven't checked out the books but I'm pretty sure BB like more total advance. The vacuum advance will need to be reduced and that will require an adjustable vacuum advance. With the HEI, it has a mechanical advance limit that is around 20 degrees. Changing it would require altering the mechanical stop on the advance weight assembly. You can run more initial advance provided the car or truck can still be started once the vacuum advance is changed. The current setup is only giving you 30 degrees of advance at WOT because you won't have any vacuum for the vacuum advance.

[GMC Jim]

ddsmith... you asked "how to determine the correct advance to use to set the distributor to"......the factory shop manuals should list these specs. In my case; the specs for a 402 engine are listed at the back of the '1971 Chevrolet Truck Service Manual'; Distributor specifications. Example for the 402 engine the centrifigal advance degrees is 0 @ 930 RPM; 2 @ 1260; 16 @ 2400 and 30 @ 4400. The vacuum advance should be 0 @ 8" and 20 @ 17". Another GM manual "1971-80 Tune-Up Specifications" states the the total advance ( vac. and cent.) should be 28-35 @ 2000 RPM. About 10 yrs ago, I had a qualified distributor rebuild shop set the proper advance curves; checked the mechanical and electrical components. This distributor rebuild firm had the specs on file also....I installed the distributor, replaced the spark wires with those designed for the higher voltage produced and gapped the new spark plugs to .045....Very pleased with the performance and increased gas mileage............ Jim