I have a 68 327 350 hp engine, 11:1 compression, no emissions. I run the car at 6000 to 7000 feet pressure alt. What should the timing be at idle, mid throttle, and full throttle? I'm NOT talking about initial timig without the vacuun hooked up or total timing. I mean as the car is setup in normal driving condition. I know there are different ways to skin this cat. I also know factory spec's are probably not the answer. Thanks

 

Suggest you copy your post to an e-mail and send it to Lars requesting his advice. V8FastCars@msn.com

 

I set my car up with manifold vacuum, new vacuum can, new springs, adjusted the total timing to about 36* at 2600 rpm's, as per recommendations, this makes my initial (static) timing 21*. If I check timing at idle, everything hooked up, it is 36*. Is the 21* and 36* at idle a good thing?

 

Either your centrifugal curve is too short, or you have centrifugal timing coming in at idle rpm. 36 actual timing at idle is not a bad thing as long as you are not getting part throttle detonation under light acceleration. Many of the GM performance engines had advance curves that allowed some centrifugal timing to come in at idle rpm. Combined with the common 18-degree vacuum advance units, it was not uncommon for a stock GM performance engine to be at 36 degrees at idle. The determining factor on whether or not you need to do anything about it is if you are seeing detonation under any load or driving condition.

Lars

 

My weights are stock in the distributor, I installed the silver spring from the Mr. Gasket 928G spring kit. It is kind of hard to tell with side pipes, but I don't think I have detonation.

 

Set total 36 degrees at 2600. Rev it to 4000 to make sure it is done advancing. Total timing is not at a specific rpm. Total timing is when centrif advance stops advancing. You can adjust curve with springs and weights, but the important part is that it doesn't go past 36 at full throttle high rpm.

 

Yep,,, total timing is 36*

 

In that case, you are getting some centrifugal advance coming in at idle, so your initial timing of 21 is not the "real" initial timing number. As long as you are not getting any off-idle detonation under modest acceleration you're good to go. Your total timing of 36 should be all-in at 2800-3000 to avoid detonation.

 

I want to make sure I understand what you are seeing, Are you getting 36 degrees at high RPM with the vacuum hooked up or disconnected? The timing at high RPM (No more mechanical advance should be 36 degrees) Once you hook up the vacuum advance you will see 40 degrees plus at high RPM. (The higher advance helps with driveability at light and part throttle)

Also since you are at 6000 to 7000 feet you may want to try more then 36 degrees at high RPM with no vacuum advance and see how the car works, the less atmospheric pressure the more advance the engine will want for best performance.

 

There are not different ways to skin this cat.....there is one way.....any other way will cause reduced performance.
You want 36 degrees @ 3000+ RPM total.......this is what it will give at WOT. Adjust the advance weight springs to do this......there are curve kits still available.
Do not concern yourself with mid range or idle.......the curve works out the midrange and idle falls where it falls........but record that number as you will need it for checking how much vacuum advance you have plugged in and unplugged......
You want to pull 12 degrees vacuum advance OVER the initial you recorded.........
Just get Lars' papers and read them.......it pretty much explains what I just said.

Jebby

 

Did you install the bronze bushing that came with the kit. That is the advance stop bushing and may account for you initial timing issue. The original stop is made of nylon and is typically gone or cracked by now.

 

Like I said, 36* total timing. Total timing is with the vacuum unhooked and plugged. BUT,,,, my timing at idle (750 RPM) every thing hooked up, as you would be to drive the car is also 36*

 

My distributor has the originl black nylon stop in it.

 

Did you compare it to the bronze bushing?
If the bushing is wider it will reduce the advance.

 

I did notice the slot that the bronze and the original nylon bushing rides in is only so wide, so it can't take a bushing much wider than stock. And my vacuum can is a B-26.

 

The bushing is designed for the width of a Delco distributor. Compare the size of the two bushings. It will also outlast the nylon bushing.

 

ok, so lots of advice here on this subject. From my personal experience
,not just with SBC, initial timing is of importance if you want a specific idle and good off idle performance.

given your engine with perhaps a cam of decent overlap in order to get that 350 hp and your altitude more timing is going to be necessary than at sea level.
You have low cylinder pressures due to cam overlap and low air density. this requires more timing lead to get the fuel burning and more completely burned vs a car at sea level.

so given these factors 36* idle timing is probably very close to what it needs.

also you may need more total timing, perhaps 38* vs the standard 36*. Because despite the high compression ( in reality it’s probably more like 10.2:1 due to GM manufacturing tolerances) you still have low air density and thereby lower cylinder pressures all through the rpm range.

i operate from 5000 to 8500 altitudes and that has required more ignition lead for my engine as well, despite the aluminum heads. Idle I run 34* with vacuum but only 34* total 51* cruise with vacuum. It’s what the engine likes. CR10.6:1, measured not advertised. And a relatively small roller cam with not too much overlap.

I had to modify both the mechanical advance total movement and vacuum advance movement to get the timing the engine wanted but it runs very well at all rpm’s now.

 

Latest update. I am useing the Mr. gasket 928G silver spring,,,,,, and the brass bushing. I am useing manifold vacuum,,,,, 12 Hg vacuum at 750 rpm,,,, I have the B-26 vacuum canister. Total timing is set at 36*,,,,, all in at about 2300 rpm. My initial timing went from 21* to now 25* with the new brass bushing "750 rpm". Timing at 750 rpm all plugged in as you drive the car is still 38* As you drive the car,,,,, 3000 rpm 51* no real load. It starts and drives nice. I can't hear any pinging. 1968 327 350 HP L79 engine. I worry a bit about detonation.

 

Your pistons, If stock, are forged aluminum. Forged pistons are very robust and can tolerate quite a bit of detonation.

If you've ever heard detonation the sound is unmistakable. It's kinda like rattling a few rocks, or many perhaps, in a empty coffee can kind of sound. In your case with the timing coming in so soon (2300 RPM) it would likely occur early in the RPM range. Might be better if you could get that to happen more in the 2800 to 3000 RPM range. However, at high altitudes you can get away with earlier timing without exceeding the cylinder pressure needed for auto ignition (detonation) of the fuel.

Your vacuum is higher than I would have expected given a flat tappet with a "high performance" cam.
From what I can find ( the numbers seem to change depending on your source) though it's a 195*/202* @ .050 cam. So not particularly high performance.

The advertised duration is pretty high though maybe 296/310 ish?. In any case the fact that the advertised is much larger than the @ .050 number is what reduces the cylinder pressure so much at lower RPM's. Long slow ramps on the cam lobes creates lots of overlap.Large overlap wants more timing lead. Buy about 3600 RPM this engine should hit max torque.

 

I can put the stronger spring in the distributor.