Timing set to 34 total - knock in highest gear at very low RPM --- Is this OK?

[XH29N0G]

I am trying to optimize my timing. If I run 36 there is a clear knock on acceleration in 3rd & 4th

I reset my timing to 34 Total (mechanical advance - no vacuum advance MSD distributor, initial is 15) (Compression is 10.5:1 on a 450 CI). 

I have no knock in 3rd and 4th accelerating from 2200 RPM or higher.  I get a knock in 5th accelerating from 1500 stomp on it - I do not notice knocking if I ease it up from 1500 RPM in 5th. 

My thinking is that this will be OK because most of the time I accelerate or run up the gears, the acceleration almost always starts well above 2500 RPM so the car will not see a knock condition.

Am I missing something? 

[Paul G]

Knock is not good, ever. You can tune the dizzy if you want to make it go away. You need to go through the timing curve of the distributor. The curve is the rate that the timing advances in relation to rpm. If your engine likes 15° base and a total of 36°, at what rpm does it reach 36°? At 2000, 2500, 3000 rpm?

Map out the timing curve on paper. Start at idle, write down the timing number, rev the engine in 500 rpm increments and record the timing number until the engine is at maximum advance. That is the curve. You may need to try heavier springs in the dizzy. A heavier spring will hold out the timing advance till a higher rpm. The lighter spring controls lower rpm advance till the heavier spring starts to be activated, controlling the upper end of the curve.

What you are after is holding out some timing until 2000 rpm, eliminating knock in high gear low rpm, then let it advance quickly. It is trial and error, recording each spring change.

[XH29N0G]

Thanks for the reality check.  It will give me a chance to map out the advance curve and learn something new.

[justcruisin]

Check your plugs, they will tell you. Detonation can't allways be herd. Anyway - whats the point in stomping on it in OD at 1500rpm

[c00nhunterjoe]

What springs are in the distributor? Might have to loosen the curve up a tad or could be lean. A simple plug reading should help. Make it knock then immediately shut it off and pull a few plugs to see what they look like. Swapping dizzy springs is easy enough to try.

[flyinlow]

The stock manual trans cars tended to have slightly slower advance curves to help protect them when the where lugged down.  

My engine will accelerate smoothly in OD and lock up with small throttle opening at 1500 rpm. Push the gas peddle down more and it bucks and knocks. Unlock the converter so the engine can rev up a little against the converter and it's happy again.

Like Paul said there is some trial and error to get the best results.

Modern cars have detonation sensors so they can run close to the limit of timing. We have to listen for knocking and read the plugs. Then when you have all you can get we back off a couple degrees so our warrenty cost don't skyrocket.

[XH29N0G]

Thank you.  

Some quick responses:

1) Springs in the distributor:  I have not had it apart.  The engine was originally set up by muscle motors, has an MSD distributor, and looking at the total advance (which looks like 18 and the advance coming in by 1500 - which I measured as 12 degrees) it looks to me like the distributor was modified from what MSD ships.  [I did one test.  I still have to measure the advance curve].   

2) Lean condition:  I have an A/F gauge and what I found was that I can make it knock if I lug in 5th (it is a manual with OD grears) and if the A/F jumps up to 15-16 with the opening of the throttle blades.  If A/F stays 14, 13, 12.8 it will not knock.  It isn't bucking and the cruise A/F is right at 14.5-15.2.  The knock would come in at A/f 15-16, advance of 28 total at 1500 under heavy load.  

3) Why 1500 in OD?  This whole thing started when I took a quick look at the total timing an noticed it was lower (32-33) than I recall it being two years ago (34) when the car was set up.  So I thought I would try it at 36 (knocked a lot) and then 34 (knock in 5th at 1500).  I tried it because I heard nothing in 3rd and 4th and decided to see whether I would also hear nothing in 5th (which put it at 1500).  I don't like driving in any gear below 2000.  That is the reason - even if not a good one.

------
Current state:

Yesterday I dropped the timing back by one degree which got rid of the knock.  This is pretty close to what it was when I started. I think I will leave it back with ~33 total and 14 to 15 initial for now.  I will check the advance curve next time I get the car warmed up. I stopped because I was working alone and worried about using the idle adjustment screw to set it at RPM to 2000, 2500, 3000 without an easy way to drop it back if something went wrong.

I have never recurved a distributor myself, but it looks straightforward from what I read.  It idles better with more initial.  I will check the springs next time I am working with the car (if I read the literature currectly and measured the 1500 rpm point correctly, my guess is I have a light and a medium spring in and an 18 degree bushing.  Do you think it is worth experimenting with slightly more initial (and final) so it is up at 34-36 and a less aggressive advance curve so it doesn't knock in high gear?

[myk]

Agreed.  Sticking your face into a fat motor screaming at 3000 RPM is not my idea of fun...

[XH29N0G]

An update: 

I bought the springs necessary to change the advance curve and took it two steps from where it was.  It had the two lightest springs in it (light silver) and I replaced with the two blue ones.  This should move the total advance from around 2000 RPM to 3000 RPM. 

I no longer have knocking and was able to set the timing at 18 for idle (instead of 14-15) and now get approximately 35-26 above 3000 RPM.   The engine likes the 18 degrees for idle (idle speed increased and (I think - but we all know this is subjective) it idles more smoothly).  It also looks like it picked up about 1 inch of vacuum at idle from about 9-10 to about 10-11.  I wonder if this makes sense.  The carburetor did not adjust much.  I am still 1 turn out from bottom. 

In the future, I will try one light silver spring and one blue to see if that makes thinks any better, but I think in the end I will need an objective way to measure differences (track or dyno) to know for sure and I have neither.