Old School distributor (dizzy) timing question

[MB750]

This one's for the old school tuners in the group.

I've got an HEI dizzy on my 302 SBF. Vacuum and mechanical advance. Mechanical is 22 degrees per the manual, and when I tested the vacuum it starts at 3"Hg and stops at 7"Hg. I actually tuned it with my Mighty Vac and a 3mm allen, so it's adjustable. Regardless though, I don't know how many degrees it pulls or adds.

Anyway, I initially set my timing at 10 degrees during cranking. The car started, and everyone was happy. Now that I've got 150 miles on her I'm fiddling with the carb and the timing.

Setting the initial timing seems pretty straight forward. Pull the vacuum advance hose, plug the carb hole, increase idle, and turn dizzy to your initial setpoint. I started with 14 degrees. Knowing that my mechanical is 22 degrees, I'm expecting 36 degrees total.

Here's the problem. I revved up the engine while watching the timing. It parked at 30 degrees and never went higher. Granted I wasn't watching the tach, but after it stopped advancing I gave the throttle a blip from the engine bay and it still didn't go any higher.

Am I supposed to lock out the mechanical advance when setting initial timing? Or am I suppose to set the total timing to where I want (36 degrees), and then lock down the dizzy and adjust my idle after re-activating the mechanical and hooking up the vacuum?

[Mike.Bray]

It's been way too many years (decades) since I've messed with a mechanical distributor but here's where I'm at on my engine if it helps.

[J R Jones]

My experience is road racing 289, 302 & 351C SBF with Ford dual point distributors. We did not use vacuum advance since it raises cruise timing for fuel economy; we did not cruise.
Our distributors had centrifugal (weight) advance. We never "locked" it. Vacuum advance will not go higher than mechanical advance.
The critical issue is too much total timing with the givens of compression, fuel mixture and fuel quality. Too much timing can cause detonation and potentially piston damage.
Your manual says 22 degrees advance, trust your experience and your eyes.
We set the timing at RPM sufficient to read the total, and ignored timing below that RPM. IMO 36 degrees is the high end of timing. You need to burn good fuel, and you can not run lean.
jim

[Dgc333]

You mention that you increased the rpm then set the initial timing, you don't want to do that and is likely why you are only seeing 30 degrees of timing when you rev the engine.

To set the initial timing you need the idle rpm to be below the rpm at which the mechanical advance starts coming in. Start by watching the timing as you slowly increase the rpm. Note the rpm at the point the the timing starts to advance. You will want your idle rpm set at least 200 rpm below that point.

You state that the distributor has 22 degrees of advance in it. You can now rev the engine until the timing stops advancing. The timing should be the sum of the initial and the 22. If you have to rev the engine beyond 3000 rpm to get the max timing the springs in the distributor that control the rate are to stiff. You can pick up spring kits for HEI distributors from Summit or Jegs and will come with instructions on which combo of springs to use for the desired curve. For best performance 2800 rpm is a good spot. You mention you want 36 degrees which is a reasonable number. Keep in mind if the engine has aftermarket heads you may want less. If you don't get the 36 you are looking for just rev the engine until the advance stops advancing and rotate the distributor to get your 36 and lock down the distributor. Keep in mind doing it this way your initial timing may not be ideal with winding up with a rough idle or tip in detonation if it is to much. They also sell bushing kits so you can adjust the amount of timing in the distributor.

To adjust the vacuum advance you first need to drive the car at a steady 45 mph and record the manifold vacuum. Then use your mighty-vac to adjust the vacuum advance pot so all the vacuum advance is in at that cruising vacuum. You may need to back this off if you get tip in detonation transitioning from cruise to acceleration. Note: there should be a number stamped on the arm of the vacuum advance, this is cam degrees, double the number for crank degrees.

To really optimize the engine performance you will need a Dyno. Some rules of thumb; cams with more duration tend to want more timing, aftermarket cylinder heads with better chambers tend to want less timing for best performance and you need to get the timing sorted before trying to dial in the carb.

[MB750]

You mention that you increased the rpm then set the initial timing, you don't want to do that and is likely why you are only seeing 30 degrees of timing when you rev the engine.

To set the initial timing you need the idle rpm to be below the rpm at which the mechanical advance starts coming in. Start by watching the timing as you slowly increase the rpm. Note the rpm at the point the the timing starts to advance. You will want your idle rpm set at least 200 rpm below that point.

You state that the distributor has 22 degrees of advance in it. You can now rev the engine until the timing stops advancing. The timing should be the sum of the initial and the 22. If you have to rev the engine beyond 3000 rpm to get the max timing the springs in the distributor that control the rate are to stiff. You can pick up spring kits for HEI distributors from Summit or Jegs and will come with instructions on which combo of springs to use for the desired curve. For best performance 2800 rpm is a good spot. You mention you want 36 degrees which is a reasonable number. Keep in mind if the engine has aftermarket heads you may want less. If you don't get the 36 you are looking for just rev the engine until the advance stops advancing and rotate the distributor to get your 36 and lock down the distributor. Keep in mind doing it this way your initial timing may not be ideal with winding up with a rough idle or tip in detonation if it is to much. They also sell bushing kits so you can adjust the amount of timing in the distributor.

To adjust the vacuum advance you first need to drive the car at a steady 45 mph and record the manifold vacuum. Then use your mighty-vac to adjust the vacuum advance pot so all the vacuum advance is in at that cruising vacuum. You may need to back this off if you get tip in detonation transitioning from cruise to acceleration. Note: there should be a number stamped on the arm of the vacuum advance, this is cam degrees, double the number for crank degrees.

To really optimize the engine performance you will need a Dyno. Some rules of thumb; cams with more duration tend to want more timing, aftermarket cylinder heads with better chambers tend to want less timing for best performance and you need to get the timing sorted before trying to dial in the carb.

That's what I was thinking was happening. I never got an RPM curve for my mechanical advance, so I just assumed any RPM would start the advancing. Since I'm idling at 1000 rpm, that's a few degrees taken up by the mechanical advance already. Quick math says 6 degrees (22 total, but it only increases 16 to the max advance, which leaves 6 degrees).

This is all difficult, but I'm learning so it's fun. Thanks for your guidance. I think what I'm gonna do is get the engine piping hot, then set max advance with my wife hitting the gas to 34 degrees. It's a little safer than 36. I've got a B303+ cam, and AFR cylinder heads at 9.3:1 CR. I also run 93 octane.

[MB750]

It's been way too many years (decades) since I've messed with a mechanical distributor but here's where I'm at on my engine if it helps.

Thanks Mike, this does give me a ballpark at all places. Once I eventually do the EFI swap I'll also be doing ignition within the system as well. Fortunately there's dozens of initial maps free to download specific to my engine that will at least get the engine running.

All that's down the road though. For now I'm playing with the carby and dizzy like it's 1968.

[Dgc333]

Vacuum advance will not go higher than mechanical advance.
jim

That is not really true. Vacuum advance is load dependant where mechanical advance is rpm dependant. If you were cruising with the engine rpm at or very near the rpm which gives you maximum mechanical advance and and the manifold vacuum is at or near the maximum in the vacuum advance pot the two sum and could easily be approaching 50 degrees of total timing.

Keep in mind when cruising with high engine vacuum (almost closed throttle blades) the cylinders are not being efficiently filled with air/fuel mixture so more timing is required to light off the mixture to obtain an efficient burn. At WOT there is little or no manifold vacuum so the vacuum advance does not add any timing.

[Avalanche325]

I agree with what you are planning. Set it for max advance at around 34.

I did that with my Pertronix manual distributor. I needed to put advance limiters on it as the total advance sweep was too much.

[Jeff Kleiner]

Unless you've got really soft springs the mechanical should be doing virtually nothing at idle. Are you sure that your balancer and timing marks are accurate? Also are you sure that the dizzy is actually capable of delivering greater than 20 mechanical (the weights sometimes come up against a stop or limiter)? If so then yes, you can work backwards and set the dizzy so that you the have desired full advance at 3000 or whatever RPM (without vacuum) and lock it down there (This is how the MDS E-Curve is done and then you adjust the curve with dip switches) If you do this and the dizzy is truly giving you 20 mechanical that will mean that your initial is 16 which may result in difficult hot starts and detonation under load at low rpm.

Jeff

[Mike.Bray]

Thanks Mike, this does give me a ballpark at all places. Once I eventually do the EFI swap I'll also be doing ignition within the system as well. Fortunately there's dozens of initial maps free to download specific to my engine that will at least get the engine running.

All that's down the road though. For now I'm playing with the carby and dizzy like it's 1968.

Just to be clear, 100 KPa on the scale is atmosphere pressure (1 bar) and 0 is full vacuum.

That's a good move doing timing control with the ECU. I'm using a Holley Dual Sync, 565-201.

With today's EFI systems like the Fast Sportsman or Holley Terminator X you can do some base configuration and you will literally be 90-95% there. It was amazing how close mine was even with stacks which can be a bear to tune.

[BornWestUSA]

Set the total timing at say 4,000 rpm is what I was taught. (34-36 degrees?)

Then see what the timing is at idle. Around 10-12 degrees would be good, but depends on how much mechanical advance you have.

Mechanical weights / springs can be changed to alter total timing.

Vacuum advance works at high engine vacuum settings like very light throttle cruise. But at full throttle it will not work, or add to "total" advance, because full throttle = low vacuum.

[J R Jones]

That is not really true. Vacuum advance is load dependant where mechanical advance is rpm dependant. If you were cruising with the engine rpm at or very near the rpm which gives you maximum mechanical advance and and the manifold vacuum is at or near the maximum in the vacuum advance pot the two sum and could easily be approaching 50 degrees of total timing.

Keep in mind when cruising with high engine vacuum (almost closed throttle blades) the cylinders are not being efficiently filled with air/fuel mixture so more timing is required to light off the mixture to obtain an efficient burn. At WOT there is little or no manifold vacuum so the vacuum advance does not add any timing.

You are right:
https://www.motortrend.com/how-to/16...nition-timing/

Like I said, our racing was all on or all off 20-40 shifts per lap. No cruise.
jim

[MB750]

Tonight I had the wife at the helm while I was in the engine bay turning the dizzy with the strobe. I parked it at 35 degrees while she held the gas at 3500 rpm.

I first had her go to 3k, then slowly increase the gas, and 3k wasn't max. I'm not sure exactly where but at "3500" I could hear some slight changes in RPM but the timing stayed parked, so I considered that full advance and locked down the dizzy.

Then I hooked up the vacuum advance, adjusted the idle, and went for a spin. Much better throttle response, and somehow the AFR's are more predictable and consistent. I didn't touch anything on the carb and now I'm lean almost everywhere except WFO. With the secondaries disconnected I'm in the high 11's at WFO, and the mid 15's at cruise. Not ideal, but much better than before.

Now it's back to the carb. I'll fatten up the jetting a bit, then when everything looks great on the primaries I'll put the secondaries back in play and tune for some real rubber-melting around my neighborhood.

[Railroad]

I am glad you have made progress on your tuning. With that said, you are far from ideal on your ignition advance.
On light weight and fast reving gear ratios, you can set total advance about 26 to 2800 rpm. You also need to verify what amount of advance your stops are set.
I like your setting of 34 to 36* of total advance.
Do you have a pic of your distributor? It seems you stated HEI design.
I would not be too concerned about high A/F ratios at cruise. I have seen 15/16 to 1 on computer cars.
In all it sounds like you have a handle on everything and made good progress. Great feeling, isn't it.

[MB750]

I am glad you have made progress on your tuning. With that said, you are far from ideal on your ignition advance.
On light weight and fast reving gear ratios, you can set total advance about 26 to 2800 rpm. You also need to verify what amount of advance your stops are set.
I like your setting of 34 to 36* of total advance.
Do you have a pic of your distributor? It seems you stated HEI design.
I would not be too concerned about high A/F ratios at cruise. I have seen 15/16 to 1 on computer cars.
In all it sounds like you have a handle on everything and made good progress. Great feeling, isn't it.

Thank you. Yes, it does feel good, especially now that my AFR's are making sense. Yesterday I pinned it and my WFO AFR's were in the high 11's, when last weekend they were in the low 10's. I never would have thought just a timing changed fixed it. I also thought it was kinda soft too, but then I remembered I took the secondaries bar off and that was just the primaries.

It's an HEI from Summit. Nothing special, I think it was $160. I never got any springs though. If I could pull off full advance lower than 3K RPM I would but I never got any other bits or springs with the dizzy like I would have with a better brand like MSD or the like.

[Avalanche325]

Springs don't determine how much advance, they determine how fast you get it. (Timing curve) Limiters determine how much. (Total advance)

[Jeff Kleiner]

Springs don't determine how much advance, they determine how fast you get it. (Timing curve) Limiters determine how much. (Total advance)

Correct.

Order this and you'll keep yourself occupied for hours swapping springs and limiters:

https://www.summitracing.com/parts/mor-72300?rrec=true

Here is some info on the various rates the springs deliver:

https://static.summitracing.com/glob.../mor-72300.pdf

And after you digest all of that then you can ponder an adjustable vacuum canister

Jeff

[Jeff Kleiner]

Springs don't determine how much advance, they determine how fast you get it. (Timing curve) Limiters determine how much. (Total advance)

Correct.

Order this and you'll keep yourself occupied for hours swapping springs and limiters:

https://www.summitracing.com/parts/mor-72300?rrec=true

Here is some info on the various rates the springs deliver:

https://static.summitracing.com/glob.../mor-72300.pdf

And after you digest all of that then you can ponder an adjustable vacuum canister

Jeff

[Railroad]

https://www.summitracing.com/parts/sum-g1059
one of these or similar will make things a bunch easier.
It's not likely your gear friends will be asking to borrow it, since nearly everyone is laptop tuning now.
I have an old Sears adj timing light. It has a dial on it, marked in degrees.
Let the fun begin.

[CraigS]

Although w/ 9.3-1 CR you sure are not in the high CR area, still you need to watch for knock in the 1500-3000 range. Years reading here and my own experience it seems idle advance of 10-14deg and full mechanical advance of the 34-36 that you have mentioned is normal range. But the vac advance can be adding quite a bit more especially in around town 1/3 to 1/2 throttle 1500-3000 driving. I once had a rattle I searched for at around 2500 when getting back on the gas after coasting a bit or after a steady light throttle. Searched all kinds of stuff like exhaust, alternator mounts, anything that might buzz or rattle. The final discovery was that my dist had 20deg of vac advance. I limited it to 10deg and the rattle was gone. My rattle was really engine knocking because the old school vac advance didn't react fast enough to dump the advance and, of course there was too much advance also. The exhaust sound, w/ the sidepipe exit maybe 22" from our left ear, doesn't help any in discerning the difference between some type of mechanical rattle and an engine knock.

[MB750]

Although w/ 9.3-1 CR you sure are not in the high CR area, still you need to watch for knock in the 1500-3000 range. Years reading here and my own experience it seems idle advance of 10-14deg and full mechanical advance of the 34-36 that you have mentioned is normal range. But the vac advance can be adding quite a bit more especially in around town 1/3 to 1/2 throttle 1500-3000 driving. I once had a rattle I searched for at around 2500 when getting back on the gas after coasting a bit or after a steady light throttle. Searched all kinds of stuff like exhaust, alternator mounts, anything that might buzz or rattle. The final discovery was that my dist had 20deg of vac advance. I limited it to 10deg and the rattle was gone. My rattle was really engine knocking because the old school vac advance didn't react fast enough to dump the advance and, of course there was too much advance also. The exhaust sound, w/ the sidepipe exit maybe 22" from our left ear, doesn't help any in discerning the difference between some type of mechanical rattle and an engine knock.

One of my engine build design ideas was done specifically to hopefully prevent that. I took a chapter from the book of Harley and went with 30 thou squish (also sometimes called quench). Typically head gaskets are 45 thou, and the piston is recessed 9 thou, which makes for a country mile between the piston and the head at TDC. The idea here with my engine is that combustion chamber turbulence is increased by the tight clearance between the piston and the cylinder head squishing the air and blowing it into the main combustion chamber area. This increased turbulence reduces the need for higher total advance and a reduced chance of knock. I'm also running 93 octane. I'll keep my ears peeled for weird noises though, for sure.

If anyone really wants to go down the rabbit hole there's something called "Singh grooves" which are small channels cut into the head in that squish area that are specifically designed to channel that compressed air/fuel into narrow passages and force it out with more velocity, creating even more turbulence. They work in many applications, but best with a tight squish. This is all theory, but there's been a lot of anecdotal reports of better mileage, better idling, more complete combustion, etc...