Another timing curve question

-
DrCharles

DrCharles

Well-Known Member
Joined
Jun 7, 2012
Messages
1,463
Reaction score
1,368
Location
West Plains, Mo.
I'm looking for advice on ignition timing of my pump-gas 451 with big cam (272@.050), currently iron headed 10.3:1 but soon to be aluminum 10.6:1.

Currently I have 22 initial (starts to fight the starter hot at 24-25). Total is 36, trying to remember the curve but think it's all in by 3000. FBO adjustable limiter plate on a stock Mopar distributor.

Also running ported vacuum advance. FBO likes manifold vacuum, but it only idles around 7-8" at 1100-1200 which won't pull the can in much if at all. FBO can sell me a can that is all in at 8".

Since it's mostly street driven (3.91 gears), locked-out timing will not only be too advanced at low speed part-throttle cruise, but also create a hot-start problem.
Cam definitely wants more than 22 to idle the best.

What I'm thinking of is one light spring and one heavy spring with a loop. So I can crank it at about 12 degrees, and have it immediately advance once running to the start of the heavy spring, 26 or 28 degrees maybe? And re-position or remove the limiter plate so total is still 36, of course.

Or, leave the distributor where it is, but get the light-spring vacuum advance canister and hook it up to full vacuum, which will raise the idle timing into the 30's but drop when I start off.

Thoughts?
(Also posted on Moparts).
Thanks.
 
Are you running an ignition box?

I don't think I'd use vacuum advance on manifold vacuum that is all it at 8 inches.

I would reduce the initial and see if I could get the advance to start at idle speed...IOW's get it pulling some timing.

For example, if you can get the intial down to 16 and you can get a set up where by 1000 RPM you are adding 4 degrees (crank degrees) you'll have 20 degrees there and most likely, you'll get another 2 degrees by 1500.

I'm for the vacuum advance if you can get it so it doesn't rattle open tip in (mostly because the mechanism reacts to slow to the vacuum dropping) and you bring it in late enough for the same reason.

I'd love to hear what Mattax has to say, because I could be out of the ballpark but those are my thoughts.
 
Thanks. I hope he'll jump in too ;)

The problem with an advance that starts below idle speed is that the idle will never be really stable... less RPM, less timing, less RPM, stall... so I have to have some way of setting it up to start advancing at no less than 1200.

On Moparts there's a reply already with similar combo, and he just uses the one light spring to crank, then all in once it's running. Says that worked ok on the street for him...

I'll sort out the vacuum advance (runs around 14-15" at cruise) last.
 
Well thanks but I usually keep my trap shut with builds that are way outside my own experience unless its something specific. I'll give you my .02 but take it for what it is.

I agree with YR on not using a vacuum advance that is all in at 8"Hg.
The way I see it, vacuum advance is best matched for lower load, low fuel density situation.

Lets say the car cruises at 16"Hg, 2800 rpm in 3rd gear, and considering that it has stock heads and cam that probably doesn't get into its happy zone until higher rpm, we wouldn't be surprised if it wants fuel enrichment around 8.5"hg or even 10.5"Hg.
So if the engine needs power enrichment at 8.5", then the extra timing for low density makes no sense to me. I could understand if there was a little bit of vacuum advance being used at the begining of power valve opening. But full vacuum advance at the power valve opening or rod step up, is clearly mismatch.

DrCharles said:
What I'm thinking of is one light spring and one heavy spring with a loop. So I can crank it at about 12 degrees, and have it immediately advance once running to the start of the heavy spring, 26 or 28 degrees maybe?
Click to expand...
That's basically what Direct Connection was doing with the tach drive race distributors.
They didn't care if the idle rpms were a little unstable.

Easier to deal with on a 4 speed even on the street. With an automatic when its shifted into gear at idle, it loses some rpms. Right? When losing rpms causes a timing change, it then loses more rpms, until it either stablizes or dies.
 
I do have a 4-speed, but still interested in a stable idle ;)
Your vacuum advance analysis makes sense... but remember my engine only idles at 8" (at best).
Cruise is about 14-15" @ 3000 rpm. A light push on the throttle drops the vac way down... and of course under heavier load there's little if any vacuum, so no advance either. The can could be limited to only add a maximum of 10 degrees or so, too.

A couple of Mopartsians have said they're running one light spring (only) so all-in once the engine starts. Both say they didn't have a problem on the street. I'm inclined to try a compromise as noted (and quoted) above.
Why would the idle be a little unstable with that setup?
 
Because it will be very rpm sensitive. Unless - maybe - the weight will be pulling that heavy spring snuggly at idle.
I'm sure you have a loping idle. So as the idle rpm lopes up and down 25 or 50 rpm, the timing will go up and down with it.

I think you've seen this, but maybe not. Its the small block version of the DC tach drive race distributor. The plots in distributor degrees and rpm.
upload_2020-2-13_20-16-5.png


Whenever the engine fall below roughly 800 rpm, the advance drops away like a rock.
upload_2020-2-13_20-22-4.png


So to do what you want, you need a long loop short enough, and a primary spring that's very light with a lot of loops so it doesn't get overstretched at full advance. This is doable. Rotating the spring perches will change the distances a little.
Getting a secondary spring to do that and have an advance that will work with vacuum advance, that may take some more effort and trial and error. There no supply of sorted and organized distributor springs publicly available.
 
Yes, there is quite a lope (probably would just call it "Rough idle" like a lot of cam charts) ;)
That was my plan (for the weight to stay firmly against the loop of the heavy spring). If it ever drops to 800 something is wrong... It does idle best at 1100, maybe as low as 1000 if I can get it stable enough.

Definitely it'd be easiest to just try the very light spring(s) so it's full advance once the engine starts. But it can get quite hot here in southern MO and I don't want to ping the crap out of my pistons while cruising around town. I have a dash-mounted retard unit so I could twiddle if needed, but I really want to make this as simple as possible and not requiring constant attention to knobs like I'm playing a pipe organ :rolleyes:
 
"That cam" IMO, probably could benefit from the old drilled holes in the primary blades trick. If all else fails to help, you might try it.
 
RustyRatRod said:
"That cam" IMO, probably could benefit from the old drilled holes in the primary blades trick. If all else fails to help, you might try it.
Click to expand...


I'm with Rusty in that you may need to leave the ignition alone for a bit and start working on the carb.

I'm 255 at .050 on 340 inches and my junk will idle at 750.

I think you need to spend some time cleaning up the idle circuit.

Did you mention what carb you have?
 
DrCharles said:
Yes, there is quite a lope (probably would just call it "Rough idle" like a lot of cam charts) ;)
That was my plan (for the weight to stay firmly against the loop of the heavy spring). If it ever drops to 800 something is wrong... It does idle best at 1100, maybe as low as 1000 if I can get it stable enough.

Definitely it'd be easiest to just try the very light spring(s) so it's full advance once the engine starts. But it can get quite hot here in southern MO and I don't want to ping the crap out of my pistons while cruising around town. I have a dash-mounted retard unit so I could twiddle if needed, but I really want to make this as simple as possible and not requiring constant attention to knobs like I'm playing a pipe organ :rolleyes:
Click to expand...
I drove with that race distributor on the street. It's not going to ping driving around town. If it detonates at WOT, then it would be trouble.
Detonation at light to moderate load will happen when trying to use vacuum advance on top a fast in advance curve like this.

The lightest springs I know of specificaly for Chrysler distributors are these:
IMG_0774.jpg


I've not put one in the distributor, but by the dimensions they calculate to be even lower spring rate than the primary spring that comes in the race distributor.
race distributor advance springs

So one of those should get a real quick advance, then find a second spring that engages at 9 or 10 degrees advance. Sketching on the graph above for convenience. Initial is 13* and second spring is getting some tension around 22*, 500 rpm.
upload_2020-2-13_23-27-50.png

(this is just a rough sketch of roughly what can be done)


Just for comparison with a high performance setup with vacuum advance, lets look at the non-emissions A134 timing. For typical conditions, the timing at 1400 rpm would at most be 26* - with the pedal to floor!
At light to moderate throttle, it would be 26* plus vacuum advance, is 15* reasonable? At 41* that is far more advance than the race distributor. Perfectly safe because the load is light, and lighting it early should put more heat into useful energy instead of warming the cylinder walls and exhaust valve.
upload_2020-2-13_23-45-27.png


Lets look at the critical highway cruising range around 2600 to 2800 rpm, and figure factory 440 pulled at least 16"Hg when cruising.
29° initial plus centrifugal, plus as much as 24° from the vacuum advance. 53° BTDC

Now lets see what Direct Connection recommended as a starting point for vacuum plus mechanical.
Pretty close, 56°. Considering the factory tech had the leeway to advance or retard the initial a couple degrees as needed/wanted based on the local fuel and the driver preference. That could bring the factory setup to 55°
Also notice the MP distributor has a curve that's in and done a little quicker. I haven't tested a B/RB version to verify that, but it was true on the small block version.
upload_2020-2-14_0-0-37.png
 
Last edited:
yellow rose said:
I'm with Rusty in that you may need to leave the ignition alone for a bit and start working on the carb.

I'm 255 at .050 on 340 inches and my junk will idle at 750.

I think you need to spend some time cleaning up the idle circuit.

Did you mention what carb you have?
Click to expand...

I don't know of anyone running the Mini-Express that would idle anywhere near that low in a 440 or 451. Mine had a few degrees taken out during the regrind (original was 278, now 272). 107 centerline, in at 105.

Working on the carb too, although only one thing at a time ;) It's an old #4780 (800 DP). Eventually I'll get tired of screwing with it and get something more modern and adjustable!

Already have 3/32" holes in all four blades, the only way I could get enough air in and keep the transfer slots square on both the primary and secondary. If I could run more than 22 initial timing I could probably reduce or eliminate some of the holes - but I still need to keep the t-slots close to square.

Incidentally I know from watching the wideband gauge that the PV is opening way too late (I'd originally followed the common and stupidly wrong advice of half the idle vac). I just picked up an 8.5 and 9.5 at O'Reilly so will be putting one of those in for my next test run.

It's been said that 90% of carburetor problems are in the ignition. Or was that the other way around ;)
 
Mattax, thanks for all the timing info and data. I'll need to chew on that for a while...
I could (when getting all the timing in really quick, if not actually at idle) screw down the adjustment in the vac can so it's basically locked out, then start putting a little back in until ping starts to return...
It's also possible to limit the amount of advance by modifying the stops or the arm, too, 15 may be too much for a fast or locked-out curve as you mention, but even 10 or less would help with part-throttle efficiency.
 
My last grind had a long-duration like that and it was always a bear on the street... It sounded cool as hell but realistically until I had it reground into something a little more Street friendly it was just a pain in the ***... No it still sounds plenty radical and I've lost maybe three tenths or less in the quarter mile and driveability is now great. I can go around the corner and second gear and pull right up from 1500 RPMs no problem.. sits there and idle just fine...
You're likely never going to find a comfortable with that duration...
 
A comfortable... what?
It does sound very manly through the dual 3" with X-pipes :D
I'm aiming for the high 10's (without the bottle) once I get everything sorted, so I did expect to give up some street manners with a cam big enough to make the required hp from a 451...
 
DrCharles said:
A comfortable... what?
It does sound very manly through the dual 3" with X-pipes :D
I'm aiming for the high 10's (without the bottle) once I get everything sorted, so I did expect to give up some street manners with a cam big enough to make the required hp from a 451...
Click to expand...
Manly..lol...
 
With your 10 second goal have you thought of getting and aftermarket distributor and ignition?..
 
DrCharles said:
I don't know of anyone running the Mini-Express that would idle anywhere near that low in a 440 or 451. Mine had a few degrees taken out during the regrind (original was 278, now 272). 107 centerline, in at 105.

Working on the carb too, although only one thing at a time ;) It's an old #4780 (800 DP). Eventually I'll get tired of screwing with it and get something more modern and adjustable!

Already have 3/32" holes in all four blades, the only way I could get enough air in and keep the transfer slots square on both the primary and secondary. If I could run more than 22 initial timing I could probably reduce or eliminate some of the holes - but I still need to keep the t-slots close to square.

Incidentally I know from watching the wideband gauge that the PV is opening way too late (I'd originally followed the common and stupidly wrong advice of half the idle vac). I just picked up an 8.5 and 9.5 at O'Reilly so will be putting one of those in for my next test run.

It's been said that 90% of carburetor problems are in the ignition. Or was that the other way around ;)
Click to expand...


Ok, now we can talk about your carb. Nothing wrong with it, except it can be a bit tedious to adjust.

First things first, if you have a pin gauge set, pin gauge every hole in that thing. Write it down.

Emulsion of ~.028 is a great starting place, but I can't remember what the 4780 had. Also, somewhere around .031 on the idle feed restriction is a good start. You can drill all that stuff out and get yourself some 6/32, 8/32 and 10/32 brass set screws 3/16 long and some very small bits and a pin vise. You can drill out all those press in brass bits and use the set screws to make easy changes.

Also, you need to measure the T slot holes, both in the main body where the metering plate goes, and where the hole in the main body meets the base plate. I'm thinking you'll need to put restricters in there.

Once you clean up the idle and transition circuits, you can go to work on your power valve circuit along with the main jets. You can make the power valve channel restricters adjustable too. Set your cruise tune with the main jet and use the power valve to adjust WOT.

Once you get that, you may find your ignition timing requirements have changed.
 
DrCharles said:
I do have a 4-speed, but still interested in a stable idle ;)
Your vacuum advance analysis makes sense... but remember my engine only idles at 8" (at best).
Cruise is about 14-15" @ 3000 rpm. A light push on the throttle drops the vac way down... and of course under heavier load there's little if any vacuum, so no advance either. The can could be limited to only add a maximum of 10 degrees or so, too.

A couple of Mopartsians have said they're running one light spring (only) so all-in once the engine starts. Both say they didn't have a problem on the street. I'm inclined to try a compromise as noted (and quoted) above.
Why would the idle be a little unstable with that setup?
Click to expand...
Sounds like the starter getting a little bucky is what is causing your spring situation to be overly complex. Couldn't you get an ignition box with a start retard built into it? Then you could eliminate using the light spring which in my experience can lead to an unstable idle.
 
DrCharles said:
Mattax, thanks for all the timing info and data. I'll need to chew on that for a while...
I could (when getting all the timing in really quick, if not actually at idle) screw down the adjustment in the vac can so it's basically locked out, then start putting a little back in until ping starts to return...
It's also possible to limit the amount of advance by modifying the stops or the arm, too, 15 may be too much for a fast or locked-out curve as you mention, but even 10 or less would help with part-throttle efficiency.
Click to expand...
Take your time chewing that over. I got to get some stuff done today!
It's not the 15* vac advance. That plus 36 is still just 51*. At highway cruise that's fine. The problem is when it comes in. Sure you may be able to delay it so it doesn't contribute until over 12 or 14", but may or may not get full benefit.
Try to think what happens in the combustion and then work back to the timing needs.

Since you have a dragstrip goal, you're definately going to want to test upper rpm timing affects on the mph and e.t.
I'm pretty sure David in St Croix has a similar stroker, but don't recall the cam. Might want to see how he's made out with his timing curve changes. There was a little problem with the drag strip on the island after the hurricanes... he's on moparts and also RFS forums...

DrCharles said:
Working on the carb too, although only one thing at a time ;) It's an old #4780 (800 DP). Eventually I'll get tired of screwing with it and get something more modern and adjustable!

Already have 3/32" holes in all four blades, the only way I could get enough air in and keep the transfer slots square on both the primary and secondary. If I could run more than 22 initial timing I could probably reduce or eliminate some of the holes - but I still need to keep the t-slots close to square.
Click to expand...
I agree YR - make it adjustable.
I'm surprised you have the secondary t-slots square unless it was converted to 4 corner idle, and even then might not want them square depending on how high up they are.
 
I just read through the thread on Moparts which you posted in:
race distributor advance springs

It sounds like I don't just want to lock it out (since I do have an electronic ignition box), since I'd be leaving a lot of power on the table. One super-light spring and one heavy spring with a loop, then.

What do you think about this one for the super-light? McMaster-Carr
Wire .018", ? coils, Coil OD 0.240, Length 0.75", lb/in 0.78
Looks pretty close to the Mopar P2932675...

I have a heavy spring with a loop that I think came from the stock distributor:
Wire .046", 6 coils, Coil OD 0.279", Length 0.985" (measured inside the wire on each end). OK for the heavy one? (of course, I don't know how much retard will come from the Jacobs ignition box).

With those springs, do I need to keep the FBO limiter plate which limits on the outside vs. welding up the inside of the slots? Or just leave the unrestricted (stock Mopar) advance range (around 26-28 degrees I think) as long as the engine will start ok with 6-8 BTDC?

I'm surprised you have the secondary t-slots square unless it was converted to 4 corner idle, and even then might not want them square depending on how high up they are.
Click to expand...

Could you explain in more detail? No 4-corner idle.
 
Last edited:
DrCharles said:
What do you think about this one for the super-light? McMaster-Carr
Wire .018", ? coils, Coil OD 0.240, Length 0.75", lb/in 0.78
Looks pretty close to the Mopar P2932675...

I have a heavy spring with a loop that I think came from the stock distributor:
Wire .046", 6 coils, Coil OD 0.279", Length 0.985" (measured inside the wire on each end). OK for the heavy one? (of course, I don't know how much retard will come from the Jacobs ignition box).
Click to expand...
Seems like worth trying.

DrCharles said:
With those springs, do I need to keep the FBO limiter plate which limits on the outside vs. welding up the inside of the slots? Or just leave the unrestricted (stock Mopar) advance range (around 26-28 degrees I think) as long as the engine will start ok with 6-8 BTDC?
Click to expand...
You may have a better idea when you open it up. With a small pry bar push weights out until you see the heavy spring take up some slack. Is right at the begining, at the near the limit or something in between?

Not sure whether your engine will love being started at 6 or 8 degrees. Might be fine. A lot of the smogged engines had initial timing like that. On the other hand, yours is a a bit more radical than even the most radical factory engine. We can make some guesses on graph paper too.

DrCharles said:
Could you explain in more detail? No 4-corner idle.
Click to expand...
As best I can tell, transfer slot exposure on the secondary side is typically none or minimal. Secondary pump shot covers any delay in supply from the secondary t-slot. I don't know the exact timing of when secondary t-slot comes into play - I assume it is mostly for very high speed cruise or lugging a load when the secondaries a only somewhat open.

Each carb is different and each need is different, but generally the secondary t-slots are shorter and higher up in the throttle plate than on the primary side. So to get the secondary t-slot square requires the secondary throttle plate to be more open than the primary side. This is possibly comtributing to why your idle speed is so high and the engine likes over 22* inititial timing.
 
Thanks, I may try the distributor recurve before digging into the carb. I can get a Mr. Gasket 925b spring at the local O-Reilly so I'll use that for the light one.

I don't recall the slots being grossly different in location, but now that you mention it... I'll eventually be removing the carb from the manifold to install setscrew-type idle air bleeds, so can look closely at the t-slots then. (It's an old 4780 double pumper for someone who's better acquainted with the fine details than I am).

The faster idle is needed because the cam is 312 advertised, 272 @.050, 107 centerline, in at 105, and that along with lots of idle timing is what it wants. Helps with cam/valvetrain oiling too, although that's a feature, not a bug ;) I would be very surprised if anyone could get a stable idle with this cam at 800 or 900 in a carbed 451, especially with the hefty electric fan load kicking on and off... Everyone who ran one of these says it idled best at 1100-1200.

I don't know the "proper" positioning of the secondary t-slots either, so assumed it should be square like the primary. Which still wasn't enough air without adding 3/32 holes in all four blades :rolleyes:

It idles much better (and with the butterflies more closed) at 30 degrees, but then the engine won't start when hot. I am wary of start retard units and want to keep the teeth on my starter and ring gear...

Anyhow, if I get my advance curve set up for, say, 28 at idle with a long slow slope to 36, I will definitely be able to close down the secondaries somewhat (and may even have to solder the drilled holes shut). All this interactive fuel/air/timing is giving me a headache! :realcrazy:
 
DrCharles said:
It idles much better .... at 30 degrees
Click to expand...
But that's out of gear. Not such a big deal with a stick. With an automatic the lack of torque and power at idle becomes apparent when put into drive and the rpms drop like a rock. My only point is borrow that lesson learned from those of us with automagics.

It may be that with that cam and combo, four corner idle would be a help.
I say that because
on the secondary side, the only fuel contribution at idle is the dribble holes - ignoring the situation where the secodnaries are opened to the transfer port.
 
DrCharles said:
I don't recall the slots being grossly different in location, but now that you mention it... I'll eventually be removing the carb from the manifold to install setscrew-type idle air bleeds, so can look closely at the t-slots then. (It's an old 4780 double pumper for someone who's better acquainted with the fine details than I am).
Click to expand...
Quick test if a richer idle may help- stick some wire down the air bleeds. Secure it under the air filter base gasket
 
Mattax said:
But that's out of gear. Not such a big deal with a stick. With an automatic the lack of torque and power at idle becomes apparent when put into drive and the rpms drop like a rock. My only point is borrow that lesson learned from those of us with automagics.

It may be that with that cam and combo, four corner idle would be a help.
I say that because on the secondary side, the only fuel contribution at idle is the dribble holes - ignoring the situation where the secodnaries are opened to the transfer port.
Click to expand...

Fortunately I do have a stick :) I much prefer to decide exactly when the clutch is going to engage and how quickly, not to mention shift when I want it to...
If I did have an automatic, the converter would be quite a high stall to go with this cam anyway, and load the engine less at idle. With a stock converter it'd be a pig.

Does the increased advance at idle reduce the torque (at idle) significantly? If it's cleaning it up and probably creating more vacuum, isn't that an indication that it's running more efficiently?

I agree that 4-corner idle would be a big help, but by the time I finish modifying (or replacing) the metering blocks and the main body, I might as well buy the carb I should have in the first place! :D But this one cost me something like $60, not $600, so I'm willing to experiment and learn a bit.

Quick test if a richer idle may help- stick some wire down the air bleeds. Secure it under the air filter base gasket
Click to expand...
Thanks. David Vizard likes a toothpick in one of the two bleeds :D
I do have a wideband - shows about 13.5-13.8 at idle currently. Not sure how reliable it is at idle with lots of overlap, since it's only measuring the oxygen in the exhaust.
 
You must log in or register to reply here.
-

Back
Top