Timing curve for my big-cam 451?

-

DrCharles

Well-Known Member
Joined
Jun 7, 2012
Messages
1,463
Reaction score
1,367
Location
West Plains, Mo.
I think my timing curve isn't quite optimal, but I'd like some recommendations. My combo: 451, 10.3:1 hand-ported iron heads, QF with BLP baseplate 950 double-pumper with 1" open spacer on Edelbrock RPM, .652/272@.050 cam, premium pump gas. 4-speed (2.66 low), 3.91 rear (3000 rpm @ 60 mph).

This cam idles at 1100-1200 rpm, 8" vacuum. I have only manifold vacuum (this carb doesn't have ported), but the vac advance can, with adjustment fully clockwise, doesn't start pulling in at idle. It is all-in at 15" which is my cruise vacuum.

Centrifugal advance is somewhat McGyvered as follows. All figures in crank degrees. I have one superlight spring, and one OEM heavy spring with loop. With the current initial timing, it cranks at 16, immediately goes to 27 at idle (the start of the heavy loop), starts to advance about 1400, and up to 35 deg at 4000 rpm. I am reluctant to get that close to the engine with a timing light at higher revs! The slots of the rotor tower aka cam plate limits the advance to 22 over initial, which is 38 max, less the inherent delay in the electronic ignition and retard unit, so should be 35 or so at high RPMs too. So basically my advance is all-in by 4000. I would like to try all-in by 3000.

Before you ask - the reason for that configuration is that 22 is the maximum initial I can run before it begins to fight the starter when hot. The idle is much better at 27 and could be even higher, but since I street drive mostly, I am wary of too much advance (such as all-in at idle, effectively locked).

Some would say to use manifold vacuum (which I have to use as noted above), and I could get a more sensitive can from FBO that would pull-in at idle, providing 35-40 deg total idle advance. But that also might be too much advance at lower rpm, part throttle operation.

The simplest solution would be a loop spring of identical dimensions except with less tension, so the advance would come in more quickly until it is limited by the slots. But those don't seem to be common. Is it possible to modify one end of the springs in the advance kits so that it has a loop? Or does the spring wire just break?

Or do I have too much low-rpm advance already? My brain hurts. It runs well and doesn't ping, and WOT operation feels good on the butt dyno, but I would like to improve the part-throttle performance below 4000 rpm as much as possible. Thanks for any assistance! @Mattax ? :D
 
Do you have a cranking pressure figure?
 
Do you have a cranking pressure figure?
I can't recall offhand, but with the 98 degrees (seat-to-seat) overlap of this cam, it was remarkably low - something like 120 psi! :eek: Edit: Yes, it's degreed in, 107 installed at 105.

It's thoroughly broken in now with 700 miles, so I could check it again this weekend but it's likely to not be significantly higher...
 
I can't recall offhand, but with the 98 degrees (seat-to-seat) overlap of this cam, it was remarkably low - something like 120 psi! :eek: Edit: Yes, it's degreed in, 107 installed at 105.

It's thoroughly broken in now with 700 miles, so I could check it again this weekend but it's likely to not be significantly higher...
Wow. That seems like a pretty low amount of cam advance. So it's a 107 LAS? Damn if I wouldn'ta tried it down around 100.
 
Well, I didn't want to deviate too far from the design parameters. I can always change the installed centerline later. The Direct Connection catalog listed the powerband as 3400-7000, presumably for a 440, so a hair lower for my 451.

Any thoughts on the timing? :poke:
 
Well, I didn't want to deviate too far from the design parameters. I can always change the installed centerline later. The Direct Connection catalog listed the powerband as 3400-7000, presumably for a 440, so a hair lower for my 451.

Any thoughts on the timing? :poke:
You already did that because your static compression is too low for that camshaft. That's deviation, right there and calls for deviation elsewhere to get it "right". 272@.050 needs every bit of 12:1 to run to its full potential, so it needs more camshaft advance to help get cylinder pressure up. The 2* advance they give on the cam card takes for granted that you have the "correct" static compression to run that cam. You do not, so you have to "make up" for it elsewhere.
 
Oh and my "thoughts" on timing is, no amount of timing is going to make a big difference....ignition timing that is. Now camshaft timing is another thing altogether.
 
I think my timing curve isn't quite optimal, but I'd like some recommendations. My combo: 451, 10.3:1 hand-ported iron heads, QF with BLP baseplate 950 double-pumper with 1" open spacer on Edelbrock RPM, .652/272@.050 cam, premium pump gas. 4-speed (2.66 low), 3.91 rear (3000 rpm @ 60 mph).

This cam idles at 1100-1200 rpm, 8" vacuum. I have only manifold vacuum (this carb doesn't have ported), but the vac advance can, with adjustment fully clockwise, doesn't start pulling in at idle. It is all-in at 15" which is my cruise vacuum.

Centrifugal advance is somewhat McGyvered as follows. All figures in crank degrees. I have one superlight spring, and one OEM heavy spring with loop. With the current initial timing, it cranks at 16, immediately goes to 27 at idle (the start of the heavy loop), starts to advance about 1400, and up to 35 deg at 4000 rpm. I am reluctant to get that close to the engine with a timing light at higher revs! The slots of the rotor tower aka cam plate limits the advance to 22 over initial, which is 38 max, less the inherent delay in the electronic ignition and retard unit, so should be 35 or so at high RPMs too. So basically my advance is all-in by 4000. I would like to try all-in by 3000.

Before you ask - the reason for that configuration is that 22 is the maximum initial I can run before it begins to fight the starter when hot. The idle is much better at 27 and could be even higher, but since I street drive mostly, I am wary of too much advance (such as all-in at idle, effectively locked).

Some would say to use manifold vacuum (which I have to use as noted above), and I could get a more sensitive can from FBO that would pull-in at idle, providing 35-40 deg total idle advance. But that also might be too much advance at lower rpm, part throttle operation.

The simplest solution would be a loop spring of identical dimensions except with less tension, so the advance would come in more quickly until it is limited by the slots. But those don't seem to be common. Is it possible to modify one end of the springs in the advance kits so that it has a loop? Or does the spring wire just break?

Or do I have too much low-rpm advance already? My brain hurts. It runs well and doesn't ping, and WOT operation feels good on the butt dyno, but I would like to improve the part-throttle performance below 4000 rpm as much as possible. Thanks for any assistance! @Mattax ? :D


You need to shorten up the curve so it doesn’t pull any timing when it starts. It needs to start on initial and not start adding timing until 500-600 rpm above idle speed. You need to use heavy enough springs to keep the timing from moving around at idle. It should add timing about 1 to maybe 2 degrees per 1k rpm.
 
You need to shorten up the curve so it doesn’t pull any timing when it starts. It needs to start on initial and not start adding timing until 500-600 rpm above idle speed. You need to use heavy enough springs to keep the timing from moving around at idle. It should add timing about 1 to maybe 2 degrees per 1k rpm.
Thanks for actually addressing my question, Rat ;)

Yes, I'm aware of the unstable idle problem that would result from advance starting to come in at idle speed. That's another reason I used the heavy OEM spring.

As I explained, I can't start on initial over 22 without kickback and I do not want to have a separate ignition toggle or a start retard box.... and 22 is not enough idle timing for that cam. It idles much smoother and cleaner at 27... 30 would be even better.

So timing is 27 at 1200 and 35 at 4000. That is +8 degrees in +2800 RPM. You are suggesting I actually run less advance than that? Then I'd need more idle timing around 30-32 and still end up at 35@4000? Not too much for street use?
 
When I first got my dart, it had two Mr Gasket springs in it. It would crank at the initial timing and then run at max advance at idle.

You could probably get away with the same thing on your setup.

Otherwise, I'd try to pull in more with the vac advance at idle and then still lighten the springs. However, that can still cause an erratic idle, especially since your vac signal is likely to be messy at idle with that big cam.

Another option would be to use a programable ignition that you can custom map. But that's not real cheap either.

I'd try option one above. Between the low cranking compression and the small amount of advance, having it effectively locked out with mechanical timing retard would probably work well.
 
Dr C,
Congrats on using MVA. Very smart & you are also smart enough to realise that more than the current 27* of idle timing will probably be beneficial.
Not quite following where the 27* idle comes from because you say the VA doesn't pull in at idle? Does the extra timing come from the centri curve starting early?
I am surprised the VA does not pull in with 8" of vac. Admittedly have not worked with many adj Chrys VA units but those on GM HEIs will generally work with 5-6" of vac.
 
I'd like to see the timing in earlier as well. Personally on a street driven car I'd like to see all of the timing in at cruise RPM, in your case, 3,000RPM.

27 initial, 35 total seems ballpark.

YOu may need a better starter, a better battery or a start retard to make your dreams come true here.

I run 26/38 in at 2500 with no start retard and a monster 1,450CCA battery.
 
Dr C,
Not quite following where the 27* idle comes from because you say the VA doesn't pull in at idle? Does the extra timing come from the centri curve starting early?
Only the single superlight spring is holding the weights back initially. So, at a few hundred rpm, the weights advance until the loop on the heavy spring engages, which prevents further advance until about 1400 rpm which is above my idle speed. :) That loop happens to be 11 degrees, which is just right for easy starting and good idle timing!

I am surprised the VA does not pull in with 8" of vac. Admittedly have not worked with many adj Chrys VA units but those on GM HEIs will generally work with 5-6" of vac.

I don't know which Mopar distributor I started with... maybe it's some emissions thing that prevents early advancing. It did have a LOT of centrifugal advance. I'm sure the spring adjustment is all the way clockwise. I may experiment with another can, or just pay FBO the $69 (I thought they actually cut them open and install a lighter spring... but maybe not!)
 
Last edited:
I believe there's a # on the vac can arm which relates to what vacuum will pull it in.I can't find the article I read . I'm not an expert but you may want to see what # you have and go with the lowest # available.Some thing you may not have considered. Also (I think you may have already done this) Turning to screw CW in the advance all the way in should make it react at 5 inches of vacuum.
 
Thanks for actually addressing my question, Rat ;)

Yes, I'm aware of the unstable idle problem that would result from advance starting to come in at idle speed. That's another reason I used the heavy OEM spring.

As I explained, I can't start on initial over 22 without kickback and I do not want to have a separate ignition toggle or a start retard box.... and 22 is not enough idle timing for that cam. It idles much smoother and cleaner at 27... 30 would be even better.

So timing is 27 at 1200 and 35 at 4000. That is +8 degrees in +2800 RPM. You are suggesting I actually run less advance than that? Then I'd need more idle timing around 30-32 and still end up at 35@4000? Not too much for street use?

IMO, when you have one (or worse both) light springs as soon as you hit the starter it adds timing. That first jolt of the starter can make the weight move with a light spring. It may start with 26-27 initial if you get rid of the light spring. Also, I forgot to say that he slowing the curve down ask suggested I would bring in the VA sooner so you are getting some additional timing before full advance. I’m not a proponent of of using the VA to add timing at idle, so you can delay it I adds timing after that.
 
I believe there's a # on the vac can arm which relates to what vacuum will pull it in.I can't find the article I read . I'm not an expert but you may want to see what # you have and go with the lowest # available.Some thing you may not have considered. Also (I think you may have already done this) Turning to screw CW in the advance all the way in should make it react at 5 inches of vacuum.

The number on the arm is the max advance in distributor degrees, i.e. half the crank degrees. So a "9L" can will advance 18 degrees when fully in. At least for Mopar cans.

I've heard (from other reputable sources) that screwing in the adjustment to the stop should also start it moving at 5"... but this one definitely does not! Maybe it's defective, or maybe they don't all work that way? Anyhow a new stock can is $25-30... I'll talk to FBO and get the specs on theirs before shelling out $69 plus shipping.
 
I think the last part of that sentence is a little typo'ed?
So why don't you like VA extra timing at idle? :popcorn:


Yeah, I can’t think of what I was thinking when I posted that last part of the last sentence. I’m stumped now. I don’t like using MV to add timing at idle because as soon as you touch the throttle it’s gone. As you roll into the throttle you start giving back timing. I think you said you have delayed the start of the VA, and I think I was getting at sorting out the curve a bit more and starting the VA a bit sooner, even using MV. I have no idea where I was going with that last sentence in the earlier post.
 
But you can also look at it this way... the idle, no-load timing would be excessive when the engine is loaded at launch. So you want the timing to drop back to centrifugal-only as the load comes on :)
 
But you can also look at it this way... the idle, no-load timing would be excessive when the engine is loaded at launch. So you want the timing to drop back to centrifugal-only as the load comes on :)


That’s true...mostly. If the timing drops back too far it will be retarded for the load and make a bunch of heat. That’s why I try and make the curve delay a bit after initial timing is where I want it. Then you add in the VA timing while the mechanical advance isn’t adding any timing. Sometimes, depending on compression ratio, gearing, converter and such I try and delay the start of mechanical advance even without using an vacuum advance. Low throttle opening and transitory loads don’t always want more advance. If you give it more than it wants, you can get a tip in rattle.
 
Lock the distributor @ 38° and run separate ignition / crank switch. Crank it over then hit the ignition switch.
 
Finally had a chance to work on it... very strange findings.
I had another vac can (9L, or 18 degrees) and tested that with a Mityvac hand pump. Sure enough, with the adjustment full CW, it barely moved at 5", about 10 degrees at 8", and full 18 deg at 12". :)

So I removed the existing can (8L) and installed the 9L. There is a little slack in the reluctor plate and it doesn't actually start to move the plate until 6-7", all in by 12".

Started the engine and got it warmed up... noticed the idle speed was a bit faster. My timing light showed it idling at 39 deg now, rev in neutral up to 50 deg. I readjusted the idle speed slightly and disconnect the vac hose, idling at 28.

With the hose still disconnected, I gradually increased the rpm and it only got to about 33 and then began to retard! :eek: A quick fast rev and it dropped to 31... on decel came back up to 33 and then down to 28 at idle. I expected the high rpm retard from the electronics, but what happened to the 35 at 4000... ?

Even more strangely - I tested the 8L can I'd removed and it also started to move at 5" and was all-in by 11". WTF. There has to be something binding or moved in the mechanical parts... Time to take the dist apart and clean/lube. Maybe reconfigure to an initial of 22-24 with two medium or heavy springs as suggested, as long as it doesn't start advancing at my 1200 rpm idle. Definitely will need the vac can to keep the idle timing up then.

Would be nice to have a distributor machine in my shop about now :rolleyes:
 
It seems you are doing this in the wrong order. Forget about the centri curve in the dist. Get the idle timing sorted out FIRST. I have a post in the Ign system about how to do that.
Once you have the idle timing sorted, you will know what your idle rpm will be. It may be lower with more idle timing because the engine is making more hp from the extra timing. If I read it right, your idle was faster with 39* at idle. Exactly what I mean. Because of the extra hp, engine may be happy to idle at a lower rpm. Whatever the final idle rpm is, you need to sort that first. Once you do this, you can look at the centri curve.
 
I have done plenty of experimenting on the idle already. The idle speed has to be 1100-1200 rpm since the cam has so much overlap, and a 950 cfm carb - because the vacuum signal is low. Whenever the rpm drops below that, it slowly "spirals in" with lower vac, lower rpm, lower vac... until it dies. 1200 is stable so that is where it needs to be. Adding timing (and tweaking the mixture screws) makes a little more vacuum and less chop, but 8.5" is about it.

So that takes us back to the advance curve. It doesn't ping even at the currently measured 28 deg centrifugal, off-idle. The question is, how fast should it climb to 36 deg? 2500, 3000, or what?
 
-
Back
Top