Dizzy help

[3406pk]

I have 72 Challenger 340 4spd with original std electronic distributor . It has FBO ecm box & coil. The engine has been rebuilt w hughes 220/224 hyd flat tappet and 9.5 cr. I broke in cam and found sweet spot for idle n timing. Idle 900 rpm and 16* btdc, 16 in-hg. I haven't got on road for full advance but want 34* limit. This will be cruiser only. I got a FBO advance limit plate to install, but have question now that dist is disassembled. Does limit plate go below slotted original? Also the weights had only one spring, a yellow. Does it need another and what color? Thanks for help.

 

[Mattax]

If its for cruising on the highway or idling around town then you need the springs strong enough to slow the advance around 1600-2200 rpm. This way you can use the vacuum advance effectively. The factory method was to use a second spring that began to engage in that rpm range. Those secondary are identified by a long loop on one end.
You may have to compromise and just go with steady mechanical advance (instead of a two stage curve). Use a pair of springs that keeps the mechanical timing so it doesn't hit your 34* until 3000 rpm or higher. Experiment from there. The problem won't be at WOT. The problem will be light detonation at part throttle moderate load.
One series of experiements will be to slow the vacuum advance down. There other will be to play with the springs.
If not using vacuum advance, then super quick advance is OK.

If you find the mechanical advance starts too soon or too late, add or reduce tension by turning the cammed spring perch.
This is almost always neccesary when changing spring rates lengths.

 

[Mattax]

'72 340 Distributor have a solenoid on it?

 

[3406pk]

'72 340 Distributor have a solenoid on it?

No. Vacuum adv only

 

[Mattax]

No. Vacuum adv only

Then its probably not original. For what you're doing, that makes things easier and don't have to deal with a rare part.

 

[Mattax]

Here's some graphics using the specs from some factory service manuals.
Notice how quick the timing advances in the first stage. That's controlled by the light spring.
The spring with the long loop slows the advance above 2000 rpm.

This is true with the pre-emissions advance curves as well.
The biggest difference is the pre-CAP timing curves used more initial timing, and shorter loop on the secondary spring.
(This is for a stronger idle with slightly richer idle mix but also more CO. This is closer to what you want.)

 

[3406pk]

Then its probably not original. For what you're doing, that makes things easier and don't have to deal with a rare part.

It is original. Part number matches spec for 72 challenger 340. Thank you for all the info and charts. I will get spring assortment to find one that works.

 

[Mattax]

It is original. Part number matches spec for 72 challenger 340. Thank you for all the info and charts. I will get spring assortment to find one that works.

In that case, the vacuum advance should have an electrical connection. @halifaxhops may know more about the details.
Here's what the factory was telling their techs late '71 - early '72
1972 Chrysler Engine Performance Facts & Fixes Service Book (Session 291 page 11
1972 Chrysler Ignition System Service Book (Session 292) page 13

I don't have a '72 FSM but since the original springs are missing, not sure it matters.

It's not likely the factory secondary spring would engage early enough to work with the FBO plate, but if you have one, try it.
Here's what I think you can do with two equal springs.
The dashed line will have no problems working with a vac advance. Might even use a little more initial.
The dotted line will definately be a problem with the vac advance.
Something in between you may be able to make work by fiddling with the vacuum advance. There's a "How To adjust vacuum advance" in the How to section of the forum.
But if you do have a solenoid type vac advance advance on there, might want to swap it out for an regular one.

 

[3406pk]

That ref must be for Chrysler or Imperial. All I have is vacuum advance can w no electrical. It has 8.5 on mechanism. So if max is 34* on mech advance, then 8.5* vac would be 42.5* total. Should be good for cruise. With the 9.5 cr and 91 premium, should be safe from detonation. Thanks again.

 

[RustyRatRod]

That ref must be for Chrysler or Imperial. All I have is vacuum advance can w no electrical. It has 8.5 on mechanism. So if max is 34* on mech advance, then 8.5* vac would be 42.5* total. Should be good for cruise. With the 9.5 cr and 91 premium, should be safe from detonation. Thanks again.

You do not include vacuum advance when figuring for total advance.

 

[Mattax]

Also Chrysler used distributor degrees and rpm.

 

[yellow rose]

That ref must be for Chrysler or Imperial. All I have is vacuum advance can w no electrical. It has 8.5 on mechanism. So if max is 34* on mech advance, then 8.5* vac would be 42.5* total. Should be good for cruise. With the 9.5 cr and 91 premium, should be safe from detonation. Thanks again.

Like mattax said, Chrylser used distributor degrees so 8.5 is 16 on the crank.

That means if you have 34 total at full vacuum you'll have another 16 degrees.

How do you know 34 is what it will want?

 

[3406pk]

How do you know 34 is what it will want

Well 34* is good starting point since 90% of sb engine dyno reports say 34* to 36* give best hp. Since I can't get car on road til spring, I need starting point that is safe to set advance.

 

[yellow rose]

Well 34* is good starting point since 90% of sb engine dyno reports say 34* to 36* give best hp. Since I can't get car on road til spring, I need starting point that is safe to set advance.

Ok. I agree with that, but how do you know your timing light is telling you the correct number.

I've personally seen timing lights that are 3-4 degrees fast, and some that are 3-4 degrees slow. That's a big spread.

I never get excited about what the final number is. I don't care if your timing light says you have 30 degrees as long as the plug says that's all it will take.

Same with the other way. Of your timing light says 40 degrees total is what the plug says you need, that's where you time it.

Timing lights are a tool and they almost never read exactly alike. That's why I tell everyone to use the same timing light, every time.

 

[rumblefish360]

Do you have a suggestion to what is a good timing light?

Ok. I agree with that, but how do you know your timing light is telling you the correct number.

I've personally seen timing lights that are 3-4 degrees fast, and some that are 3-4 degrees slow. That's a big spread.

I never get excited about what the final number is. I don't care if your timing light says you have 30 degrees as long as the plug says that's all it will take.

Same with the other way. Of your timing light says 40 degrees total is what the plug says you need, that's where you time it.

Timing lights are a tool and they almost never read exactly alike. That's why I tell everyone to use the same timing light, every time.

 

[67Dart273]

Well 34* is good starting point since 90% of sb engine dyno reports say 34* to 36* give best hp. Since I can't get car on road til spring, I need starting point that is safe to set advance.

Yeh, but that is not the ADVANCE CURVE, that is initial timing PLUS the advance curve. A good starting point for performance Mopar V8s is about 18-20 in the mechanical advance that is in the mechanical curve and that is CRANK degrees. As much as 15 intitial then gives you the about 35 total. The vacuum advance is "on top" of that

So a few things:

Timing curves in the shop manuals ARE NOT crank degrees so a late model SMOG dist curve is VERY long and VERY slow and way too long and slow for a performance engine. The numbers sometimes stamped on the advance mechanism is dist degrees, not crank.

In general you want to shorten that curve way up from factory, speed it up with weaker springs and bump the intial way up

 

[Mattax]

Yeh, but that is not the ADVANCE CURVE, that is initial timing PLUS the advance curve. A good starting point for performance Mopar V8s is about 18-20 in the mechanical advance that is in the mechanical curve and that is CRANK degrees. As much as 15 intitial then gives you the about 35 total. The vacuum advance is "on top" of that

So a few things:

Timing curves in the shop manuals ARE NOT crank degrees so a late model SMOG dist curve is VERY long and VERY slow and way too long and slow for a performance engine. The numbers sometimes stamped on the advance mechanism is dist degrees, not crank.

In general you want to shorten that curve way up from factory, speed it up with weaker springs and bump the intial way up

You got my posts on ignore Del?
3406pk sez he has the original '72 distributor in it.
That has a long and fast curve. See the spec graphs I posted above. They are not plotted on the same scale so its not as obvious at first glance.

Well 34* is good starting point since 90% of sb engine dyno reports say 34* to 36* give best hp. Since I can't get car on road til spring, I need starting point that is safe to set advance.

If you're going straight to the track in the spring, then yes, do what you gotta do. In which case, don't use the vacuum advance, and use a relatively quick mechanical advance.

16" Hg at idle is pretty good. Might even be able to idle it at 750-800 rpm once its fully warmed up.
Doing that at 16 BTC indicateds a relatively efficient combustion. Just a little worse than stock. And its going to get more efficient as the rpms climb. Its not going to need alot of lead time.

The reason timing varies with rpm and load is to get this:

from: The New Distributor (Session 136) from the Master Technician's Service Conference

A sporty engine combo gains combustion efficiency at higher rpms - thats why the secondary advance rate is slower on the factory hi-performance engines.

 

[yellow rose]

Do you have a suggestion to what is a good timing light?

Not really. The digital dial back lights have been the most off I've seen.

And it really doesn't matter. I had a customer with a dial back and I made sure he brought his timing light when we went to the dyno.

His dial back was 5 or 6 degrees slow on timing compared to my non dial back and 3 or 4 degrees compared to the shop dial back. I wanted to make sure he could see the difference.

As long as you use the same light, every time and let the plug tell you when it's happy for initial and total then it doesn't really matter.

When you start using one light on the dyno and another at home and then borrow one at the track...then you start going sideways, backwards and bonkers.

My friend Tuner has a classic story of years and years ago running into the same situation when the the shop had two different lights and the customer had his own light.

That guy should write a book.

 

[halifaxhops]

You cant go wrong with a timing tape ever!