lets revisit timing

-
exactly!
I welded the slots to .4 to get about 20 mechanical and set my initial to 13 for a total of 34.I was getting a trace of detonation occasionally so i backed down to 11 initial and a total of 32.
The dart pulls hard and runs cool and i was just wondering if i was leaving anything on the table.
 
For what its worth, here's little excercise I did using Dynomation 5 some time ago.
One green line is what the program calls the 'ideal' timing for a 340 with a cam that has roughly 223/230 duration .505 lift.
The other green line I got by trial and error reducing the timing every 500 rpm until the power was about 5% less.
Red lines are some real life timing curves that the real engine pinged mid rpm under some conditions.
upload_2021-9-9_18-23-6.png


Dynomation 5 (and its predescessors like Desktop dyno) is at least partially based on hundred if not thousands of dyno runs.
How much weight one should put on the timing results is certainly debatable. In general they follow the Chrysler curves. However I played around with the program enough to see if changing the rod-stroke ratios effected power and timing and wasn't seeing any difference. So that suggests to me that the program does not take into account all factors which would bear on valve events and ignition timing.

Just for fun* maybe later tonight I'll unlock the program and see what it spits out for 'idea'l timing of a stock early 340.
*Obviously what I consider fun or interesting may not be the same as some others.
beer-gif.gif
 
Last edited:
ir3333 said:
exactly!
I welded the slots to .4 to get about 20 mechanical and set my initial to 13 for a total of 34.I was getting a trace of detonation occasionally so i backed down to 11 initial and a total of 32.
The dart pulls hard and runs cool and i was just wondering if i was leaving anything on the table.
Click to expand...
Turn the spring perch for the long looped spring so it contacts sooner. This will engage the heavy spring sooner and may slow the mid rpm advance enough to let you run 13 at idle. More likely to work if the inner end of the slot was welded than the outer, but either way its something you could try if you wanted.
 
Absolutely...that's what's great about the hobby.My pistons are zero deck and i run a short duration cam that produces strong power from idle up.
I'm convinced a lot of guys would be happier with smaller cams and smaller tires on the street.
 
On high powered turbo stuff (700-1000hp) a change of 6 degrees of timing could be as much as 200hp. In your NA street car with 400hp the difference between 30-36 degrees could be as much as 50hp. Every combination is different. The spark plug will tell you everything you need to know.
 
i'd be surprised if my 340 produces any more than 275 hp...it's built to stock specs with the small 600 cfm carb.
When i was playing around with the carb i thought it was running lean judging by the plugs so i went richer with the metering rods.Eventually got an A/F gauge and had to go back to stock..it was spot on!
 
Even stock 10:1 340s made more than they were advertised at. I think the realistic number was likely about 320. So going from “optimal” timing (call it 34-36 deg) and pulling 6 degrees out of it I am of the opinion that you would notice a difference in power.
 
You should be our test dummy. Set it 34 degrees all in and Go run it. Come back and set it at 28 all in and run it again. It won’t hurt the engine. It’ll just feel sluggish.
 
I've seen 2 early stock rebuilt 340's dyno 276 and i think 281 and thought they were around 9.5:1 stock.
I have read the '71 340's with the big thermoquad dynoed higher at around 300?
Doesn't matter they worked great in a light A body!
 
Mopar Manipulatin’ , HRM August ‘69

blob-png.png


A more recent magazine dyno test of a '70 340 resulted in very similar in stock configuration. I generally don't trust magazine tests but having two tests, decades apart, provide the same results in stock suggests the numbers are probably fairly legit.
 
Those numbers look realistic. Too many guys get stuck on hp numbers. The 340's wide piston, short stroke and good flowing heads make for a quick revving engine which is perfect in a light A body with factory drivetrain.
The problems start with big tires and big cam which challenge a small cube engine...now many changes are needed and you move from the street to a drag strip piece!
 
@Mattax interesting that in the edlebrock dyno testing you linked they had the timing set at 39 at first and 37 after test 10.
 
ir3333 said:
exactly!
I welded the slots to .4 to get about 20 mechanical and set my initial to 13 for a total of 34.I was getting a trace of detonation occasionally so i backed down to 11 initial and a total of 32.
The dart pulls hard and runs cool and i was just wondering if i was leaving anything on the table.
Click to expand...
A cleaner idle..lol
Better gas milage.
More power,earlier, same power up top.
Improved throttle response.
Just lights it earlier aiding in a more complete burn. Less residual when the over lap let's end gas linger or revert and contaminate the intake charge at low speed under the cam range. Improves everything under the cam range.
Every engine combo coupled with tq convertor,weight,gear will want it's own curve and let you know how early it can take it.
My stuff has always ran 24-28 initial timing with 4-6 degrees mechanical all in by 2200

The more radical the cam the more the effect it will have. What it wants or positively responds to...is the ticket.
 
While we're on the subject of limiting timing and welding slots, I wanna throw this related tidbit into the mix. I was where Tony Defeo recommended welding the slots from the inside ends of the slots and not the outside. I don't agree with that and I'll tell you why. If you do that, you're giving the governor "however much" advance shortening those slots from the inside gives. You understand what I'm saying? In other words, you're moving those weights OUT however much you weld the slots up and that's advancing the rotor, so you have that much advance in at initial. I don't like that idea. I guess it would work out the same, I don't know, but I'd rather limit the top end of the advance, not the beginning. Did any of that make sense?
 
Mattax said:
Just for fun* maybe later tonight I'll unlock the program and see what it spits out for 'idea'l timing of a stock early 340.
Click to expand...
Here's the first comparison.
Dynomation's best timing vs. locked at 39 degrees.
For the modelling, the factory 340 cam specs are as close as we could estimate and get Dynomation to accept. (back story here 340 cam specs?)
upload_2021-9-9_22-7-14.png


Thats interesting!

Lets try another comparison.
Dynomation's best versus an advance that is "all in" with 32* at 3000 rpm.
upload_2021-9-9_22-24-33.png

Looks like the advance picked up some power over the locked timing up to 3000 rpm.
On the other hand, as rpm climbed above 3000 rpm the all in of 32* has noticibly less power.

Lets see what happens if the advance is altered so its 30* at 3000 rpm but continues to 36* at 4000 rpm.
upload_2021-9-9_22-44-20.png
 
Last edited:
RustyRatRod said:
While we're on the subject of limiting timing and welding slots, I wanna throw this related tidbit into the mix. I was where Tony Defeo recommended welding the slots from the inside ends of the slots and not the outside. I don't agree with that and I'll tell you why. If you do that, you're giving the governor "however much" advance shortening those slots from the inside gives. You understand what I'm saying? In other words, you're moving those weights OUT however much you weld the slots up and that's advancing the rotor, so you have that much advance in at initial. I don't like that idea. I guess it would work out the same, I don't know, but I'd rather limit the top end of the advance, not the beginning. Did any of that make sense?
Click to expand...
Nope. Doesn't make sense.
Yes welding the inside moves the weights outward in their initial position. This results in a little more initial spring tension. But that difference in spring tension is small and within the range it can be easily adjusted.

Whether its better to weld the inside or the outer end depends on the original distributor's advance curve.
You may have noticed I try not to say one or the other is always better. This is because which is better it depends on the springs in the distributor and whatever other springs you have available to work with.
However, when starting with an early to mid smog era distributor, most of the time its better to weld the inside.
1. The retarded initial timing used on CAP/CAS distributors was offset by super fast mechanical advance. Welding the inside pretty much is all that's need to convert the distrributor to a pre-emissions curve.

A good example of the change can be seen in the 1967 440 Hi performance package since it was sold with CAP in California and non-CAP elsewhere.
upload_2020-12-12_18-37-53-png.png


Take any similar CAP/CAS timing curve, weld up the inside and then initial can be set the same as pre-emissions.
Below is the timing range for the 1968 440's with CAP. Weld the inside, and set the initial for 12.5*. Done.
upload_2020-12-12_18-49-48-png.png


Lets compare this to reducing the advance by stopping the weights by welding or blocking the outer end of the slots
With points and no vacuum advance, this could be a winning set up at the drag strip.
The timing advances very quickly and then acts something like a locked unit.
We kind of saw that in the dynomation example with locked timing above - and many of us know its what was frequently done back in the day.

Using the '68 440 as an example.
upload_2021-9-9_23-28-47.png


There are two drawbacks to doing this.
1. For street use (or endurace racing) it's too quick and too much timing to work with vacuum advance. Instead of having 50* at 3000 rpm cruise, now its 50* from 1200 rpm up with light throttle.

2. With any electronics dependent ignition, the actual timing seen by the engine at high rpm is retarded. See my earlier post with the videos by yellow rose and William Baldwin.
 
Mattax said:
Nope. Doesn't make sense.
Yes welding the inside moves the weights outward in their initial position. This results in a little more initial spring tension. But that difference in spring tension is small and within the range it can be easily adjusted.

Whether its better to weld the inside or the outer end depends on the original distributor's advance curve.
You may have noticed I try not to say one or the other is always better. This is because which is better it depends on the springs in the distributor and whatever other springs you have available to work with.
However, when starting with an early to mid smog era distributor, most of the time its better to weld the inside.
1. The retarded initial timing used on CAP/CAS distributors was offset by super fast mechanical advance. Welding the inside pretty much is all that's need to convert the distrributor to a pre-emissions curve.

A good example of the change can be seen in the 1967 440 Hi performance package since it was sold with CAP in California and non-CAP elsewhere.
View attachment 1715788525

Take any similar CAP/CAS timing curve, weld up the inside and then initial can be set the same as pre-emissions.
Below is the timing range for the 1968 440's with CAP. Weld the inside, and set the initial for 12.5*. Done.
View attachment 1715788526

Lets compare this to reducing the advance by stopping the weights by welding or blocking the outer end of the slots
With points and no vacuum advance, this could be a winning set up at the drag strip.
The timing advances very quickly and then acts something like a locked unit.
We kind of saw that in the dynomation example with locked timing above - and many of us know its what was frequently done back in the day.

Using the '68 440 as an example.
View attachment 1715788520

There are two drawbacks to doing this.
1. For street use (or endurace racing) it's too quick and too much timing to work with vacuum advance. Instead of having 50* at 3000 rpm cruise, now its 50* from 1200 rpm up with light throttle.

2. With any electronics dependent ignition, the actual timing seen by the engine at high rpm is retarded. See my earlier post with the videos by yellow rose and William Baldwin.
Click to expand...
Thanks for all that. You saw I refrained from saying one is better....just that I prefer to weld the outsides. That's how Don Dulmage shows you how to do it. If anybody would know, it's him.

Also another note. I was playing around with the timing light on Vixen yesterday and noticed the mechanical advance doesn't begin until right at 1800RPM. Sounds a little high, but she runs like a bat outta hell, so I'm leavin well enough alone.
 
@RustyRatRod @Mattax

Exactly.
Welding the inside , extending the springs into more tension.. ...wtf is he thinking.lol so we want a slow curve?
Putting the spring under tension and into its max resistance..uh no thanks.
The outside is where you would close it up.

Tony is a pants shitter
 
MOPAROFFICIAL said:
A cleaner idle..lol
Better gas milage.
More power,earlier, same power up top.
Improved throttle response.
Just lights it earlier aiding in a more complete burn. Less residual when the over lap let's end gas linger or revert and contaminate the intake charge at low speed under the cam range. Improves everything under the cam range.
Every engine combo coupled with tq convertor,weight,gear will want it's own curve and let you know how early it can take it.
My stuff has always ran 24-28 initial timing with 4-6 degrees mechanical all in by 2200

The more radical the cam the more the effect it will have. What it wants or positively responds to...is the ticket.
Click to expand...
Some dont need a curve, some a retard.
5500 stall race, what's the point if the fuel is right..
 
ir3333 said:
A good curve for a 10:1 340 usually falls around 10 initial with another 24 mechanical for a total of about 34 at around 3000 rpm without vacuum advance which may add another 16 or so for a cruising total around 50.
how might the engine performance differ with totals of 30 - 36 at wot ( no vac adv.)
..would it be noticeable?
Click to expand...

Are you talking at the track or around town.
I'm a streeter so all I can talk about is street performance; so
now it becomes a question of ; what transmission, and what gears.
then, if an auto; what stall?
Depending on the combo (say; automatic trans &3.23s) you might not hit 3400 at WOT until over 30mph in First gear. If you take that to 5500 @50 mph, then the Rs might drop down to 3250 in Second@50mph, and there you are again, struggling up the power curve. By 65mph, in Second, your Rs may be up to 4200.
So; will you notice a performance loss, at WOT, at these rpms?
IMO, only if the tires don't spin, and
only if you put a stopwatch on it
that measures to at least 10ths of a second,lol.
=============
Watch Uncle Tony's videos; sometimes he hunts for a horsepower number just 3 hp more, and tries different max advance settings two or three times, and still can't find it.
============
In my combo, by seat of the pants testing, with alloy heads ( 367 cubes with flat-tops at 11/1Scr), I couldn't tell the difference between 32/34/36 degrees, so I just run 32/34 and call it done. It went 93 in the Eighth on that; surprised thechit out of me.
==============
But if yur talking Quarter-mile, running 4.30s and 1.8second 60fts and stuff. ........ that's another topic.
 
ir3333,
The only way you will find out how much difference to HP the timing makes is put the car/engine on a dyno. Anything else is pre speculation because of all the variables involved.
 
Bewy said:
ir3333,
The only way you will find out how much difference to HP the timing makes is put the car/engine on a dyno. Anything else is pre speculation because of all the variables involved.
Click to expand...
Drag strip.
 
You must log in or register to reply here.
-

Back
Top