Proper Transfer Slot Position

[TT5.9mag]

In one of those threads linked, TT5.9mag mentions that these engines peak around 3200rpm

Context young skywalker, context. I believe (I didn’t go back and look I’m working off of memory) what we were talking about was the difference in timing requirements for peak torque and peak hp which occur at different rpm points. The magnum chamber wants 32-33 degrees all in at peak hp rpm and it wants considerably less than that at peak torque rpm. So developing a two stage curve with one heavy spring and one light spring (as well as limiting the mechanical advance) becomes the hot ticket.

 

[Bewy]

About the BS in post #47. You want the most timing at idle that the engine will accept.....which is not 5 or 8 degrees. Less timing creates more problems than it fixes.
[1] By reducing timing, you will have an even greater rpm drop going from N into gear. That is because the engine is making less hp with reduced timing & requires the rpm to be increased to make enough hp to carry the load.
[2] Reducing idle timing means less hp, so that there is less hp to carry the load off idle to carry the load. Result is an off idle stumble. I would like a $1 for every off idle stumble I have fixed.....just increasing idle timing.
[3] Below is another quote from an English publication that puts it very well...

 

[Bewy]

Should have added....engine will be more likely to overheat with less timing, because it has to work harder.

 

[jmanhoff]

Should have added....engine will be more likely to overheat with less timing, because it has to work harder.

So in terms of giving the engine as much initial timing that it will accept, do I just set the carb up the way it should be in terms of t slot exposure, etc, and then set the timing to what gives me an acceptable idle?

 

[Hysteric]

The less timing the better. You have to remember that you lighting the fire before top dead centre.......

 

[jmanhoff]

I feel like two mixed opinions are developing lol

 

[Hysteric]

I feel like two mixed opinions are developing lol

He'll tell you to give it 50 initial.....Every degree you give it before top dead centre you are creating pressure while the piston is moving UP the bore NOT DOWN the bore. So its effectively fighting itself.

Here's another thing LEAN mixtures burn slow, RICH mixtures burn fast...........

 

[jmanhoff]

He'll tell you to give it 50 initial.....Every degree you give it before top dead centre you are creating pressure while the piston is moving UP the bore NOT DOWN the bore. So its effectively fighting itself.

Here's another thing LEAN mixtures burn slow, RICH mixtures burn fast...........

That part I do understand, hence why you’d want the most amount of advance possible at a lean cruise

 

[Hysteric]

That part I do understand, hence why you’d want the most amount of advance possible at a lean cruise

Because of the dilution of the mixture at high vacuum levels during cruise. But at Idle.....

I've run a big solid with 320 Adv and 256 @ 50 and all it required at idle was 21 degrees.

 

[jmanhoff]

Because of the dilution of the mixture at high vacuum levels during cruise. But at Idle.....

I've run a big solid with 320 Adv and 256 @ 50 and all it required at idle was 21 degrees.

So then I should stick around the 14-16 area like you mentioned earlier?

 

[Mattax]

This is my plan, but of course plans can change, so PLEASE give me honest input if I’m going in the wrong direction lol

It runs OK now, right?
So why would you change a bunch of things before measuring?
How would you be able to compare the effects of the changes?

 

[jmanhoff]

It runs OK now, right?
So why would you change a bunch of things before measuring?
How would you be able to compare the effects of the changes?

It’s almost there. I put a much smaller IAB in trying to get my tip in cruise richer, but then realized I have a vacuum leak which is obviously leaning the entire circuit, and also most likely the reason why it idles so high to the point where I have to cover the t slot to get it down to where I want it.

So my first step is to put the idle circuit back to stockish (0.028” IFR and .070” IAB) after fixing that vacuum leak. Square the t slot, and then see where my idle ends up. I’ll get it fully warmed up then map my timing curve and compare to the moroso charts and see where I should go from there.

 

[Mattax]

So then I should stick around the 14-16 area like you mentioned earlier?

You think the factory team might have a something more than a WAG ?
That said you want to try for a hot idle at 600 rpm like a mid 1960s truck, you might try a couple a less degrees than the factory 5.9 truck.
A carb is going to respond to the conditions creating the signal. It's going to work best when the engine works best.
What can be done better with EFI and controlled ignition and 3-way cat is make it get more complete burns of HC, with low CO and NOx, especially at closed or near closed throttle.

 

[Mattax]

It’s almost there. I put a much smaller IAB in trying to get my tip in cruise richer, but then realized I have a vacuum leak which is obviously leaning the entire circuit, and also most likely the reason why it idles so high to the point where I have to cover the t slot to get it down to where I want it.

So my first step is to put the idle circuit back to stockish (0.028” IFR and .070” IAB) after fixing that vacuum leak. Square the t slot, and then see where my idle ends up. I’ll get it fully warmed up then map my timing curve and compare to the moroso charts and see where I should go from there.

Several of us have told you that the timing may be playing into why it hangs.

 

[jmanhoff]

You think the factory team might have a something more than a WAG ?
That said you want to try for a hot idle at 600 rpm like a mid 1960s truck, you might try a couple a less degrees than the factory 5.9 truck.
A carb is going to respond to the conditions creating the signal. It's going to work best when the engine works best.
What can be done better with EFI and controlled ignition and 3-way car is make it get more complete burns of HC, with low CO and NOx, especially at closed or near closed throttle.

Like I mentioned a couple of posts ago, I do like the 600rpm range at a hot idle, as it’s quiet, but still has the power to get moving. That is my goal, 600-750 would be perfectly happy for me.

Several of us have told you that the timing may be playing into why it hangs.

I did check the timing while it hung up, and it showed the same 18 degrees at idle as it did at the hang up at 900-1000 degrees. Unless my timing light is being goofy I don’t believe my mechanical advance is coming in quite yet

 

[Mattax]

And you didn't measure when the rpm the timing advances?
Did you disconnect the vac advance?

So in terms of giving the engine as much initial timing that it will accept, do I just set the carb up the way it should be in terms of t slot exposure, etc, and then set the timing to what gives me an acceptable idle?

That would be a starting point. Which it seems like you have. Then make a stepwise change. I would not jump from 18 to 8 degrees. That's a huge jump. Yes you can change initial timing to change rpm. But with a distributor changin initial also shifts the entire timing curve. So in fact the one change is really changing the timing for the entire rpm range. Go from 18 to 16 at 750 rpm and if the idle is stronger, trim the fuel, etc then do another 2 degrees, until it gets worse again. Testing is iterative.
The thing is, at some point, even if the idle is stronger, above idle it will become sluggish.

 

[jmanhoff]

My mechanical advance with the stock weights and springs starts to come in around 1300-1500 from what I can tell. I checked this with the vac advance disconnected.

And then essentially, I need to balance getting the idle down and strong, while not causing a tip in by retarding the timing too much/changing the fueling too much?

 

[crackedback]

No way do I idle a roller cammed SBM engine at 600 rpm. JMO

 

[jmanhoff]

No way do I idle a roller cammed SBM engine at 600 rpm. JMO

If you don’t mind me asking, why not? I think the stock magnum engines hung out at that rpm. It’s not apples to apples but the magnum v6 that was originally in this truck idled at 650 when it was hot

 

[TT5.9mag]

No way do I idle a roller cammed SBM engine at 600 rpm. JMO

Magnum roller? No problem idling at 500-600. They live long happy lives at that idle rpm. ANY flat tappet cam? No way I would let a flat tappet idle down there.

 

[jmanhoff]

Magnum roller? No problem idling at 500-600. They live long happy lives at that idle rpm. ANY flat tappet cam? No way I would let a flat tappet idle down there.

That’s what I was told too. My hot oil pressure at that rpm is about 20 psi, and the magnums are speced where as low as 6 is acceptable lol. Oil gets to the top end there as well.

I’ve had the VCs off several times with the engine running trying to diag an issue of clacky valve train noise, which I’ve basically just deemed as a magnum engine thing at this point lol. Just to throw it out here again, 20k+ miles, 2 rebuilds, 3 styles of lifter (Melling stock replacement, Hughes 5007, Hughes 5006-S), 3 styles of rocker arms (stock, Harland 1.6 pedestals, Harland 1.6 3/8 studs), different length pushrods to change preload around (.060-090 for melling lifters, .120-140 for Hughes which is their spec) with the non adjustable rockers while making sure the roller was centered, which it was. Now with the adjustable stud rockers I’ve tried 1/2, 3/4, 1, and 1.25 turns and the sound is still the same lol. With the non adjustable rockers it was quieter cold louder hot, with the adjustable it’s the opposite and slightly quieter. I’ve verified several times that there’s no exhaust leaks, and I’ve tried stock and jba shorty headers, remflex gaskets, etc. I caught DeadDodgeDarage on a livestream and told him my specs and my situation, and he literally said “some magnums just do that” so I think I’m going to close the very big jar that is this whole situation lol. Anyway, back to the topic of this thread

 

[TT5.9mag]

I don’t run nearly as much preload as what Hughes or mopar or anyone else for that matter suggests. I run the factory lifters at .030 preload and they’re nice and quiet.

 

[jmanhoff]

I don’t run nearly as much preload as what Hughes or mopar or anyone else for that matter suggests. I run the factory lifters at .030 preload and they’re nice and quiet.

If it bothers me enough to make me feel like digging into the engine again then I might try to find some NOS or used good condition lifters and throw them in. 20k miles and it hasn’t gotten worse, and no sparkles in the oil, and engine is very strong, so I’m not worried about it to be honest

 

[AJ/FormS]

To my knowledge it’s 3.83 in first, with 3.55 gears and 29.5ish tall tires. First is, naturally, very short, but it’s a very snappy truck because it’s so light.

That would be like my dream truck.

 

[Mattax]

And then essentially, I need to balance getting the idle down and strong, while not causing a tip in by retarding the timing too much/changing the fueling too much?

There's a limited working range of initial throttle position where the transfer slot will function effectively as the throttle opens.
When tuning in neutral, which has to be done with a manual transmission, reducing the initial timing from 18 will slow the engine. Then if it was wanted to return the idle to 750 rpm, the throttle has to be cracked open a little more or some air needs to be bled in. Then the idle mix screws should be adjusted.

Working the other way, if the throttle is cracked open a bit more, then to bring the idle rpms down, reduce the timing a bit.

T-slot square is just an easy visual to put in the ballpark. On 4777 .020 to .040 is probably the working range.