Swap the box, that’s easy enough.

No love for the orange box from me.

I agree except get rid of all of the mopar ignition and install an msd box
And I bet your problem will be gone. I had a 408 stroker that wouldn’t pull past 5k no matter wht box i added.
Installed a msd 6al and problem solved. Ran a lot better all around.

The 26918 spring are the WRONG spring to have on there...swap them out for a nice double spring and it will eat. The valve's need to be really light for those springs to work properly, especially with a solid FT cam. Beehive's don't like the jerk motion that solid FT's have.

Boy, if that's the case, it really sucks that Comp would recommend them to me.In their defense I did say I was interested in using a beehive style spring, but why would they recommend a part that's completely wrong for the application?

Swapping the carb and ign box are easier tests than swapping the springs....... but if they make no difference at all, and it still runs into the wall at 4K......... It’s going to be tough to not start taking a harder look at the springs.

Obviously I’m not seeing or experiencing it first hand........ but from the description........ it acts just like it would if the springs wer giving up.

Yes, I agree.. it's just disappointing if the springs are in fact the culprit. I'll do some more testing and report back.

Find a chrome box. Orange box isn't much better than stock. Chrome box will go over 6k. Quicker than springs or carb. I can't see either of those causing the motor to just "Stop" at 4k. Acts more like a rev limit than anything else, to me.

Update :

I checked a few things last night,
#1. Fuel pump pushrod looks brand new
#2: verified no obstructions in the fuel filter
#3: baselined the carb back to 78's all around ,per factory

Fired it up this morning, did as Fast68 instructed, revved up to 3,800 then pushed past, it climbed about 5,200 then I let off. It felt good.
Took it for a romp,.. pulls clean to about 5k in second then manually shifted to third.. not a hiccup. I'm not sure if re baselining the carb helped, or if I was just being hyper critical (paranoid) on the first few test drives after the new head install, but it seems to be running just how it should, just with some more "umph"..

Thanks to all for your input and suggestions, I appreciate it. So, next step will be some track time to quantify my improvements, hopefully move forward past my previous 12.55 best et.

Have you tried going higher than 5k?

I’d expect with those heads and cam it’ll need about another 1000 rpm(or maybe more) to see the best track times.

those crane ductile rockers will check somewhere between 1.57-1.6 true ratio. your actual lift at the valve will be .530-.540. the .028"-.032" recommended lash is somewhat loose; i'd try .022".

No, I haven't tried much past 5k. Yes, I probably will need to get to 5,500- 5,800 to see the best e.t. maybe kinda hard with a 28" tire and 3.91's .. I'd probably need more gear.. but I like driving it on the street easy too.

I was mostly referring to the best shift point.
With the bigger heads and 112lsa cam I can easily imagine where it would want to be shifted at 6000-ish for the best ET.

Ok, gotcha... yeah on that 12.55 pass I was thru the traps around 5k.. and just left the trans in Drive. It was probably shifting around 4,200. I plan on upgrading to full manual valve body down the road to better control my shift points. I know I'm leaving a lot on the table just making passes in drive, but I'm not a very experienced track guy. I still need a LOT of practice... lol.

what car is this in? what torque converter?
a 12.55 seems pretty respectable for short shifting...and it feels alot stronger with the promaxx heads now?

Its in a 66 Satellite.. weighs 3,760 with me in it. It's a 9.5" FTI brand converter with a 3,600 stall. Yeah, 12.55 was better than I expected and that was even with a dismal 1.88 60'. I have a lot of room for improvement. I'm hooked now !

Work your way down more on the jetting as long as the plugs look OK. This is where it helps to have a wideband

going up 4 steps is about a 16% increase in main jet area (a bunch!) without prior testing. I don't think the current valve spring is good but about anything will go 4000rpm.

Good info, I've never verified actual valve lift at the rocker tip. I have them lashed at .026. A little clackity, but better than the factory spec. I think the solid lifter "sewing machine" sound is cool anyway.

UPDATE :

Just wanted to wrap this thread up. I know it's over a year later, but better late than never.What was happening after the head upgrade with the new beehive valve springs installed , the engine revved so much faster I was actually hitting the flash stall on my converter very easily. I wasn't used to it because of my old valve train, it revved much slower. So on the first couple of shakedown runs it kinda spooked me. I just wasn't used to feeling the engine flash that fast. So now, yes it flashes to about 3,800/4k rpm very quickly, then begins to pull. It pulls clean to 6k. Thanks for everyone's input. Just wanted to cap it off for anyone reading this thread in the future.

it's always good to hear about results after a problem has been solved.