Magnum head swap - rocker ratio change? Effects?

I picked up a pair of Mopar replacement Magnum heads new in the box from a friend of mine for $100. I was toying with the idea of putting them on my 78 Fury and have one major concern. My Fury is basically a stock E58 360 with the exception of a Comp XE256 cam, Hooker comp headers and 2 1/2" duals. The trans is a 727 with a 2500 stall, and the factory 3.21 S/G rear out back. My concern will be the increased lift, not as far as piston to valve clearance, but how it will affect how the car runs/performs. The rockers will basically increase my lift from .447/.455 to .477/.485. The advertised duration is 256/268, and @ .050" is 212/218. Will the small increase in lift affect anything? Im in the process of successfully redrilling the heads to use the stock LA 360 manifold, and already have the 7.65" pushrods. I figure since the Magnum heads make much more power and are more efficient than even the old X's or J's in stock form, it would be worth a shot to try them on the old boat. Anything I should be concerned about, or is this cam too small to worry about anything? I understand its a heavy car, but I was thinking it may run good for what it is when combined with the exhaust, converter, and gears. Im just bored more than anything and thought I'd mess with it

Your motor will love the whole head swap includeing the ratio increase

I would swap on a set of GM 3100 V6 valve springs and retainers, very cheap rediliy available behive spring that can very easily handle that lift for years to come.

The extra lift will allow just a tad more flow but no significant change in duration and therefor no change in drivability except more HP especially with that tiny cam.

The compression increase will be nice for drivability as will the far superior chamber and ports. Should idle smoother and make more HP, TQ and MPG.

Well thats good news! So you suggest the GM spring over the P5249464 spring? Would you happen to have a part number for the GM springs and retainers?

are these the MP heads with the .525" lift, ~1.4" diameter cylindrical valve spring on them like the crate motors had, or OEM truck beehive springs with ~1.1-1.2" retainers?

in my measuring, the '94-up GM 3.1/3.4L springs have a nice rate, but are kinda light on seat pressure, and the ones I snagged from a junkyard would only give ~.490" lift capacity. if I were to use these, I'd snag some used junkyard ones. new springs & retainers thru napa, rockauto, or GMpartsdirect.com will run ~$120-150 or so

sealed power VS1642 ($4.68 ea from rockauto, $5.99 from napa, $8.99 from summit) is the part...
retainers are common with LS1 motors, don't see them on any of the sites, but will probably be ~$50 for the set.

I used hughes #1110 springs, they give at least an additional .050" of lift capacity (for future cam upgrades?), a bunch more seat pressure, and use either their retainers, or stock magnum keepers and retainers from junkyard 2.2L/2.5L chrysler SOHC 4 cyls...spring price is about the same as the GM springs new,

or any 5/16" valve retainer and single groove keeper, which may open the door to more installed height options...

You are going to pick up about 1/2 point compression right off the bat. You probably will have good piston to valve clearance also.

I shaved my Mag heads .020" before installing them on my stock '74 360 short block. And my cam is adv. duration of 268/285, and lift at .050 is 220/236 installed at 108* intake center line and I still had plenty of valve clearance to the stock dished pistons . I was also able to use the stock wimpy Magnum beehive springs for break in and had no problem with coil bind. Make sure you check this. Swapped in the proper springs after break in, and the cam is happy as a clam.

You'll need to determine what RPM the new build makes it's peak power, then adjust your converter and/or rear gear and/or rear tire size accordingly. Ideally you'd want your cruising RPM's to be higher than the stall RPM. If your stall speed higher than your cruise RPM's you'll generate a lot of transmission heat unless you have a high quality converter. At any rate, a good auxilary trans cooler is in order if you don't have one already. If you could get a 3.55 gear it would help get the B out of the hole quicker.

I am running an XE268 in a 72 short block with magnum heads and the GM V6 springs and retainers ($125 springs AND retainers from my local GM dealer)home ported heads stock valve job and it is still pulling real hard around 6000 where I chicken out. Stock non adjustable rockers.

PS I broke it in with stock springs because they are even lighter and it took the dealer a few days to get the springs and retainers.

I think it works because the valve and retainer are sooooo light compared to normal LA stuff it just don't need as much preasure as conventional wisdom would have you think.

You should check coil bind just to be sure (mine is very close) as the heads patrick used show different heights than mine or mabey a lot more lift than you would think is lost with the funky pushrod angles. I checked mine with 1.7 adjustable rockers and could see light through the coils so I figured the stock 1.6 rockers would give me a tad more clearance. I have several thousand miles like this and it runs great

So I can use the Sealed Power VS1642 springs, and get a set of retainers off of a GM 60 degree V6? I cant seem to find a GM part number for he retainers. If youre running an XE268 with the 3100 springs, thats good news. Granted yours isnt a big cam, but my XE256 is like a babys toy compared to yours Im kinda looking forward to trying this out!

Quote:

---

So I can use the Sealed Power VS1642 springs, and get a set of retainers off of a GM 60 degree V6? I cant seem to find a GM part number for he retainers. If youre running an XE268 with the 3100 springs, thats good news. Granted yours isnt a big cam, but my XE256 is like a babys toy compared to yours Im kinda looking forward to trying this out!

---

yup....

there's a lot of things that could affect IH, one of the biggies is how high the valve job is--if the seats have been recessed some, and the valve margin reduced by valve job(s), that will increase IH, helping lift capacity, but hurting closed pressure.

with my cam (a pretty aggressive ramp HR), too little seat pressure could cause the valve to bounce on close, that was another reason why I went with the hughes springs (and the additional ~60lbs of seat pressure they give).


here is all the investigation/discussion of the GM springs.

for me, the hughes #110 springs were ~$95, and the 2.2/2.5L retainers were $5 from the junkyard.