Mopar Action On-line – Tech Questions


Tech Question

Rod Wood McConnell AFB, KS

I’m curious to know if my existing valve springs could be used with a MP P4120235 “Purple Shaft” cam. The springs in question are Lunati #73815 (singles w/dampener) that have approximately 15,000 miles on them. My current cam is a Lunati #30306, grind #H234-245. I know that MP recommends P3690933 springs for the “235” shaft, but I’m hoping to avoid the added expense of replacing them if they aren’t too strong or weak for the MP cam.

Along with the cam swap, I’d like to do some head work. At the last rebuild, I had installed 2.14/1.81″ valves, hardened seats, and bronze guides. I ordered the MP porting templates (P4120437), but I’ve learned that they’re only for standard-size valves. What would you recommend to do in the bowl area, and would a semi-polish on the chambers be worth it? The car itself is a daily driver ’68 GTX 440/727 combo, that makes an occasional spin down the quarter.


Before getting specific, I have to say that I question the logic behind saving a few bucks on the springs. But, I guess you could say that’s none of my business, and you’d be right!I don’t have the exact specs for your spring or “old” cam, so I’m just going to pass along a few guidelines on spring selection. The first, which you obviously have covered, is: do they fit the head? In other words, is the o.d. correct for the retainers and does the i.d. clear the seals, and is the free length “in the ballpark?” Next, you need to check for possible coil bind. This is something you can do yourself. You can easily measure the current installed height with a scale (precision ruler). This is the dimension from the underside of the spring retainer to the seat (head) or the top of the shims, if used. Now, take the maximum valve (not lobe) lift figure for the “new” cam and subtract it from the first number. This is the “valve open” dimension. (If the spec sheet only give lobe lift, multiply it by your rocker arm ration, probably 1.5). Put the spring in a bench vise and compress it to this dimension. While compressed, carefully check that the coils of both the wire and the sheet-steel dampener aren’t hitting each other, i.e., that you don’t have coil bind. As a rough guideline, I’d like to see bind not occur even if you compress the spring another 0.10″ inches.

Assuming that your springs pass this test (you really only need to test one), now you face the toughest calculation: spring pressure, both “seated” and “valve open”. You can’t readily test this yourself, you need to have a machine shop do it for you. In this case, I’d be sure that the springs are at least as stiff as the MP 933s. If they are, and don’t exceed the MP’s numbers by more that 15-20%, they’ll probably be fine.

A “meatball surgery” way of comparing springs is by checking the diameter (with a vernier caliper) of the wire. If it, and the coil count, is similar (or beefier) than the MPs, you’re probably okay. The MP 933 spring you mentioned uses 0.224″ wire.

One other suggestion: splurge for just one of the MP springs and visually compare them to your Lunatis.

On the heads: polishing the chambers is a waste of time. The bowls should obviously be blended into the new seats as gradually as possible. Professional, Mopar-specializing shops can find you a lot of horsepower here that you can’t readily duplicate at home. If you make any shade-tree attempt, though, heed this advice: don’t touch the short-side radius (“bottom” of the port.)


Main Index | Next Issue | Tech Q & A | Tech Archive | Subscription | Advertisers Links | Contact Info