I'm going to put a comp cams XR264HR with .48/.49 lift in my vortec 350 and I'd perfer not to machine the heads for clearance so I was going to use Crane Cam’s 10309-1 spring and retainer kit which is supposedly good for .550 lift on the vortec heads.

Now when they say the kit is good for .550 lift, does that mean it won't be able to handle the XR264 cam because after considering the rocker arm ratio, I'll be at .735 lift? Am I missing something in my calculations? I will use scorpion full roller rockers 1.5 ratio.

Thanks.

 

Are you using a stock rocker arm, roller tip rocker, 1.52 rocker, 1.6 rocker?

 

Lobe lift multiplied by rocker ratio gives you valve lift. (Calculated) actual lift at the valve may vary, especially with solid lifter cams. The .480/.490" figures are valve lift typically with a stock rocker ratio of 1.5:1.

 

I will use 1.5 ratio full roller rockers.

 

Um ya no need to add the rocker arm ratio to your cam lift. So your valves will only move .480/.490 so the. 550 springs will work.

 

The lift you have quoted is the valve lift with 1.5 rockers. .48 is .512 with 1.6 rockers. Just multiply the 1.5 lift by 1.6/1.5.

 

Ahh, ok thanks for the answers guys!

 

Your 0.480/0.490 valve lift is quoted by using a standard 1.5 rocker ratio. The lobe lift is .480/1.5 = 0.320 and the .490 is 0.326. Now, knowing the lob lift, you multiply it by the new ratio of 1.6 and get a valve lift of 0.512/.521

Instead of worrying about pushrod clearance through the head by using a 1.6 rocker which will move the pushrod closer to the pedestal, why don't you just buy a cam that will give you the same lift with 1.5 rockers?

Using an increased rocker ratio is just a poor mans cam swap. If you're going to change the cam, get a grind to match the lift you're shooting for. There is a time when an increased rocker ratio can be beneficial so that you don't have to use a real small base circle cam but you're nowhere near that range.