Rocker Arm Ratio

Hi Dr. Dino,
I read your post on this forum and I am very impressed by the wealth of knowledge you possess regarding the XJ. The article on increasing the power of the old 4.0 is most timely for me.
I am planning to rebuild my aging 4.0L, which has in the excess of 240,000 miles. To date it has only the usual symptoms of the typical 4.0 namely piston slap and valve clatter. Despite this, I still get up to 20 MPG on the highway at 75 MPH. I looked at one of your posts on this forum regarding engine upgrade to give about 250 HP. This is something I am interested in. Most (80% +) of my driving is on the highway with the occasional trip to a wheeling park.
I want to replace the OEM rocker arms with some after market ones. What I do not know is the ratio of the OEM units. Since the rest of the valve train including the camshaft will be replaced I think it is only reasonable to replace the rocker arms as well.
In short, I would like to give the old XJ a bit more get-up-and-go. However, I want to retain the normal drivability and use mid grade gasoline. Any and all tips you can give will be greatly appreciated.
TIA.

:dunno:

I belive they are 1.6 to 1 but could be wroung. I just got desktop dyno 2000 for my putter but don't have awhole lot of time or brain power to work with it right now. so can't wait to see what i can get out of my favorite motor

Thanks Patrick! The stock rocker arms are indeed 1.6 ratio and since the stock 4.0 cam has a lobe lift of 0.265", the valve lift is 0.424". Duration is 197* @ 0.050". Unfortunately there aren't any higher ratio (1.7) roller rockers available specifically for the 4.0. It IS possible to adapt 1.7 roller rockers for the Big Block Chevy to the 4.0 but it's an involved process and the HP gains are probably too small for this to be worthwhile. Mopar Performance have 1.6 ratio roller rockers for the 4.0 but they're so ridiculously expensive to make these a complete waste of time.
You could go further than a mere stock rebuild and build a stroker engine out of your 4.0. With the crank from the Jeep 258 engine and a 0.030" overbore, you can increase displacement to 4.6L. With the added displacement, the engine will produce more torque and make highway driving even more enjoyable. It's also a lot easier to get 250hp from 4.6L than it is from 4.0. If you're tempted by a stroker engine, you can find more info. from Dino's stroker webpage
Enjoy!