Re: 1.7 rocker arms

From: Gary Shook (gary.shook@mci.com)
Date: Wed Dec 09 1998 - 17:25:01 EST


>On an internally stock motor, is there any performance advantages or
>hazzards by changing to a higher ratio rocker arm? Wouldn't this
>effectivly increase the lift/duration of the stock cam since the valves
>would be physically traveling further?
>
>Jon "I'm no engineer" Smith
>
>There is a article in the january 1999 issue of Mopar Muscle that put a
set
>of 1.7 roller rockers on a 3.9l V6.They said that it would not cause any
>problems .They showed the dyno sheets on the engine picked a few horses.The
>article said that it was a fairly easy mod.
>Hope this helps.
>
>Roderic
>98 Sport+ 5.2l 4x4
>

I've got the article right in front of me... It says that the stock ratio on
the Magnum motors is 1.6, and they tested Crane's 1.6 and 1.7 ratio rocker
arms.

I found it interesting that the table they provided w/ torque & HP numbers
didn't add up.. that is, if you use the HP formula, you don't get the same
results... It appears that the base HP numbers are low (as much as 5HP below
4000RPM), but after putting the chart in Excel, and using the HP equation
to determine HP, you get the following results:

1.6 ratio rockers: peak improvement 3.5HP, average 0.8HP
1.7 ratio rockers: peak improvement 7.1HP, average 4.6HP
you do have to change the rocker covers, or trim the baffle inside the stock
ones, which add to the cost. They estimate it effectively increases the
duration of the cam by 1-2 degrees, and thought the improvement would have
greater increase on the 8 cylinder, due to more cylinders to improve. (like
a percent increase in HP, not a straight addition)

overall, I don't think it's a good bang-for-the-$ mod... but if you don't
have alot of choices... They're supposed to have less friction, but I'm not
sold that it's enough to make a difference. Incidentally, they also changed
to MP performance valve springs, as suggested by Crane- this could
contribute to decreased gains, and doesn't make much sense to me- unless the
new rockers are heavier.

Gary "engineer (but electronic, not mechanical) & skeptic" Shook



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