OHC duration measured differently?

[SEMI_287]

Ok, so my friend and I were looking at stockish tune friendly ls1 cams and for the heck of it I looked up the Jeeps. The stock cams measure 244/254, how is that possible?? I mean the lift isn't low and the valves aren't small. The cam in my LT1 is 226/234 and its a pretty healthy cam. What gives here?

 

[Steve S]

At what lift is the cam spec. measured at?

 

[FL_Rob]

The difference is in the rocker arm geometry for each vehicle,or no rocker arms for OHC.Overall you're most likely pushing alot more valve lift than that jeep engine for example,plus there's duration to consider.

 

[Astro14]

Originally Posted By: Steve S
At what lift is the cam spec. measured at?


Exactly - looks like your LT1 cam is measured at .050 lift...big difference in duration at .050 and at 0...so, since it looks like OHC and an OHV duration is measured differently, it's an apples to oranges comparison...

 

[440Magnum]

I agree that the duration has to be measured at the same lift for an apples-to-apples comparison. The 4.7 has roller finger followers, which basically serve the same function as rocker arms in that they can have a ratio other than 1:1, so the valve lift can still be different than the cam lift. But I suspect they are set up closer to 1:1 than the rockers in the LT1 are. That may be part of it. There are a thousand ways to make it hard to get a direct comparison of two cams!

 

[Duffman77]

Cam companies typically quote duration at 0.050" lift. The car manufacturers use the SAE standard of 0.006" lift. Could be the issue here.

 

[mechtech2]

Duration is duration, no matter if it is OHC or cam in block with rocker arms.
Measured cam lift would of course be way different with 1 to 1 OHC, and about 1.5 to 1 rocker arms.

So, are the stock cams the 'stockish' cams you refer to?
I think there are three different vehicles refered to.
What vehicle has what cam specs?