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Sat Aug 02, 2008 11:19 pm

fellas,
i'm wondering how to figure out actual valve lift with different ratio rockers.
Is the formula: cam lift x rocker ratio = valve lift?

Thanks!
Craig

Sun Aug 03, 2008 5:59 am

craigman wrote: cam lift x rocker ratio = valve lift?

Yep, that's it. Stock VW rocker arms are 1.1 ratio. Say you have a cam made for the stock rockers, like an Engle 110 that would have .430 of lift with the 1.1 ratio. To get the actual lift at the cam, you would divide .430 by 1.1, getting .391. Then you could take the .391 and multiply it by your ratio, say 1.25, and get .489. There are a range of cams that are ground to be used with certian ratio rockers. You probably wouldn't want to use a 1.4 ratio rocker with the Engle 110 designed for 1.1-1.25 rockers, and you wouldn't want to use a 1.25 ratio rocker on an Engle FK Series made for 1.4-1.5 rockers. The lift at the cam is also usually advertised with the cam info, then you can just multiply it by your rocker ratio.

Sun Aug 03, 2008 8:31 am

I like to take it a step further and work out the absolute best estimation of running lift, here how I figure it. Of course this will change with various types of push rods, so this is an example using choromoly push rods set to "0" cold and estimating a running hot lash of .006"

Cam lift x rocker ratio - lash - push rod deflection= total valve lift.

.430" x 1.25 - .006" - .002" = .530 actual valve lift.

Mon Aug 04, 2008 7:59 am

SRP1 wrote: .430" x 1.25 - .006" - .002" = .530 actual valve lift.

If you're using an Engle 110 as an example, isn't the .430 lift calculated at the 1.1 ratio of the stock rockers? I thought the lift at the cam was only .391. Shouldn't your calculation be;

.391" x 1.25 - .006" - .002" = .481 actual valve lift?

Mon Aug 04, 2008 11:10 am

I would think so, because I thought cam lift is given at the lobe, because the cam doesn't know what rockers are being used.

Mon Aug 04, 2008 11:27 am

Yes, it's an easy calculation.

It seems however, that many ratio rockers are not geometrically what they are advertised as. So you really need to put it all together and measure lift directly at the valve.

Max

Mon Aug 04, 2008 12:50 pm

Sigurd wrote: I would think so, because I thought cam lift is given at the lobe, because the cam doesn't know what rockers are being used.

That's what you have to verify. Some cams are advertised for their lift at the valve when used with a specific ratio rocker. If you're trying to see how a cam advertised like this would respond to a different ratio, divide the valve lift by the advertised ratio and multiply that value by the ratio of the new rocker.

Mon Aug 04, 2008 12:54 pm

Max Welton wrote: It seems however, that many ratio rockers are not geometrically what they are advertised as. So you really need to put it all together and measure lift directly at the valve.

And that ratio is also affected by the rocker geometry set-up by the end-user, variation in the location of the rocker-blocks and mounting locations in different head-castings, and the length of the valve-stems.

Some rockers are known for not meeting the advertised ratio, but other factors outside of the actual rocker-arm may affect the actual ratio. Like Max suggests, your best bet is to install and measure the lift directly at the valve.

Mon Aug 04, 2008 5:06 pm

[email protected] wrote: SRP1 wrote: .430" x 1.25 - .006" - .002" = .530 actual valve lift.

If you're using an Engle 110 as an example, isn't the .430 lift calculated at the 1.1 ratio of the stock rockers? I thought the lift at the cam was only .391. Shouldn't your calculation be;

.391" x 1.25 - .006" - .002" = .481 actual valve lift?

The numbers I posted are purely for example, I used no cam in this.
Yes you figure this from cam lift out, like this:

Cam lift x rocker ratio = theroetical lift @ valve
Then subtract for valve lash and push rod deflection and you come up with a more realistic actual valve lift.

Mon Aug 04, 2008 5:13 pm

SRP1 wrote: Then subtract for valve lash and push rod deflection and you come up with a more realistic actual valve lift.
And I suppose you should use valve lash at operating temps, not the cold lash. See, now you gotta take into account which type of pushrod is used (steel or aluminum) and what the target "operating temp" is ... ugh.
:lol:

Max

Mon Aug 04, 2008 6:04 pm

SRP1 wrote: The numbers I posted are purely for example, I used no cam in this.

Oh, OK, I just didn't want to confuse craigman that originally started the post. Thanks!

Tue Aug 05, 2008 8:58 pm

Max Welton wrote: SRP1 wrote: Then subtract for valve lash and push rod deflection and you come up with a more realistic actual valve lift.
And I suppose you should use valve lash at operating temps, not the cold lash. See, now you gotta take into account which type of pushrod is used (steel or aluminum) and what the target "operating temp" is ... ugh.
:lol:

Max

Correct I used chromoly push rods as the example in my first post and a estimated lash of .006" hot.

Tue Aug 05, 2008 9:05 pm

So forget the lash cap.

Tue Aug 05, 2008 9:12 pm

Geese, you guys are getting technical! :lol:
Thanks again on the replys. I figured i was right, just wanted to make sure.
Thanks!
Craig