Gross Cam Lift to Gross Valve Lift

[zack_280]

I was just looking for some information on total lift based on measured cam lift and didn't find anything on the site. It may be here or buried in a thread somewhere, but I figured it would be useful to someone like me who was looking (or hoping) for a quick answer. My quick answer only brought up more questions, but I did get an answer.

 

Someone here posted their cam profile and total valve lift. I used that to calculate the multiplier to get from cam lift to valve lift (see below).

 

 

-------------------Cam Lift----------Valve Clearance-------Valve Lift--------Multiplier

Intake---------- 0.333-------------0.01-----------------------0.5053-------------1.564

Exhaust-------- 0.334-------------0.012----------------------0.5053-------------1.569

 

I wasn't sure (and am still not) what my cam profile was since I purchased the car with this Sunbelt/Jim Thompson built engine already in it. But based on a post from someone else they had a similar engine built by Sunbelt and the gross valve lift was "493". Based on my measured cam using the multiplier above my intake is (0.325-0.01) X 1.564 = 0.4927 and the exhaust is (0.327-0.012) X 1.569 = 0.4942.

 

Now this was in no way a scientific experiment, and actual lift will vary based on the assembled geometry of the valvetrain. However, if you are looking for a quick answer this might be useful.

 

Another reference would be Frank Honsowetz book "How to Modify your Nissan & Datsun OHC Engine". He suggests using a multiplier of 1.5 as a rule of thumb.

 

Another interesting thing I found was the grinder used for my cam. It is an outfit in California called Integral Cams. Here is a link to their website. It looks like they have some standard grinds for street and track. I spoke to them the other day and the recommended talking to Jim Thompson (Sunbelt) or Jim Wolfe for technical info on custom cam profiles. I'm still waiting on a call from Jim Thompson.

http://www.integralc...om/nissanL6.htm

 

Enjoy,

 

edit: I pasted an excel table. It looked fine in the original write-up, but didn't translate on the final product.

Edited January 11, 2011 by zack_280

[Leon]

This multiplier is called the "rocker ratio." Searching that will yield more results.

 

FWIW, the stock rocker ratio is 1.48.

[Tony D]

The rocker ratio is actually variable, not fixed. It changes as the cam nose wipes across the rocker contact pad from tip to rocker pivot end. For 0.001" of lobe movement on the tip end you generate FAR less lift at the valve than for 0.001" of lobe movement on the pivot end of the pad. It's this variable ratio that requires the assymetric grind profile to get proper valve acceleration on opening/closing events.

[Leon]

The rocker ratio is actually variable, not fixed. It changes as the cam nose wipes across the rocker contact pad from tip to rocker pivot end. For 0.001" of lobe movement on the tip end you generate FAR less lift at the valve than for 0.001" of lobe movement on the pivot end of the pad. It's this variable ratio that requires the assymetric grind profile to get proper valve acceleration on opening/closing events.

 

That's a valid point Tony and it's a good transition into a deeper discussion, but when talking max lift (as the PO was) stock rocker ratio is 1.48.

[Tony D]

That's a valid point Tony and it's a good transition into a deeper discussion, but when talking max lift (as the PO was) stock rocker ratio is 1.48.

 

I would debate that simply on the simple geometry involved. Maximum lift will have the most advantageous rocker ratio (closest to the pivot point, slightly over 1.5 as he has found in his calculations), while valve opening will have the ratio closer to 1.48

 

This is easily graphed using a blueprint of the valvetrain and measuring distances from pivot to actual contact point on the wiping pad. Closer to the pivot you are, the higher effective ratio you will have.

 

This is how some commercially available, and similarly configured 1.4 (designed) ratio rockers are useable in our engines somewhat interchangably with the Nissan Parts---they have a fixed pivot point of contact, and therefore will return similar performance despite having 'less' ratio---less, but constant. It gives similar performance to a variable ratio which goes from 1.48 to 1.56 (or more, depending on how agressively you wipe off the end of the pad...)

[Leon]

I would debate that simply on the simple geometry involved. Maximum lift will have the most advantageous rocker ratio (closest to the pivot point, slightly over 1.5 as he has found in his calculations), while valve opening will have the ratio closer to 1.48

 

This is easily graphed using a blueprint of the valvetrain and measuring distances from pivot to actual contact point on the wiping pad. Closer to the pivot you are, the higher effective ratio you will have.

 

This is how some commercially available, and similarly configured 1.4 (designed) ratio rockers are useable in our engines somewhat interchangably with the Nissan Parts---they have a fixed pivot point of contact, and therefore will return similar performance despite having 'less' ratio---less, but constant. It gives similar performance to a variable ratio which goes from 1.48 to 1.56 (or more, depending on how agressively you wipe off the end of the pad...)

 

That makes sense, thanks for clarifying that.