Rocker ratio

[neilca]

What is the rocker ratio for these six cylinder engines?

Thanks,
neilca

[ischmitz]

1,45:1 average and 1,5:1 on 906 engines (taken from Elgin's Website)
Ingo

[Wayne 962]

FYI - the reason he's asking is to convert cam lift numbers into actual valve lift numbers...

-Wayne

[911s]

So, does this mean that the lift spec's listed in various literature are actual lift values off of the cam lobes, and that the actual lift is about 1.5 times that?

Wow, that would be impressive. I didn't realize that Porsche engines used such high lifts. So, that would mean that an E cam, with an advertised intake valve lift spec of .408 would have an actual lift of .5916" (.408" x 1.45). That's pretty high. Guess that's why they come from the factory with dual valve springs!

Get this, that would mean that a solex cam gives an actual intake lift at valve of .63655", and .66555" for an S cam!

[ischmitz]

No, the valve lift is measured on the valve spring retainer. So the lift values are actual valve lift specs. Think about how you time the cams with a dial gauge and where the dial gauge measures the lift of the intake valve at TDC overlap.


The highest point of the lobe less the lobe base circle diameter is 1.45 times smaller than the valve lift. Still the highest point on the lobes has almost the same dimension as the bearing surfaces. If it would be higher you would have to cut the cam tower in half to insert the cams

Ingo

[Wayne 962]

Actually, it's not as simple as that - this topic can be very confusing. You must check the numbers that you're looking at to see how the lift is measured. Some of the Porsche specs are measured in the actual lift of the camshaft (Cam Lift with Base Circle Radius, as dictated in the 72/73 spec book). Other measurements may be given in actual valve lift.

In addition, there is the European standard method of measurement, which dictates that you measure valve action (lift) with a rocker/camshaft clearance of 1mm (.040). While this is a standard, it's not what you set the 911 valves to normally, so lifts measured and calculated with this standard don't correspond directly to the actual valve lifts. In reality, the 911 clearance is .1mm (not 1mm), and thus the valve lift measurement is off by a function of .9 multiplied by the ratio of the rocker arms. It gets very confusing, and I spent many hours trying to get it straight (finally did) for the Engine Rebuild Book.

Where this affects measurements most is in the calculation of when the valve actually opens. With the European method of measurement, you don't get realistic numbers for opening and closing because the standard dictates the larger clearance than is normally used on the 911.

It's very confusing. Paul Frere's book does a good job of giving actual lift numbers (based on the .1mm clearance) in Appendix B.

-Wayne

[911s]

Shucks! And I thought I was going to get Pro Stock lift values out of my tiny Porsche engine. Oh well.......