Question for our engine specialists.

[Wes Bender]

Greg; thanks for discovering that,
Ron; thanks for verifying it.

++1 Isn't it great that we have folks as talented as we do, and that they are gracious enough to share their wisdom with us? I've belonged to quite a few other car groups, but none as good as this one.

[DonCichocki]

Got my engine apart and decided to make an arbor like Ron's and see the actual offset between the lifter and lobe in the horizontal plane. I used the original #15 narrow lobe cam and a new wide lobe cam to check the offset. Of interest is the wide lobe cam has slightly different C/L of the lobes than the narrow lobe cam. The end result is the offset on the narrow lobe is about .090", the wide lobe about .120".
The bottom scribe line on the lobe is the lifter C/L, the top scribe line is the C/L of the lobe. You can see how the lifter will rotate with this setup even with flat faced lifters and non-tapered cam BUT it will rotate better with a tapered lobe and radius/tapered faced lifters.

[C J Murray]

Don, don't you need to verify that all 8 lifters are offset the same amount?

[Mike Horton]

Don, whos wide lobe cam is that one? Tapered lobes? Tappets radiused, and of so, by whom? Inquiring minds...

Thanks!

[DonCichocki]

Cliff,
Out of curiosity I'll probably check them all, but knowing Porsche they will probably all be close.

Mike,
The wide lobe cam is a recent Web Cam that, because of a miscommunication by me, was ground flat. It has been sent back to have a taper ground to the lobes if the base circle will permit it. The lifters are new Web Cam radius faced to, hopefully, match the reground cam.

[M Penta]

Interesting read and good to know, thanks for posting!

[Ron LaDow]

Used the ID-end of a (plastic) dial caliper to check dims:

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I have no way of knowing if the intent was to match the contact ratio, but it is as close to 2:1 as you can measure on a screen on both of them, meaning the rotational torque should be close.
The wide lobe will obviously reduce the unit contact load on a flat lifter. A radius on the lifter means a theoretical point contact and an infinite load, but equally obviously, the elastic qualities of the material means there is no "point" contact and therefore some measurable load.
For street valve spring rates, it's probably irrelevant. Vic, CJ, Jacques, other race builders - what do you use with the rocket-launch springs? Radius on the follower? Taper?
Oh, and Don, what are the base circle diameters? Just some added data...

[DonCichocki]

Ron,
Both cams have a base circle of 1.028". Interesting because one is reground by Web the other wide lobe is new by Crower.

[C J Murray]

Cam wear in a racing engine is dependent on how the cam controls the valvetrain movement and the ability to use the lightest valve springs possible. These two issues are interrelated.

Cam wear comes from excessive spring pressure and excessive spring pressure is required when a cam is poorly designed. Race cams need to be aggressive but some are so poorly designed that they put the valvetrain into horrible gyrations that beat the parts until they fail. To compound the problem the engine builder installs stiff springs to get things controlled and that overloads the lubrication at the lifter and elsewhere. Solution, buy your cam from a company with the technology to provide smooth operation without excessive valve spring pressure. A 356 race engine will rev to 9000 rpm without using crazy spring pressure.

Porsche was never known for having valvetrain wear or breakage in the 616 street cars and always used very mild spring pressures so you can assume that they understood cam design. Some other brands did suffer problems and had to make changes. I think that Triumph had issues with the early TR cars.

[Jacques Lefriant]

Hi Ron
with the new semi finished cam blanks you have to specify the base circle to the cam grinder. if the base circle is not within a certain range the cam /rocker ratio will be different than intended. some cams have reduced base circles to get more lift from a used cam and also to get clearance to the rod. you knew that already.
j