the mystery of cam regrinding

jmshepard · 11-02-2004, 06:35 PM

Can someone please explain how a cam can be reground to develop higher lift? As an example, I was told that my stock 1980 cams can be reground to a profile that matches a 964 or 20/21 cam. How is this possible? Don't these cam profiles produce higher lift than the SC cam?
Thanks,
Jim S.

Oldporsche · 11-02-2004, 06:43 PM

Check out a couple of Hot Rod technical magazine articles.

By regrinding the base circle of the cam, the overall lift can be increased.

Good luck,
David Duffield

jmshepard · 06:50 PM

Thank you David, now I understand. The center of the eccentric is moved!

Andras · 11-03-2004, 06:03 AM

Ummmm!

Is that quite right? I would think that lift cannot be reground. The total distance from the centerline of the cam shaft to the topmost part of the cam lobe would determine the total lift.

In order to increase the lift, one would have to add more material, thereby increasing the distance from the CL to the top of the lobe. Once more material is added, THEN you would grind it smooth, and thus get more lift.

By simplly moving the center of the lobe, you would change only the timing of the lift and its duration.

Zeke · 11-03-2004, 06:25 AM

Andras, they reduce the size of the base cirlce which effectively makes the lobe higher. At the heel of the cam, the follower is closer to the center, thus when it gets to the lobe, it is pushed up higher.

The grider may take a little off the top of the lobe to increase duration and he may grind one side of the lobe to change the overlap.

RSupdate · RSupdate's Garage

Camgrinder could clear this up instantly

jpnovak · 11-03-2004, 06:45 AM

The description above is correct. Here is an overly simplified explanation to clear up a few details.

Imagine that you have a base circle of 1 inch and a bump (lobe) on one side that is 1.5 inch tall. This gives an effective lift of 0.5". If I grind off some from the base circle (-0.25") then the effective lift is now 0.75". A cam will small lift and duration values can be reground to a hotter profile be removing material from the base circle and surrounding areas. This is tricky. There can also be problems. there is only so much material that can be removed before the ramp rate is too quick. This is how fast the lobe lifts the rocker arm. To fast and the parts can't keep up. It would be like trying to climb a hill. If the hill is steep, you can't get up. If its a gradual incline you can run up really fast with little feeling for the change.

Duration comes from the bump (lobe). If the lobe is wider there is more duration, ie, the valve stays open longer. Duration is usually given in degress of crank rotation. Some profiles may take advantage of a different slope on the closing or opening rate of the valve.

since there is an intake and an exhaust valve, each with their own lift and duration there is another term called overlap. This will be the number of degrees of crank rotation where BOTH valves are open.

The basic concepts are simple. Putting it into practice is more art than science.

bigbrotherjohn · 11-03-2004, 07:00 AM

So would it be true to say that when using a reground cam that has had the lift increased that some component of the valve train has to be swapped for a longer component? For example, you could increase the length of the pushrods or even the lifters.

jpnovak · 11-03-2004, 08:31 AM

No, the difference in heights is usually small. For example the lift difference between an SC and 964 profile is 0.02" on intake and 0.035" on exhaust. This is just taken up using the valve adjuster screws.

camgrinder · 11-03-2004, 04:24 PM

Jamie is right on the money.
The real trick is knowing which profiles can be ground on which camshafts and what not to try.
Porsche used base circle sizes from 1.150" all the way to 1.270"
Most of the Early racing cams were ground on the 1.160 base circle. SC cams have a base circle of 1.265". Grinding the 964 (C2/C4) profile on SC cams reduces the base circle to about 1.225" What the rocker arm sees is 1/2 of the reduction since we measure the base circle accross the heel of the lobe. So the rocker has to take up .020" of travel by moving the adjusting screw closer to the valve tip.

Oldporsche · 11-03-2004, 05:45 PM

Yo Camgrinder,

For years the rule of thumb for cam regrinds has been that you could grind the next step up in cam grinds on original cores. Thus an "E" grind could be done on a "T" core. An "S" grind could be done on a Solex grind. What do you think?

Do you have a favorable opinon on hardweld grinds?

Thanks,
Enquiring minds want to know.

David Duffield

camgrinder · 11-03-2004, 06:34 PM

Hi Dave,
Thats a good rule of thumb. Some dont fit as well as others. Lately I have been doing the Solex on E cams using the E lobe centers (102) which gives the Solex cam a wider power band.
Nice for a street engine.
My opinion on hardwelding...... it is only needed to repair a single lobe on a cam. I have new castings for all the engines and dont need to do full weld ups.

strokher racing · 11-03-2004, 06:56 PM

Hey John I will have the complete flow numbers on my heads in the next day or so. The heads flow a ton! Exhaust was right around 230 at 500 lift on a conservative bench with no pipe

Intake is way up there is all I will say on the boards lol. Think we can come up with something for my little 3.5 turbo motor? Not sure the grind I have now will do my heads justice. I sent you the specs that were similar to the evos

Eric Hood

fredmeister · fredmeister's Garage

Remember too that if you change base circle and increase lift the rocker arm can be made to contact via the adjusting screws. However, the angular orientation of the rocker arms to the cams and valve tips will not be nominal, that is to say different then originally designed with stock cams. This effects effective lift ratios, contact points and other kinematics of the valve train that usually are detrimental to optimum performance and wear. Alot of design goes into the geometry of all these parts if done properly and small changes have sometimes big effects dynamically. What can you do? Other than in my opinion rewelding cams and then recutting the profiles is a better way kinematically since you restore cam lob profiles back to optimum.

camgrinder · 11-03-2004, 07:02 PM

Hi Eric,
Yes we can come up with a grind. Thats a nice exhaust number,
what pressure drop was it tested at? (28"?)

camgrinder · 11-03-2004, 07:14 PM

Fredmeister,
The only way to restore the factory geometry is to use original cams (ie, S profile on S cams) new castings, or reweld the camshafts.
An extremely reduced base circle may have an effect on the valvetrain, however a slightly reduced base circle is not a problem. I have compared back to back graphs on reground and factory cams and the valve lift curves are similar.

I prefer making cams with full size base circles.