SC or 964 cam regrind?
 

It looks like I should have my cams reground as part of my engine rebuild.:eek:

http://forums.pelicanparts.com/uploa...1196730278.jpg

I am keeping the CIS, with the CIS pistons at 8.5:1 CR. In this configuration, and for street driving, will there be any benefit/power increase if I have the cams reground to the 964 profile, or should I stick with the SC profile? Does anyone run this combination 8.5:1 pistons/964 cams?

Aurel

many on here consult with and send cams to "camgrinder", he will have knowledge of pros and cons of your question

just be careful with camgrinder... He does great work, but can be very slow. held up my rebuild for over a month.

I have also read that rocker surfaces will wear faster with 964 profile, and need to be resurfaced as well. That would be a reason to stay with SC profile. But all things wear eventually. The question is how faster?

Aurel

Hi Aurel,

You should contact John Dougherty at www.drcamshafts.com he will be able to answer all your cam questions. I am sticking with my CIS also, but 9.8-1 compression, so I'm going with the 964 or John's DC20 cam.

Good Luck.

I could have provided better contact detail camgrinder = John Dougherty

I had my cams and rockers resurfaced by Dougherty as well. He was quicker than the machine shop by a fair margin. I think anyone in the business can get spots where they are backed up. I think if you compare John's pricing to that of say, Webcam, it's worth a little delay.

I'm not understanding your comment about the wear; you are planning on resurfacing the rockers for your build, right? The higher lift and duration of the 964 or similar cam MAY wear rockers "faster", but it has to be negligible. Certainly not enough to wear an engine out prematurely.

also I think it is recommended when resurfacing the cam you want to have a new correct surface on the rocker - you want these surfaces to be "parallel". this brings up another connected subject and that is rebushing the rockers along with cleaning up the rocker shafts if needed... and at the same time the shafts need to fit properly in the tower - IMHO there is a number of connected issues. - I'm sure John Dougherty knows all about this and may provide all these services.

Yes, John does the rockers also for about $30 each. Just e-mail him your questions and he will get back to you the same day.

With 8.5-1 compression I usually recommend the DC15 profile. Its one step down from the 964 and does a little better in a dedicated street car.


A couple reasons to purchase new, or rebuild your rocker arms.

Bushing wear. The will have some amount of wear, with a certain amount of taper. Putting a tapered bushing rocker arm onto a new cam lobe can cause edge loading. Bad!

Also, the rubbing face can be worn or even pitted. This can cause the new
lobe to scuff during break in. Each camshaft profile puts a unique print onto the rubbing face of the rocker arm. The more aggressive profile, (964 with high lift and relatively low duration) use up more of the rocker face. Some profiles use the rocker face closer to the bushing end, and some out towards the outside edge.
Here is a picture of two used rocker arms. One is out of an early 911 T engine, the other out of an SC. If you put the 911 T rocker on the SC camshaft, the cam lobe would ride off the worn section onto the unworn section. The small lip between the worn and unworn section can squeeze the oil film out and lead to premature wear.


http://drcamshafts.com/911ra/rocker.JPG

John,
very well explained, thanks
-Henry

Thanks John ! Your DC15 is definitely what I was looking for. I`ll be in touch.

Aurel

Aurel
Take note of this thread.
John

http://forums.pelicanparts.com/porsche-911-technical-forum/200267-new-chains-without-new-sprockets-read.html

I wonder if there is a way to check how worn my chain is. I imagine that the lateral bending radius of the chain over a fixed distance would be good indicator. Then, if it was in specs, I would not change it :D.

Basically, the issue of chains and sprockets is similar to the issue of cams and rockers: it may be a wrong idea to replace only one of the wear item and not both.
It really is just a matter of investment and return on investment...
I am looking for ways to minimize my investment and maximize my return :cool:
I wish I knew instantly by looking at parts which need to be replaced and which can go for another 50k miles. I guess this is what experience is.

Aurel