Thread: cam choice

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joe, I don't think anyone is saying buy the cam by the size of the chambers. What we're saying is match the cam to the static compression ratio. Let's take a for instance doing it your way, choosing by the rpm's where you want to make power. Let's say that you have a 9.0:1 motor and you want to make power from 3,000 to 6,500, so you choose a cam that will do that. The problem is that with the long timing, the intake closing point will be too late to capture a good charge of fuel/air mixture and your motor will be a dud. Let's also go the other way and say that you have a 11.0:1 motor and you want to make power from idle to 4,500 rpm's. The short timing on this cam and the early intake closing point will capture an excessive amount of mixture and make so much cylinder pressure that there is no pump gas on the planet that will keep it from detonating. According to Crane, you want to shoot for a maximum of 165 psi cranking pressure in order to operate the motor on pump gas.

Crane Cams has the best catalog in my opinion, showing the recommended static compression ratio to run with each cam. If I want to run a different manufacturer's cam, I find the specs on a Crane cam that fits my needs and apply those specs to the other grinders published specs. Of course, the best way is always to phone the tech line and get a recommendation from the cam grinder you're gonna use, but I like to get close myself before I do that.

The first thing I do when I'm planning a motor build is to determine the operating range I'll be using the motor for, then go looking for a cam in the Crane Catalog, using their recommended static compression ratio for that cam. Then I choose parts to go in the motor that will give me the middle range of that static c.r. range listed by Crane and I have a coordinated package that will do what I want it to.

That's the problem with these first time kids who want to hop up their motor. They have a c.r. of maybe 8.5 to 9.0 in their stock motor and a deck height of maybe 0.040" to 0.070", resulting in a squish approaching 0.100" with the head gasket figured in, so they pick a cam with long timing thinking that it's that easy and they're gonna build a killer motor and the motor won't pull the hat off their head at low r's because they haven't assembled a coordinated package. Of course the motor doesn't ping in spite of the crummy squish because they're producing insufficient cylinder pressure for it to ping, even on the crummiest pump gas.

Denny posted a couple of excellent articles from Crane yesterday..

http://www.cranecams.com/?show=faq&id=2

http://www.cranecams.com/?show=faq&id=3

And here's some stuff that I started puttin' together. I particularly like the article by Dimitri Elgin....

http://streetmachinesoftablerock.com...topic.php?t=78