Thread: cam choice

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Just how would somone go about getting the size cam you would need for the engine your building. I have always went buy what rpm I would like to run and buy one for that. I have never picked one buy the size of the head. That must be wrong buy what I have seen in the past week or two.

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

Good reply here, Joe. It's all about compression ratio. It's important to get it right before you look for a cam...and know what it actually is, not what you estimate it just might be. To add a little to Techinspector, as important as the static c/r is, there is also dynamic c/r....that's the actual c/r the engine is seeing while it's running. You could theoretically have a static c/r of let's say 9/1, so you figure you're OK with pump gas...but lets say the cam you choose has a very early intake closing point...traps a lot of charge, and makes a lot of power....Well, it might also make the engine detonate like crazy, and of course that's very bad. So all this stuff requires a lot of research. It's best to talk with a good cam company that will take all the info on what you're trying to do, and build or suggest a cam that will work for you. As mentioned earlier in another post, I prefer to go with the smaller, maybe lesser known company's, mainly because I like to talk with the guys who actually grind the cam, and maybe even design it themselves. Most all the people on the cam tech lines at the big companys are just reading off a screen, and suggesting from a set list, with no real experience in anything else. Unless you're a distributor or some higher up, you wont be able to speak with a real hands on person. Im not saying that Crane is like that, I personally have no experience with them. I know I have not had luck at all with Comp Cams, and that's the first one most people think about, cause their name is all over and they're so big. Try Crane, Isky, Howards, Crower, etc. John

Real good advice John.
Joe, call Dimitri Elgin at Elgin Cams. Here's his website, about halfway down the page is his phone number and note the sentence "Feel free to call if you just need some advice"......
http://www.elgincams.com/main.html

I was always going to call but I would rather get an idea my self like you said. I know to go buy compresion but looking in jegs and summit they dont list the com. on ever cam just certain companys do.

cam size is dictated by rpm and if you have the heads that will work for that rpm the size of the engine the intake the cc on the intake side of the heads and the comp is what it takes the cam to work rigth at the rpm .you have a small engine 427 with big heads you will have to run this hard to get it to make the same hp as a 454 or 496 the bigger engine will make more torque at lower rpms and the rpm range say 3000 to 7200 +dose not mean you will have to run at 7200+ to get to work but the bigger cam in a small engine will have to be runed a lot harder when i build a engine i think what dose this thing have to do?? how much comp can i run in this dose the customer want to run race gas??is it in a lite car or truck if raced how long is the track what intake will it have. a stick or auto trans in it . do they have parts they want to use . this some times can not the best comb on big block i try to get the biggest engine they can afford and not run it as hard and it will not need so much comp to get it to run but a bigger engine can take more cam so you need to know what size big block are you goning to build 427 or 526 ?? the heads you have would work best with a bigger engine as for just picking out a cam out jeg.s you need a cam company books try to fined some that used their cams .but getting the rigth cam is not all ways easy i have used comp lunati crower crane howards isky with good luck you need cam books from more than one to lookat to pick a good cam and if you cant find one you will need a custom cam$$