Can someone post a "Picking a cam 101"

[Total Sign]

I know you would use a different cam for a blower motor than one you would use for a high revving NA motor, how would you choose? Not picking one, just a question I have never asked.

 

Thanks

[Dave1647545494]

you have to pick a cam based what you plan to do to the car. you kind of have to match the parts.

 

while cam A works in one car it may not work in another with an identical engine used in a different type of racing. IE you don't want a full on drag cam in your roadrace car becuase it won't use the power band right.

 

theres some rules of thumb but nothing is really set in stone becuase applications vary between different kinds of motors.

 

lobe separation (i'll call it tight or loose) is the distance in degrees between the top edge of the intake and exhuast lobes on each cylinder.

 

a tight cam is going to have lobe centers that are 110 degrees or less apart

 

a loose cam would be 110 degrees or more

 

stock cam profiles usually come in in the 110 to 114 range

 

then you have lift

 

most of your stock cams are at about .400-.500 lift and have a power band from right off idle to 5k or 6k rpm and then they fall off

 

the more you lift you have the more air you can flow but you sacrifice bottom end power and torque becuase the engine has to rev higher to see power gains.

 

then you have duration this is the amount of time the cam holds the valve open at a given lift. duration is measured in degrees but can be over 360 becuase cam lodes are ground as an elipse not a circle

 

a stock cam is going to have duration usually in the low to mid .200's.

 

overlap- this is the amount of time (measured in degrees) that the intake and exhuast vlave are open at the same time on the same cylinder

 

putting the numbers together

 

a tight cam with a high lift and long duration

lets say 108 lobe centers .650 lift and 300 degrees of duration is going to make power to 7-8k rpm and increases the effeciency of the engine by using the fact that the exhuast and intake valve are open at the same time to pull fresh air charge into the engine by sucking some of it into the exhuast

 

now the same exact cam with say .490 lift is going to make more torque becuase its peak rpm is much much lower.

 

now take loose cam with the same lift and duration specs as the first tight cam and you get a profile more suited to a high rpm forced induction race motor you don't need the overlap becuase the forced induction is putting the fresh air charge into the motor.

 

split pattern cams

 

a split pattern cam uses different lift an duration specs on the intake and exhuast to achive a desired goal.

 

you might run it tight but have say .590 intake lift and .600 exhust lift and say 300 degrees duration on the intake side and 312 on the exhuast side you would use a cam like this in a small valve N/a motor to help pull fresh air into the engine and hold the valve open forever to cram as much into the engine as you can past the smaller valve.

 

a forced induction car may use a split duration/lift cam to favor exhuast breathing to help fully empty the cylinder between cycles.

 

Custom grinds

 

this where things can get really crazy. to get max power out of a given engine you can vary lobe seperation lift and duration between cylinders to make them all make the exact same amount of power.

every cylinder is alittle different due to intake manifold design induction type and exhuast.

 

Advice-

 

when you build an engine first decide what your going to do with the car primarily and be realistic or you won't be happy.

 

match all of your parts to the rpm range you plan to utilize the most.

while cam choice is important here improper matching of parts may give dissapointing performance. if you just stuff a great big nasty race cam in your stock motor it'll sound cool but you'll see very little gain or possibly even power loss with out the supporting parts. engine design is important too a motor that has a redline at 6k is not going to gain a thing by using a cam that makes max power at 7000rpm.

 

call the techline at your favorite cam grinder. talk to someone about your combination and ask what they recomend, remember to be truthfull with them about your true intent with the car or they won't be much help.

[desperado]

Dave, one small thing, cam duration is measured in crank degrees and not cam rotational degrees, and a cam turns at 1/4 speed that the crank does.

 

That's how you will get durations of 300+ degrees on a cam.

 

And you're partly right about stuffing a race cam in a stock motor, any cam that is lifting over .550 at the valve, will most likely coil bind the spring in a STOCK motor, this is when the coils in the spring all touch and the spring can not be compressed any more. Having this happen will break things like the cam or the rockers, bend the pushrods and basically turn your motor into a pile of shit.

[vitamincj]

Originally posted by desperado:

Dave, one small thing, cam duration is measured in crank degrees and not cam rotational degrees, and a cam turns at 1/4 speed that the crank does.

I thought my cam turns at 1/2 speed of the crank? I thought it was that way at least for all single cam 4 stroke motors?

 

CJ

 

Edit: Westerville Public Library has a couple of very enlightening books on cams. I will have to look up the names.

[desperado]

Yeah, I am a dumbass, your right. I looked it up. It IS half speed.

[Total Sign]

Thanks, those help quite a bit.

[Total Sign]

Could you guys post a few examples...

 

A basic box stock daily built to last over 100k miles, what would the cam specs be? Just a tame, plain jane motor, lets say a 350 cubic inch small block in a truck.

 

Now, lets say the cam specs for a NA small block that is making 400hp, 400tq.

 

And, a 500hp blower 350"

 

I know there is infinite variables for this, but if you could give me some more examples, and please post what the engine is in/what all has changed between the others, and what is done to the motors.

[vitamincj]

"The Best of Hot Rod Magazine Volume 11 - High Performance Camshafts" is the best one that I read. The first chapter explains what you need to know. I will see if I can get my hands on it again.

 

CJ

[desperado]

I have found that one of the best things that you can do is get one of the Dyno SIMS (I use Desktop Dyno 2000) and put in your motor components. Then start putting in different cam specs for the common cams and see what you get. The only drawback to this is that the dyno software will do the math, regardless of what is phyically possible, IE valve lift of 1 inch on stock heads, 3000 CFM carbs, and other odd shit like that. Nor will it ask about other internal parts that will limit the operating range of the motor, like trying to spin a stock lower end at 9500 RPM, the motor WILL fail (SBC is what I am refering to) but it's completely possible to put aftermarket heads, cam and intake on a stock block that will flow the necessary air to make the motor spin 9500 or more.

 

As long as you aware of the phyical limitations of the parts you have and don't exceed the limits, then you will be able to see what cam works best.

 

As far as is it close. I did Nates GTP the other night, and it was 15% over the wheel dyno sheet of what the car actually did, so it's pretty damn close.

[Total Sign]

Pretty fun program!

[twistedfocus1647545489]

I agree, Desktop Dyno takes in just enough info that it seems to be pretty real-world as long as what you put in is fairly accurate (port-flow..etc)

It's a great tuning tool to see what kind of improvements you can make. Even if the "baseline" you make is not perfect... you can get a pretty good idea of waht kind of gains you can expect from other mods... (even if you measure it in Purple Elephants to begin with.. you can kinda figure what will make significantly more "purple-elephants" w/ changes in camshaft timing..etc.)