Ok i'm done trying to figure out how a cam works and what kind of cam will work for my engine. I need someones help. I'm buying some s/r torquer heads for my tbi 305 and since i'm going to do the heads a might as well get a new cam right? so i'm wondering what is a good AFTERMARKET cam i could get that will add a nice amount of power but will still be good to drive every morning to work. My friend tells me i should go with the powermax series by crane cams but i still dont know what type of cam, how much of this and that to make it work for me.Please please help I'm sick of trying to figure it out. thanks

 

The cam opens the valves. I'm sure you knew that, but let's start there.

An engine is 2 things: the actual "engine" function, which is, converting heat to mechanical energy; and a pump. The cam's function is most closely related to the pump part but it affects the other somewhat as well.

Torque is another name for "work" or "energy". It is exactly equal to the amount of energy released in one rotation of the crankshaft. The enrgy comes from burning fuel. Therefore, the more fuel you can burn per cylinder firing, the more energy (torque) you get. Torque, work, or energy, in a rotating object, are all measured in ft-lbs. This is why that whole {insert brand here} "is a torque monster" bunch of crap, is crap: the gasoline molecules DO NOT release energy (torque) in proprtion to the brand of the cast iron, but rather, ONLY in relation to how many of them there are. Obviously in order to burn gasoline, you have to have air. Air is big, and there's not much stuff in it. Consequently, you have to move ALOT of it (many cubic feet er minute, or CFM) in order to support lots of power.

Power is equal to the time rate of doing work. RPM is how we describe the "time rate": so many engine revolutions per minute. Recall that torque is equal to fuel burned per cyl firing. Power (measured in horsepower) at any given RPM, is the torque at that RPM, times the RPM, times a constant to make the units come out right (the constant is 1 / 5252.11, if you're using HP and ft-lbs as your units). What this is telling you, is that if you want higher horsepower, there are 2 ways to get it: either burn more fuel per cyl firing, or get more cyl firings per unit time (higher RPM).

To get more RPM you need FLOW. In fact, an engine's HP can almost be DIRECTLY calculated from FLOW. You can look at the definition of HP above, and it's pretty obvious that the # of cyls filled per unit time, = total engine flow; and that the higher the flow rate into and out of the engine is, the more times per second or per minute you can fill and empty each cyl.

OK, enough of the basics. But, think for a minute about how you would make a pump with pistons and cyls work, at different RPMs.

If it was at a very low RPM, you'd want to open the intake valve at the instant the piston reached the top; hold it open while the piston descended; and close it at the exact instant the piston reached the bottom. Likewise, on the exhaust side, you'd want to open that valve at the instant the piston reached the bottom, and close it at the instant the piston reached the top: which would of course be the same instant that the intake would re-open.

Now think about what happens as RPM increases. Air has mass, meaning it takes time to get it into motion, and time to stop it once it's moving. Additionally, air molecules aren't a "solid" thing, like a rod of metal or something; they're "springy". IOW, think about how an air molecule right above the carb "knows" that a piston is going down: all it knows, is that the air molecule right below it moved, which "knew" that it was time to move because the one right below it moved, which moved .... etc., all the way down into the cyl. This "ripple effect" is a pressure wave, which is simply another word for "sound". In fact, the speed at which the "information" is transmitted among air molecules as to where the pressure is lower or higher, passes through the air at the speed of sound.

So, as your pump's RPM increases, you have to open the intake valve earlier and earlier, so that as the piston starts to descend, the valve is already sufficiently open that air can begin flowing; and you want to hold it open for some time after the piston reaches the bottom and starts coming back up on the compression stroke, because you've already got this whole column of air rushing into there, and you can take advantage of that inertia to keep filling the cyl some more even after the piston has started moving up, because the air molecules farther up the column don't "know" yet that the piston has changed direction. Similarly, on the exhasut side, you can open the valve before the piston gets to the bottom, because it won't let out much of the spent material for a little while, because the valve is only a tiny crack right at first; and you can hold it open past TDC on the intake stroke, and let the rushing out of the exhaust help suck the remaining gas out of the cylinder, and even help draw in fresh intake charge. This is the period called "overlap", and putting it ot use is key to making high HP (RPM).

The fraction of the engine's rotation that each valve is held open by some standardized amount, is called "duration". It is expressed in crank degrees. There are 720° of crank rotation (2 full turns) per complete cyl cycle; so if the valve was open exactly ¼ of the time, i.e. the low-RPM situation as described above, then the duration would be 180° (¼ of 720°).

To make an engine work better at higher RPM, you hold the valves open longer and longer; which extracts a penalty, of making it work WORSE at lower RPMs. Always remember that: everything you do to improve high RPMs, COSTS YOU at low RPMs. So cam design is a tradeoff.

For normal street engines, you want an intake duration between about 205° and about 230°. Less than 205, you get a tractor motor (high low-RPM torque, poor high-RPM power); more than 230, you get the opposite (poor low-RPM torque, but high HP).

So, to pick a cam, you try to find one that works the best in the RPM range that your engine will ACTUALLY be running in. It is VERY BAD to be anything but hard-nosed realistic about this. Pretending that your engine is going to run between 5000 and 7000 RPM, when in reality you have a stock converter and 2.73 gears such that it NEVER exceeds 3500 RPM no matter what, is a recipe for a slow car, even though on paper, the engine should make high HP at high RPMs. But since it never SEES high RPMs, but only low RPMs, it is inefficient and weak; aka SLOW.

Other things besides the cam end up SIGNIFICANTLY affecting flow; most notably, the exhaust, and the heads. You're already talking about changing out your heads, which is good: those that came on your motor are some of the worst heads ever built, for high-RPM HP. They do what they do well, but it isn't that. In your particular car, you need to change not only those, but also the exhaust, EVERY SINGE PIECE, from the heads to the back bumper, and replace it with stuff that's NOT for the L03. Get headers, cat, and cat-back for a TPI 350.

Your 305 is a roller motor, so you need to be looking at roller cams. A good cam for use with aftermarket heads, that you can get for cheeep, is the ZZ4 cam. People buy that motor and don't want the cam and put them on eBay all the time; you can get one for $125 to $150. It's 208° on the intake side, perfect for a street 305.

Some of that series of cams from Crane are quite good, specfically the ones that are around 210-215° @ .050" of lift. Comp has some as well; their "501" grind and "302" grind for example (very similar to each other) are right in there.

But, for a good cam for your combo at a cheeep price, the ZZ4 is hard to beat.

 

ok i know that i need new exhaust...as it is all i have in it is some blowmasters. I'm going to do that as soon as i get all this stuff into my engine. the main reason i'm buying all this stuff is becuase a head gasket blew so i figure i'll go ahead and get more stuff done to my car. My question is since im getting new heads and a cam, Do i absolutely have to get new headers or can it wait till i have my car up and running? And also where's a good place to find a zz4 cam i couldn't find a single one on ebay.

thanks a lot sofakingdom

 

You can wait on the exhaust, but it's alot easier to do it all at once; since you'll be having to deal with the hard part of it already. No sense in putting yourself through that twice.

Just keep watching eBay for the cm. You won't have to wait long. Remember, eBay doesn't sell ZZ4 cams, PEOPLE do. Sooner or later, somebody will have one they don't want and will put it up on there. They're quite common.

 

Wow sofa that was a read! Very informative.

One question I had, was:

Lots of cam specs say x amount of overall duration, and y amount of duration at .050.

I guess that the overall duration would be the degree measure from where the lobe starts on one side, to where it ends on the other side, and everything else not included would be perfectly round.

But what about the duration at .050?? Any of the cams I have looked at, are well below .050" of lift. Actually, I would hazzard a guess that most cams (except aftermarket ones) are below .050.

How do you measure duration at .050 if the cam never reaches that lift?

 

I am ding a 350 .030 over with 9.5-1 flat tops. The cam I have decided to try is the CS1013R it has an ad 288 IN and 298 EX duration with a max lift of .443 in and 465 ex and 112 lobe centers. I didnt want to go to radical since I will be driving this car daily. Have to remeber I am running a HEI ignition and carb set up.
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And its like 30 bucks. I am running a tight budget. But EVERYTHING from hei, intake,carb,and a complete rebuild.