We're gonna talk cams today. The following is a question and my answer from a current Team Camaro thread:
Andy: idk if this is really the right section but here goes. i really don't know much about camshafts but i'm about to get one of those 350/290 hp crate motors from GM and i wanted to put a cam in it...something thats gunna make it sound better, not terribly worried about the power but it would be nice if it wouldn't take much away from the mileage...i don't even know where to begin looking. using an edelbrock performer manifold, thanks
Jim: Andy, a cam determines how much power an engine makes, and where in the rpm band it makes it.
An engine that get's good gas mileage is typically geared to cruise at a very low rpm Example, my 93 LT1 corvette, 2.53 gears AND an overdrive trans! At 45 mph, she's locked in OD at 800 rpm! About 1200 at hwy speeds! I believe the cam is about 216 @ 050, right around .5" lift, and a very wide 114-116 lobe separation. It will rev to 6k, makes great power with excellent drivability, has roughly the same flywheel HP as your crate motor, and will push that slipery vette to the tune of 32 mpg @ 75 mph.
And... it idle smooth as a caddy at 650 rpm.
That lumpy lump idle we all love is the sound of an engine running badly. Lots of overlap (tight LSA and long duration) has the exhaust blowing back out the intake tract at idle!
Not very efficient at low rpm, and that's what makes that lumpy sound. Up in the powerband, when the air gets moving, that lumpy cam starts making some serious power, but down low it's just blowing fuel out the exhaust pipes.
Andy: what do you mean 216 @ 050 and the "lift" 114-116 lobe seperation etc. i have an OD trans as well..as you can see i am quite clueless.
Jim: To hit a few high spots just cause I don't feel like doing the dishes or pulling more weeds...
1 cycle of a 4 stroke engine is 720 crankshaft degrees, right?
180 each for intake, compression, power, and exhaust. But wait, even my vette's mild cam keeps the intake open for a lot more than 180 degrees, what gives?
When the valve opens, it takes a lil while to get the column of air moving into the cylinder, so we open it before the piston gets all the way to the top.
Once all that air is moving, it tends to stay moving, so we can get more air in the cylinder if we leave the intake valve open a bit after the piston has started back up.
The same is true on the exhaust side. The valve opens while the piston is still on the way down, and the force of the expanding gas blows quickly out the exhaust port. As the piston passes TDC, the gas keeps flowing out the exhaust.
But, the intake is open too! This is overlap. At high RPM, the exhaust rushing out can help suck the new charge in faster. The new charge coming in can even help push the exhaust out.
But this kind of stuff happens around the engine's torque peak. It doesn't happen at idle. At idle, some of the high pressure exhaust goes up the intake ports, that's why idle intake manifold vacuum is low witch a "wild cam"
And some of that fresh gas and air goes straight out the exhaust pipe. "Kaching $$$"
Since an engine is most efficient around it's peak torque rpm, AND since the slower the engine is turning, the less fuel it is using, a high mileage engine will have it's peak torque at low rpm, and will be geared to run on the road at the lowest rpm possible.
The camshaft is the brains of the engine, it is the character. It defines everything. Every other part in the engine must be matched TO THE CAM selected for best results.
Cam lingo:
Duration: How long are the valves off the seat in degrees of crankshaft rotation.
Advertised duration: measured at "just barely off the seat" and can vary depending on the cam manufacturer. When a valve is off the seat, the engine is not compressing the mixture, but the valve may noit be open enough to actually move any air, either.
Duration at .050: is exactly that, how many crankhaft degrees from the point the valve has moved .050" until it is back to 050" while closing. This measurement is kinda a standard, and a good comparison between cams, especially from different manufacturers. At .050", there is for sure some air moving.
LSA or LDA: Lobe Separation (or displacement) angle is measured in CAMSHAFT degrees (half of crankshaft degrees) This is the amount of cam rotation between the peak of the exhaust lobe and the peak of the intake lobe.
Overlap: The number of crankshaft degrees that both the intake and exhaust valves are open. Overlap has a lot to do with how a cam sounds at idle. It also has a lot to do with how efficient the engine is at all rpm's.
Overlap is determined by both duration and LSA.
Increasing duration by 10 degrees will increase overlap by 10 degrees. (Intake opens 5 degrees sooner, exhaust opens 5 degrees later.
Increasing the LSA from 106 to 116 degrees will reduce the overlap by 20 degrees (remember, LSA is in camshaft degrees!)
Lift: is just lift, how high does the valve open? Well sorta. Advertised lift is the measured difference between the cams base circle (valve closed) and the nose (all the way open) multiplied by the "expected stock" rocker arms ratio of the engine.
And all of this is STILL terribly oversimplified, as long as it is...
The cam lobe is not square after all. It has to accelerate the mass of the valvetrain, push the valve open, not drop it closed faster than the valvesprings can push that same mass down, then set it "gently" on it's seat. An "aggressive" cam accelerates the valve rapidly, the difference between advertised duration and duration at .050" will be smaller, and duration at .2" and .4" will be lonfer. A roller can be much more radical, too.
And it all has to match the rest of the engine. A cam that produces peak torque at t5k on a 350 had better have heads, intake, and exhaust that will support that level of airflow, cause that sucker is gonna want to rev, and it's gonna make some power. A torque peak at 2k will want everything MUCH smaller.
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