How Shit Works: Cam

"TheElephant" (426cubicinches)
09/19/2014 at 02:13 • Filed to: None

Look, cams are like quantum mechanics to most people. They joke about cars that "climb up onto the cam" and comment about the "amount of cam" in a lopey muscle car. But does anyone know how they work?

So, lets do some learnin' about fuckin' cams. !!!error: Indecipherable SUB-paragraph formatting!!!

Think of a cam as a the engine's management team. It coordinates all the critical functions - basically managing the car's ability to inhale and exhale. It does this by controlling when the intake and exhaust valves open, how far they open and how long they stay open.

Cams come in different sizes - kinda like gun calibers and barrel lengths. The important things to pay attention to are the measurements for Duration At .050", Lift, and Separation.

So, ignore the technical jargon on the first one - that .050" shit doesn't matter to you - duration means how long the valve stays open. Higher number means more time open, got it? This is usually presented as two numbers (like "264/264") because you need to measure for both the intake and the exhaust stroke.

Lift describes how far a valve is lifted off its seat. By lifting the valve high off its seat, you allow more air to flow into or out of the cylinder (and make !!!error: Indecipherable SUB-paragraph formatting!!! ). Sometimes, though, you can lift the valve so far off its seat that it might hit the piston if the lift isn't perfectly timed. This is called "interference". Interference could suck.

Separation is basically the distance the between the intake and exhaust lobe on a cam. It determines how closely spaced the opening of the exhaust valve is in relation to the intake valve. Think of like this: separation is the time between opening of the intake valve and opening the exhaust valve. If the intake valve stays open for half a second, but the exhaust valve opens a quarter second after the intake valve, then you have both valves open at the same time .

This creates suction (from the exhaust) and pulls air into the cylinder. This is called overlap. Overlap sounds amazing in a high revving Ferrari. A lack of overlap also makes massive 'Merican motors lope along at idle.

OK - I'm losing you. So, here's what a !!!error: Indecipherable SUB-paragraph formatting!!! . Why's it sound like that? Cause its got the following specs:

duration at .050": 226/234

lift: .364/.360

separation: 116

You gotta look at duration and lift together - here the exhaust valve stays open a little longer (234>226) but the intake valve lifts higher (.364>.360), meaning it can move more air. READY FOR SOME MATH CONCEPTS? Lots of volume for a short time can move the same amount of air as less volume over a longer time. Make sense? It's like the run from LA to Las Vegas: you can cover the 300 miles in three hours (at 100MPH) or you can cover it in five hours (at 60MPH). Either way, you still traveled 300 miles.

So, the exhaust is lazy. And it tumbles out of the cylinder slower than the air is coming in. This creates that roundness in the exhaust note at idle. It almost sounds like the exhaust is coming out in puffs. Compare this with a !!!error: Indecipherable SUB-paragraph formatting!!! that has a little bit of lift, but lots of duration (which makes the engine want to spin).

Separation of 116 degrees is at the far end of the spectrum (generally 106 to 118). This Hemi has a lot of separation, meaning there is less potential for overlap. This creates that... "pause"... you hear between whomps in the wompwompwompwomp of the exhaust note. Again - the Ferrari has more of a murmur or slur, without the defined "womps," thanks to its much small degree of separation (usually around 106, sometimes lower).

And there it is: your cars character defined in a couple simple numbers.

I like mine with lots of separation and lift. Nothing beats a lumpy idle.

DISCUSSION (15)

AddictedToM3s - Drives a GC > TheElephant
09/19/2014 at 02:19

Very well written and informative! Thank you for this. Do you have any plans for future posts regarding the other parts of an engine like the crank or compression ratios? I'd love to read those.

TheVancen- In Pursuit of a Greater Payday and Car Parts > TheElephant
09/19/2014 at 02:19

Ive been doing it wrong this whole time, I though overlap=chunky idle. The shame... Good article. Very easy to digest.

Kinja'd!!!

TheElephant > AddictedToM3s - Drives a GC
09/19/2014 at 02:23

Yeah - I am going to explain stroke and how it produces torque next, I think.

Then compression and its impact on RPM maybe?

AddictedToM3s - Drives a GC > TheElephant
09/19/2014 at 02:25

SWEEEET!~ I'll be looking forward to those!

TheElephant > TheVancen- In Pursuit of a Greater Payday and Car Parts
09/19/2014 at 02:26

Overlap gives a Ferrari that ragged, blood curdling scream as it nears redline.

TheVancen- In Pursuit of a Greater Payday and Car Parts > TheElephant
09/19/2014 at 02:29

Kinja'd!!!

Thanks for clearing that up for me.

Nerd-Vol > TheElephant
09/19/2014 at 04:57

Please repost this in 5 or 6 hours so more people can enjoy this.

Jake - Has Bad Luck So You Don't Have To > TheElephant
09/19/2014 at 08:01

I'm still trying to figure out if I want to get $200 performance cams for the Murdersofa or save up for a Riviera... decisions...

Saf1 > TheElephant
09/19/2014 at 08:03

Excellent post! Dude, there are many articles on the net about hp and torque and I've tried many but the fifference is still not clicking with me (why torque is relatively more helpful with acceleration, hp for top speed ? But much more importantly what is with power bands in engines?? Why is the power not constantly increasing till redline - it drops before? I heard somehwere it's all about how the engine breathes it'd be cool to get a more in-depth explanation

V8Demon - Prefers Autos for drag racing. Fite me! > TheElephant
09/19/2014 at 09:02

that .050" shit doesn't matter to you -

Actually it does. Duration @ .050" is different than overall duration. It's generally measure @ .050 due to the fact that on certain setups on older cars valve lash can vary considerably. That being said are the lift numbers you posted at the cam or overall at the end of the rocker? You say it's big, so I'm guessing the cam itself.

My old Cougar is right there as well. 228/232 @ .050" with .353" lift at the cam on both sides (.565" after the rocker arms) and a lobe separation of 112 degrees.

A larger lobe separation angle will lend to a smoother idle.

http://www.compcams.com/Technical/FAQ/…

Intake and exhaust opening/closing points have effects as well. It's all in how everything works together. Walking a tightrope as it were.

http://www.superchevy.com/how-to/engines…

BKRM3 > TheElephant
09/19/2014 at 10:02

Lumpy you say? HUZZAH!

You can tell a Finn but you can't tell him much > Saf1
09/19/2014 at 11:03

As an oversimplified explanation, torque is analogous to the amount of force an engine creates. More force = faster acceleration from F=ma. This means that a big torque can give you a big acceleration.

Horsepower is basically how fast an engine makes torque. The faster an engine can make torque, the faster it can drive the car. Once a car is speeding along the mass doesn't really affect how fast it can go. The speed is limited by the drag of the car and the amount of power its engine makes.

As for the power bands you've pretty much hit on it being a factor of how the engine breaths. Everything that moves has a natural frequency. The intake and exhaust systems have their natural frequencies. These determine where the power band is. At a certain rpm the air pulses flowing through the intake and exhaust system will be at the natural frequency of the piping. This lets the intake and exhaust move the air very efficiently.

Other things do come into play though when it comes to power bands. At low rpm you have a lot of pumping losses because of the throttle being closed. The engine loses energy by having to overcome the vacuum in the intake system to get fresh air into the cylinders. At high rpm you have less pumping losses because the throttle plate is wide open, but now you have a lot of frictional losses. If you were to graph pumping loss v. rpm and frictional loss v. rpm there will be a point where the combined curves is a minimum. That point will be a high point in the power band. That point won't necessarily be the same point that the exhaust system is at its most efficient, and the intake system may have a different efficient point too. Generally these are designed to all occur at the same rpm, but constraints such as engine bay packaging and other concerns can keep everything from lining up exactly right.

The other reason that the power doesn't increase right up to the redline is that engines are designed to make the most power at lower rpm's. Manufacturers tune their engines to make power lower down in the rev range because that is where the engine spends most of its operational life.

Saf1 > You can tell a Finn but you can't tell him much
09/19/2014 at 16:33

Thank you so much!! That was a fantastic explanation - I finally get it! Not get it but get it :D

DanZman > TheElephant
09/22/2014 at 20:14

This is awesome! thank you. I have a cam in my Vette and i got so sick of now understanding the numbers when people would ask that I took a picture of the specs to just show people.

Mattbob > TheElephant
09/24/2014 at 09:53

blah blah blah... "Lift:... Separation"... blah blah blah... hehehehehehehe lift and separate. also, ever hear a nascar idle? It sounds Nasty. Such cam, much lift.