Camshafts

I know how camshafts work but I was just wonder how much force it takes to push down a valve spring, and given this required force would there not be a lot of friction? I'd think the Cam lobe would have a tough time, hopefully somebody can answer this.

Oh ya, what are lifters?

It's hard to say.

Fairly low rpm engines (I'll use low rpm pushrods like the 3800 for example) don't need very much force because it'll never have to spring back very hard so they can use a relatively low sprign constant (measured in newtons of force/m or lb/ft for you imperial freaks).

High rpm engines need a much higher k (spring constant) because if it is too low it'll suffer from valvle float, when the camshaft pushes too hard and makes it accelerate (f=ma, remember folks) too hard and actually shoot off the cam lobe and create some overlap. If the valve float is too long it could also create some major problems as far as compression and combustion go.

In the end it doesn't create much friction because the cams and the springs on new cars are microfinished for the most part.

it varies from one application to another, but to put rough numbers on it, the fitted force on a SBC-size valve spring is in the neighborhood of 165N, and the max working force is in the neighborhood of 400N. Solid force is probably in the neighborhood of 450-500N.

Originally posted by Porsche
I know how camshafts work but I was just wonder how much force it takes to push down a valve spring, and given this required force would there not be a lot of friction? I'd think the Cam lobe would have a tough time, hopefully somebody can answer this.

Oh ya, what are lifters?

open seat pressure on many of them is in the 400 lb springrate range, closed is typically 50 to 75% of that, when you start getting into modded cars turbo SBCs can have upwards of 1000 lb springrates in the open position, it all depends on your application and like 911GT2 said, the desired RPM, the faster you open and close the valve the more likely it is to bounce off the seat, meaning it needs alot of pressure holding it closed.

And a lifter is the piece which rides directly on the cam lobe of a pushrod engine (some OHC motos use them i know, the 4G63 does I believe) if you picture a cam in a smallblock chavy in the middle of the block, the lifter will ride on the cam lobes, which will move the pushrod, then the rocker & valve. they're the #1 reason for valve float at high RPM assuming these are hydraulic (99% of stock motors run hydraulic) the problem is they're oil filled, and if the oil can't bleed out of the lifter fast enuogh it holds the valve open and basically acts as if the cam timing is all wrong.

Thanks to all you guys for the hasty and informative responses.

Another Cam Question, Does the rotation of a Camshaft (The functional part, hitting the valves) take power away from the engine, opposed to a different Valve system?

Originally posted by Porsche
Thanks to all you guys for the hasty and informative responses.

Another Cam Question, Does the rotation of a Camshaft (The functional part, hitting the valves) take power away from the engine, opposed to a different Valve system?

I'm not sure if i understand, but if i do you're asking if the camshaft motion will take power? yes, it takes engine power to compress the rocker arm spring and theres inherent friction in the 2 metals rubbing against each other. a car with say DOHC will have 4 cams taking power, it'd be tricky to compare though since there is no pushrod taking power away, there is 2 times the spring pressure to overcome and 4 times the friction comparing a DOHC V8 to a pushrod V8 but less distance to the valve.

The cam is run off the crank, so it does sap some power. But it's sorta required!

Originally posted by 911GT2
The cam is run off the crank, so it does sap some power. But it's sorta required!

Ya, I was just confirming that more/less. I once came up with an idea that would eliminate the cam, put it up here only to find out it's already under development. :(

Originally posted by Porsche


Ya, I was just confirming that more/less. I once came up with an idea that would eliminate the cam, put it up here only to find out it's already under development. :(

Link? I'd sorta like to read it!

I'd have to guess solenoid-driven valves though? Totally electronic, no cam.

Nope, even simpler than that. Check the Coates Rotary Valve System. I actually pioneered the idea myself and found out later it was already done. I'm smart and stupid like that.

You can't pioneer something that's already been developed.

BTW, rotary valves have a lot of shortcomings.

Originally posted by 454Casull
You can't pioneer something that's already been developed.

BTW, rotary valves have a lot of shortcomings.

Er, they have sealing problems over long periods of time, otherwise the system is very similar to the way a rotary engine breathes through a port, only with more variable timing to it.

http://www.coatesengine.com/eGallery/images/pic06.jpg

interesting concept, however, how would they combat the blowby around the "valve"?

Originally posted by civic1784
http://www.coatesengine.com/eGallery/images/pic06.jpg

interesting concept, however, how would they combat the blowby around the "valve"?

similar to the way a valve seal keeps exhaust gasses in the head, physical contact. the rotary valves though have much hihger wear on those seals and even thought hey're teflon coated they dont last long, which is their #1 drawback. besides the they've got everything over poppet valve heads, they could even be driven by an electric gear, since theres no spring pressure to overcome it wouldn't take much force to move, plus theres no oil in the head in this case, amkes things much simpler, and oil changes could double in distances since most of the heat which breaks oil down is absorbed in the head.