What do the lifters do?

VaSteve · 03-19-2006, 04:36 PM

I've been searching for the parts to fix my hurt 944 (might be a broken valve spring - might be worse). I don't understand what the lifters do? I understand that the camshaft pushes down on the valve to open them and the springs make it close. What do the hydralic lifters do?

Thanks!

Slam · 03-19-2006, 06:20 PM

Lifters transfer movement from the camshaft lobes to the valves themselves. Lobe pushes lifter, lifter opens valve. Literally lifts valve off seat.

Zero10 · 03-19-2006, 06:21 PM

They are inbetween the cam and the valves. The head of the valve stem is very narrow, and if pressed against a rotating surface would wear a nasty groove in it.
Then, things would go out of spec, you would get decreasing valve lift, excessive play, etc.

Lifters sit inbetween, they have a broad surface up top to help distribute the load, and a narrow button on the bottom to press on the valve stem.
Then, they use oil pressure and check valves inside to take up any play that develops as the parts wear.

VaSteve · 03-19-2006, 06:24 PM

So they take the place of "rockers" in the 944 engine? I looked at howstuffworks.com and their animations only featured a rocker set up.

Razorback1980 · Razorback1980's Garage

Not quite VA...even the rockers have lifters involved. With an overhead cam, rockers are eliminated altogether which is why an overhead cam engine is more efficient. Here is how the rocker setup works. The cam has lobes or elipses on it and the lifter still rides against camshaft. As the camshaft spins around it raises and lowers the lifter. With the rocker type, there is a push rod that sits in the middle of the lifter and pushes on the bottom of the rocker. The rocker then pivots on a bolt located in the middle and pushes down on the valvle stem opening the valve. The overhead cam eliminates a couple of those parts. The cam is still the same but it is located above the valve stem now and the lifter still rides on the cam lobe and still moves up and down but because the lifter is above the valve stem, it doesn't need the push rod or rocker arm...it simply gets pushed down by the camshaft and the lifter then pushes the valve down. If you are looking at the rocker arm type, notice the camshaft is located below the top of the valves, so it needs to push up using a push rod but use a lever (rocker) to transfer the energy to a downward force on the valve. Here is a very rough sketch of how the two work and how they are different. Hope this helps clear it up a bit.

VaSteve · 03-20-2006, 04:34 AM

Thanks, guys. Makes sense now. I was also thinking "lifting" in terms of "up" and not "off" as in off the seats.

Porsche944 · 03-23-2006, 03:49 PM

Actually for these engines the word lifter is a misnomer. The more correct term is hydraulic valve tappet (As per the description in PET), just simply tappet.

However, the term lifter is more widely used because the tappet in a 944 motor or other over head cam engine is the equivelant of the lifters in a push rod engine.

FWIW, I have also heard people refer to lifters as tappets.

earlr85944 · 03-23-2006, 07:52 PM

our cars have hydraulic lifters or tappets which also take up any slack in the valve train vs solid lifters which are static and need adjustment. BTW the 944 is also stock with dual valve springs.