Noahs944

Tonight I was thinking if one was to machine down the top of the head or the bottom of the cam housing then this would be a way to gain valve lift. What I don't know are the clearances involved.

odonnell

I don't think you'd gain anything. The hydraulic lifter will expand to meet the base circle, and compress the spring as the lobe comes around. So your net lifter movement is still defined by the cam profile.

Noahs944

Thank you Michael, yes it appears I was tired & a few beers in last night when the thought came to me. This morning I must agree with you.
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However I think there might be benefit (though marginal, still worth while), here goes with the other half of the concept that I didn't mention in the original post.

Noahs944

I've talked before about a DIY hot rodding of the old bumpstick.

Essentially, my plan is to remove .030" off the base circle.

Noahs944

This will improve intake & exhaust valve lift by .030" by the time the nose of the lobe touches the hydraulic lifter which pushes the valve down ("valve lift").

Noahs944

The above can be done with stock 8 valve components, though it's best done with turbo valve springs or better to prevent valve float. .030" is basically THE MAX for factory valve train is my impression.

Noahs944

With me so far? The hydraulic lifter expands longer than normal (.030" longer than normal the case of the DIY base circle re-profile).

and that is therefore max lift-since in the garage tech re-profiling we don't want to touch the high pressure areas of the cam. But what if we could remove .040" from the base circle? Then, the valve lift would be more right? No! because the lifter won't allow it! We are already at maximum length (or "stretch", not sure what industry term is).

Noahs944

What I'm trying to figure out is can we play the cam housing height and the cam re-profiling together to make a very dynamic partnership. But I'm getting confused. Anyway, that's the thought. Opinions welcome.

SPECIFICALLY: if we shave .010" from the bottom of cam housing will that allow us to remove .40" rather than .030" from the base circle & allow us to get more valve lift than before?

jhowell371

You will get your .030 lift. The lifter will adjust for the .030 difference but the distance from base to tip of the lobe is still your true lift. The lifter once pumped up with oil to take up clearance then behaves as a solid lifter since the oil is incompressible and your gain in lift is achieved.

Noahs944

Yes, but can we increase that number (see response #8)?

V2Rocket

You can cut the base circle maybe even up to 0.050" if you want and can use BBChevy lash caps on 944 valve stems.
You would want turbo VALVE SPRINGS or better, not valves.

But valve lift is only the second half of the power that comes from a cam - you'd be further ahead by changing the profile for more DURATION (higher rpm power) and just setting the lift where it geometrically works with the new duration profile.

There's an old rule of thumb that valve lift is only really useful up to 25% of the valve's head diameter - this is because at lift=diameter(.025) the CURTAIN AREA between the valve head and the valve seat, calculated by the circumference of the valve multiplied by the lift value (think of it as the outside edge of a cylinder, which you can uncurl and make into a rectangle) makes an area equivalent to the area of the valve head itself.



You can't flow more than the valve seat/head area will allow so what's the point lifting higher for more curtain area?
Now, going a little bit beyond the 25% will get a liiiitttllle bit more flow (because the valve stem subtracts from the valve head's useful flow area) but it's not much.

For numbers...944 intake valve is 45mm diameter, stock lift for all years of 944 8-valve is 12mm or 26.6%.

So again...DURATION will get you more ahead. Even with LESS lift!

Noahs944

That's very interesting Spencer! Love learning this stuff.
Also thanks for the correction on my error (I have since corrected it to include "spring").

Noahs944

Can you answer the question from response #8 though?

V2Rocket

No...the lifter would just take up the slack until it maxes out its travel and then gets too much clearance between cam and valve.
Don't cut the housing or the head.

944CS

Additional lift won't gain you anything, and you run the risk of piston to valve contact.

If you want more performance, get rid of the air flow meter for a MAF, or modify the intake manifold. Change out the rear muffler if you replace the air flow meter.

Get 50 series tires instead of 60 series.