Danno

Geo & Dave, I think both you guys are correct. Geo is on for the 951 because a lot of the combustion chamber volume is in the piston tops. Dave's idea works for the 944NA which has a fairly flat piston top.

"Since the compression is way up and the head has been opened up a little, I wonder if the mostly stock intake and exhaust are choking it off at higher rpm. When it was first installed there was NO power above 5000 but a less restrictive cat let it go a few hundred rpm more before it "hits the wall.""

Yeah, both the 944 and 951 can use larger intake-ports for high-RPM flow. There's an optimum ratio of inlet to outlets port sizes and on both cars, it's too small on the intake. One thing I'd like to see is a cam with higher lobe-centers and more duration (lift is not as important). The ones I've been able to measure have the intake closing way too early for high-RPM flow.

Jakerx

wow talk about getting some good info off this thread i started...never thought i'd get this much off my question...thanks

Tony K

Best thread I have ever read here. Thanks, everyone.

My .02: my old 83 had the head shaved after PO broke t-belt and bent a valve. I don't know how much it was shaved.

But for speed comparison, I knew two people with 89 NA 944s (2.7), and from 0 to 75 mph, my 83 always had at least half a car length on the 89s. I know the 89s are heavier, but my 83 definitely had noticeably more power than "stock". It was surprisingly faster than any other 2.5 NA I have driven, including my current and previous 85.5.

Cheers,

Oddjob

Does anyone know what angle the camshaft is offset from the crank/piston centerline? And what the distance is from the crank to the camshaft?

Its very easy to calculate how much the cam timing will change when the head is shaved, and it wont be very much.

Geo

It will be a few degrees and a few degrees will usually make measureble changes in performance. For most road cars that's not a big deal, but when tuning a car for maximum performance it's certainly not something to be ignored.

david fracolli

With my head shaved .040" it changed the timing of the cam by 2.5 degreees.

Oddjob

David,

How did you measure that?

meanmaroon951

Tony K , the 83 should be lighter but those were 2.7litres hmm

david fracolli

I used a degree wheel.

Swagger93

my friend bought a cavalier with 211,000 miles on it. The PO blew a headgasket and decided (as he was a machinist) to mill down the head and jack up the compression. Let's just say it is WAY quicker than those cars are stock.

not related to the 44's, sorry, but it's kind of interesting

Danno

Here's some basic info on degreeing cams: Degreeing Camshafts Tech - Part 1

Basically the idea behind getting more high-RPM torque & power is to get more air into the chambers at high-RPM. A longer duration cam with a later intake-closing will do this. Without that, you can still use the stock cam and retard the cam for a similar effect.

DanG

I don't know the exact radius of the cam pulley, but assuming this relation is correct...

we can figure it out.

amount milled off head / degree change in cam pulley position = .040/2.5

2.5 degrees = .0436 radian = .04 inch
1 radian = .04/.0436 inch = .917 inch or 1.834 inch diameter

This seems much lower than it should. I'd estimate the cam pulley is approximately 4 inches diameter or 2 inches radius.


So .040" change on a 4 inch dia pulley would equate to...

.040/2 = .02 radians = 1.14 degrees

So for every .010" removed from the head, you'll see a .285 degree change at the cam pulley. (assuming 4 inch diameter pulley, if its larger than that, the degree change is even less)

Oddjob

Dan G,

My math matches your math and I also just did a quick CAD draw-up to double check. Assuming a 4” diameter cam sprocket (2” diam crank sprocket), CAD shows:

0.02” - 0.573 deg retarded
0.04” - 1.146 deg
0.06” - 1.718 deg

Geo

Actual observation seems to say differently.

Danno

You also have to account for the angle between a line drawn between the two pulleys and the surface of the block/head. Take the SIN() of this angle since as the angle decreases from 90-degrees, the differences would lessen.

".040/2 = .02 radians = 1.14 degrees"

Multiply by 2 to get the crank degrees...