L8 APEKS

Hey gang,

Dropping my head off at the machine shop tomorrow morning. Still debating how much material to remove. I recall a while back talking with Jon Milledge about headwork. IIRC, he told me that 40 thousandths removed would raise the CR from the stock 10.2:1 to 11:1. Though I can't find the notes I wrote down to verify this.

Does anyone have any info to support or disprove these numbers? I'm just trying to ballpark my resulting CR after headwork.

The car is an '88 NA,the head has never been off the car. Stock CR is 10.2:1 and I'm looking for 10.8 - 11:1. Let me know what you've heard!

Chris_924s

Sean. If you can see your skull the haircut is too close..

Machinists can get deck flatness within .001, Aluminum will twist off the bench- all depends on heat.

Compression ratio is an algorithm..

try this- http://www.dunegoon.org/compression.html
HTH

L8 APEKS

I've seen calculators like that before, but I don't know the other info to fill in (cyl volume, etc).

L8 APEKS

Lotsa reads, no posts...Bueller? Bueller?

L8 APEKS

Found my notes. Not sure how this compares to a low compression engine, but on the 87-88 cars, 30 thou brings it from 10.2 to 11:1. To make up for this, I was advised to adv. cam timing 3 deg. Hooah!

Eyal 951

i assume its different with the NA pistons (Since everything else is the same) but after milling .050 I only went to about 8.5:1 CR. (from the stock 8:1)
Eyal

tod84944

On my 88, when I had the head milled less than the .040. Mainly just had the surfaced cleaned up. The big thing I did though, I used a 951 head on it instead of the stock 944 head. You would have to drive the car to feel how much difference it made. As far as compression, on my other 88, I can get away with regular fuel, with no ping. On this car, I have to use prem, or I will get ping when I blip the throttle. The reason for the 951 head, according to Jim Pasha, the 951 head flows so much better, and more than the 944 head. I also used a 16V exhaust instead of the 8V due to it being 5 mm bigger and had a flow master muffler welded in.

Giantviper

Interesting topic. Just kind of curious. What kind of HP gains did you get tod84944? also what kind of gains do you expect Urin 2nd?

Also (i may just be mistaken, do not know alot about upping cr), don't you have to put in new pistons when you up the cr?

L8 APEKS

It's not a big gain in peak HP or TQ, but it's one of those gains that goes from "idle to redline", which is rare. I'd say probably ~5hp and ~5tq if I had to take a guess. It feels similar to advancing the timing, except there's no compromise. When you advance your timing, you pick up power down low but you sacrifice power in the high RPM range, and vice-versa. This gives you the gain all over the power curve.

I'm on my way to the machine shop after I finish typing this. The car felt rediculously strong for an NA, so I'm very eager to get it on a dyno in the near future.

Waterguy

Fairly easy to calculate using algebra.

Our cars have a displacement of 2479/4 = 620 cc per cylinder.

Compression ratio CR = (620 + X)/X, where X is the combustion chamber volume in cc. X includes head, head gasket and piston dish volumes. Assuming a starting CR of 10.2:1, solve for X = 67.4 cc.

We now reduce the combustion the combustion chamber by a volume of Y by planing the head. The new CR = (620 + 67.4 - Y) / (67.4 - Y). I won't go through the algebra, but you can solve for Y:

Y = (67.4 - 687.4/CR) / ( 1 - 1/CR)

To get a compression ratio of 10.8:1, you must reduce the volume by Y = 4.14 cc. For 11.1, Y = 6.01 cc.

Each cylinder has a bore of 100 mm = 10 cm, or an area of pi/4x10cm^2 = 78.5 cm^2. To reduce the comustion chamber volume by Y, you must shave a thickness:

d = Y/78.5*10 mm/cm.

For 10.8:1, Y = 4.14 cm^3, so d = 4.14 cm^3/78.5 cm^2 * 10 mm/cm = 0.53 mm or 21 thousands of an inch. For CR = 11.1:1, Y = 6.01, d = 0.77 mm or 30 thou. Note that the recommended limit for machining heads in the FSM is 0.4 mm. This would get you a CR of 10.65:1.

L8 APEKS

Thank you Don!

FRporscheman

Hooray for math. Will there ever be an issue of getting the valves too close to the pistons?

Giantviper

Thanks for the info, interesting thread. Urin 2nd, please let us know what its like when the job is done.

tod84944

I have no idea about the HP gains. Don't have access to a dyno. I can tell you this......my 0-60 times are between a 7.8 and 8.1. And the "seat of pants"meter compared to other 88 that I have driven, it is a lot faster.

david fracolli

I hate to throow a wrench into this but the 944 cylinder head is not symetrical. What I mean is that the first .010 inches removed will reduce the volume of the head by 2cc but the next .010 will only reduce the volume by 1.8cc.
When I did by N/A engine I used the 88 10.2/1 pistons and had my head shaved .045 inches. The mesured compression ration was a tad over 10.7/1.
When I had the headwork done I had my machinest measure the stock head and then remeasure the volume after every .010 inches removed. If I can find the information I will post it but it might take a while to find as this was done over 4 years ago.
I wanted to remove .060 inches but was not able to go more than .045 due to piston to valve clearance problems since I was running an aftermarket cam. As an FYI with .045 removed my cam timing was retarded by almost 3 degrees.