Dave W.

Update!
I had hoped to keep this build rolling along but the machine shop work took a bit longer than I expected. The block came back last week, took a few hours to measure everything. I cleaned and assembled the bottom end yesterday. It's always surprising how much gunk is stuck in the oil passages in the crank.
Laid the crank in the block.



Assembled pistons, rods, rings. Eagle rods are quite a bit lighter that stock.



Installed main girdle, pistons, oil pump



Clearance between block and rods is good as-is, still more room for a longer stroke crank.



Ceramic coated piston tops seem to soak up oily fingerprints, it's much prettier in person!



I took measurements of the final assembly. Stroke is 85.75mm, bore is 102.45mm, displacement 2826cc. Block has been shaved so the pistons are .035"/.89mm below deck. I have an offset cam key to compensate for the shorted deck height.

So now that the bottom end is assembled and measured I was able to calculate the combustion chamber volume and choose a head gasket. It's nice that Cometic offers their head gasket in many different thicknesses so I could tailor the final compression ratio. It's not easy to decide! Comp ratio will be 8.7 to 8.8 assuming 59cc heads.

refresh951

Really nice! Few questions:

- I also spent a lot of time cleaning the crank oil passages? Do you have a procedure for this?

- Did you turn the mains? What was max runout on the crank?

- What was you final deck height? I think stock is 9.05" IIRC. When you talk about cam timing, what kind of adjustment are you making and how did you calculate it? I have read about rules of thumb like "add 1 degree of advance per 0.012" reduction in distance between the cam and the crank" on some other forums.

blown 944

Nice to see it coming along. Yeah, that smaller rid combo adds a ton of clearance. After my first assembly, I had already started planning for a 95mm stroke. Then Shawn built it lol

Dave W.

Yes, my procedure for cleaning the oil passages in the crank is to first lay out a clean piece of cardboard on the floor to soak up the cleaner. Lay the crank on the cardboard and get two new cans of brake parts cleaner. To reach the oil passages that go between the main and rod journals, and to reach the plugged hole between the rod journal and the end plug I like to bend the tip of the straw on the brake cleaner at approx 45* angle. The straw will catch the edge of the passage and squirt around a 90* corner. Pulse the cleaner at full pressure, apply in multiple directions, marvel at all the crap that came out. I actually pulled out a lump that was a full cm long.

I did not turn the mains, just polished them. Max runout was .0002" Main oil clearance is .0027 to .0032. I'd prefer it to be slightly tighter but there's no other bearing sizes available to adjust clearance.

Deck height is approx 9.025". Decking removed around .015" which is quite a lot. It was a choice between keeping some quench area between pistons and head, keeping cam timing close to normal, keeping the timing belt tensioner within it's range, and getting the compression ratio close to my goal. I haven't calculated the adjustment needed for the cam gear yet, it'll be a week before the head goes on so I'll look at it soon.

I know what you mean. After researching the balance shaft issue in my Balance shaft thread I looked at using Mitsu 156mm long rods with SRP 1.18 deck pistons, which are also around 50 grams lighter. There's so many options once you start looking outside of standard Porsche replacement parts.

Dave W.

Let's see; cam gear has 40 teeth, that's 9 degrees per tooth at the cam, 18 crank degrees. Cam gear circumference is 378 MM . So that's nearly even, 378mm/360* = 1.05 mm per degree at the cam.
1.05mm = .041" so my headgasket thickness and block decking is only approx 1* change? I thought it would be more. Check my math please?

Maybe I'll start with the offset key at 2* advanced to match the displacement and spool characteristics of my 5558 turbo.

pole position

Both 5558 and 5858 share the same turbine IIRC , why not use the. 5858 , I don't think spool would be effected and you should have more top end . It also would be interesting to see what the 5862 with the billet turbine can do

refresh951

Max runnout is .0002"? That is amazing! Any feedback from the crank guy on how he did this?

Dave W.

I agree, the 5858 or 5862 would suit this engine better, but I originally bought this turbo when the engine was stock. I later found out that it's not so easy to install, it takes a lot of custom fabrication so it was delayed. Now that I'm doing a full engine and trans build I guess it's a good time to finish installing the turbo.

Dave W.

He turned and polished the crank nice and slow.
Now that the crank is installed, I can put a gauge on it and double check from both ends of the crank.

refresh951

My numbers were in line with your numbers and your approach makes sense to me but...I asked a top engine builder and he told me for every 0.011" removed from deck/head/cam tower of a 944, the cam timing will be retarded 1 degree. He seemed rather definitive about it. I tried to get this number from the math to no avail but I trust his number.

333pg333

That sounds pretty incredible. Such a small surface amount netting a whole degree. Physics still amazes me.

Dave W.

Since the crank turns twice as fast as the cam it only takes .0205" to change the crank by one degree, assuming the cam was held at it's TDC position. That's as close as I can get unless my math is off. For this exercise I prefer to think of the crank as being locked at TDC.

I checked the crank runout tonight. The best I can see it's off by less than 1/2 thou.

Thom

Since the cam tower does not sit parallel with the head/block, where does he get his number from?

I made the same observations as Dave, which is that +1° ≏ -1mm off the head/block.

Chris White

Wait - the cam tower is not parallel with the head/block? I must be building them wrong....

BTW - remember that (typically) retarding the cam gets you better top end and advancing the cam gets better low end. So you can go for a balancing act to get the widest power band or the highest HP peak.

Thom

Are you saying you see the plane between the cam tower & head and the plane between the head & block as parallel? Are you serious?