Deck Height questions

[patkeefe]

I'm putting my 2.7 engine together. The case has been together for a year; I got a little sidetracked with the turbo project. Anyway, the pistons and cylinders have been in a box, ignored. They are OK, but here is my problem (part problem and part vent):

I bought these under the impression they were factory high compression Mahles. After I have done all of the volume and geometry measurements the last few days, it seems these are just plain old 2.7 RS pistons. Did all of the cc measurements twice, or three times. My calculations show that my compression ratio will be 9.32:1 with my current deck height of 0.015 inches. If I bring the deck height up to 1 mm (0.040"), my CR will become 8.74:1.

This engine is going into the 914 track car. I have to do all of the clearance checks this weekend. I'll probably end up mocking up one side of the engine, get all of the geometry back to more or less factory geometry.

If my clearance checks come out OK, I figure my options become these:

Run a single base gasket at 0.015" deck height
Flycut some volume out of the heads, and run a thicker base gasket. This would effectively take out some volume from the chamber, yet maintain my geometry. I can probably get almost to 9.6-ish this way, but may still have the same deck issues, and exacerbate the clearance issues, if I have any.
Run the 1mm deck height and live with the 8.74:1 CR.
Cams are "S", btw

The obvious answer is get different pistons. (this is the vent part). As I have already shelled out $1600 for these misrepresented P&C, not to mention another large sum for a purchase which is yet be delivered, I really don't have any money for this. So, I'm willing to roll the dice on the lower deck height, but I would like to know "how little is too little?" on deck height.

TIA
Pat

[Eagledriver]

You will have enough trouble getting clearance for your valves with "S" cams without trying to increase your compression. I had to get the pockets cut deeper on my 2.7 with this same set-up. I think you'll find very little increase in performance from a compression increase anyway. With the lower compression you can run very aggresive ignition timing and pump gas. There are calculators on the web for calculating the increase in power vs compression ratio.

-Andy

[BURN-BROS]

Try to stay at the 1mm deck. You can get by with .035...after that you are on your own. A set of custom cams may be in order to get dynamic compression up. Try messing with the cam intake closing position and focus on dynamic compression to get a good compromise. A DC of a low 7/1 ratio is where I would want to be with pump gas

[patkeefe]

Thanks!
I'm headed to the basement to check everything more thoroughly. I started looking at the other pistons last night, and the skirts are scratched on a few of them. Looks like some of these pistons came from a blown up engine once upon a time.
I will never again buy mismatched stuff.
Pat

[Eagledriver]

Pat don't worry too much about scratches on the skirts. They are a small amount of the surface area of the skirt. If any of them are so bad that they stick up from the surface of the skirt you can clean them up with some fine sand paper.

-Andy

[911pcars]

Bieker Engineering worked on my pistons at the time. To arrive at the desired deck height, he machined the piston crown around the periphery of the piston crown.

IMHO, try to maintain as much compression ratio as you can, especially with S cams that produce less effective compression pressure due to the large valve overlap. Increased compression ratio increases the volumetric efficiency - good for added torque and fuel economy. Those in the know say 9.5:1 is doable with single plugs and today's premium fuel.

Sherwood

[patkeefe]

I'm going to put half of the engine together today, before the distraction of the football game, and see what my clearances are on the valves. I'm also thinking I may get the piston skirts coated, as it seems I have approximately 0.003" clearance in the pistons and cylinders. Matching a mismatched set is like a freaking Rubiks cube.

I really hate to leave any CR on the table, as HP is strictly a function of CR. I'll figure something out as soon as I can see the clearances, as that will be the limiting factor.

I don't think you can see the scratching in these photos, but I do have plenty of bearing surface on the skirts, well as much as can be with a little skirt.

Pat

[patkeefe]

So, I tried that solder method from Wayne's book. With a 0.5mm base gasket, I get 0.052" of deck height. I think I can live with that. Now, maybe I can get the valve clearances checked before the game starts. Then, I'll leave the remainder of the geometry til next week, and further contemplate CR.

[Eagledriver]

I ran a calculator for 8.5 vs 9.5 compression ratio. Using 230 HP at 8.5, I got 237 at 9.5. This is a 3 percent change. It was the first entry I found when googling "compression ratio horsepower calculator"

If you used a .25mm base gasket you have about .042 (1mm) of head clearance. (the solder method gives head clearance not deck height). That should be fine assuming you have enough valve clearance.

Remember when checking valve clearance to the pistons that the closest approach is not at TDC. On the exhaust side it's around 5-10 degrees BTDC and on the intake it's around 5-10 degrees ATDC.

-Andy

[patkeefe]

I am putting in the new JE pistons. With one in, I have a deck of 0.030". JE says these are 21.8 cc dome, and using my old numbers for swept volume and combustion chamber, I get a CR of 9.64:1, this with 0.25mm base gasket. I think I will try to find some 0.5mm base gaskets (there must be some in that pile of parts), which should, according to my calculations, put me at a deck of 1.0mm, with a CR of 9.37:1.

These JE's are pretty nice, BTW. Part of the experiment package:
Alusil cylinders, JE pistons with some special JE coating which JE says will work, and the Alusil ring package.