Offset bore rod bearings

[aircooledkiwi]

I'm building a 3.0 SC with Mahle 9.8:1 P&C and '78 heads, 964 cams. Deck height has not been altered by case mods etc. If the piston pin bearings are offset bored 0.030", roughly what will the increased CR be and will there be sufficient clearance? Thanks

[Eagledriver]

There are numbers in Bruce Anderson's book on this but I don't have the book with me. From memory I'd guess you'd get about .5 more on the compression ratio.

The deck height would have to be measured along with clearance to the head and valves. I suspect you wouldn't be able to safely get .030 closer to the heads than stock.

A much easier way to do this is to surface the heads by the amount you want. This will increase your compression ratio without causing the deck height to decrease. You still would need to check the clearances and if you go more than .020 inches you should shave the chain housings to match.

9.8 is about as high as I'd want to go without using twin plugs or race fuel.

-Andy

[Lukesportsman]

It's just so much fun building engines in America. We talk thousands of decking and offset boring in engines with mm shims and clearance specs. Its so much fun being the one hold out in the world

Couldn't you also achieve something different yet similar by reducing .5 on base shim (obviously your particular engine would have to have that much to give up in the first place)? Still other measurements need to be considered, but if the piston clearance is there wouldn't this be more reversible than rods?

Of course you've changed chain issues here but would it be any different than shaving the heads?

Andy, are you referring to 9.8:1 pistons or a measured 9.8:1? There certainly seems to be a discrepancy in Pcar compression claims.

[aircooledkiwi]

Thanks for the advice - surfacing of the head is probably a much better idea. I plan on twin-plugging and was just thinking of getting the CR to somewhere around 10:1 to 10.3:1 to get the most value out of the twin plug. I think I'll check the heads for any previous resurfacing and go from there. Thanks again Bern.

[ChrisBennet]

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Originally posted by aircooledkiwi
Thanks for the advice - surfacing of the head is probably a much better idea. I plan on twin-plugging and was just thinking of getting the CR to somewhere around 10:1 to 10.3:1 to get the most value out of the twin plug. I think I'll check the heads for any previous resurfacing and go from there. Thanks again Bern.

To get the compression up that high you will need to get different pistons I think. (My compression ratio spreadsheet is on my other laptop.)
Here's some things to consider when planning your project:
1) The relief for the 2nd plug will increase the head volume (lowering compression) a little.
2) Shaving the head will make the motor narrower i.e. move the cams closer to the motor centerline. Depending on how much you remove you may need to remove material from the chainboxes and do something about the extra chain slack with a special idler sprocket.
3) Deck height; the distance from the outer edge of the piston to the head, should be around 1mm (.035 to .040"). Smaller than that and you risk the piston touching the head. Larger than that and you increase the risk of detonation.
4) The factory compression numbers are invariably larger than what you get if you actually measure the motor so don't use them as a basis for comparison. Measure your "9.8:1" motor and you'll get something like 9.5:1 I think.
5) Be careful mixing and matching pistons and cylinders. The cylinders come in 4 different diameters and 2 different heights. The pistons come in 4 different weight groups
-Chris