2.2 L to 2.54 L Engine Conversion
 

I have a 70 911 E which is about to be converted into a larger engine size. I have a std/std 70.4 crank w/rods, cams reground to 1966 Solex specs, a stock refurbished mfi system, access to good cast iron cylinders which are intended to be fitted with J&E 87.5mm pistons with a 9.5:1 compression ratio, and stock 2.2 heads with 32mm ports. There is also an experienced engine builder and machine shop doing the work with over 30 years of Porsche building experience, largely in the racing circuit. We agree that the cast iron cylinders are quite stable given that the cylinders and fins are cast as one piece. At possible issue is the cylinder wall thickness (.0042 In.) especially at the spigot where the piston makes the turn on its wrist pin, the concern being that the small amount of piston rattle in this area might eventually lead to a fissure over the course of normally driven mileage. Does anyone have a comment or experience with this conversion other than using the very expensive L&N 87.5mm cylinders? There was a 2.5 L racing class of cars in the 1970's using cast iron cylinders with 86.8mm bores, but they may have had larger cylinders requiring spigot enlargements (not sure), however I would be interested in acquring these cylinders if they were larger. All comments would be appreciated.

Walt

Walt, no doubt you recognize my user name so I will get you started and then the real experts will chime in. First of all WELCOME to our forum, great to have you here.

Conventional wisdom is that a 1mm overbore is the max for both the Birals and Cast-Iron. The longer stroke (70.4) will have greater rod angularity. The JE will have zero offset to the piston pin. Factory pistons have a small offset, see here http://forums.pelicanparts.com/911-engine-rebuilding-forum/335700-piston-pin-offset.html

What is the concern here about using Nikasil 89s like the ST? Boring the spigots for a true 2,7? The price of the JE pistons is the same, I assume you have external oil cooling?

2,5 was achieved back in the day a couple ways- short stroke or long stroke
short stroke 89x66, 2464cc, with 38cc dome gets you 12,3 to 1 at 1.0mm deck
long stroke 86.8x70.4, 2500cc, 38cc dome gets you 12,6

Nickies can support 87.5mm. Birals cannot. The grain structure of the alloy in the machined Nickies is tough as bails.