PONPON的部落格

I've checked out the piston dimensions in the mahle catalog, and here are some findings: 

Both e and i have 84mm bores. 
Stroke are 81 and 75mm. 
Conrods are 130mm for e and 135mm for i. 

Mahle makes 3 types of i pistons and 4 types of e pistons. 

The i pistons have compression: 8.8, 9.4, 9.8 :1 
The e pistons have compression: 8.5, 9.0, 10.2, 11.0 :1 

(Does anybody know which e engines have 10.2 and 11.0 compression?) 

All pistons with original rods have their top at about 206mm from the crank centerline. (Not including any bumps or dips in the piston top itself, all pistons have some sort of shape there) 

Some conclusions: 
If you use i pistons in an e engine the tops of the pistons will end up 2mm lower in the bore. 
If you use i rods and pistons, the piston is 3mm over the deck. 
e pistons in i engine will result in the pistons being 3mm down in the bore. 
e pistons and rods in i engine will put the pistons 2mm over the deck. 

This means that you'll probably have to use the pistons meant for that specific engine, i.e e rods and pistons for e crank and i rods and pistons for i crank, if you don't want to grind the top of the pistons down. 

So the question is then: for the ultimate M20 with stock parts, what will the compression of the combination e crank, 11:1 e pistons e rods and i head?? 

UPDATE!!

I just did some calculations to figure out the compression ratios of different combinations. I used this formula to try and find the combustion chamber volume:

Formula(1)

Where:

CBC = Combustion Chamber Volume.
VOL = VOLume of one cylinder. 
CR = Compression Ratio.

Transformed this equation gives:

Formula(2)
 

From which we can calculate that the CBC of an 325i engine is respectively:

Early Euro 9.7:1 = 47,7 cm³
Late Euro/US 8.8:1 = 53,2cm³

If we then change the crank to an i crank with 81mm stroke the compression ratio can be calculated with this formula derived from formula(1).

Formula(3)

From this we get the following results:

Early Euro with e crank (81mm stroke): CR=10.4:1
Late Euro/US with e crank (81mm stroke): CR=9,4:1

IMPORTANT: The only way to achieve these results is with custom conrods, since the i rods are too long and the e rods too short. The 6mm longer stroke of the e will make the i pistons end up 3mm to high with the stock i rods. The solution to this is to use custom rods with length 132mm, which will provide the correct piston-to-deck height. I don't know what is the most expensive, custom pistons or custom rods, but this is one solution that would give the stock combustion chamber shape.

-Paal Steinar Berg.

(To be continued)