This may be a stupid question but i've never been really sure on this one. I'm buying a recently rebuilt(4000km's) off my buddy who's putting a V8 into his GT. My 2.5 is tired so i'll be putting the 2.8 in. So here's the question. He said that the motor is a 3.1 block done 40 over. He also has a good cam in it. So is it a 3.1? What makes it a 3.1? Is it the heads or the stroke? Thanks

Bore X Stroke yield the engine displacement.

2.8 and 3.1 block are the same. Difference between a 2.8 and 3.1 is stroke(crank throw, I think it's 76mm for 2.8 and 80mm for 3.1 crank, could be wrong).

fiero56 is correct in that the difference between 2.8 and 3.1 is the stroke:

2.8: Bore 89mm Stroke 76mm
3.1: Bore 89mm Stroke 84mm

Pistons are different for the 3.1, but the diameter is same as 2.8 pistons.

To calculate engine displacement, take the bore diameter, square that, multiply by pi (3.14159), and divide by four million. Then multiply by stroke, and multiply by number of cylinders. Really.

BTW, bore and stroke must be in mm, and gives you the engine displacement in liters.