Josh B

I've got a question that has been puzziling me for a while. How is it that by adding a 3 liter crank to a 2.5 liter block that displacement is increased?

87944turbo

Throws are longer increasing the stroke, therefore increasing displacement.

Really one of two ways to make the cylinder larger, you either increase the length or increase the diameter or both.

Josh B

That is the part i don't understand though. I've had my head off and the pistons come right up to the top of the cylinder. There isn't any room for it to go any higher into the head.

BGA9512

My understanding is that the 3 l crank has more offset than the 2.5 crank - the distance the crank journal bearings (where the piston connecting rod big end connects) are further away from the centre line of the crank. The increased length there would push the piston up higher and pull it down lower as the crank rotates, thus giving it a longer stroke. Because the bore stays constant, you could probably calculate it quite easily.

Josh B

Here is a pic of a turbo short block with the head off. See how the pistons are right even with the top of the cylinder walls?

87944turbo

I hear what your saying Josh. But I believe with the 3L crank, you have to change pistons and rods as well to change the crank to wrist pin distance. Here is link to Clark's that is probably outdated, but I think covers the basics well (http://www.clarks-garage.com/shop-ma...rfmod.htm#disp).

KuHL 951

Shorter rods and pistons plus a longer stroke will increase displacement without effecting TDC position. I don't think a 3.0L crank with 951 rods and pistons will work-ouch

fast951

If using the stock length rods, the pistons used with the 3L crank have a different pin offset than the stock 2.5L pistons. If you use the stock 2.5L pistons + stock rods + 3L crank the pistons will stick out of the block.
Some use the factory 2.5L pistons + 3L crank but use shorter rods. Not ideal as the rod/crank angle becomes questionable.

Longer stroke = larger displacement.
Larger bore = larger displacement
Add both and you have a big smile

95ONE

This is funny.

reason bigger (to a 2.7 I believe) is the longer stroke. its a 3liter because in addition to stroke, the bores are 4mm bigger on the 3.0 motors.

HOW: They changed wrist pin position higher up on piston. fine for NA. Not as good for turbo. leaves less "meat" to handle the heat. e.g. Thicker ring lands.

But. to shorten rods is to give a large rod angle ratio. So I think piston pin height is the way to go.

adrian1

The stroke on the 2.5 crank is 78.9 and 88 mm on the 3.0 crank. Without changing the bore from 100mm you will have a 2.8 L displacement. Shorter connecting rods or wrist pins solve the TDC problem. If You are go to go through all that trouble just build a 3.0 with 104mm bores and 88mm stroke. More power with less boost.

95ONE

even funnier. Kuhl,fast, and I were writing and posting at the same time.

Josh B

Hmm - still having trouble with this. Is the stroke defined as the distance from the center of the crank to the center of the rod journal? I thought displacement was the measure of the volume of the cylinder with the piston in the lowest position?

blown 944

stroke is the total throw distance of the crank.

When figuring pin height you divide it in 1/2.


dispacement is: bore x bore x stroke x .7856 x number of cylinders.

blown 944

to put it into numbers

1.555 (1/2 of) stroke
5.960 rod length
1.485 pin height
all added together = 9"

now use the 3.0 crank
1.732
5.960
1.48 pin height
=9.178

so you have to either move the pin up .178 or shorten the rod to get back to 9'

these all numbers off the top of my head I don't think the pin height is correct so don't shoot me, but it all works the same.

95ONE

or. maybe you need a visual.

The piston travels further down (not up) the cylinder wall. = more volume. Moving the wrist pin position further up allows the piston to sit at top of deck. but it will travel a longer distance down. Of course it doesn't have to be much in a cylinder because you get to multiply that space by 4.

Pics would probably be better also.