Is it a 2.4 or a 2.2?

[72targa]

meet in the middle and call it a 2.3

The 2.4 is literally a 2.3...

[Harvey Weidman]

But is the larger cc got by longer rods or longer crank throw ?

You can make the rods a foot long and the stroke still will be the same.....
It doesn't need to be said but I'll say it anyway, the stroke is determined by the crankshaft...(stroke).
That is why when you put 2.2 pistons (made for 66mm stroke) with a 2.4 stroke (70.4mm) the compression increases....
(9th grade auto shop)
H

[Boosted3s]

What does the compression ratio end up being with the 2.4 T heads?

[NeunElf]

You can make the rods a foot long and the stroke still will be the same.....
It doesn't need to be said but I'll say it anyway, the stroke is determined by the crankshaft...(stroke).
That is why when you put 2.2 pistons (made for 66mm stroke) with a 2.4 stroke (70.4mm) the compression increases....
(9th grade auto shop)
H

Harvey is 100% correct, but maybe it will be a little more clear to say that stroke is the distance between the crankshaft Bottom Dead Center (BDC) and crankshaft Top Dead Center (TDC). At these points the connecting rod makes a straight line from the crank to the piston pin.

Making the connecting rod longer or shorter moves the both piston's BDC and TDC positions up or down by an equal amount which leaves piston displacement unchanged.

Of course in a Porsche it's Right Dead Center and Left Dead Center but it works out the same.


Changing connecting rod length does a lot of things (you can google connecting rod length to learn more than I as an electrical engineer can easily explain). A longer connecting rod would increase compression ratio if nothing else is changed but compression ratio's normally changed via piston design. In a Porsche, changing the shims between the cylinder and the case can change compression ratio a bit.

[jac1976]

"Changing connecting rod length does a lot of things (you can google connecting rod length to learn more than I as an electrical engineer can easily explain). A longer connecting rod would increase compression ratio if nothing else is changed but compression ratio's normally changed via piston design. In a Porsche, changing the shims between the cylinder and the case can change compression ratio a bit."

How does this work? The stroke has not changed. The max and minimal volumes have not changed. The compression ratio would not change. Am I incorrect in this thinking?

[TWJ]

jac1976,

I could be wrong here (experts please chime in...) but I would think if the rod is longer the stroke would start (BDC) higher in the cylinder and therefore end (TDC) higher in the cylinder. This would leave less space between the top of the piston and the cylinder head thus creating greater compression. Yes/no?

[NeunElf]

jac1976,

I could be wrong here (experts please chime in...) but I would think if the rod is longer the stroke would start (BDC) higher in the cylinder and therefore end (TDC) higher in the cylinder. This would leave less space between the top of the piston and the cylinder head thus creating greater compression. Yes/no?

Yes, exactly.

Moving the piston towards the top, whether it be by lengthening the connecting rod, removing shims below the cylinder, or milling the cylinder head decreases the volume at TDC.

Displacement is volume displaced by piston movement: stroke * area (where area is π/r²).

Compression ratio is cylinder volume at BDC/cylinder volume at TDC.

Cylinder volume at TDC, which I’ll call combustion chamber volume obviously decreases if you move the piston towards the cylinder head.

Let’s look at a 2.2 liter 911E. It has an 84 mm bore and a 66 mm stroke for a 365.7568 cc piston displacement. The compression ratio’s 9.1 : 1. So, designating cylinder head volume as X we can find it from:

X/(365 + X) = 9.1 =>
(365 + X)/X = 1/9.1 =>
365 + X = X /9.1 =>
365 = (X/9.1) – X =>
365/8.1 = X = 45.15 cc

As a recheck, 365.75 + 45.15 = 410.91 and 410.91/45.15 = 9.1

Now, if you move the piston 2 mm into the cylinder head (by lengthening the connecting rod, removing shims below the cylinder, or milling the cylinder head), you decrease the volume at Top Dead Center by π*(84/2)² which is 11.08 cc making the combustion chamber volume 34.07 cc.

Now the compression ratio’s (365 + 34)/34 = 11.73 : 1.

[TWJ]

Great explanation Jim! Thank you.

[boba]

(9th grade auto shop)
H

But they don't teach auto shop in school any more.

[boba]

Don't forget the head shape and piston dome shape in the compression effect.

If you are dealing with an interference motor changing rod length to a longer rod has other impacts. pun intended