Hmmm...not sure what you mean about the issues with the actual stroke length...
Let me see if I can break it down into numbers (being that, as we know, words in English are not my strong suit
![Wink ;)](./images/smilies/icon_wink.gif)
).
The
stroke is the distance between the center of the crank pin at TDC, and that same center at BDC. That distance determines the distance the piston will travel inside the cylinder.
So let's put some numbers to it.
Let's use the KL-ZE, so we don't have to deal with the cupped pistons.
Compression ratio is the ratio between the room inside the cylinder when the piston is at BDC, and the remaining room at the combustion chamber, when the piston is at TDC.
Now:
Being that we have a stroke=74.2mm, and a bore (inside diameter of the cylinder)=84.5mm, we can do some math.
The capacity of the portion of the cylinder swept by the piston (aka the "displacement") will be, then: (r2 x π) x H, in which "r" is the radius of the cylinder (bore/2), and "H" is the stroke. For the KL that will be:
(42.25^2 x 3.1415926) x 74.2=(1785.0625 x 3.1415926) x 74.2=416.109 cc approx.
Now, we know the ZE compression ratio is 10:1, which means the combustion chamber capacity will be 1/10 that of the
TOTAL cylinder capacity (the volume swept by the piston, plus the combustion chamber volume itself), which means the volume swept by the piston will be 9/10 the total cylinder capacity. So we know the combustion chamber capacity will be 1/9 the volume we just calculated, or 46.234 cc.
Now, we replace the KL crankshaft for a K8 unit. All of a sudden, our stroke went down to 69.6 mm, which means the volume swept by our piston is now (1785.0625 x 3.1415926)
(because "bore" and "π" didn't change) x 69.6=390.312 cc.
That leaves us with 3 possible scenarios:
1. We leave the original rods.
In this case, now the piston is stopping 2.3 mm shorter of where it did with the KL crankshaft (because of the shorter stroke), which will increase the combustion chamber volume (at TDC) in (1785.0625 x 3.1415926) x 2.3mm= 12.898 cc. So now the total combustion chamber volume will be 46.234 cc + 12.898 cc = 59.132 cc.
That means, now, our total cylinder volume is 390.312 cc + 59.132 cc = 449.444 cc, and the compression ratio becomes 449.444 cc/59.132 cc =
7.6:1!!!
2. We lengthen the rod, so that the piston at TDC leaves the same combustion chamber volume as it did with the KL crankshaft.
That means, now, the total cylinder volume will be 390.312 cc (the volume swept by the piston on the K8 CS) + 46.234 cc = 436.546 cc, which will leave our compression ratio at 436.546/46.234 = 9.44:1. Close, but no cigar.
3. We lengthen the rod, so that we end up with the same compression ratio as with the KL crankshaft. For that, we will need to reduce the total combustion chamber volume to 1/9 that of the volume swept by our piston, or 390.312 cc/9 = 43.368 cc.
That means that
besides lengthening the rods, we will have to modify the piston so that it intrudes almost 3 cc into the combustion chamber. So either we do some very careful engineering, or we can kiss our "non interference" status good bye.
Now, let's go for the "stroke to rod" issue.
Just for the sake of argument (as I don't know the length between centers of the real KL connecting rod), we will establish the "rod" value arbitrarily at 126.14 mm, so we end up with a nice 1.7 rod to stroke ratio .
So now that we replaced the crankshaft for the K8 one, our stroke to rod ratio went, all of a sudden, up to 126.14 mm/69.6mm=1.81 approx. That's great, as it means our engine will rev up higher than before, but, as everything else in mechanics, it comes at a cost:
1. Now we have a shorter stroke, which means the piston has a shorter lever to push on the crankshaft, so we lose torque all over the RPM range.
2. Remember our CR went down considerably, and with it our power and thermal efficiency, which means we will be making less power at the same RPM than before (although we may end up making the same or more power at higher RPM), but our fuel consumption won't go down accordingly, as we will need more fuel to heat up the engine, now that the cylinder pressure went down.
But then, we can partially solve the problem by lengthening the rods...
Not quite. Let's suppose we lengthen the rods by the 2.3mm we talked before, and we get new pistons to get our original CR back. Sure, our stroke to rod ratio just went up again, to (126.14 mm + 2.3mm)/69.6mm = 1.845, but now the rods are heavier, which will partly or completely counteract (for lack of a better word) the advantages of the higher stroke to rod ratio, and we still can't solve the lower torque problem, which we can only solve by changing the gear ratios either in the transmission or in the differential.
So, to finalize, unless you can come up with a ton of money, to get some custom pistons and lightweight rods (besides beefing up the mains and camshaft bearings, replacing the valve springs and retainers improving the oiling system, and a lot of other things), and unless you're building an all-out racing engine, replacing the KL crankshaft for a K8 unit doesn't really make sense, at least in my opinion.
Now, as you know, I AM planning on lengthening the rods on my future KL build. But I'm also planning to slightly increase the engine stroke, not to decrease it, as to make more power and torque in the
lower RPM range, which is where the KL engine is lacking a little.