K8 Crank vs KL crank?

[MrMazda92]

I keep forgetting to ask, and search has failed me enough times that I will go for it now.

How much does the K8 crank weigh?

[mikeinaus]

im not too sure but if your looking for the lightest the 2000+ millenia (klg4) are supposed to be the lightest.

[Inodoro Pereyra]

MM92: just in case you're thinking what I think you're thinking the K8 and KL cranks are not interchangeable. The K8 crank has a shorter stroke than the KL crank (69.6mm against 74.2mm). I don't know about other measurements (distance between cylinders, journal diameters, etc.), but the shorter stroke alone will get your specs all out of whack.

Mike: as far as I know, the KL-G4 only came on the 626, never on the Millenia.

[Ryan]

Inodoro is correct, the KLG4 was installed in 98-02 626's. The 2.5 milli engine was a strange combination of ZE and DE.

[MrMazda92]

Mike, you're right about light weight, but it's the 626 as Inodoro stated It also seems to be inconsistent... as if only SOME of them were lighter.

Inodoro, you're partway there... I was also thinking about longer rods... Idk, maybe my brain rattled a bit too much on my last road trip. It's only 4.6mm as you pointed out, so I imagine slightly longer rods would be a feasible fix...

Ryan, I remember seeing that you have 2 gutted K8s laying around? Any chance you could weigh the crank for me please?

I might be crazy, but it couldn't hurt to check huh?

[Inodoro Pereyra]

Inodoro, you're partway there... I was also thinking about longer rods... Idk, maybe my brain rattled a bit too much on my last road trip. It's only 4.6mm as you pointed out, so I imagine slightly longer rods would be a feasible fix...

There are 2 parameters (among a gazillion more) to consider here, that are related: compression ratio, and final cylinder pressure.
True, you can compensate the extra final distance between the crankshaft and the cylinder head by using a longer rod. That would have the extra advantage of giving you a higher rod to stroke ratio, which will translate into a higher redline, and more power at high RPM. Simply put, a rev happier engine.
But that's about it. Providing you can get longer rods, and modify your pistons to reach the same CR the engine had originally, you'd lose torque pretty much all along the power curve, and your power at lower RPM would also suffer (remember now the lever your piston has to move the crankshaft is shorter), and, in the meantime, to reach the same CR as before, you will need your piston to go higher into the combustion chamber (because now it has a shorter travel distance than before), so you're probably gonna be kissing your non interference status good bye.

Now, if you were building an all out racing engine, and planning to slap a big turbo on it, that'd be a great way to decrease your CR (if you don't use longer rods), so you could increase the boost accordingly.
That is, of course, assuming the K8 crankshaft can take the abuse, which is a whole different animal...

[MrMazda92]

Maybe I'm not looking at it the right way, but wouldn't longer rods eliminate most of issues with the actual stroke length? Well the piston also wouldn't move as far down in the cylinder, so would that decrease in travel have adverse effects as well?

After re-reading your post, I think that's exactly what you meant by this statement: "to reach the same CR as before, you will need your piston to go higher into the combustion chamber (because now it has a shorter travel distance than before)"

I'm still curious about the weight though, whether or not the application would be correct!

[Inodoro Pereyra]

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 ).

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.

[MrMazda92]

That covered every question I had on the matter, except the original one I understand that there's more to it than I originally hoped(Figured there would be, but the only stupid question is the one that's never asked).

I still wonder what it weighs, out of curiosity if nothing else.

[Ryan]

sorry, I scrapped my K8's.

[Inodoro Pereyra]

I understand that there's more to it than I originally hoped(Figured there would be, but the only stupid question is the one that's never asked).

Definitely. But you already know I'm of the same philosophy...
Don't get me wrong: I think it's great that you chose to ask these questions, because they not only help you, but may help somebody else in the future. Not to mention it feels great to see REAL mechanics is not a lost art, after all...

I still wonder what it weighs, out of curiosity if nothing else.

Sorry, can't help you on that...yet.
But who knows? Maybe in the future...

[MrMazda92]

Ryan,
No worries, I'll figure it out!

Inodoro,
I'm almost done fixing the little issues with my car, so hopefully I'll start making greater progress on this particular build soon. I'm also working on an outdoor "carport" of sorts so I can indulge and paint my car.

[Inodoro Pereyra]

Pictures! Or it's not happening...!

[mazdags94]

Just so happens I have a K8 crank sitting in my garage doing nothing but staring at me when I walk in. I could easily plop it on a scale to get a general weight measurement if nothing else...

[Inodoro Pereyra]

I could easily plop it on a scale...

So...What are you waiting for???

Let us know what you find out...