Destroking A K Engine

[ATOYOTA]

Has anyone ever put a 3K crank in a bigger bore motor such as a 4K or 5K? I know the valvetrain doesn't really allow these engines to handle high revs, but having a higher bore/stroke ratio sure would improve the responsiveness. It'd be a pretty cheap mod if everything bolted together.

 

Are the cranks actually interchangeable? Which rods are longer than others to bump the compression ratio? Are all the pulleys/flywheels interchangeable too? What would need doing aside from the bottom end swapping?

[Clapped out]

Bore size is the same in a 3k and 4k, 5k have the 80mm. Ive got a 4k crank in a 3k block, so yeah, you can swap the cranks over. 5k's run a 4k crank. 3k crank and rods are different to 4k's. 5k's run the same length rod as a 4k. Hope this helps,

 

cheers!

Edited December 18, 2014 by Clapped out

[GJM85]

 

5k block decked 7mm. 3k crank. 5k rods and shallow pistons. 4k head.

 

I would estimate displacement at 1.4 litres.

[GJM85]

Crankshaft journal diameters are the same throughout.

[ATOYOTA]

The bore/stroke I read up were as follows:

 

3K:

75.0mm X 66.0mm

4K:

75.0mm X 73.0mm

5K:

80.5mm X 73.0mm

 

So a 5K with a 3K crank should in theory make it 80.5mm bore X 66.0mm stroke, which is crazy over square.

Edited December 18, 2014 by ATOYOTA

[Clapped out]

Oh and don't worry about the valve train with high revs, I've seen k motors hit 9g and have heard of them screwing out to 10, don't know how true this is, would like to hear from someone on here who has/had a k motor handling these reves,

 

cheers!

[ATOYOTA]

Mine has handled ~9000rpm, but it has a twin cam head :P and also does not run now :(

[coln72]

8000 was seen on occasion, but that's where the tachometer stopped....

[Super Jamie]

To continue ATOYOTA's post above:

 

3K/4K pistons with 3K crank:

75.0mm X 66.0mm = 1166cc

 

3K/4K pistons with 4K/5K crank:

75.0mm X 73.0mm = 1290cc

 

5K pistons with 4K/5K crank:

80.5mm X 73.0mm = 1486cc

 

5K pistons with 3K crank:

80.5 x 66.0mm = 1343cc

 

So a 5K with a 3K crank should in theory make it 80.5mm bore X 66.0mm stroke, which is crazy over square.

 

That's part of the idea, so you can rev it harder.

 

If I remember correctly, 3K rods are also shorter, so there's less rotational mass because there's less metal. Same theory as lightening a flywheel.

 

There was a webpage a long time ago with the different rod lengths measured out, maybe it was a post Stewart made on the oldcorollas Yahoo Group.

 

This would be a neat thread to capture in the FAQ.

 

Oh and don't worry about the valve train with high revs, I've seen k motors hit 9g and have heard of them screwing out to 10, don't know how true this is, would like to hear from someone on here who has/had a k motor handling these reves

Easily possible with good double valve springs.

 

I'd had my old 4K to 8 and a half grand fairly regularly, even 9 grand on occasion, though it stopped making good power well before that so there wasn't really much point. I only revved that hard because it was quicker than changing gear down the final corner of Mountain View (Grafton) Hillclimb. I really should have sourced a 4.1 or 4.2 diff instead of the 4.3 I used for street driving. Anyway.

 

Another friend's accelerator cable broke and his motor sat at 10500 before he killed the ignition. That motor only did that once, but it also saw regular 8500rpm.

 

I have heard that you crack blocks if you venture over 8500 regularly because of resonance, but I'm not sure how true that is.

[ATOYOTA]

If you properly balance the crank, you should be able to do away with the cracked block myth.

 

I don't know if the 3K rods would work as far as keeping the compression ratio goes, sounds like a lot of decking.

[Super Jamie]

You can also use 4K rods, plus pistons with a shorter wrist pin height, and not deck the block at all.