ka rods vs sr rods and my silly idea

[Vkoslak]

I saw a picture of a ka rod and an sr rod next to each other. Other than length, they look damn near identical. Will a sr rod fit on a ka crank?I couldnt find the diameter of the rod where it attaches to the crank.

89 bore over 86 stroke = 2.1 L

Compression would be real low, might be able to use SOHC pistons to raise it and then turbo that mutha. Should rev pretty good theoretically.

Getting the ecu tuned for that might be a biatch though.

I ask because I have a couple of KA blocks, rods and pistons laying around (dohc and sohc) and I was going to rebuild one to put in this 91 super hicas car with a hole in the block. I initially was going to just put the sohc pistons in the de block, but that looks like it will require ecu programming as well.

Thanks in advance for any info!

[InsanityInc]

compression would be absurdly low. Too low for anything.

an 89mm bore losing 10mm of rod length will expand the combustion chamber volume by 62.2ccs.

Consider that each cylinder (without the combustion chamber) displaces 599.13ccs. Since the CR stock is 9.5:1, that means that the combustion chamber stock is 63.07ccs, meaning that you're almost doubling the combustion chamber volume and therefore the new compression ratio would be:

599.12/125.27 or 4.78:1.

No dice.
Modified by InsanityInc at 8:54 PM 1/4/2006

[AeonTorpor]

You'd also consider that the deck height of the KA is one key factor in determining how long the rods will be. If the rods are too short, you might end up having a piston exit the cylinder out the bottom. Even if it didn't exit, the rod would be at a very steep angle and would probably cause the piston to bind against the cylinder wall w/ a good amount of force. I can imagine it, if it even fit, "Crank Crank Crank, BAM!". lol.

[Vkoslak]

Now I know, and knowing is half the battle!

[InsanityInc]

Oh, and I forgot to mention.

Changing the rod length won't change the stroke. The stroke is just the distance from the crank centerline to the crank journal.

[Vkoslak]

So to get the end result I was after, I would use stock sized (or close to) rods and get a custom crank. f that. :p

if only 11.1:1 compression ratio would work well with stock ecu.

[InsanityInc]

Problem with a custom crank to destroke is that it also reduces the height the piston comes up to. To destroke a KA, you need to use a different block and also different rods generally (to attain a more desirable R:S ratio).

[Vkoslak]

Too much work at that point.

[SimpleEnigma]

Why would you have to reprogram your ECU to work with 11.1:1 compression?

I could understand having to do it in order to maximize power, but you would see a ~20hp jump just by installing the E pistons and goin on about your business.

Everything i have read thus far about E piston into DE block swaps is that they work just fine without any other sort of additional modification, but big increases in performance are noted with used in combination with your simple basic upgrades, i.e. Intake, header, exhaust.

[Vkoslak]

I was hearing that the high compression with sohc pistons and using stock ecu would greatly reduce longevity. If I use the sohc pistons, I was also going to use a 248 exhaust cam turned 4 teeth for the intake cam, and throw on an injen cold air kit. Car came with a 2.5 inch catback, but I borrowed that for my other 240sx

If that setup poses no problems with stock ecu, then Ill probably go for it. especially since I already have everything except the injen intake.

[InsanityInc]

Well, you heard wrong. Higher compression can only reduce engine life if you're detonating more. If you're really worried about that, just retard your timing a bit.

[Chezedik]

Well, to be fair, he may not be TOTALLY wrong. I expect since he is MODIFYING a working factory design, and will likely be raping it like Kobe, it will not last as long. But all things being equal, that is you beating your motor like Ike, even in stock trim, then no, it shouldn't make a considerable difference. If anything, it may get a little more life with new bearings, rings, and valve seals with the replacement.

[S13 240SX]

hrmmmmm... I would really like to see someone someday build something like that for the KA, if it can be done, it would be really sweet.

or instead of sr rods in the ka block with ka pistons, how about sr pistons, and ka rods???

[Chezedik]

Nope, I am afraid that won't work either, the SR is smaller bore.