Overbore

[SimpleEnigma]

I'd like to increase the displacement of the KA from 2.4L to like 2.5L or a little greater in prep for a custom turbo kit.... a lot is going to be done to the engine in preparation for the kit so i can get the most Hp with the least amount of boost possible (going to shoot for ~350rwhp). Is the only option to bore out the cylinders and use oversize pistons?

[TrunkMonkey]

if you're going to run boost and you're only shooting for 350rwhp, then overboring for more displacement won't be necessary.

-demetrius

[All_Motor_KA]

I'm going NA and I know a few people on here have bored over in 2.5. I was wondering what piston size do they use, what rings, and what exact bore and stroke they have. (if stroked) Thanks in advance.

[deviousKA]

I think im the only one with 2.5l ka? Mine is 2502cc and i achieved this with a 91.1mm bore and stock stroke. It is also rumored that the nismo built CORR racing ka's are 2.5l, although i think this is a marketing ploy, the 2005 frontiers have the 2.5l qr and the 2005 bodied race trucks use ka's.

The reason i built the 2.5l was simple at the time, I needed high compression sohc ka pistons that I could find affordably. Boring to 91.1 was a side effect of the 10.5:1 compression ratio that i was after. In order to fit the pistons they had to be modified, both the skirts and the tops. The top edge is machined away so that it clears my headgasket and edge of combustion chamber. The block is a japan rwd 240sx sohc block, which is just about the worst candidate for the 91.1mm bore, the mexico cast ka blocks have thicker cylinder walls. The main point of concern on both blocks is the cooling passages that run between bores 1/2 and 3/4, at 91mm bore the cylinder wall is the thinnest at this point. I was very concerned with hotspot issues, although everything is speculation until it is tested, so i did. It has been running perfectly for 15k miles, 2 years.

The pistons i used are mitsubishi 2.6l I4 pistons, not the starion, but the early model of the mazda/mitsu 2.6l with the flat tops. They are .060" taller compression height, compared to oem nissan 2.4l. The piston pins are also 22mm vs. the 21mm stock nissan pins. My rods were machined for the pins and then press fit. The skirts also had to be clearanced and all piston weights matched after modifications.

These pistons are also used in the early model of the dodge caravan 2.6l.

Also, a headgasket with 100% proper bore spacing is required, youd actually be surprised how off some are when your comparing a stack of different manufactures. I found ROL to be the best and thats what i use.

I can sell the same pistons that i used, non modified, if your interested. They cost the same as ka24 replacement pistons.

[SimpleEnigma]

The reason i wanted to look into overboring was the simple fact that, "There is no replacement for displacement" (to a certain extent, that extent would be of course boost, then again that is also to a degree). Anyway, as yall know, the larger your displacement, the less boost you have to use in order to gain the targeted power level, which in turn leads to a longer lasting engine. Now of course, it kinda looks like overbore might be a little overkill in order to gain my projected Hp....I am trying to achieve it by 15psi or less which is why overboring came to mind....I had also planned to get the cylinder head and intake mani ported and polished amongst a few other mods

[Ajax]

How do you figure out the total displacement when overboring?My block is being bored .03 over (I think that's what my mechanic said).

[Veriest1]

Hey Devious, you said you were "after" 10.5:1 CR. Is there a particular reason you decided on this number? Why did you not aim for something higher?

Is it because with the proper setup you can still get away with 89 octane gas? I know my Z3 does this and it's kind of nice.

[deviousKA]

10.5 is a reasonable level that allows me to still run premium pump gas in my sohc. I can run no less than 92 octane with my non-modified combustion chambers.

Compression ratio/detonation/octane level is purely dependent on the combustion chamber and cylinder head design. On sohc ka for instance, a very "primitive" design in these areas only allows a mediocre compression ratio to put it on the brink of detonation. You Z3 has a much higher tech cylinder head, with modern quench theory most likely used.

This is why ka24de can handle a bit more compression ratio than the sohc ka24e. An approximate figure here but 11:1 in ka24de will have same tendency to detonate under same conditions as 10.5:1 in ka24e with same fuel. Ka24de has quench design implemented into the combustion chamber design.

This is also why honda engines can tolerate such a high compression ratio and still not detonate with pump gas. The combustion chamber/cylinder head design has modern theory applied. Mainly this is in the "quench" area but valve arrangement and volumes also.

BTW, damn, nice Z3

[Dattebayo]

Are you all also aware that you can buy NISMO 9.1 to 1 SOHC pistons (or just go conventional lesser CR 8.6 SOHC pistons from a early hardbody) and drop them into your DE? I think the compression comes out to about 11.1 - 11.6 to 1 or something due to therod to piston connection pushing the piston higher and the pistons shape being flat-top. I reasearched this alot a few years ago.