piston and compression selection....

[AntonioG]

So, I have the motor out of my 91 240sx. There is a bad ridge in cylinder 4 and my machinist said it was bad enough that he would suggest I go with 90mm pistons (.04" over). The only place I've been able to find 90mm pistons is from wiseco. And They're about $500 for the set.
The price is about the same whether I go with 10.5:1 compression or the original 9:1 compression.

My question is, could I safely go to 10.5:1 compression?

I will already be getting stage2 cams from BC, cometic metal HG, emissions delete, and an OBX 4-2-1 racing header. Any advice is appreciated.

[compactfean]

It depends if you ever plan on turboing the car. 10.5-1 with a turbo would most definately need race gas all the time but if not, hells ya! Do the high compression with 91 octane fuel. You'll be fine. you'll need at least a chip tune though unless you run the stock compression.

[AntonioG]

No turbo for me. Not on this motor at least... >.> I figured I'd have to chip whether I went with high or low compression because of the bore, cam and exhaust change. And, thankfully, we have 93 octane here!

[compactfean]

Oh dude! Lucky dawg. Lol. Id say go with the high compression...... Start smashin on some vtec! haha.

[AntonioG]

My main concern is valve clearance. I'm just worried that with the head getting decked(it had a blown HG) and with the smaller HG, I'm worried the hi-comp pistons may hit the valves or head, etc.

[compactfean]

I wouldn't worry about it too much. When they make the pistons, they take that into account. what is the lift on the cams your getting? Id contract wesico and ask them what is the max lift/duration cam compatible with those pistons. Usually a good piston company will shape the piston to avoid issues like this. As far as having the head/block resurfaced, usually its only like .001 .003 inches and can be countered with a thicker hg if desired. If the valves do hit, when rotating the engine, a machine shop can machine the piston to have more clearance.,(I doubt you will have this issue)

[AntonioG]

Alright then, sounds like a pretty good plan to me. Any suggestions on what type of chip/fuel management I should look into?

[compactfean]

I personally am a loyal customer of jasons at e-mance. There is jwt and enthalpy that still do chip tunes as well. Fyi the ka motor is jasons favorite! Im sure he has a good tune for that setup.

[AntonioG]

Alright, I'll check it out. Thanks again!

[240Timmons]

Hi AntonioG I just read your post and i have a few things to say. I'm running a 91 ka24de as well and ironically pretty much the same setup as your described above. if the only pistons you can find are wiseco then at least you know that they are a good reliable brand, as long as they are installed correctly. if you don't plan on going turbo then i recommend using the 10.5:1 pistons. this is only a bit over stock (9.5:1) for your original pistons.

my set up consists of this:
11.3:1 compression ratio
E85/flex fuel
brian crower stg. 2 cams (264/264)
modified JWT tune running SR injectors
4-2-1 header
and a few other small other things, but that's irrelevant

now, my set up is a bit extreme for what you're planning but i'v been down the same road as you're about to take so this helps you .. not me haha

Pistons:
the wiseco 10.5:1 pistons seem very well designed and i have no doubt that they will clear your BC stg. 2 cams b/c the wiseco pistons have clearance notches machined into theme for valves / large cam lift. I'm running SOHC pistons that are completely flat top and don't have any fancy valve clearance notches. so your pistons will be fine

fuel and tune:
you can actually get away with 91 octane fuel. for the longest time i ran 93 oct... but that's b/c i have higher compression. the combustion chambers were designed with detonation resistance in mind so that's a plus, but i still have detonation here and there sometimes. Honestly, it wouldn't even waste your $ for a tune unless you want just a couple more HP, b/c if you have everything set up correctly it's going to run nearly the same. trust me on this, i made plenty of usable power off of a stock tune. . it's not necessary to convert to E85 with your set up, it's not going to be as extreme as you think, but it should be a bit more peppy.

emissions:
you want to keep your EGR, i made that mistake and i regret it (helps with detonation resistance). trust me, it's not going to hurt that much. Make sure you triple check that you won't have any vacuum leaks after you delete the emissions, you're motor WILL run like crap and probably melt if it's bad enough. make sure you don't delete you FPR, you need it.. don't remove your 02 sensor either. i'm not trying to be rude, i just don't know how much you know. better safe than sorry, right ?

head gasket:
deffinately worth spending the extra $ for a metal head gasket, so good on you! the extra cylinder pressure will put more strain on it so it's smart to beef it up.

headers:
good, even the ebay ones work quite well and they are cheaper. i know a handful of people with them. ebay turbo headers suck btw.. i'm just referring to the N/a headers.

extra advice:
invest in a wide-band 02 sensor, it will save your a**. trust me on this.
you'll want a better flowing intake, if you don't already have one.
after all this is said and done you might as well get your head ported and polished ( i can do this, pm me for more info)
you can also just pick up a used block for like $30 and have better luck with it haha

good luck and lmk if you have any other questions!

[AntonioG]

I really appreciate the detail on your post, thanks for that!

I do currently have the stock intake manifold for it. And as far as an EGR delete, I didn't have much of a choice. The previous owner did the deletion. A very poor one at that. I'm sure it was leaking pretty bad. So, I've pretty much just improved on his deletion, by doing it properly.

I will look into the wide band O2. I'm assuming since the wide band sensor can handle more flow change, it would be enough to correct the fuel trim and keep me from needing to chip it? Even if I get a better flowing intake(Also, do you mean the entire intake, including the manifold)?

[240Timmons]

if you can, i would recommend finding an egr and putting it back on.. but that's just what i would do.

the wide band is actually a completely separate system on it's own. you want the wide band with the meter so you can see where your AFR's are at. the stock 02 sensor works fine for a stock ecu. a wide band 02 sensor is not compatible with your stock ecu.

and even if the wide band could work with your ecu it would have no way of changing the fuel trim. the only time an 02 sensor is used is when you're crusing or light throttle.. and it will try it's best to go to 14.7:1 AFR.

also, with the pistons, you wont see an increase in flow or a need to change fuel or timing maps. your motor wont really know the difference.. the timing on the other hand will might need to be retarded... you can do this via cap and rotor if need be

[AntonioG]

Alright, I was planning on getting an AFR meter anyway. I'll check around on the EGR, though. Again, I appreciate all the info.

[240Timmons]

^^ no problem sir, just here to help

here's a link to my work:
affordable-cryo-treated-port-polished-h ... 55412.html

lmk if you have any Q's.

[240alex420]

im running flat top pistons in my ka24de im not sure if you guys are doing the same thing, but im having a hard timing getting my engine running correcting, i have base timing atm, please help