Rb28 With Rb25 Neo Head Build

[Borci88]

Hey guys,

Currently building my motor which was originally an RB26. It now has an RPM 2.8 kit in it and I'm adapting the RB25 NEO NA head onto the block. Due to the NEO head having a head cc of 51-52cc this means I have to change what pistons I use instead of the normal RB26 pistons.
I'm aiming for a compression ratio of 8.5:1 to ideally 8.7:1. I'll be running E85 at all times. I need a piston pin of 21mm and a compression height of 30mm.

What pistons does everyone use? Everything is pointing towards getting a custom piston made. I've also heard there is a Honda piston that would work with small modification but I can't find anything that would suit. ACL no longer make the pistons to suit. I believe I'm looking for a flat top piston or one with a small amount of dish.

[Ben C34]

Are you aware the ports on the na neo head are heaps smaller than the turbo head?

As for pistons, getting custom pistons made is extremely easy.

Use a neo turbo head.

[Ben C34]

http://www.sau.com.au/forums/topic/284632-rb25de-neo-head-differences/

[Piggaz]

Why such low compression if you're using E85 all the time?

[Borci88]

Are you aware the ports on the na neo head are heaps smaller than the turbo head?

As for pistons, getting custom pistons made is extremely easy.

Use a neo turbo head.

That was the reason for buying the NA head. It's been proven to flow over 500kw without porting, it's more than enough for what I need.

Good question Piggaz, I guess because I will have the option of Flex Fuel. Realistically it's around standard compression with a torquier head with VCT, 2.8 litres and E85. I shouldn't have a problem with bottom end torque and don't necessarily need to go higher comp? Thoughts?

[iruvyouskyrine]

There is zero reason not to go higher comp, especially if you are full time E85.

Have you ever heard an RB owner say "man I wish I went with a lower CR, there is just too much torque everywhere for me"

[Piggaz]

9.0:1 minimum even on petrol.

Evo 9's are 8.8:1

R35 is 9.0:1

RB25 is 9.0:1

9.0:1 - 9.5:1 would be nice is predominately on E85. You are going to flex it so the 98ish map(s) can be relaxed.

[Borci88]

I think we went 8.7:1 or so on the last build as just an RB28. There was nothing wrong with street driveability. I will take what you've said on board though and swing it past my engine builder to see what compression ratio would be ideal and what we can achieve. But I can't start working out compression ratios until I can find a piston that can be made to suit. Otherwise I'll just go most likely a custom Wiseco piston. Have you dealt with any of that sort of thing before?

This will be a track car as well as a street car.

[sneakey pete]

9:5 is what I'm shooting for. Easy to control with e85 but not getting silly to the point where other issues start to build up with the fact the head is a 30+ year old design

[Ben C34]

I'm not talking about the combustion chamber. The actual ports are tiny.

Click the link I posted

Whoo did 500kw with a na neo head?

[Borci88]

9:5 is what I'm shooting for. Easy to control with e85 but not getting silly to the point where other issues start to build up with the fact the head is a 30+ year old design

Fair enough! Thanks for your input

I'm not talking about the combustion chamber. The actual ports are tiny.

Click the link I posted

Whoo did 500kw with a na neo head?

I think it was Big Red (the time attack 32), but I can't remember off the top of my head, I'll have to ask. Theres no reason why they won't flow 500kw anyway. Regardless though I'll be aiming for 400kw not 500, but the scope for future would be nice.

[bigmikespec]

I'm not talking about the combustion chamber. The actual ports are tiny.

Click the link I posted

Whoo did 500kw with a na neo head?

It seems crazy.

Borci88, I don't know why you would persist with not porting the head (only if you could not afford it), you can make power through small ports but not efficiently. Put VCT on the RB26 head if you really want, done.

[Borci88]

It seems crazy.

Borci88, I don't know why you would persist with not porting the head (only if you could not afford it), you can make power through small ports but not efficiently. Put VCT on the RB26 head if you really want, done.

While you are correct porting it would probably make it more efficient, I'm more interested in torque rather than peak power, and porting the head isn't necessarily going to help that.

I've done research into putting VCT into a 26 head and from all accounts you can't do it without going the Otomoto kit or going VCAM. I know of one person who did it himself in his garage but he hasn't revealed details of how he did it.

[Hadouken]

What a waste.

Big $ bottom end and oh yeh let's slap a $2 head on it

[Borci88]

Do we all know that the NA head won't flow enough or are we just looking at pictures and going nah I don't think that will work? Because it doesn't sound like it's been done before and I'm building it for torque (thus the 2.8, thus the E85, thus the VCT, thus the smaller ported head). The car made 385kw earlier, this head will more than flow that, it's not a waste. I'm doing it to test a theory,

Regardless of all that it doesn't answer my main question of Piston. Port size is irrelevant, I need to find out what piston will work with a 52-52cc combustion chamber and an RB26. RB30 stuff doesn't work I understand. Custom is looking like the only option, but thought I'd swing it past you guys to see if there was an off the shelf option.

[Borci88]

Also to settle your minds, I'm a mechanic and other than machining the head, this isn't anymore expensive or cheaper than going a turbo head. If it works and its good then we've opened up a new line of thinking with RBs in Australia. If it doesn't work then it puts that idea to rest, this is more R&D than anything as I can swap the heads over on a weekend at work.

[KiwiRS4T]

You may be fine with the pistons you have. If you can calculate the CR for the 26 head then you can substitute 52cc for the 26's 63cc and see what you come up with. I am guessing it will be around 9.6:1 which should be fine or you could just use a thicker gasket to reduce it slightly.

[Ben C34]

I'm very interested to see how you go.

My thoughts are the small ports will be too restrictive to make decent power, and not worth the response gain.

If the small na neo ports were good, and could flow well, Nissan would have used them with the turbo models.

[SS8_Gohan]

is RPM looking after the build?

they've done NEO heads before, just ask them, surely that would be easiest?

[Borci88]

You may be fine with the pistons you have. If you can calculate the CR for the 26 head then you can substitute 52cc for the 26's 63cc and see what you come up with. I am guessing it will be around 9.6:1 which should be fine or you could just use a thicker gasket to reduce it slightly.

Possibly, we don't think it'll work though, more investigation is needed definitely.

I'm very interested to see how you go.

My thoughts are the small ports will be too restrictive to make decent power, and not worth the response gain.

If the small na neo ports were good, and could flow well, Nissan would have used them with the turbo models.

Same here, you have to remember that the NA had a higher compression than the Turbo models, and therefore longer and thinner intake runners were a benefit to that already gutless motor

is RPM looking after the build?

they've done NEO heads before, just ask them, surely that would be easiest?

They are, unfortunately it's not that simple as RPM usually keeps it simple in the head department, only a very small amount of motors have been built like this, and whether or not an NA head has ever been used, I'm not sure. That's how I knew about the Honda piston, the builder said that there is a Honda piston that will do it but he couldn't remember it off the top of his head. Therefore I've been searching to find one but haven't had much luck. This is unexplored territory at this point I believe.