Jump to content
SAU Community

Recommended Posts

Hi guys I'm new to this forum,

Ok iv purchased an rb30 block, it's been deck,honed, blue printed, balanced ,

New stock rb30 rods, race bearings, acl dura light pistons ra9301y 1.00 rib 0.025

Iv got a rb25det neo head that I plan on using for the build, I'm wondering what head gasket

I should use? Any info would be great please.

Did you know that the Neo head has approx 10cc smaller combustion chambers than the normal RB25 head and that you must therefore use suitable pistons, not the same pistons that you would use for a normal 25 head? If you don't, compression will be sky high.

  • Like 1

^^^ Exactly.

IIRC Neo heads have approximately 50-51cc chambers. R32/R33 RB25 and RB26 heads have 62-64cc chambers. This difference will shoot your static C.R. into the 10's if you dont get pistons suited to this head. That might be OK if your running E85 or race fuel, but definitely not on 98.

Read the RB30 thread and associated RB30 DOHC pdf. Its there for a reason.

It even states the brand of head gasket recommended for this build by many others on the forum. Im sure if you searched the R33 RB30 conversion thread, you might even get a part number.

Has anyone done the calcs to see what the static CR would be with a Neo head on a std RB30E or RB30ET short engine ?

I have wanted to, just haven't found the details regarding the OEM RB30 pistons. TBH I havent looked that hard as I already have a built RB30. But one day I wouldn't mind swapping the motor out to use the Neo head :yes:

From memory when I disassembled my RB30, the RB30 OEM pistons have a dome but to what extent (cc wise) I'm unsure.

Attached is my own calculator I used to work out my RB30's static C.R. Bare in mind, I used all metric units. So some converting may be required.

static compression ratio calculator.xls

If anyone can shed some light on the OEM RB30 piston specs, would be greatly appreciated.

Edited by R32Abuser

Ok but I've just read a tread where a guy is doing the same build. And has used the exact same pistons as me. The acl rb30et pistons with 1.1mm head gasket and it gave him a 8. Something compression ratio. I've seen 2 different forums where guys doing this build have used the same pistons that I am

How can you expect an answer unless you give all the information including which pistons have been used and you have cc'ed the head?

Everyone will be guessing about your final comp ratio.

Just read the first post again (are the pistons you have listed 30ET pistons?)

Edited by wolverine

At the start of this post I put what my build was. Pistons etc. yes they are rb30et acl pistons . I haven't flow tested the head yet but guessing it will be around the same as every other factory rb25det neo head

Hey theres nothing wrong with using a neo head I know I have.

I used ACL 8.5:1 pistons on my 1st motor and Mahle 8.5:1's on my second. On the second I decked the head and also decked the block to a 0 deck height. standard nissan rb25 head gasket & I also run BP98 with no problems. I don't think you will have any problems unless your trying to put 65 psi of boost into it. but hey thats just my experience so far.

I wish I had a Neo head. With stock pistons and the 33 head my compressions are 132psi (leak down test is fine) so I am guessing about 8:1 if I'm lucky. Only making 230-awkw at 15psi so soon to be retuned to 25 psi or as much as poss till it stops making more power (GT3540). I'm using a Tomei metal head gasket.

I wish I had a Neo head. With stock pistons and the 33 head my compressions are 132psi (leak down test is fine) so I am guessing about 8:1 if I'm lucky. Only making 230-awkw at 15psi so soon to be retuned to 25 psi or as much as poss till it stops making more power (GT3540). I'm using a Tomei metal head gasket.

Likewise.

Only thing I would change about my build.

In a perfect world a compact chamber with a flat or slightly dished piston giving the desired static CR and stopping approx 25 though short of the heads deck face works pretty well . IMO the dama is people using all kinds of weird and wonderful ways to get the CR they want ie thicker or thinner head gaskets . Few of these methods works best and ends up being a compromise . Making changes the worst possible ways can actually give you a low compression ratio engine thats not very detonation resistant .

I think if you could emulate a Neo engines pistons and static CR with a deeper dish or whatever you'd have a pretty good solution in a Neo RB30DET .

I would have thought by now plenty of people would have buretted std RB30E and ET pistos to get those volumes . Getting either engine to top dead center and placing a feeler gauge between the piston edge and a steel rule across the bore is not exactly rocket science .

I bet plenty an RB26 engine builder would have loved smaller chambers and flat or dished pistons .

A .

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now



  • Similar Content

  • Latest Posts

    • Thanks! Supercheap has a sale on this weekend so might be a good opportunity to get a new one. I do have a decent multimeter so I'll give it a try either way.   Hah, you got me! I do not. All I know is that the battery holds charge and sits at an okay level before I start the car. Doing the full charge and rejuvenation cycle on a trickle-charger seems to not really make a difference to the cold start behaviour. I'll try and measure it during cranking and see what it does. It's likely terrible; it certainly feels terrible.  I'll be happy if it is really just the battery and I don't have to dick around with the alternator or starter motor.
    • how do you know voltage is fine?  quick easy test is connect DMM up and put it on max/min mode, remove fuel pump fuse, crank car for 3-5s, go check min voltage You'd be surprised how many "healthy" batteries are showing under 10v during this test in cold weather. 
    • Not properly. You need to be able to dissipate 100 amps or so to doing it meaningfully. You can do it indirectly by watching to see how far the voltage falls during cranking. Unplug the coils or something else to prevent it from starting so you can get a good couple or three chugs. It also helps if you have the multimeter set up reading before you start, and that it has max/min functions. So you can catch the real minimum without having to watch the screen, which often doesn't update fast enough to show the real max/min in dynamic situations. Or use a digital oscilloscope, which can be obtained for <<$100 from Aliexpress (although I'd argue for paying up to ~$200 for a nicer one). A >4 yr old battery will very likely be well down the path to the knackery. Many only last 5-6 years these days. The cold weather lately will definitely make it worse.
    • Hmmm, what do you mean by getting weaker? And where did you get that test done? I've been wondering about my battery because the cold starts crank super slowly. The voltage is fine. Not sure if cranking amps can be checked at home. Battery is 4+ years old.
    • No. The simplest wastegate hookup, with no solenoid or other form of "boost control" (ie, control over your boost control, if you know what I mean) is a single hose, direct from the turbo outlet/hot pipe, straight and only to the wastegate actuator. It is that pressure signal that drives the wastegate to open, providing the boost control (and by "boost control" here, I mean, limiting how high it can go, which is essentially the spring pressure of the actuator). You only end up with tee pieces and alternate flow paths once you start adding things to the boost control system to allow you to determine how much of that boost signal makes it to the actuator. There are so many ways to do that that there is no single way to run the hoses and tees and the like. If you have a stock boost solenoid, then all it does is either allow all the boost signal to go to the actuator, or open up to allow some of it to bleed off. There needs to be a restriction in that bleeder to allow only a small amount to bleed off. And in a stock system, that would then be plumbed back to the turbo inlet (for "emissions control" reasons). That is actually what that nipple on your BOV return pipe could/would be for. If you have an aftermarket boost controller and solenoid, then the above is mostly true, but there is no need for a restrictor in the bleed, because the solenoid is pulse width modulated to create a variable bleed off. The air that escapes from the bleed can either be vented, or also returned to the turbo inlet. For emissions reasons it should be returned to inlet, but the amount of air being vented is so small that it really doesn't matter (either from an emissions perspective, or from an air-fuel ratio affecting perspective).
×
×
  • Create New...