Jump to content
SAU Community

Recommended Posts

We have an Ex Zoom GTR from Vic in bits now that has RB30ET pistons,

Now this has to be a nastie low comp ratio.Worst part is we have to re use the pistons as the owner only wants to replace the bearings.

Its going to be interesting to see how laggy this thing is.

If you use the CP pistons its easy to get your machine shop to get you 8-1-8,4 to1 comp.

well as soon as i get the block and head from machining i will mock fit the bottom end and check the measurements and see how much i want to deck the block, if at all. but 8:1 or 8.3:1 will be fine either way.

I'm unsure exactly where it origionated, but it is fairly close to the mark.

Using my 62cc combustion chamber, 86mm pistons, 85mm stroke, 1mm headgasket, 0 deck clearance, -1cc dish - the calc shows a comp ratio of 8.175:1.

Same figures with the 55cc rb30 sohc's combustion chamber and I get 8.988:1. Nissan claim 9:1 so close enough for me.

If I then directly enter my specs.

87mm pistons

87mm h/gasket bore

1.397mm head gasket

62.2cc comb. chamber

14cc dome

1mm deck clearance

= 9.09:1

--

If I were to dial in the stroke, 1mm headgasket and deck clearance those pistons run in the rb25 (according to wiseco) it returns the comp ratio of 8.2:1 as indicated by the wiseco data sheet.

So... I must be running a 9:1 comp IF the engine builded did as he said and pulled 0.020" off the block.

Even if he didn't I would still be running an 8.7:1 comp ratio.

Interesting.... It still returns damn good economy considering it runs such a large quench gap. 'Maybe' the higher comp negates the larger quench area vs a lower comp nice quench.

Andrew, you wouldn't have a 240rwkw dyno sheet laying around? I'm interested to see how the low end torque as you initially jump on the throttle responds.

Its difficult to compare with a gt35r slapped on the side of yours. :(

If I am indeed running a 9:1 comp ratio it always leaves the door open to drop a 1mm headgasket in there that will bump comp ratio up to 9.4:1 and run a turbo such as the xr6 item. The higher comp will work well with the free flowing hotside, how well. No idea. :)

I'll have to check the deck clearance when the head is off. I have not pulled the head off for the springs and restrictors yet. :thumbsup:

No time during the semester. :D

On the very first RB30E / RB5DE we built (5 years/80,000k's ago) we decked the block and the cylinder head to get ~8.3 to 1 compression ratio. No big deal, the blocks are always skimmed for the o'rings and the head to check flatness. I chose 8.3 because of the standard RB30E cast pistons and using 1 bar of boost maximum.

Our later RB30's have all had 9 to 1 compression ratio and they are much more torquey off boost and built boost noticeably faster. Some of that is compression ratio and some could be attributed to the better quench. Which also helps with the detonation problem.

:ermm: cheers :D

Decking the block is fine and has been done since the dawn of time.

Its most definitely NOT a big no no.

However decking too much... I can't see any problems with it UNLESS the blocks design causes problems with quench, water and or oil galleys.

So block dependant I would say.

I really don't know for sure, which is why I asked some one in the know to clear it up.

With regards to the vg30 pistons. I can't see how you have achieved such a high comp raito with a piston that runs a 9cc dish and sits 0.040" down the bore.

The CP's run flush with the top of the block, and no dish yet achieve around the 8.2-8.3:1 ratio. :)

On the very first RB30E / RB5DE we built (5 years/80,000k's ago) we decked the block and the cylinder head to get ~8.3 to 1 compression ratio. No big deal, the blocks are always skimmed for the o'rings and the head to check flatness. I chose 8.3 because of the standard RB30E cast pistons and using 1 bar of boost maximum.

Our later RB30's have all had 9 to 1 compression ratio and they are much more torquey off boost and built boost noticeably faster. Some of that is compression ratio and some could be attributed to the better quench. Which also helps with the detonation problem.

:) cheers :D

Gary,

How do you achieve a 9:1 ratio, what pistons?

Sky30,

I've been looking through the specs of various piston manufacturers.

It appears the JE VG30DET pistons are suitable. (22mm pin vs rb30's 21mm)

4.8cc dome, 1.255 compression height, the only down side is they are only available in 87.5mm.

http://www.jepistons.com/pdf/2006-sportcomp3.pdf

Without decking the block it 'appears' they will achieve an 8.2:1 compression ratio with a 0.045 deck height and 1mm headgasket.

IF you were to bring the pistons up flush with the top of the block (rip 0.045" off the block) compression will raise to 9.06:1.

I do know Gary has used JE's in the past. So I assume this is how they get their 9:1 RB30's?

I'm also assuming the pin dia. issue is sorted with a nice set of rods.

87mm CP flat tops grab an 8.4:1 comp ratio with 0 deck height. (Requires 0.020" off the block)

87.5mm JE 4.8cc domes grab an 9.06:1 comp ratio with 0 deck height. (Requires 0.045" off the block)

Having just finished my 25/30 conversion and having to modify the exhaust side to suit a bigger turbo and external wastegate, i have decided to make a few more systems if people are interested to bolt straight onto your stock r32,r33 exhaust manifold with the 25/30 engine. This will be an adaptor plate which allows a bigger turbo t04 or gt3540 to sit lower, looking stock as well as clearing everything while also enabling the use of an external wastegate. Now i can offer these adaptor plates which cost me $200 to make in all stainless steel as thats what they cost me to make, also if people are interested i can make the adaptor plate and dump/ front in one with a tial 44mm external wastegate all attached so all that would have to be done is a turbo bolted on, let me know what you guys think i have attached some pics below.

post-26467-1146580496.jpg

post-26467-1146580505.jpg

post-26467-1146580517.jpg

post-26467-1146580527.jpg

post-26467-1146580538.jpg

Decking the block is fine and has been done since the dawn of time.

Its most definitely NOT a big no no.

However decking too much... I can't see any problems with it UNLESS the blocks design causes problems with quench, water and or oil galleys.

So block dependant I would say.

I really don't know for sure, which is why I asked some one in the know to clear it up.

With regards to the vg30 pistons. I can't see how you have achieved such a high comp raito with a piston that runs a 9cc dish and sits 0.040" down the bore.

The CP's run flush with the top of the block, and no dish yet achieve around the 8.2-8.3:1 ratio. :O

my error was not increasing the bore size with the vg running the 87.12 and not putting in the - for the cc's of the piston

gary has mentioned to me a while back he uses the JE as his first pick and has also used acl pistons, im one whos very interested in getting some part numbers for both

i think the vg's will have a similar problem to the rb30et pistons with horribly low compression. it seems we really need something with a positive cc to get a nice comp ratio.

Edited by SirRacer

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

    • Came here to say, put all the wiring from the new motor and gearbox in that you can, then throw the stock ECUs in the bin, and get an aftermarket ECU. Should be pretty easy if you can use a multimeter and read a wiring diagram to then use a PNP aftermarket ECU to suit the motors wiring loom, and make the minimal changes you will to get it to work in with the body loom (If any). This will mean you can very easily circumvent/bypass the Park/Neutral start disable switch, and get everything running really easy!
    • If the roof is dual skinned the whole way, IE, there's a "top" metal piece, and a "bottom" metal piece, to slow it down as much as you practically can, you should be able to get an attachment for a spray can/your spray gun, where it is a long, thin flexible hose, and when you're "spraying" it is spraying it in every direction possible. The I'd get that, and feed it through the roof as much and as far as you can. It's basically like fish oiling the car, but you're soaking it in rust converter. Then do the fix like Murray has described having cleaned up the existing metal as much as you humanly can. I'd also throw as much rust converter on that exposed metal before putting the fibreglass/metal filler over everything.   As for welding a replacement in. I've owned my own MIG welder for about 10 years. I've also worked in an industry doing MIG welding for a job for about 3 months dead straight, and we were doing 11.5 hour work days 5 days a week, plus a Saturday 6 hour day. (I then moved over to running the massive CNC plasma as I could understand the technology, and work with the main guy out there). I also f**k around with my welders a bit at home. So what I'm saying here is, I've probably got more hours on a MIG gun than you'll manage to get under your sleeve doing home sorts of jobs over the next 5 years. I also have an ACDC TIG that I got myself a year or two back. I've got a short amount of experience on the TIG only. My home MIG is also presently setup for doing thin sheet metal. Unless I didn't care about how that roof looked, and I just wanted a functional metal roof, and it being out of alignment, warped, and bowed, I would NOT attempt a roof replacement UNLESS I could do it as a whole panel like Murray described where the spot welds were.  Welding has this REALLY annoying thing, where if you want something to be perfectly square, unless you can clamp that thing to damn perfection (Welding fixture table), it is NOT going to be square, so you start to learn, the type of metal you're working with, how thick it is etc, and weld in VERY specific ways, and by knowing how YOU are as a welder, so that as the welds cool, the metal work pulls itself into place. If you want to see some cool tricky shit done, Bennets Customs is an Aussie guy, and he mentions a guy a lot call "Kyle", who is from "Make It Kustom". Watch some of their welding videos, especially on sheet metal. You can use the welder to shrink the steel in, and you can also use the welder to stretch the panel out. When you have the skill level that I have, you can shrink the metal in and out... But never on purpose like those two guys do. You just manage to f**k it all up. Then I smack it around with a hammer till it sits lower than I will want it too, then I shove filler on top and then pray to deitys that I can sand it into some form of sane shape that doesn't look like a dog has taken a shit, after eating a tonne of pumice stone... I'm all for DIY, and for learning, and please, feel free to give it a go, but be aware, you need to live with the consequences of how time consuming it is to do, AND that it's going to look no where near as good as what you can make it look with just some filler now. Oh AND, even once you replace it, it's like to still rust away again eventually, because you'll have missed putting primer and paint on some part of the newly welded in sheet metal, or against part of the existing metal you couldn't get to...   Oh, and to weld all that in, you will need to pull the window out, and strip at least the roof and A Pillars of interior trim and wiring. You'll then need things like the big fire proof/weld spatter proof mats to lay down, OR you'll end up needing to strip the ENTIRE interior to avoid sending it all up in a ball of flames. If you want to see how annoying sheet metal is to weld, head to bunnings, buy there 600x600 1.6mm mild steel (Not GAL!) sheet, and cut a few pieces, and try and weld them together. Then understand, 1.6mm sheet is nearly 50 to 100% THICKER than the cars sheet metal.   The photos I posted before, I'm replacing with 1.2mm thick mild sheet, and it's very easy to blow through both the original steel (Especially if I hit an area that should have probably been cut out a bit more) or straight through the new sheet metal. And I'm doing the floor, which can be hidden easily, and doesn't matter how pretty I make it, as long as it's damn strong! I'm also doing it in a 4WD, that has seen many off road trails, and doesn't need to look that pretty ever
    • This sounds like a perfect excuse to install a Haltech R3.
    • I do believe from context he is talking about a S0/S1 R33 RB25 with associated ECU and Wiring for that, and a manual gearbox into a R34 N/A Auto. I don't have the knowledge of all the pinouts and such but my gut feeling from doing my own conversion is to use as much of the R33 stuff that you can. The "car" wiring is quite seperate from the "Engine" wiring when all things are considered. The only things to truly consider 99% of the time is the cluster, reverse lights and potentially disabling the 'not in P/N' immobilizer circuit.
    • A realistic expectattion of how long it has to last also comes down to.... when do you think you will be banned from registering and driving old petrol powered shitters? It's 27 years since that thing was built. It probably rusted out 15 years ago. It was probably repaired and looked OK for another 10. If you do a similarish bodge job now, or perhaps slightly better with some actual rust conversion and glassing, then.... get another 10-15 years out of it, after which you'll only be permitted by the CCP to drive electric cars manufactured in their Shenzen zone anyway. 
×
×
  • Create New...