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Old 01-01-05, 21:23   #51
JuZi
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Quote:
Originally Posted by LukeVL
My final figure is 8.48:1. Sorry about the confusion. Thats with an Rb25 head (VVT if it makes any difference), nissan gasket and NA psitons.
why did u put a vvt head on an rb30 block? i've been told that the vvt wont work so its not worth it, i would go for the non-vvt rb25 head, i've been told it fits straight onto the series 2 rb30 block, which would do gr8 with a blower!

also i guess the vvt head might give higher compression than the non-vvt head
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Old 01-01-05, 21:34   #52
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Quote:
Originally Posted by JuZi
why did u put a vvt head on an rb30 block? i've been told that the vvt wont work so its not worth it, i would go for the non-vvt rb25 head, i've been told it fits straight onto the series 2 rb30 block, which would do gr8 with a blower!

also i guess the vvt head might give higher compression than the non-vvt head
VCT picks up around 10rwkw through the mid range.

Its basically giving the cams more overlap at the top end to give the top end performance while minimising the overlap at the bottom and mid range to improve torque.

It is most definately worth while for a streeter that is going to see only a mild set of 260 or so duration cams.

Tomei/HKS don't make cams that work well with VCT with anything from MEMORY around the 260duration mark.

The VCT head also has slightly larger ports, BUT from my measurements it appears the R32 RB25DE Non-VCT head has ever so slightly larger valves.
My RB25DE and another friends I have measured up at 35mm for the inlet and 29.5mm for the exhaust.

ONE R33 RB25DET I checked out measured up at 34.5mm for the inlet and 29.5mm for the exhaust.

Unsure which is better. The larger valve or larger port.
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Old 02-01-05, 08:34   #53
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Done some stopwatch testing last night. I done several 3rd gear roll-ons from 2000rpm up to 4500rpm (where VCT switches off). With the VCT plug left connected, i got an average time of 7.6 secs, with the VCT plug off 8.6 secs.

So yes VCT is worthwhile. Mind you this is on 5psi, with 15psi the difference should be larger again.
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Old 02-01-05, 14:05   #54
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I've seen a stock RB25DET with exhaust, fmic, boost & a microtech (just over 200rwkw) that with the VCT on literally flicks to redline and fry's its 255's through second.

Disable the VCT and there is no wheelspin through second.

For a mild streeter it is definately worth while.
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Old 05-01-05, 11:57   #55
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Quote:
Originally Posted by R32-Joel
What deck clearance are you looking at?

Having built an engine once I would always set the quench/squish gap to around 40-50thou. Hence have a zero deck clearance and use a 40thou headgasket to give you your 40thou clearance.

If you go too large you will loose power, economy and the benifits regarding the 'free' octane boost of a well setup quench/squish.
can you elaborate on the free octane boost theory?
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Old 05-01-05, 16:53   #56
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Quote:
Originally Posted by duz360
can you elaborate on the free octane boost theory?
A good quench improves combustion efficiency.

Quote:
When properly designed, the quench areas can have a tremendous effect on the quality of combustion, and allow higher compression ratios, and due to this they are considered "artificial octane" by scientific types.
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Old 05-01-05, 17:34   #57
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where did you get that from?
id like a read of it
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Old 10-12-05, 11:31   #58
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hi ya all just a quick question im doing a conversion soon rb25 head on to a rb 30 block now do i have to change the pistons or can i keep the rb 30 pistons and its not a turbo
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Old 10-12-05, 11:44   #59
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duz360, google is your friend.

vl_86boy, use the N/A pistons, with the dohc head it drops the comp ratio to a nice 8.2-8.3:1.
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Old 10-12-05, 18:44   #60
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Quote:
Originally Posted by Cubes
duz360, google is your friend.

vl_86boy, use the N/A pistons, with the dohc head it drops the comp ratio to a nice 8.2-8.3:1.
cheers dude
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Old 16-12-05, 22:57   #61
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wow this thread has been goign for two years now
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Old 01-04-06, 20:52   #62
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So the bottom line is:
Rb25t pistons will fit straight into a rb30 and run the same compression (with rb25det head)
Rb30 na pistons will provide sufficient low compression for an rb25det head
Rb30t pistons will make to lower compresion ratio.
Is this correct?
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Old 10-04-06, 10:46   #63
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RB25 pistons have a different pin height (the block needs to be decked)
RB30E pistons give a C/R of 8.3
RB30ET pistons 7:1 or something way too low.

Most people use N/A RB30E pistons or the forged oned to suit the conversion are usually around 8.5.

Im going to use some high comp RB30E pistons and machine the tops so its around 9:1, as most new BB turbos these days dont need to boost more than 15psi.

Last edited by vhr32; 11-04-06 at 11:36.
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Old 12-04-06, 22:32   #64
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Quote:
wow this thread has been goign for two years now
Yeah, and its the first time ive actually read it since i made it, man there is some interesting stuff said in here.....
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Old 27-04-06, 14:42   #65
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Originally Posted by carcrazy
Yeah, and its the first time ive actually read it since i made it, man there is some interesting stuff said in here.....
Just out of curiousity does any one knnow maximum diameter for a rb25 piston

and if any one knows off the top how much these wiseco pistons are worth and there biggest size?
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Old 24-05-06, 23:31   #66
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hey im plannin a build with the 26 head on the 30 block and wondering wot pistons ud recommend for it. it should be seeing round 1.5bar tuned
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Old 24-05-06, 23:40   #67
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CP's are definitely the most suited.
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Old 04-07-06, 00:00   #68
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im buying a big dollar rb30 bottom end thats been built with rb30 forgies and are converting to a rb25 head. as all parts are new and block is already oringed, balanced and decked. is it goin to be a big fuss or do i have to change to rb30/25 forgies and is it possible to trade in or have to start gain
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Old 11-07-06, 20:48   #69
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hi i have done the rb30-rb25det head conversion cr was 8.3:1 and instead of using a turbo i have used a procharger supercharger c2 rated at 725hp.
max boost 24psi.
intercooled at 15 psi using the LinkPlus G2 plugin ecu for vls i have tuned it with my computer it realy needs to go on the dyno couse its a litle rough still but it eats my mates rb26dett with out trying.
i will be getting it on the dyno hopefully soon and will post the figures for you to have a look at should be interesting
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Old 24-07-06, 11:53   #70
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The compresssion of those forgies may be a bit low maybe 8:1 or there abouts, all depending on what power output you want though, if you want redicuousboost youll need it but if you want stockish boost try opt for a 9:1 compression
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Old 19-08-06, 12:57   #71
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9:1 is fine for up to ~18psi using the twin cam head and pump fuel providing you don't have excessive turbine back pressure. i.e trying to push 18psi through a highflowed rb20/25det turbo or something silly.
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Old 27-08-06, 20:58   #72
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just wondering about doing this combo.

at current i am running a standard rb30et with high mount t04e, fmic, 2 extra injectors, 19psi.

If i convert over to the rb25det head the comp ratio will drop down to around 7.1:1, would this compression ratio still be to low if i up the boost to around 22psi.

Not sure if it matters or not but the car is manual not auto.
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Old 27-08-06, 21:10   #73
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Apparently CP now do a piston that achieves a 9:1 comp ratio.
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Old 27-08-06, 21:28   #74
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Quote:
Originally Posted by VL_30_LT

If i convert over to the rb25det head the comp ratio will drop down to around 7.1:1, would this compression ratio still be to low if i up the boost to around 22psi.

still want to use the original rb30et pistons, not all of us can afford cp pistons and all the rest the would go with doing a rebuild of the bottom end.
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Old 26-09-06, 07:36   #75
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Im looking at purchasing a RB25DE head, and im going to also be performing a RB30E->RB30ET, So the RB25DE head can bolt straight onto the RB30 block without to much **** around? a big dirty RB30DET would chop! thanks for your help.
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