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Owner of the MSNSPS
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5,199 Posts
Discussion Starter #1
Thought I'd post this up here as it may be of use to someone.

A little background - I got thinking about having a bottom end refresh (worried that my early R32 doesn't have the wide oil pump drive mod) and it got me thinking about improving things (seems rude not too). I like the way the engine works currently but it would be nice to have more everywhere. I'm not too keen on revving the engine any higher (usually go to 7,500 and keep 8k for special occasions) so more air in and out in the same time (ie revs). I should also add that I'm not aiming to spend lots of money nor do I want masses of power. Reliability and response are the goals with more power if possible.

I started thinking about increasing the capacity of the engine. First off I'm assuming that whatever happens I'll want to replace my pistons. Forged items seem no more expensive than stock items so that sounds like a good idea. However I don't really want to over-bore the engine (beyond usual 0.5-1mm to correct the bore if required). I'd like to retain the stock rods. NB I should also add that I am assuming that other supporting mods take place at the same time.

This leaves the crank. I read about the RD28 which is purported to be very similar to the RB (specifically the RB30). RD28 cranks are available for peanuts. I started wondering if this crank could be made fit into my car without too many heroic measures. I had a look at SAU and have done some schoolboy calculations and this is what I've got (NB this may not all be correct so please take with a pinch of salt!):

RD28 is 85x83, the crank pins are larger than the 26

Bore is irrelevant so that leaves the 83mm stroke. If I'm keeping the stock rods then the crank will need some work. If the crank pins are reground with an offset circle the stroke could be brought down to 81mm. Why 81mm well apparently Apexi do/did a stroker kit with this throw which fits into the normal block.

With the 81mm stroke and 121.5mm rods the piston would clout the head. The additional stroke would have the stock piston protruding by 3.65mm. If the Gudgeon pin is moved up by 3.65mm this should sort this out. I should add I'm assuming that having a gudgeon pin 3.65mm closer to the top of the piston (26.35mm rather than 30mm) won't cause problems with piston strength etc.

My main concern is that at BDC the bottom of the piston would end up clouting the crank. Even assuming that the RD crank is the same size as the stock item the bottom of the piston will be sitting 7.3mm lower. Though the Apexi kit would suggest that this is OK it could be that the Apexi kit's crank is shaped to allow this (and the post on the SAU seems to suggest this too).

Anyway that's about as far as I've got. It looks to me that you'd have to do some machining on the RD crank and I guess by the time you've done that you might as well have just bought a kit or one of Rob's RB30s. Pity really as a cheap 2.8 option sounded like just the thing for a road car.

Hopefully this will be useful to somebody.
 

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Ive seen an RD28 crank offset ground, into an RB25 block, made a 2.9
Not 100% sure what rods, but could find out (im fairly sure they were RB26 rods)
The pistons were very marginal for falling out the bottom of the bore....
But it made good power.
So I would guess the unstroked RD28 would be ok.
 

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Owner of the MSNSPS
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5,199 Posts
Discussion Starter #4
I've seen a few RD28 crank conversions in Japanese books.

I'll dig a few out and check the pics.

From memory, Rising often do this.

Japanese Used Car Exporters / OVERDRIVE JAPAN

Discussed here before

http://www.gtr.co.uk/forum/98485-interesting-new-3-0l-stroker-kit.html
Oddly the second link was the one which originally sparked my interest.

Regarding the first one I'm not sure if that one uses a head spacer. I'm trying to get away from that sort of thing. I guess that once you get into that sort of bracket (with custom rods too) cost goes up. I am/was hoping that the additional costs (beyond a 'normal' refresh) for doing this would be limited to buying a second hand RD crank and having it machined. Perhaps some more mapping work too I suppose on reflection.
 

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Owner of the MSNSPS
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5,199 Posts
Discussion Starter #5
Ive seen an RD28 crank offset ground, into an RB25 block, made a 2.9
Not 100% sure what rods, but could find out (im fairly sure they were RB26 rods)
The pistons were very marginal for falling out the bottom of the bore....
But it made good power.
So I would guess the unstroked RD28 would be ok.
If you have any more info I'd love to know about it. My concerns are really about the viability of the 81mm stroke without longer rods and a head spacer.

Again I'm thinking that the crank is pretty cheap (US$2-300 at most), shipping might be pricey. I have no idea on what the machining work would cost but again I'm thinking that a couple of hundred pounds. If you're already replacing all the other items...
 
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