Rb20 --> Rb24 Head Question

[carl h]

hey all.

contemplating rebuilding the old rb20 into a rb24 as the engine has terrible compression (140psi across back in jun; still pulled 288rwkw's tho ) and if so to go in with nice forged bits for longevity.

everyone knows to make a rb24 you use a rb26 crank with rb26 rods and tomei/custom/4agze pistons but it seems that everyone always forgets about the head.

the rb20's head isnt exactly a fluid dynamics wonder, so what can be done to improve the head flow?

also since we would be boring the snot out of the rb20 block to fit the new pistons whats to be done with head as the CC would need to be worked to suit the over bore right?

im not really familiar with this kind of stuff so i would like to learn as much as i can.

thx.

carl

[R31Nismoid]

well starting off you already know the RB20 head isnt the most friendly to even the 2ltr

I think the figure is 30-35% that generally is thrown about.

To get the flow required would definately require a lot of head work, not a cheap endeavour.

Im not down with all the terms but its not going to be something that small.

Cams selection is also going to play a very large part in it all i would think

This will be another great thead for sure

[DRIFTER]

i'm thinking of doing this conversion, but i still need alot more infofmation before i start. i would like to know how much more response it gives, how much is a crank worth, is it necessary to go for a 33 crank for the wider oil pump drive, what inlet manifold should i use. what cam specs should i use. so far the power i have is enough to drift with but it would be alot easier with better response.

[carl h]

main reason im looking into it is because i want reliabilty as well as having good lowdown response.

not to mention my rb20 has seen much better days and with the 2.4 it should spool a large turbo fairly quick like.

from what i gather you should use a r33 crank because of the oil drive fix, as for intake manifold ive got a custom RIPS one so im good on that front .

since i was planning on building the engine to be 9k capable i was gonna run some kind of 260/270 combo or 270's.

ive already got 260 tomei procams in the car now......

[Sydneykid]

Hi guys, I will try and answer the questions, but before I do I should point out that you can swap most of your good bits onto/into an RB25DET and end up with a better result than building an RB23/RB24. And it will cost a damn site less as well.

On to the questions;

The standard RB20 bore is 78 mm, the most common RB23 (2,280 cc's) piston upgrade is 4AGZE which are 81 mm (from memory). This means you will need to relieve the combustion chambers radius at the bore by 1.5. Then taper it into the normal combustion chamber shape. Pretty simple really, any machine shop could do it.

You will need to relieve the combustion chamber volume to achieve your target compression ratio. From memory about 3 cc (slightly more than a good polish) gives 8.8 to 1 for an RB23. The combustion chamber volumes will have to be equalised of course.

Porting is pretty much essential if you want to use the higher rpm effectively without huge amounts of boost. There is a lot of work on both the inlet and exhaust ports needed. If you do a really good job, you should end up around 85% of what a standard RB25 cylinder head flows.

Depending on the experience of the machinist and the workshop hourly rate you are looking at around $1K to do the above.

There are larger valves available for RB20's , I have seen 2mm larger inlet valves and 1.5 mm larger exhaut valves. I have a recollection that they were for a VG30 or CA18 but they fit the RB20. Since VG30's are popular in the US there are quite a few valve, valve spring alteratives. A quick check of the sizes should confirm the suitability.

From memory the VG30 valves where around $US20 each, so that's $640 for the set plus freight.

RB26 cranks vary considerably in price, I have seen R32GTR cranks around $400, rods around $200. Up to double that for R33GTR cranks and rods. The best suggestion I can make is to shop around the For Sale forum and check out the historic prices.

As for cams, 256 to 264's would be fine for using with the hydraulic followers (260 Poncams for example) . But to really use 272's or higher you would need to do a solid follower conversion. No, RB26 bits won't fit, well not without some serious engineering and machining. But CA18/VG30 may well fit, worth checking anyway.

By the time you have the necessary RB23/RB24 parts purchased, get it all machined and assembled you are looking at a bit over $5K. Then you have to add the accessories, manifolds, turbos etc.

Hope that was of some help

cheers

Edited October 9, 2005 by Sydneykid

[carl h]

and of course as usual sydney kid is on point with the info....... cheers gary!

at this point ive realized that even tho the rb24 would be loads of fun for bragging rights, it really isnt practical anymore.........

while rb25's are the better alternative they arnt as reddily available over here as they are round you guys.

not to mention we have to pay super inflated prices for stuff, thats the way of capitalism i suppose.

what i think ive decided to do is tear downt he old girl and rebuild it with slight over bore pistons to clean up the walls and get a hair more displacement (rb20.5 comes to mind ) and get the rods cleaned up and new bearings and gaskets with gtr pumps.

new hydro lifters and some decent springs and i should be good for a while.

besides being a student and trying to build a fast car isnt too practical atm.

thx for the info.......

[Roy]

Do we know the difference in piston height between std RB20 pistons and std RB26 pistons. Have doen a search on a few piston sites and cant find the info. Before i have a go at building my proper motor, im tempted to use a bunch of second hand pieces to build a long stroke, std bore RB20 using R32 GTR crank and rods. Use std water.oil pumps etc, std RB20 pistons, perhaps with a shaved head to bump up compression to closer to between 8.7:1 and 9.0:1...

Ii dont want to go learnign expensive lesson on a motor that is coting me lost of moiney in new partds.