Doing Research On Rb20 Bitsa

[BLSTIC]

Ok I'm considering getting a skyline or cefiro. Or even a VL crumpledoor. Here in nz they come with RB20E's...

So I'm assuming that there are three RB20 bottom ends available, with the pistons being my biggest concern. Based on theorising the DOHC engines will have the highest crown height (due to multiple small, low lift valves, as opposed to the SOHC's larger valves that probably open further). So given the same head n/a DOHC pistons have the highest compression and SOHC pistons have the lowest compression. Is that correct (ie will SOHC pistons should give me lower compression than any of the factory DOHC items)?

And can I use the RB25 head on the RB20 bottom end without hassles? This is due to the higher flow of the RB25 head and hopefully lower compression. Assuming that the combustion chamber in the head isn't larger than the bore, which is the thing most likely to stop me.

Basically the goal is to assemble the lowest compression RB20 I can screw together. Mainly because my 'tuning methods' are a little agricultural so I need a little leeway in regards to detonation. And wiping the floor with my mates while costing half as much is a pre-requisite.

Any thoughts or suggestions?

Thanks

Ben

[R31Nismoid]

25 head and 20 bottom end is well covered - simply not worth the sheer effort to make it fit (it is not direct by any means of the word)

I don't think anyone has actually gone through with it

[BLSTIC]

I did a search and couldn't find it in the first two pages "rb20 block rb25 head".

I can however try my hand at porting and modifying the combustion chambers of the rb20 head. Won't be as good as a stock RB25 head though.

Alternatively there's a 2mm difference in stroke between the two, meaning I might be able to de-stroke an RB25 for lower compression while keeping the good head. Dropping 70cc in the process... I'm about to head home so I'll do the compression ratio calculations when I get there.

[Chris-06R]

if you keep looking you will find gary aka sydneykid talks about doing a 20/25 at some point - apparently the water galleries or something don't match up and its a major job to get them to match... much easier to just use a 25/25

[BLSTIC]

Caution - Maths follows

Well here are the numbers for lowering compression by de-stroking. (after re-calculating due to brain fade while calculating initial combustion chamber size. Starting with a 10:1 compression RB25 (as they all are I think).

compression ratio = (swept volume + combustion chamber) / combustion chamber

2498cc / 6 cylinders = 416.333333333cc/cylinder.

10 = (416.333 + x) /x

10x = 416.333 + x

9x = 416.333

x = 46.259

so it appears that combustion chamber volume = (compression ratio - 1) / swept volume

416.333 * (69.7/71.7) = 404.72

416.333 - 404.72 = 11.613cc (new swept volume, but only half of that gets added to the combustion chamber) / 2 = 5.807

46.259 + 5.807 = 52.066

new CR = (404.72 + 52.066)/52.066 = 8.77:1

So a de-stroked RB25DE has lower compression than an RB25DET (while becoming an RB24). Not by much but it's cheaper getting an RB25DE and an RB20 crank than it is to get a genuine DET. I'll do it again starting with a DET as a base (9:1 compression)

x = 8/416.333 = 52.042cc combustion chamber

52.042+5.807 = 57.849cc new combustion chamber

new cr = (404.72 + 57.849) / 57.849 = 8:1 compression

[/math]

8:1 compression is highly desirable for big boost, and can be dropped even further if there are pistons that have a lower crown height (compared to the gudgeon pin) available out of any 86mm bore RB... It might be worth comparing the factory RB30 pistons to the RB25 items.

[discopotato03]

I don't want to burst the bubble but resisting detonation is not just about low compression ratios , it is possible to have a low CR and have it detonate sooner boost pressure wise than another better set up slightly higher CR one .

I think it's more about the effective combustion chamber and piston crown form allow the charge to burn , fuel octane plays its part as does charge temperature .

There are two schools of thought on CR and boost , high CR/low boost and low CR/high boost .

I would rather keep the compression ratio up and find ways to make the engine breath as well as possible so that you don't need high boost pressures to adequately charge your cylinders .

I think it's a bad idea to use an RB20 in any form if it's in any way avoidable . They have small ports and valves and there isn't room in the chamber (for larger valves) or casting (for larger ports) to do anything significant with them .

Any RB25 head including the R32 RB25DE one has a lot more valve and port area than you could ever get into any RB20 one .

Stretched RB20's don't work in my book because when the head runs out of flow all the cubic inches in the world won't help .

A .

[BLSTIC]

I'm aware of that. Squish/squash has a lot to do with it as well. Simply lowering the pistons (what de-stroking does in regards to compression) will make any squish area ineffective. I haven't actually seen an RB25 head off an engine though, so whether or not it actually has a combustion chamber that relies on squish is something I don't know yet. If it has no sqish pads (for reference I am talking about the flat parts of the head that are inside the bore area. The piston comes up to within a millimeter or so of these and it creates turbulence in proportion to the size of the pad) de-stroking can provide a win. If not, it won't work as well.

But does anyone know what RB*20* pistons will give the lowest compression in an RB20DET? Just acedemic interest now, because I can't use the high flowing head on the RB20 I won't bother. Still want to know though.

***EDIT: Where's my edit button for my previous post? I made a mistake and want to fix it. This:

so it appears that combustion chamber volume = (compression ratio - 1) / swept volume

should read

so it appears that combustion chamber volume = swept volume / (compression ratio - 1)

*** Edited September 15, 2009 by BLSTIC

[Sydneykid]

From memory there is an 8 mm difference in the bore size between an RB20 (~78mm) and an RB25 (~86mm). I don't see how you can reduce the combustion chamber diameter in an RB25 head by 8mm to fit the RB20 bore. Conversely the RB20 block won't handle an 8mm over bore to fit the RB25 combustion chamber.

For clarification, way back we used to use RB20DE or RB20DET cylinder heads on RB30E and RB30ET bottom ends to achieve a DOHC RB30. It was a prick of a job that took hours of welding and re-machining as the water jackets don't match up and, more importantly, we had to increase the diameter of the RB20 combustion chamber from ~78mm to ~86 mm to suite the RB30 bore diameter.

Back then RB26 heads were the only alternative for RB30 DOHC conversion and they were unbelievably expensive and very rare. Once the RB25 heads became readily available at low cost everyone stopped using RB20 heads on RB30's.

To even bother to do any work on an RB20 you would have to be either forced to by motorsport or road registration regulations or psychotically attached to RB20's (yes, Roy that's you). Personally I have all the parts necessary to built an RB24 (complete RB20, RB26 crank, RB26 forged rods, 4AGZE forged pistons etc etc), have had for years, but I have built 3 x RB30's in that time for my own use. The only reason I continue to keep the RB24 stuff is because there are motorsport regs that benefit an R32GTST with standard block and head and one day I might take advantage of that. Until then I will continue with RB30/26's because value for money wise they are the most effective.

Cheers

Gary

[BLSTIC]

I'm not going to do an RB20 any more, since the RB25 head is so hard to fit. If I could edit my first post to change the topic I would. But I can't.

So budget rb25 info is now what I'm after