Rb24

[BLSTIC]

After some research, I discovered the existence of the RB24S, a combination of RB25 block, RB20 crankshaft, RB30 SOHC head, a carby, and unknown rods and pistons. Which leads me to believe that the RB20 crank can fit into an RB25 without dramas.

I want to do this for two reasons, the stroke-rod length ratio improves slightly, and it drops compression significantly. I won't go into the maths (epic long calculation) but if the compression was 10:1 to start with (I think that's what the n/a R33 RB25's have) it drops to 8.77:1. If I start with 9:1 (the turbo version) it drops to 8:1. The only issue that might prohibit big boost is the combustion chamber shape.

So three questions:

1. Does the RB25 combustion chamber use squish (also known as quench) in it's design, or is it closer to a hemi style of head?

2. Has anyone done this before?

3. Are there any reasons (such as oil pump drive, or similar) why this wouldn't work?

Yes I'm aware that it's ideal to use lower compression pistons (or a genuine DET engine), but for a budget bitsa approach it makes sense, especially when the combined cost of an RB25DE and an RB20 crank is far less than the cost of an RB25DET. And I only give away 70cc for the lower compression. And even if it's not ideal, it's probably still way better than an RB20DET.

Thoughts?

Ben

[R31Nismoid]

Why are you so concerned about lowering the comp ratio to silly levels that are of 1980's proportions?

I just don't get it.

Unless you intend to use the chip oil as fuel... there is no reason to in this day and age.

If, as your other thread said, tuning 'isnt the best' or whatever, even lower comp won't save you from detonation necessarily.

[StealthX]

even lower comp won't save you from detonation necessarily.

there's no shortcuts in the car modding game unfortunately...

[blind_elk]

"budget bitsa" is an oxymoron. Trying to mix'n'match parts is just asking for trouble.

Why would it be cheaper - you would have to get a RB25DE, and a (whole) RB20 to get the crank. A RB25DET isn't really all that expensive.

A bit of history. In the good old days - before supertaxis - the Nissan race GT-Rs ran 9:1 c/r and 1.8 bar boost, and were a very tractable race car. The Ford Sierras ran only 6:1 and 3 bar boost. They were an absolute pig of a car to drive - gutless off-boost and uncontrollable on-boost. So, low c/r is not the be-all-and-end-all of safe turbo driving.

[discopotato03]

Personally I think the only instance of a shorter stroke crankshaft in an RB25 being useful would be to increase it's rev potential or to possibly if legal suit a class/capacity limit .

I reckon I'd do the cylinder capacity calcs because guestimate thinks you may be less than 2400cc .

To seriously destroke an engine you have to look at the shorter stroke and possibly longer connecting rods to improve the bore/stroke AND rod/stroke ratios - if you want all the benefits available .

I think destroking an engine to reduce the compression ratio is a bit out there somehow , I wouldn't be aiming for anything less than 8.5:1 in anything that saw much part throttle road type usage .

The manufacturers had all sorts of reasons for using silly low 7.5 or 7.7:1 ratios in the late 70's to mid 80's era OEM turbo engines . It was the era of "power kerosene" octane ULP and turbochargers/engine management systems were crude by todays standards . They also hadn't gotten around to realising that intercoolers AND more sane 8-8.5 CR's gave much better overall results .

Lastly having the piston further down the cylinder at top dead center is probably the worst way to reduce CR because the piston is further away from the heads quench pads so you may reduce the CR and raise the detonation threshold . Lose lose situation for no very good reason .

Imo cheap and quite reasonable = ex VL RB30 , close to equal bore and stroke and good rod length to give very acceptable rod angularity . Arguably better than RB25 or 26 because can make the power with 20% less revs in some cases .

Just my thoughts , cheers A .

Edited September 18, 2009 by discopotato03

[BLSTIC]

8.7:1 is not that low. Factory rb25det is 9:1. See the difference? It isn't much. The 8:1 figure was for comparison purposes, (ie if I started with an rb25det). 8.7:1 also more compression than what japanese (8.5:1) and australian (8:1) Legacy RS's use. I wouldn't call it rediculously low.

One of the questions was about the quench pads. I asked you whether they have them or not, and noted concerns about the combustion chamber shape. I know if they use them it's not ideal to pull the pistons away from them, but if they don't use them it doesn't have nearly the same effect.

I have done the calculations. here.

Just so you know, the difference in stroke is 2mm, the major displacement change (RB20-RB25) comes from the bore change. I am moving the pistons down 1mm from where they previously sat. Nowhere near as bad as the spacer plates they used to use in the 80's on 202's and so on, and it's only a little worse than using a thicker head gasket (as some people do to drop the compression).

Another question - Do the pistons in an RB30E come all the way to the top of the bore (or very close)?

[bbe]

Hay mate, i think its a great idea. I found the nissan article were the rb25 block uses 20 crank and sohc head and carby lol.

Im a big fan of the 20 and only dont like the 25 because of its slow revving nature without hi rpm compared to the 20, revs isnt everything but its fun an sounds good lol.

If you find out anymore about what bearings, rings,pistons sizes etc throw us a pm as im interested in doing the same thing sometime.

Stupid question, but does a lower compression turbo'd car help make it rev quicker ?

Good luck with it all.

Edited September 19, 2009 by bbe

[paulr33]

i agree with ash it just sounds like a lot of hassles, effort and mish mash parts and costs

it would end up cheaper, easier, quicker and a better result to just get an rb25det

too often we see people "trying to be different" or "trying custom" and its just a waste of time, effort and $

[ARTZ]

Stupid question, but does a lower compression turbo'd car help make it rev quicker ?

Good luck with it all.

From my knowledge lower comp will allow you to run higher boost with out fear of detonation , but its the shorter stroke that will allow it to rev faster. On paper it seems like you could be on a winner. In practice, well it would be interesting to see.

[BLSTIC]

It all depends on whether I get a car that can easily take an RB. If I get something like a VL or a cefiro, we will probably find out. But I don't like my chances of finding one in good nick for next to nothing.

[_Ryan_]

It all depends on whether I get a car that can easily take an RB. If I get something like a VL or a cefiro, we will probably find out. But I don't like my chances of finding one in good nick for next to nothing.

School holidays...

[BLSTIC]

what does school holidays have to do with anything?

[Lukas 33]

what does school holidays have to do with anything?

Means he thinks your just another kid trolling on SAU to waste time and talk shit.

Not saying you are. Just thats what I think he thinks.......catch me?

[Cjmartz2k]

i agree with ash it just sounds like a lot of hassles, effort and mish mash parts and costs

it would end up cheaper, easier, quicker and a better result to just get an rb25det

too often we see people "trying to be different" or "trying custom" and its just a waste of time, effort and $

And too often on this board people aren't willing to accept new ideas or try new things just because they are different. "Just go with something proven" gets thrown around a LOT on here. 8.7 to 1 isn't that low of a compression ratio. The change in quench height is a valid concern, but he already noted that and that's the main point of his post I think. I do think Discopotato is right on though that the only real meaningful advantage is the ability to rev higher, but the 25 hydrualic head is going to be the limiting factor anyways, so unless he is planning to do something about that, it's kinda pointless.

there's no shortcuts in the car modding game unfortunately...

Wrong, but by all means, keep spending all that money on overpriced jap namebrand parts and stay slow. I'll find better deals, new things, and cheaper parts as long as the work well and go faster for a lot less thank you very much.

[BLSTIC]

Nah I'm not still in school. Although I might mentally still be 15 (at times) I'm 22 and have done a few things with cars. Mainly auto-manual conversions and engine swaps (for identical engines and factory gearboxes though). But I am trolling for information before I go out and buy a long term project car (if I'm building an engine for it, it's going to stay with me for a while).

Incidentally I found a picture of an RB25 head, and it does have squish pads. But they cover a very small portion of the bore (at least compared to engines like the later 20v 4A-GE's, and f-a-r less than most 2 valve per cylinder engines) so I'm not sure what to think.

[robots]

I can supply photos of opened rb24 block ? Anyone needs it ? If so, let me know. Gonna go to garage and take some photos.

[daisu]

I can supply photos of opened rb24 block ? Anyone needs it ? If so, let me know. Gonna go to garage and take some photos.

As in a factory RB24S or an RB20 stroker? or a destroked RB25?

[James_03]

Buy a shagged rb25det, (ringlands gone etc) then purchase forged pistons at desired CR.

Get it bored OS, crank work done etc and you have a cheap rb25det with the CR you want.

That would probably end up costing the same as a rb25de/rb20crank purchase as you will need to have machine work done on that too more than likely so that it all works properly.

my 2c.

[discopotato03]

I'll say this then I'm out because this could go nowhere forever .

IMO torque is what drives you down the road and the more you have the better off you'll be to a point (traction) .

I think it's brave to say that an engine that revs (as in picks up revs) faster makes for a better car .

In the real world compromises have to be made and no one drives everywhere in 2nd gear at 4000 + revs .

Money is always an issue to most mortals and chasing small changes in stroke length IMO is not worth the effort unless your hands are tied by class regs .

If I was that way inclined I'd put any any extra money into RB25 head work and cams because I reckon that would work better than a de stroked one with a lesser CR .

The theory around using a "decompression plate" on an archaeological find (Olden 202) was not a good one and while it got the static CR down it killed all the bottom end torque as well .

The only reason CR's are lowered is to fight off detonation and there are better easier ways to do that anyway .

While ever you have an engine that has to be run at part throttle it is stupid to have real low CR's , the fact that the engines throttled (strangled for want of a better term) means that the cylinder filling ability is lower so the "dynamic" or effective compression pressure will be as well . If you like having no torque and doughy throttle response this is the way to go about it .

I really wish people could grasp the fact that if you make an engine breathe better by removing avoidable restrictions you can often make lots of power/torque without high boost pressure . "Big boost" makes for lots of problems so if you can make adequate power without it then you're not making problems that are difficult/expensive to solve .

Your call .

[robots]

As in a factory RB24S or an RB20 stroker? or a destroked RB25?

I have a few real RB24S lying in my friends garage. So its RB24s. Not a rb20stroker or rb25 destroker )))