Rb20 Crank And Rods In Rb25 Block?

[Tool26]

Im planning a build in the very near future for a s13 240sx here in the US. I have been reading a ton about the rb series motors and know i want to use one.

I like the higher revs of the rb20 but dont like like the lack of torque and amount of lag I'd be looking at to use the turbo I'd need for ~300kw at the crank. And i thought about the rb2x build with the rb20 block, but its pretty expensive.

So since i can get an rb25 for very close to the same price as the rb20 (here in the states) I was thinking about putting an rb20 crank in the rb25, with solid lifters, and maybe 8.5:1 CR?, in order to find a good middle ground between high revving (from the shorter stroke) and decent low end torque and displacement (from the larger bore) for spooling purposes, not to mention better flowing head. I know a rb24 was made similar to this from the factory in cefiros but with a rb30 head and carb'd so im hoping this would be simple.

Is this an alright plan? I figure i could get a straight up trade with someone for the rb20 gear since a lot of rb20 owners want the rb25 stuff, so it would be cheap for the two of us. Plus i like the car staying a true 240sx lol.

What id like to know from some more knowledgeable people is what kind of rpms could this be able to handle safely? Im hoping ~8-8.5k is this wishfull thinking? Would the larger bore compromise the integrity of the block at high rpms?

And what would be the most responsive turbo to make 300kw at the crank which i guess would be ~250-270rwkw at a conservative 12-14 psi. I want the longest power band i can get.

[Justa32]

would b a kool engine, lots of ppl here with bag u and say go 4 the rb25... im with u try sumthing diferent

[Lithium]

Its an interesting idea, and its always interesting to see something different but seeing as you are asking for an honest opinion I am going to be predictable and say I think its a ridiculous idea. I have no idea why people think that RB20s rev better because they need to rev - being torqueless doesn't mean they are better up high. Yes it has a SLIGHTLY shorter stroke, but the same as an RB25 has a slightly shorter stroke than an RB26 - no one complains about how they rev.

To be honest, do the same work on a true RB25DET (people will just call it a destroked RB25 anyway) and get all the revs plus the bigger displacement - you get the best of both worlds. If you do the same work to an RB25DET and your hybrid, then drive two cars with the different setups - one will make more torque than the other... and a little more power. Thats all that will stand out.

My opinion, save your money - its going to be an aweful lot of money with the only positive at the end being the ability to say you have something a little different.

[dano4127]

Its an interesting idea, and its always interesting to see something different but seeing as you are asking for an honest opinion I am going to be predictable and say I think its a ridiculous idea. I have no idea why people think that RB20s rev better because they need to rev - being torqueless doesn't mean they are better up high. Yes it has a SLIGHTLY shorter stroke, but the same as an RB25 has a slightly shorter stroke than an RB26 - no one complains about how they rev.

To be honest, do the same work on a true RB25DET (people will just call it a destroked RB25 anyway) and get all the revs plus the bigger displacement - you get the best of both worlds. If you do the same work to an RB25DET and your hybrid, then drive two cars with the different setups - one will make more torque than the other... and a little more power. Thats all that will stand out.

My opinion, save your money - its going to be an aweful lot of money with the only positive at the end being the ability to say you have something a little different.

+1

Anyway RB30DET is the winner here it was discussed yesterday and shown to be the clear winnar in every way possible...

There is no replacement for displacement

Edited June 19, 2008 by dano4127

[Justa32]

just do it, im sure u will b happy with it its not like your chasing 500rwhp anyway

[Roy]

Its an interesting idea, and its always interesting to see something different but seeing as you are asking for an honest opinion I am going to be predictable and say I think its a ridiculous idea. I have no idea why people think that RB20s rev better because they need to rev - being torqueless doesn't mean they are better up high. Yes it has a SLIGHTLY shorter stroke, but the same as an RB25 has a slightly shorter stroke than an RB26 - no one complains about how they rev.

To be honest, do the same work on a true RB25DET (people will just call it a destroked RB25 anyway) and get all the revs plus the bigger displacement - you get the best of both worlds. If you do the same work to an RB25DET and your hybrid, then drive two cars with the different setups - one will make more torque than the other... and a little more power. Thats all that will stand out.

My opinion, save your money - its going to be an aweful lot of money with the only positive at the end being the ability to say you have something a little different.

I am with you, it would be cool thing to do, and i have considered it with all teh shit i have layign around, lol but i cant build engines so But if i raced in a class that meant with a turbo i had to stay under a 2.4L or something then i would seriously build the engine. But that is the only reason reallu.

But regarding the whole revs thing. Of course the RB20 revs way better. It has smaller and ligther pistons meaning less interia. Its stroke is a 69.7mm vs 71.7mm which all helps reduce piston speeds and they have thicker blocks meaning far less chance of warping. I personally would never rev an RB25 as hard as an RB20 for those reason alone. There is a tad of we need to rev them that hard to get a usable power band, but there is some physics and mettalurgy on its side as well

[Lithium]

But regarding the whole revs thing. Of course the RB20 revs way better. It has smaller and ligther pistons meaning less interia. Its stroke is a 69.7mm vs 71.7mm which all helps reduce piston speeds and they have thicker blocks meaning far less chance of warping. I personally would never rev an RB25 as hard as an RB20 for those reason alone. There is a tad of we need to rev them that hard to get a usable power band, but there is some physics and mettalurgy on its side as well

OK you are talking about RB20 blocks - in the case of an RB20 block, it comes with a head which takes reasonable work just to match a stock RB25 head in terms of flow, but alas you can do work to the RB25 head to take that further too. As much as your bottom end is able to rev is no good if it doesn't have good VE up there.

In terms of what the topic is about, the block thickness (which I am unaware of anyone not shooting near 1000hp having issues with anyway...) "issue" will apply to this guy anyway given he is using the RB25 block. Unless he is specifically on a mission to build a motor to rev as high as possible without any concern for performance etc then seriously man, 2mm of difference in stroke is not going to be noticeable on a decently built motor.

Food for thought - Hondas F20C (S2000 motor) which has a stock rev cut of around 9200rpm and generally makes a RB20DE feel like a joke in terms of willingness to rev and has a almost RB30ish stroke of 84mm, go figure.

Edited June 19, 2008 by Lithium

[Roy]

Not a fair comparison. The RB20DE was a mass produced poo commuter engine. The F20C was always built as a performance engine with variable vale timing and light internals. But i get what you are saying. LOL, and again i agree with you, but was just commenting on your reference to "I have no idea why people think that RB20s rev better because they need to rev " .... The simple fact is that they do rev easier.

Also the VE isnt really directly the right comparison. How do you know the VE is better for the RB25? Fact is the RB20 could have a higher VE as that would explain why it has a narrower power band? Typicall race engine often have higher VE , tuned for certain rpm ranges which makes them better for that power band, but overall a peaky engine that isnt as good an all rounder. So thinking about the RB20 could have a slightly higher VE which explains why it is so poo.

Alos, consider that for a single cylinder's cc the valve surface area and i know the RB20 has more valve area then an SR20, but i am gussing its less then the RB25 even though i have never worked it out.

The RB25 flows more, but it needs to as its cylinder displacement is higher, and VE is a ratio of actual flow into a cylinder vs the volume of the cylinder. Its the mass flow rate and pressure ratio that matter. They could very well have a similar VE but the RB25 simply has a higher volumetric flow rate because of its displacement, and in turn higher mass flow rate because of the higher volumetric flow rate.

LOL, so not really arguing or disagreeing with you...just bored and dribbling on

[Tool26]

I guess what im really trying to do is get full boost (~1bar) low enough and then rev high enough to have at least a 5,000rpm streetable power band(rb20's with this seem to go from nothing to everything in no time, not being ver ystreetable). And i was looking at the few actual dyno sheets posted in the rb25 upgrade thread and it seems they can come onto full boost ~3000-3500rpm but then die out by 6500-7500 at the latest. So i was hoping to keep the relatively quick spooling but then stretch the power band to 8000-8500 (even more if safe). So i thought trade a little displacement for a couple thousand rpms(rb24).

However if i can just squeeze the extra couple thousand rpms out of the rb25 then I would gladly do that.

Can it be done? can i get somewhere between 8000-9000rpms safely?

What would i need to do in order to achieve this reliably? Clearly i wouldnt be taking it to 9k all day every day but when the urge strikes me id like to be able to without worrying.

Solid lifters? Cams/cam gears? Lower compression? forged pistons?

Also it looks like the hks gt-rs can make 270rwkw @ 15psi, with full boost coming on around 3200 with pretty normal supporting mods. Would this turbo be able to make this power efficiently all the way up to 8-8.5k or more?

Thanks in advance for any info as i am pretty ignorant when it comes to this sort of stuff.

[Lithium]

Roy - I didn't mean outright VE, I meant maintaining a good VE up into the higher revs... the engines VE changes throughout the rev range, peak VE in my understanding should be where peak torque is and then it drops. If you want to have an engine which CAN rev to 9000rpm a large amount of the focus will have be be put on making sure the head is going to support the flow at those revs. Both RB20s and RB25s are dropping off hugely by then. And I understand what you mean by dribbling on, I am a big fan of it - its what keeps these forums going and conversations like this can end up in really interesting things happening. Or shitfights haha

And i was looking at the few actual dyno sheets posted in the rb25 upgrade thread and it seems they can come onto full boost ~3000-3500rpm but then die out by 6500-7500 at the latest. So i was hoping to keep the relatively quick spooling but then stretch the power band to 8000-8500 (even more if safe). So i thought trade a little displacement for a couple thousand rpms(rb24).

However if i can just squeeze the extra couple thousand rpms out of the rb25 then I would gladly do that.

Can it be done? can i get somewhere between 8000-9000rpms safely?

Also it looks like the hks gt-rs can make 270rwkw @ 15psi, with full boost coming on around 3200 with pretty normal supporting mods. Would this turbo be able to make this power efficiently all the way up to 8-8.5k or more?

Thanks in advance for any info as i am pretty ignorant when it comes to this sort of stuff.

No worries Seeing as it sounds like you have perused the dyno threads, where are the RB20s falling over? I know people who have spun RB25s to over 8000rpm, even on hydraulic lifter heads - 8500rpm is the highest I know of but I'm not 100% sure on all the supporting mods done. I know a lot of it was pretty typical, forged pistons (lower than stock compression, ~8.6:1 but thats just for high boost friendliness), rods, balance all the bottom end stuff, get an oil pump which can handle the revs and generally make sure you are always going to have enough oil in the right places. Get it all balanced!! And you will have to run a decent exhaust and inlet manifold as well, a nice high mount manifold can actually help them wake up in the high end quite nicely once you've got some decent cams etc.

I'd probably swing towards going a Neo solid lifter head if I were to try something like that myself, though I've been in RB25s running 8200rpm and I tell you - with a GT3582R bolted on the side and a decent set of cams they feel a LOT angrier than what you'd associate with the power delivery on an RB25 running a stock turbo or even a GT-RS. And that brings me to the next bit, how much boost/power are you aiming to run?

I think GT-RSs are struggling to flow enough air efficiently at the high end of the rev range of a stock revving RB25DET, much less an RB of that displacement area which wants to rev to 8500rpm or so. You are going to need a turbo with a reasonable amount of compressor to not choke the engine on its mission up there, you can probably forget any GT28 or potentially even GT30 based turbo - and unfortunately as a result of that also forget any chance of making full boost under 3500rpm. This kind of stuff is why we rarely get to have the best of both worlds

Edited June 19, 2008 by Lithium

[ehh]

i'm actualy building exactly such an engine(i'm just in the process of buying/getting money for the internal parts)

it'll be running big revs with solid lifters and serious cams(will be in a dedicated track car)

i have to be honest, yes i could make slightly more power keeping the 25 crank and it would be slightly more responsive

it would also be a bit cheaper

but there's also fun to be had with being different

it'll have a different character and will sound un like any other rb out there

anyway if i remember i'll post a video and or pics when it's all done

[KiwiRS4T]

RB30s can be made to rev to 10,000 if enough money is spent but a stock RB25 can do 7000 no sweat so 8000 is not too hard. If you really want a wide power band then twin charging the Rb25 is the way to go - supercharger and turbo (and that would be a little different although I believe it has been done!).

[GTSBoy]

The money spent on doing the various mods required to destroke the RB25 should be spent on a very good port job (not just a typical hack out - find an engine builder/head developer that knows RBs and what they need) and a very good exhaust manifold. These things will in turn improve the boost onset and extend the rev range at the top too.

There is no point having vague aims of just making a high revving destroked 25 unless you have a real need to go that way. I see that you are talking ~300kW at the crank. That target is well within the reach of pretty standard RB25s. Hell, the standard injectors will go that far. If you were spending money on a build, especially the largish amounts needed to do something odd, then you'd be wanting either a lot more power than 300kW, or you'd be looking for an engine that was pretty much bulletproof at 300kW.

[discopotato03]

Imo big revs are not the answer , big revs mean a lot more wear and tear and a higher chance of handgrenaded engines .

Bottom line is that a 2mm stroke difference is SFA and while a slightly shorter stroke could work better with a large bore diameter they don't go into RB anything blocks easily .

The reason why big bores work so well is that you can fit a lot of valve area and breathing ino the head above easily . Probably what limits an RB20 most is the inability to do much with bore and valve size bcause of that 78mm bore .

As for 12-14 pounds of boost and that power level the turbo would have to be really big and really laggy so not a wide ranging thing .

The practical thing is good rods and pistons in an RB25 or DOHC 30 . 100Kw/litre should be quite straightforward and I wouldn't think RB30s would be grass seeds in the US .

A .

[DR 32R]

Consider going the other way more stroke, rb26 crank or after market stroker crank. Potential earlier spool (less lag), more power with less boost. I think Upper Rev range in the RB 25 is improved more by quality springs retainers and head work than bore stroke ratio.

[aerofocker]

nobody in their right mind would do this to a working engine surely,you need a slap in the face lol not a destroked rb25, why the hell would people use rb30s? oh i just realized...school holidays... has to be