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I'm researching possibilities for yet another project and this involves building a modified RB20DE NEO. Probably with a larger bore for slightly more displacement in oversquare configuration. Not quite 2.5l, though, but as the power will be have to be made with revs, the breathing ability of the setup is crucial.

The jump from 155 to 197 crank hp between RB20 NEO and RB25 NEO is significant so I've been trying to research what, exactly, Nissan has done to accomplish this in addition to larger displacement. There's really no point in reinventing the wheel when engineers at Nissan have already done most of the work for you.

Has anyone compared the heads, intake manifolds, throttle body and exhaust manifolds side-by-side? Port sizes and volumes, valve sizes etc. are of interest in this case but simple observations are useful as well.

Using aftermarket parts is out of the question because of racing class rules so I'll have to make do with stock and modified Nissan parts.

All help is much appreciated.

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@Savman might be a good source of info, although he was interested in turbo not NA differences. From memory there was an RB20VE somewhere along the line which was even better then DE with the variable valve timing.

I don't know what the class rules/restrictions you are looking at, but there are a lot more efficient 2.0l na engines than an RB20.

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14 hours ago, hq_ said:

The jump from 155 to 197 crank hp between RB20 NEO and RB25 NEO is significant

No it is not significant.

2500/2000 = 1.25.

197/155 = 1.27

ie, the 25Neo makes a little bit more than the 20Neo does, but only ~3 HP. You could make that difference with 2° of timing, or just from the 25 breathing a tiny bit better in the peak power rev range than the 20 does. Or it could be that Nissan deliberately trimmed the 20s power output a tiny bit in order to achieve some fuel consumption or emissions goal.

Whatever the case, it does NOT represent a major design or specification advantage in the 25.

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I've seen RB2*DE NEO engines called VE on some occasions even though the NVCS doesn't quite justify the moniker. Nissan VE-engines like SR20VE alter valve lift along with timing, which AFAIK isn't available on RB heads.

The difference in specific output is negligible but in absolute terms it's 42hp. Atmospheric RB25DE NEO does have small port volumes to maintain intake charge velocity and high BSFC efficiency, but the real question is whether they (and manifolds) are identical or even close to those of RB20DE NEO.

I can't alter the stroke, the crank will obviously have to be knife-edged and balanced, but the block can be bored out to a degree. Not quite to the venerable RB24 specs, though, so the difference has to be made up with hot cams and revs. At that point, a lot more air will have to be moved to make power, not unlike when increasing displacement.

The ability of the head and manifolds to move a sufficient amount of air to make at least 170 crank kW is the key. That's been done with unmodified RB25 NEO manifolds and head so if they're the same (or close enough) in RB20, we're in business.

The point is that we'd love to race an HR34 coupe in a class dominated by E36 BMW:s and VTEC Civics and in order to do that we'll have to use OEM 2.0l, naturally aspirated engine. This is endurance racing so the NEO head being a semi-lean-burn design is also an advantage. We can map it for optimum BSFC but it also has to be able to make power at top end to be competitive.

I raced an E36 in the same series earlier for several years. It made 174kW and was very competitive in terms of speed, but fuel consumption held it back in longer, 6-24 hour races.

Edited by hq_

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Jeesus. If you can't make nearly 300HP with a 2 litre 24 valve modern engine like a 20Neo then you should set fire to it. They used to make nearly that much from 2 valve 4 cylinder engines with dirty dribbly Webers on them.

You won't need any RB25 parts for this. Just weld up the cams and regrind to 300° total duration and 11mm of lift, relieve the cam bores and do something to stop the shims from jumping out. Port the bejeezus out of it and get the compression up to 11:1. Job done.

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Well I don't have specific experience with the non-turbo engines, but I always love to see a skyline racing :) It is hard to see how you will end up quicker than a 2.0l honda as engine and whole car will weigh a fair bit more.

If you want to stay nissan (and who wouldn't), what about an n/a slivia with that SR20VE you mentioned. Must be lighter, reasonable suspension geometry etc.

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Good points, but there's a major problem. While the rulebook follows FIA guidelines, there are some notable exceptions. Engine and drivetrain are very restricted. To sum it up:

MY 1998 or older car model, original naturally aspirated engine, less than 132kW in stock form. Displacement affects minimum weight. OEM intake and exhaust manifolds and throttle body. OEM transmission, no LSD, final drive ratio from same model series. Cams, pistons and head porting are unrestricted, but there's just so much you can do when manifolds are the limiting factor. Pump fuel only, which effectively limits the compression around 11.5-12:1 at most.

Fortunately this rules out most Hondas with 1.8l or larger displacement so competition is less than fierce, technologically speaking.

Then there's the fuel capacity limit. 26l tank, 20l maximum per refueling. If you go all out for HP, you'll be refueling at least twice more than the competition during a 6 hour race and each stop costs you around 3.5 minutes.

So, now that the background is sorted out, it may be obvious why I've contemplating RB20DE NEO. Dare to be different too, but after some 26 years of tinkering with RB26:s, trying to make power without stroking, resleeving and boosting the heck out of RB sounds like an interesting challenge.

Not to mention the R34 bodyshell with functional GT-R/Z-Tune aero bits is one heck of a sponsor magnet compared to a mundane BMW sedan or Honda Civic.

Still... anyone have an idea about the differences between RB20 and 25 head/manifolds/etc.?

Edited by hq_
Typos

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Sounds like a smart set of regs to prevent an arms race.

You will have trouble getting reliable input on na rb20 vs rb25 because pretty much everyone here is turbo. I suspect you need to buy a motor and have a good look....

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Back to your original question, the RB20DE NEO has butterfly valves for swirl (see page EC-3 in the R34 manual). I remember seeing some pics once but so many pics have dead links now I can only find the discussions by googling. So I can't remember exactly where the valves are - in the runners maybe? The RB25DE NEO doesn't have them although both have the variable air intake butterfly valve in the inlet plenum.

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I have a 100% complete 60xxx KM RB20DE Neo in my shed that i would be happy to take as many pics of as you want. I am very busy at the moment though so may have to wait a couple of weeks.

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