RB20 and 25 NEO differences

[hq_]

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.

[Duncan]

@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.

[GTSBoy]

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.

[hq_]

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 September 12, 2020 by hq_

[GTSBoy]

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.

[Duncan]

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.

[hq_]

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 September 13, 2020 by hq_
Typos

[Duncan]

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....

[DatsunBanana]

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.

[iruvyouskyrine]

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.

[DatsunBanana]

This article appears to be talking about the swirl valves ...

https://carscanners.net/nissan-skyline-r34-and-laurel-c35-with-rb20-de-neo/

And this is one of the old discussions with dead pic links.

https://www.skylineowners.com/threads/rb20de-r34-gtt-neo-engine.155290/

[Neostead2000]

Hopefully I can shed some more light into this. I was sent an RB20DE Neo by mistake (instead of a 25) so now I am stuck with it, trying to build it up for +T and also learning all the specifics about this "little" engine. I made some pics when I tore it down, hopefully this will help people. Note that I took a video of the swirl valve actuator and then took a picture from it to upload here. The manifold is technically a five piece design: 1. Throttle Body ---> 2. Y-shaped divider ---> 3. Top part of manifold including the big central torque valve ---> 4. Lower part of manifold ---> 5. Flange with swirl valves on one common shaft in each runner that mates up to the head.

 

 

I am still uncertain whether I should keep the swirl valves and torque valve when turbocharging or if I should somehow remove them or hack them to stick open permanently.

So far the crankshaft looks to be identical to a standard RB20 however it has the SHORT snout, but matched to an N1 oil pump (took it apart to confirm). The water pump is the smaller impeller one. Solid lifters, no front head drain/breather, coil on plug setup.

I just test fitted the stock 25 turbo manifold and it bolts on perfectly.

[Dose Pipe Sutututu]

On 18/02/2022 at 8:48 AM, Neostead2000 said:

I am still uncertain whether I should keep the swirl valves and torque valve when turbocharging or if I should somehow remove them or hack them to stick open permanently.

would be fun to have that controlled by your aftermarket ECU when you're off boost, once you hit a target boost level that then fully opens.

That, along with VCT and proper twin scroll turbo setup would probably make it pretty fun.

[DatsunBanana]

The main torque valve shouldn't need hacking as it defaults to open without the vacuum pipe connected (at least on the 25DE). Not sure about the swirl valves but in your pic they look open by default. On the 25DE I used a GTT ECU but it doesn't have the signal to control the valve. Other discussions seem to suggest at boost the valves won't do much but would still be cool to know what it's like controlled by an aftermarket ECU.

[Neostead2000]

On 2/18/2022 at 9:46 AM, DatsunBanana said:

The main torque valve shouldn't need hacking as it defaults to open without the vacuum pipe connected (at least on the 25DE). Not sure about the swirl valves but in your pic they look open by default. On the 25DE I used a GTT ECU but it doesn't have the signal to control the valve. Other discussions seem to suggest at boost the valves won't do much but would still be cool to know what it's like controlled by an aftermarket ECU.

Correct, the swirl valves are normally open when the actuator is at rest. As I pull the shaft towards the actuator, it moves the valves closed. So that would imply under vacuum they will be fully closed and 100% of the airflow will be redirected through the two little Mickey Mouse ports. However, this is an N/A engine, so *naturally* it is always under vacuum. That's why there is a separate controller under the manifold that looks like a big vacuum pump of sorts, with three vacuum lines. I didn't spend too much time tracing the vacuums before I disassembled it, but I suspect at least one vacuum line goes directly from manifold to the pump, then the other two lines go to the actuator and a solenoid perhaps.

As far as I know the swirl effect was designed to be operation under low-RPM operation, but I have not found a specific value. I would guess anywhere from 0-3000 RPM. The swirl effect helps air-fuel atomization while entering the combustion chambers as well as providing a leaner burn at those conditions (Which is why this engine in the factory manual is referred to as "L/B" - Lean Burn).

My worry is, since this engine will be seeing boost, it is going to slam shut both valve mechanisms and severely restrict airflow to the head. I HIGHLY doubt Nissan designed the solenoid and vacuum pump to operate with boost in mind, as that system is not present on the turbo NEOs. So this will be an interesting task to tackle.  BMW use the same exact system on their N57 turbodiesel engine, so there must be a way to configure it for proper turbo operation also.

[DatsunBanana]

That 'pump' is a vacuum tank, I guess just a reservoir for holding the vacuum. On the 20DE looks like it's used for both the swirl and torque butterflies. One line is to the plenum and the other two will go to your torque valve solenoid and swirl solenoid. The solenoids will have their own lines to the actuators. The solenoids seem to switch the actuator hose to either the vacuum tank or the air filter side of the throttle body. This is from page EC-3 in the manual. On 25DE the tank is just for the torque valve so I binned that and the solenoid and related hoses and used the old tank take-off for my boost gauge I think.

From experience I know the torque valve is under spring pressure keeping it in the default open position. I don't know about the swirl valves but if they are sprung to open position then removing the vacuum tank, solenoid and hoses from the actuator would (I'm guessing here) let them spring quite firmly open (??)

Page EC-44 in the manual gives info on the RPM threshold for the solenoid ON/OFF signal.

 

 

[Neostead2000]

On 2/18/2022 at 3:04 PM, DatsunBanana said:

That 'pump' is a vacuum tank, I guess just a reservoir for holding the vacuum. On the 20DE looks like it's used for both the swirl and torque butterflies. One line is to the plenum and the other two will go to your torque valve solenoid and swirl solenoid. The solenoids will have their own lines to the actuators. The solenoids seem to switch the actuator hose to either the vacuum tank or the air filter side of the throttle body. This is from page EC-3 in the manual. On 25DE the tank is just for the torque valve so I binned that and the solenoid and related hoses and used the old tank take-off for my boost gauge I think.

From experience I know the torque valve is under spring pressure keeping it in the default open position. I don't know about the swirl valves but if they are sprung to open position then removing the vacuum tank, solenoid and hoses from the actuator would (I'm guessing here) let them spring quite firmly open (??)

Page EC-44 in the manual gives info on the RPM threshold for the solenoid ON/OFF signal.

 

 

You're correct. Actually I didn't know this info was explained in the manual, but I opened it up and those pages do talk about it a little bit. Basically the spring inside the actuator is always pushing the valves open, but if the engine is running (running always under vacuum), it should be pulling the diaphragm against the spring, essentially closing the valve assembly across all 6 runners. I took a closer look at the diagrams, it doesn't seem to be a clear ON/OFF scenario, but rather the ECU determines when and how much to open the valves by selling the solenoid to let air past it and start "pulling" on the diaphragm.  It's a bit convoluted but after a few minutes of study, it appears clear.

 

I believe that with a decent aftermarket ECU, the SCV (Swirl Control Valve) and it's solenoid can be configured to remain CLOSED up to the pre-spool RPM range of the selected turbocharger, then slightly open just as it starts spooling in conjuction with advanced timing and leaner AFRs. Then as full boost is attained, the valves should go 100% open (boost hitting the diaphragm) and richening the mixtures and dialing back the timing.

In theory, and based on my own experience of ECM tuning, this should work nicely and offer a good economy "round town" operation whilst picking up on boost once power is demanded.

 

I attached a video I took just to show physically how the arm moves and valves open for anyone curious.

 

[Kapr]

Hi.

a little later but if the topic is here.
So...the exhaust is the same and you can use 25/Turbo or any aftermarket?

What about intake side? Is it too "plug n play" or? (i do not remember for 100% but i think that one intake to head gasket was little bit different than on the RB25 - i bought gasket kit way back)

I know that even if it "plug" those butterfly swirls would cause ECU to be confused or?

 

Just asking for the info 🙂 and of course the RB25 things are easier to come by
 

[Neostead2000]

3 hours ago, Kapr said:

Hi.

a little later but if the topic is here.
So...the exhaust is the same and you can use 25/Turbo or any aftermarket?

What about intake side? Is it too "plug n play" or? (i do not remember for 100% but i think that one intake to head gasket was little bit different than on the RB25 - i bought gasket kit way back)

I know that even if it "plug" those butterfly swirls would cause ECU to be confused or?

 

Just asking for the info 🙂 and of course the RB25 things are easier to come by
 

I have up to date pics of the engine build and turbo conversion over on my instagram @skpro_jects if you want to take a deeper look.

 

Basically the exhaust manifold is straight bolt on and the ports match (they arent any smaller than a 25 or 26 head). 
 

the intake ports are different shape so the 20/25det manifold will bolt up but wont match properly or seal. You will need to port match using the 25det gasket as a stencil. Personally i ran the entire stock 20neo intake setup and have configured the swirl valves and torque valve to work based on my custom parameters to help with off boost response and spool. But this is using a standalone aem v2 ecu. The stock 20det ecu wont be able to control them. 
 

 

[Dose Pipe Sutututu]

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