Foreword
Part 2 of the tuning menu was the engine. The degradation of the Air Flow Meters (henceforth will be called AFM) was becoming increasingly noticeable over he last year.
While just a swap of the AFMs with OEM ones would have been the more budget friendly option the ongoing goal for improving my car’s (Vivian) efficiency the R35 AFMs + injectors present on the Nismo S2 RB26 has always been a want. Currently without the funds to commission an engine at Omori Factory I had been considering finding a way to make these S2 upgrades to the S1.
RB26S1改MkI
This means my Nismo S1 RB26 will have a slight change of flavour, the RB26S1Kai(改). 改 prounced “kai” is the short form for “modified” in Japanese. I will swear here to keep the fundamentals of the S-tune engine the same such as Turbo architecture, camshafts going forwards. Primary focus for this engine as is with the car to improve efficiency of the native RB26 architecture.
Followers of the instagram account gt_grand_touring will have seen the purchase of the Nismo RB26DETT Fuel Kit (product no. 17500-RSR41) last year, I could not contain my excitement for that product. I went down to my local Nissan Performance Centre in Yokohama to go place an order the moment it was announced in August 2021.
However then finding out that Nismo only does complete engines (as they need to warranty their work) tuning of the engine around this kit would need to taken up by the aftermarket. So the search began for a tuning shop.
The contents of the aforementioned kit is a custom fuel rail by Nismo designed for the OEM R35 injectors included in the kit to be fitted as well as OEM R35 AFMs and R35 AFM adaptors and all the required wiring and hoses.
The tuner
After consulting with Best R’s mechanic, Konishi-san within the Tokyo-Kanagawa area there were not many tuning shops with extensive experience tuning the Hitachi factory ECU to work with both the R35 injectors and AFMs together.
The most experienced with the most data turns out to be Mine’s. Best R has worked with Mine’s on a number of customer cars over the years as well just to add some extra piece of mind. Mine’s it was then!
DPi system
Having seen many R35 injector tuned RB26s Konishi-san recommended we try something new.
He came across a product being sold by the same company, Jura Tech whom I got my R35 coilpack adapter kit years back.(Ed.They were not the first to market here but that is a story for another day)
I have decided to nickname DPi, Direct to Port injection system. In short it is a modification to the stock RB26 intake manifold in order to optimise it for the R35 injectors spray pattern.
In order to best explain how the DPi system works we need to back up and look at the R35 injector design. As we know it is a 12 hole design, with 6 holes per left and right port.
As shown here the spray pattern is directed at each port.
The RB intake manifold designed for a single-hole injector. Bolt on the R35 injectors still do offer an improvement in fuel atomisation.
While In a single-hole port injected engine it makes sense put some distance between the injector and the port to allow the spray to diffuse out alittle.
Evident in the above two pictures the injector port width and distance of the injectors from the ports restricts the full potential of the R35 injectors.
Closing the distance from the injectors to the port would optimise for the R35 injector spray pattern
From here, the stock position and diameter
The Jura Tech manifold modification has increased the diamter and brought the injectors closer to the ports, favouring the V spray pattern of the R35 injectors. Spraying more directly to each port, a DPi system.
The final major piece of the Jura Tech kit are these adaptor/spacers to accept the R35 injectors as well as bridge the increased distance between the fuel rail and the now deeper seated injectors. As a final touch they come with this silver coating.
To get this done they require a donor factory intake manifold (I imagine It can be applied to other RB series engine manifolds too). Best R happened to have a spare which they sent in so I could still drive my car while the other was sent in to be machined.
Then later they sent it back boxed nicely as a kit including the newly machined manifold!
Here is the DPi system fitted to the S1 (rotated so the spray pattern will align with the left and right ports).
You may also notice that the stock fuel rail is being used instead of the Nismo rail. This was because the adapter/spacers are designed to work with the stock rail. Jura Tech did say if provided with the rail they can create a spacer to match the Nismo rail however I thought it unnecessary so decided to go with the original rail.
The tune
As previously mentioned the tuning after the parts were fitted at Best R (While the intake plenum was out I also had all the water lines replaced) would be conducted at Mine’s. Usually for most setups Mine’s can mail out a ROM based on your setup (that is how much data they have) as they will most likely have the data for your combination of parts. In my case however they had never done an engine with the DPi system. Requiring the car to be transported to their shop and an additional 100,000JPY for custom tuning fees.
The result
As this happened during the summer there was a rainstorm which delayed the tune by a week or two. In the end though it was well worth the wait.
To summarise the specs of the RB26S1改:
Nismo S1 ECU w/ Mine’s VX ROM
R35 injectors
R35 AFMs
Jura Tech DPi system
SARD Fuel pump 500hp rated (an S1 engine uses the factory fuel pump and injectors)
Driving impressions
In a word, even easier to drive.
Around town you can dawdle about effortlessly. When reved out it feels beefy, noticeably now has a torque kick while still maintaining it’s linear power and torque characteristics. Throughout the rev range and especially on boost (which now begins at 2k rpm) much less throttle is required than before.
A final Easter egg is the range has definitely increased. This is a factor I also wated to improve continuously as more range=more fun. Previously it would do about 300kms per tank now at a pinch will do 350kms. This I confirmed with a drive up to Lake Chuzenji with both before and after setups in the same Summer season. I am very happy to say the least.
Now my wallet needs to regenerate so won’t be doing anything major for a while. Just little bits here and there.
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