Author: GATER KAZUKI

Hello everyone, GATER KAZUKI here.

Today's topic is "Why is the RB26DETT's displacement 2.6 liters?"

2,568cc. Hasn't this number always bothered you? Not 2.0, not 2.5, not 3.0, but a curiously in-between displacement of 2.6 liters. And to be precise, it's 2,568cc, not even reaching 2,600cc.

There's a reason for this, and quite an interesting one at that. Today, I'll delve into it.

First, let's review the RB26DETT's specs.

The RB26DETT is an inline-six DOHC twin-turbo engine, achieving a displacement of 2,568cc with a bore of 86mm and a stroke of 73.7mm.

With a bore of 86mm and a shorter stroke of 73.7mm, it's what's known as a "short-stroke" design. While short-stroke engines are good for high revs, they tend to produce less torque at low RPMs. Indeed, the RB26DETT has a somewhat thin torque feel at low RPMs, and only truly comes to life above 3,000rpm when the twin turbos kick in. This characteristic is a matter of preference, but I personally like it.

Let's get back to the main point: why 2,568cc?

The "1.7x Rule" curse of Group A

To understand this, you need to know about the Group A regulations.

Under FIA Group A regulations, an "equivalent factor of 1.7" was applied to the displacement of turbocharged engines. In essence, turbocharged cars were classified based on their actual displacement multiplied by 1.7.

FIA documents from that time show class boundaries at 3,000cc, 4,000cc, and 5,000cc. The converted displacement listed in the BNR32's homologation documents was 4,367cc. This exceeds 4,000cc but doesn't reach 5,000cc. This means the R32's displacement was calculated backward to fit into the "over 4,000cc, under 5,000cc" class.

If the actual displacement had been 2,500cc, the converted value would be 4,250cc, barely putting it in the same class. So, what was the point of increasing the displacement to 2,568cc? The actual displacement equivalent to the class limit of 5,000cc (converted) is approximately 2,941cc. The fact that the displacement was stopped at 2,568cc without using up the full potential up to that point is, I interpret, a natural outcome of designing the bore and stroke combination for high-revving performance. I believe it wasn't a case of "we wanted 2,568cc," but rather "it became 2,568cc as we refined the design."

The "Why not 2,500cc?" question

This raises a question. If they had chosen a clean 2,500cc, the converted displacement would have been equivalent to 4,250cc. There must have been a strategy to push the very limits of the class boundary. Why did they increase the displacement to 2,568cc?

This is my conjecture, but I believe it was "purely because they wanted more displacement for more output."

In Group A racing, outputs eventually exceeded 600ps, and for that, a larger displacement is always advantageous. In Group A, where the engine of the base production car had to be used as is, displacement directly impacted the race car's performance.

The difference between 2,500cc and 2,568cc is 68cc. While this number seems small, when high-output tuning is applied, a 68cc difference is by no means insignificant. I believe Nissan's development team chose that 68cc as a sign of their intention to "secure as much leeway as possible, even 1cc, for racing."

The choice of 86mm bore and 73.7mm stroke

Another interesting point is the combination of bore and stroke.

An 86mm bore is relatively large among RB series engines. A larger bore increases the combustion chamber area, improving intake and exhaust efficiency at high RPMs. On the other hand, a shorter stroke of 73.7mm reduces piston travel, making it easier to handle high revolutions.

This balance can be described as "a design predicated on maintaining high RPMs on the circuit." In other words, leisurely driving on public roads was never a consideration from the start.

Indeed, when you drive an RB26DETT slowly in the city, the engine honestly doesn't feel particularly pleasant. The torque at low RPMs is thin, and accelerating from a stop feels sluggish. I interpret that, if favorably, as a message saying, "This engine isn't for city driving."

Why "2.6 liters" seems like an odd number

Finally, let's talk about why 2,568cc seems "odd."

In the Japanese automobile tax system, the tax amount varies by displacement. 2,001cc to 2,500cc is one category, and 2,501cc to 3,000cc is the next. Since the RB26DETT is 2,568cc, it falls into the latter category.

In other words, as a result of prioritizing Group A regulations, the displacement became disadvantageous from a tax perspective. It's a case of "optimizing for racing resulted in higher taxes." The displacement was chosen for racing, somewhat at the expense of the users who would buy the production car. The R32 GT-R is that kind of car.

Do you consider this "unfriendly" or "perfectly fine"?

I think it's "perfectly fine." The GT-R should be that kind of car; it's not a car whose displacement is determined by worrying about user maintenance costs. Rather, the fact that "they chose this displacement even if it meant higher taxes" seems to tell the true essence of the R32.

Behind the number 2,568cc lies the persistence for Group A and a commitment to racing. That's what I wanted to talk about today.

By the way, the R33's RB26 also has essentially the same displacement, but I consider it completely different due to its weight. We'll save that discussion for another time.

That's all from GATER KAZUKI.