Phantom bike introduction (2)

(Phantom bike) 200kph+ VT250F Turbo! Pops Yoshimura was also keen on Honda turbos

  • 2018/10/14

Although there are many machines that don’t appear after being discontinued mid-development, it is rare that we ever get to take a proper look at what might have been. So we would like to pick up on some of these phantom bikes. This time we will introduce the Honda VT250F Turbo.

Phantom VT Turbo discovered!

While turbochargers for cars were experiencing a boom, in 1981 Honda had started selling the CX500 Turbo exclusively for export. However, this is the VT250F fitted with a turbo that was intended for the Japanese domestic market. Although this bike was picked up in a scoop here at YM, it was never put on the market because there was no concrete reason for its existence. *Young Machine: December, 2000 edition 

(Honda VT250F Turbo 1984) This is the VT250F turbo that was scheduled to go on sale. While it might look like a standard VT at first glance, there is a turbine positioned under the front cylinder. It recorded 53bhp with a top speed of more than 200kph. However, it was not approved by the Ministry of Transport, instead becoming a phantom machine.

Even the speedometer is marked out as far as 240kph. To the left and right of the meters are the voltmeter and a clock, but it isn’t fitted with a turbo pressure gauge.

Pops Yoshimura showed interest in the NR250 Turbo in 1987

Here we reprint an interview from 1987 between YM and Pops Yoshimura, founder of Yoshimura Japan. The discussion focuses on the wider issues around turbochargers and GP race regulations.

YM: Are four-strokes not permitted to compete in GP races?

Pops: It is possible, for sure. It’s just that normally-aspirated engines are not allowed.

YM: So this means…

Pops: It’s fine to enter provided you have a turbocharger fitted. There is a rumour now about a Honda 250 turbo, so if that appears it will be able to participate for real, then surely Yamaha and Suzuki will all compete against each other, too. If these kind of bikes actually become able to compete as a competition model, then I think the two-stroke engines will likely disappear after a couple of years. What we’re talking about here really, is whether it will get the go-ahead or not.

YM: But we’ve been hearing how these machines are pretty much completed…?

Pops: Oh no that’s not what the problem is here, instead it’s the issues between the FIM and the manufacturers regarding regulations. In a nutshell, it’s all the makers other than Honda that are now in a panic and opposing this. When regulations were made such as having anything up to a 250cc engine with superchargers being able to participate in GP500, everyone made light of how no-one would be crazy enough to make such a thing. However, as soon as Honda made their move to produce one of these, it seems like the other manufacturers went all faint, and they’ve brought up the matter of changing the regulations.

YM: When you think about it, technology these days is absolutely amazing, right? Honda managed to win even on four-wheels in the F1 championship in merely their third season running…

Pops: Indeed!! Technology truly is amazing. However, just because it looks like you won’t win, to go and have regulations changed is the lowest of the low. Now they are saying about having non-turbo 3.5 litre cars in F1 from 1988. Well, I say this might prove a blow for the conspicuous Honda. If I’m not mistaken I think it was Renault that instigated F1 cars to have turbos in the first place, but as a few years passed and they realised how they’d been overtaken by the Asian makers technology, the European teams all ganged up to have a change of rules. I’d like to say to the Europeans how they ought to be ashamed of themselves!! If much of the same technology is removed, then the technicians should be getting fired up and saying “Right, just you wait and see next year!! We’re not going to lose in the technology race!!”

Well, it’s the same thing on two wheels. Despite it already being decided beforehand whether turbo chargers or superchargers can be used in racing, instead of teams challenging themselves with new technology they are trying to change the rules, which is just downright cheating in my book.

YM: Wouldn’t you fancy giving turbochargers or superchargers a go yourself?

Pops: Yes, I would like to give them a go, but I cannot prepare adequately being a small scale privateer. It costs a huge amount of money on basic research and parts alone. Well, it’s not completely decisive. However, as a technician myself, what I’d like to see in particular is a four-stroke bike equipped with a supercharger that reaches a certain point of evolution.

YM: And yet there are road bikes coming out that will be fitted with turbo chargers, right?

Pops: No, that is difficult to say if it’s true. Whether it’s a turbocharger or a supercharger, if you want to use any of its real power freely it won’t be satisfactory. Back when I used to tinker about with aeroplanes in the war I witnessed the unused power of the B29 turbo chargers, so it was frustrating but there was nothing I could do about it. As for the planes that I had serviced, there were no enemies for the B29 that flew comfortably at high speed.

Well, I’ve not ever made anything myself that can be used for practical purposes, but I feel that I understand the sheer amazingness of a turbo engine. That power, the heat, the strain, and durability… whichever aspect you take there is a completely different perspective in terms of technical issues. To cut a long story short, horsepower is about torque and number of revs, but these days the most common method being adopted is for raising the revs to increase power. Increasing horsepower by 20-30% is a difficult thing to do.

Boosting horsepower means that torque is made to increase. So, to put it briefly this means that pressure is forced onto the cylinders and the fuel-air mixture is crammed together. Also, the mass of fuel-air mixture jumps up greatly for each revolution, but this is not to say that it would be a two or three times increase.

YM: Whoa, so they really are incredible things then…

Pops: That’s right, so in terms of evolving a four-stroke engine I think the technology would be a real breakthrough. It would be very difficult to manage, and just because something can be raced doesn’t mean that it will convert well to a road-going production machine. However, because of this point in particular, I feel I ought to give it a go. I cannot forecast what will happen ahead of time, as various issues would likely crop up one after another. If it was done… it would be on the basis that it would cover all aspects of safety. Regarding the scope of engineering evolution, this is not something that can be compared with a two-stroke.

YM: As guessed it can’t be done then, with a two-stroke?

Pops: Ahh, such a thing would be like the same as an animal which doesn’t have many functions like an earthworm, as it would be inferior. There will not be a machine in the future that has its lungs and intestines placed together. So, as for the question you pose, the idea of being able to simply compare between two-strokes and four-strokes is nonsense, so best to give up on this one pronto. Right, are we done then? *Young Machine February, 1987 (Hard talk with Pops Yoshimura)

Illustration of the commonly named NR250 Turbo engine that appears in the talk with Pops. An NR500 engine is halved in size to make a V2, with turbines placed either side of the front and rear banks making for a twin turbo, exhibiting 53bhp/18,500rpm at 2.0 turbo boost pressure!

Hideo Yoshimura (7th October 1922 – 29th March 1995). Founder of Yoshimura Japan. Besides developing the world’s first exhaust collector pipe, he played a large role in the racing scene of the 1960s to 1980s. Despite being a privateer, he managed to triumph over the powerful Honda Works team, and was known as the bike tuning god. His nickname “Pops Yoshimura”, is from “Father” (Pop).


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■愛車:BMW R100GS(1988)

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