You don’t see many oval piston sport bikes for sale these days, or any other days, for that matter. The Honda NR750, an outgrowth of the NR500 GP bike, was a rare specimen back in the early 1990s when it was new and I just got a note from Tim, one of our regulars here, that he spotted one of these exotic machines for sale on eBay over in the UK.
The NR750 has four oval pistons in a V4 configuration. Two connecting rods for each piston and 8 valves per cylinder make this engine really unique. It’s sort of like a V8 where someone knocked down a few walls while remodeling. The motorcycle press was all over this when it was launched and people still scratch their heads just thinking about simple things like piston rings, (piston ovals?). The styling of the bike doesn’t jump out at you unless you know your bikes and you see the letters NR on the side.
They’re asking £115,000.00, that’s $184,448.50 at today’s exchange rate. Yep, pricey, but you are pretty much guaranteed never to see another one show up on bike night.
UPDATE: As comments below show, these aren’t quite as hard to find as I thought. Interesting. Still a neat bike, though.
Link: eBay NR750 – auction over
SK says
Umm, maybe overpriced?
Just searched and found one (with 1K less miles) for £35000.
See link:
http://www.autotrader.co.uk/classified/advert/200739295340680/sort/priceasc/usedbikes/model/nr/make/honda/postcode/nn32by/radius/1500/page/1?logcode=p
SK says
sorry, typo, £38000 in fact.
B50 Jim says
I wondered what happened to them! Not one of Honda’s better ideas, although it seemed right at the time as a means to achieve V-8 power for race bikes under 4-cylinder rules. The motorcycle press was over the moon about it, though. A few critics pointed out the obvious shortcomings (there are good reasons pistons and cylinders are round), but pushing boundaries is the sort of thing Honda is famous for and what made it the powerhouse it is. I believe they had problems making those oval rings set properly — ironic, given that Soichiro Honda began his career manufacturing piston rings.
kneeslider says
SK, the one you pointed to is no longer available and it isn’t clear how old that ad is. When they’re in stock, they usually cost more. 🙂
Gen Kanai says
There’s almost always a few NRs for sale in Japan. Here are 3 for sale (and one of the stores has 2 copies…) and they’re priced at a fraction of the one for sale in the UK. You could fly to Japan first class, buy all 3, ship them back, and still get back to GB for less than the asking price of the one you’ve listed.
http://www.goobike.com/cgi-bin/search/spread.cgi?8700323B20080819009+
http://www.goobike.com/cgi-bin/search/spread.cgi?8201078B30080621001+
http://www.goobike.com/cgi-bin/search/spread.cgi?8700214B20090820001+
Gen Kanai says
One more!
http://page22.auctions.yahoo.co.jp/jp/auction/l40496106
kneeslider says
It looks like they are more available than at first appears and certainly more so than I thought. Since there aren’t that many to begin with, these might be turning into some sort of commodity for trade, like those bottles of wine that get sold at auction over and over that no one ever actually consumes. Pretty interesting, really, but I should have known, I even wrote about the phenomenon some time back.
HoughMade says
Despite the fact that there seem to be a few out there, that bike is a true collectible. It is significant technically and somewhat rare. I don’t know what a good price is, but I can’t see the market falling out from under them.
Paulinator says
I’m sorry about the negativity….but an oval piston is like a square watermelon. They were available at my local grocers for a while at about 5 times the normal price. I heard that they’re collectors’ pieces now – no one actually consumed them. The oval piston exercise was Honda Racing’s version of urban trials riding.
Dallara says
Oval pistons like square watermelons? Hmmmm… Well, perhaps you can describe a better way to get the valve area of a V-8 out of a V-4 of equivalent displacement? There are other advantages to the oval-piston concept, too. So many that the FIA feared Honda using oval-pistons in F-1 that they outlawed all but round pistons. And then there was the FIM and MotoGP… After the FIA closed the loophole allowing 5-cylinder engines to race at the same weight as 4-cylinders the FIA just the mere rumor that Honda was working on an oval-piston MotoGP engine had the FIA outlawing oval-pistons there, too. In fact, just about every motorsports sanctioning body in the world has outlawed oval-pistons, and the reasons have a lot to do with performance.
todd says
If Mazda can get rings to seal in a Wankle, I’m sure Honda would have no problems getting these to seal if they were allowed to develop if further. It’s too bad racing is NOT a forum for technology development – more like figuring out ways to bend the rules.
-todd
Paulinator says
Dallara, Hmmmm? Lets see…sleeve valves?
I designed a bearing using a rectangular piece of high performance plastic once. It heated up and changed shape enough to seriously degrade performance even though it was within PV limits of the material. I tweaked it and made it work adequately, but a cylindrical shape would’ve been far better. I know that fundamental premise doesn’t apply to Honda, though.
Aerion says
“The initial application of superior principle is almost inevitably defeated by the developed example of convention.” I don’t recall the exact phrasing or who first stated that, but he nailed it.
Such was the case with Honda’s oval piston engines. The piston ring sealing problems were overcome by adding some curvature to the front and back sides of cylinders, pistons, and rings. On early engines these were straight. Developed engines could run to 20000rpm and set a number of records. The FIA and FIM did and do fear the implications. V8 power in an engine little wider than a conventional V4 but with frictional losses approximating a V6.
Why hasn’t Honda done more with production applications for this technology? Maybe they expect it would simply be misunderstood or even unappreciated by most. Any marketing effort for a production motorcycle could not capitalize on potential racing successes thanks to bans by sanctioning bodies. The NR was certainly misunderstood initially. Most expected a hyper-powered sports bike. Instead they got a powerful, sophisticated engine that pulled like an electric locomotive. As for tuners and builders, boring an oval piston engine certainly would not be trivial and would be beyond the abilities of most machine shops.
Paulinator says
I don’t want to rant…but this technology was conceived with a suicide gene. It would NEVER enter the world in mass-production form. It can’t. The only reason oval pistons saw the light of day was to skirt around rules of engagement limiting the application of practical design in the first place. It is a band-aid applied to a self-inflicted wound. I find it funny that the rule writers had to run around comparing notes and building a defense against this unnatural menace. Ya I know. Racing brought us the CV coupling AND disc brakes….
Nice collector piece, though.
Aerion says
Many technologies were developed to skirt around the rules of racing, including: active & semi-active suspension, semi-automatic (paddle) shifting, traction control, ground effect (literally featuring “skirts” in some designs), etc. You named a couple yourself.
Paulinator, you seem very dead set against oval pistons. They did see production, although not mass-production. Maybe that will change as society becomes increasingly intolerant of 2-stroke pollution but still wants/needs higher power in smaller packages.
Klaus says
I lean towards paulinators argumentation: “The only reason oval pistons saw the light of day was to skirt around rules of engagement limiting the application of practical design in the first place”. Simply put the above pictured engine is a V8 but features connected pairs of cylinders/pistons because it had to be a four cylinder. An interesting piece of technology and definitely a collector’s item, but applying this idea to mass produced street bikes doesn’t make sense since there’s no restriction to build a V8.
Aerion says
I don’t think the primary application for most street bikes would be V8 round piston to V4 oval piston. In many ways V4 round piston to V2 oval piston would be preferable; the free revving power of a four with the pulling torque of a twin, and the frictional losses of maybe a triple. It seems like those are the engine characteristics most riders want. Besides, variable valve timing and lift can only gain so much and some of its incarnations have garnered extensive criticism (ie. VFR800 VTEC). Anyway, time will tell what, if anything, Honda decides to do with this technology.
Tom Lyons says
I find it an interesting technical undertaking.
From what I can see of the design, there are some very serious compromises taking place, which came along with whatever benefits were anticipated.
For one, the combustion chamber size and shape presents very difficult challenges for combustion efficiency.
Also the piston crown is a poor shape for combustion efficiency.
Another glaring one is that the valve recess on the end of the piston crown is just a hair above the top ring groove, and that is not conducive to longevity.
The pistons are inherently heavier with this design, so reciprocating mass becomes an issue.
The valve stems are extraordinarily long
The valves inherently shroud each other because of close proximity to each other, and they are also shrouded by the chamber wall due to their steep angle.
I have alot of respect for the Honda engineers, and they probably did as well as could be done to overcome these things,and it probably works real well for a racing design.
I think the reason it is not used today, is because there is more known now about the importance of combustion chamber design, and this design comes up short in that area.
Nothing is perfect, and perhaps the advantages outweighed the disadvantages in the overall result. I have some doubts about this being competitive with smaller chamber designs in round-bore engines.
rohorn says
There are drawing out there of a Honda V2 oval piston design – with the thrust faces being the round ends rather than the flat sides. There were stories of them developing a V2 turbo oval piston 250 – back when the FIM said that a 250 turbo would be legal in 500GP.
Something about the valve area / combustion chamber area ratio…
Oh yes – there are also pictures out there of a Honda twin turbo 250 sportbike that supposedly almost went into production as well – round pistons, though.
Sick Cylinder says
The ring sealing problems were overcome by Honda before this model was introduced.
One problem with using one regularly is the cost of parts – one was owned by a professional UK snooker player – he had a minor off in the damp on his way to a bike magazine shoot / test and it was £5,500 for a replacement seat unit.
No way is this worth £115,000 – a few years ago (during the previous UK recession) these bikes were selling for £15,000. I beleive they cost £38,000 new and are probably worth about £45,000 now..
It’s a bike I would have in my collection if I won the lottery jackpot – just to keep on my staircase landing to admire each time I passe on the stairs!
John S says
If you owned it you wouldn’t ride it. It’s difficult to get parts for early GoldWings, Can you imagine convincing Honda to cast you up a set of oval pistons? Or just a set of rings?
Aerion says
Tom Lyons,
Actually, the combustion chamber shape is practically ideal for twin spark plugs, with each plug nestled in the center of a 4 valve cluster. The very center between the 2 valve clusters would be a perfect spot for direct fuel injection.
The piston crown is typical of high compression engines: a shallow v-dome with machined cut-outs for valve clearance. The pistons themselves might be a shade heavier but they are supported by twin connecting rods. In the end the engines could reliably speed to 20000rpm or more, something a round piston engine of the same displacement and number of cylinders could not do.
The thermodynamic ideal is to minimize the distance between the top ring land and the top of the piston. This helps reduce the quantity of air-fuel mixture trapped between piston and cylinder that is largely hidden from the flame front. This in turn maximizes power while minimizing emissions.
The valve stems only look long because the valve heads are relatively small by comparison. Their absolute length is comparable to what is seen on most modern performance engines, with the length dictated by the need to clear an inlet tract that has as little curvature before the valve head as possible. The valve angles follow the current trend toward smaller included valve angles. The same features are apparent on drawings and cutaway engines by Aprilia, Ducati, Kawasaki, Yamaha, etc. Some engine designers have even argued for zero included valve angle with valve stems parallel to cylinder bore. I don’t think valve flow shrouding is any greater a problem than with round cylinders.
Cheers.
rohorn says
That 1990 VW diesel engine that also must have been done to get around those racing rules:
http://www.ibiblio.org/tkan/audi/audi-misc/oval2.jpg
http://www.ibiblio.org/tkan/audi/audi-misc/oval1.jpg
Tom Lyons says
Ok, Aerion,
Let’s take a look at each point you made individually, bearing in mind that I said that the issues may have been dealt with as best as could be, by the excellent Honda engineers.
“Spark plugs in perfect locations”.
Yes, that appears to be maybe so, but let’s understand that the location of the spark plug is only one minor aspect of the situation. Combustion efficiency concerns go far beyond the location of the spark plugs.
Piston crown shape is typical”
Perhaps nominally, but there are sharp ‘hot spots’ that can be seen in that dome’s irregularities, and the irregularities in the crown will affect the combustion path more than most normal pistons. There’ alot of shadowing on those crowns.
“Pistons may be a shade heavier”.
They are definitely heavier because they have the weight of 2 pistons in one. The fact that there are 2 rods on each piston makes it even worse. Reciprocating mass is definitely higher than in a similar displacement round bore engine of same number of cylinders because the round bore engine has only one round piston, and a circle has the smallest perimeter for a given area. Therefore a round piston has less skirt mass than any other shape. The 2nd rod adds more reciprocating mass, and it’s steel.
No contest there.
“speed to 20000rpm or more, something a round piston engine of the same displacement and number of cylinders could not do.”
This is simply false.
First I must point out that the NR500 was capable of 20krpm, but the NR750 redlined at 14krpm and 125hp. The MV Agusta 750-F4 SPR redlined at 13,900rpm in street trim with round pistons, with 146bhp.
The safe engine speed is determined by the piston speeds, which are defined by the stroke length. I assure you that a round piston engine of similar stroke length and proper design can achieve these same rpms in an engine designed to do that. Oval pistons do not equate to higher rpm capability in terms of mechanical integrity redline. It is impressive that they are getting this rpm, and they did a great job, but that does not preclude that a round piston engine could reach that rpm.
“The thermodynamic ideal is to minimize the distance between the top ring land and the top of the piston.”
I get what you are trying to say, but you are describing parasitic crevice volume effects, not “thermodynamic ideal”. In the pistons shown, the parasitic crevice volume above the top ring is not significantly different than most pistons, and this is because the top ring land must have sufficient thickness to space the top ring far enough away from the chamber to shield it from so much heat. And the top ring land must be thick enough to withstand the heat and pressure sufficiently to avoid collapse of the top ring groove, so the ring doesn’t bind. So the parasitic crevice losses are still there for the most part.
But, the end of the piston has a valve recess which intrudes deeply into the top ring land, almost to the top ring groove. This is a danger zone for pinching the ring from ring land collapse in that location. It is not conducive to longevity of the piston, and is far from ideal. I’m sure that was a compromise to fit all the valves in there. And a scary one, too..
“The valve stems only look long because the valve heads are relatively small by comparison.”
I understand this, but they are still very long. I also understand that many other engines have long valve stems, and I understand that steep valve angles assist in attaining better downdraft shape in the intake port. This is not lost on me, since I work in engine performance design.
However, in this layout, valve shrouding does occur on the chamber wall side of all 4 valves, and I can see it, even if I don’t see the cylinder head. This layout is designed for the typical multi-valve tumble effect, and it flows mostly out the long side. This does not require as steep a downdraft angle because it’s losing half the valve area for flow anyway. And this is exacerbated by the narrow chamber shape to fit these narrow pistons, and a round piston would have less shrouding on the short side turn.This was a compromise to fit this piston shape, and is less than ideal, even though they tried hard.
No design is perfect. I think these guys did a fantastic job with what they were trying to achieve with this. They are great engineers. But that does not mean that there are no compromises in this design, with some being serious compromises.
I must also point out that this design was from the late 1980s to early 1990s, and that was prior to many of the recent developments in understanding combustion chamber shape and design. The useful version of the wet-flow bench was not even invented at that time. There have been so many things learned since then, that this is just an archaic design.
Beautiful, and daring in its complexity, but archaic.
In the end, it’s just something that didn’t make it.
Even with a rule change, I seriously doubt that any of the modern designers would venture into that again, knowing what they know now.
Not a knock against the bike. It was done at a time when fewer things were known.
You could go down to the local bike shop and by a modern 600cc street bike that would spank it on the track. With round pistons. Even from Honda.
Paulinator says
Rohorn, You made your point well. That engine is everywhere 🙂
rohorn says
Yeah, it’s hard not to trip over those these days…
Seriously though – as I understand, it was designed to make a 2.3 liter engine with the same block footprint as the 1.6. I’m guessing that the eliptical bores/pistons/rings aren’t that much more expensive to machine than the round ones. Round pistons aren’t round – or straight – either.
Paulinator says
True about the pistons, but the rings and bores are…and will always be. DAMN IT.
Aerion says
Hi Tom,
“Pistons may be a shade heavierâ€.
I meant a single oval piston may be a shade heavier than a pair of round pistons sweeping equal total displacement.
“speed to 20000rpm or more, something a round piston engine of the same displacement and number of cylinders could not do.â€
I was referring to development and racing engines, not the limited production NR. Unlike the MV-Agusta F4, the production NR wasn’t designed as a hyper-powered sports bike, something I had mentioned. The biggest reason Honda pursued oval pistons was to increase 4-stroke engine speeds to compete with 2-stroke engines, without exceeding 4 cylinders. As a side benefit, although friction losses from piston rings and valve train were comparable to a V8, crank shaft losses were closer to a V6: a net gain.
“You could go down to the local bike shop and buy a modern 600cc street bike that would spank it on the track. With round pistons. Even from Honda.”
Agreed, but again see note above.
“The thermodynamic ideal is to minimize the distance between the top ring land and the top of the piston.â€
Minimizing parasitic crevice volume effects ultimately goes back to fuel efficiency and emissions, which lead to thermodynamics. That’s all I was trying to say. Thanks for clarifying. It would be interesting to see what, if any, problems would surface if someone took a production NR to 100,000 kilometers or miles and then did a detailed engine tear-down.
“However, in this layout, valve shrouding does occur on the chamber wall side of all 4 valves, and I can see it, even if I don’t see the cylinder head. This layout is designed for the typical multi-valve tumble effect, and it flows mostly out the long side. This does not require as steep a downdraft angle because it’s losing half the valve area for flow anyway. And this is exacerbated by the narrow chamber shape to fit these narrow pistons, and a round piston would have less shrouding on the short side turn.This was a compromise to fit this piston shape, and is less than ideal, even though they tried hard.”
I’m not sure I follow this argument. From my perspective, the intake flow for the 2 end valves, one at each extreme of the chamber, is shrouded or masked and therefore compromised to a similar extent as the same flow would be in a 4 valve round cylinder head. The valve head diameter to piston end radius ratio seems comparable to what is found in 4 valve engines. In contrast, the intake flow for the 2 middle valves is shrouded much less so, because the cylinder wall is straighter in that area and doesn’t hug or shroud the perimeter of those 2 valves to as great an extent. Overall, this would seem to be a net benefit.
“The useful version of the wet-flow bench was not even invented at that time.”
Commercially marketed wet-flow benches may not have been available. Whether or not Honda had one of its own proprietary wet-flow benches is open to debate, at least I’m not privy to that information. I’m fairly sure they had other visualization and evaluation tools available to them.
Design is all about a balance of compromises. Maybe the biggest compromise is simply manufacturing cost. Anyway, it’s interesting to debate with someone familiar with engine design.
Cheers.
Tom Lyons says
Aerion,
Cheers!
OMMAG says
This was …. wayyyyy out there at the time it was introduced.
I don’t think that it is a practical idea … but it sure is an interesting one.
What a great piece for a collection.
Tom says
Thanks Tom & Aerion for a nice in depth explanation of the technicalities of this machine. For a bike thats 20 years old now, it is still quite awesome, if a little pricey!! But if you think how much money Mr Honda poured into the project, its no wonder they cost so much. But definitely one for the collection. Saw one recently for sale here http://nr750.com
peter says
Dear Rohorn,
Can you tell me where to find these pics of Honda twin turbo 250 sportbike.
cheers,
Peter
rohorn says
All my old links on that are dead. The best one I had was a copy taken from MCN or some other non-US newsprint weekly.
The bike looked a lot like the VT250F, but with more plastic – sorry I can’t find anything helpful.
Honda recently released pictures of the advanced prototype inline 750 that was developed into the first CBR900 to some of the motorcycle media – I’m hoping they do the same with the 250 project.