Oval pistons?!?!? Oh yes...

[GPracer2500]

Undoubtedly the most technically sophisticated and complex motorcyle engine ever produced, the 1992 Honda NR750's powerplant boggles the mind. What makes the NR750's 750cc V4 so special? Its pistons are not round, they are oval. It has 8 valves per cylinder (32 total) and two connecting rods for each piston. Special indeed...

http://static.sportbikez.net/bikepics/medium/1048073526.jpg

http://static.sportbikez.net/bikepics/medium/1048073544.jpg

http://www.motorrad-bild.de/images_textbildarchiv/lexikon/v_kunde/nr750.jpg

http://us.tnpv.net/pv/2003/10/30/HON2003103002263_pv.jpghttp://www.diseno-art.com/images/nr750_oval_piston.jpg

http://211.141.107.26/HONDA/PIC/NR/9.jpg

http://photos1.blogger.com/blogger/5890/2526/320/NR%20750.jpg

http://www.diseno-art.com/images/honda-nr750.jpg

Honda's purpose for pursuing oval piston technology was to compete with the two stroke Grand Prix road racers of the 70's with four stroke technology. Honda had vowed to not pursue the two stroke road racing technology that was then dominant. The class rules of the time limited engines to 4 cylinders. And unlike current motorcoss racing classes, displacement limits where absolute--it didn't matter whether your engine had 2 strokes or 4, the displacement limit was the same. So, in the late seventies Honda began figuring out a way to make a 500cc four stroke competative with 500cc two strokes.

They found that the key was valve area. 5, 6, 7, and 8 valve (per cylinder) configurations where tried. 8 valves where found to achieve the necessary flow. But it proved impossible to arrange that many valves around a conventional cylinder and have them operate satisfactorily.

According to Honda lore, Honda engineer Mr. Irimanjiri was driving home from work one day and looked up at a stoplight. He saw two rows of four lights each. Eureka! The idea of an oval piston, 8 valve cylinder was born. Over the next 10+ years a variety of oval piston race bikes were built by Honda. Overall, they were only marginally successful. Desperate for domination in GP racing, Honda eventually relented and developed the wildly successful NSR line of two stroke GP bikes.

Around 200 NR750s were built. Sticker price was an astounding $60,000. The last one I saw for sale from a private collector had an asking price of over $70,000.

:D

[400exrules]

wow, i didnt know that was possible

[EXevan91]

thats very well awsome right there i wish we knew the hp numbers but boy i bet that thing go the no tomarrow until u have to get new wrist pins.

[JOEX]

The oval pistons have been brought up here in the past but not with the pics and details you've provided. Thanks:)

Any ideas on why this technology hasn't been more fruitful?

[GPracer2500]

Originally posted by JOEX
The oval pistons have been brought up here in the past but not with the pics and details you've provided. Thanks:)

Any ideas on why this technology hasn't been more fruitful?

I'm not entirely sure. But here's what I think:

The high costs of oval pistons don't help their chances of ever becoming mainstream. Apparently, it is far more difficult and time consuming to machine oval pistons and rings. Plus I don't know if they ever completely figured out how to get oval piston rings to seal (over the long haul) as well as round rings.

It's alot easier to simply add additional conventional cylinders to get the valve area desired for any given displacement. The problem for Honda in GP bike racing was they didn't have that option because of FIM rules. Four cylinders was the max they could have at the time. I suspect they would have never bothered with oval pistons if they could have built a V8. In a way, the oval piston was just a way to build a V8 that technically was only a V4.

When Honda entered Formula One racing as an engine manufacture, oval pistons were soon banned by the FIA. This was done to keep Honda from using the technology. The concern was that engine development costs could spiral out of control and hurt the sport. I'm sure that didn't help brighten the future of oval pistons. Someday our quads will rev to 20,000 rpm with pnuematic valves. We'll look back and thank F1 for providing a place to develop that technology. Not so with oval pistons....

I *believe* there is a volutary ban on the use of oval pistons in MotoGP racing today. I don't think it's an FIM rule--rather, the MSMA has chosen not to allow them. Or something like that. The MSMA is a manufacture member orgnaization.

[sidebar: Now that I think about it, this highlights how much racing rules/regulations influence the direction technology takes. Maybe if 4 stroke MX race bikes were limited to 250cc instead of 450cc we would see a resurgence of oval piston technology from Honda. Hmmm....]

And lastly, Honda owns patents on oval piston technology. If you want to build oval piston engines, you better talk to Honda first.

Add it up and oval pistons just aren't meant to be anything more than a technical curiosity. For now, anyway....;)

[400exstud]

GPracer2500, I nominate you the best, new member here. I would rank you right up next to Wilkin:eek:

[2004exrider]

Originally posted by 400exstud
GPracer2500, I nominate you the best, new member here. I would rank you right up next to Wilkin:eek:

Agreed 100%. I always enjoy reading your posts, your a good contibution to this site!

Jimmy

[Rich250RRacer]

Almost as amazing as their technology is the extent Honda goes to protect it. Honda actually patented the precise engine location in a three-wheeler, which stood until '84. Remember the early Yamaha tri-motos with the engine almost completely under the seat. This wasn't by their choice, they weren't allowed to put it in the conventional position. Made for one hell of a wheelie machine.

[JOEX]

Originally posted by Rich250RRacer
Almost as amazing as their technology is the extent Honda goes to protect it. Honda actually patented the precise engine location in a three-wheeler, which stood until '84. remeber the early Yamaha tri-motos with the engine almost completely under the seat. This wasn't by their choice, they weren't allowed to put it in the conventional position. Made for one hell of a wheelie machine.
This is the kind of stuff I like to read about. Helps you understand why mfg's do things a certain way.

[wilkin250r]

GPracer nailed it, with only a few minor details left out.

A few pitfalls that GP racer didn't mention were the rods and valves. You have to realize that these engines were designed to compete with a 2-stroke engine of the exact same displacement. In order to compensate for having only half as many combustion cycles, these engines needed to rev really high (in theory, twice as high), like in excess of 20,000rpm, to achieve the same output. With two connecting rods per piston, the rods had a tendency to twist at such high rpms, not to mention the complications of getting your valvetrain to spin that fast without floating a valve.

Like the article mentions, the key is valve area. The simple solution is to just increase the number of cylinders. Instead of a single-cylinder 600cc (5 valves max), you end up with four 150cc pistons (8 valves minimum, but can be made for more), and you don't have the same pitfalls of twisting rods and odd-shaped rings that don't seal as well. This is why a Raptor 660 can be modified for 65hp, but a 600cc inline-4 streetbike motor can be modified to make well over 90hp.

The oval-piston technology was developed because displacement AND cylinder numbers were fixed. It's just not cost-effective to go through all that hassle if you don't have those limitations. A 750cc V8 could be produced MUCH cheaper, easier, and more reliably than an oval-piston V4 with about the same horsepower.

[JOEX]

Now i'm confused.....

Is the comparison for displacement or number of cylinders?

[wilkin250r]

Originally posted by JOEX
Now i'm confused.....

Is the comparison for displacement or number of cylinders?

Both. The Grand Prix rules limited absolute displacement, but didn't specify whether it was 2-stroke or 4-stroke. If you wanted to race a 4-stroke, you had to do it against 2-strokes of the exact same size.

Not only that, the rules also limited to a maximum of 4 cylinders.

Now, here's the key, a little in-depth knowledge on engines. For 4-strokes, the limiting factor for power is a number of things, but one of the biggest contributers is the ability to breath (your porting and valves). If you could get more valves and breath easier, you could get higher revs, more horsepower, and get close to the same power/displacement ratio as the 2-strokes. :eek:

Unfortunately, you just can't squeeze 8 valves into a single round cylinder. 5 is pretty much the maximum. If you could find a magic way to get 4 more valves on your 400EX, you could probably achieve well over 75HP at approx 18,000rpm. :cool:

its sweet, none the less

[nakomis0]

I wish I could remember where i read it but...

About a year ago there was rumors of the oval piston going into a Honda quad.

I either read that on Exriders, or in an ATV magazine.

[wilkin250r]

Originally posted by nakomis0
I wish I could remember where i read it but...

About a year ago there was rumors of the oval piston going into a Honda quad.

I either read that on Exriders, or in an ATV magazine.

Not a chance. It will NEVER happen. You won't even see them in dirtbikes, and we all know they get the technology first.

On a side note, I wonder how well they would work on a 2-stroke. I could see you getting a lot more port area for a given displacement size, but I wonder if you could figure out the complexities of properly filling the cylinder.

[parkers30]

I would say the biggest challenge would be in ring seating, with a round ring the spring of the metal applied even pressure, but with a oval of this nature you would end up with pressure points and high wear areas. Which might not be such an issue on a race engine, but for a mass produced engine I cannot imagine the reliability would be there.

[GPracer2500]

I'm building an engine with a square piston--so there! Take that Honda...

:p

[Iliketogofast]

Originally posted by GPracer2500
I'm building an engine with a square piston--so there! Take that Honda...

:p

I wonder why they don't do that. It would be like the Kicker Solo speakers that are square for more surface area. Wouldn't you be able to fit more gas/air into a square combustion chamber than a round one?

I guess maybe it would leak, and the edges would wear too easily, and gas would get trapped in the corners...

Anyway, NS750 was a really, really sophisticated motor, but it didn't make much horsepower, and had a comparitively low top speed and 0-60. It's amazing on paper, and the idea worked good enough to remain the favorite of many riders even today... But there are many more bikes today with a lot more HP per cubic inch.