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post #1 of 6 (permalink) Old 02-07-2002, 01:24 PM Thread Starter
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Engine Design

In 1979 Honda introduced to the racing world the NR500. Why did it use oval shaped pistons?

This should be an easy one.

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post #2 of 6 (permalink) Old 02-07-2002, 01:56 PM
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To fit more valves in the combustion chamber of each piston.
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post #3 of 6 (permalink) Old 02-07-2002, 02:51 PM
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there is a theory that says that for a given displacement, the motor with the most cylinders (all things being equal) will make the most power. it's 750-4's are in the same class as 1000cc twins.

since the rules didn't allow more than 4 cylinders, they made oval pistons to fool the motor into working like a 32 valve v-8, even though it was only a 4 banger (a 32 valve one though). honda learned a valuable lesson with that bike. do it your way, lose. do their way with the most money, then use your influence to not only do it your way, but get a major displacement break as well.

oh well, that's enough gp-1 complaining for now.


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post #4 of 6 (permalink) Old 02-07-2002, 04:22 PM Thread Starter
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I think the main reason is to get more valves per area like Desmo said. However Tony's explanation was the answer I was looking for. They wanted to make a V-8 engine.

Here is the link: http://integra.vtec.net/carmag/soichiro3.html


Last edited by Andy; 02-07-2002 at 04:29 PM.
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post #5 of 6 (permalink) Old 02-07-2002, 06:13 PM
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Desmo isn't quite wrong but Tony is very right. The valves make it work and are critical but they weren't the goal.

Letting the engine think it was an 8 cyl with the same cc's as a 4 equals EXTREME oversquare. IE, it's got a short stroke so can spin a zillion rpm's which is where HP comes from. Similar to the 6 cyl 250 they had back when (spun like 18,000 rpm). It's all about RPM's and to spin faster you have to shorten the stroke to keep piston speeds down cause past ~4500-5500 feet per second (I think thats the unit of measure) rods give up the fight. The eight valves just made it possible to fill such a long thin combustion chamber. Short strokes and high RPM are where most of the power improvemment per displacement has come from in the last 20 years. I think why it ultimately failed to succeed (for now anyway) was because of difficulty in getting rings that could seal such an odd shaped cylinder so compression sucked.
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post #6 of 6 (permalink) Old 02-07-2002, 07:46 PM
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Hi guys, nice topic!
The story goes that they got the idea from the shape of traffic lights, while waiting at a red light

The NR was the most evolutionary bike to be seen in GPs so far.
Much more than the current RCV!
It was the greatest exercise of Honda, but the time was wrong. Honda didn't expect the 2 strokes to gain over 30 Hp in 3 years time!
Like Apexismaximus said, the technology existed already for Honda, the only problem was the rules.
The rules were made by FIM to restrain Honda mainly, and the other Japanese, who were using multicylindre engines.
The 250/6 was extremely fast revving, but the 125/5 and the 50 twin were the absolutely extremely revvy engines with a very very narrow band. They were using 14 gears on the 50cc at some stage!
The NR was designed to run like an V8 but there were other problems. They had to get even more revs as the 2 strokes were beyond the 100hp mark already and they were lighter also!
They ended up getting to around 18000 revs but the bike had no torque at all at anything below 5 figure revs, st the point that it was very difficult to start it!
The NR except of the most technologically advanced motorcycle (or other ) engine ever made (still) had other firsts.
It had a monocoque frame, 16 inch front wheel, side radiators, and other features. The idea was great but the results were so poor that they couldn't even qualify to race!
In 1987 thy had perfected the bike and engine (see Trivia 'Name that Honda' and finally produced the road bike to prove their superiority. Soichiro Honda disliked the 2 strokes, he always wanted to beat the 2 strokes with a 4 stroke. He will be watching with a grin this years GP from above.


Last edited by ariszr7; 02-07-2002 at 07:56 PM.
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