300cc Inline 6 Cylinder by Guy Coulon – a 22000 RPM Race Engine Personal Project

Guy Coulon with his 300cc 6 cylinder racer

Guy Coulon with his 300cc 6 cylinder racer

Guy Coulon is Colin Edwards' crew chief on the Tech3 Motogp team. When Guy isn't directing the building and maintenance of exotic race bikes, he spends his time exercising his skills building exotic engines and bikes for himself like this tube frame, 300cc inline 6 cylinder racer, an extremely impressive engineering project. I hesitate to use the word "homebuilt" since he obviously has the shop resources of a race team at his disposal but it's, nevertheless, his own design and work. Starting, I believe, with two CBR250 engines, he machined his own cylinders and crankcase, machined a crankshaft from solid alloy and even designed his own gearbox.

Guy Coulon 300cc inline 6

Guy Coulon 300cc inline 6

The dyno says 60 horsepower at 18,500rpm, it will spin to 22,000rpm! The video below gives you a small sample of what those little cylinders sound like though not at the extreme of the rev range.

There's not a lot of information about this project and Guy is obviously a busy fellow so what you see here is as much as we have so far.

Guy Coulon 300cc inline 6

Guy Coulon 300cc inline 6

Guy is also a friend of French builder, Daniel Mercier, who specializes in fabricating parts for and building replicas of the 1970 Honda CB750 Daytona racers. Along with those beauties, Mercier now says he will offer a replica of Coulon's inline 6.

Whether it's a Ducati V8 or an inline 300cc 6, projects and builders like these just make the mechanical problems you or I might encounter look a lot more manageable.

Big Thanks for the tip, Michael!

Link: Guy Coulon
Link: MotoMatters
Link: Daniel Mercier

Video below:


  1. rafe03 says

    Some Monster Talents & Skills! Can hardly be called machinery. More like rolling performance art. And for his day job, he’s Colin Edwards’ crew chief on the Tech3 Motogp team. So building beautiful things like this is something he does in his spare time. WOW!

    Also, I went to look at the site of Daniel Mercier (see link above) & among the many fantastic bikes shown there is a complete V6 engine/trans that appears to be made from 2 BSA Rocket 3’s!


    Obviously, another bunch of guys brim full of skills & talent! I guess I was standing in the wrong line cause I only got a bit of that. I stand in awe!

  2. dosed says

    Hmm, that engine looks huge for a 300cc.
    And it has the same hp/liter as an “ordinary” jap 4cyl 600cc

  3. Jim says

    Fantastic! I’m a huge fan of inline sixes, from old Plymouth flatheads to the indestructible Slant-6 to Chevy’s tough-as-nails 292, and of course Mike Hailwood’s Honda. Small, high-revving 6’s in bikes take the form to a new level, and this little gem really does it! Notice the fellow holding his ears in the video; I’ll bet it cranks out some serious decibels. I’d love to see the innards; it must look like jewelry. I would make one change — if the exhaust were 6-into-2-into-1 (a configuration that works extremely well on inline 6’s) it would be highly tuneable and sound like a tiny demon school bus from hell.

  4. rohorn says

    Nobody gets the CBR250 anymore – they quit making them years ago. The entire 250/400 4 cyl sportbike market has been gone for a while. Only the old cheap-o twins are left.

  5. dan says

    Trick set up no doubt from the sounds of it he figured out how to make it smoother then any engine Ive ever heard! Creative minds in the right environment sometimes makes magic! This is not the last time we will hear this or see this! John Britton smiles!

  6. joe says

    Fantastic engineering project and design creation from a guy that must be a genious.Honda made a lot of these multi cylinder, small capacity racing engines in the sixties. They had a multi billion $ corparation and a huge development team dedicated to the projects. To think that this guy designed and built this machine almost single handedly is absolutely amazing.

  7. Klaus says

    Sometimes less is more (cc)!

    I’m more interested in this bike than in the 1600cc BMW six.

    Too bad it’s not available to the public.

  8. Kevin says

    What? Normally i am very impressed by what I see on kneeslider. But this started out great then ended in big disappointment. Only 60hp?!? It has to rev to 18,500rpm to make 60hp?!? A 300cc SINGLE CYLINDER 2-stroke dirt bike engine with mild tuning will make 60hp at under 10,000rpm. A modified 450 single cylinder 4-stroke dirt bike makes over 60hp and so does KTM’s factor 350SX-F.

  9. Wave says

    Kevin, what do you mean ‘only’ 60hp?! My VW Beetle only has just over 60hp from 1600cc and weighs 750kg and that still feels quick and nimble to drive.

  10. Scotduke says

    Seen this bike before – like it just for the pipes – reminds of the first time I saw a Benelli 750 back in the 70s. With up to 22,000rpm, I bet it screams when gassed and being a six it’ll be super smooth too. Who cares if it ‘just’ produces 60hp, I bet it’s very light and handles superbly. I know Aprilia’s RS250 two stroke that we still see over here in Europe puts out 60hp and I’ve seen them on track days outcornering the latest sports 600s, 750s and 1000s and make them look like lumbering trucks on sharp bends.

  11. zipidachimp says

    having heard honda’s 250/6 up close and personal, it is the loudest bike I have ever heard. ear-drum shatteringly loud ! would sure love to here this one the same way.
    we need an international prize set up to honor mechanical achievers, in all fields.
    britten would get the first posthumous award for transportation!

  12. Jim says

    Looking at the pics of this 300cc six and those of BMW’s six and noting the relative size of each motor makes the design of the BMW more impressive. This motor sounds wonderful, but as others have pointed out similar power can be achieved by less complex solutions.

  13. dan says

    The power band is likely long and flat with smooth acceleration, not to mention bulletproof. Mission accomplished.

  14. dan says

    Check that it probably starts out that way then hits hard half way home! Anybody care to speculate!

  15. dosed says

    “I know Aprilia’s RS250 two stroke that we still see over here in Europe puts out 60hp and I’ve seen them on track days outcornering the latest sports 600s, 750s and 1000s and make them look like lumbering trucks on sharp bends.”
    There is no way this could compare with an RS250. I don’t think cornering is what this bike is about, with 6cyl and the extra long crank…

  16. MikeC says

    This bike is not about POWER. It is about doing the business of building something that you want but can’t buy. Period. Great job. Who cares if it is competitive with the latest whatever. HE BUILT IT because he wanted to, and could. End of story. Again GREAT JOB.

  17. Mule says

    Paul, I agree. This bike, although an incredible amount of work, ended up as visual and sensory work of art. A job that be disected and appreciated at all levels.

    However, the previous thread on the Ducati V-8, which appeared to be done just to see if it would work, reminds me of a recent quote from the most respected man in the Hot Rod building world. “You can put just as much time, effort and dollars into a bad idea as a good one”. While I can appreciate the Duc effort, I can’t for the life of think of why it would be undetaken. Yes there is viewing and and a morbid entertainment value, “Wow! Look what that guy did!”

  18. says

    I think it’s very cool!
    And this guy is obviously a master, and he’s running the tech show on a MotoGP team.
    How many other machines around are running 22k rpm?

    As for the power level, it’s P-L-E-N-T-Y.
    Is anybody really going to say that people who run 250 GP bikes are not going fast, or having fun?
    Most old tymers will tell you if you want to learn to go fast, ride a 250 for a few seasons. It’s alot different than crawling around corners, and squirting down the straights, using big engine power for a crutch, which seems to be the “de jure” riding technique for most typical riders these days.
    Put Edwards on this machine, and he’ll make mince-meat of of just about anybody around on a GSXR or whatever.

    It’s not all about big hp, folks. Although this thing has very good hp for a 300cc 4-stroke bike.
    I ride a bike with just a little more than half the power of this, and it is a blast to ride on the back roads. The hp wars are over-rated.
    This guy and this bike are cool. It’s an awesome hand-made machine with very high tech, built by a guy with one helluva lot of talent and experience in the state-of-the-art form of motorcycling today(MotoGP).
    I’d LOVE to ride it!

  19. Boog says

    Cool bike!

    How about an inline 8 or 16 using Wen-mac or Cox .049 engines? Just kidding…

    itty-bitty multis were a Japanese specialty back in the ’60’s.

  20. mxs says

    It’s amusing NOT how some people here will compare a 300cc 6cyl 4 stroke with a 300cc single cylinder 2 stroke power output ….. can you say apples and nuts?

    What I find funny is that Yamaha tech using a Honda equipment. He will get the papers soon … LOL

  21. todd says

    If it’s putting out “only” 60 HP then it’s fairly close to being in stock tune (except pipe and filter). The original 250 makes 45 HP at the crank as is where the RS250 is measured too. This bike has its power measured at the rear wheel. In that case, it probably makes 70+ at the crank.
    Give the guy some time to tinker with it and it should put out a little more.


  22. rohorn says

    It is odd how some will gush mushingy over a 30 hp (if that) air cooled fake cafe racer, yet get all snotty over a bike that makes twice that with less displacement.

    I’m guessing a 6-1 pipe could bump up the numbers, but there’s no way to measure music. And a 6-6 can play some lovely tunes.

    Even if there is more power in store, there’s a lot to be said for doing this work only once, rather that finding the limit and starting over. There are no trophies involved here – just a whole lot of satisfaction and fun. Or so I’m thinking.

  23. Kenny says

    I’m kinda confused,
    How was it made from a CBR250 engine. If he used the pistons and whatnot from the 250 it would be displacing closer to 375cc. Did he use the honda’s engine architecture as a starting point for the design and build it all from scratch.
    That said, the cylinder head and carburetors, and water pump all look identical to those on my ciblet

  24. rohorn says

    I’m guessing it is a 300 and not a 375 (And 22,000 rpm rather than 19,000 rpm) due to a shorter stroke. By reducing the stroke from 1.33″ to 1.065″, you end up with 50 cc/cyl and even lower piston speeds at 22k (3889 fpm) than the ones produced at the stock “long stroke” 19k redline (4195 fpm).

    Again, that is just a guess – I have no info beyond that from my TI-36.

  25. Scotduke says

    The ground clearance on this bike looks pretty good and given his race experience (more than everyone on this thread combined), I bet the bike’s lightweight and has the best suspension components available. In other words, despite the six pot motor I’d be surprised if it’s cornering isn’t close to, or better than, an RS250. It’d be also somewhat less likely to need new pistons and/or a rebuild after a track session. Incidentally, the RS250’s a v-twin and not a single. I’ve heard those old six pot racing Hondas being gassed and they’re pretty loud indeed.

  26. says

    i develop the electronic ignition of Guy Coulon’s engines ( 6 cylinders and V4 ) you could see on my website the video of first running 6 cyl on the bench and few steps of machining V4