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Discussion Starter #1
anyone get their vintage racer on a dyno?
what sort of hp/torque you guys are getting out of your vintage stuff.
what are reasonable expectations for a two valve engine, is it really any sort of Major hinderance to be short two valves?
 

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What kind of bike nad what modification level are you looking for?

CB/CL160s with stock(ish) parts optimized make about 14.5 hp at the rear wheel on a dynojet.
CB/CL175s with stock(ish) parts optimized make about 18.5 hp at the rear wheel on a dynojet.

The pull out the stops open your wallet wide limit on a 160 is probably somewhere around 18 hp. Same on a 175 is probably about 28 hp.

Aermacchi 350s built to 392cc make about 43hp.

Michael
Michael
 

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My bike at 201cc made a solid 23.5 hp on the dyno.
That's 116hp per liter.
With some additional mods since then, and upped to 209cc it should make a solid 25hp. Which would be 119hp per liter.

Compare that to a modern 4v sportbike, fuel injected, water cooled, better everything making about 140 hp per liter.

I'd say about 7.75 to 8cc per hp would be a ball park limit for a two valve, air cooled engine. Excepting any really exotic modifications.

Whereas a stock modern sport bike would be closer to 6.5 cc per hp.
JohnnyB
 

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As it happens, the local Honda store had their open house this weekend and Dyno runs were available, so I went home and brought back my '79 Yamaha XT500. The motor has been bored and stroked to approx. 589cc. It pulled 40.8 hp and around 38.8 ft/lb. I was pretty happy with those numbers, the power and torque were there from around 4800 up to peak at 6500 rpm, but I discovered that I had safe overrun up to about 8000 rpm. (I never knew this because I don't have a tach)
That motor was rebuilt about 3-4 years ago, has about 16 race weekends on it and it hasn't lost any power or significant compression.
Not the same power as a twin but stone simple and fun as hell to ride.


FR

ps, It was a mobile dyno, the same guys show up at lots of events around the Northeast, so I don't know how accurate it is. Close enough for dirttrackin' though.
 

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Discussion Starter #5
i was just comparing my bike to some new bikes, figuring their power to weight, and power per liters, and mine is really behind contemporary bikes, even that damn ex250. i didn't expect my bike to stand up to a modern bike, but the figures were interesting.
the one liter race replicabikes are putting out like 190hp/liter though. damn.

stock hp on an xs400 is 36
and weight is 360, it probably doesn't weight 360 anymore but i am sure its still above 300.
who knows what gain or loss in power i have had so far while tweaking with it, but i was just curious what sort of numbers i could expect w/ reasonable mods.

at the 8cc/hp i can expect about 50. what all does it take to get here, new cams, porting, or does it take machining the heads and forged pistons etc.??
 

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8cc per hp would require a pretty complete race tune on an engine. Cam, carbs, exhaust, porting, perhaps oversize valves, optimal ignition timing, dyno tuning. Not necessarily pistons or machine work...depends on how suited the engine is to begin with.

And you'd also have to compare engines with similar bore/stroke ratios... you can't expect the same cc/hp numbers with a long stroke thumper type engine...they make their power a different way. You'd have to do something more like comparing area under the hp graph / per cc.

Shouldn't have to get too exotic.

Bear in mind my comparison was a pretty highly tuned air cooled, two valve vintage race engines against a stock, water cooled, modern sport bike.

A modern four stroke GP bike is going to be closer to 3.5-4cc per hp.
JohnnyB
 
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