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Bike Profile - Posted 1st April 2011

1977 BMW R80/7 Dyno Testing - Part 9
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If it ain't broke, don't fix it. Or at least measure it before you do. Martin Gelder puts his classic 800cc BMW Boxer Airhead on the dyno. Disappointment? Disaster? Despair? Place your bets now...

Let's start with a recap. I have a 1977 BMW R80/7 that's done a hundred thousand miles and is starting to suffer from what I think is valve seat recession due to running on unleaded petrol. I have also recently bought a 1979 BMW R80/7 which has a knackered engine, but which came with boxes and boxes of semi-knackered parts, engine and otherwise. The plan is to make the best good bike out of whatever I have and then sell the rest of the bits as spares. The good bike will hopefully end up with a 1000cc engine, twin front disc brakes, less driveline slop, and sundry other replacement bits and bobs that are currently on the worn side of serviceable.

Roxanne at the front, The Vicar's Bike behind.... 1977 BMW R80/7 at the front, 1979 BMW R80/7 behind

The 1977 bike - Roxanne to her friends - runs very well considering her age and mileage. She doesn't burn much oil although a fair bit leaks out past the pushrod seals, the base gaskets and the sump gasket. As far as I know she's still on her original pistons, rings and bores, and it's only really the valve clearances needing constant tweaking that have brought on my eagerness to start rummaging around inside the engine. I'm usually a great believer in "if it ain't broke, don't fix it".

I'm also a great believer in comparing Before with After to see if improvements really are an improvement, and to that I end I decided to subject Roxanne to a couple of runs on a rolling road dynamometer to see how much power she was making before I delved into her innards. What could possibly go wrong?

Dyno-testing is a bit of a brutal affair; the bike is strapped down with its rear wheel pressing on a very big, very heavy roller and then it's accelerated hard through the gears at full throttle. The bike drives the roller, the rate at which the roller is accelerated is measured, and this is used to calculate the amount of power that the bike is putting into spinning the roller. A sensor is unceremoniously stuck up the bike's exhaust pipe to measure the emissions and thus give an idea of the full throttle air/fuel ratio when the bike is under load, and provided nothing explodes, showering the operator with lethal con-rod shrapnel, a computer printout is generated showing a graph of horsepower against engine speed or road speed, corrected for air pressure and temperature on the day.

The calm before the storm. Or before the clouds of oil smoke, anyway.... 1977 BMW R80/7 strapped to the Cambridge Motorcycle dynamometer

Testing like this has always been widely used in the development of race bike engines, and more recently it's become common when setting up the carburetion or fuel injection on modern sports bikes that have been tuned or tinkered with. But these are bikes that are quite happy - that are in fact designed - to be taken repeatedly to the redline at full throttle under load.

Roxanne the BMW is thirty four years old, has covered over a hundred thousand miles, has a slack and possibly weak transmission, and was originally conceived back in the 1970s as a gentlemen's express, thrumming serenely along German autobahns, whiffling up Swiss Alpine passes and bumbling down quiet back-roads. Being strapped down in a Cambridge back-street bike shop and thrashed to within an inch of her redline was never in her design brief.

Of course, doing the testing is only half of the process; the data then has to be interpreted. What would be a good result? What would be a disappointment? How much power should an old Airhead Boxer make?

As standard, and when new, BMW claimed 55bhp at the crankshaft for the 1977 R80/7. Thirty years later Roxanne is still healthy enough to comfortably crack an indicated 100mph (private test-track, etc) but what would this equate to in measured, rear-wheel horses?

Only one thing to do; run a sweepstake on the Real Classic.co.uk message board.

BMW R80s on Right Now......

The answers ranged from the quietly optimistic (SteveMoto, with 48bhp) through the well informed but pessimistic (L.A.B with 38bhp and previous owner Graham W with 40bhp) to the slightly insulting (Paul Grace and Pathfinder with 32bhp) and the downright derisory (Paul The Destroyer, 20bhp). The nice man in the bike shop, possibly preparing me for later disappointment, told me anything over 40 would be good.

Bwaaaaaaaaaaaaaaaaaaaaaarghhhhhhhhhh. Cough, cough, cough, cough, cough... Quiet please, examination in progress

Oh ye of little faith. After a bit of faffing about to get a clean reading off the ignition system, the oil smoke (and there was quite a lot of it, particularly on the overrun) finally cleared to reveal that Roxanne the 100,000mile BMW R80/7 is developing a healthy 43.9bhp.

Dynojet dynamometer graph for 1979 BMW R80/7 1979 BMW R80/7: 43.9bhp at the back wheel

The standard claimed 55bhp at the crank would probably, allowing for 15% of losses through the gearbox, shaft drive and bevel box, translate to about 48bhp at the back wheel, when new. So 34 years and a lot of miles later, an output within 10% of that figure is pretty good going, in my view.

The power curve is a nice straight line from 2,000rpm to 5,000 rpm, which is what you feel on the road; a progressive and linear build up of power before things go a little flat at the top end. The graph shows a bit of a stutter above 6,000rpm which the nice man in the bike shop said was probably down to the points ignition, but he confirmed that the fuelling was pretty good across the rev range.

Nothing fell off, nothing blew up, and the bike ran as smoothly afterwards as before. Whether that will still be true after I've taken it all to bits and put it back together again is another question entirely...

Words and Photos: Martin Gelder

Dyno testing done at Cambridge Motorcyles, www.cambridgemotorcycles.co.uk

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