racerdave
^ Gone not forgotten ^
This has been around a long time. I just came across it while deleteing stuff off my laptop. It may be of interest to those that have not seen it. I did my best to copy and paste. I do still have it in a PDF. I will have to post this in several post to comply with the 1,000 character rule here.
OU-72/OW72
Yamaha’s Answer To The Harley XR750
(There was a time when the XS650 shed its wimpy reputation and put on Superman’s cape!)
This 90 horsepower hybrid was the final evolutionary step in the XS650’s
development as a race engine. In 1975-1976 a really talented team of Southern
California tuners, fabricators and specialty suppliers engaged in a crash program
to create a version of the engine that could take on the factory Harley XR750s.
The nomenclature is seen as both OW72 and OU-72 in publications from the
time, however a Yamaha International Corporation publication in 1976 refers to it
as OU-72, so that’s what I’ll use.
At the time, Kenny Roberts was Yamaha’s only fully sponsored AMA dirt track
competitor, and Yamaha was eager to do whatever it took to help him retain the
number one plate. Everyone understood that the number one goal was to make
the XS650-based dirt tracker a more effective weapon. Essentially, this meant it
needed more power. In late 1974 Yamaha USA had hired Tim Witham, founder
of S&W Engineering, and a very highly respected ex-dirt track tuner, to manage
an engine improvement program. He was somewhat successful and managed to
squeeze another approximately 10 horses – probably reaching the mid-70s if
tested on a modern dyno - while still using highly modified stock castings.
Reliability was becoming a problem however, and the final versions had
strengthened crankcases, stronger gearboxes, clutches, beefy connecting rods
and upgraded valve trains. Unfortunately, the alloy Harley XR750 was evolving
also and continued to retain a small but very real advantage.
Facing 1976 with few tuning options left, the stock head castings became the
obstacle that couldn’t be overcome. Every trick was tried by the very best air
flow specialists available, but they couldn’t make the stock head flow any more
air and still retain any kind of a manageable, linear power band.
Consulting with C.R. Axtell and a number of other specialists, Witham had a set
of engineering drawings put together that depicted a new head casting that would
have higher flow potential while retaining the necessary interfaces with the stock
barrels and cases. The new design featured revised intake and exhaust valve
angles and straighter, larger ports with optimized shapes. Also included was a
new barrel with some detail changes including “750 cc” replacing the stock
engine’s “650 cc” raised lettering on the casting.
In July 1975 Pete Schick, Yamaha’s Racing Manager in the US, met with
Yamaha engineers in Japan and asked them to cast at least 24 prototype heads
per the drawings he’d brought with him (24 or more heads were needed because
per the racing rules in force at the time the AMA had to see 24 actual, running
units in Yamaha’s Southern California distribution warehouse before permission
could be given for use in the 1976 racing season). The timing was good because
Yamaha was just getting into four strokes and Yamaha’s Engineering
management thought this would be a good exercise for the production team and
also, of course, because it would assist Roberts to retake the number one plate
and sell more Yamahas in the US. They agreed to supply the castings.
It was already October 1975 and there was no time to waste. The team only had
eight months before the engine’s scheduled debut in May 1976 at the San Jose
Mile.
A few months later the first castings showed up. They had no real ports – more
like small tunnels along the port’s centerline, no valve seats or guides, and of
course no camshafts, valves or valve springs. Everything had to be spec’d,
sourced, fabricated, or cut. Vendors were developing and producing improved
pistons, gears, rods and even the crankshafts had to be specially weighted,
balanced, and fitted with larger crankpins. Dozens of pairs of cases were
magnafluxed to find those with the fewest casting flaws.
On April 14th, just a little more than a month before San Jose, the first engine
fired on the dyno at Yamaha’s race shop. Bud Aksland had assembled the
engine himself and after four hours on the dyno he saw larger torque and
horsepower numbers than any prior Yamaha XS650/750 race engine had ever
produced. The 24 complete running bikes with OU-72 engines were frantically
assembled by borrowing mechanics from Yamaha’s snowmobile division. This
was completed just in time for Earl Flanders, an AMA tech inspector, to check out
the bikes at Yamaha International Corporation’s facility at Buena Park.
To make
sure he wasn’t being given a snow job, Earl had them tear apart some of the
engines and also started a few others. Everything passed and the OU-72 would
be recommended for approval at the AMA Professional Rules Committee
meeting on May 4th.
Actually, none of the engines Flanders inspected were the race versions. Six
race engines, including three earmarked for San Jose, were receiving special
cams, pistons and other components as rapidly as they could be fabricated and
delivered. C.R. Axtell and Mike Libby were flowing and finalizing the new head’s
ports and zeroing in on desired camshaft lobe contours. Kel Carruthers in San
Diego had three race bike chassis ready awaiting arrival of the completed
engines. Sig Erson, Forged True, Webster Racing, Pete Smiley and a host of
others worked incredible amounts of overtime to make it all happen.
On May 6th the engines were run on an engine dyno (not a chassis dyno) and
recorded numbers that translate to approximately 90 horsepower. The still hot
engines were immediately trucked to San Diego so Carruthers could install them
in the three race bikes. On May 9th the fully assembled bikes were taken to the
deserted Santa Clara County Fairgrounds (which had been rented for a day of
private testing) so Kenny Roberts could have an initial outing on the bikes. To
everyone’s relief and joy, the test was a complete success.
The race weekend began on May 16th. In practice, Roberts and the OU turned
laps around 38 seconds; only one of the Harleys was as fast. Engine power
3
wasn’t the problem … there was plenty of that. However, once Robert’s fresh
tires began wearing traction became an issue. By the mid point in his heat race
Robert’s bike was slewing dramatically, getting too sideways to maintain
momentum and losing traction on each drive out of a corner. He had to make a
last lap pass to transfer directly to the main event.
Unfortunately, the story doesn’t have a fairy tale ending. As a final exclamation
point on a frustrating day, the bike refused to start for the main. A later
inspection revealed a coil wire coupler had failed and caused a short. The tiny
laps around 38 seconds; only one of the Harleys was as fast. Engine power
wasn’t the problem … there was plenty of that. However, once Robert’s fresh
tires began wearing traction became an issue. By the mid point in his heat race
Robert’s bike was slewing dramatically, getting too sideways to maintain
momentum and losing traction on each drive out of a corner. He had to make a
last lap pass to transfer directly to the main event.
Unfortunately, the story doesn’t have a fairy tale ending. As a final exclamation
point on a frustrating day, the bike refused to start for the main. A later
inspection revealed a coil wire coupler had failed and caused a short. The tiny
part may have cost ten cents. After a few more race weekends the frame
geometry and suspension were tweaked and tuned to provide a better balance,
traction wasn’t a continuing problem and the OU-72 went on to win its share of
AMA national and regional events.
During 1975 - 1976, additional batches of heads – totaling perhaps 50 or 60 -
were sent from Japan to the Yamaha Race Department. Most were sold to
privateers and non-factory tuners. Cutting ports into a head from scratch is a
daunting task and few are up to such a challenge. It is unquestioned that the
best OU-72 heads were those ported by C.R. Axtell and Mike Libby. Axtell’s
shop continued to develop and evolve the OU-72’s porting and component
specifications throughout 1976. Their engines routinely produced 80 – 83 rear
wheel horsepower as we would measure it today, and featured very progressive,
powerful torque and horsepower curves. But, there weren’t very many of those
as a percentage of all the OU-72s being run at flat tracks across the nation. The
average, so-called privateer OU was generally not as effective as a high level,
expertly modified AMA Grand National caliber XS750 with a stock-based head,
and many OU-72s were simply expensive underperformers.
As Yamaha exotica goes, an OU-72 engine in reasonable shape is the cherry on
top of a XS650 sundae. An unmolested unit from Axtell’s shop is the Holy Grail
for serious collectors and vintage competitors.
OU-72/OW72
Yamaha’s Answer To The Harley XR750
(There was a time when the XS650 shed its wimpy reputation and put on Superman’s cape!)
This 90 horsepower hybrid was the final evolutionary step in the XS650’s
development as a race engine. In 1975-1976 a really talented team of Southern
California tuners, fabricators and specialty suppliers engaged in a crash program
to create a version of the engine that could take on the factory Harley XR750s.
The nomenclature is seen as both OW72 and OU-72 in publications from the
time, however a Yamaha International Corporation publication in 1976 refers to it
as OU-72, so that’s what I’ll use.
At the time, Kenny Roberts was Yamaha’s only fully sponsored AMA dirt track
competitor, and Yamaha was eager to do whatever it took to help him retain the
number one plate. Everyone understood that the number one goal was to make
the XS650-based dirt tracker a more effective weapon. Essentially, this meant it
needed more power. In late 1974 Yamaha USA had hired Tim Witham, founder
of S&W Engineering, and a very highly respected ex-dirt track tuner, to manage
an engine improvement program. He was somewhat successful and managed to
squeeze another approximately 10 horses – probably reaching the mid-70s if
tested on a modern dyno - while still using highly modified stock castings.
Reliability was becoming a problem however, and the final versions had
strengthened crankcases, stronger gearboxes, clutches, beefy connecting rods
and upgraded valve trains. Unfortunately, the alloy Harley XR750 was evolving
also and continued to retain a small but very real advantage.
Facing 1976 with few tuning options left, the stock head castings became the
obstacle that couldn’t be overcome. Every trick was tried by the very best air
flow specialists available, but they couldn’t make the stock head flow any more
air and still retain any kind of a manageable, linear power band.
Consulting with C.R. Axtell and a number of other specialists, Witham had a set
of engineering drawings put together that depicted a new head casting that would
have higher flow potential while retaining the necessary interfaces with the stock
barrels and cases. The new design featured revised intake and exhaust valve
angles and straighter, larger ports with optimized shapes. Also included was a
new barrel with some detail changes including “750 cc” replacing the stock
engine’s “650 cc” raised lettering on the casting.
In July 1975 Pete Schick, Yamaha’s Racing Manager in the US, met with
Yamaha engineers in Japan and asked them to cast at least 24 prototype heads
per the drawings he’d brought with him (24 or more heads were needed because
per the racing rules in force at the time the AMA had to see 24 actual, running
units in Yamaha’s Southern California distribution warehouse before permission
could be given for use in the 1976 racing season). The timing was good because
Yamaha was just getting into four strokes and Yamaha’s Engineering
management thought this would be a good exercise for the production team and
also, of course, because it would assist Roberts to retake the number one plate
and sell more Yamahas in the US. They agreed to supply the castings.
It was already October 1975 and there was no time to waste. The team only had
eight months before the engine’s scheduled debut in May 1976 at the San Jose
Mile.
A few months later the first castings showed up. They had no real ports – more
like small tunnels along the port’s centerline, no valve seats or guides, and of
course no camshafts, valves or valve springs. Everything had to be spec’d,
sourced, fabricated, or cut. Vendors were developing and producing improved
pistons, gears, rods and even the crankshafts had to be specially weighted,
balanced, and fitted with larger crankpins. Dozens of pairs of cases were
magnafluxed to find those with the fewest casting flaws.
On April 14th, just a little more than a month before San Jose, the first engine
fired on the dyno at Yamaha’s race shop. Bud Aksland had assembled the
engine himself and after four hours on the dyno he saw larger torque and
horsepower numbers than any prior Yamaha XS650/750 race engine had ever
produced. The 24 complete running bikes with OU-72 engines were frantically
assembled by borrowing mechanics from Yamaha’s snowmobile division. This
was completed just in time for Earl Flanders, an AMA tech inspector, to check out
the bikes at Yamaha International Corporation’s facility at Buena Park.
To make
sure he wasn’t being given a snow job, Earl had them tear apart some of the
engines and also started a few others. Everything passed and the OU-72 would
be recommended for approval at the AMA Professional Rules Committee
meeting on May 4th.
Actually, none of the engines Flanders inspected were the race versions. Six
race engines, including three earmarked for San Jose, were receiving special
cams, pistons and other components as rapidly as they could be fabricated and
delivered. C.R. Axtell and Mike Libby were flowing and finalizing the new head’s
ports and zeroing in on desired camshaft lobe contours. Kel Carruthers in San
Diego had three race bike chassis ready awaiting arrival of the completed
engines. Sig Erson, Forged True, Webster Racing, Pete Smiley and a host of
others worked incredible amounts of overtime to make it all happen.
On May 6th the engines were run on an engine dyno (not a chassis dyno) and
recorded numbers that translate to approximately 90 horsepower. The still hot
engines were immediately trucked to San Diego so Carruthers could install them
in the three race bikes. On May 9th the fully assembled bikes were taken to the
deserted Santa Clara County Fairgrounds (which had been rented for a day of
private testing) so Kenny Roberts could have an initial outing on the bikes. To
everyone’s relief and joy, the test was a complete success.
The race weekend began on May 16th. In practice, Roberts and the OU turned
laps around 38 seconds; only one of the Harleys was as fast. Engine power
3
wasn’t the problem … there was plenty of that. However, once Robert’s fresh
tires began wearing traction became an issue. By the mid point in his heat race
Robert’s bike was slewing dramatically, getting too sideways to maintain
momentum and losing traction on each drive out of a corner. He had to make a
last lap pass to transfer directly to the main event.
Unfortunately, the story doesn’t have a fairy tale ending. As a final exclamation
point on a frustrating day, the bike refused to start for the main. A later
inspection revealed a coil wire coupler had failed and caused a short. The tiny
laps around 38 seconds; only one of the Harleys was as fast. Engine power
wasn’t the problem … there was plenty of that. However, once Robert’s fresh
tires began wearing traction became an issue. By the mid point in his heat race
Robert’s bike was slewing dramatically, getting too sideways to maintain
momentum and losing traction on each drive out of a corner. He had to make a
last lap pass to transfer directly to the main event.
Unfortunately, the story doesn’t have a fairy tale ending. As a final exclamation
point on a frustrating day, the bike refused to start for the main. A later
inspection revealed a coil wire coupler had failed and caused a short. The tiny
part may have cost ten cents. After a few more race weekends the frame
geometry and suspension were tweaked and tuned to provide a better balance,
traction wasn’t a continuing problem and the OU-72 went on to win its share of
AMA national and regional events.
During 1975 - 1976, additional batches of heads – totaling perhaps 50 or 60 -
were sent from Japan to the Yamaha Race Department. Most were sold to
privateers and non-factory tuners. Cutting ports into a head from scratch is a
daunting task and few are up to such a challenge. It is unquestioned that the
best OU-72 heads were those ported by C.R. Axtell and Mike Libby. Axtell’s
shop continued to develop and evolve the OU-72’s porting and component
specifications throughout 1976. Their engines routinely produced 80 – 83 rear
wheel horsepower as we would measure it today, and featured very progressive,
powerful torque and horsepower curves. But, there weren’t very many of those
as a percentage of all the OU-72s being run at flat tracks across the nation. The
average, so-called privateer OU was generally not as effective as a high level,
expertly modified AMA Grand National caliber XS750 with a stock-based head,
and many OU-72s were simply expensive underperformers.
As Yamaha exotica goes, an OU-72 engine in reasonable shape is the cherry on
top of a XS650 sundae. An unmolested unit from Axtell’s shop is the Holy Grail
for serious collectors and vintage competitors.
