I'm sure there are a lot of people here that aren't on the 650performance.com email list... I got this email and thought some of you might find it interesting. So here's a little free advertising for 650performance.com heads:
Too Much XS650 Power Can Be A Problem
Now that people are putting their modified engines with the CNC'd heads on the street and race track I'm becoming a bit uncomfortable with what I'm hearing about how radical some of their modifications are.
The XS650 cases, clutch and transmission were designed for an engine that would make around 42 rear wheel horsepower (RWHP). When Yamaha was in the middle of the dirt track wars in the '70s, the engines built for Kenny Roberts and others on the factory team (as well as the best privateer engines) were putting out right around 70 RWHP and reliability wasn't generally a problem. The best engines built by Bud Askland, Harry Lillie and others were inspected after every race but would typically last a half season or more before any major replacements were required. Compared to the BSAs, Triumphs and Nortons this was a real luxury.
When Harley took the next step in XR750 power Yamaha responded by pushing the XS650 power envelope even further. With Tim Witham in charge of development the XS reached the 75 RWHP threshold with stock head castings. While the bikes were rockets, things began going wrong. Broken transmissions, cases and connecting rods were the worst, but clutches and valve trains were breaking too.
You probably know the story about how Yamaha's response to this was the fabled OU-72. What some people have forgotten is that the OU-72 didn't just have a sophisticated revised XS650 cylinder head with previously untouchable flow numbers, it was also accompanied by all the reliability tricks Tim Witham had learned - strengthened cases, Webster transmissions, thickened and deeper clutch baskets, alloy rods and a host of other upgrades.
The first XS engine that Harry built for me was exceptionally powerful. It was a full AMA-spec build and was the strongest XS engine I've ever run. After about a half year of racing it at Sears Point with AFM another racer was looking at the bike and pointed out a small crack in the cases, right in front of the cylinders around the oil up tube fitting. When Harry dismantled the engine the cracking was found to have extended all the way down into the cases and around the crankshaft webbing. We tossed those cases and took the edge off the rebuild by lowering the compression a little bit, richening the Lectrons a little bit, advancing the cam a little bit for more midrange, etc., and it dyno'd out at 71 RWHP. Perfect.
Since then I've always run my engines in a state of tune that gives them 69 - 72 RWHP and I've never had a catastrophic failure.
The point of this message is that the stronger cases, clutch baskets, etc. aren't available today. They are either worn out, broken or lost. And the specific lessons learned from piles of broken parts about how to modify your engine to live happily at 75 RWHP are long forgotten by the men who developed them.
If you have a good OU-72 head, or you are one of the people who recently received one of the new CNC'd heads (or you have any head that really flows well), you have the potential to reach 75 RWHP.
For the reasons above, I caution you to resist the temptation to raise the compression a bit higher than discussed in my XS engine modification guide (see 650performance.com if you don't know about this), or tune it a bit sharper, or bias the power curve to toward the top end, etc.
If you have a solid 70+/- RWHP or less your bike will run really hard on the street or race track and (if it's put together correctly) will be reliable.
Remember: 69 - 72 RWHP = reliable. 75+ RWHP = expensive things break.
Good luck on your project.
Craig
Too Much XS650 Power Can Be A Problem
Now that people are putting their modified engines with the CNC'd heads on the street and race track I'm becoming a bit uncomfortable with what I'm hearing about how radical some of their modifications are.
The XS650 cases, clutch and transmission were designed for an engine that would make around 42 rear wheel horsepower (RWHP). When Yamaha was in the middle of the dirt track wars in the '70s, the engines built for Kenny Roberts and others on the factory team (as well as the best privateer engines) were putting out right around 70 RWHP and reliability wasn't generally a problem. The best engines built by Bud Askland, Harry Lillie and others were inspected after every race but would typically last a half season or more before any major replacements were required. Compared to the BSAs, Triumphs and Nortons this was a real luxury.
When Harley took the next step in XR750 power Yamaha responded by pushing the XS650 power envelope even further. With Tim Witham in charge of development the XS reached the 75 RWHP threshold with stock head castings. While the bikes were rockets, things began going wrong. Broken transmissions, cases and connecting rods were the worst, but clutches and valve trains were breaking too.
You probably know the story about how Yamaha's response to this was the fabled OU-72. What some people have forgotten is that the OU-72 didn't just have a sophisticated revised XS650 cylinder head with previously untouchable flow numbers, it was also accompanied by all the reliability tricks Tim Witham had learned - strengthened cases, Webster transmissions, thickened and deeper clutch baskets, alloy rods and a host of other upgrades.
The first XS engine that Harry built for me was exceptionally powerful. It was a full AMA-spec build and was the strongest XS engine I've ever run. After about a half year of racing it at Sears Point with AFM another racer was looking at the bike and pointed out a small crack in the cases, right in front of the cylinders around the oil up tube fitting. When Harry dismantled the engine the cracking was found to have extended all the way down into the cases and around the crankshaft webbing. We tossed those cases and took the edge off the rebuild by lowering the compression a little bit, richening the Lectrons a little bit, advancing the cam a little bit for more midrange, etc., and it dyno'd out at 71 RWHP. Perfect.
Since then I've always run my engines in a state of tune that gives them 69 - 72 RWHP and I've never had a catastrophic failure.
The point of this message is that the stronger cases, clutch baskets, etc. aren't available today. They are either worn out, broken or lost. And the specific lessons learned from piles of broken parts about how to modify your engine to live happily at 75 RWHP are long forgotten by the men who developed them.
If you have a good OU-72 head, or you are one of the people who recently received one of the new CNC'd heads (or you have any head that really flows well), you have the potential to reach 75 RWHP.
For the reasons above, I caution you to resist the temptation to raise the compression a bit higher than discussed in my XS engine modification guide (see 650performance.com if you don't know about this), or tune it a bit sharper, or bias the power curve to toward the top end, etc.
If you have a solid 70+/- RWHP or less your bike will run really hard on the street or race track and (if it's put together correctly) will be reliable.
Remember: 69 - 72 RWHP = reliable. 75+ RWHP = expensive things break.
Good luck on your project.
Craig
I was planning on making about 80whp with my build. 
