HD XR 750 & Full Auto port conversion

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Schwany said:
I'll take West coast USA "f___" lets just try it "any day of the week. Harry Lilly made Rotax motors go pretty darn good for a lot of flat trackers, and I don't think he had a flow bench anywhere nearby. At least I don't remember seeing one. Maybe it was in a different room from the machine shop.
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If he did not have one, he must have just made up numbers.
HD XR 750 & Full Auto port conversion
 
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@Schwany

Having lived for considerable time in Santa Cruz i'd Say that some of the most smartest tuners in California are/were operating flowbenches in the backroom.
For sure I do know: cr Axtell, Jerry Branch and Kenny Augustine.
Doesn't mean that one could not do without it but it sure makes life a lot easier in particular if one know a little physics's, math and how to use a lot/prandtl tube.
I'd assume that their is at times some bodacious macho bravado involved for marketing reasons but in the backroom most (I actually think all of them) of those guys knew how to operate profoundly a calculator.

Kind regards

Christian

Kind regards

Christian
 
Bernhard said:
Once again you have quoted something dubious, the Triumph 650 Saint was a single carb version with E 3134 cam on the inlet side only. In their time they caught enough speeding motorists.
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If you are building a hot triumph and need a one-piece crank, you can buy the light one cheap. It will do the job, but not as well as the Bonneville crank. With a heavier crank there is more stored-up energy, which is used as you change up through the gears, however the heavier crank does not respond as well to the throttle. When you ride fast, it is normal not to let the revs drop so far that throttle response becomes important. With a standard gearbox, that is not so easy. The best way to get a Commando-engined bike to accelerate fast, is to use a close-ratio box and don't let the revs drop too far as you change up. Even with a close box, if the revs drop to 4000 RPM, you will wait too long for them to come back up., because the heavy crank becomes more throttle-dependent.
With my bike, I try to only use 5,500 RPM to 7,000 RPM, even in slow corners. I usually change up without using the clutch - I simply back-off slightly and stand on the gear change, then get back to full throttle If you are peaking at 7000 RPM, the revs drop 1500 RPM and you get a leap forward due to the crank inertia. It is much easier to come up from 5,500 RPM to 7000 RPM on the throttle, than it is to come up from 4000 RPM to 7000 RPM.
There is a big difference between racing with a light or a heavy crank. I raced with a light short stroke crank for many years. I like the heavy long-stroke crank much better. The bike is much less likely to grab you by the throat, so you are faster everywhere.
 
With most race bikes acceleration is more important than top speed. You can have a lot of horsepower with less torque and your bike might be faster than others, if the straights are long enough. However getting to your top speed faster is usually better then getting to a higher top speed slower. When you fit a race cam into a normal bike, you usually get an increase in torque right across the whole rev range, unless you make the inlet ports too big. Then you get more top end with less mid-range.
 
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Dan Kyle said:
If he did not have one, he must have just made up numbers.
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I probably wasn't paying enough attention when I was in the machine shop. There was another glassed off room, I never went into. I'm fairly sure he didn't bother putting my Rotax head on a flow bench. I think he had Rotax head porting well under control by then. Plus I wasn't a popular racer like Chris Carr and it was just a mild port for a dual sport bike. On top of that horrid mistake about saying I never saw a flow bench, I spelled his last name wrong, and didn't get back here in time to correct it. The mistakes just keep piling up.
 
Hudriwudri said:
@Schwany

Having lived for considerable time in Santa Cruz i'd Say that some of the most smartest tuners in California are/were operating flowbenches in the backroom.
For sure I do know: cr Axtell, Jerry Branch and Kenny Augustine.
Doesn't mean that one could not do without it but it sure makes life a lot easier in particular if one know a little physics's, math and how to use a lot/prandtl tube.
I'd assume that their is at times some bodacious macho bravado involved for marketing reasons but in the backroom most (I actually think all of them) of those guys knew how to operate profoundly a calculator.

Kind regards

Christian

Kind regards

Christian
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The tuners bodacious macho bravado disappears on track as long as the riders know what they're up to, so I guess that would sort the wheat etc...
 
When you use some of these tools, there is often a presumption that they are telling you what you need to know. In the end it is what happens when the motor runs which is important. With ports, loss of vacuum and low gas flow speed must be compensated for. On a flow bench, larger ports probably increase flow, but in a motor larger ports drop the gas flow velocity. It is the mass of the total incoming charge which is important.
You don't transport electricity across country using low tension cables. It comes across at much higher pressure.
 
cliffa said:
The tuners bodacious macho bravado disappears on track as long as the riders know what they're up to, so I guess that would sort the wheat etc...
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True that. I could have the best handling and powered bike on the starting grid and still figure out how to come in dead last.

I raced one time in my life. It was a hare scrambles with all the classes running on the same course. Sportsman started last so the Pros could get away from the fluster cluck of the amateur riders. My bike ran strong. I was lapped 3 times by the Pros. Some of them literally jumping over my head on single track rock strewn downhills. I never raced again. My balls were apparently not even close to large enough. There is always a faster gun... always.
 
Schwany said:
True that. I could have the best handling and powered bike on the starting grid and still figure out how to come in dead last.

I raced one time in my life. It was a hare scrambles with all the classes running on the same course. Sportsman started last so the Pros could get away from the fluster cluck of the amateur riders. My bike ran strong. I was lapped 3 times by the Pros. Some of them literally jumping over my head on single track rock strewn downhills. I never raced again. My balls were apparently not even close to large enough. There is always a faster gun... always.
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" fluster cluck" - Love it :D:D:D
 
This is very interesting... the talk of the XR 750 making in excess of 90 HP, I read the engines were turning near 9000 rpm, is this correct?
 
I rode on a race circuit when Don Emde was riding the XR750TT. That one was fast enough, but it did not have 90 BHP. A TZ750 would make it look stupid. With any big furstroke twin, 75 BHP would be excellent. You would make up plenty in cornersgainst two-strokes.
 
Hillbilly Bike said:
This is very interesting... the talk of the XR 750 making in excess of 90 HP, I read the engines were turning near 9000 rpm, is this correct?
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Yes.
One of the better ones we had was 93HP, 90 was more common, we suspect the factory bikes made as much as 95HP. These were all in flat track bikes, as the last time I can recall the roadrace version being raced at a factory level was around 1974.
I do not recall the exact rpm, but that is most likely correct, which is why they had a hard time lasting even 25 miles.

The Kawasaki 650's bored to 700 or 750 could make 95 to 105 HP all day.
 
In my yuth in SJ CA USA, I was a big fan of Ron Wood's Norton ridden by Jorgensen going around the roundy round. He did alright at the time. I should copy the exhaust Woods was using on the flat track bike to use on my P11. Unfortunately, I never learnt myself how to weld. Might be time to do it.

Dan you must have been there, unless you showed up after they shut down the mile at the San Jose fairgrounds because of noise.
 
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I was at the San Jose mile, one time I helped Ron Wood "carry" his Beautiful Norton back to the pits when the swingarm broke.

I was not that much of a flat track guy but I went to a few back then.
I have done some suspension stuff in the last 5 or so years.

My friend Jeff Haney, knows all about that old stuff.
 
@Schwany

My dad is currently still back home in europe but should return around December.
My old man has a pretty decent Miller Tig machine and loves to tinker in case you wanna make a Saturday trip up to Auburn/Georgetown.
I assume he would be happy about some visitors ;)

@acotrel

To my knowledge don emde did not have the latest revision aluminum head XR not even talking bout the oval port heads (that came out in the 80's and were the basis for buels work).
One can not compare pears to apples in my book this xr750 early (or even GG) to xr750 late, different volumeflow, different air speeds and I would have to look it up even slightly different valve sizes).
But yes I very much believe that with factory optimized no effort spared equipment 90-100 horses would be possible.
Equally I believe knowing the commando.heads a lil that with a big bore engine 75-85 horses should/could be possible without considering reliability of the bottom end.



Good evening to everybody and kind regards

Christian

Ps: @ dankyle

About which Kawasakis are you talking about?
Or are you perhaps talking bout the yamaha xs650 twin?
 
They typically stroke the Kawasaki Ninja 650s to 750cc to make the HP Dan quoted, and have been raced that way for at least the past 8 years. I believe for 2020 AFT racing in the production twins class they can increase the displacement of this vertical twin to 800cc. But as is well known in flat track, there is little if any relationship between displacement and winning races.
 
Torque is power relative to the revs and power is the time rate of doing work. . When you road race a Commando with a close box, there is probably no point in revving the motor past the point at which maximum torque occurs because the acceleration rate must be slower. I wonder how many guys ever short-shift to find out the best revs to change up at ? If you use a 2 into 1 pipe and advance the cam to suit, you probably get maximum torque, however the revs at which maximum torque occurs might change. Enlarging the inlet ports helps make more power at higher revs, at the expense of midrange. If max power occurs above 7000 RPM with a Commando engine - BANG. On most race circuits, two things are important - the slowest corner and the longest straight. If you get to your maximum speed fast enough, it does not usually matter much if you run out of puff towards the end of the longest straight. And getting to max speed fast enough depends on how fast you can get around the slow corners.
Lowering the overall gearing on a Commando does not necessarily mean you will accelerate faster, because you become more dependent on throttle response than crank inertia as you change-up through the box. It is very deceiving. You might think the bike is going as fast as it can, when it is not.
 
If you know a particular race circuit extremely well, you usually know when the corners are arriving faster.
 
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