What type of exhaust for power.

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Maybe, but for me the results were conclusive enough to ditch the ever cracking balance pipes.
I believe Norton introduced the balance pipe together with the quieter bean can silencers.
My test was with open peashooters.
baz: the sound changed, but quiter ? .. hard to tell while riding the bike.
Anyway, it is all a long time ago. Still have that pipe lying around somewhere.
If worntorn likes to repeat the test, he can have it..
I have two 850 commando mk ones at the moment. One on balanced headers and one without. One has Emgo peashooters, the other on no-name peashooters. Other than a webcam #12 in the balanced header bike they are standard in terms of anything relating to the exhaust note. The balanced bike is definitely quieter, and crucially to my ears, a much nicer exhaust note, throughout the rev range. It doesn't lose any of the lovely character, just takes the harshness off. Subjective but everyone who has listened to both, agrees. It's also noticeably stronger in the midrange though that could just be the cam? Swapping peashooters makes no difference. Webcam would make engine note harsher if anything?
This is also my experience with Triumphs. My '68 bonnevilles is noticeably harsher (to the point of not sounding that nice imho) than my brother' s balanced trophy which sounds just lovely. Both identical mufflers and 650 engines.
Cracked headers? Copper grease the balance pipe joints and don't tighten one to take out the stress?
Just a noise law mod? Clearly people who say that know more than Doug Hele who fitted them to the Daytona winning 500 racers!
 

maylar

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My experience with a balance pipe was better mid range torque and better gas mileage. Same silencers. Don't recall an change in noise.
 
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Maybe, but for me the results were conclusive enough to ditch the ever cracking balance pipes.
I believe Norton introduced the balance pipe together with the quieter bean can silencers.
My test was with open peashooters.
baz: the sound changed, but quiter ? .. hard to tell while riding the bike.
Anyway, it is all a long time ago. Still have that pipe lying around somewhere.
If worntorn likes to repeat the test, he can have it..
I have two '73 850 Commando mk one roadsters at the moment. One on balanced headers and one without. One has Emgo peashooters, the other on no-name peashooters. Other than a webcam #12 in the balanced header bike they are identical in terms of anything relating to the exhaust note. The "balanced" bike is definitely quieter, though not by that much, but crucially, to my ears, a much nicer exhaust note, throughout the rev range. It doesn't lose any of the lovely exhaust note character, just takes the harshness off.
Subjective I know, but everyone who has listened to both, agrees. To give my particular views on exhaust notes some perspective, If you think Harleys with straight through pipes sound good you'd probably disagree with my findings, I think they sound shit, and I ran a 95 rwhp ex racing Harley XR1200 on the road that sounded lovely!
Balanced Commando also noticeably stronger in the mid-range though that could just be the cam? Swapping peashooters makes no difference to results. Webcam would make engine note harsher if anything with it's slightly higher lift and overlap?
This is also my experience with Triumph twins. My unbalanced '68 Bonneville is noticeably harsher (to the point of not sounding that nice imho) than my brother' s balanced '71 Trophy which sounds just lovely. Both identical mufflers and 650 engines (aside from single vs dual carb, a non-event acoustically).

Cracked headers? Copper grease the balance pipe joints and don't tighten one to take out the stress?

Just a noise law mod? Clearly people who say that know more than Doug Hele and his team who fitted them to the Thruxton Bonnevilles in the mid-sixties when noise laws weren't a consideration and track results were everything. Quote from Les Williams on this mod:-
"We also introduced the balance pipe (between the two exhaust pipes) on this bike, again for mid-range power, and it was an immediate success. The silencers were another Hele success. They kept the bike quiet but still allowed good breathing."
 
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Maybe, but for me the results were conclusive enough to ditch the ever cracking balance pipes.
I believe Norton introduced the balance pipe together with the quieter bean can silencers.
My test was with open peashooters.
baz: the sound changed, but quiter ? .. hard to tell while riding the bike.
Anyway, it is all a long time ago. Still have that pipe lying around somewhere.
If worntorn likes to repeat the test, he can have it..


Albert Einstein said that insanity is doing the same thing twice and expecting different results.
I'm going with Albert and will accept Ludwig's result.
I'm also thinking that Doug Hele was correct, the balance pipe allowed him to meet noise regulations with a slightly less restrictive silencer than would otherwise be required.
If the bike is a bit noisier without the balance pipe it doesn't really matter in this case.
From Doug Hele's obit,-

His first project was to improve the Triumph Bonneville T120. Drawing from his success with Norton Dominator twins, Hele raised the power ouptput from 47 bhp to 52 bhp on open megaphone exhausts by careful modifications to the design of the camshafts and cam followers. Keen to keep the power gains for road and production racing use, he added a balance pipe between the two exhaust pipes where they exited the cylinder head adjacent to the ports, quieting the engine and allowing use of a less-restrictive silencer. A decrease in exhaust-gas velocity caused by linking each cylinder into effectively two silencers was addressed by reducing the exhaust pipe diameter from 11 /2" to 11 /4".[4]

1 1/4" pipes on the T120?
I thought the Commando 1 3/8" were small!



Glen
 
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storm42

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My 500 does that (rear wheel), it's an AMC box with Manx outer cover and Hemmings internals. Will check my TTI box on the other bike and see if it is the same. I suspect it is.

Hi Steve, did you get the bike sorted and are you coming out to play with the BHR this season?
 

storm42

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For the record - Moto55UK said he got better performance with a long 11-3/8" intake tract from the head (not the valve) to the end of the velocity stack. This is for a 920 with 37mm carbs and assuming that bigger proportions work better.

I tried 1-1/2" pipes on my 750 cafe racer and got zero improvement. For an all out racer things are different. Ron Woods used 1-3/4" pipes on his 84HP short stroke. Harley also used 1-3/4" pipes on their 90 - 100 HP XL 750s with huge ML cams and D ports. The HD ML cam is even bigger than the Sifton 480.

Jim, do you think the dynos Harley used might have been a bit patriotic or a bit downhill ? I have seen your drawing for the ports and whilst I can appreciate the design, I am struggling to come to terms with 100bhp from a 750 twin.

Just for comparison, Ducati claimed 84bhp for Paul Smarts 750 and that had desmo valve gear, I realise that was around 1974 and you may be referencing Harleys from around 2008 but an extra 16 bhp from a pushrod twin seem a bit optimistic.

What fuel did they use?
 
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If I wanted to find out where in the rev range the balance pipe affected engine performance, I would replace it with a 3 inch diameter expansion chamber with tapered ends. When the engine is running the balance pipe probably resonates at a frequency related to the firings. If you amplify the effect with a chamber, you will probably feel where it occurs more easily. I think the chamber would knock the top off the usable rev range. Most noise reduction modifications do that. However, where the balance pipe fits is about the same distance from the spark plug as the length of the inlet tract, so it might be a tuned length.
Wavelength equals speed of sound divided by the frequency. A tuned length is usually a multiple of the wavelength.
 
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Jim, do you think the dynos Harley used might have been a bit patriotic or a bit downhill ? I have seen your drawing for the ports and whilst I can appreciate the design, I am struggling to come to terms with 100bhp from a 750 twin.

Just for comparison, Ducati claimed 84bhp for Paul Smarts 750 and that had desmo valve gear, I realise that was around 1974 and you may be referencing Harleys from around 2008 but an extra 16 bhp from a pushrod twin seem a bit optimistic.

What fuel did they use?

In a discussion with a friend yesterday, he mentioned he has been communicating with riders in the US who race Triumph twins. He said that for some circuits, they change lobe centres of the cams as well as intake lengths. I presume they are racing on dirt. If that is the case, the biggest variable is the circuit surface preparation. If you have maximum torque coming from the motor, the gearbox is a torque multiplier, if you need more drive or more slide. I think they are kidding themselves. The same applies to racing on bitumen - once you are getting the best out of your motor, the rest is gearing. However with most motors, you can set them up to have either midrange or top end power. You would not change that to suit the circuit. Getting it right for one circuit is difficult enough.

That 84 bhp for the Paul Smart 750 was probably at the motor and the stroke would be shorter, so the revs would be higher.
 

storm42

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In a discussion with a friend yesterday, he mentioned he has been communicating with riders in the US who race Triumph twins. He said that for some circuits, they change lobe centres of the cams as well as intake lengths. I presume they are racing on dirt. If that is the case, the biggest variable is the circuit surface preparation. If you have maximum torque coming from the motor, the gearbox is a torque multiplier, if you need more drive or more slide. I think they are kidding themselves. The same applies to racing on bitumen - once you are getting the best out of your motor, the rest is gearing. However with most motors, you can set them up to have either midrange or top end power. You would not change that to suit the circuit. Getting it right for one circuit is difficult enough.

That 84 bhp for the Paul Smart 750 was probably at the motor and the stroke would be shorter, so the revs would be higher.

The Ducati info said the power was at the wheel, good power for a 750 twin with desmo valve gear, which is why I am struggling with the Harley claims. Always happy to be wrong though.
 
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The best British 350cc single cylinder four-stroke of the 1960s turned out about 44 genuine horsepower. 100 bhp for a 750cc Harley sounds optimistic. If it was a 1000cc Vincent on methanol, I could believe it.
 
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Hi Steve, did you get the bike sorted and are you coming out to play with the BHR this season?

I got the 750 sorted, (ign and clutch), though that needs to be fully proved! The 500 is waiting on ignition unit from RTD and exhaust mods. Planning on doing CRMC mainly though, might fit in a race or 2 at BHR. Would be a logistics choice. So hopefully see you round the bazaars! Roll on spring
 

storm42

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I got the 750 sorted, (ign and clutch), though that needs to be fully proved! The 500 is waiting on ignition unit from RTD and exhaust mods. Planning on doing CRMC mainly though, might fit in a race or 2 at BHR. Would be a logistics choice. So hopefully see you round the bazaars! Roll on spring

BHR test day at Mallory on 20th March.
 
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googling 100hp xr750 harley gets alota easy hits
Jim, do you think the dynos Harley used might have been a bit patriotic or a bit downhill ? I have seen your drawing for the ports and whilst I can appreciate the design, I am struggling to come to terms with 100bhp from a 750 twin.

Just for comparison, Ducati claimed 84bhp for Paul Smarts 750 and that had desmo valve gear, I realise that was around 1974 and you may be referencing Harleys from around 2008 but an extra 16 bhp from a pushrod twin seem a bit optimistic.

What fuel did they use?
 
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" The Complete Harley Davidson " makes a much lower claim for the xr750.

"1970-80 XR 750 Specifications Engine Dpi vt train Engine: OHV 45' V-twin
It was approximately 100 lbs lighter than a kick-start Sportster and made roughly 1 0 more horsepower in stock form."

Wasn't the Sportster 883 of that era rated at 55bhp?
I also read a later claim of " well over 100 HP" and " just 290 lbs" for the xr750 , however those claims were made by a collector salivating over his own bike.

Glen
 
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i think you are suggesting ~ 65hp?

" The Complete Harley Davidson " makes a much lower claim for the xr750.

"1970-80 XR 750 Specifications Engine Dpi vt train Engine: OHV 45' V-twin
It was approximately 100 lbs lighter than a kick-start Sportster and made roughly 1 0 more horsepower in stock form."

Wasn't the Sportster 883 of that era rated at 55bhp?
I also read a later claim of " well over 100 HP" and " just 290 lbs" for the xr750 , however those claims were made by a collector salivating over his own bike.

Glen
 
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I have no idea, just quoting the publication " The Complete Harley Davidson"

It does seem to say about 65 bhp.
Wikipedia says 82
Others claim way over 100

Harley never released an official number.



Glen
 
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Your quote is from page 96 & applies to the early ironhead that also had overheating issues, lower C/R as well as showing stock HP #s, they then went with alloy heads, higher C/R & lots more..

To the left of the same page shows 90hp (eventually) for the model (made from 70-80) with 'around' the same HP #s & more details on the next page (97).

https://books.google.ca/books?id=9i...oQAQ#v=onepage&q=100 HP Xr 750 harley&f=false

prior thread
https://www.accessnorton.com/NortonCommando/norton-intake-ports-compared-to-harley-xr-750.14445/

The wiki also includes
https://en.wikipedia.org/wiki/Harley-Davidson_XR-750
"The official horsepower was never published, but estimates for the early 1972 engines were in the high 70–79 hp (52–59 kW) range, increasing to an estimated 100 hp (75 kW) or more by 2008."

" The Complete Harley Davidson " makes a much lower claim for the xr750.

"1970-80 XR 750 Specifications Engine Dpi vt train Engine: OHV 45' V-twin
It was approximately 100 lbs lighter than a kick-start Sportster and made roughly 1 0 more horsepower in stock form."

Wasn't the Sportster 883 of that era rated at 55bhp?
I also read a later claim of " well over 100 HP" and " just 290 lbs" for the xr750 , however those claims were made by a collector salivating over his own bike.

Glen
 
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xbacksideslider

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I won’t namedrop but my hearsay knowledge is north of 80 HP but to achieve that level they have to be spun to speeds that so shorten the life of the rods’ big ends and cases that they are impractically short fused and prohibitively expensive
 
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