Maximum torque from a Commando.

concours · May 6, 2018

Nater_Potater said:
I found a local speed shop with a dyno; they only "allow" Harleys to run. ??? Maybe they're afraid of being embarrassed by a lil ol' 850 antique.

Nathan

Or, like the stoner who could not kick start my RZ350 (fires first goddamn stab warm) and then blew right by the 10,000 rpm redline I mentioned to him twice...
It's a "hate everything that's not a cookie cutter dump truck engine" mindset.

acotrel · May 6, 2018

Harleys have a cult following, some businesses make money by specialising in them. Anyone who puts a Harley on a dyno must be joking. They don't go, stop or handle, but they sound and look really great. Even the XR750TT did not do much.

worntorn · May 6, 2018

Nater_Potater said:
I found a local speed shop with a dyno; they only "allow" Harleys to run. ??? Maybe they're afraid of being embarrassed by a lil ol' 850 antique.

Nathan

That's pretty common, I've encountered the same thing.
They play the hi-lo game with the dyno to sell expensive tuning and aftermarket parts to Harley owners. The low initial reading shows its needed and the high reading proves that it worked, money well spent.
Bigger bhp number, everybody is happy.
Actual performance- maybe no different than before.

So they offer inexpensive or even free dyno runs for Harley owners.

Glen

bill · May 6, 2018

if harleys could convert decibels to forward motion it would be a rocket ship. it is the most efficient machine invented by man to convert gasoline to noise

Dances with Shrapnel · May 6, 2018

acotrel said:
Harleys have a cult following, some businesses make money by specialising in them. Anyone who puts a Harley on a dyno must be joking. They don't go, stop or handle, but they sound and look really great. Even the XR750TT did not do much.

Actually, the HDXR750 is a bench mark in terms of two valve air cooled twins of the era. Show me a Norton that can come close to the power characteristics of a HDXR750. Furthermore, tell me where you can get women panties with a Norton logo and how many people have a Norton tattoo?

Danno · May 6, 2018

Make a Norton run by using XR750 port shapes. What are we talking about here? I've ridden 130-hp Big Twins and I can tell you they will go and they can be made to stop and handle as well, size being a consideration.

Some people can only appreciate one brand. that is definetley cultish.

Dances with Shrapnel · May 6, 2018

Not sure what you are saying Danno. If it were just the ports then everybody would be doing it. A good example is the stroke. You’re not going to touch an HDXR750 with an 89mm stroke. Friction and ring flutter would eat your lunch. Then there is the factory cases - gotta go Steve Maney (while they last) or NZ billet. Not saying a Norton 750 twin couldn’t achieve power comparable to an HDXR750 but HDXR750s achieved it decades ago.

Danno · May 7, 2018

What I'm saying is, it's silly to diss a brand of motorcycle you don't own, have never ridden, know little about (except to repeat unsubstantiated wild rumors, like 'Harleys are made in Indonesia now' or that they're oil-leaking, breaking junk.) All the brands that still exist (and some that don't) have contributed things that the others can and have used to make their machines better. And all of us who love our typically leaky old British twins should know better.

worntorn · May 7, 2018

Not sure if you are referring to my comments, but I'm not saying anything negative about the bikes, just the Dealers who do this " Harley only" dyno routine.

I've read a couple of reports of a similar thing being done with Dealers of other brands. They know that a lot of owners are hooked on BHP bragging numbers. It's so easy to manipulate those numbers and make $ in the process.

Dances with Shrapnel · May 7, 2018

I'm with you Danno. Agree.

Danno · May 7, 2018

worntorn said:
Not sure if you are referring to my comments, but I'm not saying anything negative about the bikes, just the Dealers who do this " Harley only" dyno routine.

I've read a couple of reports of a similar thing being done with Dealers of other brands. They know that a lot of owners are hooked on BHP bragging numbers. It's so easy to manipulate those numbers and make $ in the process.

No, not at all. but the idea that a Harley could be embarrassed on a dyno by a 250 anything is ludicrous.

worntorn · May 7, 2018

That wasn't one of my claims.

Glen

acotrel · May 8, 2018

Do you think a Honda RS125cc two-stroke road racer might embarrass an HDXR750 on either a dyno or a bitumen race track ? What do dyno figures really mean ? I've not seen a genuine Harley road race bike in Australian since Don Emde rode the XR750TT at Calder in about 1973. It was quick but not extremely so - I was able to keep him in sight. Is the HDXR750 to which you refer a modern replacement or is it a dirt bike ? In that situation a good dyno figure might mean more.
I once considered playing with a Sportster motor, but everything seemed to cost a bomb.

acotrel · May 8, 2018

If you put all the bikes in this race on the dyno, do you think Hailwood's Ducati would have the highest BHP ? I think a lot of guys believe, the biggest most powerful motor ensures racing success. You obviously need enough power to keep up, but it is not the be-all and end-all.

Dances with Shrapnel · May 9, 2018

acotrel said:
Do you think a Honda RS125cc two-stroke road racer might embarrass an HDXR750 on either a dyno or a bitumen race track ? What do dyno figures really mean ? I've not seen a genuine Harley road race bike in Australian since Don Emde rode the XR750TT at Calder in about 1973. It was quick but not extremely so - I was able to keep him in sight. Is the HDXR750 to which you refer a modern replacement or is it a dirt bike ? In that situation a good dyno figure might mean more.
I once considered playing with a Sportster motor, but everything seemed to cost a bomb.

Alan,

Well sorted later model XR750 engines were putting out something likely closer to twice the power your Commando based Seeley puts out. Good enough?

Fast Eddie · May 9, 2018

acotrel said:
If you put all the bikes in this race on the dyno, do you think Hailwood's Ducati would have the highest BHP ? I think a lot of guys believe, the biggest most powerful motor ensures racing success. You obviously need enough power to keep up, but it is not the be-all and end-all.

That video is irrelevant to anything, apart from arguing what a superb rider Hailwood was. He would have won on ANY of those bikes.

mdt-son · May 9, 2018

Dances with Shrapnel said:
Not sure what you are saying Danno. If it were just the ports then everybody would be doing it. A good example is the stroke. You’re not going to touch an HDXR750 with an 89mm stroke. Friction and ring flutter would eat your lunch. Then there is the factory cases - gotta go Steve Maney (while they last) or NZ billet. Not saying a Norton 750 twin couldn’t achieve power comparable to an HDXR750 but HDXR750s achieved it decades ago.

Your comment made me curious to read the 1972 XR750 specs. BxS = 79 mm × 76 mm; 2-valve head, big valves, special porting; Capacity 748 cc; CR 10.5:1; power output 82 hp (61 kW) @ 7,700 rpm according to Wikipedia (unsubstantiated); Max. permissible revs 8500 rpm; All-aluminium motor; Triple chain primary; Unit Sportster-derived engine; 4 speed transmission (its weak point); 2x 36 mm Mikuni carburetors rubber mounted on long inlet manifolds.

A big advantage of the XR750 (as with all HDs) is the narrow crancase which more or less eliminates crankshaft flexing. This and the transverse V-tvin layout ensured a wonderful absence of vibration throughout the entire rev range, according to reports.

-Knut

Dances with Shrapnel · May 9, 2018

Hi Knut,

Crankshaft flexure is in my opinion, overrated. If it becomes a problem (breakage), remedy it through better materials of construction and/or billet design. The power you cited from Wikipedia sounds about right for a 1972 XR750 purchased from HD. The key is the term "well sorted". I cannot recall when the factory discontinued development and support of the motor but later versions power output was considerably more and benchmarking against a rider such as Jay Springsteen on the track is really the next best thing to dyno data and that bike and others like it had motor!

Numbers I have heard were closer to mid to high 90s hp and even over 100 hp; probably all further developed motors by people like Carl Patrick and all unsubstantiated.

worntorn · May 9, 2018

Absence of vibration in a Harley VTwin.
I dunno about that one.
The Harley roadbikes have a narrow crankcase and certainly have made a lot of vibrations over the years.
I guess these xr750 engines would be balanced and blueprinted plus they are of fairly small displacement, so that might help.
Almost every British parallel twin test from the 50s and 60s claimed an absence of vibration so it seems to be one of those questionable claims that was thrown around, but wasn't necessarily based on fact.

Glen

mdt-son · May 9, 2018

Dances with Shrapnel said:
Hi Knut,

Crankshaft flexure is in my opinion, overrated. If it becomes a problem (breakage), remedy it through better materials of construction and/or billet design.

Hi Dances,

Using certain assumptions (e.g., no deformation work produced by bearings and crankcase, axial angular adaptility of bearings), the amount of crakshaft flexure can be computed and the resulting vibrations compared to those produced by the inertial forces. The superimposed amplitudes may be compared to a V-twin of equal cylinder capacity and the narrowness of a HD crancase. I have no doubt the HD will excel in this comparison.

The crankshaft deflection is essentially posssible due to the spindly shafts and stretching of the bolted connection. Both can be mitigated.

-Knut

Maximum torque from a Commando.

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