Available engine performance

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Hi Mar
The Kawasaki was light, as was the Commando.

In 1970 BSA were testing what would be the works 750 3 I expect against the power graph of an A65 twin with A10 crank and almost matching it. The twin had 78hp @ 7,000 at the crank. They were revving the 3 to 9,500 and still shy. And even a hot 650 A65 was beating it to 5,500rpm. The biking world would have been different if only the 3 cylinders were based on the A65. Thunderbolts have 27mm ports like the triple except they flow 108cfm each. If they used the Spitfire cam, same bore, short stroke for a 750cc, pushrods inside, with the oil, the motorcycling world would have been a different place. On the mildest hp calculator that flow would enable 77hp.

With A65 stroke and '71 porting for a 981 triple 87hp and massive torque would have decimated the Z1 with technology they had, and those heads can be made to flow enough for that to be around 120hp in a race motor with fierce mid range. Yet they chose a 500 Triumph base with ports crowded with head studs, as if BSAs development didn't exist. And no less than 4 external pushrod tubes and separate rocker boxes to leak oil which were so unnecessary, as was all the complex crank cases with the expense and time intensive assembly.

And the head of the BSA/Triumph company was thinking they needed OHC motors to catch the modern Japanese advances.
hi Mark,
I like your post. As a long time Trident owner I am biased towards the bikes but what you say is correct regarding the seperate push rod tubes, rocker boxes and complex crankcase. To me they always feel like a prototype awaiting final improvements.
Im not familiar with BSA twins but if the performance figures for the A65 are correct it would of made a hell of a bike with an overall more modern appearance that might have been easier to live with.
As it is, I have had my trident bored out to 830 cc with spitfire cams and three into one which certainly improves it’s lower end and mid range power.
Please note, this post is not intended to be a comparison between Nortons and Tridents, just an interesting observation.
The 74 Mk 2 is a lovely bike to ride but to me it does seem a little breathless above 5000 rpm, not that I want to ‘ring it’s neck with high revs
Al
 
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Which magazine did this come from Mark? The 1970 Cycle shootout test perhaps?
Probably. I got it from the net. A friend has a LSR A65 with very high compression, it had a stock '71 head with 34mm TMs. Set records with it and got me to do a head which he is yet to test. But they were surprised at it's output, not top end, but grunt.

The head should extend top end and maybe add a little midrange. But I drew his dyno graph on this one for comparison. Remembering the dyno's are a bit different and the stock bikes had air filters and mufflers. But his bike really needed power peaking past 7,000 and I'm waiting to see a dyno graph with the new head and overlay it on the other graph when he gets to it. So this is with a stock '71 port.

In 68 BSA built a Spitfire with a specially ported head better in the bowl area. They used a 32 concentric for 56hp with 9-1 comp. The parts book showed a std head but it had it's own number. In 70 Umberslade's guys built a Firebird and I think used that porting and drawing, measuring 8hp or so difference with production line engines. A stock 71 head I have has that porting but with 30mm carb, it flows around 122cfm. That's what they were running for this red graph with 34mm carbs somehow adapted.



 
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Hi Mar

hi Mark,
I like your post. As a long time Trident owner I am biased towards the bikes but what you say is correct regarding the seperate push rod tubes, rocker boxes and complex crankcase. To me they always feel like a prototype awaiting final improvements.
Im not familiar with BSA twins but if the performance figures for the A65 are correct it would of made a hell of a bike with an overall more modern appearance that might have been easier to live with.
As it is, I have had my trident bored out to 830 cc with spitfire cams and three into one which certainly improves it’s lower end and mid range power.
Please note, this post is not intended to be a comparison between Nortons and Tridents, just an interesting observation.
The 74 Mk 2 is a lovely bike to ride but to me it does seem a little breathless above 5000 rpm, not that I want to ‘ring it’s neck with high revs
Al
The Tridents are great with a bigger bore. Shame the factory didn't do it in a Rob North. I had one for years and loved it. (Not a Rob North, I would have never sold it) It was only I was doing a Thunderbolt head for a race bike and decided to measure the std ports minus manifold, before I started, 27mm not 30mm and 1cfm less than a Lightning?
 
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The main thing to to is eradicate anything impedeing performance .Thus ensure all the chain wheel alignment is good .
Ensureing the gearbox is top line and secure . If you cant handle the basic gearbox , steer clear of interfearing with the engine etc .

I regard the intakes manifolds as the biggest ' baulk ' to free breathing / running .
You cant regard anything with that wall length differance , top to bottom , as other than ' kinked ' . ensureing a degree of tumbling / stalling , at times .
A free curve / radius , through from the head to near level - with the carbs up against the frame gusset , gets a far more uninterupted tract .

The 72 JPS F-750 ran semi downdraft GPs at times . good for 155 mph . There was a 5600 or 5800 ' continuous ' rateing , in the Poore Era .
Tecnically the oil capacity is sup par . 3/4 Imp. Gallon with 1/4 breather space . I.E. ONE GALLON TANK . but use say your Std. amount for short trips ,
Full for touring / Summer use . And Racing .

The Oil Tube post 70 Triumphs , were a bit the same . a oil cooler adds capacity . But theres a ' ratcheting ' heating / cooling oil - continuously , thats bad for it .
So a thermo thingo helps .

18 in rear wheel will get better rubber . T.C. was a 16 in fan , for 5.00 16 tyres . But some might say he was heavy on the throttle .

So its mainly fettling it to get it all functioning top line . Like a better maintained Top Line fighter aircraft . An hours polishing should be worth a few m.p.h. ;)
Hi Matt (and everyone) for the really interesting replies.
Dont get the wrong idea, I am basically very happy with the bike (Feb 1974) Mk 2. It is fitted with a late model Boyer ignition.
I have no intention of turning it into a stove hot racer. If I can sympathetically lift it’s performance a little to offset the extra weight it’s carrying. I will be fitting new Premier carbs shortly.
It seems to me that some mild head work and slightly higher compression might just give it a little lift.
thanks everyone
al
 

Fast Eddie

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Probably. I got it from the net. A friend has a LSR A65 with very high compression, it had a stock '71 head with 34mm TMs. Set records with it and got me to do a head which he is yet to test. But they were surprised at it's output, not top end, but grunt.

The head should extend top end and maybe add a little midrange. But I drew his dyno graph on this one for comparison. Remembering the dyno's are a bit different and the stock bikes had air filters and mufflers. But his bike really needed power peaking past 7,000 and I'm waiting to see a dyno graph with the new head and overlay it on the other graph when he gets to it. So this is with a stock '71 port.

In 68 BSA built a Spitfire with a specially ported head better in the bowl area. They used a 32 concentric for 56hp with 9-1 comp. The parts book showed a std head but it had it's own number. In 70 Umberslade's guys built a Firebird and I think used that porting and drawing, measuring 8hp or so difference with production line engines. A stock 71 head I have has that porting but with 30mm carb, it flows around 122cfm. That's what they were running for this red graph with 34mm carbs somehow adapted.



Wow. That’s a sharp looking tool !
 

Fast Eddie

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Hi Matt (and everyone) for the really interesting replies.
Dont get the wrong idea, I am basically very happy with the bike (Feb 1974) Mk 2. It is fitted with a late model Boyer ignition.
I have no intention of turning it into a stove hot racer. If I can sympathetically lift it’s performance a little to offset the extra weight it’s carrying. I will be fitting new Premier carbs shortly.
It seems to me that some mild head work and slightly higher compression might just give it a little lift.
thanks everyone
al
Head work should include bigger valves, especially on an 850. Read Comoz’ head flow thread, he dispels the myths about this and says bigger valves give gains throughout the rev range.
 
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We tend to focus on modifications that might produce more power. The problem with that is the gearbox, first designed to handle about 14 bhp, is already at the breaking point.
A friend who specializes in AMC gearbox rebuilding tells me that the 850s are a lot harder on gearbox internals than the 750s.
I would guess that a 920 will be worse yet, will find out.
So if one is serious about hotrodding the engine, a TTI gearbox (6-7kNz?)should figure in the budget.
Im hoping that my combo of major weight reduction and s
added engine power will work with the standard box.
The thought is that weight reduction guarantees added performance all over, but should also lighten the load on the trans.
On edit- I really don't understand why a standard Commando would be anemic above 5k rpm. In my experience, accelerating through the gears, they like to rev quite quickly up to a place they shouldn't go...

Glen
 
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grandpaul

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I think @Alan L might need to chime in on where his line of thinking has gone by this time...

...He MIGHT have a budget, you know...
 

Fast Eddie

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We tend to focus on modifications that might produce more power. The problem with that is the gearbox, first designed to handle about 14 bhp, is already at the breaking point.
A friend who specializes in AMC gearbox rebuilding tells me that the 850s are a lot harder on gearbox internals than the 750s.
I would guess that a 920 will be worse yet, will find out.
So if one is serious about hotrodding the engine, a TTI gearbox (6-7kNz?)should figure in the budget.
Im hoping that my combo of major weight reduction and s
added engine power will work with the standard box.
The thought is that weight reduction guarantees added performance all over, but should also lighten the load on the trans.
On edit- I really don't understand why a standard Commando would be anemic above 5k rpm. In my experience, accelerating through the gears, they like to rev quite quickly up to a place they shouldn't go...

Glen
Tis true, that ole ‘box is well past its designed limitations. But…

I’d say 850s are harder on the ‘box cos they have more torque and folk use that torque to chug about on. 750s probably uses higher revs on average, which is MUCH kinder on the ‘box.

Lots of racers and specials use them, inc chopped Vincent’s. So I reckon power with revs (up to a point) is praps less of a killer than chugging around like a Harley!
 
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The 850 power band is way above the 750 then they pretty much come together at the top.
So unless you are at 6500 revs, which even the sportiest rider only uses for a few seconds per ride, the 850 has more available power.
So every time you twist the throttle to accelerate in a gear, the 850 twists on things just a little harder.

Glen
 

baz

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Tis true, that ole ‘box is well past its designed limitations. But…

I’d say 850s are harder on the ‘box cos they have more torque and folk use that torque to chug about on. 750s probably uses higher revs on average, which is MUCH kinder on the ‘box.

Lots of racers and specials use them, inc chopped Vincent’s. So I reckon power with revs (up to a point) is praps less of a killer than chugging around like a Harley!
I did know of a bloke that was running a Harley shovel motor with an AMC box and it was always tearing the box up
He changed to an STD BSA A10 box and no more problems
 
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Tis true, that ole ‘box is well past its designed limitations. But…

I’d say 850s are harder on the ‘box cos they have more torque and folk use that torque to chug about on. 750s probably uses higher revs on average, which is MUCH kinder on the ‘box.
The added weight of 5 bronze plates in the 850 clutch didn't help either..
 

cliffa

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Has anyone ever put a poweregg in a Commando frame (utilizing the iso's)?
 
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Here's a quick zero to 70 mph+.
This is a stock MK3 Interstate, so it's dragging the extra 30 lbs of starter gear along.
It's also dragging me along. I weigh about the same as Cook Neilsen did in 1970....if we add his girlfriend's weight :)
So a lot of weight.
Still, it doesn't take long to get to 70mph. This is a higher speed than the speed limit on about 90% of our roads. The other 10% are at about 70 mph.
I count 7 seconds at full bore.
Makes you wonder about the need for power modifications.
Silly thought, of course we think we need more power!

 
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Here's a quick zero to 70 mph+.
This is a stock MK3 Interstate, so it's dragging the extra 30 lbs of starter gear along.
It's also dragging me along. I weigh about the same as Cook Neilsen did in 1970....if we add his girlfriend's weight :)
So a lot of weight.
Still, it doesn't take long to get to 70mph. This is a higher speed than the speed limit on about 90% of our roads. The other 10% are at about 70 mph.
I count 7 seconds at full bore.
Makes you wonder about the need for power modifications.
Silly thought, of course we think we need more power!





Some foam placed in front of or wrapped around the microphone might help to diminish or eliminate wind noise.
 
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Thanks. One day I'll figure out how to make a proper video.
There is no external mic here, it's just a cheap Android phone clamped on.
There have been comments over the years about 850s "running out of puff" above 5 k and so on. I don't see that on mine, and as mentioned , it and I are a heavy combo.
So if the wind noise is annoying, maybe turn the sound down and just watch the tach.
One day I'll get a GoPro. And an external mic. With a wind sock:)
Glen
 
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I don't wind my 850 up that tight, but it certainly gets to 75mph fast, and pulls well until way past enough. It pulls my 235# carcass just as well, it seems, as it did when my carcass was a young 185# a long time ago. Got the torque of of an old inline six on a pull.... More than adequate for an aging Commando and operator.... What it does for what it is always amazes me. Guess I'm easily entertained.... by all the racket and roar.
 
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Had a better look at the YouTube time clock. It's only six seconds full bore, not seven.

Glen
 
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