Flywheel weight?

marshg246 said:

Please take with a grain of salt - I have only minor personal knowledge of porting...

Around 1970 I was talking to the Triumph 500 tuner for the 1966/67 Daytona 200 winning bikes. A Triumph 500 with twin carbs produces pitiful HP but they handled excellent and had excellent riders. He told me that they tried a few things for more power. One was larger, straighter, polished intake ports - made the bike slower. Then they tried standard ports but polished - still slower than stock. Then they just cleaned them up and made them a little rougher than stock - that made them faster. He said the only other big things they did were "650 racing springs", exhaust porting, and high-compression pistons (I think 13:1). At the same time, there was factory-made a 5-speed gearbox for the 500 sitting on the shelf with me drooling over it. Only two existed and none on the road so it wasn't allowed to be used for the races.

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With a polished inlet port the fuel droplets tend to cling to the wall giving poor distribution of fuel droplets in the air in this region. With a rough surface there is a turbulent region close to the wall that gives improved fuel/air mixing at the expense of slightly reduced volumetric efficiency.
Most people however, prefer the look of a polished port.

Percy Tait went well against the MV 500 on Spa, with the 500cc Triumph. It was the A type motor with the 65.5 mm stroke. They have very little torque but will rev to 10,500 RPM, and winde out forever, if geared high. If geared low, they accelerate like a rocket, but have limited range. With a close ratio gearbox and lean needles, the main jets in the commando carbs do not need to do much. Triumph 500s are a long way different to a Commando 850. Percy Tait was a genius rider. A Commando 850 is an easy ride.

Schwany said:

When I first installed a cam in a 750 with two S's on the drive side exhaust lobe, I did not have adequate crank case ventilation since the cam did not support a timed breather. That really messed with the top end. No amount of carburetor tuning made a difference. Probably has nothing to do with it though, because I don't think I could reach 6000 RPM when crankcase ventilation was poor. Maybe check whatever PCV (breather) plumbing you are running anyway?

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I have a XS 650 type PCV valve fitted so i am happy with that, on standard cam it would only reach 6000 rpm down a long hill, the PW3 has increased mid range rather well and going from a 21 to 19 gearbox sprocket has slightly raised top end revs as you would expect without really loosing much in the way of mph, [i checked with a gps speedo], it does run rich looking at the plugs but doing a plug chop is tricky with no kill switch fitted and the key tucked away under , i will persevere though,,,,,,,,,, Thanks all

Colin , this is very strange as the stock 850s rev beyond their max power point so easily.
The other thing is that the PW3 is not known for improving midrange, at least not on a dyno, although the 21 t to 19t change would make up for most of that.
Jim Comstock wrote-
"The last time I did a PW3 in an otherwise stock motor the back to back dyno tests showed a loss of about 5 horse at 4500 and a gain of about 3 horse at 6500.

However the owner was thrilled by the rush between 4500 and 6500 and said he liked the extra power - no doubt due to the steep raise in power between those two points. Jim"

I can't help but feel something else is afoot with your bike.
Have you tried a compression leak down test on the bike? The leak down test can be very illuminating.

Glen

worntorn said:

Colin , this is very strange as the stock 850s rev beyond their max power point so easily.
The other thing is that the PW3 is not known for improving midrange, at least not on a dyno, although the 21 t to 19t change would make up for most of that.
Jim Comstock wrote-
"The last time I did a PW3 in an otherwise stock motor the back to back dyno tests showed a loss of about 5 horse at 4500 and a gain of about 3 horse at 6500.

However the owner was thrilled by the rush between 4500 and 6500 and said he liked the extra power - no doubt due to the steep raise in power between those two points. Jim"

I can't help but feel something else is afoot with your bike.
Have you tried a compression leak down test on the bike? The leak down test can be very illuminating.

Glen

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A close ratio gearbox makes the difference between usable and not usable, if you want performance. If you race using the normal Norton box, the bike will always be a slug. I tried it and was very disappointed. Manx ratios fit the Norton box, and are much better. However the high first gear is a problem in clutch start races, unless the Commando first gear is used or you rev the tits off the motor to get mobile. Once the bike is rolling, the Manx ratios are excellent everywhere. They give much better acceleration because there is less reliance on throttle response, For $700, the Manx ratios are a good purchase. Try driving a four speed manual car and change from 1st gear into 3rd and see how it accelerates - or from 2nd into top. You get the same thing as the wide ratio Commando gearbox. The steps in the Manx ratios are smaller than Commando ratios by about one third. In a race my motor stays between 5,500 RPM and 7000 RPM. I often see about 7,300 RPM on the tach. I suggest for the investment, the close ratios give more bang for the buck than any engine modification. The 6 speed TTI box costs about $5000 - that would get any Commando really going. Forget all your cams and bullshit - the motor is fast enough, except for the crank balance factor.

These last few topics on this forum have reminded me of what I have not done. For some time, I have wanted to make an on onboard video of my Seeley 850. I think you guys need to see what it does. It is actually a bloody joke. I love it - 'somethings are so bad that they are good'. It is an interesting ride. I just need to get around my wife. I was talking to her yesterday about riding, and she still has the wrong idea - she worries for nothing. I stopped crashing motorcycles in about 1978.

acotrel said:

A close ratio gearbox makes the difference between usable and not usable, if you want performance. If you race using the normal Norton box, the bike will always be a slug. I tried it and was very disappointed. Manx ratios fit the Norton box, and are much better. However the high first gear is a problem in clutch start races, unless the Commando first gear is used or you rev the tits off the motor to get mobile. Once the bike is rolling, the Manx ratios are excellent everywhere. They give much better acceleration because there is less reliance on throttle response, For $700, the Manx ratios are a good purchase. Try driving a four speed manual car and change from 1st gear into 3rd and see how it accelerates - or from 2nd into top. You get the same thing as the wide ratio Commando gearbox. The steps in the Manx ratios are smaller than Commando ratios by about one third. In a race my motor stays between 5,500 RPM and 7000 RPM. I often see about 7,300 RPM on the tach. I suggest for the investment, the close ratios give more bang for the buck than any engine modification. The 6 speed TTI box costs about $5000 - that would get any Commando really going. Forget all your cams and bullshit - the motor is fast enough, except for the crank balance factor.

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Colin's 850 that doesn't want to rev past 5000 won't be fixed by changing the gearbox ratios. I'm almost certain that his machine is a regular roadbike, not a racebike.
The standard 4 speed gearbox ratios work really well on the road.

Glen

Even with a road bike, performance could be improved by using a Commando first gear with the upper three gears as Manx ratios. However I would not rev a Commando high unless the crank balance factor was above 72 %. The low balance factor is not good for high revs. Close ratios for 2nd, 3rd, and top would help the bike to accelerate better. However the bike needs to be going a bit quicker when you change from 1st to 2nd. You would need to rev the motor a bit higher in 1st for normal riding. I could have raced using that combination, but I did not think about it. These days, most road races are clutch starts, so a racer is used a bit like a road bike. One of my friends used to race a Manx - when I discussed this with him, he told me he has 3 first gears for his bike which he changes to suit the circuit. Like an idiot, I paid $5000 for a 6 speed TTI box. I have got about three Norton boxes which all have different but interchangeable first gears. However 6 speeds close ratio might be slightly better. Good Japanese bikes have that. It is what makes them quicker.
The Suzuki XR69 which Crosby raced had race cams and a 6 speed close box. These days the replicas have Katana motors and I think - 5 gears. But the GSXR 500 L has a close ratio 6 speed cluster which might fit.
I once tried to race my Seeley 850 with a normal Norton gearbox - it was woeful. I could not do anything with it.
It might be difficult to believe but close gear ratios make bikes faster.

If I had a road-going Command, I would trick the gearbox before I tried to improve the motor. The crank balance factor is a problem. It is not only about vibration through the handle bars. If you did the mod I have suggested, and you were riding with your mates and decided to have a go, they would not know why you would be quicker.

The crankcase breather on my 850 is the normal pipe, but the tube attached to it has a PCV valve from a Ford Falcon, and a catch tank.

SteveA said:

36mm, you know, I didn't know that!

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acotrel said:

A close ratio gearbox makes the difference between usable and not usable, if you want performance. If you race using the normal Norton box, the bike will always be a slug. I tried it and was very disappointed. Manx ratios fit the Norton box, and are much better. However the high first gear is a problem in clutch start races, unless the Commando first gear is used or you rev the tits off the motor to get mobile. Once the bike is rolling, the Manx ratios are excellent everywhere. They give much better acceleration because there is less reliance on throttle response, For $700, the Manx ratios are a good purchase. Try driving a four speed manual car and change from 1st gear into 3rd and see how it accelerates - or from 2nd into top. You get the same thing as the wide ratio Commando gearbox. The steps in the Manx ratios are smaller than Commando ratios by about one third. In a race my motor stays between 5,500 RPM and 7000 RPM. I often see about 7,300 RPM on the tach. I suggest for the investment, the close ratios give more bang for the buck than any engine modification. The 6 speed TTI box costs about $5000 - that would get any Commando really going. Forget all your cams and bullshit - the motor is fast enough, except for the crank balance factor.

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Looking at the common Manx gear ratios, I can't see this working well on the road. If , as you suggest in a later post, the Commando first gear is ratio is used with Manx 2nd 3rd and forth ratios, that jump from a 2.6 first to a 1.33 second would feel like the engine fell out.

Glen

worntorn said:

Looking at the common Manx gear ratios, I can't see this working well on the road. If , as you suggest in a later post, the Commando first gear is ratio is used with Manx 2nd 3rd and forth ratios, that jump from a 2.6 first to a 1.33 second would feel like the engine fell out.

Glen

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Made me laugh in a fully supportive way.

Schwany said:

Made me laugh in a fully supportive way.

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The jump from zero to 1st gear in a Manx cluster makes the bike too slow from a standing start, but 2nd gear in a Manx cluster is only one third high than 2nd in a Commando box. The increased step from 1st to 2nd would only cause minor difficulties in stop-start traffic.
I raced a fair bit in the 1970s and a little after 2000 - the gearbox was always the handicap. I am fairly certain it is the only thing which stops the Commando from being extremely fast.
If I lived in America, I would stay with wide ratios, but in Europe 6 speeds close ratio would probably be much better.
My friend loves Harleys because they are good for eating miles of boring roads, of which we have a lot in Australia.
It depends on how you want to use the bike.

I often wonder about engine tuning mods. After everything is optimised we often end up with almost zero improvement. If we get more torque we raise the gearing, but when we do that the spacing between the gears gets larger - so we lose more revs on upchanges ? That means increasing throttle response problem.

The two things which most improved my Seeley 850 were installing the Manx gear cluster and increasing the trail in the steering geometry. I have not used 6 speed TTI box in anger, but I know what the result would be - the Manx cluster was good enough, but too slow in a clutch start. I only bought the TTI box to complete the bike - I sold a TZ350 to get it. With the Commando engine, the carb jetting is as critical as it is with any two stroke.

I was at a race meeting at Mount Gambier in about 2004, there was a guy there with a 650SS. He told me he had a bike similar to my Seeley 850, but did not bother to race it. He has more fun with the 650SS. But I don't think he raced against Wolfenden's creations.

It was really funny riding with the Manx cluster. From a standstill, the bike used to lumber-off, then go like buggery. It was absolutely perfect everywhere, except at the start of a race. I really enjoyed it - it is fun to ride.
I am going to have to get myself organised and make the on-board video - it might cause some amusement.
If you want to road-race a Commando, I can recommend a Seeley - they are different to what you might expect.

A great racebike, not so peppy as a roadbike. Just starting to get moving at the end of the quarter. By then a regular Commando has disappeared over the hill.
This is with the overall top gear ratio similar to a Commando with a 21 tooth sprocket.
A good ordinary 4 speed Commando with 21 tooth has much stronger acceleration to 100 mph. Above that the Manx is superior.
We don't go above 100 mph on the roads very often. Never for most road riders.

Since this thread seems to have morphed into gear ratios I will stick my two half pennies worth in.
The gear box I used for racing in my domi with both 500ss ish and 750combat+( balanced for rigid mounts) motors was an early AMC 4 speed with Barton 4th gear pinions (a rare box, I must say). For most short circuits the overall gearing I used gave approximate speeds in in each gear at 7000 rpm of 50, 75, 100 and 110 mph. This was great for racing and bump starts but, for road use, 50 in first with a tick over of 2000 rpm was too high in slow town traffic. My stock (relatively) '68 Commando was a superior road bike. Incidentally, although my commando is a rapid road bike, I could lap quicker on my 500 domi than the commando.

acotrel said:

The jump from zero to 1st gear in a Manx cluster makes the bike too slow from a standing start, but 2nd gear in a Manx cluster is only one third high than 2nd in a Commando box. The increased step from 1st to 2nd would only cause minor difficulties in stop-start traffic.
I raced a fair bit in the 1970s and a little after 2000 - the gearbox was always the handicap. I am fairly certain it is the only thing which stops the Commando from being extremely fast.
If I lived in America, I would stay with wide ratios, but in Europe 6 speeds close ratio would probably be much better.
My friend loves Harleys because they are good for eating miles of boring roads, of which we have a lot in Australia.
It depends on how you want to use the bike.

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I have zero experience with a Manx cluster. I'll take your word for it. I am getting what Molnar refer to as a Standard 5-speed TTi gearbox sometime in the first quarter of 2024. I think the gearing is going to be a little on the high side compared with what I have in the older AMC gearbox, but I'll get use to it I hope.