Recall that a year or 2 ago on this forum we debated ad nauseum the HP required to increase speed from X to 2X (for some given object, maybe a motorcycle?), which revealed that HP must increase by the cube of the change in speed. For this discussion a plot was created using this cubed relationship and a starting datum value of 47 HP = 117 mph. Why was the 47 HP = 117 mph datum chosen? Because this puts the 45-50 HP range in the 115-120 mph ballpark which various people and articles suggest might be the correct ballpark. If the relationship is extrapolated to much higher speeds (150-160 mph), the HP numbers remain in the reasonable ballpark and do not get really crazy. I’m sure some of the LSR people here could weigh-in and indicate whether the plot has some basis in reality, keeping in mind that any given dyno number might be + 5-10% of reality.
Dyno run (2017)
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Recall that a year or 2 ago on this forum we debated ad nauseum the HP required to increase speed from X to 2X (for some given object, maybe a motorcycle?), which revealed that HP must increase by the cube of the change in speed. For this discussion a plot was created using this cubed relationship and a starting datum value of 47 HP = 117 mph. Why was the 47 HP = 117 mph datum chosen? Because this puts the 45-50 HP range in the 115-120 mph ballpark which various people and articles suggest might be the correct ballpark. If the relationship is extrapolated to much higher speeds (150-160 mph), the HP numbers remain in the reasonable ballpark and do not get really crazy. I’m sure some of the LSR people here could weigh-in and indicate whether the plot has some basis in reality, keeping in mind that any given dyno number might be + 5-10% of reality.
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You've spent some money. Wisely spend a little more; my suggestion would be get it on a dyno and start with OEM ignition advance (28 degrees BTDC or leave it at 29 - a reasonably safe starting point) and find the ignition setting where it makes best power. Best to do this on a brake dyno if available. Find a dyno owner/operator who knows what they are doing and who has an exhaust gas analyzer.
Are you tuning to race? Have you set up any squish? A 10.25:1 is not very aggressive for race? What fuel do you plan to run? All these aspects will impact where the motor will be happiest with respect to ignition advance.
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Ah yes, fond vague memories of that handbag war.
WZ507 being the good straight man has reproduced a curve here conveniently starting at 108 mph. Assuming it is a reasonable proximation of a Commando characteristics, one can see that it would only require an additional 6.5 hp to get the bike from 108 mph to 114 mph. Again, 1up3down only saw 108 mph and some literature stated 114 mph to 124 mph. We don't know the state of tune nor local atmospheric conditions with 1up3down's speed trials but using WZ507 chart above if we bump compression from 8.5:1 to 10:1 we should gain 2.3 hp in thermal efficiency alone (assuming we don't do something stupid like inserting a big wonky dome on the crown of the piston). There are usually other benefits to increasing compression on a Commando if properly done but we will not go into that here. So 6.0 - 2.3 = 3.7 hp is the deficit we would need to make up for to get 1up3down from 108 mph to 114 mph. Improving valve seats will improve volumetric efficiency and jetting for better power certainly move you closer to closing and maybe exceeding that 3.7 hp deficit.
If we want to discuss Triumphs, HDXLR or HDKR flat heads, they are all somewhat different kettles of fish.
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I've noticed on this forum, some people claim that to get the best out of JS3 cams, you need to raise the comp. ratio. It might be my imagination, but don't engines run cooler when fitted with race cams due to the better breathing ? Perhaps a race cam allows you to run a slightly higher comp.ratio ? In the end, when you use petrol as a fuel, you don't have unlimited antiknock as you do with methanol. The head design of the Commando engine is a limiting factor when compared with a modern four valve motor. I suggest that a modern four valve motor can run higher comp. ratios than an early commando because of the better mixing within the combustion chamber. In two valve motors, there are more hot spots that cause detonation, while in other parts there is unburned fuel. The difference between a two valve 500cc Jawa Speedway motor and a four valve is about 10% in power, but a very good two valve is almost as fast as an ordinary four valve.
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Good questions Alan. As the story goes, with greater cam duration there is a loss of mid range torque due to the shift of volumetric efficiency further up the rpm range. One way of looking at increasing the compression ratio in this instance is to take advantage of the greater resistance to knock (due to the lower volumetric efficiency) in the lower to mid range with greater cam duration. So what you loose in lower volumetric efficiency you gain through greater thermal efficiency.
As for better breathing equating to cooler running, I don't know about that. I equate better breathing to greater volumetric efficiency which equates to greater charge density in the combustion chamber which equates to greater heat. Maybe better breathing on the exhaust for better blow down may reduce heat. Interesting concept. Another way of looking at it is if you employ a cam with greater duration, the mid range will have less volumetric efficiency thus less charge density thus run cooler. Do you want to run cooler or make more power?
As for the four valve versus two valve, one should also consider the total area under the torque or power curve, not peak power in isolation. This is where a four valve shines as it has more valve time-area without getting crazy with valve lifts and durations.
As for better breathing equating to cooler running, I don't know about that. I equate better breathing to greater volumetric efficiency which equates to greater charge density in the combustion chamber which equates to greater heat. Maybe better breathing on the exhaust for better blow down may reduce heat. Interesting concept. Another way of looking at it is if you employ a cam with greater duration, the mid range will have less volumetric efficiency thus less charge density thus run cooler. Do you want to run cooler or make more power?
As for the four valve versus two valve, one should also consider the total area under the torque or power curve, not peak power in isolation. This is where a four valve shines as it has more valve time-area without getting crazy with valve lifts and durations.
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Whenever I have fitted race cams to a road bike in place of standard cams, I have gained more go below the cam spot and a lot more go above it. That is, even with mufflers fitted and if the inlet ports have not been enlarged. Enlarging the ports can be a one-way trip to losing. I have run 650 Triumphs at up to 11 to 1 comp. on avgas, but not seen much benefit. Overall the biggest leap in power has come by fitting the race cams. I've run a 650 Triumph with race cams on methanol as low as 7 to 1 comp. and it was still very fast. However we have also stroked a two-valve Jawa engine to make it 600cc - the comp.ratio is about 17 to 1 and it is an absolute blur on methanol.
Fast Eddie
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The main link between big duration (race) cams and CR is that the race cam loses ‘effective CR’. The increased duration, and overlap, result in some of that compressed charges escaping. Increasing static CR helps to correct this.
We discussed this before Alan when you mentioned you wanted to fit a 2S in your otherwise standard motor, I and many others advised against this without at least bumping the CR.
My road going 850 is 10.5:1. When I first built it I got kinda paranoid about heat and had lots of ceramic coating done. Some of it flaked off though, and when I rebuilt it again I had all the coating removed, now I just use a DiamonDyze treatment (a type of anodising) on the pistons (I don’t know if it’s really adding value, but at least it can’t flake off, and it should help).
FWIW, I have not noticed any difference with / without the ceramic coating.
With a big Norton, there is also a relationship between CR and squish band. These engines can achieve a high CR and a functioning squish band all with little or no machining and whilst maintaining a flat top piston. Having spent years having Triumph heads welded up and machined to get a squish band, to me, this is a revelation and the natural squish band built into the Norton head is too good to ignore. It’s well reported that squish bands help with lower temperature. I believe any extra heat of my increased CR is counterbalanced by the functioning squish band, and in combination, it works well together.
I have not seen any of the downsides often attributed to raised CR, heat, pinging, difficult starting, poor low rpm running, poor tick over, etc. Quite the contrary actually, the only downside to the 10.5:1 CR that I can find is that it does require a hearty swing on the kicker, but I’m a big ish fella so that’s not the end of the world, and as it normally starts first time, ticks over like a Honda, pulls smoothly from tick over, and goes like the clappers, it seems an insignificantly small price to pay!
However, one caveat: we are blessed with the availability of good, high octane, pump petrol here in the U.K. so I cannot claim this set up would work for everyone, everywhere.
We discussed this before Alan when you mentioned you wanted to fit a 2S in your otherwise standard motor, I and many others advised against this without at least bumping the CR.
My road going 850 is 10.5:1. When I first built it I got kinda paranoid about heat and had lots of ceramic coating done. Some of it flaked off though, and when I rebuilt it again I had all the coating removed, now I just use a DiamonDyze treatment (a type of anodising) on the pistons (I don’t know if it’s really adding value, but at least it can’t flake off, and it should help).
FWIW, I have not noticed any difference with / without the ceramic coating.
With a big Norton, there is also a relationship between CR and squish band. These engines can achieve a high CR and a functioning squish band all with little or no machining and whilst maintaining a flat top piston. Having spent years having Triumph heads welded up and machined to get a squish band, to me, this is a revelation and the natural squish band built into the Norton head is too good to ignore. It’s well reported that squish bands help with lower temperature. I believe any extra heat of my increased CR is counterbalanced by the functioning squish band, and in combination, it works well together.
I have not seen any of the downsides often attributed to raised CR, heat, pinging, difficult starting, poor low rpm running, poor tick over, etc. Quite the contrary actually, the only downside to the 10.5:1 CR that I can find is that it does require a hearty swing on the kicker, but I’m a big ish fella so that’s not the end of the world, and as it normally starts first time, ticks over like a Honda, pulls smoothly from tick over, and goes like the clappers, it seems an insignificantly small price to pay!
However, one caveat: we are blessed with the availability of good, high octane, pump petrol here in the U.K. so I cannot claim this set up would work for everyone, everywhere.
ntst8
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I bought my 850 in well used condition, ran it for years on low octane fuel assuming it was in normal trim.
Eventually found the head to be very heavily planed, deep valve pockets in the pistons and a serious race cam.
And the point is the norton engine seems very fuel tolerant as despite its state of tune mine ran well on the low octane fuel for all those years, not that i ride it very hard.
Eventually found the head to be very heavily planed, deep valve pockets in the pistons and a serious race cam.
And the point is the norton engine seems very fuel tolerant as despite its state of tune mine ran well on the low octane fuel for all those years, not that i ride it very hard.
Yes, I noticed back in the seventies that my Norton always ran hot. So did others. I guess modern fuels make the difference. Even when living in the Pilbara region of Western Australia in riding temperature of up to 50 degrees C that the motor never tinkled like the old days. In fact it ran well in the heat and didn't seem bothered. Back then, I had a MKII 850. The previous owner had a 2S cam fitted, a 2 into 1 exhaust, and the Anals jetted for velocity stacks. That thing ran hard with a definite power step at 4500rpm. As far as I know, it didn't have a raised compression ratio.
I'm waiting for someone to order a head with either no squish band to make a hemispherical combustion chamber or one with a reduced depth. No one so far!
SteveA
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I have raced the bike since 2015, fairly successfully for a 60 plus year old! will be 64 next year, might do 3 race meetings a year in the future, investments need to be balanced with the fact that I am not going to get much faster, and as an early retiree I have no real income!
The squish is not as tight as it could be, it was 60thou, but I went 10thou thinner (0.030" copper) on the head gasket this year. 10.25:1 is not very aggressive agreed, but I have no real desire to go higher. I run european pump fuel, which means variable 98 Octane.
Higher compressions are not that easy to achieve with a short stroke without more radical work than I have equipment for, or easy access to someone with equipment for, and in any case I have never used more than 10.5:1 on a Commando, possibly because I am from the generation that had to push start them
Current piston to valve clearance is as tight as I am comfortable with, take more off the head and I would need deaper piston pockets. My cast iron barrels have 20 thou off them, the fullauto also 20 though off. I may move to Maney barrels at standard height which would lose a little compression! I would be reluctant to skim Maney barrels because I know the sleeves are left a little proud and this would be difficult to achieve, so it is likely the head will have another 20 thou off in this configuration!It was on a Dyno once, when fresh built in 2015 to get basic fuelling sorted on 34mm Mikunis, no real time for incremental ignition adjustments then. Having moved to France dyno access is a little harder. It had a PW3 at that time, which hasn't lasted, I now have a Webcam from Jim Comstock with assymetric followers and 36mm Mikunis. Of course dyno time would be helpful. With the PW3 it revved to 8K with little drama, except the cam didn't last! With the Webcam it does not rev so high, initially 7K then with retarding the cam, 5 degrees, 7300, then 7500 with the 36mms, exhaust mods suggested as next step, which I will do, but limited scope means shortening 1" is about it.
But it is making good enough power to pass a whole range of bikes in a straight line with 100kg of me onboard! getting me a little lighter and fitter probably has more payback than 3 rwhp. Really I think my objective is to get it to run best it can without any more major work. So, shorten exhaust, refine jetting, a degree or so more retarded.
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I feel your pain. 100kg, that is 1/10th of a tonne if that makes you feel any better.
I suggest you close up your squish clearance a little bit more.
SteveA
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Well, no it doesn't
but really I am only 95kg, plus boots, leathers, gloves, back protector, helmet....and lunch 
Will be looking at squish in any case next time head is off, because it is likely there will be a barrel and piston swap at the same time.
This will happen as soon as a couple of other projects are progressed a little.
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Perhaps getting on the gas earlier when coming out of corners might be more important than trying to out-drag the opposition in a power-based competition ? If you are o0n the gas when half-way around the corner, you have an earlier start to the drag down the next straight. I don't think many guys ever really experiment by changing their steering geometry. If the bike turns quicker, it can be disconcerting until you get accustomed to it. The other thing is gearing, if you gear low, the bike is usually better in corners, but might not have the legs toward the ends of the straights. With a Commando engine, I tend to gear high and use torque rather than horsepower. With a two-stroke, I do the opposite.
PommieJohn mentioned that the Dyno Dynamics Dynomometer which Ken used is a type that gives low numbers, or at least doesn't give pumped up numbers.
One operator of this brand of Dyno found that the same vehicle showed 12 percent higher HP when run on a competitor's Dynojet Dynamometer.
So Ken, if you want a better dyno result, the least expensive and easiest way might be to find a Dynojet.
With 12 percent added your peak number would be 52 HP +-.
Glen
One operator of this brand of Dyno found that the same vehicle showed 12 percent higher HP when run on a competitor's Dynojet Dynamometer.
So Ken, if you want a better dyno result, the least expensive and easiest way might be to find a Dynojet.
With 12 percent added your peak number would be 52 HP +-.
Glen
I'll keep that in mind if I want some cheap horsepower, Glen!
I'm quite enjoying it.
Try to please everybody and somebody is not going to like it.
If it's dribble as you say, why read all three pages of it?
Glen
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