Dyno runs before and after engine mods

I wanted to share my recent experience. I rebuilt my 850 MkII engine last year. Before I took the engine apart I did a test on the dyno to have a base line. After the rebuild I did another run last week. The engine has around 8000km on it now. See below comparison of power and torque. Same Dyno was used.





There seems to be a shift in the rev range between the two runs. This may be related to my tacho being broken this time so the Dyno guy somehow ‘calibrated’ the scale by the roller speed. Not sure if that was correct.


Condition prior to engine rebuilt:

• Compression approx. 8:1 (measured)
• Piston: original flat top pistons 0.2 oversize
• Barrels rebored approx. 10,000km ago; immaculate condition
• Camshaft: stock; appeared to be in good condition
• Valves: stock; head was rebuild approx. 20000km ago
• SAE 50 monograde oil used when I had the bike and my mate before that
• Amal 932 sleeved; probably original; main 260, needle jet 106, needle at top position
• Ignition: Pazon
• Ignition timing probably at around 30 degrees

The bike ran great, normal fuel consumption. Dyno showed that the bike was running a bit rich though. I did a 1/4 mile race for fun not long ago against a mate, I managed 14.5s. Only reason for the engine rebuild were dodgy noises from the primary side of the engine. I found the inner main bearing ring loose on the crank shaft.


Condition after rebuilt:

• Compression: 9.4:1 (no base gasket, thin JS copper head gasket, head milled down 1mm)
• Piston re-used with new rings
• PW3 camshaft (cast iron from AN); timing had to be corrected by 5 degrees
• Inlet ports widened around the guides, guides narrowed down on the sides
• Outlet ports cleaned up
• Valves redressed and re-used; rings re-cut; valves probably sit quite deep in the head now.
• Trispark ignition with Trispark twin coils
• Rockers lightened
• Pushrods shortened
• Followers reground
• Amal premier (230 main, 106 needle jet; needle at top position; slides No 3)
• Ignition timing at 26 degrees
• Full synthetic oil 20W50

Draw whatever conclusions out of the above. I am happy. I put a lot of time into this engine and somewhat got something out of it. Thats at least my believe.

Looks as if you've gotten your time and moneys worth for sure. I'll wager she's got a helluva howl when the steam is poured on.
Mine needs a good teardown and improvements, but I shake too much to fiddle with it.... Perhaps my body's ISO's need replacement.

Nice work! That torque up high must be really nice with that camshaft.

If you raise the comp. ratio, it can have the effect of leaning-off the mixture. If the jetting was previously rich, raising the comp. could be the reason the motor might now give better performance.
If you have more torque available it might not change your bike's performance much until you raise the overall gearing. With more torque, the motor can do the same revs, but pull harder.
If you want more acceleration, you probably need closer ratios. I don't think lowering the overall gearing helps a Commando to accelerate much faster. The motor always gets to the same revs at pretty much the same rate, regardless of the gearing.
Your carb needles should not be in the top notch unless the needle jets are too large. However with your comp. mods, you might have corrected that. It might be worth trying slower taper needles.

Nice!

It's good you were on the same dyno.

Nice result. Looks like a nice ridable torque curve.

That’s great. The summary is simple: more of everything from 3,000rpm upwards.

That‘s a fast and fun set up you got there. Enjoy !

If I'm reading the dyno chart correctly, those mods produced around 15 more HP than the stock setup - around 40% more power. That's an absolutely amazing figure for the work done...

MexicoMike said:

If I'm reading the dyno chart correctly, those mods produced around 15 more HP than the stock setup - around 40% more power. That's an absolutely amazing figure for the work done...

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When I did ’version 1’ rebuild on my 850, although I did a lot to it, the main performance enhancing items were 1) a JS#1 cam (essentially in between a stock cam and a PW3) and 2) a raise in CR to 10.5:1.

So I had a slightly milder cam and a higher CR than Joachim. IIRC that gave circa 56-57 rwhp and the same ‘more of everything above 3k’ summary. So Joachim‘s results do seem to correlate with my own.

My version 2 was as above but with ‘the full monty’ Comnoz head job on the RH10 inc bigger valves. That unleashed another staggering 9 rwhp. And ‘even more of everything above 3 k’.

So, my conclusion is that these Norton lumps have a surprising amount to give in the early stages of tuning which will be VERY noticeable (unless you ride much below 3k rpm). Of course, that will taper off into diminishing returns the more serious one gets, but those early steps of cam, compression and (skilled) cylinder head work, do seem to unlock a lot. Well, that’s my experience at least.

Also, regarding Joachim‘s cam, the PW3 often gets bad press, now, I’ve never used a PW3 myself, but Mick Hemmings swore by the PW3 as does Norman White (who describes it as ‘the only cam to consider’) and it was designed by PW himself… so it really should not be a bad cam. And then we have Joachim‘s evidence, how can anyone argue against his before and after graphs?

Thanks for the post, there's some fairly solid information here, a refreshing change from what most of us do: use the ARSE dyno, which usually tells us that we have done a fantastic job everywhere

Did your dyno man also do the air-fuel ratio readings to check the carburation?

Steve… +1 x 10 !

Unless you’re a Percy Tait, Peter Williams, etc, never, ever, ever trust the arse dyno !

SteveBorland said:

Thanks for the post, there's some fairly solid information here, a refreshing change from what most of us do: use the ARSE dyno, which usually tells us that we have done a fantastic job everywhere

Did your dyno man also do the air-fuel ratio readings to check the carburation?

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Hi Steve,

here the AFR check:



I dont know much about the AFR ration. Dyno man said its on the rich side but probably good for the Amals as they dont have an accelerator pump and might not run well leaner. I think he is quite experienced. I may try with the needle lower by one groove.

IIRC, you want around 14?

Joachim said:

Hi Steve,

here the AFR check:



I dont know much about the AFR ration. Dyno man said its on the rich side but probably good for the Amals as they dont have an accelerator pump and might not run well leaner. I think he is quite experienced. I may try with the needle lower by one groove.

Click to expand...

I don't know much either, but there are some people here who do know rather a lot about this, so I'm hoping they will chime in!!

SteveBorland said:

I don't know much either, but there are some people here who do know rather a lot about this, so I'm hoping they will chime in!!

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14.7

14.7 is optimal yes… but that’s too lean for safety with most old air cooled lumps.

Anywhere between 12-13 is good as I understand it.

Below 12 is indeed too rich. So your Dyno man is right IMHO and you are too rich between 2,900 and 5,500… which is where you’ll spend a LOT of time!

But, many Dyno operators don’t understand old carbs, they don’t ‘feed the throttle’ they just whack it open to WOT and hold it there. That makes it difficult (ok, impossible) to estimate which carb circuits to change.

Best to mark the twistgrip at 0, 1/4, 1/2, 3/4, WOT and get him to hold the throttle steady, with the AF probe in place. That’ll give a much better indicator.

But also nice to see the Amal fuelling nicely to this HP with 230 main jet.

The effects of air/fuel ratios are probably reverse calculated. i.e. somebody uses a dyno to measure power while monitoring the oxygen content of the exhaust. Then we know the optimum oxygen content of the exhaust to get best power with the fuel we are using. Dyno runs made at full throttle only use the main jets in the carbs. Under normal circumstances, the main jets should always be slightly too rich. However the needles and needles jets need to be as close to too lean, as possible, without doing damage. - 'There is a fine line between pleasure and pain' ?
Always start rich and work towards leaner, with mid-range jetting. And don't change the fuel type, ignition timing or comp. ratio. while you are doing it.

Madnorton said:

But also nice to see the Amal fuelling nicely to this HP with 230 main jet.

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When you are accelerating and changing up through the gears, the main jets are irrelevant. And very few people ride with full throttle for long periods anyway. If the mains are slightly too rich, that is good.

Nigel's 5 speed gearbox was probably a more significant change than all of his engine improvements. Commando engines have poor throttle response, but deliver massive torque. Most gearboxes have gear ratios which are evenly spaced and first and top are usually in the same place in road bikes. 5 gears mean you are less dependent on throttle response.