Dyno Run

A friend and i had our Commando's on the Dyno today.
In his case to do a baseline run before making some mods. Currently a stock 850 except for a single 34mm Amal MkII, boyer ignition, 750 type headers and mufflers with minimal baffling.
In my case to check the tuning of the FCR carb's, currently set to BrianK's settings off this forum. The bike also an 850, with a fullauto head, pazon altair, mufflers are claimed to be a replica of the original Norton ones but i cant confirm or deny that claim, 750 type headers.

The initally lower curve peaking at 48hp is my bike, not that impressive at present. Briank's settings are pretty close on the fuel/air ratio plots i am told.
The middle curve peaking at 39hp is the single Amal bike, the air filter was restrictive and it showed a couple of extra hp with that removed.
The final plot is my bike with with the free-er flowing mufflers off the other bike fitted - wow. The carb was then very lean over the full range, so perhaps the original jetting would be nearer the mark in that case. We didn't alter the carb tuning since they weren't my mufflers.
My bike misfired from 5000rpm, hence the jagged curve at that point - the suspicion is exhaust rather than electrics related.
Yes i will be shopping for a new exhaust.

That jagged bit of the curve looks more electrical to me. I'd be surprised to see an exhaust make a misfire come on so suddenly and then go away so quickly leaving the curve looking like it's missing a bit in the middle.

Good to see some real runs.

I have used dynos a lot to tune my race bike.

Some observations.

dont get hung up on the hp numbers. If you go to another dyno down the road you could easily see plus or minus 5 hp.

Second 48 BHP rear wheel is actually a good number for a standard 850. There is a lot of posing about inertia dyno hp numbers.

That misfire looks a bit electrical to me as well. Also check that your not going through a vibration period at that rpm.

I have open megaphones with reverse cone on my bike but did try some Commando mufflers at one time and they did well. Especially in the mid range which is where you want it on a street bike. So a good well made set of standard mufflers could suityou well if you are looking for good flexilbility street riding.

Once you have solved that misfire problem you should concentrate on carb tuning, cam timing, ignition timing optimisation. Based on my experience you could expect 5 bhp just by getting these things exactly correct for your bike. Standard settings are not alway the best.

advancing the cam timing say 5 deg can be good. Whats your compression. If it is higher then try retarding timing to about 28 deg deg advance. Aircleaners, bellmouths, extended inlet tracts etc can all help a lot.

johnm said:

.......... Also check that your not going through a vibration period at that rpm.

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Good call. That's where I'd be looking.

ntst8 said:

My bike misfired from 5000rpm, hence the jagged curve at that point - the suspicion is exhaust rather than electrics related.
Yes i will be shopping for a new exhaust.

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I had a similure issue with my 40mm Pumper. Went down 2 sizes in the main and smoothed it right out. This was after numerous attemps to increase the main to no avail. So much for logic.

So much for the assertion by some that a single carb gives equal or even greater performance than twin carbs......

worntorn said:

So much for the assertion by some that a single carb gives equal or even greater performance than twin carbs......

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Hijack alert!
This assertion is still valid by my experiences.

A single Amal, 34mm MK2 to be exact, was always one of the worse of many scenarios.
After experimenting with them, I have since sold my twin 32mm Premiers and have reverted back to the tried and true awesomeness of the single TM40 (aka HS40).

I may not be an authority, but I sure as hell know what i have. This is not a willy nilly experiment but an effort that has taken many years to attain.

Even if I post a dyno run in the mid to high 50's, and maybe more, the sceptics would deny it and the purists could never except it and besides that, it appears to be too risky for any others to try.

A high performance single carb upgrade kit? It just doesnt sound right, does it? Oh well, there you have it.

Ha! Didn't take long to flush out the single carb heretic! :mrgreen:

worntorn said:

Ha! Didn't take long to flush out the single carb heretic! :mrgreen:

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Heretic? Don't you mean couragous explorer or perhap one who goes where no one else dares? An optmist. A seeker. Norton tuning as an artistic medium.

I think those who accuse of heresy are closeted. Tis a pitty! :mrgreen:

So much for the assertion by some that a single carb gives equal or even greater performance than twin carbs......

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thank you...

fact: it is not a question that a single does not flow enough gas as a twin, but it absolutely cannot flow
as much AIR at upper rpms as a twin carb setup

even after repeated dynometer comparisons by Comnuz and others including myself, and even after
staring at the fact that the guys racing Commandos do NOT use single carbs....

even after all this, there are still a few, granted very few, that still cling to denying the laws of physics

two plus two will always equal five in their minds, and all them Elitist world class math people are just wrong

It's strange some have not tried one large nostril rather than the two they were born with.

Thanks for the feedback.
The aim with both of these bikes is usable street bikes so the peak hp is not of great concern. My initial aim with the Dyno run was getting the tuning right on the FCR's.
The interesting thing for me was how much difference a change of muffler could make. I think Joe Seifert has said in the past that he runs standard Andover peashooter's on a race bike so i'm thinking mine aren't as good a copy as claimed.
At the moment the single Amal would be the better bike for everyday street use with the power coming in nice and early, but his mufflers are too loud for my taste.
When you say a vibration period - what are you suggesting i check?
Comp ratio is now standard for an 850. I have assumed a standard cam also, but when i bought the bike 30 yrs ago the RH4 head had been planed (i had a lot of grief with that so treated myself to the FullAuto head) and it had a full Dunstall exhaust (earlier version not the 2-1-2) so it is possible that there is a different cam in there too.

'I have open megaphones with reverse cone on my bike but did try some Commando mufflers at one time and they did well. Especially in the mid range which is where you want it on a street bike. '

I suggest that is where you want it on a commando based race bike too. I haven't raced my Seeley 850 that much, however the major problem has been gearing, not so much about getting an increase in top end performance. There is an old saying - 'torque wins races', and I believe in it implicitly. I noticed the comment about advancing the cam, mine is advanced 12 degrees however I am using a two into one exhaust. I suggest the matter of gas inertia in the two into one exhaust has an effect when you consider that the gas in the tail pipe must vibrate at twice the rate of the individual header pipes. When you use separate pipes with either mufflers or megaphones you are playing a different game with the potential to make the bike savage and more difficult to ride. There is a friend of mine who is currently racing a Molnar Manx. At Goodwood this year he used a megaphone and finished 16th against the Brits however ahead of Wayne Gardner. Last weekend in the state historic championships he had four wins from four starts. His comment was that these days with the muffled exhaust, the modern manxes are much easier to ride fast. He is a guy who raced an Atlas in the 60s, and progressed to sponsored racing in A grade on a TZ700 Yamaha. He has also mentioned the difference when they started running the manx on methanol, he said it pulled from 2000 RPM instead of 4,000 RPM. It is something I did not experience with my commando based bike, I never use petrol in a race bike unless it is a two stroke, and even then if you want to win with one of those it is worth doing.
Incidentally, I've used a single carb on a race twin, I think the only advantage is you only have one lot of jetting to play with. I look at the inlet tract, combustion chamber and exhaust as being all part of the same vibrating system, - with a single carb, it must vibrate at twice the rate of an individual one of a pair, so the inlet length should be dramatically different ?

1up3down said:

fact: it is not a question that a single does not flow enough gas as a twin, but it absolutely cannot flow
as much AIR at upper rpms as a twin carb setup

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OK!

However, the motor will not run on either alone. I submit the a single carb of a certain nature can deliver a mixture at high RPM that will perform as well if not better than twin Amals. Now if you speak of twin FCR 35mm flat slides then I believe that these can be overkill and require jetting down to run well.

Please concider that we are not talking about racing motors here, but it is not out of the question. This is about a smooth operating road machine that runs like a bat out hell, particularly at higher rpm's.

If my twin 32mm Amal Premiers would have performed as well as the single TM40 then the later would be on the shelf, Period! Not to mention the PWK things, Long gone!

My main testing zone is WOT from 80 to 90 mph in forth gear. With my current gearing, 4000 is 70, 4500 is 80 and 5000 is 90 give or take a couple points. Running up to 90 in third where rpm's run up into the 6's is sort of overwelming.

But these are just numbers and we all know how we feel about numbers. I am alone with this with no others who have tried this senerio.

Can 2 carbs deliver more air. I suppose! Can a single 40mm flatside deliver an optimum mixture all the way to redline and beyond on a 750 Commando with a stage 1 cam and a massaged RH1 head and 1 1/2" exhaust (not to mention stump pulling torque down below)? Well, who gives a crap what I think or know.

Here is the point. I don't expect anyone to believe everything they hear particularly in this regard and what would be the point in lying, but to go so far as to say to someone that they' are full of shit because the numbers do not add up is just .....I'm not sure what the right word is. 2+2 may add up to 4 but I think there is a little more to it than that.

Just because JimC thinks he can blow more snot out of his 2 nostril does not mean someone else cannot blow more out of one big one, but that's just Jim.

And just because what I now to be true to me doesn't match ones math doesn't give one a right to judge. You may think I am wrong, but you do not know it.

ntst8 said:

......
When you say a vibration period - what are you suggesting i check?
.....

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Essentially, check that vibration is not shaking an electrical connection loose.

worntorn said:

So much for the assertion by some that a single carb gives equal or even greater performance than twin carbs......

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Knock knock knock, anybody home in there?
Actually reading what someone says before commenting on it goes a long way.

FYI the bike that made a bit more horsepower has a $3000 replica cylinder head on it, which makes comparison between the two bikes at least silly. And that is beside the fact that the owners of the bikes seem to know little about them, so we certainly know even less.

As far as single carbs go, funny how Nascar guys used to get 2 bhp per cubic inch out of pushrod chevys with one carb on one plenum feeding eight cylinders, so why can't anyone? To dismiss it is simply ignorance.

Actually, the textbooks say that one carb per cylinder gives maximum acceleration (torque in the mid range), but one carb per engine still gives the same top speed.

This rule works pretty well, when you look over all the engines ever made.
All other things being equal...

Very emotional what works best in a particular combo. I'm more in the big single camp myself even if it don't make more power anywhere it sure made me use caution on low down throttle snaps and pulled faster than I often wanted per wrist twist and part the reason I got a bit handy on leaning break frees and some care to snick 2nd w/o tire spin hesitation. Peel will be a single 40 mm with and w/o a belt applied but also 2-1 fat exhaust. Would take the same dyno and conditions or one location drag times in similar conditions to know what worked best. If not for ongoing non Norton time/expense recoveries I'd already gotten a device to put on our bars to record the G's per mass over time in real life for a sense of which way to go on a particular cycle. There are a lot of tales of best dyno tunes going sour in road contests d/t various other factors applied. How boring if we all had to build to suit everyone's approval. A pumper single sure helps make up for lack of draw on its bigger throat till its bigger throat flows like crazy. The 1>2 manifold design might come in to play too.

Vibration resonance can foam carb fuel feed and/or give intermittent electrical contacts, or shake air intake or exhaust seals to leak just then.

On New Zealand roads 50 HP is plenty as we can only do 100kph, as for the one carb, are not the pistons taking turns sucking? :roll:
Hey NTST8, those leather throw over bags with Norton embossed on them...you want them as I'll never use them.Rather then go to a good home that stored under the house.

72Combat said:

.... as for the one carb, are not the pistons taking turns sucking? :roll:
....

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I believe it's a bit like a two into one exhaust where pressure waves can work for or against you. It's simpler to get it right with one carb per cylinder.