From: Peter Aslan
Date: Mon, 16 Jan 1995 17:52:32 +0000 (GMT)
I was reading an Old copy of 'The Classic Motorcycle' the other day, it's funny what some of us get up to in bed, and I came across an article titled 'lets have more revs'. In said article there's a set of formulas for all sorts of stuff, and in there was the following:
Where N = rpm, T = measured torque and pi = pi or 3.14 Etc.2pi x N x T
-----------
33,000
This is, unless you have already realised, the formula for calculating BHP. It being a function of the torque of the Engine at a given RPM. Needless to say, or should that be obviously, the higher the RPM, the more the BHP, That number we all refer to in connection with the performance of the engine of our beloved. Also the figure all the manufactures want to improve on, its a bit like sex and the size of your d***.
Anyhow, if you can push the max torque higher up the Rev range, you get bigger BHP. This is discussed with an example in the article as follows:
Lets look at an Engine that pulls 20lb/ft at 4000rpm.
(The result is 15.25hp, take my word for it). If we can maintain the torque figure of 20lb pulled at 4500rpm, the power output rises to 17.14hp - a very valuable gain indeed. Unfortunately its not that simple (wouldn't you just know it !) As crankshaft speeds rise past a certain point, torque begins to decrease. If the torque of our engine at 4500rpm falls to only 17.80lb/ft then there is no gain in power. And there's absolutely no reason to run at the higher speed - especially when the increase in inertia is considered. Yet if the torque pulled at 4000rpm can be increased from 20 to 21lb then the power will rise from 15.2 to 16hp. It can readily be seen that gaining an increase in torque is of more value than a mere increase in rpm. And torque can only be improved by better breathing.
Keep that final sentence in the front of your mind.
I recall the Norton Notes concerning the different Heads that have been used on the Commandos. There was a Change in the size of the carbs from 30mm to 32mm, for the Combat I believe, and the 32mm size was retained on later models, particularly the 850 as I remember. However, although the Carb size and therefore the inlet junction was increased to 32mm the actual manifold on several types of head remained at 30mm. NB. I have this type of head, I have checked the notes. Now, when you put this fact together with the bit about torque and breathing, it makes you start to think about increasing the inlet manifold to 32mm to gain the maximum results from the 32mm Carbs.
Anyone done this ? Any suggestions in this area ?
As an aside, I recall discussing single carb conversions with a chap from Minuki last year, and he said that they had dino'ed (Power Tested) a Commando with Dual Amals and Single Minuki and there was no difference. I wonder which type of head the bike had ?
Peter Aslan (aka Captain Norton).
From: Pete Serrino
Date: Mon, 16 Jan 1995 14:39:22 -0500
Peter Aslan Wrote:
>And torque can only be improved by better breathing. Keep that final
sentence in the front of your
>mind. I recall the Norton Notes concerning the different Heads that
have been used on the
>Commandos. There was a Change in the size of the carbs from 30mm to
32mm, for the Combat I
>believe, and the 32mm size was retained on later models, particularly
the 850 as I remember.
>However, although the Carb size and therefore the inlet junction was
increased to 32mm the actual
>manifold on several types of head remained at 30mm. NB. I Have this
type of head, I have checked
>the notes. Now, when you put this fact together with the bit about
torque and breathing, it makes you
>start to think about increasing the inlet manifold to 32mm to gain
the maximum results from the 32mm
>Carbs.
>
>Anyone done this ? Any suggestions in this area ?
Hello Peter, I have tried similar experiments and so far have failed to improve on my original 30mm 750 setup. First tried dual 32 Mikunis with the supplied 32mm manifolds and enlarged the intake ports to accommodate. Lost 10mph top end. This was probably my first mistake. Changing the intake ports in any way should be left to a specialist. Then switched to 32mm Amal MKIIs and a combat cam. Little increase in top end and lost some low end. Currently running the MKIIs with a megacycle cam and another head flowed by Leo Goff. Still haven't topped 110mph. Originally saw 117mph at 7400rpm (19 tooth CS sprocket).
Torque is dependent on the maximum charge that can be ignited. Increasing flow comes at the expense of velocity which will affect combustion chamber turbulence. I know all of fast Nortons use bigger carbs, intakes, exhausts, etc. but I think the whole system has to be designed together. Simply increasing intake size may not result in more torque unless a new cam is fitted, exhaust changed, or valves re-angled. I believe the combats had 32mm manifolds and ports enlarged. 1/4 mile times were within .1 second of the 30mm bikes.
Cheers,
Pete Serrino
'71 Fastback
From: Dan Milhone
Date: Mon, 16 Jan 1995 10:45:57 -0800
Great info. about the importance of torque.
This probably explains why race car builders spend so much energy on
port matching/head flow projects.
I imagine Robin T. has spent a few hours at a flow bench.
As far as:
-(Power Tested) a Commando with Dual Amals and Single Minuki and there
was no difference.
Personal experience would lead me to a somewhat different conclusion. I've run both set-ups on a stock 850, and the Mikuni did not have the top end of 2 correctly adjusted Amals. The bike felt stronger in the +4,000 rpm area with Amals. The funky all on / all off choke on Mikunis is none too slick. Expensive air-filter, too.
The dyno test may indicate some things, but I can't find any reason to go back. Besides, 2 carbs look cool....
Dan Milhone
'74 Commando Roadster
From: Mike Taglieri
Date: Wed, 18 Jan 1995 02:33:19 -0500
Peter Aslan Wrote:
>There was a Change in the size of the [Commando] carbs from 30mm to
32mm, for the Combat I
>believe, and the 32mm size was retained on later models, particularly
the 850 as I remember.
>However, although the Carb size and therefore the inlet junction was
increased to 32mm the actual
>manifold on several types of head remained at 30mm . . .
>
>when you put this fact together with the bit about torque and breathing,
it makes you start to think
>about increasing the inlet manifold to 32mm to gain the maximum results
from the 32mm Carbs.
Knowing what extraordinary cheapskates Norton were (though maybe blaming AMC is more fair), I wouldn't be surprised if they kept using the 32's because they had a lot of them. They milled the head to increased the compression in the Combat [causing untold harm to the rocker geometry because the pushrods weren't milled as well] just because they had a lot of standard pistons on hand, and didn't want to spring for a taller piston.
If by "increasing the inlet manifold," you mean tapering the thing from 30 to 32, this would eliminate the "step" and probably help. Norton sold/sells(?) an intake manifold that goes from 30 to 32, and you could probably turn yours into that with a bit of file-work [or even better, a stone for an electric drill with a taper like an ice cream cone]. However, if you mean increasing EVERYTHING to 32mm, you're talking about porting the head as well, an operation that can supposedly do wonders for an engine and horrors to your wallet.
>As an aside, I recall discussing single carb conversions with a chap
from Minuki last year, and he said
>that they had dyno'ed (Power Tested) a Commando with Dual Amals and
Single Minuki and there was
>no difference.
No disrespect meant for the nation that drove the UK's bike industry into the ground (with help from the leaders of the UK's bike industry), but that claim is bullshit. Two Amals mounted on the standard manifolds are virtually a straight line into the head, and should give better breathing and a higher top speed than any single carb on a manifold, assuming both are set up properly, because the manifold requires a zigzag path for the intake. (In fact, you get slightly more power by moving the needle out of the way -- that's what Amal did in their Grand Prix racing carbs).
This WAS a man from Mikuni after all, and I've never yet heard a fishmonger yell, "get your rotten fish here," but what he MAY have meant is that there is no PRACTICAL difference for a typical rider. This is probably true, since few of us ride Commandos with the carb wide-open. I can go at speeds sufficient to put me nicely behind bars on about half-throttle with twin Amals, and probably would have quite a bit of wrist left over with a Mikuni also, plus better low-rpm performance and idle. However, if you use your Commando to chase the Ninjas with, I suspect the Amals are the better bet.
Mike Taglieri
Raul -- '72 Commando Interstate