One carb vs duals, theoretically speaking

I think these JS carbs breach the gap from low end torque and top end performance. Flatsides meter better and could be concidered ,at least by me, as the best of both worlds.

I have cam that is suppose to like the mid to upper range, and believe you me, it does. With these carbs I attain a good pull off the bottom as well.

Perhaps a pair of Reed valves would help that problem.?

Rohan said:

comnoz said:

Dual injectors do not equal dual carbs. Air is the concern- not fuel.

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I'm disappointed you saying that - that is utter nonsense.

Multi point injection appeared decades ago - a single injection point is the equivalent of a single carb on a manifold, .

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Sorry Rohan - You are wrong and Jim is right. Air flow is the critical matter NOT how many or what type injector attached to the manifold . The entry aperature of a single manifold, its cross sectional area and length from the inlet valve head concern gas flow efficiency. If a single spray nozzle or multi spray nozzle injector is fitted it is of little consequence to the volume and flow of air into the cylinder. And it is the amount of air captured in the cylinder that dictates combustion energy.

Mick

So are you saying that Jim could achieve the same torque figures and horsepower, or better, out of a single injector as with his dual injectors ?

A lot of manufacturers would beat a path to his door if that were true.....

I think Jim meant fuel injector count has no relation the intake throat count, so apples to horse shoes confusion of terms. Good point about the fuel sprayer location and count as seems F-I is more sensitive/difficult to dial in on each engine map 2D & 3D, than a carb that can self adjust via their mechanical fluid analog processing circuits. Ain't no one size fits all and reason in past for staggered opening multi throats or in two sized staged throats plus accelerator squirters.

Single Carb's are O.K. ,

if you fit a Supercharger .

P.S. Harley abandoned their single injection system and went dual, after a lot of trying...

Now a V-twin doesn't breathe evenly like a parallel twin, but from what you are saying it shouldn't matter ?

This site applies to the subject line and is cool reading regardless not to mention the phallic trumpets sticking straight up.

http://www.classictrucks.com/tech/1005c ... ewall.html



The Hilborn injection systems utilize a Carabine ECU that doesn't require a laptop to tune. It doesn't require a MAP, MAF, or O2 sensor either, and it is quite easy to install. The fuel curve is pre-programmed at the factory and simple tuning requires only a screwdriver.
At idle when air speed is low, the fuel droplet pulled from the booster is much larger than the one pulled when air speed is greatest or wide-open throttle. This has a profound affect on mixture distribution. If a carburetor's venturi is sized for low to midrange torque and strong acceleration, it will be too small to produce top end power; conversely, if the venturi is sized for top end power, low to midrange torque and throttle response will suffer. The correctly sized carburetor for some applications will be a compromise of low to midrange torque and top end horsepower.

The Eight-Stack Manifold "In a test conducted with a common plenum intake against the same engine with a Hilborn eight-stack manifold, with both adjusted to provide the same air/fuel ratio, the eight-stack manifold made more power. There are numerous reasons why.

"Without a carburetor's booster in the way, the eight-stack manifold will flow more air than a comparably sized carburetor venturi. Without the need for a pressure drop to supply fuel for the engine, the bore size of the eight-stack can be increased to supply the required air for top end horsepower, yet have increased throttle response along with increased low to midrange power. The mild turning radius of the eight-stack intake helps promote line of sight for the air/fuel into the cylinder head. Since there is no common plentinum, the cylinders no longer need to digest an air/fuel mixture that is contaminated with pulses from companion cylinders. Since each cylinder is separate, there is no dilution to companion cylinders from reversion, eddy currents, fuel puddles, and the tight bends for the air/fuel to follow causing mixture distribution concerns.

"The eight-stack manifold employs a converging tract design, meaning the larger ram tube top is reduced in size as it enters the lower portion of the manifold before entering the opening of the cylinder head runner. With this design, air/fuel speed is increased when going from the larger opening to the smaller opening, ensuring that the air/fuel stays in suspension, unlike the tract of the common plenum manifold, which allows the air to slow when going from the small venturi of the carburetor to the larger opening of the plenum. Around the point that the intake tract is constricted, fuel is injected under pressure, allowing it to take maximum advantage of the air speed. In the converging tract manifold, most of the fuel that falls out of suspension as the wall is constricted is picked back up and put into suspension instead of being pushed along the walls.

Rohan said:

So are you saying that Jim could achieve the same torque figures and horsepower, or better, out of a single injector as with his dual injectors ?

A lot of manufacturers would beat a path to his door if that were true.....

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NO I am not saying or suggesting that merely providing a correction to your assumption. Keep it at the technical matter and don't go down the Carbonfibre path

But you have constricted the discussion and lost the subject of the discussion....

ML said:

Sorry Rohan - You are wrong and Jim is right. Air flow is the critical matter NOT how many or what type injector attached to the manifold . The entry aperature of a single manifold, its cross sectional area and length from the inlet valve head concern gas flow efficiency. If a single spray nozzle or multi spray nozzle injector is fitted it is of little consequence to the volume and flow of air into the cylinder. And it is the amount of air captured in the cylinder that dictates combustion energy.

Mick

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Air alone does not burn.

This reminds of the story of Jack Williams (father of Peter Williams) tuning the AJS 7R racer in the early 1950s (ohc 350cc GP race bike). The Manxes could beat it, so he did some work including on the airflow bench. What he found was that if he sprayed ink (instead of fuel) into the incoming charge, and tweaked it so that the ink was kept off the cylinder walls as the mix was swirled in, horsepower improved, considerably (when fuel was used !). He gained a +10% torque improvement just from this method, enough to beat the manxes at the time. Rod Coleman won the IoM 350cc TT in 1954....

The discussion has been suitably muddied with the introduction of EFI. EFI is simpler than carbs.... Im sure that you could get a single injector on a Commando to make as much power as two or four, the Commando does not make that much power so the injector would not be stupidly big, hence allowing reasonably precise delivery at lower power outputs, this would be one of the limiting factors in using a single injector in an engine that makes a few hundred hp. When it comes to the single carbs on the Commando the killer has to be the manifold, it is ugly, you can have tuned runners and a single carb which in some circumstances could make more power than individual carbs due to the resonant tuning, you can also have plenums and all sorts of other harmonic devices to make either system work differently. Although you will be limited in carb size due to the air velocity over the jets, EFI doesnt have this problem.

Indeed , The mighty ' column of air ' gathering momentum , to arrive at the valve as it opens , is unobtainable with a single carb , unless its on a single cylinder
or a generous plenum is downsteam of it , which would provide further disruptive effects , so isnt used. Unless upstream of Dual Carburators .For a single carb
a Weber or Dellortto Twin Choke would provide the answer .

With a motorcycle ' coming on the mega ' at 7.500 rpm , with perforated baffles that become incandecent , it acts as a Augmentor .
The suction created by the exhaust energy creates a force that starts the intake atmosphere in motion , combined with the pulse effect of the Dual Intakes
the mass of Fresh Air / Fuel is moved or thrown , into the evacuated cylinder . The fuse is lit and the ouput further enhaced , but its not quite . :mrgreen: :shock: :wink: . Nor is it particulary responsive below 2 - 3.000 rpm , requireing considered use of the throttle and a clear road ahead .

The increased airflow ( Doubled ) the a single instrument of the same size will enhance the responce so that about town manadgeability is enhanced .
But it can only flow half of what two will , so will retrict output at higher r.p.ms. thus is usually tuned , jetted & camed differantly. Not the thing for a
Combat or High r.p.m. Camshaft . The stock S Commando cam being the Dominator S S , may be in advance of its requirements .

For the Triump interplod , the Saint , the Bonneville inlet cam is advanced , something a bit hard to do on a Commando . The powerband of the Triumph
is definately Flat in the last 1.000 r.p.m.s plus , even fitting a 36 mm to a 750 .The benefits being obtained by ' ram induction ' not being fully available .

As said , for touring , this is no bad thing , if the carburator is of adequate capacity .Fitting a undersized single carb is a good dodge to get a flexable engine with good responce that is near incapable of wearing itself out through excessive heat generation and R.P.M.s ,presumably some idiots still capeble of wrecking it though . :roll: Just wait till I get my Hands on . . . :cry: WHEREES my BOOTs . :evil:

Cheesy said:

The discussion has been suitably muddied with the introduction of EFI. EFI is simpler than carbs.... Im sure that you could get a single injector on a Commando to make as much power as two or four, the Commando does not make that much power so the injector would not be stupidly big, hence allowing reasonably precise delivery at lower power outputs, this would be one of the limiting factors in using a single injector in an engine that makes a few hundred hp. When it comes to the single carbs on the Commando the killer has to be the manifold, it is ugly, you can have tuned runners and a single carb which in some circumstances could make more power than individual carbs due to the resonant tuning, you can also have plenums and all sorts of other harmonic devices to make either system work differently. Although you will be limited in carb size due to the air velocity over the jets, EFI doesnt have this problem.

Click to expand...

We'd just have to keep the r.p.m.s up then , and get the gearing up to get the economy . And at that speed , just think of all the time we'd have , as we'd get there in half the time , otherwise a Supercharger should give better mixing / distubution , through the turbulance , This'd be quicker still and well have to be looking for other things to do, as we'll have so much time left . He'll be saying " a computor says so " next . :wink:

:idea: :roll:

Matt Spencer said:

But it can only flow half of what two will , so will retrict output at higher r.p.ms.

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Do you want to put money on this, Matt ?
This clearly is not right.

Plenty of bikes, with fairly low output admittedly, have come out with either single or twin carbs, and outright speed is very little affected. So the single carb version must be flowing near the same as both of the twin carbs combined...

The Norton Mercury and Norton 650SS being recent(ish) examples, the Mercury was standard with a single carb. I don't have a quoted top speed handy for the Mercury, but it wasn't far behind the SS.

P.S. Remember that most (brit) bikes that sprouted twin carburettors had initially only had one, so that the size of carb chosen gave adequate flow in single carb form. For whatever reason, (marketing, sales blurbs ?) when twin carbs were added the choke sizes remained the same - but for flow purposes could have been reduced in size.

??

Cheesy said:

When it comes to the single carbs on the Commando the killer has to be the manifold, it is ugly, you can have tuned runners and a single carb which in some circumstances could make more power than individual carbs due to the resonant tuning,.

Click to expand...

It would be interesting to play with this and see what was required to make it work successfully. ?
And what range of revs it worked over, could be rather limited ? Unless you have variable length manifolds, electrically even, like some race motors...

A reminder that the first Model 7 Norton Dominator (500cc 1949) had a long bifurcated manifold in the head that appears to be shaped along the lines you mention. When it was tried on the road it was dreadful apparently, it delayed selling this new model until they figured out a fix. Which was cutting off the long manifold, and fitting the U-shaped manifold like all later single carb Norton engines used. (until the studs changed from vertical pairs to horizontal pairs anyway.).

Cheers.

Doubt it pulled cleanly past 8.000 r.p.m.s though . :shock:

Carburators are usually rated in c.f.m. ( cubic foot per minute ) under std. atmospheric conditions .

two = 2 x 1 , 4 = 4 x 1 , and 3 = 3 x 1 , Trident . 3 x 30 mm Magnesium Concentric , with the offset stacks so the float is vented to atmosphere .
250 c.c. x 3 , 10.500 . at what biggles and cohorts designate " Through the Gate " 10 Mins maximum .
Combat Rateing , overhaul engine Reqd .

But you get to keep your aeroplane , we hope . Theres also speed rateings for things like undercarrages , usually under 200 then , or they blow away .
Flaps it was about 150 . However , activateing them at high speed , could drag speed of considerably and instantly ( if it didnt fall to pieces ) alowing
you to pug the sucker up youre ass , as he whistled past . IF you were a ' crack shot ' and didnt muck about .

So , with our little ' infernal combustion engines ' , some colt is always useing the throttle as a switch . OFF or ON . The harmonics in the later position
generate lots of intresting resonances and shock waves , and can be heard miles off . ( pity I didnt record it ) .

This , I believe , is why Norton did away with the Combat . LIABILITY .The likelihood of certain types to knock hole in the Countryside , or observor's
was somewhat mind boggling . Particularly as profits had erroded and manadgement was not permanantly inebriated at this stge . :?

back to our Squadron Chums . :lol: a Aeroplane cruiseing along at full thrttle ( not emergancy , through the gate ) indicates say 200 airspeed at 30.000 ft altitude.

this is about 400 / 450 true.


Sq Ldr , Surveying rubber marks on tarmac , holes in headges ,
The general drift is , sevre use may well have severe service ' complications ' .

A relaxed reliable road machine , and a relaxed reliable ' Combat ' ( RACE ) Machine , would appear to differ in several respects .

Current tecnology is still falable , as you just said . " It Wouldnt Work , Unless " .

So with all due respect , It Was the Same Then . Stuff with it At Youre Peril .

The majority unfortunately did not emulate the successes of the few .( back in the previous era , 40s , they had Bombs Dropped on them . :shock: )
There were many superb pre war machines . all what would be Called , in the 70s Jargon , ' Tool Room Specials ' .

Now , the experts alledge , " todays production standards " ( youll still pay through the Orifice for todays grand Prix & the like , Machines )

" Are Vastly Superior " well , I should hope so , but still , looking at a lot , the manner of construction , is engendered to Volume Production .

If You want something hand built , Hand Drawn , and manufactured by hand , as you appreciate the Artistry and Arcitecture of it , Blokes Like Us ,
are looking at stuff thats origans were concieved in the ERA I was drawn to in the analogies .
Wot Ho .

The typewriter ribbon appears to have gone . ! bother . Theres many a bloke spent many a day sorting them , as evident from the links .Use the Correct Tools . If youre starting off . Optimum outputs of configurations , are another story . Merlin went from under 1000 hp to 2200 hp ,This is not untipical in a
design over a period of time , though remidies , 20 years after the fact , are easier to spot . The XR 750 is still available , at 100 H.P. @ 750 c.c.

The Comando was Sorely Tried , at F-750 Level , but to deride it as being obsolete is the result of ignorance and closetted thinking .Theres development
left in it still , though ordinarilly by this stge , itd be a Jeffersons Axe , on Paper .The design would have been altered , refined and honed ,
retaining its inherant advantages , Not Discarding them .Though in 3 Generations ( youres and mine ) since its arrival , its fair to assume
that things wouldnt be the same .

for the next exciteing installment ./ . . :mrgreen:

Rohan said:

comnoz said:

Dual injectors do not equal dual carbs. Air is the concern- not fuel.

Click to expand...

I'm disappointed you saying that - that is utter nonsense.

Multi point injection appeared decades ago - a single injection point is the equivalent of a single carb on a manifold, with all its inherent fuel distribution problems - the problem is that the fuel drops out of tha airstream when the airstream dithers about which cylinder its going to. As someone says, the resonant pulsing is lost. Multi carburettor engines show much better mid-range torque on a dyno (cylinder filling - BMEP), thats why the acceleration is so much better on a multi carbed engine.

If multipoint injection was no different to single point injection, why would anyone go with the more complicated system ?. Yet you will barely find a road engine using single point injection, let alone a race engine.

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And that is why I said I used two injectors. So I could adjust the amount of fuel going to each cylinder individually and insure a good mixture in each. However twin injectors do not influence how well the cylinders fill with air which is what determines how much power can be made. A single throttle body will flow and charge the cylinders with air just like a single carb will. There is certainly an advantage to injection in that you do not have to be concerned about going too large with the throttle body and killing the ability to carburate at low velocities. The twin injectors just let me get the same mixture in both cylinders. That is something that a single carb or a single injector does not do very well because of small variations in porting, cam timing, compression ratio, and exhaust sytem tolerances. Jim

comnoz said:

However twin injectors do not influence how well the cylinders fill with air which is what determines how much power can be made.

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Not entirely. Reference my note above about Jack Williams experimenting with spraying ink into the airstream, and swirling it in through the port into the cylinder without the ink getting on the walls (or a minimum of, anyway).

Fuel distribution in the fuel/air charge is equally important to making good power. Or air compressors would win in the power making stakes !!. And why twin carbs make better torque than a single carb on a manifold, all on full throttle, since the fuel distribution gets less disrupted ??

As someone said above, if air was all there was to it then the biggest ports and valves would win.
And since this is most definitely not true, there must be more to it.....