One carb vs duals, theoretically speaking

An example of a manifold and head port that is a compramise in its original state, and most are, and what can be done to improve what is there.
This flowed a little less than 80 cfm with a 40mm Dellorto and 42mm valve as it was originally.
After adding material and changing the internal shape it flowed a little over 100 cfm, which proves bigger is not always better.









graeme

Aw come on guys, lighten up, Navy Seal P+++ hard ball .45 acp, as example of a bigger single throat's hi power punch. Nicer than a plain bolt and closer at hand to to grab to prop up Lake Injector slide. Fuel Injectors don't need no stinking venturi restriction like carburetors do. Kind of scary to me though so will start out slower with a single 34 Miki carb. Drouin adds about 15 lb mass, water injection another 10 or so but I want to try to get an off road trials bike version of Peel below 300 lb. Most the Lake Injectors were sold and still used on non blower applications. Will add a Mad Max on/off lever/handle like the movie.

[video]http://www.youtube.com/watch?v=KAROcbv4gco[/video]
[video]http://www.youtube.com/watch?v=PRtymuRMZiE&feature=related[/video]

GRM 450 said:

After adding material and changing the internal shape it flowed a little over 100 cfm, which proves bigger is not always better.
graeme

Click to expand...

Nice example Graeme, pity its not Norton so we learn something...
(Ken and FullAuto have previously shown some worked Norton port shapes)

"which proves bigger is not always better."

That saying is extremely well known to folks who do good work gas flowing heads.
And erasing all the work of well-meaning home tuners. ?

Organising the flow so it goes where it needs to go is WAY more important than just big ports going nowhere. The gas needs a smooth path into the cylinder, without leaving a trail of liquid gas on all the port walls, and the gas flow not bouncing back into itself to hinder further flow forwards...

hobot said:

Who into Commando's want a pussy cat economy cruiser only when ya can have both at once, which I think is the point of this subject. A single carb/throat should have straightest shots into the head so the split in path should not be right at the head as most our 1 into 2 manifolds do, with a pair of allen head bolts cluttering it up right there to boot. Here's what Tom Drouin came up with.

Click to expand...

Can't quite see in this shot how the manifold splits the gas flow?

Supercharging is not a necessarily a good example to follow for gas path flow anyway - because the gas is forced under pressure, the flow path doesn't have to be optimised or even good - the gas has no alternative but to flow there anyway.

Under atmospheric flow conditions, it has to be coaxed and encouraged and optimised to flow...

Cheers.

I'm hanging with ya Rohan, I very aware the Drouin issue/features don't really apply to this post or most everyone's C'do including my delightful Trixie Combat. If I didn't have the Drouin I'd go with JIm's dual carbs and about anything else he offers, then a Comstocke Fullauto head and a dialed in 2-1 exht>megaphone. If cost and time no limit Comstock's fuel injection could over power carb or carbs.

But I want to out squirt the current and coming crop of squids and bait fish Ms Peel has in her sights. So will just blow though restrictions and harmonics.

hobot said:

Who into Commando's want a pussy cat economy cruiser only when ya can have both at once, which I think is the point of this subject. A single carb/throat should have straightest shots into the head so the split in path should not be right at the head as most our 1 into 2 manifolds do, with a pair of allen head bolts cluttering it up right there to boot. Here's what Tom Drouin came up with.

Click to expand...

Steve, for atmospheric flow, the closer the split in the manifold is to the inlet valves, the better a manifold will perform ? When each valve opens, the vacuum wants to be able to draw straight away from a source of fuel/air, not have the vacuum travel back a long separate tunnel to the carb to start getting the flow it needs.

Gas flow paths are not all intuitive - this is what the early Norton Dominator taught folks - its long y-shaped manifold back to the carb didn't work at all well, it needed a short inverted U-shape manifold near the ports to work. Supercharging don't care...

Rohan said:

hobot said:

Who into Commando's want a pussy cat economy cruiser only when ya can have both at once, which I think is the point of this subject. A single carb/throat should have straightest shots into the head so the split in path should not be right at the head as most our 1 into 2 manifolds do, with a pair of allen head bolts cluttering it up right there to boot. Here's what Tom Drouin came up with.

Click to expand...

Steve, for atmospheric flow, the closer the split in the manifold is to the inlet valves, the better a manifold will perform ? When each valve opens, the vacuum wants to be able to draw straight away from a source of fuel/air, not have the vacuum travel back a long separate tunnel to the carb to start getting the flow it needs.

Gas flow paths are not all intuitive - this is what the early Norton Dominator taught folks - its long y-shaped manifold back to the carb didn't work at all well, it needed a short inverted U-shape manifold near the ports to work. Supercharging don't care...

Click to expand...

Actually forced induction will benefit massively from having well designed manifolds and ports, the problem is, well with turbos anyway that in the chase for more power it is simpler to just wind up the boost, exactly the same flow dynamics apply for a forced induction engine as they do for a natural aspirated one, it is a given that air density and temperature will be different, this will also happen when you go for a ride in the alps as well though.

What we are debating is the volumetric efficiency (being the ability of the cylinder to fill itself) of the engine, well more specifically how manifold design effects the volumetric efficiency. It seems that most people stop at using a flow bench to optimise their port design, I feel this is not much more than a waste of time given that the actual flow is not steady state (as in a flow bench) and the actual VE will be strongly influenced by dynamic/transient responses. For example the U shaped manifold may have been the correct length that there was a reflected wave from the closing inlet valve at the junction while the other inlet was opening, there is the possibility that this even 'supercharged' the opening cylinder in much the same way as a 2:1 and or megaphone exhaust can significantly lower the cylinder pressure over certain speed ranges

Cheesy said:

Actually forced induction will benefit massively from having well designed manifolds and ports,

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Definitely.
BUT, with surpercharging and turbocharging, the blower is usually down there, and the ports up thar, and you need a freeeway of plumbing to connect them. Not optimal.

You sometimes see the blower right behind the cylinder, and about 2" of plumbing connecting them. Be interesting to know if thats better, or doesn't give the flow path enough time to sort itself out. No plenum capacity either, how does that work.

[video]http://www.youtube.com/watch?v=hbVKRLFW2tg[/video]
A single 34mm Mikuni VM works well on my Mk3. Reliable easy starting, good gas mileage and low end punch. Not to mention a nice smooth idle.
>OK

Here is a dyno graph of a buddies 880 Commando. It is the green line. It is a twin to my engine, same pistons, cam and head and built by me. The difference is he has twin FCR flatslides.



Here is an older dyno report from the same dyno on my engine. Since this was made I have replaced the 42mm body with a 47 mm body and done a little fine tuning and gained around 5 more horse.





Looks like my long Y manifold doesn't suffer too much. Jim

comnoz said:

Looks like my long Y manifold doesn't suffer too much. Jim

Click to expand...

Can we see a pic of this long y manifold, or setup ?
Obviously the difference is in the detail.

BTW, for gentle street riding below 4500 rpm ( if anyone does that), that red graph is the strongest.

The manifold is the one shown on page 5 of this subject. There are many hours of developement in that manifold.

The red graph is Bob Hermans high compression 880 with stock MK3 cam and head and 32mm Amals. Same pistons as my bike and also built by me. It is indeed strong through the midrange.

The bottom graph is a stock 850 with a single 34 flatslide carb on a MAP manifold. Jim

Here's more views of manifold end.



To rephrase my throat sizing remarks - in regards to maxed out rpm power, what was found on the single carb/throat/jug - the cfm ratting had to be higher than calculations implied or practice dictated, by the piston volume x rpm. Ie: if an engine with 4 100 cc jugs could be feed well by a 400 cfm carb, going to stacks would require each stack to flow over 100 cc to feed as well. This is not saying the single stack couldn't be sized even bigger to surpass the single 400 cfm carb. This is not d/t to the ram air effect of straight shot intakes but the lack of any plenum excess to draw from.

I can see the principle behind a manifold divided close to the valve opening vacuum, but also the inertial to change direction and still keep fuel suspended.
In my meager Norton experiments by far the best power I had on Peel Combat was with the single 34 MIki with the curd rough bolt heads on either side of the cleavage. Yet up to the point I shot out the supertrap end plate on Dunstall long dong mega I was sorely disappointed and glad I had a 70 hp/360 lb SV650 to actually have some fast fun with others. After the exhaust matched the rest no way on Gods grey tarmac could I even conceive of taking on Peel with a mere SV-wheelie prone- corner cripple, nor inline 4's pissed off to dangerous risk taking.

Hard to say what combo works best till tried and tested. I found small head ports were needed in Peel too or she became less than everyone else. Some day I will have to reproduce Peels combo in Trixie as have all the kit on hand, standard 28 mm head, single Miki carb and the 2-1-megaphone. But Trixie is un-linked so know better than hold WOT through 35 mph 270' sweepers past 75-80 on lumpy surface. Good sports bikes accelerate over 90 on this dangerous tester. They could not catch Peel there though, where she gained up more than they caught up in the prior straight.

Well, we seem to have clearly established that a properly sized and set up twin carb WILL flow more, produce more horsepower, and accelerate faster in the higher rpm range, say above 4500 than a single MIkuni.

It MAY also be true, but not (yet) factually established, that the twin carb set is also stronger as in faster accelerating than the single, even right off idle and all they way up, rpm wise.

At least, there is no evidence presented yet that confirms that the single mikuni produces more torque, or lower rpm muscle than the twin set up.

But there remain verified dyne evidence that the twin set up produces more hp and out accelerates the single carb.

Thank you, Jim, for providing conclusive evidence.

1up3down said:

Well, we seem to have clearly established that a properly sized and set up twin carb WILL flow more, produce more horsepower, and accelerate faster in the higher rpm range, say above 4500 than a single MIkuni.

It MAY also be true, but not (yet) factually established, that the twin carb set is also stronger as in faster accelerating than the single, even right off idle and all they way up, rpm wise.

At least, there is no evidence presented yet that confirms that the single mikuni produces more torque, or lower rpm muscle than the twin set up.

But there remain verified dyne evidence that the twin set up produces more hp and out accelerates the single carb.

Thank you, Jim, for providing conclusive evidence.

Click to expand...

Not really, three high compression 880 commandos, two with different cams by the look of it make massively more power than a single 34mm mikuni. Of the two highest power examples one has twin FCRs and the other has a single throttle body and EFI, both look pretty close to each other in terms of power. To make a valid comparison we need a stock 850 with amals or maybe Jims carbs (the FCRs are kind of in a different league price wise) to be run on the same dyno, Im guessing the amals at least wont make much more power if at all

comnoz said:

The manifold is the one shown on page 5 of this subject. There are many hours of developement in that manifold.

Click to expand...

Pardon me for asking if this is out of line, but why are we making all those different combinations of carbs and manifolds if they are not being offered for sale ?

i.e. where can we buy that manifold ?
Does it need a 45mm to make it operate - that seems a hefty increase over stock.
Would a slimmer version with a (smaller) amal / mikuni / delorto do anything similar ?

What doth it do for fuel consumption. ?
Yeah I know, questions questions, but enquiring minds like to know these things ?

P.S. prototype for EFI doings ?
Cheers.

hobot said:

Who into Commando's want a pussy cat economy cruiser only when ya can have both at once, which I think is the point of this subject. A single carb/throat should have straightest shots into the head so the split in path should not be right at the head as most our 1 into 2 manifolds do, with a pair of allen head bolts cluttering it up right there to boot. Here's what Tom Drouin came up with.

Click to expand...

Looks very nicely done.............what sort of times does the bike run at the strip in comparison to Jap fours and Italian twins?

Just for reference, this is the alloy manifold that Nortons came out with for the 1949 Model 7 dommie twin. This replaced the long Y-shaped manifold inbuilt in the iron head, that was apparently unsatisfactory.

All early dommies had the studs above and below, not in a horizontal line.
(Why did they change this setup ? Does it help fitting twin carbs ?)

Rohan,
The manifold was developed for the fuel injection system and I have a limited supply of them. I have only been supplying them with the fuel injection.

I don't know if I will make them available for carb use for several reasons
.
1. they can't be used with a slide type carb as there is not enough room and a tall carb will hit the backbone tube.

2. The only carb that will fit is the Keihin or S&S carbs designed for a Harley in the 45 to 50 mm range.

3. A big carb is needed as it is designed to be part of the plenum. When a smaller carb is used the velocity of the
air exiting the carb causes major turbulence at the split.

4. Further refinement will be needed to get the mixture even in both cyclinders. I think that is due more to carb
design than manifold design as the ports are a mirror image but the setup needs more work.

5. I have been doing the casting myself and can build enought for the limited fuel injection market. I would need to
turn the casting over to a casting facility if I were to up the quantities.
Maybe someday.... Jim

Looks very nicely done.............what sort of times does the bike run at the strip in comparison to Jap fours and Italian twins?

Click to expand...

Hehe who the hell knows yet le Carbono, [do appreciate your kindly slapping me attitude here] but you and similar sneering nay sayers are directly in Ms Peel's sights to find out dramatically. I want to break under 10 sec 1/4 mile ET. What I've already found out in spades is new bikes that can break under 11 sec 1/4 miles can't corner worth a damn compared to tri-linked Peel. So my goal ain't highest top speeds or best bee line sprints, its staying in dragster acceleration into and out of turns. The extra fun part will be trying to do this with vintage technology. I think Peel's 130 narrow tire will be the limiting sprint factor, but if I can believe the online calculators Peel/me will have equal or more torque lb ft per total mass than a lot of the elites out there. What I've found is the torque hook up accelerates ya while the hp determines how long that pull can continue.

There is also the wheelie prone-ness I ran into contesting with '04/'05 liter bikes. They'd pop up and have to back off as Ms Peel just leaked ahead of them into turns. Really blew my mind as it should those reading it here. I never popped wheelies on my SV650 until I had a flat on Peel and just jumped on SV to go back to work. My seat of the pants was still programmed on Peels pull and pull downs so almost crashed the first time I pulled down on SV for blind crest and damn tail lifted instead of slowing like Peel. Then the next shocker was entering easy sweepers at Peels mere commuting speeds to have my suspension upgraded SV on non DOT fat race tires skipping out in shock to recover and back off. UGH. At this point I'd already had Codes corner school under belt and thought as everyone else did - no way a quaint C'do could match em in any way but looks and sound.

Ms Peel must also run good w/o the Drouin so gives opportunity to experiment with various carb on Big D's dyno and my own data logging. Much as it may confuse you I'm not really that trilled anymore by road race acts on Ms Peel, she's so stable secure easy to control on tarmac there's little adrenalin rush compared to trying to handle the harder- weirder off road stuff. Another shocker to me was playing with trials and MX bikes, to find what helped off road paid off on tarmac in spades. i've got pair of 32 mm Lectrons, a 38 mm QuickSilver, a 34 Mikuni and 932 Amals to play with - if I live that long.















The real test of bike and pilot is the corner handling,