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Why duel carbs?
- Thread starter Lonnie
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Bruce Bowker
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Not only MG but dozens of cars use twin or even triple carbs. And then of course there are the V12 cars with as many as 6 Webers.
Can't answer your question as to exactly why. More input more power. Designer/engineer's choice? Availability of larger carbs? Philosphy of the era?
Many twin carbed cars went to a larger single later on.
Bruce
Can't answer your question as to exactly why. More input more power. Designer/engineer's choice? Availability of larger carbs? Philosphy of the era?
Many twin carbed cars went to a larger single later on.
Bruce
D
Deleted member 3577
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Good question, Maybe it is because it allows the cars designer to lower the bonnet line. Most British sport cars I have worked on have very little space between the underside of the hood and the valve cover/ air filters ( in fact the Elva bangs its K&N's if it runs on and it sounds pretty awful )
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aerog
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I'm going way out on a limb with this - and it isn't exactly an answer...but ideally wouldn't it be better to have one carb per cylinder (I think some race cars were/are set up that way?), that way you can tune each cylinder for maximum power/efficiency/etc. As it is having one carb for four cylinders is a huge compromise as you're inevitably going to have one or more cylinders leaner or richer than the others due to all kinds of factors. Having one carb per two cylinders like the MG gets you a little closer.
I know with a 4-cylinder aircraft engine and one carb you can lean the engine down until one or two cylinders aren't really firing anymore but the others are happily firing away. With a properly set up fuel-injected aircraft engine (with the right tuned injectors) you have all cylinders working equally.
I'm sure there's a more practical reason - just a thought though.
I know with a 4-cylinder aircraft engine and one carb you can lean the engine down until one or two cylinders aren't really firing anymore but the others are happily firing away. With a properly set up fuel-injected aircraft engine (with the right tuned injectors) you have all cylinders working equally.
I'm sure there's a more practical reason - just a thought though.
Nunyas
Yoda
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good analogy Aerog. I never thought of it before but modern day multi-port fuel injection would be similar to having an individual carburetor for each cylinder in your engine. However, I think you hit the nail on the head with better tunability. With a single carb setup you have to rely on the efficiency of the manifold to split the air-fuel mixture equally among all cylinders. Since the air flowing through the manifold is turbulent there is no guarantee that each cylinder will recieve an air-fuel mixture that is identical to its siblings.
Anyways, I think the multiple carbs is for better tunability of the engine. As aerog noted about "tuned injectors", properly tuned multi-carb setups would allow you to have all cylinders in the engine work equally.
Anyways, I think the multiple carbs is for better tunability of the engine. As aerog noted about "tuned injectors", properly tuned multi-carb setups would allow you to have all cylinders in the engine work equally.
77MidgetMkIV
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I am not an expert mechanic by any stretch, but I always thought that it was a matter of geometry. The distance that the fuel needs to travel through the manifold on a single carb is higher on the outer cylinders. When a fluid (liquid/vapor, etc) travels through a body (manifold), it is going to experience drag on the inner surface of the body and loss of velocity. By going to a multiple carb setup, the geomtery of the manifold is different, and allows for more efficient and balanced flow of the fuel to the valve. I suppose the trade-off is in the tuning and configuration. With multiple carbs, you must tune them together. Any discrepencies in tuning (mixture, etc) will be compounded. With regards to aircraft, I think you are on the right track. Mixture is much more important, given the changes in density of the air that is taken into the carb, and it is critical that your engine cylinders are performing in synch. Having multiple carbs, howver, means multiple points of failure. That, I think, is why you can adject mixture to each valve. At least, this is how I understand it. I have been flying on and off for a couple years, and have helped on the annual inspections a couple times, during which you normally get a whole ration of tech talk from the A/P mechanic.
I am by no means an expert, but at altitude, a tuned engine is peace of mind. On the ground, you can just pull over.
I am by no means an expert, but at altitude, a tuned engine is peace of mind. On the ground, you can just pull over.
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Re: Why dual carbs?
I agree with Aerog & Nunyas; The main reason is the more even fuel distribution provided by multiple carbs. A secondary benefit with SU's is that for engines which require more air flow, the SU is limited to a two inch bore which gives around 3.14 sq inches of bore area.
Two 1.25" H2 SU's would give 2.46 sq inches.
Two 1.5" H4 SU's would give 3.5 sq inches.
Two 1.75" H6 SU's would give 4.8 sq inches.
Two 2.0" HD8 SU's would give 6.3 sq inches.
Note; Many 40 mm Weber DCOE type carbs actually have something like 32 MM (1.26") venturi chokes installed, which is equivalent to two SU H2 carbs. They look impressive but actually flow no more than a pair of SU's. The huge triple Webers that you sometimes see actually have much smaller chokes inside. Impressive, but no more flow than the SU's.
D
I agree with Aerog & Nunyas; The main reason is the more even fuel distribution provided by multiple carbs. A secondary benefit with SU's is that for engines which require more air flow, the SU is limited to a two inch bore which gives around 3.14 sq inches of bore area.
Two 1.25" H2 SU's would give 2.46 sq inches.
Two 1.5" H4 SU's would give 3.5 sq inches.
Two 1.75" H6 SU's would give 4.8 sq inches.
Two 2.0" HD8 SU's would give 6.3 sq inches.
Note; Many 40 mm Weber DCOE type carbs actually have something like 32 MM (1.26") venturi chokes installed, which is equivalent to two SU H2 carbs. They look impressive but actually flow no more than a pair of SU's. The huge triple Webers that you sometimes see actually have much smaller chokes inside. Impressive, but no more flow than the SU's.
D
John Moore
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Plus they look cool! Down draft webers look like tractor carbs! lol! /ubbthreads/images/graemlins/yesnod.gif
A
aerog
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[ QUOTE ]
Mixture is much more important, given the changes in density of the air that is taken into the carb, and it is critical that your engine cylinders are performing in synch.
[/ QUOTE ]
Yes but that isn't true. Generally speaking, the traditional non-injected aircraft engine only has a single carb and you cannot adjust the mixture to each cylinder. If you were to put an EGT (exhaust gas temp) and CHT (cylinder head temp) gauge on each cylinder you'd find they all (again, generally speaking) run at different temps as you adjust the mixture because there is a natural tendency for them to be mismatched. As you lean the mixture it will start running rough (usually) because one or more cylinders is starting to run leaner than the others. There's more to it of course, but that's just "in general".
Fuel-injected engines (still talking in generalizations and aircraft here) have the same problems, and when the engine is leaned you'll still get the same "rough running" engine before it dies. The difference is when you match the injectors to each cylinder, either mechanically with tuned injectors (see "Gami-injectors") or electronically then you in effect tune each cylinder individually. Put an EGT and/or CHT gauge on each cylinder and you'll find they now all run happily together burning the same amount of fuel and run at the same temps. Lean that engine and eventually the whole thing just dies with little or no "roughness" beforehand.
The problem becomes a little more simple if you consider each cylinder as an independant engine that needs to run the same as the rest of the cylinders as a team. Each individual "engine" needs to be tuned to each other for the team to work at its best.
Find information on "Gami injectors" for some interesting information (and debunking some old myths) about setting up injected aircraft engines. It also goes a long way to understand some of the theory behind engine setup and management. I did an in-depth analysis of two new 300hp aircraft engines a few years ago when we compared running them with stock injectors then switch to the Gami injectors (that are matched to the cylinders). I graphed the EGT/CHT curves of each cylinder on each engine (12 total) using an engine monitor. We leaned the engine to the point where it was barely running. The graphs showed each cylinder running at different temps and dropping off (due to leaning) at different mixture settings. At those settings the engine ran pretty rough, but it ran.
With the Gami injectors (tuned to each cylinder) the same tests were run and the graph showed that each cylinder was running in synch with each other cylinder. We could then lean the engine an incredible amount to where it was burning substantially less fuel than normal, and consequently much cooler -- all because all the cylinders were balanced to each other. When the engine was leaned to the point of fuel starvation there wasn't a hint of roughness, it just stopped.
Mixture is much more important, given the changes in density of the air that is taken into the carb, and it is critical that your engine cylinders are performing in synch.
[/ QUOTE ]
Yes but that isn't true. Generally speaking, the traditional non-injected aircraft engine only has a single carb and you cannot adjust the mixture to each cylinder. If you were to put an EGT (exhaust gas temp) and CHT (cylinder head temp) gauge on each cylinder you'd find they all (again, generally speaking) run at different temps as you adjust the mixture because there is a natural tendency for them to be mismatched. As you lean the mixture it will start running rough (usually) because one or more cylinders is starting to run leaner than the others. There's more to it of course, but that's just "in general".
Fuel-injected engines (still talking in generalizations and aircraft here) have the same problems, and when the engine is leaned you'll still get the same "rough running" engine before it dies. The difference is when you match the injectors to each cylinder, either mechanically with tuned injectors (see "Gami-injectors") or electronically then you in effect tune each cylinder individually. Put an EGT and/or CHT gauge on each cylinder and you'll find they now all run happily together burning the same amount of fuel and run at the same temps. Lean that engine and eventually the whole thing just dies with little or no "roughness" beforehand.
The problem becomes a little more simple if you consider each cylinder as an independant engine that needs to run the same as the rest of the cylinders as a team. Each individual "engine" needs to be tuned to each other for the team to work at its best.
Find information on "Gami injectors" for some interesting information (and debunking some old myths) about setting up injected aircraft engines. It also goes a long way to understand some of the theory behind engine setup and management. I did an in-depth analysis of two new 300hp aircraft engines a few years ago when we compared running them with stock injectors then switch to the Gami injectors (that are matched to the cylinders). I graphed the EGT/CHT curves of each cylinder on each engine (12 total) using an engine monitor. We leaned the engine to the point where it was barely running. The graphs showed each cylinder running at different temps and dropping off (due to leaning) at different mixture settings. At those settings the engine ran pretty rough, but it ran.
With the Gami injectors (tuned to each cylinder) the same tests were run and the graph showed that each cylinder was running in synch with each other cylinder. We could then lean the engine an incredible amount to where it was burning substantially less fuel than normal, and consequently much cooler -- all because all the cylinders were balanced to each other. When the engine was leaned to the point of fuel starvation there wasn't a hint of roughness, it just stopped.
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Re: Why dual carbs?
A similar & very noticable example of poor fuel distribution is the LBC engines which have cylinders 1 & 2 sharing an intake port & cylinders 3 & 4 sharing a port. With the common 1,3,4,2 firing order, Cylinders 3 & 4 share a port & are intaking & firing sequentially. This causes the #3 cylinder to "rob" some of mixture that "should be" going to cylinder #4.
D
A similar & very noticable example of poor fuel distribution is the LBC engines which have cylinders 1 & 2 sharing an intake port & cylinders 3 & 4 sharing a port. With the common 1,3,4,2 firing order, Cylinders 3 & 4 share a port & are intaking & firing sequentially. This causes the #3 cylinder to "rob" some of mixture that "should be" going to cylinder #4.
D
trrdster2000
Luke Skywalker
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Agree Aeroq, and that is way I go to a lot of troble to CC all the heads I work on. For those not familiar with this term, it requres you put in the plugs, lay the head level on a table and use a needle with CC measurements, fill it up as many times as required with thin oil putting it into the combustion chamber to get all the chambers the same CC's. It requires a bit of grinding to smooth out these areas to get it right. I'm sure there are beter ways of doing this but I can handle it in the basement on my own. I find most stock engines are really close, but feel better just knowing I'm starting out right. Wayne
coldplugs
Darth Vader
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This is a fascinating topic - people have built careers around it.
I'm not sure that the Siamesed ports and the firing order favor cyls 1 and 2, since you could just as easily describe the firing order as 4,2,1,3 - now two and one fire sequentially.
It gets complicated in a hurry and I think Dave's earlier point about SU's vs Webers is accurate. The big advantage Weber sidedrafts have over most other carbs is reconfigurability. They're basically a box full of jets and venturis, all of which can be easily changed. Whether a single carb (or carb barrel) per cylinder is the ideal setup depends a lot on the engine configuration, port geometry, valve timing, and so on. Lotus Twincam engines ran as well or better with 2 Zeniths than with 2 sidedraft Webers. Go figure. One theory was that gas flow was better - rather than the start-stop-start-stop of a single barrel per port, there was a smoother flow with 2 cylinders drawing through a single port.
I strongly suspect that MG went with multiple carbs simply because of availability and cost. They were already in the parts book.
Re the original question - American manufacturers (and hot rodders) did the same thing back in the days. It was rare to see a tuned American six cylinder engine without two or three carbs. Before we got the Holley 600 cfm four barrels we used "dual quads" or "three deuces" on V-8's. In the Chevy world this meant 2 Rochester 4 barrels. It wasn't until the early sixties that you started seeing high performance engines with only one carb.
Steve mentioned two strokes with a single carb per cylinder. Very common - lots of two stroke bike engines did this. I always thought it had to do with the sealed, isolated crankcases etc but some two stroke, two cylinder bikes ran nicely on one Amal. Actually, most two strokes have such a hopeless intake path that the number of carburetors probably doesn't matter. Maybe it was a packaging issue with Saab and the bikes?
I'm not sure that the Siamesed ports and the firing order favor cyls 1 and 2, since you could just as easily describe the firing order as 4,2,1,3 - now two and one fire sequentially.
It gets complicated in a hurry and I think Dave's earlier point about SU's vs Webers is accurate. The big advantage Weber sidedrafts have over most other carbs is reconfigurability. They're basically a box full of jets and venturis, all of which can be easily changed. Whether a single carb (or carb barrel) per cylinder is the ideal setup depends a lot on the engine configuration, port geometry, valve timing, and so on. Lotus Twincam engines ran as well or better with 2 Zeniths than with 2 sidedraft Webers. Go figure. One theory was that gas flow was better - rather than the start-stop-start-stop of a single barrel per port, there was a smoother flow with 2 cylinders drawing through a single port.
I strongly suspect that MG went with multiple carbs simply because of availability and cost. They were already in the parts book.
Re the original question - American manufacturers (and hot rodders) did the same thing back in the days. It was rare to see a tuned American six cylinder engine without two or three carbs. Before we got the Holley 600 cfm four barrels we used "dual quads" or "three deuces" on V-8's. In the Chevy world this meant 2 Rochester 4 barrels. It wasn't until the early sixties that you started seeing high performance engines with only one carb.
Steve mentioned two strokes with a single carb per cylinder. Very common - lots of two stroke bike engines did this. I always thought it had to do with the sealed, isolated crankcases etc but some two stroke, two cylinder bikes ran nicely on one Amal. Actually, most two strokes have such a hopeless intake path that the number of carburetors probably doesn't matter. Maybe it was a packaging issue with Saab and the bikes?
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Early Corvettes & my Chrysler 300D had dual fours.
D
D
The primary difference between GM, Ford and Mopar dual-quad and Tripower 3X2 (like my '69 Corvette came from the factory) and Six-Packs as compared to European/Japanese multiple carb setups is that American multiple carb setups all feed into all cylinders, and European/Japanese have each carb feeding a set of cylinders. Obviously there are differences between constant velocity/variable venturi carbs (SU/Stromberg 175 CD (Constant Depression)) and fixed jet carbs (Holley, Carter, Rochester Quadrajet etc..) But that's a different discussion.
Early dual quads, such as the aformentioned solid-axle Corvettes, Chrysler 300letter cars, Hudson Twin-H Power and the like were synchronized to idle and dump fuel at the same rate, much like our SUs and Strombergs (but into all cylinders)...but later 3X2 barrel setups were designed to idle on one carb, and wide-open-throttle accelerate on all three. This actually often was a problem for light-footed drivers (women!), since the fuel often sat in the bowls of the outside carbs for months, gumming them up. Of course, most people who bought tripower muscle and sports cars weren't known for driving with a light foot.
Multiple carb setups went away, because carb technology allowed for larger CFMs, and then electronic fuel injection came in and made carbs obsolete for non racing applications.
And actually, it should be mentioned that multiple carb applications are as old as cars themselves. Dual SUs are found in some of the earliest British sporty cars. Flathead Ford V8s were frequently fitted with three-two-barrels.
But the bottom line for all these setups is the answer to the initial question: why multiple carbs?
To get more fuel and air (in a correct mixture) to the engine to develop more horsepower and torque, yet still make it tractable at different RPMs.
Early dual quads, such as the aformentioned solid-axle Corvettes, Chrysler 300letter cars, Hudson Twin-H Power and the like were synchronized to idle and dump fuel at the same rate, much like our SUs and Strombergs (but into all cylinders)...but later 3X2 barrel setups were designed to idle on one carb, and wide-open-throttle accelerate on all three. This actually often was a problem for light-footed drivers (women!), since the fuel often sat in the bowls of the outside carbs for months, gumming them up. Of course, most people who bought tripower muscle and sports cars weren't known for driving with a light foot.
Multiple carb setups went away, because carb technology allowed for larger CFMs, and then electronic fuel injection came in and made carbs obsolete for non racing applications.
And actually, it should be mentioned that multiple carb applications are as old as cars themselves. Dual SUs are found in some of the earliest British sporty cars. Flathead Ford V8s were frequently fitted with three-two-barrels.
But the bottom line for all these setups is the answer to the initial question: why multiple carbs?
To get more fuel and air (in a correct mixture) to the engine to develop more horsepower and torque, yet still make it tractable at different RPMs.
D
Deleted member 3577
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Yada, yada....yak,yak,yak.....Do you really think they were interested in tun-ability, efficiency or air/fuel ratios back in the 50's &60's. heck no...They were interested in the same thing we are now...AESTHETICS.... Can you pick up chicks in it? Think about it....4 star was pence/gal. and the finer points of intake manifold design were the least of their problems. ( push rods, restrictive exhausts and litany of other performance impediments) The Brits if nothing else were practical, pragmatic and KISS sensitive designers. Put a Skinners Union carb on the car and when they break down along some rural road, they can ask a farmer for assistance. The Americans & Italians can make cars with tall hoods/ bulges but ours will be low and sleek & sexy.
lesingepsycho
Jedi Warrior
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/ubbthreads/images/graemlins/iagree.gif Now that sounds like the truth to me, (or at least a heck of a lot more likely)!! /ubbthreads/images/graemlins/yesnod.gif
Reading the development history of the engine (and the people involved with development) for TRs, XKs, MGAs/MGBs, it would have been very easy to throw a single SU on there, but the duals carbs were used as a cost-effective way of improving horsepower.
The aesthetic issue doesn't seem to ring very true, since the dual carb cars were generally pushed by competition heads and engineers.
And the comment about aesthetics doesn't speak to single vs. dual carbs, it's more a comment regarding side vs up/downdraught carbs. It's not just aesthetics that caused British manufacturers to utilize SUs, it was that the engines were designed for SUs on the side (or any sidedraught carb.) The contracts signed specified SUs.
And certainly Ferrari, Maserati/OSCA, Alfa
[ QUOTE ]
Yada, yada....yak,yak,yak.....Do you really think they were interested in tun-ability, efficiency or air/fuel ratios back in the 50's &60's. heck no...They were interested in the same thing we are now...AESTHETICS.... Can you pick up chicks in it? Think about it....4 star was pence/gal. and the finer points of intake manifold design were the least of their problems. ( push rods, restrictive exhausts and litany of other performance impediments) The Brits if nothing else were practical, pragmatic and KISS sensitive designers. Put a Skinners Union carb on the car and when they break down along some rural road, they can ask a farmer for assistance. The Americans & Italians can make cars with tall hoods/ bulges but ours will be low and sleek & sexy.
[/ QUOTE ]
The aesthetic issue doesn't seem to ring very true, since the dual carb cars were generally pushed by competition heads and engineers.
And the comment about aesthetics doesn't speak to single vs. dual carbs, it's more a comment regarding side vs up/downdraught carbs. It's not just aesthetics that caused British manufacturers to utilize SUs, it was that the engines were designed for SUs on the side (or any sidedraught carb.) The contracts signed specified SUs.
And certainly Ferrari, Maserati/OSCA, Alfa
[ QUOTE ]
Yada, yada....yak,yak,yak.....Do you really think they were interested in tun-ability, efficiency or air/fuel ratios back in the 50's &60's. heck no...They were interested in the same thing we are now...AESTHETICS.... Can you pick up chicks in it? Think about it....4 star was pence/gal. and the finer points of intake manifold design were the least of their problems. ( push rods, restrictive exhausts and litany of other performance impediments) The Brits if nothing else were practical, pragmatic and KISS sensitive designers. Put a Skinners Union carb on the car and when they break down along some rural road, they can ask a farmer for assistance. The Americans & Italians can make cars with tall hoods/ bulges but ours will be low and sleek & sexy.
[/ QUOTE ]