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Why duel carbs?
- Thread starter Lonnie
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lawguy
Jedi Knight
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So most of you are thinking the reason is based in engineering? My take is- no.
I would say "economics". By manufacturing a few sizes of relatively small carbs, the manufacturer can cover everything from tiny industrial engines on up to larger auto engines.
Need more capacity? Use two. Need more yet? Use 3. That is much cheaper than manufacturing a dedicated single carb. for all the engine sizes you want to cover. Think about it this way. Think of the range of engines that SU could provide carbs. for with 4 sizes of carbs and some jetting choices. Tooling needed: 4 sets.
By contrast, what if SU built a dedicated single carb. for all engine sizes it wanted to cover. Now how many sets of tooling do you need? 20? 30?
Now. Why did the American manufacturers go a different way? They didn't, they just adjusted their means of attaining the same goal of efficiency. To cover American V8s, you need a carb to use on engines from about 4.3L (262 ci) to 7.5L (460 ci). How do you span the range efficiently? Use a few sizes of carb. bodies and then use different sized venturis and jets.
Same idea, different execution. Keep in mind, though, that U.S. manufacturers, over the years, did use multiple carbs. from time to time. Hemi anyone? Cobra? Why not just make one huge carb. instead of two lage ones? The large ones are on the shelf. Now you just need to make a new manifold and linkage instead of all that and a complicated, expensive, limited use carb.
I would say "economics". By manufacturing a few sizes of relatively small carbs, the manufacturer can cover everything from tiny industrial engines on up to larger auto engines.
Need more capacity? Use two. Need more yet? Use 3. That is much cheaper than manufacturing a dedicated single carb. for all the engine sizes you want to cover. Think about it this way. Think of the range of engines that SU could provide carbs. for with 4 sizes of carbs and some jetting choices. Tooling needed: 4 sets.
By contrast, what if SU built a dedicated single carb. for all engine sizes it wanted to cover. Now how many sets of tooling do you need? 20? 30?
Now. Why did the American manufacturers go a different way? They didn't, they just adjusted their means of attaining the same goal of efficiency. To cover American V8s, you need a carb to use on engines from about 4.3L (262 ci) to 7.5L (460 ci). How do you span the range efficiently? Use a few sizes of carb. bodies and then use different sized venturis and jets.
Same idea, different execution. Keep in mind, though, that U.S. manufacturers, over the years, did use multiple carbs. from time to time. Hemi anyone? Cobra? Why not just make one huge carb. instead of two lage ones? The large ones are on the shelf. Now you just need to make a new manifold and linkage instead of all that and a complicated, expensive, limited use carb.
AltaKnight
Jedi Knight
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I know little of aircraft engines but always fancied knowing more about them (and flying them). Sure sounds like some folks here know quite a bit.
I think aircraft engines started using FI in the late 30's or early 40's or maybe even earlier. But carb aero engines, how do they deal with fuel floats and needles coupled with inversion/banking issues? To us land lubbers it doesn't sound possible but there must be a way!
BTW I think 77midgetmk4 hit the nail on the head about multiple carbs, I think it's all about equalizing runner length (as best you can) from carb to inlet valve, too much distance and that mist turns into a river! doesn't burn right.
So who's going to step up and give me the aero carb float quandry answer?
I think aircraft engines started using FI in the late 30's or early 40's or maybe even earlier. But carb aero engines, how do they deal with fuel floats and needles coupled with inversion/banking issues? To us land lubbers it doesn't sound possible but there must be a way!
BTW I think 77midgetmk4 hit the nail on the head about multiple carbs, I think it's all about equalizing runner length (as best you can) from carb to inlet valve, too much distance and that mist turns into a river! doesn't burn right.
So who's going to step up and give me the aero carb float quandry answer?
AltaKnight
Jedi Knight
Offline
Thanks Bugeye for the pressure carbs tip!
Ok I've got the concept of a sealed carb that doesn't leak fuel out all over when you invert, but how does the float/needle bit work on our not so hypothetical carb aero engine?
I would guess it's not the same as an auto engine where the vacuum and air speed at the venturi coupled with the needle/jet draws the right amount of air/fuel but rather it has to meter the right amount some other way so you don't need a float?
Ok I've got the concept of a sealed carb that doesn't leak fuel out all over when you invert, but how does the float/needle bit work on our not so hypothetical carb aero engine?
I would guess it's not the same as an auto engine where the vacuum and air speed at the venturi coupled with the needle/jet draws the right amount of air/fuel but rather it has to meter the right amount some other way so you don't need a float?
My Cortina GT was advertized with 85 hp when new. It dynoed with 135 hp. when I had it tested a couple years ago. The dual Delorrto 40's helped a lot. The stock carb was not going to get within 25 horsepower of that.
The later LBC's that went with single carbs were not a performance improvement from their earlier multi carbed relatives.
Dual side drafts on a 4 cylinder give a pretty straight shot from the butterfly to the head, too.
Many a hot detroit V8 runs 4 two barrels too. Lots of them came stock with 3 dueces.
The later LBC's that went with single carbs were not a performance improvement from their earlier multi carbed relatives.
Dual side drafts on a 4 cylinder give a pretty straight shot from the butterfly to the head, too.
Many a hot detroit V8 runs 4 two barrels too. Lots of them came stock with 3 dueces.
