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MG TD TF 1500 - Single carb?
|Perhaps a dumb question:|
Why do superchargers only need one barb? Seems to me if anything, it would need more, not fewer.
I get it if it's a bigger carb, but if it's that simple why hasn't someone been offering an intake manifold that mounts a single larger carb for a normally aspirated engine?
Anyone doing a manifold for a downdraft Webber or 2bbl Stromberg?
I'm running a driver and my wife's been driving it. The closer to 'get in it and it runs great' I can get, the better.
|the single carb is larger, not by much but enough, as it is still only delivering a dose of fuel to 1 cylinder at a time, be it at increased pressure in a single manifold. as to single over double carbs, its a minefield, 3 worse I believe as some mini people get into fights over the merit of a single/dual setup. depends on the type of carb and manifold etc state of tune .... Mg Y types only ran 1 carb.|
|Think about it - a single carb serving four cylinders is never going to serve all of them equally - manifold geometry at the very least will see to that. With two carbs you can attempt a balance between them. You fit one supercharger and only need one carb to serve it. |
|Building a manifold that puts the carb equidistant from the front and rear intake ports seems simple. |
Every thread that contains the words "balance" and Carburetors" in the same sentence would instantly be obsolete.
I've seen threads on Healy, MG midget, B forums talking about fitting a single SU HIF44 to their engines (+-1250cc in later years) with good results.
This can't be more troublesome that fitting a new cam & roller lifters, or a supercharger.
| All the old American straight 4s and 6s used only one carb and they worked very well. Dumping fuel and air in one chamber below or above the carb and distribute it to the cylinders was never a problem, until we started hopping up these old engines and wanted more HP out of them. Under normal driving conditions, the cylinder sucking air will get what it's looking for from a stock setup. The air/fuel mixture in an intake manifold on a running engine is a violent turbulence of continuous action. |
Racing manifolds, porting and relieving plus other mods tend to quiet this turbulence and improve performance.
|Paul S Jennings|
|There is a huge difference between a normally aspirated engine and a supercharged one, as far as single/multiple is concerned. A normally aspirated engine having multiple carbs does indeed enable the engine to get the most even charge into each cylinder at the same time.|
A supercharger supplies a charge of compressed gas/air mixture to an intake manifold that supplies all the cylinders. The compressed gas/air mixture is waiting at each closed valve for the valve to open, while the intake of a normally-aspirated engine is (broadly) limited by the volume of each cylinder, and the size of the intake valves.
With a supercharger there is no limited sucking by the engine of gas and air mixture; it is forced into the engine under pressure. Two carbs on a blower would not distribute any better the mixture that is already pressurized and there, waiting for the intake valves to open.
I have never seen a multiple-carb supercharger. The only way multiple carbs would work is if you had a supercharger for each cylinder; however, the power loss to run them would negate the advantage (and it would be almost impossible to fit or balance properly!).
Turbos, by contrast, can benefit from multiples, since they are run by the exhaust gasses that normally vent out the exhaust and are otherwise not used. The complexities of fitting a turbo on a T-type are such that an entirely new exhaust manifold to house a turbo would need to be designed and cast.
Four carbs would probably help a normally-aspirated TD, but probably not by much (and as anyone with 6-pack carbs on their American car or hotrod can tell you, it's a pain to set up right). The TYPE of carb would make a difference - 50's T-series speed equipment used single or double Holley or Ford carbs, and because mechanics knew more about how to tune them, people felt they were more responsive, smoother or got better mileage. On my Wall o' Speed, in addition to almost a dozen manifolds from US makers, I also have a single Weber carb for racing applications (there is usually a new Pierce/Weber 32/36DGV conversion for sale on eBay), and I even have an Arnolt dual Solex manifold and carbs for the XPAG, all in an attempt to get more power. My experience is that because it has an accelerator pump the Weber has more grunt but worse fuel mileage than dual SU's, but is MUCH easier to tune and modify for dialing-in the best performance. The Mark II TD and TF1250 used stock 1-1/2" SUs, and got a few more HP, but there feels little difference between the two, unless in a racing application.
There's good reason why de-tuned cars use a single carb (Y-type), and more highly-tuned engines (T-series) use dual!
Pic is of a Bentley with dual carbs on a supercharger.
|How big is the Bentley - 3-1/2 or 4-1/4 liter? Quite4 a bit bigger than out 1250's!|
Certainly 1250cc running at up to 4,500 rpm's can be handled by a modest sized SU or similar. So, why not mount a modern carburetor on a driver?
|No problem at all - as I said, a Weber is much easier to tune and properly jet than an SU, but probably gets worse mileage..|
|Beware of slapping on just any old bigger carb. The supercharger sucks smoothly 100% of the time versus gulping air/fuel one cylinder at a time.|
I spent many hours machining a new carb manifold to test various carbs, specifically Harley Davidson carbs with a real choke, adjustable main & idle jets and accelerator pump. Considering they power 1200cc V-twins, a 12500cc 4 cylinder wouldnt be any problem, right? Wrong.
I tested 2 very good carbs (not together, one at a time), and the engine would not pick up from idle- it wanted to bog down. You couldn't step on the gas, only ease down on it very slowly. Once up to speed, they'd work great, but with the smooth drawing supercharger, they were horrible taking off in traffic. I gained a whole lot of respect for the "constant velocity" SU carbs!
|The stock SU setup has super short and straight carb to intake valve distances. I would think any single carb setup would have to have equal length intake "tubes" for each cylinder, so the average length from carb to valve would be longer and also with curves or angles. The resulting volume of mixture would be greater, with more resistance to flow due to curving and greater wall surface area causing friction. For a "stock" car, seems MG knew what they were doing. George|
|Jim - yes, but where are you going to find a 12,500cc 4-cylinder engine?|
LOL Tom Lange
Well here is 28,400 CC engine so a 12,500 one is possible.
|I don't think manifold tube length is THAT critical, we're only going about 7". I'm pretty sure I can find a manifold more balanced than this one from a Chrysler slant 6. The one at the bottom is more what I'm after. It's for a 1 3/4" SU setup for racing. |
|Anyone notice that 28,400cc engine was started with a hand crank?|
|As were most aircraft engines, either with a crank at the side of the fuselage or by kicking the prop.|
|The crank on the side of the fuselage was geared to a heavy flywheel that when up to speed was clutched to the engine turning it over just like an electric starter would do but using the inertia of the heavy flywheel for the energy.|
That's how we started the largest single engine aircraft, the TBM
Don H USN WW II TBM Aircrew
Oh and by the way, the original question for this thread, how would you put dual carbs on a single blower? Think about it!
Note that the carb has to be in front of the supercharger, not between it and the engine to work properly
|Don - Some superchargers had a pressurized carb - the McCullough comes to mind, but I am sure there are others (also Paxton, but they bought McCullough, or vice-versa)..|
|I was not aware of pressurized carbs, Fitting two seems like a nightmare to tune|
|Don't turbo chargers pump down into the carb?|
This thread was discussed between 07/06/2015 and 11/06/2015
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