The 100 mpg Carb.

Finally here’s what  gas companies have been hiding all these years!

Now  you can forget  rumors – here’s reality.

I’ll make a bet here that there is barley a performance enthusiast in the English speaking world that has not heard of the rumored 100 mpg carb that the oil giants bought up so you, the poor consumer, would keep on buying lots of their fuel. Bet you have also heard arguments both for and against the existence of such a carb. Well folks here is what could possible be the end of the controversy. It’s going to take a little math toward the end but I have taken 99% of the pain out of that so here goes.

First off let me give you what I believe qualifies me to address this subject. In 1975 I was the lead engineer on an economy project. The subject was a 1275 Mini GT. That, as many will know, is an extremely close relative of the original 1275 Mini Cooper.

The goal was to see if we could get this car, as un-aerodynamic as it may have been, to do 50 mpg at 50 mph. Once that had been achieved it would be taken on a 3 month round USA drive at normal highway speeds (not fuel economy test speeds which would take forever to get anywhere) and see if we could get 50 mpg. An independent test at MIRA (Motor Industry Research Association) on a truly lousy wet and windy winter day recorded 55 mpg at 50 mph and 99 mpg at 30 mph. OK that 99 mpg is real close to 100 but do we all want to drive at no more than 30mph – no! Not only that but that 99 mpg was achieved with a 1600 lb car. The same size car today weighs in at over 2100 lbs and the average mid sized sedan at 2800 lbs plus.

I am not sure my team was the first to break the 50 mpg at 50 mph but we did it before Honda and all for less than $100,000 in today’s money. We failed to see that 50 mpg with our round USA trip but did record a close 49.88 mpg. But in all fairness this figure included full throttle road test by Road & Track plus 300 miles of snow drift bashing in 2nd and 3rd gear as we drove through one of the worst winter storms in Wyoming. Added to that the car saw a hurricane in the New Orleans area which we had to out run. But that’s not the end. The car had run-flat tires which were magazine tested by driving at 50 mph for about 50 miles plus it was also driven off road. How many economy drives include such? Answer – about none!

Figure 1 This high performance AED carb (Richmond, Virginia) may not look like an economy carb and in true it is not. But because it can be accurately calibrated it can still do well. If a 4 barrel Holly style carb is in your future and mileage is a high priority then you should strongly consider a vacuum secondary carb.

Ok so now you all know where I’m coming from lets get technical. Unless you are really into carb design the inner workings of a carb look more like a collection of parts for executing a little black magic. Don’t feel bad about that – most of today’s top carb designers were there once. That AED four barrel street performance carb you see nearby is complex but not un-fathomable. Nor for that matter is the giant 1 barrel SV1 carb also shown. These are the product of nearly a century of carburetor development. With hundreds of different types out there – some good – some not so good- we should as a technical species have a pretty good handle on carb design. In short we should know a lot but not everything when it comes to carbs.

In this feature I am showing carbs that are definitely leaning toward the high performance end of the scale. That however should not preclude a carbs ability to deliver at least decent mileage. I had four 48 DHLA downdraft carbs on my 1974 small block 350 V8 Chevy truck. Because these carbs could be accurately calibrated and were able to deliver a well atomized mixture at part throttle mileage was surprisingly good (about 18 around town and near 20 on the freeway – engine was a short cammed 10.5/1 build). Needless to say power was really good but the mileage figure were only realized when the desire to stand on it was curtailed (it would smoke the tires well into second gear).

Figure 2. A set of 48 sidedraft Webbers may deliver all you need in the way of hp but can be fuel hungry at part throttle unless the boosters(auxiliary venturi’s) can do a good job of atomization. This along with good progression calibration can be a big redeeming factor.

If the argument for or against the existence of the 100 mile per gallon carb is to be understood then absorbing the next two points are critical. In reality the function of any carb can be reduced to two simple functions that it must perform. The first is to deliver a certain ratio of fuel and air to the engine. The second is to atomize that fuel so that a significant proportion is vaporized and the remaining wet fuel is sufficiently well atomized to turn to a vapor as the piston reaches Top Dead Center on the compression stroke.

OK so now we know what the carb has to do we no longer need worry about how it works so lets get down to the nitty gritty.

Any respectable carb can be calibrated to deliver whatever fuel air ratio you want to the extent that it will be either too rich (has too much fuel for the air) to burn or will be too lean (insufficient fuel for the air). However it’s not what you may want to see in the way of mixture ratio that counts – it’s what the engine wants to see. Most engines run into lean misfire at about 18/1 to 19/1 air/fuel ratios. Just for the record with some fine development details we managed to get the ‘Round USA Economin’ to run effectively at just over a 22/1 air/fuel ratio. Now consider this; if we can get any decent carb to deliver a fuel air ratio significantly leaner than say 30/1 the problem of finding fuel mileage is not constrained by that carbs ability to deliver a lean enough mixture. In other words any decent carb (and there are dozens out there) will deliver an economy mixture far leaner than any engine can burn.

Figure 3. The 48 IDA Webber is dwarfed by it’s look alike 55 mm unit from Gene Burg Enterprises, of Orange, California. In spite of the size involved both the carbs shown here and the 48 DHLA Dellorto have good part throttle mixture qualities. If used with a cam that gives a decent cruise vacuum then mileage can be pretty respectable.

Let’s move on to the other aspect – namely fuel atomization.

Some carbs such as the constant vacuum SU and Stromberg carbs can atomize fuel better than a 45 psi fuel injection nozzle so the question of whether or not fuel can be adequately atomized is – yes it can. But not all carbs are as good as these at doing the atomization deal. But there is a fix for that. It’s called intake manifold heat. If a carb fails to atomize the fuel well enough then a little manifold heat fixes the problem Also the fact that fuel economy driving takes place at part throttle also means that there is a lot of intake manifold vacuum present. This also has a strong influence on vaporization.

So where are we at now?

I’ll tell you – any decently designed and correctly sized carb can cover a far wider range of fuel mixture conditions than the engine needs so what is this supposedly 100 mpg carb going to do that the ones we currently have available cannot already do? Answer – Nothing! It’s the engine that is the limiting factor not the carb.

From the forgoing you can see the argument for the existence of a 100 mpg carb is starting to get somewhat tenuous. But I have not finished yet.

Figure 4 .Shown here is a Quadrajet carb. For about a quarter of a century it was a standard fitment on most GM V8’s. This one has been modified by the Carb Shop in Ontario California. Among many mods is a part throttle mixture adjustment screw that is situated in the hole just by the nearest float bowl vent tube. This can be adjusted until best mileage is seen. This carb delivers good hp along with very acceptable mileage figures on a big V8.

We don’t need to consider the 100 mpg carb in terms of a 50 cc moped as that is not in context with what is so often refer to. When the 100 mpg carb crops up in conversation it’s in connection with a reasonably sized sedan and speeds we could reasonable expect people to drive at. Let’s say such a vehicle is a typical 2300 lb sedan (and that’s on the small side) and the speed concerned is say 50 mph (and that’s on the slow side). By picking these conservative numbers I’m giving the benefit of the doubt to the 100 mpg carb. To power such a car at a steady 50 mph, assuming reasonable aerodynamics, will typically take about 15 hp. With Brake Specific Fuel consumption figures being what they are at part throttle ( about 0.4 lbs per hp per hour)such a car would turn in right around 45-50 mpg at that speed. Also it would be utilizing the fuel’s energy at close to 25% (that is the engines thermal efficiency is 25%). If the engine could convert 100% of the fuels heat into mechanical energy the mileage would go up to about 180-200 mpg. Now that shows what room there is for improvement but the problem is it is the engines inability to convert the fuels total heat capability into power not the carbs ability to deliver a suitable mixture. It’s already doing its job near 100% so that 100 mpg is going to come from engine development not from fuel delivery development (i.e. the carb). This means that long ago carburetors had already reached near maximum potential and there is little room left for improvement. From that we can safely conclude that the 100 mpg carb never has existed and never will!

OK – by now I should have convinced you that 98% at least of any mileage improvements will come mostly from engine development and maybe a little extra from fuels. Of course you could drive more carefully by easing down on the gas pedal, avoid overtaking, drive 5 mph slower and back into the driveway at night. But if you are anything approaching a type ‘A’ personality like me the very fact there is a car in front means that some overtaking is due! I have to race something and the only way I have been able to consistently drive for economy is to race a trip computer that reads out in mileage to date and instantaneous mileage. The bottom line is I need to get there now not five minutes later. All this adds up to one thing for me – and that is the vehicle I drive better be able to go fast on next to no fuel!

If I had to put my name on a carb that most nearly represents a real world 100 mpg carb it would be the SU as used on so many British cars up through to the late 80’s early 90’s. Prep one as I detail in my big 528 page ‘A’ Series book ( Tuning the ‘A’ Series Engine by David Vizard – published by Haynes and available from APT (www.aptfast.com) and you are set for the nearest thing to a 100 mpg carb possible. Stick one on a high compression 850 mini and you might just see that 100 to the gallon – if you keep a close eye on the speed!

As things stand as of now I can detail how you can increase the output of a typical Detroit built street V8 engine (and the principles apply across the board to all engines) by some 50% while improving mileage by some 40% That means if your vehicle is making 250 hp and doing 18 to the gallon now, I can show you how this can be bumped to 375 hp and 25.6 to the gallon.

You can continue to put money into the pockets of foreign oil cartels or lobby whoever in some attempt to get the cost of fuel down – if you believe that will work that is.

Figure 5. The Edelbrock carb has, much like the Quadrajet, a flow on demand secondary system. As such this allows the primary barrel to be calibrated with a strong bias toward economy. My tests have shown good results with this carb.

But there is a third alternative that really will benefit you and that is you can hop up your engine using the proven tech that I can supply you with. However it cost me a lot of time as well a seven figure money number (not all my money might I add) to do the research and testing needed to verify what I am proposing. If I am going to give this info out for free (I repeat for those who may be sight impaired – ‘for free’) I need you to do something in return. If I am going to spend about two months writing this all up and producing a lot of fancy artwork to boot I would, in return, appreciate you spending a few minutes of your time sending out a blanket email to all of your auto enthusiast friends informing them about this web site and what it has to offer. When I have a big enough audience I will spill all in great detail with extensive text, color drawings and a lot of top notch photo’s.

Now I know someone is going to say ‘what a cheap shot to advertise his books (heaven help me for actually wanting, as a professional writer, some form of re-imbursement for my work) and to get hits on this site’ but consider this. If each of you guys does your bit to reduce overall fuel consumption the savings you can bring about as an individual may only amount to some 5 gallons a week. What with my books and magazine articles I have about a million readers a month but this site is new. If I could get all my paper readers to follow just a few of the mods I propose and each one saves very conservatively just one gallon a week that means I will have brought about a saving, just for the US as a whole, of 50,000,000 gallons a year. Add Europe to that and the fuel savings figures really look impressive. Even so you may feel even a 100 million gallons a year is still a drop in the ocean but it’s a hell of a lot bigger drop than most individuals can bring about.

So what will this power boosting fuel saving tech cover?
Here’s just a short list in alphabetical order, of some of the subjects:

• Big bore short stroke or small bore long stroke.
• Big valves or small?
• Camshafts for mileage.
• Carb Calibration.
• Cold air intakes.
• Effects of Compression Ratio
• Exhaust system lengths and diameters.
• Fuels.
• Gearing.
• Ignition timing.
• Intake port dimensions
• Intake port finish.
• Intake to exhaust size ratio.
• Internal friction,
• Mixture preparation.
• Nitrous – yes or no.
• Oils and lubes (want to know how to go three quarters of a mil before a rebuild?)
• Optimum valve sizes.
• Piston speed for optimum mileage.
• Rip offs.
• Spark intensity
• Swirl & tumble
• Super chargers and turbo’s
• Thermal Barriers
• Two valve heads versus four valve heads – which is most fuel efficient.

That should be enough for a while.

David Vizard.

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