Inliners International Forums General Discussion Engines carb circuits and gearing/tire sizing

This is how I see the relationship of gearing and performance/milage.

Carbs got "circuits" which take care of certain conditions that happen to engines.

Idle circuit
When the throttle is at idle it is barely open, and there is manifold vacumm sucking gas from tapered screw adjusted discharge ports just below the butterfly-VERY CRUDE metering system, very rich, very uneconomical. gas continues to be sucked out until throttle is open enough for manifold vacumm to lighten up and/or butterfly moves away from these ports less and less draw at the idle discharge ports 'til no gas is being drawn out. A very rich, wasteful circuit.

Main Metering circuit
When trottle is opened enough to get some air flowing down the barrel of the carb, through the venturi fast enough to draw gas up from the float bowl through the main jets and out through the main metering discharge ports. This is the carb's best working circuit, very accurate, (as long as the main jets are properly sized)very economical, though the idle circuit can still be functioning as the main metering circuit is comming on line (depends on how close the butterfly is still to the idle circuit discharge ports. This circuit will handle most of the carbs duties-its the best its got.

Power circuit
To get a load going,or maintain speed up a hill, we step deeper into the throttle (vacumm goes down until the carb goes into a richer mode to produce more power called a power stage) at this point additional fuel is drawn into the engine (just because the vacumm dropped)(bigger cams, bigger carbs do this too!) making the air fuel mixture richen (by additional jets or a richer potion of a metering rod) from a general 14:1 ratio to a powerful (but uneconomical) 12;1 ratio as long as your foot is into it. good for power- bad for economy. Back off the throttle and higher vacumm returns and you are still on the main metering, but not with the additional power stage, and back to economical cruising.

Choke circuit
Just that big top butterfly that limits how much air can flow down the barrel that causes the fuel being drawn out of the idle and main circuits to have very little to go with the drawn gas-thus real rich mixture and no economy and a no chance for much airflow down the barrel so no real power either.
Only necessary 'til motor warms up.

Low numerical ratio rear ends will keep a motor's rpms down (at any given speed)and in general, more throttle will be used to enable the car to perform acceptably, and with that, lower vacumm will be seen, causing the power stage to be be in use more often, and with its less economical air/fuel mixtures less mpg.

If a car with 30" tires and a 3.08 rear gets some 26" tires, it will think it now has 3.55 gears-this is how tire diameters affects gearing.

Any buyers?

1. idle circuit: not particularly 'wasteful', and VERY accurate fuel metering. Not crude at all. MAINS are crude, since it's just a fixed-orifice jet. Idle is fine tuned with a screw very accurately. It keeps flowing throughout the rpm range, but the % of all fuel metered through the idle circuit diminishes as the main jet circuit takes over. (venturi velocity at the idle hole is reduced too but overall velocity still exists, drawing small amounts of fuel through the idle circuit, it does not close or stop flowing at WOT)
Idle circuit has an important role at cruising speeds, light throttle.


Lower geared rear ends mean the engine has to work harder to accelerate the mass of the whole vehicle. At cruising speeds once the speed has been reached, the lower rpms required from the engine "often" means better fuel economy... so you save a little fuel after spending more during acceleration. Unless you are way overgeared for the speed and lugging the engine too low. An automatic transmission would then kick into a smaller gear to stay in the powerband.

just what do you think these very accurately set air/fuel mixtures are at idle conditions? And ahh here comes the stinger, what do you define as "way overgeared", what is "lugging the engine too low? Not a very well chosen rear gear if the automatic has to geardown to negotiate a hill at 70 mph (Pikes Peak perhaps).

But the nugget to salvage from all this and the principle purpose of the thread is captured in your sentence about..."idle circuit has an important role at (low-my opinion) cruising speeds,light throttle", as no one has spoken this fact on this bulletin board pror to now.

To me overgeared with a too low numerically ratio will keep you under the grip of the "very accurately' adjusted idle circuit far too long.

Idle circuits do work very well, not overly rich and crude as you point out. Try adjusting one someday using a air/fuel ratio gauge and you will see how sensitive they really are. I do believe they shut down for the most part once the throttle is open. Even if they don't shut off totally, they supply very little fuel once the engine gets up in rpm.

Gearing has a more to do with the motors torque output at the given rpm then just the rpm itself. If the motor make tons of torque at low rpm. then gear to run at the point, but if it make all its torque at a high rpm, low gearing will kill any fuel mileage and driveability.

Lugging a engine down is gearing it to low for the available torque. This requires your foot to far into the throttle just to keep it going. If its just a simple gear swap for mileage, then every car out there would have 2.41 gear ratio's. Some of the early 80's Chevrolet Malibu's had very low gears with lock up converters and small V6 engines, on flat ground at 55 mph they got decent mileage, but put in the hills or at higher speeds, the converters wore out shifting in and out so much, the little engines just didn't make to torque to pull them.

Just as example of gearing and rpm, my 2001 Beetle has a 1.8L turbo engine that runs 3100 rpm @ 70 mph and still gets 31 mpg. It was design for this, if I was to gear it different, the engine wouldn't have enough power to pull it at any lower rpm. Thats why you just can't change gears and expect the carb and engine to respond with better fuel mileage.

Go for a ride in a car with a big cammed engine with a low gear, you will see what lack of torque means!

Joe

Idle mixture is precisely controlled because the idle adjustment screw is not metering raw, solid fuel past it. Before the fuel can even exit the idle metering slot it passes through an emulsion well, a small chamber in the bowels of the main body, that introduces air into the idle fuel stream through a series of precise orifii that breaks up the liquid fuel into smaller and smaller droplets. This emulsion passes by the metering screw where it is further regulated. Most high end carburetors (e.g. Webers and racing Holleys) have removeable adjustable air bleeds to further tailor the fuel delivery needs at idle. This is best done on a wet flow bench. Few of us have one of these in our garages so most production and aftermarket carbs settle for fixed air bleeds that are a best guess fit for most applications. I can tell you from my experience that this 'best guess' calibration is usually miles away from what a multiple-carbed inline six wants. Modifications to both the air bleed circuit and the idle transfer slots are imperative for the well tuned, multi-carbed hot rod six.

Anyway, back to the topic of gearing for fuel economy. Generally speaking, running the engine at a lower speed and at lower manifold vacuum (higher manifold absolute pressure) will produce BETTER fuel economy than spinning the engine higher at a lighter load setting. This is mainly for two reasons: 1) less "negative work" is done by removing the high vacuum from the tops of the piston during intake charging and
2) less frictional horsepower is lost due to the internal friction of the engine bearings, valve train, etc. Of course with a carbureted engine factors such the power valve / power enrichment circuit and optimal timing vis-a-vis engine octane tolerance at various loads also come into play. That is why the industry has gotten away from carbs and switched to EFI and has explored the fast-burn characteristics of combustion chambers to such a great extent.
There may be those who say, "progress was great at one time, but it went on for too long". We (the automotive engineers of the world) don't have all the answers yet, but we're sneaking up on it.

6re6

I agree with this.

Back to carbs, I think there is something called a "transfer slot" that delivers fuel in between the idle ports and main jets. GM Rochester carbs I've messed with have these.

X2, but the major reason it doesn't always produce the expected results is that the power system (enrichment for high load) has not been re-tuned, and is brought on-line prematurely.
This is not the same as the actual metering device (jet, metering rod step, PVCR), but the point where they are initiated.
You typically need a lower PV point for Holleys (4.5 instead of 7, etc.), and a weaker spring for metering rod carbs (AFB, QJ).

Whatever vacuum level you have at normal cruising, the PV must be at least 1" (IMHO) below the indicated vacuum and closed, and the economy rod step still in the jet (the "down" position).

Some years ago, I had a '57 Chevy with overdrive. When the overdrive went out, I found that I got the same mileage. Probably because I was in a better power range.

If you have a vacuum gauge on your engine, you will probably find you will get the best mileage at minimum vacuum.

BTW - If you are going up Pikes Peak at 70 M.P.H., you are not interested in fuel economy. lol

Will Willis

The main metering circuits generally uses a main jet and an air bleed orifice. Without the air bleed orifice, the main jet tuned for mid range rpm's would tend to run too rich in the upper rpm range. So if you increase the main jet a few sizes and don't change the air bleed, your specific tuning will not stay in the best fuel/air ratio band throughout the mid to high rpm range, especially as the weather temperature changes.

You caught me.

Just testing you.

Will Willis

Hi Preacher edski . . .

I love this post - I am a big fan of pondering things from a practical standpoint.

I agree with your analysis of the various circuits in a carb - but I am not sure I follow the conclusions.

First, depending on carb design - most driving is done on the idle circuit or a combination of the idle circuit and the main meter.

I somewhat agree with you about the power circuit - but only if we're talking about a vacuum driven power valve (diaphragm) like in a Holley 94. Not so in a mechanical carb like a Stromberg 97.

Same with a carb equipped with an accelerator pump - it pumps only when your foot transitions the throttle - not when a throttle position is held.

You conclusion about the effect of RPM is the exact opposite of mine. Your analysis of circuits is a discussion of cause - let's focus instead on effect at cruise.

I did a little math on Sunday morning over coffee to demonstrate my reasoning. As you point out in your post - fuel air ratio is critical to both power and to efficiency.

Let's look at the numbers on efficiency first. I find it useful to expand fuel air ratio into something conceptually meaningful to ones every day senses.

Some weights and measures:

Weight . .pounds/gal . .pounds/oz . . .ounces/oz
Gasoline . . . 8.66 . ..0.06765625 . . . .1.0825

Air . . . ounces/cu ft ..cu-in/cu-ft
. . . . . . . 1.2 . . . . . ..1728

Fuel air ratios are calculated by weight:

. . . . . . Air (ounces) .Gas (ounces)
14 to 1 . . . . 15.155 . . . . . 1.0825
12 to 1 . . . . 12.990 . . . . . 1.0825
10 to 1 . . . . 10.825 . . . . . 1.0825

But they are consumed into an engine by volume:

Weight in Ounces . . . . Air volume..| .Total ..|...Air. . . Vapor* . .Fuel consumed
|Air| . . |Fuel| . Ratio...(cu/ft).....|.(cu-in)..|.(cu/in)...(cu-in) . . . . . . (oz)
14.00 . . 0.0000 . 14:0 . . 16.800 . .|. 29030 ..| .29030 . . ..0 . . . . . . . . 0
15.16 . . 1.0825 . 14:1 . . 18.186 . .|. 31462 ..| .31425 . . .36 . . . . . . . 1.00
15.16 . . 1.2629 . 12:1 . . 18.186 . .|. 31468 ..| .31425 . . .42 . . . . . . . 1.17
15.16 . . 1.5155 . 10:1 . . 18.186 . .|. 31476 ..| .31425 . . .51 . . . . . . . 1.40

*Assuming the volume displaced by one liquid oz gasoline = 1.80468 (cu-in) and a Vapor Liquid ratio of 20.

Now keeping in mind that engine is essentially an air pump - we can see the effect of running these volumes or 'helpings' through an engine with a particular displacement.

Take for example my personal favorite:
Engine. . . . . . . . . . . Per Rev
1949 216.5 (cu-in) . . . . 54.12500
. . . .. . v/e 0.85 . . . . . 46.00625

Now Let's assume that we consume these three theoretical 'helplings' of fuel/air a total of 384 times each over the course of an hour:

Ratio . . Volume (cu-in) . . Revolutions . .RPH . . . . .RPM . . Fuel oz . . . . Gallons
14:1 . . . . 31462 . . . . . . 683.85 . . . 262599 . . . . 4376.66 . . 384 . . . . . . 3
12:1 . . . . 31468 . . . . . . 683.98 . . . 262650 . . . . 4377.49 . . 448 . . . . . . 3.5
10:1 . . . . 31476 . . . . . . 684.17 . . . 262720 . . . . 4378.67 . . 538 . . . . . . 4.2

Again we're looking at these numbers from a common sense perspective - so the question is - do these numbers pass the 'smell' test?

Consider the RPM required to consume 384 helpings - how fast does it imply the drive train is carrying the vehicle?

In the case of a '49 chevy fastback with a 4.11 rear-end and 26 inch rubber we'd be traveling in excess of 80 miles per hour when cruising at this RPM.

And now that we have added a distance dimension to our 'volume' model we can calculate efficiencies:

Ratio . . . .MPG @80 @4377 RPM
14:1 . . . . 26.67 (3.0 gals)
12:1 . . . . 22.86 (3.5 gals)
10:1 . . . . 19.05 (4.2 gals)

What happens if we change the rear gear to a 3.55? But we hold the RPM constant:

Ratio . . . .MPG @92
14:1 . . . . 30.83
12:1 . . . . 26.42
10:1 . . . . 22.02

What happens if we add a .72 Overdrive on-top of the 3.55 gear? But again holding the RPM constant:

Ratio . . . .MPG @128
14:1 . . . . 42.67
12:1 . . . . 36.57
10:1 . . . . 30.48

If at this point you are thinking 'balderdash' - then we're on the same page!

Let's instead add the 3.55 gear while holding the speedo constant. RPM drops by (3.55/4.11) as does the volume of the fuel/air 'helpings' consumed:

Ratio . . . .MPG @80 @3780 RPM
14:1 . . . . 30.87 (2.6 gals)
12:1 . . . . 26.46 (3.0 gals)
10:1 . . . . 22.05 (3.6 gals)

And then add a .72 over drive again while holding the speedo constant. RPM drops by (3.55/4.11)*.72 as does the volume of the fuel/air 'helpings' consumed:

Ratio . . . .MPG @80 @2722 RPM
14:1 . . . . 42.88 (1.9 gals)
12:1 . . . . 36.75 (2.2 gals)
10:1 . . . . 30.63 (2.6 gals)

What is interesting to me is the symmetry that results from such single dimensional analysis. A 10:1 ratio can be used either to go 80 MPH @2722 RPM or 128 MPH @4377 at the exact same efficiency 30.5 MPG?

But this too perhaps belies the smell test when other dimensions are considered.

What about the power contained in a gallon of gasoline? The theoretical number is 49HP/hour - but physics says about 65% of that is lost to heat in the process - leaving perhaps 17 HP/hour per gallon.

Is 71 HP enough to maintain 128 MPH? (17 X 4.2 gallons)
Is 44 HP enough to maintain 80 MPH? (17 x 2.6 gallons)

When viewed from this perspective the ability of an OD to extend efficiency at lower RPMs seems more likely then it's ability to extend top end speed at a given RPM without actually burning more fuel. The former is about efficiency while the latter is about power (which an engine is either making or it isn't). Moreover, in the same way that on OD doesn't make power - it doesn't rob it either at cruise.

I am including some links that I have found useful:
Calculating Wheel HP

It is surprising how few HP are really required to maintain speed at cruise - and that wind resistance is at play - not weight.

The impact of small things on an engine at cruise is equally interesting:
Day Time Running Lights and MPG

head lights, pumping losses? . . . so many dimensions to get one's head around when pondering efficiency at cruise.


See where I am coming from?

regards,
stock49

Wow....maybe I need to start drinking coffee in the mornings.

Gasoline weighs about 6.15 pounds/gallon, water weighs about 8.3 pounds/gallon.

stock49,

I like your thought process. To improve your analysis you need to add volumetric efficiency. VE is how much mixture is actually trapped in the cylinder when the intake valve closes divided by the amount that could be if the valve closed at exactly BDC and wide open throttle.

VE will be higher at low speed open throttle compared to high speed closed throttle.

Engine performance equation: Brake HP = fuel heating value x fuel/air ratio x inlet density x suction volume x volumetric efficiency x indicated efficiency x mechanical efficiency x engine speed.

The value of this equation for a hot rodder isn't to start from scratch. Rather it is to understand the effect of changing one of the variables.

You're are correct Titen. The site I visited was comparing gasoline to water - and I cut/pasted the wrong value. Duh!

This means my model is 'rich' because it over estimates the weight of gasoline. This doesn't really impact the 'concept' of the model which quickly moves back to liquid ounces.

StrokerSix - I used a constant v/e of .85 in the model to make the math easier. But your point is well taken. Cruising involves bringing just enough HP to bear in order to overcome wind resistance. At lower RPM the the engine brings more fuel to bear per revolution - which is more efficient - then revvving higher at a lower v/e.