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Carburetor Facts and Correcting Myths

First a couple of things that are seldom mentioned but are useful to know.

The cubic free per minute, CFM, rating for 2 BBL and 4 BBL carburetors can not be directly compared.   2 BBL carburetors (and 1 BBLs) are tested with a 3 inch vacuum while 4 BBLS are tested with a 1.5 inch vacuum.   In other words the CFM tests for 1 and 2 BBL carburetors are done with the testing equipment sucking twice as hard.   If you want to compare the CFM rating of a 2 BBL to that of a 4 BBL multiply the 2 BBL rating by 0.707.

Air temperature directly effects power.   Cool air is denser so a more air (oxygen) actually enters the engine.   A 7.2 degree change in air temperature causes a 1% horsepower change.   So if the air temperature under your hood is 180 degrees and the air temperature in front of your hood is 90 degrees then by ducting the cooler air to your air cleaner inlet you can give your engine a 90 degree incoming air temperature drop for a 12.5% increase in horsepower (90/7.2 = 12.5).

[Back in 1974 on my 1969 429 Galaxie 500 I installed an electric temperature gauge and several sending units: one into the air cleaner in the air stream, one just sticking out into the air by the air cleaner inlet, and another near the front grill.   I used a rotary switch to flip between the sending units.   Under the hood temperature on a hot day with the AC running was almost always over 200 degrees and in traffic was usually over 240 degrees.   The temperature of the air inside the air cleaner was the same.   Grill air temperature was never over 105.   I installed a 3 inch diameter air duct from the grill to the air cleaner snorkel and was able to drop the air temperature in the air cleaner to a maximum of 175 degrees at 25 MPH in traffic and down to 120 degrees on the highway.   The result was a better acceleration.   Note: this was in Omaha, NE and I had to disconnect the cold air duct in the winter.]

Most air cleaners restrict air flow significantly.   Testing has shown that the average stock air cleaner assembly with a new filter drops the typical 700 CFM carburetor to 480 CFM.   Adding a second snorkel got it up to 550 CFM.   A 14 inch in diameter, 2 inch tall open element air cleaner gave 675 CFM.   Two such air filters stacked gave the full 700 CFM.   And dirty filters always lower the air flow.

Part I What a carburetor does

A carburetor meters fuel in proportion to the air flowing through it.   A carburetor meters best over a specific air flow range and has extra circuits added to cover idle, power (acceleration), and cold starting.   A fuel air ratio of 12.5 : 1 is good for maximum power, 17 : 1 is good for fuel economy and 14.7 : 1 is the ideal for complete burning and lowest emissions.   Most people try for 14.7 since this is where the engine runs best and since too rich (12.5) wastes fuel and too lean (17) makes an engine run hot.   There are other things such as air temperature and barometric pressure which vary quite a bit so a carburetor is not set up for peak power (or maximum economy) unless you are prepared tweak it like racers do just before each race.

Ignoring idle speeds and the transition from idle to open throttle, a carburetor meters the fuel based on the volume of air flow through the carburetor and the pressure difference between the air outside the engine and the manifold vacuum.   Sounds easy enough; just open the throttle plates and sense the air flow and then dump the fuel into the air stream.   And usually that works fine, especially on a stock engine where these signals, air flow and intake vacuum, work as designed.   The engine runs smooth and pulls hard when the throttle is wide open at low to medium RPM.

Part III Reversion (Greatly Simplified)

The catch is we modify our engines for more power and that changes things.   One common modification is to change the camshaft for more power.   Most after market camshafts have higher lift and longer duration.   Higher lift is not a problem (for this discussion) but this duration thing is.   Duration is the length of time the valves are open.   RV and high performance camshafts increase the intake valve duration by 10 degrees and may it may be increased up to 60 degrees, with more increase for hotter camshafts.   And increased duration increases reversion.   The rump-rump sound of a engine with a "hot" cam at idle is mainly reversion in action.

Reversion is the flow of the air fuel mixture out of the cylinder and into back into the intake manifold.   Reversion happens even on a stock engine because the piston is moving up the cylinder before the intake valve closes.   Leaving the intake valve open a bit as the piston starts back up the cylinder makes more torque (and horsepower) at medium to high RPMs since every camshaft closes the intake valve when the piston is moving up the cylinder on the compression stroke.   A cam with a longer duration increases reversion since more of the air-fuel mixture in the cylinder is pushed back into the intake manifold just before the intake valve closes.

Reversion, if increased very much, disrupts the fuel metering process of the carburetor.   This is because the air flow is no longer smooth.   Some of the air is being pushed back toward the carburetor and if too much air is pushed back, air stops flowing for a tiny bit of time and then starts again.   The problem is that carburetor reacts slowly to this, so air gets through the carburetor with the incorrect amount of fuel.   Specifically the fuel metering becomes inaccurate when reversion is very high.   So the engine runs rough and torque drops a lot.

At low (500 to 600 RPM) idle speeds the air is moving very slowly in the intake manifold.   A slight increase in cam duration can easily push some air back to the carburetor.   To over come the reversion problem with hot cams, the idle speed is increased.   It may not seem like much but raising the idle speed from say 600 to 800 RPM, for a 25% increase in the air flow, is enough to reduce reversion effects so the engine will idle and not die from poor fuel metering.

Another common performance improvement trick is to raise the compression ratio.   This too this causes more of the air-fuel mixture in the cylinder to be pushed back into the intake manifold just before the intake valve closes.   Not a major reversion increase but every little bit hurts idle and low speed performance.

[In1969 there were two 390 2 BBL passenger car engines one with 9.5 and the other with 10.5 compression ratio.   These two engines were identical except for their compression ratio and camshafts.   The low compression engine had a camshaft which opened the intake valve at 13 degrees before TDC and closed it at 63 degrees after BDC.   The high compression 2 BBL engine and the 4 BBL engine camshafts opened the intake valve at 16 degrees before TDC and closed it at 60 degrees after BDC.   Thus the higher compression engine's camshaft closed 3 degrees earlier to reduce reversion and to keep the engine running as smooth as possible.   These specifications are in the 1969 Ford shop manual.   This conclusion is mine.]

Part IV Choosing a Carburetor

Almost any carburetor will work on any engine.   Well, within reason, a Holley 850 won't work on a chain saw and a lawn mower carburetor won't work on at NASCAR.   But one that is too small will limit peak power and one that is too large will hurt low RPM torque.   A carburetor that is too small is usually easy to detect; the engine accelerates well at low RPM but does not reach the RPM expected under load.

An over sized carburetor is harder to detect.   A carburetor meters the fuel based the volume of air flow and on the pressure difference between the air outside the engine and the manifold vacuum.   At low RPM and wide open throttle there is less difference and not much air flow.   The larger the carburetor the harder it is for the carburetor to measure the small air flow and the these pressure differences.   So if you want the carburetor to work well at low RPM (good low RPM throttle response) you select a small carburetor.

The 1967-68 390 2 BBL and 4 BBL (non GT) engines provide a good example of carburetor size impact.   (There were two 390 2 BBL engines but the one with the 10.5 compression of the 4 BBL engine is the one I'm referring to in this example.)   These 2 BBL and 4 BBL engines used the same heads, valves, and exhaust manifolds and the camshaft was almost the same.   The 2 BBL engine had a 351 CFM Autolite 2100 carb and the 4 BBL engine had a 430 CFM Autolite 4100 carb.   Using the conversion factor the 2 BBL carburetor only is rated at 248 CFM.   So the 4 BBL flows 73% more air than the 2 BBL.

2 BBL   250 CFM HP of 280@4400   Torque of 403@4400

4 BBL   430 CFM HP of 315@4600   Torque of 427@4600

The RPM given above was where the highest Torque or HP occurred.   Notice that the larger carburetor made more HP and Torque but this was at 200 higher RPMs.   (But only 200 more RPM for 35 more HP from a 73% larger carburetor.   Well the stock FE exhaust manifolds are the problem as they are a major restriction.   Add a decent set of headers and pick up 20 or more horsepower as those exhaust manifolds are like corks above 4000 RPM.)

An engine draws air into the combustion chamber every other revolution (and exhausts it on the next).   So a 390 cubic inch engine at 5000 RPM can theoretically use 390*5000/2 or 975000 cubic inches of air per minute.   Lets divide that by the number of cubic inches in a cubic foot (12*12*12 or 1728) which gives us 564 CFM.   (The short cut formula is: Engine size in cubic inches times peak RPM divided by 3456 to get CFM of airflow.)

So 564 cubic feet of air is being pumped through a 390 cubic inch engine at 5000.   But that's theoretically.   Because the intake side of the engine restricts air flow and so does the exhaust system.   Thus the actual air flow is closer to 80% of the above number.   So 80% of 554 is 451.8 and a 450 CFM carburetor would work fine.   Well, actually only for a truck at low to moderate RPM is this true.   My experience has taught me to use the 564 value for street use.   For street-strip add 10 to 20% (but remember that low RPM performance will decease as carburetor size increases) and for racing go as big as the rules will allow but not more than twice the CFM capacity.   (Bigger than twice the CFM capacity and the carburetor signal strength won't be high enough to meter the fuel accurately.)

If your are running empty and want to run hard on the street or track at high (over 5000) RPM then a big cam and large carburetor are great.   But if you're carrying a heavy load or pulling a large trailer then you should be more concerned with low to medium (1600 to 3500) RPM performance; a smaller cam and a 550 to 600 CFM carburetor would probably work a lot better.

Part V Adjusting a Carburetor

The carburetor should be adjusted or tuned for the engine it is installed on.

Changing the jets or the power valve (or on a Edelbrock carburetor metering rods and their springs) is done to adjust for any unusual engine air flow optimizations or inefficiencies.   Changing the jets in a carburetor adjusts the air fuel ratio and we normally want 14.7 or so.   Re-jetting the carburetor for the load and driving style (economy or max acceleration for jack rabbit starts) is part of tuning the carburetor also.   Re-jetting does not change the amount of air that will go through the carburetor.   It just changes the amount of fuel; all re-jetting does is modify the mixture ratio.

Re-jetting to save fuel is often done but the adjustment range is limited and effects how well the engine runs.   Remember a lean carburetor mixture usually makes the engine run hotter; if it doesn't run hotter it may have been too rich.

A good way cheap way to detect if the mixture is OK is to put in new plugs and make a run down a nearby flat stretch of highway under a light load.   Just accelerate up to highway speeds and hold it for a few miles.   A very slight up-hill grade is OK but down hill grades won't work for this since you want the engine to be working a small amount, not coasting.   Pull off on the side of the road or into a rest stop but do not idle for more than a few seconds.   Remove your spark plugs and see what color the tips are now.   A light tan is ideal.   Brown is rich.   Black is way too rich.   If the tip is still uncolored (white) the carb is too lean.   You may not have a lot of color build up if you only went a short distance but the color will be there.

To adjust for acceleration or pulling up hill you will need to repeat the above on an uphill stretch of highway or by accelerating from a stop.   You don't however change the jets; for a Holley you change the power valve to adjust fuel metering under acceleration.   On the Edelbrock carburetors you change the metering springs.

Here are some factory specifications for the Autolite 2100 and 4100 Carburetors.

In response to someone's question regarding a bogging problem on the starting line I wrote the following:

Here's some thoughts on your double-pumper carburetor.   First, I've never used double pumpers.   And I've never raced automatics so this may not apply to in your case and that's one of the reasons I haven't commented until now.

First your bogging may be a converter stall issue and deciding that is not something I would even comment on.   But if its a carb problem then the following may help.

When you dump the throttle to leave the line you have air in the manifold that had fuel that was metered based on a non-accelerating engine.   IE the engine was reved up and held at a certain RPM while you waited for the green light; when you opened the throttle the air in the manifold does not have a good mixture for acceleration (max power).   And it takes a several hundredths of a second for the carburetor to change modes and start metering the fuel for max power.

The accelerator pump is supposed overcome that delay and richen the mixture until the carb starts providing the right mixture.   The accelerator pump system has three variables, the outlet hole size of the squirter, the shape (and placement) of the cam, and the volume of the pump. (Double pumpers have a pair squirters, two cams, and to pumps making tuning twice as much fun; most people just make the same adjustments to both.)

The squirter outlet size determines the amount of fuel that is delivered.

The cam determines when the extra fuel is delivered.   For example you don't want the pump stroke to be completed before the green light comes on; you therefore determine what the throttle shaft position is when you are staged and reved up and then chose a cam and position for the cam that lets it push on the accelerator pump's lever (add fuel) when the light turns green and the throttle is opened.

If you have a large carburetor or a large volume of air in the manifold because it has a large runners you may need a 50 cc pump instead of the normal 35 cc pump.

Double pumpers are carburetors with mechanical secondaries and I have only hear-say knowledge since I always use vacuum secondaries.   Double pumpers were designed for race only cars and originally were used on tunnel rams.   Most cars don't need them as they only work well if you launch at very high RPMs about 75-80% of your RPM at the end of the track and have a high stall converter.   Of course there are exceptions to every rule and again I have no personal knowledge as I don't use them.

Regarding carb spacers, usually you match the spacer to where you want to make power.   The open spacer helps the high RPM end, 4 holer helps the low to mid RPM.