Carburetor selection
I came across an article I wrote back in 2001 for the old BEEF mail list, thought it might be of interest to anyone considering a carby change or rebuilding an engine.
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There must be many a V8 Bristol out there with a tired or non original carburetor, so for those who are thinking about fitting a new carburetor I thought I might share a few facts I which learned recently about carburetor selection, and possibly explode a few myths in the process. This is a fairly simplistic view but it should at least point people in the right direction!
Carburetors are rated according to the airflow capacity, that is the volume of air which they can supply or 'flow' in one minute, this is measured in Cubic Feet per Minute or 'CFM'. The air of course combines with a measured quantity of fuel on the way through, which ideally will be vaporized by the time it gets into your engine's cylinders. There is a 'rule of thumb' formula which can be used to match carburetor size or capacity to engine size;
engine capacity (CID), multiplied by maximum engine speed (RPM), divided by 3456 = Carburetor CFM
CID x RPM
__________ = max CFM
3456
For example for a Bristol 411 series 4 or 5 the formula would be (400 x 5500) / 3456 = 636 CFM
However this formula assumes 100% volumetric efficiency (VE), which means the 400 CID engine actually consumes 400 cubic inches of air/fuel mixture for every two revolutions. Which in the case of a normal 400 CID Chrysler engine produced in the 1970's, it doesn't. In fact of the Chrysler V8's used by Bristol, the 400 CID engine, fitted to the 411 S4 & S5 and 412 S1, probably has the lowest VE because of the anti pollution, or 'smog' measures in force in the USA at that time. It is effectively 'de tuned' and probably has a maximum VE of about 70%. All other V8's used by Bristol probably have a maximum VE in the region of 80% to 85%. An engine's VE varies with engine speed and is highest when peak torque is produced.
So taking into account the VE of the 400 CID engine fitted to Bristols, it really only NEEDS a carburetor which flows about 445 CFM.
CFM ratings are only a guide and it is possible for a four barrel carburetor to flow at least 20% more air than it is rated. At the other end of the scale they can of course flow a lot less, in fact the secondary throttles on some four barrel carburetors are activated by manifold vacuum controlled air valves (commonly known as 'Air Valve Secondaries'). If these are working properly the carburetor will only flow the amount of air required by
the engine. (Manifold vacuum varies with engine speed and load)
However, most carburetors are fitted with jets, metering rods, etc, which provide a fuel supply in keeping with their air flow rating. So the greater the CFM rating the bigger the standard jets will be and vice versa, (to allow the correct maximum air/fuel mixture). Jets are of course changeable (as are often metering rods ) so there's nothing stopping you from re-jetting a carb that is slightly too small or too large for your engine to make it more suitable. There are however, other specifications which may need to be modified to achieve optimum performance from a wrong sized carb. Including but not limited to; metering rods, air valve adjustment, inlet 'needle and seat' size, air bleed, float level, float drop, accelerator pump adjustment, enrichment circuit timing. Most of these modifications should not be attempted by anyone other than an experienced specialist.
When a carburetor is significantly larger or smaller than the engine requires, these issues are exacerbated because the manifold vacuum becomes too high or too low. Larger carbs, for example, have larger throttle bores and venturi area, which means the manifold vacuum will not reach anywhere near that which the carb is rated at WOT wide open throttle (1.5-inch/Hg), unless the engine is highly tuned to a greater volumetric efficiency and probably higher maximum engine speed. Generally speaking any significant variance in manifold vacuum from the standard at which a carb is rated will make carburetor tuning very difficult.
Many people have been, and probably still are, led astray by the carburetors fitted as standard by American muscle car manufacturers in the 1960's. According to the formula given above, it would seem they fitted oversize carburetors to some particular production model Corvettes, Camaros and Mustangs. Well they did! But that's because these cars were frequently entered in sanctioned competition events where the standard equipment carburetor had to be retained. So when competing they tuned the engine 'up to' the capacity of the carb.
The moral of this story is, if you need a new carburetor for your Bristol, unless the engine has been seriously modified for performance, use the formula above to correctly size your carburetor and it will likely work very well 'straight out of the box' with very little alteration.
NB: Carter, and most other manufacturers of 4 barrel carbs, made different models for different engines (Chevrolet, Pontiac, Ford, Chrysler etc). Make sure you get the correct one for your engine or the throttle linkages won't be correct, the choke actuation will likely be incorrect, and it probably won't fit your intake manifold without an adapter plate!
Kevin Howard
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