Welcome to TMP Carbs' High Performance and Racing Carburetor Blog

TMP Carbs is dedicated to the continuous advancement of the internal combustion engine for pleasure, sport and environmental purposes. Enjoy the blog and feel free to contribute or comment as you desire.

Racing Carburetor CFM / cubic inch displacement

I'm always surprised at how many people running high performance and racing engines run small or conservatively sized carburetors.  Many do not realize they are.  They believe a double pumper 750 Holley on a 383 / 406 / 408 / 427 / 434 is a high performance package.  Still others who are building upwards of 500 cid engines and larger believe a 1000 cfm 4150 or a 1050 Dominator is a massive high performance carburetor on those engines.  The torque and horsepower loss can be measured in a manner of speaking very simply with a vacuum gauge @ at launch and WOT.

Optimizing power production on high performance and racing engines is largely about air flow and cylinder filing - with regard to induction systems / carburetors.  Optimized high performance and race engines will see not more than .5" lbs. of vacuum @ WOT.  I've personally observed other engines which drop vacuum momentarily to zero (with a hand held, auto parts store vacuum gauge) at launch and perform exceptionally well.  If the an engine is pulling more than 1.0" lbs. of vacuum @ launch there is power left on the table.

Not all carburetors are capable of metering / delivering fuel at these levels of vacuum.  It does take an exceptional carburetor design and tune to take maximum advantage of an engine's pumping abilities.  Enter, TMP Carbs...

                                                    Click on imagine to enlarge

Truth is, most carburetor size / application recommendations are sized based upon first; limitations of the carburetor's design and second; the extent or degree of tuning the carburetor will likely receive optimizing it to the engine / application to ensure customer satisfaction.  The larger the carburetor, the more critical tune is which brings me to another part of the equation, baseline tune or 'out of the box' tune'.

I still hear people talking about how great "X" brand of carburetor worked right out of the box, that they didn't even need to turn a screw and it ran great.  As a carburetor guy of 32 years, the only conclusion I can draw from those types of comments are that the person giving the apparent "unsolicited" testimonial hasn't got a clue about what 'running great' is.  Certainly, there are those times when a 'bench calibrated' or carburetor which is built for a given application is on the money, but experience has taught me that by far it is certain there is more performance and throttle response left on the table.

I began my foray into building high performance / racing carburetors returning carburetors built by big name carburetor builders / modifiers and know that at best, all any of us can do is provide an educated guess.

Along those lines, I should point out with regard to high performance and racing engine's that the more "generic" a carburetor's design and tune or calibration is, the less performance that carburetor will optimize any given engine's performance.  The more "aggressive" the carburetor's design and tune, be it pushing venturi size or in the fuel curve, the more critical and attention must be paid to tuning.  If you are looking to make the most of an engine's performance or maximize consistency the results are are not only cost effective, but well worth the effort.

Back to carburetor sizing: if we stop and consider the "rated" CFM of any given carburetor divided by the engine's displacement you can get a more realistic idea of degree of "high performance" a carburetor offers a particular engine displacement.

For example, a stock early 1970's 350 SBC equipped with a four barrel Rochester Quadra-Jet rated at 710 CFM provides 2.03 CFM per cubic inch displacement.  Granted, the Quadra-jet is a vacuum secondary carburetor, but let's also consider the specifications of a Camaro or Monte Carlo or whatever your favorite Chevy of the era is: weight in the 3500 +/- lbs. range; exhaust manifolds; stock non-performance mufflers and exhaust system; sleepy hydraulic tappet cam; cast iron intake manifold with heat cross-over; low flow air cleaner and filter, conservative rear gear ratio; and all designed for grandma to drive it cross-country; drive over the Rocky Mountains in the heat of summer with four or more people in the car; full tank of gas; luggage in the trunk and to do all this for 100,000 miles reliably.

A high performance / race car will most likely be lightened; have low gears; headers; high flow mufflers and exhaust system; aggressive solid flat tappet cam if not a hydraulic or solid roller using modern cam lobe technology (way beyond what existed in the 1970's let alone in an OEM applications); high flowing modern technology cylinder heads and intake manifold; modern racing ignition system; stall converter; etc.

Consider the following carb applications:

  Carb        Engine          CFM /             
  CFM         CID               CID
  710            350                2.03
  750            350                2.14
  750            351                2.14
  750            377                1.99
  750            383                1.96
  750            406                1.85
  750            408                1.84
  750            427                1.76
  750            434                1.73
  750            455                1.65
  750            468                1.60
  850            350                2.43
  850            351                2.42
  850            406                2.09
  850            408                2.08
  850            427                1.99
  850            434                1.96
  850            454                1.87
  850            468                1.82
  850            500                1.70
 1000           350                2.86*
 1000           351                2.85*
 1000           406                2.46
 1000           408                2.45
 1000           427                2.34
 1000           454                2.02
 1000           455                2.02
 1000           468                2.14
 1000           500                2.00
 1050           350                3.00*
 1050           351                2.99*
 1050           377                2.79*
 1050           383                2.74*
 1050           406                2.59*
 1050           408                2.57*
 1050           427                2.46
 1050           454                2.31
 1050           455                2.30
 1050           468                2.24
 1050           500                2.10
 1050           540                1.94

Okay that's enough, you get the idea.  Those CFM / CID numbers listed with the (*) Astrix will drop manifold vacuum @ WOT sufficiently to maximize the flow potential of a high performance / racing engine.

Here's another more generous calculators recommendation for the same 650 horsepower 406 SBC used in the previous calculator above.  Interestingly, it seems a disingenuous as the web page this calculator is placed promotes two companies who DO NOT actually recommend the large sizes predicted by this calculator.

                                                      Click on imagine to enlarge

Unquestionably, racing engine's running in the NHRA's Super Stock / Stock Eliminator, F.A.S.T. or NMRA's Edelbrock Hot Street limiting carburetor venturi and butterfly diameter and the like are producing a significant amount of torque and horsepower.  Nevertheless, similar racing engines with unlimited CFM and induction systems produce significantly more.  Unless, you have the desire or need to limit carburetor CFM / venturi and butterfly diameters, more is better.

TMPCarbs.net carburetors provides the carburetor design to enable the successful use and maximization of torque and horsepower production over a very broad rpm range with carburetors providing well over 2.40 CFM per cubic inch displacement.

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