Packaged performance is one way to describe the interrelated changes that should be taken into consideration when any part or system if modified. Example: If I increase my engine’s horse power output by 28% (the net HP difference between a MKI 260 and MKII 289) generally no changes are needed! The best way to approach increasing or improving performance is to realize that the entire car is involved. When an automobile is presented for sale by a manufacturer, it is designed to fit the needs of the broadest segment of buyers in its class. This means that a number of compromises are interrelated throughout the vehicle. Example: generally when handling is improved through greater roll stiffness, ride becomes firmer and up the scale to hard. Carburetion works the same way. One of the most common mistakes made in an effort to obtain more horsepower is over-carburetion; that is installing a carburetor that is too large for the application-which results in poor throttle response, decreased economy and a loss of torque and horsepower. The proper size carburetor for an engine is basically determined by the engine’s efficiency as an air pump or its ability to intake air and fuel, bum it and expel exhaust gases. An engine’s efficiency in doing these three things is largely a function of:
(1) manifold type and design.
(2) intake port size and finish,
(3) valve size,
(4) valve timing (camshaft),
(5) compression ratio,
(6) exhaust valve size,
(7) exhaust port size,
(8) exhaust manifold type and design.
For the purpose of this article and performance tuning. we start with the assumption that the engine’s systems are 100% efficient. True efficiency is determined by the above eight basic variables. Now we can set up a formula that will give us a base line or starting point.
RPM= Engine Revolutions Per Minute
CID= Cubic inch Displacement of the engine
1728=the number of cubic inches in one cubic foot
CFM= Cubic Feet Per Minute
First we set RPM at the point in the scale in which we are interested such as 4400 (the horsepower peak for the 260-289 Tiger). Second, since the engine is a four cycle and air is taken in and exhausted on every other stroke, we must divide by two. Third, enter the engine displacement figure (CID) and multiply (4400 / 2 = 2200 x 260 = 572,000. Fourth, divide again, this time by 1728 = 331.018CFM air pump rate. (RPM/2 x CID) -1728 = CFM. These air flow rates (assumed 100% efficient) appear in the following chart, column one. A number of recent dynamometer tests have shown that maximum performance (HP) is derived from the Ford small block engine, single carburetor version when an efficiency factor of .5 is used in determining carburetor size. This provides the air flow rates found in column three.
NOTE: The use of carburetor sizes found in this column REQUIRES improved intake manifolding and cam timing. Column number two, air flow ratings, are found using an efficiency rate of 1.25. These ratings are based on the Ford muscle parts program which calls for a 600 CFM rated carburetor for engines operating in the 5500 to 6500 RPM range with displacements of 289 and 302 cubic inches. It must be remembered that Ford has been generally conservative in carburetor sizing, preferring more response torque at lower RPM ranges than can be obtained with larger carburetors (which will provide more top end horsepower.) Before we go further, remember 2BBL and 4BBL carbs are NOT rated in exactly the same way. The pressure drop or rating used in rating a 4BBL is three pounds and pressure drop used in rating a 2BBL is 1.5 pounds; this in effect makes the 2BBL look like it will flow more air than it really will in comparison to a 4BBL. For example, a Holley 750 CFM #R4779 4BBL is a square carb (all the venturis and throttle bores are the same sizes). Venturis are 1-·3/8″and throttle bores are 1-11/16″. Now divide 750 by 2 and get the rating for each 2BBLS, 375 CFM. O.K., so far. The Holley R4412AAS 2BBL is one of three performance 2 BBL’s offered. This carb has venturies of 1·3/8″and throttle bores of 1-11/16. Its CFM rating is 500, a difference of 125 CFM or 25% due to the rating procedure difference. Therefore, it is reasonable to re-rate these carburetors so we can apply them to the chart. Holley 350=262.5 CFM, Holley 500=375 CFM, and the Holly 650 = 487.5 CFM (Due to design factors this carb is not recommended for street use or under full power under 4000 RPM).
|Tiger II||Tiger I|
|Acceleration in Top Gear|
|20-40 mph||5.4 sec||5.8 sec|
|30-50 mph||5.2 sec||5.8 sec|
|40-60mph||5.0 sec||5.8 sec|
|50-70 mph||5.4 sec||6.2 sec|
|60-80 mph||6.2 sec||7.2 sec|
|70-90 mph||6.8 sec||8.8 sec|
|80-100 mph||8.4 sec||?|
|Accereration Through Gears|
|0-30 mph||3.0 sec||3.6 sec|
|0-40 mph||4.2 sec||5.0 sec|
|0-50 mph||6.3 sec||7.0 sec|
|0-60 mph||7.9 sec||9.2 sec|
|0-70 mph||11.0 sec||12.3 sec|
|0-80 mph||14.0 sec||15.3 sec|
|0-90 mph||17.8 sec||21.7 sec|
|0-100 mph||22.7 sec||?|
|Mean 1/4 mile times||122-125 mph||117-120 mph|
|Average consumption on Edge Hill course at average speed of 26 mph||21-24 mpg||18-23 mpg|
|Fuel Consumption At Constant Speeds|
|30 mph||31.1 mpg||33.8 mpg|
|40 mph||31.2 mpg||33.1 mpg|
|50 mph||29.9 mpg||29.6 mpg|
|60 mph||26.9 mpg||27.2 mpg|
|70 mph||23.5 mpg||24.0 mpg|
|80 mph||20.9mpg||21.7 mpg|
|90 mph||18.2 mpg||19.6 mpg|
|100 mph||15.6 mpg||?|
The basic guidelines for choosing a carburetor are:
(1) For factory stock engines with compression ratios in the 8′ s, stock cams and cast iron, low rise manifold, use column #1 for best overall performance.
(2) For engines with some modifications such as high rise or Torker type manifolds with stock internals, column #2 should be used. 470 CFM Ford or 450·500 Holley is the best on high rise manifolds, good for 30 HP or 289 and 360.
(3) For engines with improved manifolds, factory type Hipo cams (solid or hydraulic), best results overall will be had using column #2. If headers are used, maximum power will be obtained.
(4) Column #3 will provide maximum power when engines have compression ratios raised to 10.5- 10.7 (289 HP), larger valves, headers, high rise or Torker type manifolds and cam shaft timing on the order or the Ford LeMans Cam (C7FE-62S0·A). Only when complete modifications of this sort have been made should this column be used.
The proof of this can be found in the Edelbrock test files on the 289 Torker. Between 2500 and 6000 RPM the maximum HP difference between a 600 CFM carb (R 6619) and the Shelby 71S-72SCFM (R4118) was as little as 1 HP at 2500 and 6HP at 5000 on a column #2 engine. Remember that the smaller sizes will provide slightly better throttle response, torque and economy than will larger ones in all three columns of the chart. The following is a list of the CFM ratings and the part numbers for those carburetors. A + indicates dual feed and ++ indicates dual feed, double pumper.
|CARBURETOR SIZING CHART|