Recently, Bubbette and I were sittin' around discussing the molecular structure of a watermelon when Billy Ray stopped by.  Billy Ray had just bought a like-new disassembled engine (most of which was in a bucket), but he didn't know what size it would be once it was assembled.  Now, since the class he races in has a cubic inch rule, this was very important.  After some thought, I figured that with the correct mathematical equation we could formulate the computation needed to calculate a formula to cipher out this problem. Although this sounds pretty simple, it did prove to be quite a task for Billy Ray to understand it all.  Since this did work out, I thought I would share this equation, and one or two others that have proven to be correct and sometimes hard to find.

WE FIGURED THE  CUBIC INCHES THIS WAY

Bore x Bore x Stroke x .7854 x Number of Cylinders=C.I.
Example in this instance: 350 Chevy
Bore = 4.00"
Stroke=3.48"
Number of Cylinders= 8
4.00x4.00x3.48x.7854x8=349.8486 cubic inches, Rounded to 350

This formula proved to be helpful, so we looked for a few more that are little known, but somewhat useful in engine building or whatever, and, if not useful, just interesting to know.

ROCKER ARM RATIO CHANGES

If you are wanting to add more performance to your engine without changing camshafts, you can simply add more valve lift by changing rocker arm ratios.  You now ask, how much lift will be added?  First, you must know what ratio rockers you currently have and the valve lift of the camshaft utilizing that rocker ratio.

EXAMPLE:
ENGINE- 350 CHEVY
VALVE LIFT- .500
ROCKER RATIO- 1.5

First divide the valve lift by the rocker arm ratio to get the cam (lobe) lift.  Next multiply the cam lift by the new rocker arm ratio (in this example we are using a 1.6) to get the new valve lift.  Caution should be used in doing this.  You should be sure your valve springs can handle the additional valve lift and that you have enough piston to valve clearance.

FORMULA:

valve lift/rocker ratio=cam lift
.500/1.5=.333
cam lift x  rocker ratio=valve lift
.333x1.6=.532

AIR CLEANER SIZE FORMULA
This formula can be used to determine the size your free flowing K&N filter needs to be.
EXAMPLE:
ENGINE--CHEVY 350
RPM--5500
Formula:
A=(CID X RPM) /20.839
A= effective filtering area
CID= cubic inch displacement
RPM= engine revolutions per minute at maximum horsepower
A=(350X5500)/20.839=92.374 SQ. IN.

To convert the square inches to a round filter use the following formula to determine the height.  We have chosen our diameter to be 14".   The reason for this is because most aftermarket cleaners of various styles and colors are available in primarily 14", therefore, there are more choices available.

H=(A/Dx3.14) +0.75
A= effective filtering area
H= height
D= outside diameter of the filter
3.14=pi
.75= rubber end caps on filter
FORMULA
92.374 / (14x3.14) +.75=2.85" rounded 3.00"
We need a minimum element for our 350 at 5500 RPM to be 14"X3"

CRUISE RPM: If you know your axle ratio and tire diameter and want to know the RPM you will be turning at a pre-determined speed, use this formula. If your transmission has an overdrive, final calculations must include the over drive ratio to be accurate. 

MPH x AXLE RATIO x 336
         TIRE DIAMETER
Example:
MPH=60
Rear end Ratio = 3.23
Tire diameter =30"
60 x 3.23 x 336/ 30=2170 RPM

This formula can be used to determine your Axle Ratio calculating this way.
Axle Ratio= RPM x Tire Diameter 
                          MPH x 336
Example:
RPM= 2170
Tire Diameter 30"
MPH =60
2170 x 30 / 60 x 336 = 3.22916 Rounded 3:23

DETERMINING REAR AXLE RATIO

Raise both rear wheels of the vehicle off the ground, and for safety reasons set in on jack stands.  Put your vehicle in neutral.  Make a reference point on the driveshaft and on the rear housing.  Also mark a reference point on the wheel and on the fenderwell.  Have a "friend" watch the driveshaft rotate both tires one revolution.  The number of turns the driveshaft makes indicates the ratio. Example 3-1/2 turns = 3.50

A quick and easy way to verify gear ratio, according to Bubbette, is to check your owner's manual.  We found that to be boring. 

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