In this book we will restrict discussions to street performance applications within a range between 300 and 450 hp. We will primarily be covering builds based on 360, 390, and 428 engines. You can certainly make a lot more power using aftermarket or factory high-performance parts, and the comparatively rare 427 block, but that type of build falls outside the context of this volume.
A group of formulas and equations are employed in both stock and high-performance engine building. Some of these are used to determine the basic configuration of the engine, such as displacement or compression ratio. Others are used during assembly to verify measurements such as deck clearance or ring gaps.
These days most of the calculations are readily available on the Internet or as part of engine-building software packages. Instead of listing them all out in complete mathematical detail, I will describe the purpose of the most popular ones, give a simple overview of the math and the impact they have on the build, and provide a couple examples. I will cover the measurement and clearance numbers during the assembly chapters.
Displacement
This is the measurement of “how big” an engine is in terms of cylinder volume. Displacement is referenced in cubic inches (or cubic centimeters). It is a simple cylinder-volume calculation, multiplied by the number of cylinders in the engine. Neither the combustion chamber nor the piston shapes affect displacement.
To calculate this number we need only the diameter of the cylinder and the stroke of the crankshaft (the distance the piston moves up and down during each crankshaft rotation).
To calculate displacement, we take the cylinder bore’s radius squared (the bore diameter divided by 2 then multiplied by itself); multiply that value by PI (22 divided by 7); and then multiply the result by the stroke. You now have the displacement of one cylinder. An increase in bore diameter or stroke gives you more displacement.
This formula can be simplified and calculated as: bore × bore × stroke × 6.2832 inches.
An example for a .030 over 390 would be:
Compression Ratio
This is a comparison of the total volume in a single cylinder when the piston is at the bottom of its stroke versus when it is at the top. The combustion chamber and the piston have a great deal to do with this more complex calculation.
Compression ratio is expressed as a value over 1. We are comparing the total volume above the piston when it is at the bottom of its travel to the total volume above the piston when it is at the top of its travel. If we have 10 times more total volume at the bottom of the stroke than we do at the top of the stroke, we have an engine with a 10:1 compression ratio.
To perform this calculation we need the bore diameter, the combustion chamber volume, and the head gasket volume. You’ll also need the deck clearance volume (the distance from the top of the piston to the top of the block when that piston is at the uppermost end of its stroke travel). The last thing we need is the effective dome volume of the piston; add this if it’s a dish or subtract it if you have a dome.
Take the individual cylinder volume number calculated in the displacement discussion. Add in all the head gasket, combustion chamber, crevice, deck clearance (volume of that small space calculated as a cylinder), and dome volumes (some of those are usually given in cubic centimeters, which you’ll need to convert to cubic inches). The total number is “on top” of your ratio. Now take all those volumes except for the displacement and you have the bottom number in the ratio. On a street engine you should end up somewhere between 9 and 10 to 1. Higher compression ratios will deliver more power, but will not tolerate low-octane pump fuels. On the risk versus reward scale, an extra point in compression might get you another 30 hp but cost you the need for race gas or a detonation-prone combination.
The typical calculation for a normal 390 Ford with flat top pistons (rounded numbers) is:
Bore = 4.050
Stroke = 3.780
Cylinder volume = (calculates to 48.71 ci)
Chamber volume = 72 cc (converts to 4.39 ci)
Deck clearance = .030 (calculates to 0.39 ci)
Gasket volume = 10.2 cc (converts to 0.62 ci)
Piston dish volume = 6 cc (converts to 0.37 ci)
Total volume with the piston at the bottom of its stroke = 54.48 ci
Total volume with the piston at the top of its stroke = 5.77 ci
Compression ratio = 54.48 divided by 5.77 = 9.44:1
Deck Clearance
This clearance is the result of a stack up of component dimensions. It is the distance between the top of the piston at its uppermost travel and the head gasket deck surface of the block. These days we seem to prefer to get this as close to zero as we can without going positive. The currently accepted ideal range for best power and combustion is for the piston to be between .040 and .060 away from the cylinder head at top dead center. To achieve this with the common .040-thick head gasket, we need to have the piston somewhere between .000 and .020 below the block’s deck.
Add up one half of the crankshaft stroke, the center to center length of the connecting rod, and the compression distance of the piston. Compression distance is the measurement from the centerline of the piston’s pin hole to the upper flat surface of the piston.
Example (using a 390 Ford FE and rounding up for simplicity):
Block deck height from factory measured from main center to deck surface = 10.17
1/2 of the 3.78 stroke = 1.89
Connecting rod center to center 6.49
Piston compression distance 1.76
Total = 10.14
Block deck height minus the total of parts = .030 deck clearance
This means that a simple clean-up machining of .010 to the block’s deck will get you into the desired range.
Piston compression distance is a different dimension, and is determined when the piston is manufactured. This is measured from piston pin centerline to the top flat portion of the piston’s head. This may not be the highest point on a piston; a domed piston will have a portion protruding above this point. Your piston manufacturer will usually provide this dimension for you in the instructions or packaging. It can be difficult to accurately measure on your own.
Ford FE Specific Design Choices
With some of the basic concepts in the background, we can take a look at the stuff that makes an FE Ford engine build unique. This will involve a bit of history to provide context to the possibilities within the factory architecture and using factory parts. While several aftermarket stroker packages are available now, this book is focused more on rebuilding and upgrading popular factory-style engines.
The engine pictured here is a completely original 1969 390GT. It has on it about 11,000 miles. The owner installed the vintage Cal Custom valve covers shortly after purchasing the car new, and it had been in storage for decades before he decided to refresh it and put it back into service.
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