Understanding Second Chance Garage's Vintage Auto Engine Specs
As requested, here a short explanation of the headings shown in our engine specs pages. Many people use the specifications information for tune-up purposes, but there are some who would like to understand the terminology a little better. Note: specifications are presented as written the year of the car manufacture; some items, such as spark plug designations, etc. will most likely need to updated to those currently available. So, line-by-line, here's what all this means:
Model Designation: This refers to the model (or models) in which a given engine was installed, as well as the range of transmissions offered.
Wheelbase: The wheelbase is the distance from the centerline of the front wheels to the centerline of the rear wheels.
Valve Location: Flathead engines have valves (and springs) imbedded in the cylinder block and overhead valve engines have valves (and associated rocker arms, springs, etc.) mounted in the head.
Bore and Stroke: The bore is the cylinder diameter in inches. The stroke is the maximum distance the piston moves in the cylinder, from top to bottom.
Compression Ratio:This is the ratio of the volume in the cylinder with the piston at the bottom of the stroke to that when the piston is at the top of the stroke.
Maximum Brake Horsepower: This is the amount of power the engine produces on a test stand with no accessories. Considerably less power is actually measured at the drive wheels.
Maximum Torque Lb-Ft @ RPM: The torque is measured on the test stand as well and typically is the greatest amount at a specific engine speed. Torque is measured in pounds-ft.
Normal Oil Pressure: Oil pressure is measured in psi (pounds per square foot) and in engines is measured at running speeds.
Spark Plug Make: This refers to the OEM supplier that provided spark plugs for this engine. The plug's model # gives the size, heat range and other important information. You do NOT have to replace these with the same make/model. You may use any plug manufacturer as long as their plugs match the OEM characteristics. All plug manufacturers have interchangeability charts.
Spark Plug Gap: The plug gap, expressed in thousandths of inches, is that specified for the engine. It is the theoretical "perfect" gap for maximum life and minimum fouling.
Firing Order: The engine's cylinders have a specific firing order designed by the engineers who developed it. It must be followed exactly when installing ignition wiring.
Ignition Timing: This information tells you where the correct timing reference marks on your engine's crankshaft are located. It usually indicates the number of degrees off TDC (top dead center) that the distributor must be set, using a timing light.
Engine Idle Speed, RPM: Most manufacturers specify an idling speed, at which all tuning adjustments should be made and which puts the least load on automatic transmissions and other components. This speed should not be set any lower, but may be set higher if one wishes to do so.
Cylinder Head Torque, Lb-Ft: This important number is the final torque that the head bolts must be set to in order for the engine to run properly and retain structural rigidity. Bolts must be set with a torque wrench. Setting bolts at lower than spec torque readings can result in failure of head gaskets and higher settings can warp the cylinder block and result in engine failure. The head bolts should be tightened in the particular sequence shown in the shop manual for each specific car.
Compression Pressure & Cranking Speed: The compression pressure is the engine's designed-in pressure during cranking, expressed in PSI. It is used to determine individual cylinder condition, such as leaking valves or damaged rings. The cranking speed is the rotational speed that the starter motor drives the crankshaft.
PISTON AND RING SPECIFICATIONS
Fitting Pistons: These measurements indicate the piston-to-cylinder clearances and/or a measurement of the force needed to slide the piston down the cylinder.
Rings: The number and type of rings and the end clearances are given in this specification.
Wristpin Diameter: The diameter of the pin that holds the top end of the connecting rod in the piston
Operating Clearance: The clearance between the top of the piston and the valve heads.
Valve Seat Angle: The tapered angle to which the valve head and valve seat are ground in order to create a perfect seal when the valve is closed. This might be a two or three angle figure.
Valve Timing: The number of degrees of rotation of the camshaft that the individual valves are open, as well as any overlap between intake and exhaust valves.
Valve Spring Pressure at Length: The pressure needed to compress a spring to a specific length. It is used to determine whether a spring is fatigued enough to require replacement.
Valve Life: The number of operating hours or cycles of operation that a valve might be expected to endure before failure.
Valve Stem Clearance: The clearance between the valve stem and the valve guide in the head.
Valve Stem Diameter: The diameter of the valve stem.
ENGINE BEARING SPECIFICATIONS
Connecting Rod Bearings:
Journal diameter — The diameter of the rod journals on the crankshaft.
Bearing clearance — The designed-in clearance between the bearing surfaces and the journal, typically between .002 and .006 thousanths inch.
Journal diameter — The diameter of the crankshaft main bearing journals.
Bearing clearance — The clearance between the bearing surface and the journal surface.
Crankshaft endplay — The amount of movement fore-and-aft of the crankshaft as it is installed in the engine.
Part number: Each engine has a distributor specific to its operation and it is given an individual part number.
Cam angle: The angle in degrees of the surfaces on the breaker point actuating cam.
Breaker point opening (gap): The amount of open space that the breaker points must be set while the wiper is on the tip of the actuating cam. This is expressed in thousandths.
Condenser capacity: The size, expressed in microfarads, of the ignition condensor.
Breaker arm spring tension: The amount of spring tension in the breaker points assembly. This dictates the ability of the points to recover from extremes of movement.
Starting at degrees — The number of degrees of rotation of the crankshaft at which the centrifugal advance weights on the distributor shaft begin movement.
Full advance at — The number of degrees of rotation of the crankshaft at which the centrifugal advance is at its limit of movement.
Inches of vacuum to start movement
Inches of vacuum for full plunger travel
Maximum vacuum advance in distributor degrees