ENGINE BUILDING TIPS AND SPECS WE THOUGHT YOU OUGHT TO KNOW COMPRESSION This is not a how-to book. It’s basically a listing RATIO of currently avail able Ford Racing Performance CALCULATION Parts. The pieces can be bought by professionals, professional amateurs, weekend hobbyists or rank beginners. A certain amount of automotive skill is assumed in presenting the parts. Modifying an engine, be it a complete assembly or a bare block, requires experience and know-how. If you don’t know, ask someone who does. Read up and find out all you can before putting down your bucks for those long dreamed of pieces. And if at all possible, consult an experienced engine SYMBOL DIMENSION VALUE REMARKS builder. You may find it to your advantage to have him do a portion or all of the heavy B Bore 4.000 in B2 = 4.000 x 4.000 = 16.000 ma chin ing and wrenching. G Gasket Bore 4.100 in G2 = 4.100 x 4.100 = 16.810 What we have here are just a few of the key P Piston Top Land Diameter 3.965 in P2 = 3.965 x 3.965 = 15.721 bits of information and specs. The idea is to help S Stroke 3.500 in keep midnight thrashing to a minimum, because S/2 Crank Throw 1.750 in parts don’t go together right, or there’s more to a L Con Rod Length 6.000 in job than you imag ined. H Compression Height 1.440 in COMPRESSION RATIO Dh Deck Height 9.200 in Increasing the compression ratio (CR) is often one of the first en gine performance r Ring-to-Top Piston 0.250 in modifications. Squeezing the air-fuel mixture d Piston to Deck 0.010 in Dh – H – l – S/2 into a smaller space increases its temperature t Gasket Thickness 0.040 in and ease of ignition; thus the rate at which 2 x S x 16.387 /4 x B720.7ccCylinder VolumeV heat is ex tracted from the fuel. Engineers call it “thermal efficiency.” Sim ply put, it means Vt Volume Above Top Ring .9cc /4 x (B2 – P2) x r x 16.387 that increasing the compression ratio Vn Valve Notches Volume 4.0cc increases horsepower. Vd Dome Volume 10.4cc Henry Ford’s Model “T” has a CR of 3.6:1. Vp Piston-to-Deck Volume 2.1cc /4 x B2 x d x 16.387 High-performance engines operate in the area 2 x t x 16.387/4 x G8.7ccGasket VolumeVg of 12.5:1. Most of today’s stock pro duc tion engines are about 8.5:1. Vh Volume Head 60.2cc NOTE: Turbocharged engines typically have Vcl Volume Clearance 65.5cc Vt + Vn + Vp + Vg + Vh – Vd a lower CR than normally aspirated CR Compression Ratio 12.0 V + Vcl engines. Thus, if you add a turbo, Vcl you may want to lower the CR, NOTE: 1) Math reduction; /4 x 16.387 = 12.87 de pend ing on performance level. DETONATION The precise amount is limited by block deck Cylinder Head (Combustion Chamber) Increasing the CR changes the rate at which h)—The irregular shape of theVolume (Vheight, casting thickness, valve-to-piston fuel burns. Spark knock (detonation) will occur clearance, etc. combustion chamber requires measurement if certain modifications are not per formed. Here are two of the most important: NOTE: Also modify the intake manifold (popularly called “cc”ing) with a glass burette Ignition Spark Timing—In creas ing the CR to maintain port alignment. and colored liquid, such as A.T. fluid. This catalog lists “nominal” values for Ford Racing heads. requires in stal la tion of new distributor springs COMPUTING Valve Notches Volume (Vn)—Fill notches to change advance curve. COMPRESSION RATIO with soft clay and make level with top of piston. requires gasoline with a high octane rating Compression ratio is defined as the ratio Remove clay with small knife and drop into—Increasing the CR Fuel Octane Rating (with anti-knock components to control between the Total Volume (Cylinder Volume plus graduated cylinder (filled with liquid to det o na tion). This is not a prob lem with engines Clearance Volume) above the piston at BDC convenient point). Note change in level of liquid that burn al co hol, because it has a naturally and the Clearance Volume above it at TDC. (indicating volume of notches made by clay). high octane number. Engines that run on alcohol Calculations for a 351 CID engine are illustrated. Domed Piston Volume (Vd)—Dome values require a high CR to compensate for the fact The formula is: CR = V + Vcl are combination “net” values of Vd and Vn. that they generate less heat. Vcl For compression ratio calculations, they should Pay particular attention to the following points: be used as follows: MODIFICATION TECHNIQUES Clearance Volume (Vcl)—This is the volume • Pop-Up pistons have a “positive” dome Common techniques to increase CR include: above the piston (actually above top piston ring) value, which reduces the volume above (1) Installation of a thinner head gasket. at TDC. It consists of several small volumes. the piston and thus must be subtracted (2) Installation of “domed” or “pop-up” pistons. Cylinder (Swept) Volume (V)—Determined (see example above). Check for adequate “piston-to-valve” clear- by cylinder bore and stroke (indicated by • Dished pistons have a “negative” dome value. ance at TDC. Camshafts with more overlap movement of piston from TDC to BDC). It must be added to compute clearance volume. require more clear ance. A good rule of thumb is 0.080" for intakes and 0.100" for exhausts. MAKE ALL CALCULATIONS WITH ACCURATE MEASUREMENTS (3) Removal of metal from deck face of block or cylinder head. You can safely mill off 0.010" OF ACTUAL PARTS. CATALOG VALUES ARE “NOMINAL” to 0.040" (0.050" max.) from most engines. SPECIFICATIONS AND MAY VARY FROM ACTUAL SIZE. For important information about the proper usage of performance parts, please see page 14. 78 Techline (800) FORD788 See pages 286-292 for important safety, emissions and warranty information.
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