WE THOUGHT YOU OUGHT TO KNOW COMPRESSION …

1 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 B Bore in B2 = x = have him do a portion or all of the heavy ma chin ing and wrenching.

2 G Gasket Bore in G2 = x = P Piston Top Land Diameter in P2 = x = What we have here are just a few of the key bits of information and specs. The idea is to help S Stroke in keep midnight thrashing to a minimum, because S/2 Crank Throw in parts don't go together right, or there's more to a L Con Rod Length in job than you imag ined. H COMPRESSION Height in COMPRESSION RATIO Dh Deck Height in Increasing the COMPRESSION ratio (CR) is often one of the first en gine performance r Ring-to-Top Piston in modifications. Squeezing the air-fuel mixture d Piston to Deck in D h H l S/2. into a smaller space increases its temperature t Gasket Thickness in and ease of ignition; thus the rate at which V Cylinder Volume /4 x B2 x S x heat is ex tracted from the fuel.

3 Engineers call it thermal efficiency. Sim ply put, it means Vt Volume Above Top Ring .9cc /4 x (B2 P2) x r x that increasing the COMPRESSION ratio Vn Valve Notches Volume increases horsepower. Vd Dome Volume Henry Ford's Model T has a CR of :1. Vp Piston-to-Deck Volume /4 x B2 x d x High-performance engines operate in the area Vg Gasket Volume /4 x G2 x t x of :1. Most of today's stock pro duc tion engines are about :1. Vh Volume Head NOTE: Turbocharged engines typically have Vcl Volume Clearance Vt + Vn + Vp + Vg + Vh Vd a lower CR than normally aspirated CR COMPRESSION Ratio V + Vcl V cl engines. Thus, if you add a turbo, you may want to lower the CR, NOTE: 1) Math reduction; /4 x = de pend ing on performance level. DETONATION The precise amount is limited by block deck Cylinder Head (Combustion Chamber).

4 Increasing the CR changes the rate at which height, casting thickness, valve-to-piston Volume (Vh) The irregular shape of the fuel burns. Spark knock (detonation) will occur clearance, etc. combustion chamber requires measurement if certain modifications are not per formed. (popularly called cc ing) with a glass burette Here are two of the most important: NOTE: Also modify the intake manifold to maintain port alignment. and colored liquid, such as fluid. This catalog Ignition Spark Timing In creas ing the CR 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. Fuel Octane Rating Increasing the CR COMPRESSION RATIO with soft clay and make level with top of piston.

5 COMPRESSION ratio is defined as the ratio Remove clay with small knife and drop into requires gasoline with a high octane rating graduated cylinder (filled with liquid to (with anti-knock components to control between the Total Volume (Cylinder Volume plus Clearance Volume) above the piston at BDC convenient point). Note change in level of liquid det o na tion). This is not a prob lem with engines (indicating volume of notches made by clay). that burn al co hol, because it has a naturally and the Clearance Volume above it at TDC. Calculations for a 351 CID engine are illustrated. Domed Piston Volume (Vd) Dome values high octane number. Engines that run on alcohol are combination net values of Vd and Vn. require a high CR to compensate for the fact The formula is: CR = V + Vcl Vcl For COMPRESSION ratio calculations, they should that they generate less heat.

6 Be used as follows: Pay particular attention to the following points: 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).

7 It must be added to compute clearance volume. require more clear ance. A good rule of thumb is " for intakes and " 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 " OF ACTUAL PARTS. CATALOG VALUES ARE NOMINAL . to " ( " max.) from most engines. SPECIFICATIONS AND MAY VARY FROM ACTUAL SIZE. VALVE TRAIN FORD RACING CAMSHAFT USAGE. When modifying production engines for performance, The durations shown in this chart are durations. The descriptions within each group of cams here are a few things to keep in mind. show performance characteristics and basic modification recommendations required to achieve desired performance. CAMSHAFTS.

8 When replacing a cam, it's a good practice DURATION PERFORMANCE ENGINE/VEHICLE USAGE. to install new related components such as a (SAE) CHARACTERISTICS AND MODIFICATIONS. distributor gear, tappets, springs, retainers, 270-290 Good idle quality Use with stock or slightly modified engine, etc. It's especially important that new tappets and low rpm torque. stock axle gears and with or be installed. Never use hydraulic lifters with a mechanical 290-300 Fair idle quality. Good low-to- Will work with stock or modified en gine. cam or solid tappets with a hydraulic cam. mid-range torque and horsepower. Can use stock axle gears and with or The ramps are not compatible. 300-320 Rough idle quality. Good mid-to- Use with or high stall Requires improved Be sure your valve train can handle the timing high rpm torque and horsepower.

9 Carburetion, ignition and exhaust systems. events and lobe lift of your performance cam. Engine will have lower vacuum than stock. Check for adequate piston-to-valve clearance, 320-340 Rough idle quality. Good mid-to- Use with or very high stall Re quires spring bind and retainer-to-valve clearance, high rpm torque and horsepower. improved carburetion, ignition and exhaust systems. spring bind and retainer-to-valve For all-out competition only. Engine will not provide enough vacuum for accessories. Axle gear ratios must be properly selected. seal clearance. Be sure to use camshaft and lifter prelube when installing the cam to prevent scoring ROLLER TAPPET CAMSHAFT Roller Tappet Longer than flat tappet, the lobes during break-in.

10 Engine oil by itself Most engines are designed with hydraulic or because of roller. Hydraulic portion functions (regardless of quality or viscosity) is mechanical flat tappet camshafts, which meet like a standard flat tappet. not enough! the needs of regular production engines that Roller Tappet Camshaft Machined Mechanical cams require lash adjustment. seldom see 6000 rpm. Flat tappet cams are from steel, instead of typical iron used for flat If production head is designed for hydraulic more than adequate for many competition tappet cam. Cam lobes specially ground and cam, modification is usually required. engines. For ultra-high-performance applications hardened to withstand loads of roller tappets. Many design changes have occurred over where durability and high rpm capability are Do not attempt to use with flat tappets!