Jeep Engines - Oljeep

- -1154 MOPAR PERFORMANCE PARTSBecause of gasoline limitations, compression ratios shouldnot exceed 12.5: 1 for competition; 10.5-1 1.0:1 for generaloff-road use, unless you have access to high qualitypremium fuel (103 octane).The most important consideration about pistons is weight.Forged pistons are stronger than stock cast pistons and tendto be equal to or heavier than stock pistons. For racingpurposes, lighter weight is where the performance is.Forged racing pistons at less than 500 grams can beobtained. However, they have to be custom made and aremore expensive. Once the piston is lighter, the pin can belighter, the rod lighter and the balanced crankshaft lighter.A word of caution, however! You can go too far andweaken the piston in a vital area, causing it to fail. This typeof failure tends to make an expensive mess.There are many clearances related to a racing piston. Wehave outlined most of these clearances, such as piston-towall,valve-to-piston, etc. However, many engine buildersforget about the piston-to-head clearance. We suggest .040"(low minimum) and .060" (high minimum). The problem isthat if the engine builder cuts it too close, say .030", thepiston will hit the cylinder head. This can result in a blownhead gasket, broken pistons, broken pins or pin locks. Theclearance becomes very important in high compressionratio engines, especially with domed pistons.Reversing Pistons With Offset Pins(Refer to Offset Pins later in this section.)CC'ing the BlockWith an ordinary flat-top piston, the block is not cc'd. Theneed for cc'ing the block arises only if the piston is domedor if the valve clearance notches or reliefs are quite large.COMPRESSION RATIOAlmost everyone talks about compression ratio, but veryfew non-professionals really know what it is. The followingprocedures show you how to make compression ratiomeasurements, plus how to calculate your actualcompression ratio.The formula for determining compression ratio is as follows:VBDCCompression Ratio = VTDCThe first step is to look at the cylinder volumes. (See Chart 1 .)This is the volume of one cylinder in cc's.Chart 1- En- Bore x StrokeCvl. Head Vol. (cc)2.51,4 cyl. 3.876 x 3.188 6 16.454.01, 6 cyl. 3.88 x 3.44 666.524.21,6 cyl. 3.75 x 3.895 704.955.91, (360 CID) V-8 4.08 x 3.44 737.00The volume above the piston at BDC (Bottom DeadCentler) is equal to the piston displacement plus thechamber volume. (Chamber volume is the volume abovethe piston at TDC [Top Dead Center].)Next, break down the volume above the piston at TDC intoits separate parts. This volume is the total combustionchamber volume. It consists of four basic parts: cylinderhead volume, gasket volume, piston deck clearance andpiston dish or dome. The head volume is typically measuredseparately with the head off the engine. The gasket volumeis a function of its thickness, and that is measured using amicrometer. Head gasket volumes run in the 10-15 cc range.(We'll discuss that in more detail shortly.) The deckclearance and dish or dome measurements are best lumpedtogetlher and measured under the 1/2" down fill volume. Inshort., as long as you have a completelypat piston (no dish,dome or notches), you can measure deck clearance with adial indicator and treat it as another (or thicker) gasket. It'sa quick and easy method, but it only works with aPat piston.The 1/2" down fill volume method for measuring the pistondish or dome is based on a calculated volume. Refer to Chart 2.Chart 2Enaine 1/2" Fill Down Volume Tvpical Head cc's2.5L 96.68 49.94.0L 98.85 64.54.2L 90.50 64.55.9L 107.12 58.6(360 CID)With the 1/2" down fill volume method, the piston isbrought to TDC using a dial indicator placed in the middle(flat spot) of the piston. The piston is then lowered in thecylinder bore SOO" (1/2").Example: If the original deck clearance was ,022" at TDC,then the piston is lowered to S22" below the deck surface.Now the piston is sealed and the cc's are measured similarto the cylinder head. Once you have the actual, measured1/2"down fill volume, you can compare it to the one listedin Chart 2.