Jeep Engines - Oljeep

T ‘11256 MOPAR PERFORMANCE PARTSNominal NumbersNumbers are fun to play with. However, they can be usedin more than one way. We all know what a number is, sowhat is a nominal number? A nominal number is similar toan average. Nominal numbers are used to describespecifications that are found in production manufacturing.In the automotive industry, we use them to describe enginespecifications such as compression ratio. The advertisedcompression ratio of any production engine is a nominalnumber. If you measure the size of the combustion chamberin cc’s for hundreds of production engines, you end up witha nominal number for the combustion chamber. If youmeasure the size of the intake port (port volume) forhundreds of production engines, you end up with a nominalnumber for the intake port (port volume).A nominal number does not mean that it is the biggest orsmallest, just the most common. Although it isn’t a trueaverage, it indicates the range that you can expect. The keyto all this is not to consider nominal numbers as absolute.They can go up or down (plus or minus). In most cases,much of the total variation (or observed variation) is notdue to the part itself. For example, let’s assume that thenominal (or advertised) port volume for a head is 170 cc’s.Let’s also assume that you want a large port volume foryour race engine, and the reason that you bought this headis because of the 170 number. You measure it at the shopand your new head measures 170.1 cc’s. You’re happy andtake the part home. A week later you measure the same portin the same head again in your own shop at home and find165 cc’s. Now you’re mad! What happened? Temperature?Maybe. Different burette? Maybe. Most likely youswitched valves. Originally you measured it with anail-head or flat valve, but at home you measured it with astock valve which was semi-tulip (for example). The portdidn’t change. Valve shape changes can affect themeasurement of the combustion chamber. A nail-head orflat valve makes the chamber smaller, and an undercut tulipvalve makes the chamber larger.Remember, nominal numbers are for reference only. Forcomplete accuracy, measure the part itself with all the partsthat you are actually going to use in the final engineassembly. If you change parts at a later date, don’t forget tomeasure again!Dynamometers (Dynos)A short example may help illustrate the point. If you builtan engine that made 200 horsepower and you took it toDyno service A and it made 225 hp, you’d be pleased. Ifyou took the same engine to Dyno service B and it made245 hp, you’d be even more pleased. Dyno service C - 260hp. But if you took it back to Dyno service A and it made225 hp you’d be disappointed. Unfortunately, you havenow spent thousands of dollars, used up your engine, andlearned nothing. You’ve been racing dynos. However, all isnot lost. If you stay at one dyno service and make yourchanges, relative gains should translate to real at-the-trackperformance gains. You just can’t get too hung up onabsolute numbers.DISASSEMBLYBefore the engine assembly and blueprinting procedure isstarted, the engine has to be disassembled. This disassemblyprocedure is usually considered only a necessary evil and is,therefore, usually done as quickly as possible. This is notrecommended. The correct way to disassemble an engine isslowly. During teardown you should check for fluidcontamination (mixing of oil and water), cross-threaded orstripped bolts and nuts, bolt holes that aren’t tapped deepenough, and be sure to turn the crankshaft withoutconnecting rods (but with all the main bearings in place) tosee if there are any tight spots. Careful disassembly can giveyou a lot of clues as to what has to be fixed, replaced, orchanged in the engine when you rebuild it.Things to CheckWhen building a new engine assembly, which for most of usreally means rebuilding a used assembly with some newparts, there is a lot of information the parts can tell us thatwill help us with our project. Unfortunately, we have tocheck these things ourselves. To this end, there are severalmeasurements that should be made during disassembly, orwhen the new parts are received, which should be recordedin the notebook that you should keep on each engine that youbuild. They will be helpful when the engine is rebuilt. Someof these measurements are crankshaft end play, connectingrod side clearance, deck height (top of piston to top of block),camshaft lift, and actual installed camshaft centerline. Youshould check the camshaft lobes and valve guides forexcessive wear and check the pushrods for straightness.Dynamometers (dynos) allow you to measure an engine forhorsepower and torque output without putting it in avehicle. This has many advantages but it also has manyshortcomings. You don’t want to end up “racing dynos”because dyno testing can be very expensive and uses up lotsof parts.