Jeep Engines - Oljeep

4.0L POWER TECH IN-LINE 6 (ENGINE ASSEMBLY) 253On the cylinder head, the valve work is the most importantaspect and is usually sent out. The special equipment that isrequired to do valve jobs is not usually found in the homeshop. This is an area where the extra money spent willresult in more Performance. Like the block, the head will besent out for milling. If new guides or seats are required inthe head, these operations should also be sent out to aqualified shop. (Not all valve grinders do guides.) Lappingthe valves can be done at home but, a word of caution-iftoo coarse a lapping compound is used, you’ll destroy agood valve job. It’s best left to the headhalve expert.Once the machine operations are performed, assembly of theengine can be done at home. Certain basic tools are required.Take your time and be extra careful. Never hurry or try tobuild an engine quickly. Leave that to the professionals.What Parts to UseThe most commonly asked question in the engineperformance business is, “What parts should I use?”Typically, the question is asked in the following manner:“I’ve got a 4.0L Power Tech engine-what parts do yourecommend?” Obviously, there is more than one answer. Inreality, the answers are almost infinite. The parts inquestion are the induction system, exhaust manifolds,camshaft, and ignition system. The easiest way to answerthis general question is to refer to earlier sections of thischapter.To develop a list of general engine parts recommendations,the important questions to answer are as follows:What type of fuel is going to be used?0 What’s the engine’s compression ratio?0 What type of transmission is to be used?What type of induction system is desired?Do you want to use headers?The answer to the fuel question (typically gasoline) willdictate the maximum compression ratio that can be used andis the most important of the five questions. It will determinewhich piston can be used and how the heads and block canbe modified, if at all. The transmission type (manual orautomatic) will affect both the carburetor and the camshaft.Remember that automatic transmission pieces will workwith a manual, but manual transmission pieces won’t workwith an automatic without other changes. The inductionsystem can be fuel injection or carburetion. If carburetion isselected, an intake manifold that will work with thecarburetor must be selected. Most performance applicationswill use exhaust headers. The ignition system should alwaysbe a high performance electronic. In general terms that givesyou the “what parts to use” answer, except for the camshaft,which is covered in its own section of this chapter.Engine Torque vs. HorsepowerTorque and horsepower relative to performance engines arenot the same thing, but they are very closely related. Theactual equation is as follows:Bhp =brake horsepowerT = torque in lbs-ftrpm = engine speed in revolutions per minutek = constantIn racers language, horsepower refers to the high rpm scaleand torque refers to the low rprn scale. Usually, the dividingline for mild performance engines is 3,000 to 3,500 rpm.Unfortunately, in practice, parts that help horsepowergenerally hurt torque and vice-versa. This is true when onlyone engine part is changed, keeping the rest of the enginethe same. Racing engines with drastically modified rprnscales are much too complicated to analyze in this manner.However, it can be useful for dual purpose machines.Let’s choose a specific example-camshafts. Here the ruleis: the bigger the camshaft, the more horsepower it makes.However, this can also be stated as: the bigger the camshaft,the less torque it makes. The question you should askyourself is, “Does my engine need more torque or morehorsepower?’ You can answer this question easily bydeciding whether you want to improve your performanceabove 3,500 rpm or below 3,500 rpm. Keep in mind that animprovement in one area comes at the expense of another;that is, more torque, less horsepower (and vice versa). Thesesimple guidelines can be very helpful in making many partsselections, but remember that you must have abaseline - some standard you want to compare against. (Themost common baseline is some version of a stock engine.)