Jeep Engines - Oljeep

162 MOPAR PERFORMANCE PARTSINTRODUCTIONOne of the most important parts of any engine assembly isthe cylinder head. It holds the key to making power. It alsooffers the opportunity to make more horsepower.Camshafts, headers, and carburetion can only go so farmaking horsepower without the cylinder head. Cylinderheads can be expensive; however, this may be one place inthe engine where spending the extra money is worth it.The 4.0L engine uses a cast iron cylinder head with acombustion chamber volume of 64.45-67.45 cc. It containstwelve valves made of chrome plated heat resistant steel,valve stem seals, springs, retainers and keepers. Thecylinder head, valve seats and guides can be resurfaced forservice purposes.The cylinder head has a quench-type combustion chamberthat creates turbulence and faster burning of the aidfuelmixture, resulting in good fuel economy. Valve lengths are(for intake) 122.4-122.8 mm (4.822-4.83”), and (forexhaust) 122.8-123.2 mm (4.837-4.852”).For more cylinder head specifications, refer to 4.0L EngineSpecifications, ‘Engine Assembly’ section of this chapter.SELECTIONFifteen years ago, cylinder head selection was very limited,so the picking process was pretty easy. Today, this situationhas reversed. Aftermarket suppliers today offer many typesof cylinder heads with different materials, different valves,bigger ports, different valve gear, unique combustionchambers, revised valve angles and locations, etc. The goodnews is the engine builder or racer has more to choosefrom, but the bad news is he must make a choice!With so many heads to choose from, we have to find amethod of choosing the head that we want. Descriptionssuch as “Race,” “Street,” or “Stock” are no longeradequate. There are many comparisons that can get acustomer to choose the wrong head, or spend money that hedidn’t have to. We can’t solve all these problems, but wecan help clarify the situation.One of the biggest mistakes that a racer can make is tomix-and-match features. This approach can be veryexpensive and is best left to the professional engine builder.So for this discussion, we will assume that we can do valvework or even porting, but we don’t want to re-machine ourcylinder heads.Cylinder HeadThe key to making any major selection for a race engine(such as a cylinder head) is to list your priorities. To keep itsimple, answer the question, “Why do you want new headsand what do you think they are going to do for you?’ Theanswer should be your top priority. The second thing toconsider is sequence. If you have five questions or concerns(1 through 5) and you ask them in sequence (1 through 5),you may end up with an answer. However, if you reversethe process and ask the same questions in the reverse order(5 through l), you may end up with more questions! MoparPerformance Parts wants to help you pick the best head foryour application. Other sources/articles want to sell you thehead that they happen to have or are discussing. That is thetrap that you tend to fall into when you look at the specificson any given head. But now that there are so many, youneed to look at the overall picture.Regarding cylinder heads, the FIRST decision that youshould make is material: cast iron or aluminum? There areso many of both kinds with so many different features thatyou need to make this choice up-front. Aluminum heads arelighter, more expensive, and offer repairability. Cast ironheads are less expensive, more durable, heavier, and put upwith more abuse. In general, aluminum heads take heat outof the combustion chamber, so an engine will act like it haslost 1.5 to 2 points of compression ratio. Therefore, if youhad an 8.0: 1 compression ratio cast iron head engine, youwould have to run an aluminum head at 10.0: 1 (estimated).The second decision is combustion chamber shape. Mostcast iron heads have the same chamber shape as production,but there are exceptions. Almost all aluminum heads aredifferent. The trick here is the piston. If the piston has adome on it or if it sticks above the deck surface, then youMUST have a head that will work with your dome. Mostdomed pistons are designed for a production cast ironcombustion chamber. If you have a flat piston, then you areOK. ’Yes, you can replace the pistons, but this is an addedexpense and could cost up to $1,000 or more. If in doubt,go back to your priorities list and write down the cost.The third decision is valves. There are several aspects ofvalves, but the most obvious is diameter. Bigger is better,right? Well, almost! Bigger also means bigger valvenotches in the piston. This is a machining operation that canbe added to the piston, but cost must be considered. Add thecost tmo your list. Bigger diameters can also hit the bore wall.Bore walls can be notched, but this is not your first choice.You also get more shrouding if you use big valves on smallbore sizes. Valve location and valve angle must also beconsidered. Not only is the dome related to these valveaspects, but also the valve notches. They must be fixed togive the proper clearance. Even a flat piston can be aproblem if there is no material under the piston top whereyou want to put the new notches.