Part 2: Building Jeep's 4.0L Six
Last month, you got the skinny on stroking the 4.0L Jeep inline Six. Now you'll get to see it in action and learn about all the goodies we bolted onto the outside.
You'll see and hear varying numbers on what power and torque the various stroker conversions produce. Many we've seen on the Web were derived from "desktop dyno" computer programs, which we have usually found to be on the optimistic side. Some claims are derived from actual dyno tests, most via chassis dynos. Chassis dyno numbers can be subject to a lot of error and are most useful for before-and-after testing to see a percentage of improvement rather than determining raw, useful numbers. Our tests were performed on a highly calibrated, state-of-the-art engine dyno by Technical Services, whose major operation is fuel-injection R&D for the major vehicle manufacturers, race teams, and crate engine manufacturers.
Technical Services is one of those behind-the-scenes companies that will develop systems and products for other companies to use and tout. Especially when dealing with the OE manufacturers, they have to be accurate to the proverbial gnat's bootie. The numbers we got are accurate and a real representation of what our particular combination of parts will deliver, with no particular products to sell and no motive other than passing along results.
We were also aided by Quadratec, Craig Jaros in particular, who provided parts for testing. Quadratec is one-stop shopping for Jeepers, with all the major brands represented, and its willingness to provide parts for testing purposes helped get the University of Northwestern Ohio (UNO) engine running like a top.
By the time the UNO High Performance Machining Class had finished, we had an engine that was, for all intents and purposes, blueprinted. That's probably a higher standard of work than most people can afford, but select portions of it are within reach of all. Balancing is one. The ARP rod bolts are another. The rest is not beyond a machinist who takes his time-time you'll pay for. Our UNO students took the extra steps of align-honing the crankshaft saddles and boring the cylinders perfectly perpendicular to, and decking the block perfectly parallel with, the newly aligned crankshaft bores. None of that will add all that much to the power output, nor to engine life, but it was cool and good training. Let your paycheck be your guide.
We knew we were starting off with the worst H.O. head of the three types available, but the UNO class was determined to make it better. The work started with airflow tests of the stock head (see page 69) on the UNO's Superflow 600 flow bench, and they largely matched other tests we've seen published.
The ports were massaged according to the generic formulae that work for all heads. Nothing radical was attempted, but the exhaust ports were enlarged to match the earlier size; the valve guide bosses were made more flow-friendly; and the port walls were smoothened. After the ports were done, a five-angle valve job on the school's Sunnen seat grinder was performed. You've all heard about the three-angle valve job, and the extra angles are further enhancements to airflow.
The end result on the flow bench can be seen on page 69, but some of the gains showed up on the dyno in an unusual way. Technical Service's top-drawer dyno can measure airflow and volumetric efficiency (VE). This is the theoretical amount of air an engine of a given displacement would use versus what it actually can use. In the old days, 80 percent was really good. Nowadays, 85 to 90 percent is fairly common. Our engine was at 85 percent at under 2,000 rpm, over 90 percent at 3,000, and hit 100 percent at 3,500 and climbed to a high of 103.4 percent at 4,500 rpm. Part of that was due to the head, and part was due to the ram-tuning effect of our 2,000-and-up intake manifold.