Wrapping up our 4.6L stroker
We put the wraps on our 4.6L Golen Engine Service-built stroker back in the December '05 issue (Insane Inline, Part III) with a trip to the chassis dyno to determine the rear-wheel horsepower. Considering our engine belted out 268 hp and 324 lb-ft on Golen's engine dyno, we were expecting bigger numbers than the 172 hp and 224 lb-ft we got. Given a standard parasitic drivetrain loss of about 30 percent with an auto tranny, we were hoping to see at least 188 hp at the tires. Our problem was in the computer-controlled air/fuel ratio. But before we could bring the vehicle in to be tuned, the engine ate a camshaft on the way home from work.
In short, we still have no concrete explanation as to why the No. 1 cam lobe went flat and why both of the No. 1cylinder's pushrods got mangled. At the time, the engine had about 9,000 trouble-free miles on it. There was plenty of oil pressure, we weren't over-revving the engine, and it happened right out of the blue at about 3,000 rpm on the highway. Whether a lifter pumped up, the spring seat pressure was off, the valvespring bound, an oil passage became blocked, or the camshaft wasn't hardened correctly, there is no end to the theories. However, no one explanation fits the evidence. We're just going to call it one of those flukes and say that we've still got confidence in our Golen Engine Service stroker and our Comp Cams camshaft.
In order to get the Cherokee ready for a new date with the dyno, we took it to the Jeep experts at Jeeps R Us in Laguna Beach, California. As a former drag racer and current off-road enthusiast, owner Larie Tales has gasoline in his blood and Jeeps on his brain. Plus, every time we visit Jeeps R Us the service area is littered not only with newer Jeeps but old MBs, Willys wagons, FSJs, CJs with RamJet 502 conversions, and a whole slew of insane projects that lesser shops couldn't handle. We don't trust a whole lot of shops to wrench on our junk, but the gang at Jeeps R Us are the real deal and build and sell enthusiast Jeeps that are trouble-free and that work. We trust them.
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After putting about 3,000 miles on the new camshaft, we took the Jeep to The Dyno Shop in Santee, California, to be dyno-tuned by owner Mark MacNeil. He installed a Unichip ECU module piggybacked onto our XJ's ECU so the air/fuel ratio and timing could be dialed in to exactly meet our engine's needs. The Unichip doesn't rely on reflashed chips but rather can be manipulated by an experienced tech to create a custom MAP to operate your vehicle not only at wide-open throttle but at all rpm and load ranges across the board. It's a process we happily turned over to MacNeil, who strapped our XJ to the dyno for a few hours to find our stroker's hidden horsepower.
On the RollersHaving a vehicle dyno-tuned delivers immeasurably more accurate results than any reprogrammer or driveway tune job. Using real-time data generated with the vehicle driving on the rollers, The Dyno Shop's Mark MacNeil adjusted our vehicle's fuel and timing not only at full-throttle blasts, but under part-throttle load conditions and for normal driving so that we get the most performance no matter how much we're into the throttle. There was probably a few extra horsepower to be had on the top end, but MacNeil pulled some timing back because he was encountering some pinging and detonation caused by our higher compression. Although we saw impressive differences in peak power between our newly established baseline and with the Unichip tune, the real story is the impressive increase in driveability. We really notice a much bigger difference in part-throttle passing and midrange acceleration than hammer-down, full-throttle use.
As for our NOS system, MacNeil suspected the fact that our nitrous bottle was at ambient temperature was lowering the pressure and affecting the power. To simulate running a bottle heater and blanket, MacNeil submerged the bottle in a tub of warm water, and repeated the dyno run. The resulting 20 hp and 47 lb-ft was enough to make us order a pressure gauge, bottle heater, and bottle blanket from NOS.