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Ask Anything - May 2014

When It Won't Rev

Byron Bates; Bakersfield, CA: I purchased a 1973 Jeep CJ-5 with a 383 Chevy by the way of another guy's nasty divorce, so I don't have good details. I will drag race it, use it as a local daily driver, and I'd also autocross it if I could figure out a way. I'm about to swap the Jeep three-speed for an M-22 or some form of Tremec overdrive. The Jeep currently has 4:56:1 gears. The engine has great torque but really runs out of air at about 5,200. I want more cam. Currently, the engine has the ZZ Hot Cam. No specific hp or torque goals—I just want it to run stronger. I suppose one begets the other. Couple early rpm exit with the gearing, and acceleration is weak.

Jeff Smith: If you've ever seen the movie The Graduate, Dustin Hoffman is wandering around his backyard college graduation party put on by his well-meaning parents when a relative pulls him aside and tells him to remember one word: plastics. It's a classic movie vignette. The Car Craft version of this scene is we pull you aside from your garage bench-racing session and whisper, "valvesprings." The Chevrolet Performance HOT cam is an excellent street hydraulic roller with specs of 218/228 degrees of duration at 0.050-inch tappet lift with 0.492-inch of valve lift and a lobe-separation angle of 112 degrees. But let's start with the fact that the engine seems to "run out of air" at 5,200 rpm.

You didn't include specifics on the cylinder heads, but let's assume that they are decent. This is a very popular camshaft and somewhat aggressive in terms of valve lift relative to the duration. The advantage of a roller is the lift curve is generally greater than a flat-tappet version. This more aggressive lift curve demands a stronger valvespring to control the quicker valve acceleration rate. Chevrolet Performance sells a complete HOT cam and kit that includes a set of springs that are a 1.320-inch diameter (1.250 inch is stock for a small-block Chevy) with 101 pounds of load on the seat at an installed height of 1.780 inches. The rate for this spring is 332 pounds per inch. This would produce roughly 270 pounds of load at 0.500-inch valve lift. While this is Chevrolet's recommended spring, a slightly stronger spring is probably a good idea. Crane suggests valvespring seat loads of between 120 and 145 pounds for a typical street hydraulic roller camshaft. What you should do first is remove one of the valvesprings from your engine and measure its installed height. This is the distance between the spring seat in the head and the underside of the valvespring retainer with the valve closed. Next, measure the distance between the lowest portion of the retainer and the top of the valve guide. There will be a seal located on the guide, and you are measuring the distance between the seal and the bottom of the retainer. Since your engine is running OK, we will assume that this distance is at least 0.550 or 0.600 inch so the retainer clears the seal at full lift.

Next, take your valvespring to a machine shop and have them measure the load at the installed height you measured. We'll assume this distance is 1.80 inches. Let's also assume a valve lift of 0.500 inch. If the spring measures less than 100 pounds of load on the seat, then you need better springs. Let's also estimate that you measure 250 pounds or less at full valve lift. This tells us you need better springs. You don't have to buy the GM springs (PN 12551483, $127.52, Summit Racing) as there are several other opportunities for better springs. For camshafts similar to the HOT cam, Comp Cams recommends a larger, 1.430-inch-diameter dual spring (PN 986-16, $79.97, Summit Racing) that offers similar load (132 pounds installed at 1.750 with around 290 pounds at 0.500-inch lift). This spring is less expensive but will require new retainers (PN 740-16, $52.97). If your engine has aftermarket heads, it's possible that the seat diameter is large enough to accommodate these larger springs.

Of course, it's also possible that there is nothing wrong with the valvesprings and the engine is wheezing due to other problems. The best way to approach this is to eliminate all of the easy fixes first, so I would suggest looking at the throttle linkage. With the engine off, have a buddy look at the carb while you stab the throttle from inside the car. Are all four barrels opening fully? Don't laugh, because it happens all the time. Next, install a fuel-pressure gauge to ensure that fuel pressure is at least 5 psi at 5,500 rpm at WOT. If not, the problem may be something simple like a restricted fuel filter or a weak fuel pump. If the fuel pressure is not consistently at least 4 to 5 psi throughout the rpm curve, this could easily contribute to your engine's failure to rev. It's also a good idea to check ignition timing and make sure you have at least 32 to 36 degrees of total timing. Assuming the engine is in good shape and everything is working properly, your engine should rev to at least 6,000 rpm and probably as high as 6,500, although the power peak is probably at around 5,800 rpm. You didn't mention the exhaust system, but we'll assume that you are not trying to push all this exhaust out of a single 2-inch exhaust. A dual exhaust of 21⁄2 inches with free-flowing mufflers is what we would consider the bare minimum.

More Info
Chevrolet Performance; 800/450-4150; ChevroletPerformance.com
Comp Cams; 800/999-0853; CompCams.com
Crane Cams; 866/388-5120; CraneCams.com
Summit Racing; 800/230-3030; SummitRacing.com

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