I’ve got a 454 H.O. crate engine and we installed a small ProCharger centrifugal on the engine to bump the power. I thought this would work well with the engine’s 8.75:1 compression ratio. I then took the combination to the chassis dyno but ran into all kinds of problems. The engine just lays down at any rpm over 5,200 and it barely makes any more power now than it did without the blower. We’re getting 8 lbs of boost out of the blower but the engine seems really lazy yet it doesn’t misfire. Any ideas?
Jeff Smith: The 454 H.O. from Chevrolet Performance is a great mild street engine but it has an Achilles Heel. Luckily, the fix is relatively easy and not that expensive. The 454 H.O. is rated at 438 horsepower at 5,300 with 500 ft.-lbs. of torque at 3,500. As you mentioned, it has a relatively low 8.75:1 static compression and uses a hydraulic roller cam with 211/230 degrees of duration at 0.050 with 0.510/0.540-inch valve lift. A clue to this engine’s weak point is in the peak horsepower rpm point. Note that while the cam is a little short on duration, those large rectangle port heads will certainly breathe and support power all the way through 6,000 rpm. Yet the engine’s peak horsepower occurs a very low 5,300 rpm.
A number of years ago, we had access to one of these engines and played with a bunch of different parts in an attempt to make more horsepower. We added a bigger cam and yet the engine still couldn’t make any power past 5,300. That’s when we decided to change the valve springs. The stock springs measure something like only 110 pounds of load at 1.880-inch installed height. This is a valve spring seat load you would use on a mild performance flat tappet camshaft – not what would be recommended for a hydraulic roller engine.
Those big, heavy, 2.19/1.88-inch 3/8-inch stem big-block valves need a little more load to control them. In the past, a heavy-handed dual spring would be called upon todo the job. The idea is to have sufficient seat load to prevent the heavy intake valve from bouncing off the seat at higher engine speeds. This valve bounce is really the onset of what is commonly called valve float – or loss of valve control. But in order to have sufficient seat load, this requires a spring to have rather high max lift loads as well, which tends to increase valvetrain wear, especially for a street engine.
While these heavy dual springs do the job, there is a high-tech alternative. Comp Cams has been slowly perfecting the art of building the beehive and conical valve springs. There are multiple advantages to the beehive spring. The biggest advantage is that it tapers to a smaller diameter at the top. This makes the retainer smaller and much lighter. Since the top of the spring travels the greatest distance (compared to the stationary bottom of the spring) this reduced mass means less spring load is required to control the spring itself. This allows more of the spring load to control the valve. So a higher seat load with a lighter upper mass of the spring combines to give the valvetrain more control.
We tested this combination several years ago on a 454 H.O. engine with a bigger cam and a better spring than the stock 454 H.O. and still managed to improve the peak power by 20 horsepower and increase the peak engine speed from 5,200 to 5,600 rpm. This really came as no surprise. What was much more interesting was that around 2,400 rpm the engine also gained torque. This could also be attributed to improved valve control. The main point to take away from this is that simply by improving control over the valves we were able to reduce the seat bounce and improve power –both of which are positive gains.
In the case of your engine with a supercharger, the issues are similar yet the difficulties are enhanced. With 8 to 10 psi pushing on the back side of the intake valve with a diameter of 2.19-inch, this just adds to the spring’s difficulties in making sure the valve closes at higher engine speeds. The back side of that big block valve creates an area of 3.76 square inches. With 10 pounds per square inch (psi) of force pushing on the back side of the valve, that produces a load of 37.6 pounds that’s attempting to push the valve open. That’s the same as reducing the spring’s seat load by 37.6 pounds. With a seat load of only 110 pounds on the stock spring, that boost reduced the spring load by more than one third! This should make it easy to see why your engine didn’t like the boost from the supercharger. The boost just effectively killed your valve spring load and pushed the engine right into bouncing the valve off the seat at higher engine speeds.
As a generic recommendation, I’ve talked this over with Steve Brule’ at Westech and in his experience, a big-block Chevy with an aggressive hydraulic roller camshaft will want between 150 and 180 pounds of load on the seat and 520 to 530 pounds of open load. The Comp Cams beehive spring offers comparable seat load but lower max lift numbers than Steve recommends, but it seemed to work for our test. The beehive will put the seat load at 155 pounds at 1.880-inch installed height and 377 pounds of force at 0.600-inch valve lift. We should emphasize that Steve’s numbers are generic. There are way too many variables to get too deeply into this but these numbers are still useful as excellent reference points.
So to put a cap on this, just adding a better spring with a higher seat load should solve your problem. You will also need retainers and 10-degree locks for the 3/8-inch stem valves. There are other springs that will also do the job for those who don’t like the beehive springs, but in our testing the beehives have worked flawlessly.