Q&A / Tech

Ask Away: Diagnosing High-RPM Power Loss

I have a small block Chevy in a ’66 Chevy II that seems to run pretty good but now I’m not so sure. It’s a 383 small block with a set of no-name aluminum heads, a Lunati hydraulic flat tappet Voodoo cam (227/233 degrees at 0.050 inches with 0.489/0.504-inch lift), a Performer RPM intake, and a 650 cfm Holley carburetor. The engine runs fine – doesn’t misfire or do anything funny  but I am concerned that it seems to stop pulling once the tach gets above 5,000 rpm. The ignition is an HEI with good Moroso wires and new spark plugs. The motor has been in the car for several years but about a year ago we changed to that Lunati Voodoo cam. It just seems like it lays down in high gear. Any ideas?

J.B.

Jeff Smith: One of the challenges of the whole hot rodding hobby/sport is that there are literally millions of combinations of parts from which to choose. With hundreds of companies making thousands of parts, the permutations of just cylinder heads, camshafts, intakes, and compression ratios becomes a blur of options. So it’s not surprising that you may have difficulties.

But I have some possible solutions.

You didn’t mention whether you were feeding the engine with a mechanical or electrical fuel pump. I’ll assume that the pump is probably mechanical. First thing to do is to check the fuel filter for obstructions. The fuel filter is often an afterthought and cheap, restrictive filters can reduce output from the pump. The pump may be capable of supplying fuel for low rpm and part throttle operation but at higher rpm, there is insufficient fuel pressure and volume due to a filter restriction, so start with that inspection first. Your 650 cfm Holley is big enough that it’s not really a restriction with regard to airflow.

The best way to check for adequate fuel flow is to install a fuel pressure gauge in between the pump and the carburetor and upstream of the fuel filter. This does not have to be an expensive Auto Meter gauge permanently mounted on the cowl – although this is a good idea. Instead, you could borrow or buy an inexpensive fuel pressure/vacuum gauge and plumb it into your fuel delivery system. Use a long length of rubber hose and place the gauge on the cowl where you can read it while driving. I’ve actually duct taped gauges to the lower portion of the windshield when doing diagnostic work like this. Next, run the car somewhere safe where you can make multiple passes past 5,000 rpm and monitor the gauge. Decent fuel pressure for a carburetor is 4 to 6 psi. It’s possible that at the higher engine speeds the pressure will begin to drop. If it falls below 3.5 psi, it’s possible that you’ve found the problem. Holley and several other companies make really good mechanical fuel pumps for a small-block. For example, Holley offers an 80 gallon per hour (gph) mechanical pump (PN 12-834) that will deliver more than enough fuel for our application. This pump should be able to easily supply 4.5 to perhaps 6 psi at WOT while supplying more than enough fuel volume.

If after checking fuel pressure you learn that your existing pump is working just fine, then we have another suggestion.

To be more accurate we would need more information on the valve springs and cylinder heads. But with an emphasis on the budget aspect, often these heads come with valve springs that are not up to the task of controlling the valves at high rpm. This is aggravated somewhat by the Lunati Voodoo camshaft. The Voodoo line offers great performance from a relatively short duration. Lunati accomplishes this by being fairly aggressive with the rate at which the intake and exhaust lobes open the valves. The Lunati description for this cam is “Makes unequaled power to 6,200 rpm with proper valve springs.” The key here is the connection to the valve springs.

ASK-02-02Our experience with the Voodoo line underscores this load on the valve springs. You didn’t mention if you were running stock 1.5:1 ratio rockers. If so, that’s fine, but with 1.6:1 rockers, the additional ratio puts even greater forces on the springs because the rocker accelerates the valve even faster to achieve the greater lift. A combination of weak valve springs with a Voodoo cam and 1.6:1 roller rockers could easily cause the problem you are experiencing. Most guys would call this valve float. What they don’t tell you is how to fix this. That’s what we’re going to do.

The most accurate definition of valve float is when the spring loses control of the valve. When this happens, the first indication is when the intake valve bounces off the seat as it closes. This may not sound like a big deal, but the results will kill power. When the valve bounces off its seat, this occurs at the same time that the piston is rising in the bore and creating cylinder pressure. But when the valve bounces off the seat, it allows some of this pressure to escape. The reason the valve bounces is because the spring cannot exert enough load to keep the valve closed. When the valves bounce off the seat multiple times, the engine loses power and often can’t even achieve the rpm. This is classic valve float. Of course, the end result is a dramatic loss of power.

The most common fix for this problem is a stiffer valve spring, especially if the existing springs are on the weak side. What you should do is remove one spring and have it tested at a local machine shop. This will tell you the status of your springs. Stock replacement valve springs generally do not produce much more than about 90 to perhaps 100 pounds of seat load with an installed height of 1.750-inch. For a stock or mild performance cam, this is sufficient load. But with a performance camshaft like the Voodoo cam even Lunati’s base replacement spring is rated at only 98 pounds with at a 1.750-inch installed height and 286 pounds of force at 0.500-inch valve lift. My guess is that this spring will probably not work as well as something with a little more closed pressure, especially if the plans call for a 1.6:1 rocker. While a dual spring could do the job, this often places excessive load on the cam at peak lift. Instead, Lunati recommends using its new line of Signature Series Ovate Beehive springs. The complete valve spring and retainer kit (PN 74818K2) includes new beehive springs, retainers, and the necessary locks all under one part number.

This beehive spring offers a substantial increase in seat load from the previous spring’s 98 pounds at 1.750 to the beehive’s 130 pounds at a taller 1.800. Now that we know how valve float starts, perhaps you can see that a spring with essentially 30 percent more seat load will have a much better chance of keeping that intake valve closed at higher engine speeds. There are other added benefits to using a beehive spring but in the interest of brevity, we’ll save that discussion for another time. Changing over to the better valve springs will probably be the thing that will improve your engine’s performance and bring your small block back to life!

 

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8 Comments

  1. I had the same issue with a BBC engine. The low end power was great but it would lay down at high rpm.
    The problem was fuel pressure. My electric pump was not up to the job so it was swapped out with a higher end pump. Also the pump needed to be lower than the fuel tank. Electric pumps are pushers and not good at suction.
    Hope this helps.

  2. Thanks to Jeff AND Doug for the info. Your articles are always stated in an easy to understand way!

  3. Randys engine building says:

    Get a new set of compcams springs, take the heads off and reface the valves and re grind the seats and port out the heads and gasket match and aslo adjust the timeing chain 2 or 4 degrees and see how she runs.

  4. Randys engine building says:

    2 or 4 degrees advance*

  5. Randys engine building says:

    If you really want to make some power try a bigger carb a 650 is kinda small for that engine and not everyone knows this but their needs to be zinc in the oil for the older cam you have hope this helps

  6. I’d also add a fuel pressure regulator. If the pressure isn’t stable your carb settings and jetting can be way off from what you think it is. A hard launch or hard braking can disrupt the amount of fuel being sprayed into the carb venturi. The fuel floats need to be adjusted high enough to keep the bowls full during a hard launch or rolling at a high speed pass. If the gas cannot be metered into the engine your car will go lean and fall on its face.

  7. I have a mechanical Hilborn injected, blown small block Chevy that
    produced plenty of horsepower on the dyno, but when installed,
    it would not produce power over 4,000 rpm. I have an MSD with blower retard
    (just replaced box and external coil). It also has MSD plug in timing module,
    Everything is new including crank trigger and multiple Optima batteries.
    Everything runs through an ARC ignition module. The car has not run in 4
    years. That is why everything got replaced, including the fuel tank.
    It almost feels like it is retarding itself.

    Any suggestions? Running out of ideas?

    • alan sparks says:

      What is your initial timing and how much advance do you have? If your timing is advancing too much, it can cause a loss of power in the upper rpm ranges. I fought this for a long time and ended up changing the springs in my mechanical advance distributor, problem solved!

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