FRUSTRATING VALVE FLOAT
I’m the owner of a ’78 Aspen with a four-speed overdrive manual transmission. Up to this point, I’ve replaced the 318 engine with a 400 horsepower 360 with an Air Gap intake, added headers and exhaust, Edelbrock aluminum heads, and a hotter camshaft. To keep up with my results, I’ve become a regular at the chassis dyno, and currently the engine is producing 303 horsepower at the rear wheels. Without a doubt, I think these Mopar small block engines are the best bargain for performance since the big blocks of the ’60s.
During my last dyno session, which by the way was the occasion when the engine finally crested the 300 rear wheel horsepower number, there seemed to be a high rpm problem. At roughly 6,100 rpm, the power curve dropped off sharply, with the engine definitely sounding unhappy. My tuner identified the problem as valve float, and limited any further pulls to below this rpm. I’ve heard the term valve float before, and know that the answer is usually an upgrade in the valve springs, but am not really sure about what causes this condition, technically speaking. Can you provide some information on valve float, and if possible, provide me with a recommendation on what valve springs to get to solve this problem. I think the engine will make even more power if I can get it to rev higher. Thanks
Louis, valve float is kind of a broad term for a loss of control in the engine’s valvetrain. Operating the engine above an rpm where float occurs, creates the potential for catastrophic engine failure, so it needs to be avoided or corrected. The ability to maintain valvetrain control is one of the challenges and limitations in pushrod engine performance, and as a result, it is one of the most highly developed areas of an engine. Starting with the basics here, in a pushrod engine, the camshaft profile determines the motion of the valves by virtue of the shape of the lobes. The tappets (lifters) are raised and lowered by the lobes, which in turn operate the pushrods, transferring the motion to the rocker arms that actually operate the valves. Essentially, the valvetrain in a pushrod engine represents a system of linkages to connect the valves to the camshaft, providing the mechanism to open the valves. The valve springs act against the valvetrain to control the system and close the valves.
As rpm increases, the demand on the valvetrain becomes exponentially greater. Any part of the system that imparts false motion or instability can create or exasperate problems with valvetrain control. Classic valve float is a situation where the loads provided by the valve springs are not high enough to ensure that the valvetrain follows the profile of the camshaft accurately. Typically, insufficiencies here will show up as the valves bouncing or oscillating on the seat upon closing. It also allows the lifter to lose tension on the camshaft as the lobe goes “over the nose,” or through the full-lift position, changing direction to begin closing. With more rpm or more intense camshaft profiles, the inertia of the valvetrain components becomes higher, resulting in higher demands on the valvetrain. So the key factors to balance are valvetrain inertia, vs. the countering valve spring loads.
Clearly, more spring load is the most obvious solution to valve float problems, however, there are other factors at play that affect valvetrain control, such as inertial and deflection. A reduction in valvetrain weight can reduce the demands, via a reduction in valvetrain inertia. The Mopar small block engine offers plenty of room for improvement with things like lightweight valves, springs, retainers and low inertia rockers. Improvements to the valvetrain can be accomplished with more refined aftermarket pieces, such as titanium retainers, and aluminum roller rockers. False motion stemming from deflection can be reduced with stiffer pushrods and.
To summarize, a successful valvetrain utilizes low-deflection components, is light weight, and has adequate spring loads. Actually, there is a further level of complexity involved in the dynamics of an operating high-rpm valvetrain, where other sources of false motion such as valvetrain harmonics and spring surge are explored. Top aftermarket firms invest in sophisticated testing to help design their components to minimize the negatives of these effects. Cutting-edge designs such as Competition Cams’ “beehive” valve springs are the result of such development.
HITTING THE BAR
I am restoring a ’64 Polara, and I need to get the torsion bars out. I am going from the stock 318 to a 426 Wedge. Do I need to replace the torsion bars with bigger bars for the bigger engine? If I do, I can hammer them out with the pipe-wrench method, or will need the special tool to remove them properly. Where can I get the tool? Also, what is the maximum “safe” cylinder bore for that engine? Currently it is 4.25-inch
Bill, I like to remove the tension by backing the adjuster all the way off. I then remove the shock, and take the nut off of the lower control arm pivot shaft, and at the strut rod. Remove the clip at the back of the torsion bars. To make things easier to handle since you are going to go through the whole suspension, you can separately unbolt the brakes, and then detach the lower ball joint and pull the upper a-arms and spindles. You can then just use a large pry bar to push the lower control arm along with the torsion bar back until the bar pops out of the rear cross member once free at the rear, the bar will usually slide easily out of the lower control arm socket. If not, some taps forward on the socket where the bar goes in with a heavy brass mallet will free it. I found this system to work much better than the factory tool.
About the over bore dimension, I wouldn’t be looking for the “maximum” overbore, but the minimum. Since it is a factory bore now, you should be fine with a clean-up overbore and rebuild. If your plan is an unusually large overbore for a substantial increase in displacement, you are going to need to have the block sonic tested to find out how much meat is available.
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