Sunday, May 13, 2007
Do-It-Yourself Valve Spring Tester
Robert S. Hoover © 2003
A Volkswagen cylinder head contains seventy-seven individual components, the majority of which are capable of rendering the engine inoperable should it fail. Some of the components, such as the studs and the head casting itself are static and not subject to friction but due to the large number of dynamic components and generally poor valve train lubrication, they make up a significant portion of the engine's pumping losses. Since the pumping losses represent the engine's 'overhead,' any reduction in the pumping losses appears as an increase in the engine's output, usually for no increase in fuel consumption. By focusing on the details of those pumping losses, experience has shown that it is possible to achieve a significant increase the output of the engine.
Complex by modern-day standards, where an increasing number of engines are OHC, despite its high parts-count the VW valve train is reasonably robust thanks to seventy years of use during which the most failure-prone components have been identified and re-designed to improve their durability. That is, durability in vehicular terms. When compared to features found in aircraft engines, Volkswagen heads are something of a joke. When the displacement of the '1600' (actual displacement is 1584cc) is increased, as is commonly done when converting the engine for use in aircraft, durability takes a further hit. Fortunately, it takes only a modest amount of effort to improve its durability by an order of magnitude.
Most of the valve-train durability enhancements are covered in the so-called HVX modifications, previously posted and discussed. Although rarely seen on engines built for the Kiddie Trade and not found on any of the commercially available VW's converted for flight, the HVX mods have proven their worth through forty years of use in professionally built, high-output engines. Most recently, the use of thick-film lubricants have enhanced durability even further. (Specific How-To information for applying thick-film lubricants to valve train components will be found in the chapter on Coatings.)
Valve Train (Springs)
Poppet valves are a one-way sort of creature The cam pushes them open but they are closed by the action of the valve spring. The spring needs to be strong enough to close the valve tightly enough to make a leak-free seal but the valve spring merely initiates the sealing process. The real sealing is accomplished by the tapered sealing surface of the valve being wedged into the cone of the valve seat by the enormous pressure of combustion.
Modern-day valve springs are coiled compression springs installed around the stem of the valve and connected to it by a retainer that is free to rotate. The retainer is secured to the stem of the valve by a pair of keepers in the form of a cylindrical wedge which mates with grooves machined into the stem of the valve.
The strength of the stock VW valve spring is determined by measuring the amount of force needed to compress the spring to a height of 31.0mm (~1.220"). A number of factors can effect the strength of a coil spring and like all other VW specs, the tolerance is quite large, ranging from 117 to 135 pounds.
The valve's spring must be compressed when the valve is opened. The energy needed to compress the spring is part of the Otto Cycle's 'pumping losses' and anything that helps reduce those losses will improve the engine's efficiency. For a low rpm engine the lower valve spring value is more than enough to ensure proper operation and since the lower value reduces the pumping losses, it also serves to improve performance. Further enhancement occurs when the strength of all eight springs is equal or as nearly so as possible. For those reasons, a standard practice in any properly built engine is to use a set of springs that have been closely matched.
Matching a set of valve springs to within a pound or so can be quite difficult if you're drawing upon used parts. Not only are there different varieties of VW valve spring, each time a VW engine is stopped at least two valve springs will be compressed. In a vehicle that is driven daily this is seldom a problem but in an airplane engine that may sit for weeks between flights, the compressed spring is liable to weaken. When doing a valve job on a VW engine modified for flight, it's a good idea to re-test the valve springs.
Ideally, a new engine or a rebuilt head should include a set of new valve springs but with the number of registered air cooled Volkswagens in steady decline, it has become increasingly difficult to locate quality parts. It isn't uncommon to find after-market VW valve springs which are not square, in that the ends of the spring are not perpendicular to their axis. Such springs do not provide a symmetrical force when compressed and should not be used, an item mentioned in the factory service manual. You will also find new springs wound of lighter gauge wire than stock springs and which fail to provide the required strength when compressed. Springs longer than stock are also fairly common, often needing excessive pressure to be compressed to the specified height. Such junk is often advertised as 'racing' equipment, clearly meant for mechanically naive youngsters.
Volkswagen valve springs are progressively-wound, with the coils being closer together at the bottom than the top. Some after-market springs are not progressively-wound. (It pays to inspect all after-market VW parts before you buy.)
Twenty years ago I would never put used valve springs in an engine. Nowadays, used stock springs are often better than new, after-market stuff. If a used spring isn't rusty and shows no signs of fretting or jamming, I'll go ahead and test them.
New or used, it is extremely risky to use any valve spring without testing..
Valve spring testers are commonly available but even the least expensive model is several hundred dollars if purchased new. Fortunately, a common bathroom scale may be used to make your own spring tester. Unfortunately, inexpensive bathroom scales are not very accurate. Accuracy - - at least enough for the task at hand -- is assured by calibrating the scale with a mass of known weight, such as your own body, immediately prior to use. That of course assumes you know your own weight to within a pound. Balance beam type scales tend to be more accurate than low-cost spring-type scales. To calibrate the valve-spring's scale simply weigh yourself on a balance-beam scale then adjust the bathroom scale to read the same amount.
If you do not have access to a balance-beam type scale you'll have to create a test-mass of known weight. Having a specific gravity of 1.00, water is the handiest mass but you'd need at least fifteen gallons to verify the accuracy of your scale and the container would introduce some amount of error.
Lead is a very handy mass, having a specific gravity 11.34 times that of water and if you have a graduated beaker (which is easy enough to make) it's fairly simple to determine the volume of a given lump of lead. Unfortunately, pure lead is rather rare stuff and since other metals often make up as much as half the mass of wheel weights and other common lead alloys, it is impossible to calculate the weight of such alloys based volume alone.
If you have an accurate scale, such a laboratory type, you can of course weigh a sample of melted wheel weights, plumber's solder or other lead alloy, determine it's specific gravity and apply that to the mass as a whole.
When you are forced to create your own calibration mass without access to a precision scale you'll probably find plain old fashioned mild steel to be the best choice. This is because the amount of carbon and trace elements is typically less than 1%, allowing you to use a specific gravity of 7.93 or about 495 pounds per cubic foot ( about 4.4833 ounces per cubic inch ).
Since mild steel comes in standard sizes, even when purchased as scrap you can determine it's weight with good accuracy by simply measuring the piece, calculating its volume and applying the figures above. Then too, many scrap yards now use electronic scales accurate to a fraction of a pound, allowing you to simply buy a test-mass of the appropriate weight. Of course, being able to calculate the weight is a handy means of keeping them honest. (Hint: Weigh yourself on the junkyard's scales. Everyone does :-)
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NOTE At one time it was common for EAA chapters to maintain a tool crib and test-mass for use by its members. The test-mass was usually pigs of lead- alloy cast in convenient sizes from five to twenty-five pounds, clearly stamped with their weight after being accurately weighed. The fact EAA headquarters no longer puts any emphasis on such basic needs is good evidence of their growing disinterest in supporting grass-roots aviation.
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(I use a mill-end of 6" steel bar as my test mass. It is about 16" long and weighs 128 lb, 4-3/4 oz).
Volkswagen's valve-spring specification calls for a compression of 117 to 135 pounds at a height of 31mm. I made a gauge of this dimension that allows me to set the height of a bolt screwed into a pallet which sits atop a bathroom scale. The scale sits on a wooden base to which a fulcrum has been attached. The spring being tested is sipped over the bolt and a lever is used to compress it. When the lever touches the bolt I know the spring has been compressed to a height of 1.220" (ie, 31mm). And I know precisely when that happens because I've rigged the lever to turn on an LED when it touches the bolt. The LED is taped to the dial of the bathroom scale; all I have to do is keep my eye on the dial. When the light comes on I read the dial and jot down the weight on a stick-up. To eliminate human error each spring is tested at least three times. Any obvious flyers are thrown out and the testing is repeated until I have a cluster of similar values.
I try to do forty or fifty valve springs at a time. The first step is to clean them and inspect each spring visually for scratches or pitting anything that might serve as a stress-riser. They are then gauged for total length, then for squareness, both tests done on a surface plate allowing me to do a handful of springs at a time. Alas, when dealing with new, after-market springs those two tests may reduce the batch by half.
Any springs that pass the initial tests are then tested for compression height. They are then sorted according to their stick-ums and made up into matching sets, coated with preservative and put aside until needed. It isn't the Bureau of Standards but it's better than guess-work, which is what you have if you don't test your springs.
In making up a set of springs for a low rpm engine I want the lowest strength and the narrowest range. Of the two, I think matching the range is the most important factor. If I can't make up a set within a pound or two of a given strength, I'll generally keep looking.
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NOTE: Many fail to appreciate the importance of 'balance' in an engine. The reason professionals put so much emphasis on balancing is because the engine must use power to overcome any imbalance before any usable power can appear at the crankshaft. That means any imbalance is effectively multiplied by two. Using springs of equal strength is part of the balancing process.
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If you are building just one engine you should try to find someone who has a valve-spring tester. Baring that, you should cobble up your own using a bathroom scale.
So what happens if you simply buy a new set of springs and throw them in? Hopefully, not a lot. There is a chance the set may contain a spring having a radically different value but with a tolerance of 18 pounds, the odds are the engine is going to run. Sorta :-)
I should also mention that I don't know of a single non-professional engine- builder who tests their valve springs. This is another of those details they deem 'unimportant.' And when addressed in isolation, perhaps it is. But a professional engine builder addresses all of those 'unimportant' details, picking up a little torque here, better fuel consumption there, optimizing each unimportant detail for better efficiency, more power, cooler running and slower wear. No single one of those unimportant details results in a dramatic change. But add them all together and it isn't uncommon for a professionally built engine to produce up to 25% more power than a poorly built engine of exactly the same displacement. And to last twice as long as well.