Friday, November 24, 2006

VW - Free Horsepower

(written about 1995)

A subscriber of this list recently took me to task for my repeated use of the expression ‘properly assembled engine,’ insisting that some ‘shade tree mechanic from California’ could not possibly improve on Volkswagen’s own methods and their millions of engine’s-worth of experience.

He failed to consider the fact that I build just one engine at a time. And where VW specifies an acceptable range of tolerance in their parts, a necessity for high-volume production, I don’t have that particular burden. When setting crankshaft end-play, for example, I dress the thrust-face of the #1 bearing until I achieve a clearance of .0025 exactly. This takes as long as it takes, from a few minutes to a day or more if I have to track down another set of bearings. And I didn’t learn this out of books, I learned it from VW mechanics; guys who took considerable pride in being able to improve on a ‘factory job’.

What remarkable improvement in horsepower or longevity results from assembling an engine to better than factory spec? In the case of crankshaft end-play, I doubt if there’s any at all but the truth is, I don’t really care. I know that if all the shims are dead flat, the flywheel and crank are true and the thrust surface of the bearing brought to truth with dye and judicious flatting, I get a nicer fit that holds spec longer. (The wear limit here is .006; you can check it without dismantling your engine by taking the measurement from the pulley-end of the crank. It’s a quick check on the general state of health of the lower end.) No quotas. No time clocks. When it’s right, the engine tells me so. And I derive great satisfaction from that. This applies to fitting the rods, adjusting ring-gap -- to everything involved in ‘proper’ assembly of an engine. And you can do it too.

Chances are, if you’ve never built an engine before, you won’t do any worse a job of it than Volkswagen and may do even better, which is why I think everyone should build their own engines. But most of you don’t. A lot of you are satisfied with a short-block from a reputable manufacturer. And that can be a costly mistake.

Back when the Volkswagen engine was designed, the importance of balancing, both dynamic and volumetric, was not well understood; by modern standards the VW balancing specs are quite crude. The best proof of this is to compare the output of a properly assembled engine against one assembled to stock specs. The difference is never less than a 5% increase in horsepower and often as much as 10% when careful attention is given to proper balance. This shows up as improved efficiency, with the balanced engine requiring less fuel for the same output, and producing less heat. The balanced engine also lasts longer.

To properly (there’s that word again!) balance an engine the clutch pressure plate, flywheel, crankshaft (with the cam gear installed), and fan pulley are balanced as a unit. This cannot be done with a short-block without dismantling the engine. In effect, each time you elect to use a short-block you are accepting the fact the engine will not be as efficient as it could be. Another burden I escape by building one engine at a time is that I can do as many pre-assemblies as I feel are necessary; VW puts each engine together once, as does all of the high-volume after-market engine builders I know of.

With new parts, held to a reasonable tolerance and assembled -- one time -- with reasonable care, the finished product will be no worse than the stack-up of those tolerances. You could get a real dog. But tolerance stack-ups tend to cancel themselves out, which puts the odds in your favor. Even so, I prefer not to play the odds when it comes to engine assembly. Poker, yes; but with engines I’ll cheat until I get it right.

Getting the volumetric balance right is something VW would rather not talk about since they don’t time all four cylinders to fire at the same point of rotation. Even an engine with perfect volumetric balance will be out of balance when the #3 cylinder is timed to fire 4 degrees later than its three brothers. (Delaying the timing of #3 cylinder was Volkswagens solution to the chronic overheating caused by the internal oil cooler.) (Ed. Note: That ended with the introduction of the doghouse oil cooler. Without the internal oil cooler to block the flow of air to #3, Volkswagen was able to do away with the retardation.)

Volumetric balance means all of the cylinders have the same internal volume, meaning each will have exactly the same compression ratio and will deliver the same specific impulse when the spark plug fires, assuming they fire at the same degree of rotation. In theory, this isn’t very important if the engine is designed to operate at slow speeds and has a massy flywheel. In practice, the difference is as I’ve stated above. It is quite common to achieve an overall 10% gain in horsepower by properly assembling the Volkswagen engine. No tricks and no add-ons.

Volumetric balance starts with the heads. Their chamber volumes must match. In practice, you increase the volume of the three smaller chambers to match that of the largest. You do this by re-seating the valves and removing metal from the chamber by burnishing, grinding and polishing. (As a matter of course, all chambers are polished mirror-bright using a felt hob and polishing compound.) With the combustion chamber volume known, the crankcase is then assembled and the deck height measured. Cylinder spacers are used to arrive at a standard height, chosen according to the desired compression ratio. The final adjustment to a ‘fat’ cylinder may be made by shaving the head of the piston by up to .005", all other adjustments made by grinding the cylinder spacers, if needed. (Cylinder spacers are available in a wide range of thicknesses for all cylinder diameters.)

When measuring deck height it’s important that the engine be properly torqued, including heavy plates used to simulate the heads. The plate is usually cast iron or precision ground cast aluminum plate, 1/2" to 3/4" thick, pierced for use of a depth gauge; usually a dial indicator. Steel spacers, their ends machined square, are used to take up the different length of the head studs. (Sections of water pipe are perfectly adequate.)

After deck height is determined, the pistons and rods may be balanced. Rods and pistons are statically balanced; the rods are balanced so as to have a common center of mass (i.e., big-end vs little-end balancing).

Sounds pretty sexy, eh? Lots of exotic tools? Frankly, no. You can cc your heads with a turkey baster. And the polished cast plate I used to make my deck-height fixtures cost $4.20 at a scrap yard. The dial indicator is the same one I use for determining end-play, runout, cam lift and so on. The head-work is done with a standard kit of Cratex tools, common stuff to anyone who has ported or polished a head. (about $24 from Enco)

But dynamic balancing can be expensive. I use a guy who does only racing and aircraft engines; his minimum spec is an order of magnitude better than VW spec and he charges accordingly. A regular automotive engine balancer usually charges between $50 and $75, depending on where you live. Their typical spec will be much better than the VW factory spec. For example, within a set of four stock connecting rods the weight variation can be 8 grams (580-588). That’s ‘low-spec’ for VW rods. (VW does not have a spec for center of balance (i.e., center of mass; big-end vs small-end).) A set of balanced rods will vary no more than .01 gram, and their center of mass will fall within .005" of the same point. (‘Balanced’ rods from an after-market source usually means they’ll meet the minimum VW spec, which is to say they are not balanced, in modern-day terms. Buy the best rods you can afford [SIR offers good value] then have them balanced.) If you’re unlucky enough to have an engine that was casually rebuilt using rods from different weight groups, the difference between the heavy and light rod could be as much as 16 grams. (Engines that have two heavy rods on the same side have a characteristic lope at idle that conventional wisdom says is the sign of a ‘hot’ engine, mistaking the lope for the valve overlap of a high performance cam.)

Central to balancing is the use of a counter-weighted crankshaft. This too was unknown territory in the early 1930's when the VW engine was designed; conventional wisdom of that period said an opposed-type engine did not require further balancing. Thanks to modern computers we now know that the stock VW crankshaft flexes in a peculiar way around the center main bearing. The flexure is called phlugoid motion (ask an engineer) and causes the bearing to wear in an oval pattern as it pounds out the center main bearing web. A properly balanced counter-weighted crankshaft, fitted with balanced rods, spins with less phlugoid motion. The energy that was being transferred to the bearing (and pounding it out) is delivered to the flywheel; engine efficiency goes up, heating goes down and the bearings last longer.

In my opinion the best VW crankshafts are those made by Gene Berg. They are straight -- you seldom find any measurable runout, and are finished to extremely close standards; the journals match. If you assemble a lot of engines you’ll run into new cranks that have one journal half a thou tighter or looser than its fellows; I’ve never seen that sort of sloppiness with Gene’s cranks. And their finish is beautiful.

Gene will probably tell you his cranks are balanced, which means they probably are. But they are not balanced as an assembly, with clutch, flywheel, gearing and pulley attached. You gotta do it over.

(This was written before Gene’s untimely death. I’ve since been using cranks from other sources.)

If it rotates, it must be balanced. If it’s designed to rotate as an assembly then the assembly must be balanced as a unit. (Hint: Dismantle your alternator. Take the rotor with the blower attached, to the balancer. The result is more air. Less driving power. Better cooling. And your alternator bearings last longer.)


There’s no such thing as a free lunch. (I’ll let someone else explain that to our foreign subscribers.) But here I am telling you where to find five or more ‘free’ horsepower and get a lot of other benefits at the same time. And since the engine isn’t using any more fuel, how can it be developing more horsepower? The answer lies in what those ‘free’ horsepower were doing in the unbalanced engine. They were making heat -- about 3,500 watts of it. In the case of volumetric balancing (and proper firing time for all four cylinders), the engine was working against itself, using some of its energy to counter the effects of late timing and an uneven specific impulse. In the case of static and dynamic balancing the losses appeared as increased friction, internal heating due to flexure, and dealing with the stored-energy loads induced by the uneven specific impulses.

There’s no such thing as a free lunch but this is one lunch you’ve been paying for all along. Balancing your engine allows you to get some benefit from what you’ve been paying for.

1 comment:

Anonymous said...

Somehow, subconsciosly, I always new it. But you have put it together so nicely.