Conversion - T2 Hoover

Rev - Nov 11, 2017

By Bob Hoover

The late Bob (Robert) Hoover, was one of the smartest aircraft VW gurus.
Bob hosted one of the most respected aircraft VW forums until 3mo
before succumbing to cancer - his opinion was rarely challenged.

See Also

Introduction - By Jan Zumwalt - (EAA 66327)

In Bob's own words..."No, not THAT Bob Hoover :-) (ie, Robert A. "Bob" Hoover from Tennessee and perhaps the best pilot in the history of flight.) The problem is that all Roberts get Bob-ed at birth and there isn't much we can do about it. When posting something about aviation I generally use 'R.S.Hoover' to prevent confusion."

R.S. "Bob" Hoover is best known for his fire side chat blog writing style and comfortable humor mixed with a common sense approach to aircraft use of auto engines. Even though he was a very qualified A&P, his approach to conveying information was more like a "grandfather" then a technician that knew what he was talking about. He was without a doubt the best aviation liaison we had for spotting new technology in the California Sand Rail & Dragstrip circles and keeping the rest of us informed on what was appropriate to aviation users. We lost a lot when we lost Bob...

Bob felt an obligation to leave important information he had accumulated in a manual for the rest of us to benefit from. His "VW Aircraft Engine Building" manual was started in 2000 but was so important to him that he took time to completely flesh it out and provide the final release just prior to his death. To post 119 pages of information on this web site would not be informative or good use of resources so a link to the entire 119 page pdf document is provided at the top of this article. It is divided into the following sections...

  • Engine History - 4 pages
  • Dismantling - 10 pages
  • Cleaning & Inspection - 15 pages
  • Parts Blueprinting - 40 pages
  • Pre-assembly 40 pages
  • Cam - 10 pages

The single most important achievement of Bob was when he and several friends examined factory fresh engines and reported improvements. He did not stop with automotive VW models. He also carefully examined Volkswagen's "Industrial" motor factory models used for air compressors, ground power units and other applications. As a result he was able to inform us of improvements and missing data provided to industrial customers but missing from automotive engines and manuals. 

Be sure to download his manual and read it thoroughly. So that we can provide a small glimpse into the depth of his knowledge we now present a sample of his recommended VW modifications for aircraft use - he coined the phrase "HVX" mods.


By Bob Hoover - MAY 29, 2007

The air-cooled Volkswagen engine doesn't’t have a very good lubrication system. (Which isn’t surprising, seeing as the design dates back to Xavier Reimspiess’ original 1931 ‘boxer’ engine.) Its inadequacies became evident in the late 1950's when me and a few other fools started hot-rodding the things.

The Ford Motor Company had recently published a study of the effects of oil filtration systems on engine wear and the results were so impressive that by the time you could say ‘Jack Robinson’ we’d retro-fitted our bugs with oil filters. But the main problem wasn't dirty oil but not enough oil, especially at high rpm. The inadequate amount of lubrication reaching the heads resulted in excessive friction, leading to high temps and failed valve-train components, which put you out of the race.

Auditing the engine’s lubrication system we found that all of the oil for both heads came through a single 5mm drilling. In theory, a hole that size should have provided more than enough oil. And it did, but only for the left-hand head. And then, only at low rpm. Which was fine for a stock 1200cc 36hp engine, but we’d already bored & stroked that puppy to nearly 1700cc and were running them at over 5000 rpm. But not for very long.

Volkswagen was aware of the valve-train lubrication problems and added a drilled oil channel to the rocker arms and a larger main oil gallery on later engines but the basic problem was that not enough oil was reaching the heads, especially the one on the right-hand side of the engine.

(NOTE: VW orientation is always relative to the driver. The right-hand head is the 1 & 2 cylinder bank, left-hand is the 4 & 3 bank.)

To get oil to the right-hand head, VW cuts a square groove in the bearing saddle for the #2 cam bearing. All of the oil to the right-hand side of the engine gets there through that channel. (And still does, if you haven’t modified the crankcase.)

To make matters worse, the oil to the heads gets there via the cam-followers... but only when that particular valve is actuated. In effect, the VW cam-followers act as a valve, shutting off the oil to the heads for approximately 92% of the time.
To get an engine that could run flat-out for 24 hours we had to get more oil to the heads. To do that we tried opening up the oil channel in the cam-bearing web. That worked but we still weren’t getting enough juice. So we modified the cam-followers to allow oil to reach the heads 100% of the time instead of only when the valve was actuated. Major improvement, but we were still seeing galling on the right-hand rocker-arm shaft, plus an occasional hair-pin fracture. What we needed was still more oil... especially to the right-hand side of the engine.

To do that, on the right-hand case half we extended the oil gallery for the cam-followers to intersect with a new oil channel we drilled into the #3 cam bearing web. Big, BIG change. And lower head-temps, too.

With that as a clue, we grooved the rocker-arm shafts to match the oil channels drilled into the rockers. Fitted with the Ford/Subie type swivel-foot adjusters, which have a matching oil channel, we could now provide the heads with approximately eight times as much oil as before. Heads ran much cooler... which meant the oil coming fromthe heads was hotter, so we had to come up with a better oil-cooling scheme. Which we did, moving the oil cooler outside of the blower housing.


All of which is pretty old news to anyone hanging out at the finish line. But a total blank to just about everyone else. The magazines were only interested in the mods when there was something tosell, such as an oil filter bracket or an oil cooler core. All of the fiddley bits that made the system work, such as drilling the new oil channels or grooving the rocker shafts, were seen as just more of those ‘unimportant’ details professional engine-builders are always messing with. (Most magazine 'technical' articles are nothing more than infomercials, intended to sell whatever product is being touted. )

A key point here is that a high-output engine needs all of the modifications described above: 100% filtered oil, increased oil volume to the right-hand side of the crankcase, 100% oil flow through the lifters, grooved rockers & rocker-arm shafts, and the Ford/Subie-type swivel-foot adjusters, which act as spray-bars. Some guys would modify the rockers and say they didn’t see any improvement. Others would modify the lifters and say the same. But not one in a thousand incorporated all of the modifications. And still don’t. But it’s interesting to note that Volkswagen included all of the modifications to the Type IV engine. In fact, you can find them in every modern-day engine. Which is just another of those ‘unimportant’ details.