Sunday, November 26, 2006

VW - Heat Risers & Education

Frustrated in Indiana wrote:
>No matter what I do I cannot get both sides of my preheat tubes to heat
>to the same level. The passenger side gets very hot while the drivers
>side gets only lukewarm. I was told that this was not a problem.
>Anyone who knows different, please let me know. The preheat tubes do
>not connect correct???????


Dear Frustrated,

Yes, they do connect. But you may not have a problem. The heat-riser gives up its heat to the manifold in order to ensure proper vaporization of the fuel. That means one side should be cooler than the other.

The manifold pre-heat pipe is in fact one pipe about four feet long(!) that runs from one of your exhaust stacks to the tailpipe of your stock muffler. That puts the inlet at in a zone of high pressure (ie, the exhaust stack) and the outlet in an area of low pressure (ie, the peashooter on the driver's side of the muffler).

The pressure differential is important because the hot exhaust gases give up their heat to the center section of the intake manifold casting (ie, where gasoline absorbs the heat and changes its state from an atomized liquid to a true vapor). Once the exhaust gases have given up most of their heat, you must have that pressure differential to insure adequate gas-flow through the pipe.

Fail to pay attention to those fundamental rules of physics and the system won't work.

And as you may have noticed, virtually all of those kewl after-market exhaust systems ignore those fundamental rules.

What you get with those oh-so-kewl after-market exhaust systems is an inlet on one exhaust stack... and the outlet on another exhaust stack. Which guarantees the system won't work.

Why not? Because you are trying to dump from a high pressure area to a high pressure area. Instead of a manifold heater, you end up with a fluidic yo-yo. And a hole in your wallet. And a VW that doesn't run very well.


When exhaust gases are cooled they can no longer support a host of combustion products. The combustion products, from water to soot to metallic compounds, are deposited on the walls of the exhaust system plumbing. When the plumbing is small, such as the manifold heater pipe, the combustion products will eventually block the pipe.

"Eventually" can be up to a 100,000 miles on an engine with a stock exhaust system or as little as thirty minutes (!) with one of those kewl after-market systems and their fluidic yo-yo.


If you thought all that was a gas, you'll love what comes next :-)

So the kiddie installs one of those go-fast/make-noise exhaust systems that all the magazines say are really, really kewl and worth at least 10hp, mebbe more... and the kid quickly discovers his ride runs like shit. Of course, it can't be the really, really kewl exhaust system so it hasta be that stock distributor or the carb or the fact that as a mechanic the kid would make a fine janitor and hasn't cracked a book since sixth grade, when he learned how to type and now learns everything he needs to know via the internet, including how to fix his ride :-)

The 'fix', according to all the instant experts, is to install a really kewl 009 dizzy and a set of Kadrons, complete with failure prone linkage and cheese cloth air-cleaners. And sure enough, the thing now runs at least as well as it did before, mebbe even better. Of course, he's now spent about $500 ( which is what all those instant experts really cared about). And with those grossly deficient air-cleaners, mechanical advance distributor and those rich-running Kabrons, the service-life of his engine just took a header into the porcelain fixture, which is also counted as a Big Win by all those instant experts drooling to sell him a new engine and all the neet stuff to go on it.


If you've got a stock exhaust system and aren't getting an adequate amount of heat to the manifold, drop the engine, pull the manifold and clean the heat-riser.

If you've got an after-market exhaust system, chances are the heat riser flanges are simply tacked on to the exhaust stacks, which guarantees the heat riser pipe will clog up.

Back in the sixties EMPI offered a header-type exhaust system that really worked. It carried the heat-riser outlet all the way to the 4-into-1 collector. Worked great.

If you run a center-mounted carb on a horizontally-opposed engine you need supplemental heat to insure proper vaporization in the runners. No mystery to any of this, it's in all the books and has been for seventy years.


But I saved the best for last :-)

Virtually all of those really-really kewl after-market exhaust systems address only two of your four cylinders. So long as you run the stock heat exchangers, all you've done is upset the volumetric balance of the engine.

Wanna go-fast/make-noise? Pull your peashooters and install a zoom-toobe. Most of the restriction in the stock system is in the peashooters, the big canister is just an expansion chamber. Get rid of the peashooters and you'll have about the same reduction in back pressure as you get from installing an 'extractor' on only two of your cylinders... but without upsetting the volumetric balance or causing the heat riser to clot shut.


I'll leave you guys to figure out why you don't see these unimportant details mentioned in the magazines :-)


> What is your reccommended method for opening a plugged pre-heat tube? >Nasty chemicals? The old ex-clutch-cable-in-the-drill trick? The >oxy-assisted burn it out>


You let out 'None of the Above' :-)

The fastest method is to use media blasting and a flexible wand.

I know St. Muir sez to use oxy-assisted burnout but take it from me, never use oxygen-blast. You'll melt the aluminum jacket that bonds the manifold to the heat riser long before you burn away the deposits inside the heat riser. (This is another example of the many destructive errors in the Muir book.)

Oxy assisted burnout works fine with cast iron manifolds and was the standard procedure for Model A's and V8 Fords but it simply does not work on the VW intake manifold. Why? Because by the time you raise the heat-riser pipe to the required red heat, the aluminum jacket will have long since melted away.

Soak & Poke (ie, carb cleaner and spinning a cable inside the pipe) works but is slow and messy. And it doesn't have to be a clutch cable. E-brake cable will work... as will any multi-strand steel cable. Just rotate the cable in the direction opposite of the lay and the end will 'fluff up', make a pretty good brush.


>I never cease to be amazed at the things that I never thought about. Thanks, Bob!


You're welcome.

For me, one of the most rewarding aspects of veedub-dom is reading all those neat reasons the kiddies use to support their totally invalid conclusions, such as how much better their ride handles now that they've dropped it nine inches and increased the caster angle to sixteen degrees :-)

A favorite is the 'fweem' crowd :-)

Block off the heat riser and guess what happens to that 'fweem' sound :-) or, for the serious experimenter, work that equation backwards and figure out where that 'fweem' sound comes from.

It's really unfortunate that so many present-day owners of aircooled Volkswagens never had the opportunity to drive one out the showroom door. Today, most veedubs are rolling wrecks and their owners don't even know it. They've never driven the Real Thing, have no idea how well the VW can handle, how reliable and economical it can be. And they never will, thanks to all the swell advice they get from instant experts who will do literally anything to sell their junk to mechanically naive youngsters, even if it means destroying the vehicle or risking the kid's life. (Go on kid, tear them flaps outta there! Then lemme show you how to lower that thing :-)


>So where should I, new to VW's, be looking for info? I've got pretty
>good sense, am mechanically competent, and am willing to learn. I've got
>a a copy of Muir, and the Bentley for my '70 transporter is on the way.
>Any suggestions on where to get more, reliable info?


You've asked a good question, not just about Volkswagens but about life in general. Unfortunately, there is no pat answer.

The generic answer is that the answers to all questions will be found within yourself. You gather information, use it to formulate what appears to be the most correct answer and then test your conclusion against reality.

In effect, the answer will reflect your perceptions. The problem is, in modern-day America, the typical youngster does not understand this extremely important learning process.

At one time all youngsters were exposed to the elements of logical reasoning in the public schools, usually between the ages of 11 and 13. This provided the foundation that allowed them to acquire knowledge from information, rather than simply memorizing the material and vomiting it back up during Finals. Mastery of Logical Reasoning (under various names) was the Rite of Passage between Primary schooling, which was devoted mostly to rote memorization, and High school, in which the student was expected to actually learn something.

Nowadays the process of learning by rote extends even to our colleges and the fact that a parrot is not a scholar is politely ignored.


As to your specific question, the best source of information is the Volkswagen Workshop Manual. The 'official' manual sold by Bentley is an abridgement and has omitted a great deal of vital information.

Muir is a good means of demystifying the automotive arts but it is larded with factual errors.

The Haynes manual for the bug and Ghia (Haynes #159) is probably the most cost effective source of VW information for the beginner.

If you intend to overhaul your engine you should hold a copy of Tom Wilson's "How to Overhaul Your Volkswagen Air-Cooled Engine"

For some insight into gaining more performance from a VW engine you should study Bill Fisher's "How to Hotrod VW Engines". For body work, painting and so forth there are other very useful manuals, some of which I've mentioned in previous articles.

If you are a rank beginner your best bet is to start with lawnmower engines(!) They are small and you can usually get all you want for free, simply by running an ad saying you'll haul the thing away. You will need the Briggs & Stratton handbook or some similar tome (Haynes offers one) and a few tools.

People who don't understand automotive engineering often laugh when I suggest they start with smaller engines but an air-cooled one-lunger has about 85% of the 'DNA' of your VW engine... or your Pratt-Whitney, Lycoming or what-have-you. They use the same Otto-cycle, the same poppet valves, the same cam/crank relationship, the same principles of ignition, carburetion, intake and exhaust. These things are the foundation you need if you want to be able to maintain your veedub.

Once you've mastered the basics, be it engines or suspension systems, you will have established the framework of knowledge that will allow you to understand and appreciate the differences between, say, a lawnmower engine and a Corvair engine.... or a Volkswagen engine... or any other air-cooled engine. I know that sounds a bit fey but in engineering terms, the differences are merely matters of detail rather than principle. The most significant gulf is not between a 4,000cid aircraft engine and a VW but between liquid- and air-cooled engines, in that air cooled engines operate at significantly higher temperatures and their maximum sustainable output is determined by their fin area.


As for my comment about driving the 'Real Thing,' it was not a joke. Most Volkswagens that I see suffer from some horrendous problems of which the owner is not even aware. But by the time I see them the accumulation of such problems have reached the point where it would be economically impossible for the typical VW owner to have the vehicle returned to spec. For the very wealthy, it's not a problem to cough up seventy bucks an hour for a hundred or more hours of skilled labor. They come away with an antique bug or Ghia that is literally in better-than-new condition. But for the average owner the only option is to do the work themselves and the record shows most are unwilling or unable to do so.


The real problem with cleaning the heat riser is when it's completely blocked. All of the usual methods work best when the pipe is still partially open. Once it becomes blocked your chemicals can only work on the end of the plug, a relatively small area. Your cable-brush is most effective scrubbing the wall of the tube rather than boring a hole through a solid plug.

And finally, the tube isn't round. Under the aluminum casting, where the two pipes are siamesed together, the heat riser is crushed down to a D-shape. This insures the largest possible contact area between the pipes but the D-shape prevents any brush from getting into the 'corners'. And if you don't get the pipe really clean, the residue will act as a 'seed', attracting combustion products and quickly clogging up again.

Media blasting works better than any of the other methods.

Vinyl or rubber tubing works fine as the applicator wand. Once you break through the plug it takes only a few minutes to scour the pipe clean.

But don't use heat. And don't remove nuts with a chisel. The fact a procedure is valid for a steam engine or a Model T doesn't mean it can be applied to a Volkswagen :-)

-Bob Hoover


Unknown said...

Hi Bob,

I read a post attributed to you about proper orientation of the distributor shaft in the VW engine case. In that post you emphasize the important of having the oil groove/hole in the dizzy shaft aligned with the oil passage window in the case. I checked my SVDA dizzy and the oil groove is at about the 6 o'clock position, far from the oil window in the 3 o'clock position. If I were to rotate the dizzy 90 degrees the vacuum can would be on top of the fuel pump. Are you sure that groove needs to be in the window? Old cast iron dizzies don't have the groove. They apparently get their oil through the base. Any reference to VW literature that supports your view about the dizzy groove orientation is much appreciated.

Jon said...


Really interesting article. I'm just starting the rebuild of the 1200 bug I've owned for 15 years, and thankfully, the engine is stock except for an 009 dizzy that's been on there since I bought it.

Me heat risers are pretty well blocked and I intend to clear them soon. I've got a question that I haven't seen answered in the books I've got....

When I removed the exaust, the gaskets for the heat risers had different sized holes on each side. I guess this is where the pressure balance along the pre heater comes in. The problem is, I can't remember which side had the smaller holed gasket. (Guess I need to write more stuff down as I dismantle ;-))



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