Paint your 'wagen

Another popular aircooled question, whith a lot of opinions, a lot of anectdotal information and bugger all science around.

It is well know in physics that a black item will radiate heat better than a silver one, but I think the first question is whether that is what we are looking for.  The majority of heat energy is conducted away by air flowing across the fins, very little is radiated. Any paint or colouring is unlikely to provide a meaningful increase in heat dissipation in any case.

Bob Hoover pointed out the advantage of preventing rust, which he points out quite correctly is an effective insulator.  I personally have not seen cylinders rusted to this degree, including my Kombi which spent most of it's life by the coast (and didn't have much metal left in the chassis, to be honest, but I was a poor student at the time.) Of course it looks good, so if you are of the "Cal-Look" crowd nothing else might matter and you need read no further.

Some guys here in Aus came up with a test rig which they claimed proved paint will make the cylinders run hotter, but there are a number of problems with their test method:

1. The temperature sensors were mounted on the outside of the cylinders. The outside of the cylinders and the fins are where we want to transmit the heat from - a higher surface temperature could indicate more efficient cooling! (Refer Newton's Laws of Cooling.)
2. The method involved filling the cylinder with water, then cooling it with a fan, which suggests questionable accuracy and repeatability.
3. The sensors, I believe, were thermistors which are not the most accurate device (using accurate in the scientific sense.)
4. The tests do not appear to have been performed a number of times.

 The main problem here is what we are measuring - we are trying to measure the efficiency of removing the heat energy out the outside surface of the cylinder (preferably with an airflow of similar characteristics to those produced in a running engine,) while this energy is being constantly supplied to the inside surface.  While using boiling water to supply heat to the inside seems appropriate, we want to know how efficiently that heat is being removed from the same region, not the temperature of the outside surface.

I would like to make some tests of my own at some point in the future, probably along these lines:

1. Drill a hole into the cylinder wall and insert a thermocouple, probably at more than one point and preferably out of the area exposed to the airflow, or inside.
2. Heat the cylinder to a set temperature, probably in a similar way, as the boiling point of water is pretty constant, so the cylinder can be filled for a set period, drained and capped, and the temperature should be pretty constant.
3. Simulate the air pressure, flow and speed of the VW fan.
4. Monitor the rate of temperature drop of the actual cylinder wall under these conditions.

This still may not be repeatable to give reliable results.  I recall when I was young there was similar debate about efficiency in heatsinks (used to cool electronic components.) 

When a heatsink used in a non forced air situation was treated with anodising to make it black, there was little doubt there was an improvement and other tests showed advantages in painting (usually a very light coat from a common spray can of flat black.)

As soon as fan forced air came into the equation (and sometimes convection flow) the advantages would disappear in fact the efficiency of a painted heatsink was usually lower!  The main reasons cited for this were usually insulating properties of the paint, or the reduction in turbulence from the smoother painted surface.  Those cases where efficiency appeared higher with a painted surface could not be discounted as being from the surface preparation for the paint rather than the paint itself.

As an aside, producing a rough finish, a la Bob Hoover's blasting, does tend to improve the efficiency of either radiating or conducting surfaces, but you are unlikely to be able to measure the difference.



So, my opinion?

As anodising is not practical on cast iron, you are not likely to gain any improvement in cooling by painting your cylinders.

A thin enough coat should not cause any significant loss in cooling however, but such a coating may not be enough to prevent rust either.  I'd probably try a combination of phosphoric acid treatment covered by a spray of highly-thinned flat black paint, or a straight (maybe tinted) zinc primer if I was going to try.

PS - I have to arguments with Bob Hoover's comments about chrome. It will significantly reduce radiant heat dissipation on valve covers and pushrod tubes and personally, I'd like to keep all the cooling I can get.

PPS - if anyone can afford gold plating, I'd love to know the results - or you can do it to my engine and I'll let you know.

Rod Ratio

We all hear a lot f advice about rod ratio - usually variations on the old "use the longest rod that will fit."  Gene Berg actually recommended the opposite, on the argument that it will increase torque in the usable range.

Why a high rod ratio? There was an excellent explanation on the net I will have to find,but basically he made a model with a rod ratio of 1 for the extreme of short rod and an infinite rod for the long.

Remembering these are impossible extremes for the sake of illustration: 

The low rod ratio model:
Starting from TDC, the piston will start moving immediately and will rapidly reach it's lowest point when the crank reaches 90 degrees ATDC. It will then stay at this point until the crank reaches 90 degrees BTDC.

Advantages here are - the more rapid movement of the piston in the early part of the stroke creates greater vacuum and higher intake velocity. At lower engine speeds, this pulls harder on the carburetors allowing them to operate in a better part of their curve, and the increased intake mixture speed will have more inertia, so will keep flowing longer after the piston ceases it's effective movement.

Disadvantages here are - the rod reaches an extreme angle, in the example 90 degrees, which is cocking the piston and pushing it against one side of the cylinder. This increases friction, wear and can interfere with ring operation. The increased piston speed at particular engine speeds limits the maximum rpm for a given set of components.  This also increases the velocity of the intake mixture, meaning large ports are required for a given rpm, though this is not linear, as the inertial fill can make up for some of this.

The high rod ratio model:
Starting again at TDC, the piston will start moving rather slowly (as the top part of the circle is almost level,) and this time will take the entire 180 degrees to travel to the bottom of the cylinder where it will immediately begin the return journey.

Advantages here are - as the piston uses the entire stroke to travel the length of the cylinder, it does not pull the air as fast, meaning the engine can reach a higher rpm before the limit of the intake system is reached. The rod in the theoretical example remains vertical, removing the side loads.The maximum piston speed is reduced, as is the change in piston speed at the top and bottom of the stroke.

Disadvantages here are - while the piston moves throughout the entire stroke, it doesn't move very fast in the early part of the stroke so does little work, then the maximum speed is lower, so produces less inertial fill later in the stroke when the piston is again moving more slowly.  So while we will get higher peak rpm, hence power, we will struggle to fill the cylinder at lower engine speeds.

Which brings us to the most important part of the question What rod ratio should I use?

In the real world, we are going to largely limited by the physical characteristics of our engine and engine bay- no-one is suggesting a stroker is not a good idea - even if you are not tempted by the torque, you will eventually need to go this route to achieve power through cubes.  This will largely determine your rod ratio - eg an 82mm stroke with stock rods will give you about 1.67, down from the stock 1.99. If we make the engine a little wider, we can fit a 5.5" rod which brings our ratio up to 1.7 - how much difference will this .03 difference make - not much. Either through calculations or driving, we will not find much difference, except in an all-out strip monster and even then it will only appear at the top end.

So choose a rod that suits your engine - personally I'd aim for minimal or no cylinder spacers and an appropriate copper "Head Gasket" as you will get the best sealing at both ends of the cylinder this way.

Save your "Porsche Length" rods for your stock bore stroker, where you can't get 'B' pistons, and longer rods for longer strokes where you have no choice.  If you want to fit an 86mm stroke into your road-going car, you are going to have to attack the body to make it fit!

Cams Part II

So, what am I running?

If you have read my previous posts, you'll know I'm starting my first build in a while - with a heavy  leaning towards being streetable.

I have a cam left over from the engine I was planning to build before I married - a Web-Cam 111.This gives 238 degrees at 50thou' and 476thou' (12.1mm) lift with an "advertised duration" of 272degrees - i.e. what would be termed an  aggressive high lift short duration cam.  The rate of lift is pretty high, but in reality .476" is not that high as a stock VW head can tolerate at least .5" of lift.

I know experts will tell me this is not enough duration for an engine of this size but  the  truth is duration is only part of the equation and should actually be related to the speed of flow through the heads, not the size of the engine.

While it is tempting to use longer duration to compensate for the reduced port speed of smaller intakes, if the flow is high enough it should not be necessary.


Higher lift, while accelerating the valve more (and increasing the danger of float,) helps to increase flow in any good head, and increases the area under the curve - i.e. the time when the ports are actually producing meaningful flow.

Aircooled.net say "it's pretty harsh on the valvetrain,don't expect it to last 50k miles" - but who does on a performance engine?  For all the respect I have for the late Gene Berg, engines with high lift (i.e. those that perform as we'd like) tend to need a head job by 50k - funnily enough, about the point that VW mechanics here said they would...

Cams

Straight into a bit of controversy - most VW people put far too much cam into their engine.

Quite simple - the extra duration of  a "performance cam" is to allow for the inertia  of the mixture or exhaust to traverse the ports.

Mention the specs of a stock VW cam to a Ford guy and he'll think you are talking about a "sports cam."

Face it, the mathematics and science of fluid dynamics are known.It's a bit pricey these days (I run my old copy on an old OS on an old PC) but Performance Trends "Engine Analyser" can really open your eyes!  - doesn't matter how many cylinders engines have, the maths are the same! Engines are engines, guys.

Look at the originators of VW performance - it is possible to almost double the original HP of a VW engine whilst keeping the stock cam! Of course you will need ratio rockers, a decent exhaust, carbs, some headwork and maybe 88mm cylinders to achieve this - and your engine won't liive long, but it is doable. In such a configuration the engine can pull well to 6k or more, but the rotating assembly and valvetrain aren't really
up to it.

This is the Gene Berg/HTVHRVWs method - 85.5 bore, 69 stroke, stock cam with up to 1.5 rockers, good carb/manifold combo flowing about 250cfm, free flowing extractors and work the heads. Won't look so good "on the dyno" - i.e. the classic peak HP figure marketing guys love,but it will be a lot of fun to drive if you get it right - tons of torque when you need it, and just keeps winding when you want it to!

Eventually it would run out of puff and the easy fix was tofit a cam with longer duration - ever noticed the popular Engle cams are advanced 4 degrees, almost the same as the stock cam?

Trouble is; your valves  are floating, your crank is pounding your centre main and everything is running a bit
hotter than what is healthy.

But shoot - guys put 50,000 MILES or more on those things, and had a lot of fun doing it!  you're wondering what my problem is, right?
 
Trouble is then, even more than now, good heads were expensive, so the easiest and cheapest way to get more power (or at least peak power) was to fit a wilder cam. So we have a world where 300 degrees at 50thou' is considered by some to be "streetable!"  Think about it - a full revolution (two strokes) is 360degrees -where the heck is your induction  or compression going to occur at any normal engine speed?

I  love drag racing as much as the next guy, but it's not the same world as any kind of driving on the road.Top Fuel guys admit the actual parts are all off the shelf, according to the rules - races are won by fuel delivery (they use mechanical injection,)  clutch setup (a multiple plate setup - no gears,) a little bit of suspension/tyre setup and quite a bit or driving.  But let's face it - these engines can't idle as we know it, and have no useable torque in the range we use!

AA Pistons

These actually turned out to look pretty good, and going through some old magazines, it turns out AA have been producing VW pistons for about a decade.

They weigh about 10-20g under a stock piston (but are 90.5 "B",) have the graphite on the skirt, which is a "short skirt" - this is supposed to produce less friction and result in a lighter piston.  I'd like to end up with a little "fuel economy" on this engine if I can.  Pin fit is excellent and the ring set looks good for my application - street use, with ability to blast up to 6000 rpm or so (if I can afford it at the time, I will have it balanced to 8000, but my cam and heads won't produce too much at that level.)

Ok, they are "cast" rather than "forged" pistons, but outside of crazy levels of supercharging, that is what most of the world is using these days...  They also come with proper circlips rather than the wire things VW supplied.  If anyone out there doesn't know, always fit these with the "wider edge" out - look at the ring from the side, you will see a slight chamfer from where they were pressed, so the bottom of the pressing is slightly wider than the top. If you fit it with this side out and the groove in the piston is good, it should be impossible for it to come out unless the piston is destroyed - any lateral pressure on the gudgeon pin is pushing the clip further into the piston.

The cylinders themselves are beautiful - measure and look better than the VW originals I bought from Gene Berg a few years ago...  ...OK they measure as good, but they don't have the casting flash of the South American "genuine VW" cylinders.  They also appear grey, rather than black, but as they are cooled by fan forced air, not radiation, this should not affect performance.

I have the total seal seating compound which I intend to use - does anyone know if this contains abrasives?

I will post some pictures of the pistons up here shortly if it turns out anyone is reading this blog.

VW heads wanted.

Hey, If anyone actually reads this and has a pair of old dual port heads, let me know, of course - I'm not afraid to get the die-grinder out and buy some valves and guides!

VW engine

Much to the wife's disgust, I've dragged out the old crankcase and what parts I have.

With wife and son, I don't know how I'll afford the heads, carbs and exhaust, but if  we need to get a second car, I'd like it to be a VW.

Not all of it was in the shed, but with the Aussie dollar at an all-time high, I picked up cheap pistons and cylinders and a crank from cip1 - boy am I in trouble when she finds out!

It's actually amazing how well my old "uni-tech" CB rods fit - I only need to take about 1.5mm (60thou) peak from the bottom of the case, the same from the back of the cam thrust and centre journal web!  So the original 78.4mm I bought them for would have truly been a drop-in (almost.)

Worried that the Crankshaft says "empi" - but what can you expect for $230?  If I get this engine on the road it won't be seeing high revs anyway. According to the Engine Calculator, the old WebCam 111 I have is about the ideal cam for this combo on a street engine anyway - so I'll be setting the redline at 6000.

If I can afford it, I'll still have the rotating assembly dynamically balanced, of course.

I'll be periodically posting notes, pictures and thoughts here - if anyone else reads it, well and good, but mostly I want a record and to sort my thoughts out as I go.

I discovered the guy who machined my case fitted 10mm case savers instead of 8mm, so I have two sets of 8mm studs to get rid of and I'll need to get 10mm set Grrrr...