Looking for info on "Drilling holes in tin for cooling"

[nogoodwithusernames]

So I have heard over on the Samba several times mention of Jake discovering at some point that drilling holes near #3 cylinder in the stock T4 cooling tins can aid cooling air flow at higher freeway RPMs. I have yet to see anyone mention details such as where, how many, and size of holes. I have not found it in the pinned topics either so far. Anyone know if that is documented somewhere?

[Lo Cash John]

I've never heard of that mod but I suppose it's possible. The key question is WHERE are the temp readings taken? A reduction in one portion of the head could result in an increase in another section of the head. It's possible that at certain RPMs and with the resulting airflow at those RPMs, there are minor optimizations that can be made. But, these same mods may increase temps at lower RPMs and make it not worth the gain. No such thing as a free lunch and all that.

Some folks have made very dubious (even laughable) claims over the years on these forums so apply logic and reasoning before you follow along.

[dstar5000]

JAKE'S NOT HERE, MAN!

DON

[raygreenwood]

Long ago on this site when Jake Raby was still here...and was still developing the DTM....there were discussions about the air directional vanes inside of the DTM.

Parts of those discussions talked about the fact that the 3/4 side runs hotter than the 1/2 side on stock cooling system type 4 engines.

I commented that I have seen this as well back in the late 90's. I did not know exactly why at the time but did a lot of experimenting with a shroud I was willing to scrap. I had access where I was working to both a vane type anemometer and a skinny hot wire anemometer.

I drilled quite a few holes here and there ...first in the underside tin....to see where the temperatures of the air that went through the head and cylinders was hotter or colder. Yes, #3 was hottest and #4 next hottest, #1 was next hottest and #2 was coolest and the difference between #1 and #2 was not very much. Between 10* and 20* maximum. But #3 could be 50-75* hotter on a hot day.

At some point drilling holes around the top shroud area to measure...something changed. #3 got better.
Since I was also tuning fuel mixture, ignition and tweaking a few things on the fit of the shroud to the fan...I passed it off as changes to those items.

Years later in these forums Jake was speaking of the fact that on the #3 and #4 side, the head space between the top side sheet metal narrows down rapidly toward the rear #3. And, on the backside of #3 (actually the forward side of the car)...the gap between sheet metal is so narrow that....between the head space rapidly getting tighter and that back lower sheet metal wall getting tighter....the high static pressure of the cooling air flow gets crammed into the gap and creates turbulence trying to negotiate both the 90* downward turn on the backside/forward side of #3 as well as the space between the cylinders.

So....you get a stack up of air...static pressure....causing turbulence right in the area around #3. I saw that with my anemometer ....but did not know the significance of it.

Jack commented at one point that drilling a few vent holes near #3 in the upper or forward sheet metal alleviated some of the static pressure which cut down the turbulence and increased cooling on #3 and #4.

He took this knowledge and incorporated it into his DTM. A handful of people back then were questioning where he drilled the holes in the stock sheet metal to alleviate this excessive static pressure. He never publicly answered this because that was hard won knowledge.

I believe it was somewhere around 2008 and should be in his locked thread.

Just from my memory.....the holes that I drilled that caused the most change in temp and turbulence ...were just forward of the #3 spark plug right before the curve of the sheet metal starts to go downward.....and slightly inboard of the spark plug hole in the sheet metal.

Ray

[BugUgly]

Turbulence gets a bad rap. It's good for forced convection. Fully developed turbulent flow should also have better flow rate than slowing things down to laminar flow on this scale. But Ray mentioned the narrowing head space and the hard 90° that ...that's a flow-killer. Please pardon me if mis-stated this...it was El Jimador.

[raygreenwood]

Turbulence gets a bad rap. It's good for forced convection. Fully developed turbulent flow should also have better flow rate than slowing things down to laminar flow on this scale. But Ray mentioned the narrowing head space and the hard 90° that ...that's a flow-killer. Please pardon me if mis-stated this...it was El Jimador.

Yes....turbulence "CAN" get a bad rap. It depends on what kind of turbulence it is, where it is and in what "fluid" it is.

Turbulence is "energy".....but its not generally good for clean directional flow. Its most easily seen in liquid. If you have a high flow volume coming from a larger volume area and trying to enter a smaller volume hole or pipe, there will be a stack up of pressure (static or head pressure).

This is a good thing if pressure is what you need. But change this up....try to make a turn into the smaller pipe at this same point. You have fluid trying to change direction into the face of oncoming fluid. That causes turbulence. Certain areas of the liquid will be higher and lower density. Static pressure increases and you get back loading of the "pump" and a reduction of flow through the smaller pipe.

This would be much easier if the spaces under the shroud WERE smooth and uniform pipe. However its a jungle of fins, multiple direction changes and multiple volumes. Ray