Jake Raby: Cooling system testing, reichert tuning? Options

[Brett W]

Yeah, a leaf blower can make a bunch of flow but it can't generate 1 psi. Pressure is how these engines are cooled.

[Aaron Cox]

what if you ran with the 7 blade 2.7 fan... not TOO much air then,
or run a smaller crank pulley to spin fan slower
your take?

[Jake Raby]

Been there, done that and also used 4 different drive ratios..

Its still got beat by a little ole high velocity Type I fan and correct air delivery...

Axial fans are designed for 911s- thats where they belong..

I keep getting emails from guys begging me to design and test my own 911 system, but I just don't want to do it- its so much work for hardly any return, and the DTM works so damn well!

[Reiche]

Pressure is how these engines are cooled.

I am trying to get my head around this whole process. Are you saying that the pressure drop from the top of the engine to the bottom cools the cyls./heads? (physics...PV=nRT...brain hurts...) That doesn't make sense to me. I would think you need a lot more pressure than the fan could produce to make that work. Besides, as I think Jake wrote somewhere, lowering the pressure in the 911-style shroud by cutting a hole in it helped with cooling the t4.

Pressure is caused by volume pushing against a restriction. Some restriction of the air movement through the cooling fins (actually a by-product of re-directing the air to the right places) may be beneficial to consistent cooling top-to-bottom (or side-to-side,) hence the under-cylinder tins and matched volumes. But isn't it convection that pulls the heat from the cooling fins to the air? And doesn't that require flow of cooling air that passes over them at a high velocity (think "wind-chill factor")? Too much pressure over the cylinders is just the air flow stagnating from having too much restriction, and won't have any cooling effect since it can't move past the cooling fins.

Am I on the right track, or am I on crack?

[Jake Raby]

You'll never understand it that way! I thought about it the same way till the engine and system taught me differently!

Until you have enough pressure in the upper plenums of the shroud it is impossible to direct the air. Thats why on my Type I DTM I ended up making the inside volume of the shroud 800ccs smaller and only then did I go from 5PSI to 9 PSI of plenum pressure. This pressure overcame to resistance of the vanes and then coupled with airfoil shaped vanes the problems were solved.

Reducing the plenum sizes and changing their shapes will boost the pressure made by the fan and help force the air out of the cylinders because its a radial fan with low volume and hi velocity..

The 911 system is different because it uses an AXIAL FAN which has alot of volume and less pressure. The holes being drilled in the shroud created a lower pressure area over the cylinders nearest the holes and the air did its best to take the path of least resistance and found its way to those parts of the shroud and cylinders. This is also impacted by the design of the under cylinder tins which we found on a TIV to create issues when using upright cooling.

The 911 system not having any vanes or deflectors benefitted from these holes being drilled, because without them all the air blows straight back on #3 and #1 and nothing to the cylinders below and underneath the fan- #2 and #4... The 911 engine does not have this issue because the fan is far enough away from the cylinders that the air does not have to bend 90 degrees to find its way there to do its job...

Without air pressure there is no way the air can be directed. A very perfectly created airfoil may overcome some of it but you still need pressure.

Lets just sum it all up by saying that I'm going to make the cylinder tins work better! Plus they will look cool! Plus they will be LIGHT!

[lapuwali]

Pressure is generated by volume trying to move through a space that's too small to contain it. The fan (any fan) generates volume. The pressure is generated by the restrictions in the system after the fan. At some point, the pressure on the "blown" side of the fan (as opposed to the suction side) will reach a point where the fan stalls aerodynamically, and no more flow (volume) will be generated. The difference between a "low pressure fan" and a "high pressure fan" is how high this point is: how high the pressure on the blown side will be before the fan stops moving air.

The 911 axial fan has a lower pressure point than the Bug centrifugal fan, so it can't generate as much pressure inside the shroud, and quickly stops delivering volume due to the higher restriction present in the Type IV cooling shroud than the larger 911 shroud. The Bug fan keeps delivering air despite the restriction.

It's still volume that's doing the cooling, and the Bug fan can provide it into the smaller space. The 911 fan can't. It works on the 911 because the shroud and the air exits are larger, so the pressure doesn't go up as high, and the fan never stalls.

[Jake Raby]

And thats why 911 fans belong on 911 engines- they were designed for them...

[Reiche]

Thanks for the answers guys and sorry for the thread hijack. I think I am with you now on the pressure issue. What threw me was the statement that "pressure is how these engines are cooled." Your context explains that statement. Pressure is just one of the many factors involved in these systems. If I understand you, the pressure drop doesn't actually provide the cooling. Pressure inside the plenum enables the air flow to be more evenly distributed through the system. On the other hand, it sounds to me like the 911s work more like a free-air system than a forced-air one.

But we still have an issue with 914s. Bug fans can be over- or under-driven (within limits) to match the volume or velocity needs of a particular system. Since the 914 fan is direct-mounted we are stuck with it and its output. Design-wise I would guess it is more like the bug fan: high pressure and high velocity. So any improvements to the cooling system itself will result from changes to how the airflow is managed. I guess from your statement that the pressure under the stock tins may be either/both too high or low, or unevenly distributed. You're working on the top tins. Could anything be done on the bottom ones? They seem really restrictive.

[Jake Raby]

The stock 914 system needs restriction under the cylinders due to the air being laterally fed over the cylinders.

In upright conversions we toss the stock TIV tins and replace them with stock Type I tins that are way less restrictive.. In the testing this freed up 3HP+ and gave more even running temps.

All in all:

Air volume and velocity are not just the key...

Pressure is not just the key..

The key is the perfect mix of both, and finding that mix is not cheap or easy. It will drive a person insane.

[Britain Smith]

I got Type-1 tins like you suggested and they are just two flat plates that fit between the cylinders. Is this correct? Should there be any angled pieces that wrap under the cylinders just a bit kinda like the ones used between the cylinders in a 911 engine?

-Britain

[Jake Raby]

Yep, they are just flat... Thats the key, you don't need the extra restriction with the TIV. The engine is much smaller and builds its own resistance.

[Reiche]

All in all:

Air volume and velocity are not just the key...

Pressure is not just the key..

The key is the perfect mix of both, and finding that mix is not cheap or easy. It will drive a person insane.

Yeah it's never just one thing. It's about finding the compromises made by the original engineers and trying to figure out why they were made. Worse, they often have to do with factors outside the cooling system itself like the engine power, fuel grades, rpm range, expected operating conditions, cost factors, etc. etc. Coming up with a solution that can accommodate every reasonably foreseeable condition inevitably results in something that works for most but is good for none.
So no wonder it drives you crazy: it's one person against a whole team of engineers and designers, not to mention executives and accountants.

We got your back though.

[Jake Raby]

They weren't making 50HP per cylinder and still trying to cool it either!

[Mueller]

that is why water cooling is better !!!

flame suit on, hahahahaha

[Britain Smith]

Yep, they are just flat... Thats the key, you don't need the extra restriction with the TIV. The engine is much smaller and builds its own resistance.

Does not having any deflector to the bottom of the cylinders cause a hot spot on the underside of the barrels? Are you telling me to just put the flat plates in between the cylinders and forget about it...cause if you are I will.

-Britain

[Mark Henry]

Does not having any deflector to the bottom of the cylinders cause a hot spot on the underside of the barrels? Are you telling me to just put the flat plates in between the cylinders and forget about it...cause if you are I will.

Yep that's about it...use Type 1 deflectors if you have a DTM...If it's stock use the stock deflectors.

[Britain Smith]

Does not having any deflector to the bottom of the cylinders cause a hot spot on the underside of the barrels? Are you telling me to just put the flat plates in between the cylinders and forget about it...cause if you are I will.

Yep that's about it...use Type 1 deflectors if you have a DTM...If it's stock use the stock deflectors.

Sorry, Jake knows that I have a 911-style cooling shroud.

-Britain

[Reiche]

that is why water cooling is better !!!

Not for the soul....

[Mark Henry]

With the 911 type I don't know which deflector Jake likes, but either way you have to have them.

One of the most common problems I see with backyard mechanic's engines is they don't install defectors.

Mmmmmmm....fried pistons.....Doh!

[Britain Smith]

As Jake knows, I also have the advantage to be running aluminum pistons and cylinders.

-Britain