Auto conversions & gear boxes

On 20 Feb 2004 14:19:16 -0600, Barry S. wrote:

On Fri, 20 Feb 2004 18:08:35 GMT,
(Corky Scott) wrote:


Auto engines are tiny when compared to direct drive airplane engines.
Take a 180 hp Lycoming. It's cubic inch displacement is 360. They
turn the prop at around 2600 to 2700. The Ford V-6 in airplane trim,
puts out 180 hp also. It displaces 232 inches and makes it's power at
4800 rpm. No prop will work at that rpm. To harness the power, it
needs to be turned slower. Enter the prop speed reduction unit.

Speaking of Fords! How's your project coming?

__________________
Note: To reply, replace the word 'spam' embedded in return address with 'mail'.
N38.6 W121.4

Slowly. I have the engine assembled and is currently mounted in the
airframe. But there's everything else to do. The airframe has yet to
be blasted and painted. I think that can happen this summer. On the
other hand, we are planning some major kitchen redo's and trust me,
ALL of my attention had better be on that.

I've built an engine test stand that will allow me to wheel the engine
outside and run it, with the prop installed. I'd like to get some 30
or so hours on the engine before it gets it's final installation onto
the airframe. I decided this after listening to a crusty old DAR
speak at a local EAA meeting. It sounded to me like he'd be REALLY
unhappy with such an engine unless I could show him that it had been
thoroughly tested.

At this point, I'm being educated about headers. I was going to just
bend up a bunch of tubes, weld them to be what I need, get them jet
coated and call it good. Then I started doing some research.

It turns out that the diameter of header tubing is critical to the
performance of the engine. Larger diameter is not necessarily better.
In fact in almost all aircraft type applications, bigger is virtually
for sure not better. The exhaust header flange has openings that are
1.75" in diameter. This matches the exhaust port opening in the head.
But the tubing diameter should be 1.5", or possibly even 1 3/8" in
diameter. Also, the length of the runners should be at least over 30
inches, and 36 would be better. In addition, each tube should be as
close in length to each other as possible. Finally, the collector
needs to be about 1 78" diameter and it should be 18" long.

Reality is rearing it's ugly head. The lengths I mentioned literally
won't fit without welding the headers into loops. Not going to
happen.

I think the best I can do is get the runners as long as I can make
them and make sure they are of equal length, and get the proper
collector as that also has a huge affect on engine operation.

Why is it so important to have the runners be the same length?
Because different length runners cause different scavenging effects
within the combustion chamber. You will end up with an engine that
does not respond to ignition adjustments nor mixture adjustments as
some combustion chambers will run rich and some lean. "A series of
single cylinder engines flying loosely in formation." Quote from John
Deakin.

Many builders of the Ford V6 have complained that their engine ran
rough at maximum power. Huge effort was made to modify the intake
manifold to correct the problem. But I have not seen a single picture
of an exhaust manifold where the effort was made to create equal
length exhaust headers of the proper diameter.

I talked with a header manufacturer who told me he had heard of Dave
Blanton because a bunch of builders had asked him about headers. He
told me they all wanted to ignor his advice about tubing diameter.
They all wanted to use bigger tubing than was dictated, because they
all thought bigger was better. It's not.

Why is it so important to have the proper diameter tubing? Because
the bigger the diameter the slower the velocity of the gasses inside
it, and visa versa, up to a point. Eventually you can have exhaust
tubing in a diameter too small such that exhaust flow is restricted.
Large diameter tubing tends to cause the engine's power to peak at
extreme rpms. The smaller the diameter of the tubing, the more low to
midrange power you have.

But everyone wanted to use 1.75" tubing because that's what the
exhaust port was. 1.75" tubing would be what you would use if you
wanted flash horsepower from the engine at 8,000 rpm, like at the
dragstrip.

The header manufacturer also had a lot to say about "Zoomie" type
headers. These are headers without collectors, basically straight
pipes. Not only are these tubes also usually too large a diameter,
they leave off the collector which is crucial to the proper design of
the header system.

So with all this information, I'm taking my time with the header
design. Obviously something so important to the proper running of the
engine is not something I'm going to throw together without using
proper design criteria.

Corky Scott