REFLECTOR: N4PE Testing Status

John Dibble aminetech at bluefrog.com
Wed Apr 10 08:20:35 CDT 2013


On 4/9/2013 10:44 PM, Brian Michalk wrote:
> As I mentioned in my earlier email, my last flight was cut short due 
> to high oil temps.
>
> I've had five flights so far, and after each flight have been 
> addressing the squawks.  The biggest danger to flight so far was a 
> manifold pressure tube that got hot in flight, and appears to have 
> collapsed after landing with low manifold pressure.  I replaced it 
> with a larger tube good for 250F in the engine bay.  That should keep 
> the EFI computer happy.
>
> On my flights I have been having high CHT's, and of course the high 
> oil temps that follow.

In my experience, it's the other way around.  High oil temps cause high 
cylinder temps.  If you have the standard oil cooler, it's too small for 
the Franklin.  You need a larger or second cooler.

>   I have made several changes, but next on my list is to richen up the 
> mixture a bit more from what I have now, which is just rich of best 
> power.  I'm using electronic fuel injection.  I am also going to try 
> the following:
>
>   * Seal up all of the gaps around the plenum.
>     When I tuned the engine, I used leaf blowers connected to the
>     plenum.  This was more than adequate for engine cooling, and also
>     kept my oil temps down, so I do believe my CHT temps are the root
>     of the problem.  With the blowers, there were obvious leaks around
>     the plenum.
>   * Clean up the sharp corners from the armpit scoops to the engine
>     I'm not giving up on the armpit scoops yet, and really don't want
>     to.  I have my reasons.
>   * Reduce the timing.
>     I've got everything at 28BTDC until I get to 2600RPM, which
>     advances to 32BTDC.  That should give me a little bit more headroom.
>

This is something I'm experimenting with.  I've wondered why the 
Franklin timing is 28 BTDC while Lycoming is 25.  They're both running 
at the same rpm, using the same fuel.  If anything the Franklin timing 
advance should be less since the compression is higher and fuel burns 
faster at higher pressures.  I asked a Franklin mechanic that question 
and his response was that the timing number is what the engine was 
certified at and nothing to do with what it should be.  I think this 
situation is due to the magneto which operates at constant timing.  At 
low altitude less advance is needed while at high altitude, more.  So 
the timing is a compromise.  At low altitude, we run ROP (in an attempt) 
to avoid detonation that can result from the timing being too advanced.  
With an EI, it should be possible to have the timing exactly where it 
should be at all altitudes.  It's a mistake to set the Ei timing to 
match the mag timing when you have the opportunity to set it at more 
appropriate settings.  Detonation, especially, mild detonation which is 
difficult to detect, is common to Franklin engines and I think that's 
due to the advance being so much.  I've reduced my mag to 26 deg and EI 
to 25 (arbitrary numbers) and saw a 2 kn INCREASE in airspeed at low 
altitude.  (detonation reduces power)  There's a whole lot more to this 
that I'm working on, but I'll leave it at that for now.

John
>
>   * Richen the mixture
>   * Cylinder baffles
>     I can try this, but I'm not sure how it would really work. The
>     Franklin cylinder castings are such that there is really not much
>     room for air to flow between cylinders.  On a Lycoming, the
>     cooling fins go all around, but on a Franklin, the castings are
>     much different.  It's difficult to explain, and really requires a
>     picture.  Has anyone added the under-the-cylinder baffles to a
>     Franklin?
>
>
>
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