REFLECTOR: Exhaust change - the data is in!

[Geoff Gerhardt]

All,

I was able to get out this morning to test the new 4-4 exhaust from Custom
Aircraft Parts.  As I mentioned yesterday, I saw no increase in my static
RPM with the new exhaust, indicating that you should expect no HP increase
(over the stock 2x2-1 Velocity exhaust) with this exhaust format.  It did
sound a little different, maybe a bit quieter and smoother (I'm talking
about when I'm taxiing around).

Now, for the real test - do I go any faster?  I first took it up around the
pattern and climbed to 5K' for a few speed runs above the airport.  Just in
case sometime came loose, at least I'd be able to glide down.  It did seem
faster, but that was a low altitude, and I was not going for very long runs
before turning around.  Put it back on the ground, ensured everything was
still strapped in, then took it up for a real test.  I climbed to 10,000',
got on with approach so they made sure I didn't hit anything, and did some
speed runs.  Bottom line, an honest 5kts increase.  Normally, at 10k',
~60deg LOP, which is ~7.6gal/hr I cruise at 150kts.  Last weekend, when I
took it out, tho, it was a little faster, more like 152ktias.  Today, at
10k', 7.6gal/hr it was 158kias.  At 8g/hr, 160kias.  As I richen it up and
approach peak, my CHT's skyrocket.  At 8gal/hr my hottest cylinder is
~390F, but at 9gal/hr it shoots up to 415F, and the others aren't far
behind.  8gal/hr at 10k' seems to be the best balance of speed, fuel
economy and engine temperature.  I can live with 160kias for awhile.

I didn't see any improvement in engine cooling - I thought that would help
a lot, but it doesn't seem to do much.  My engine temps are not bad.  In
cruise I'm usually under ~385F.  Oil temps <210F.

I suspect that a Std RG would see a bigger improvement than my 173RG.  Most
of my excess drag is in the wing.  A sleeker Std would probably see a
greater overall percentage drag reduction with this exhaust change than I
saw.

On Friday, I'll be leaving for my longest XC, from Worcester MA to
Minneapolis.  I'll be there a couple days, then heading up North to Canada
to visit my folks.  I'll certainly have an opportunity to get a stable
cruise numbers then.

Let me know if you have any other questions about this.  I'll post some
pictures later.

Geoff
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