Thorp Air Command - T18.net

Interesting results of recent exhaust swap.

My T-18 was built using a piper pacer exhaust on o-290, crossover with a muffler mounted behind the motor. I came across a dual X over exhaust made for T-18 on B stormers so I purchased it. New pipes have no muffler, 6lbs less weight!

I installed the new pipes and cut holes in the fiberglass to accommodate new exit points and built supports for down pipes.

Ran the engine and noticed the expected louder throaty exhaust note and went flying. To my surprise my cruise speed was lower and top RPM was 150 lower. After measuring the tubes I noticed the pipes were about 4in shorter on the x-over and the bends were much tighter.

I think this is a great lesson about tuned exhaust. The Piper unit is tuned better despite using a muffler.

Going back to old exhaust and patching the nice new holes I made. Lesson learned.

Anyway you can put in an extension to replace the muffler to see what results you get ? I would think it would reduce the backflow and reduce the weight ? Also repairing the muff or replacing it is a major expense.

RB

You are right about the cost of repairing mufflers Rich.

I have a plan to eliminate the muffler from the system but now I am wondering if the back pressure plays a role in the tune. I will let you know how it works out.

I have also looked into aftermarket tuned exhaust for the PA-18,20 they claim a 14 hp increase over stock, but at over $3000. I have a hard time justifying.

I have the Custom Aircraft Parts SS exhaust on my 0-360 and to date it has performed perfectly . It fit perfectly on my engine and came with the cabin heat muff . They even welded on the exhaust bung for the EGT at no charge . They were local to me in San Diego and very nice people to work with . The exhaust came with all the gaskets and flanges to install . The only change I had them make was to shorten the heat muff by about 2 inches . This past annual I took off the exhaust as requested by Custom Aircraft (they recommend it every other annual) to check the slip joints . The cross over was frozen and they broke the joint loose . They (Custom Aircraft) had initially recommended the copper anti seiz , but the Thorp group recommends the Nickel anti seiz . That is what I now use . BOOM !

Cost for the entire SS exhaust was about $1500 + calif tax , plus shipping . Custom Aircraft Parts.com

RB