Powerflow as an option
"You don't want powerflow, at least at this time in my opinion. Their "claim to Fame" is they figured a way out to make a 4-into-1 collector with tube lengths in the proper range and than a **secondary, the long thing sticking out the cowl on many of their systems. That's a pretty good trick while keeping the general original muffler and cabin heat exchanger. That is also why they charge, I don't know, over $3,500.00 for their exhaust, because they can. We have other options as experimental aircraft."
More options? Absolutely! But PF does have a track record that makes it worth exploring. I talked with them several years ago and came away rather disappointed that their system would be difficult to incorporate into my RV. At the time, they didn't (at least IMHO) understand the market potential of the RV community and were not really ready to assist in the effort, either. After an extensive talk this year at OSH, that may be changing.
The price on their existing experimental systems is $2695 and $2895/ceramic coated. An RV system should not exceed that and perhaps be somewhat less due to market potential.
"The 23.75 HP increase is OVER a really really poor factory exhaust which is a little better than your car exhaust system with a potato stuffed in the tail pipe. It is not that their system is great, it is just the factory systems they are replacing are that bad."
Again, right on. Their system will only give an 8-10 hp increase over the standard X-over found on most RV's. That's only about as much as changing to 10:1 pistons!
"The "standard" unmuffled cross over typically found on RV's is way better than a stock Piper or Cessna exhaust. You would not see an appreciable increase in power going from a RV crossover exhaust to their exhaust, even if that was possible. At this time it is not even possible, because the powerflow with not fit as you guessed."
The problem here isn't the exhaust system but the cowl. Non-standard cowls are pretty common on the flight line these days. Sam James has quite a following, and rightly so. He has eliminated some of the problems with the standard cowl, added some speed, and people are happy. True, the PF won't fit into his cowls either. We would need a new cowl.
While we making a new cowl, why not address the pinhole issue, improve cooling airflow and put in a decent inlet/filter system?
"A RV crossover is about $800, and a 4-into-1 RV exhaust which has some power advantage, go for $1,200 but does not fit "A" models.
The 4-into-1 for the RV is better than a Powerflow for a Cessna, Piper, Grumman or Mooney. The reason is Powerflow has to make compromises to fit their pipes in factory planes and incorporate mufflers. They are clever but ugly with the big primary pipes sticking out the cowl (draggy)."
Mufflers aren't a bad idea, but there are options here. The secondary pipe can be changed easily and doesn't have to be ugly. Personally, I would opt for a muffler, even at the expense of a few hp/mph. Regulations may change our thinking here.
"There are plenty of great exhaust companies for RV's. You have your choice, 4-into-4, crossover and 4 into 1, not to mention custom made to order. The simplest, easiest and most common is the crossover. The 4-into-1 is technically better, but does not fit on RV-"A" models. The average RV'er will be happy with a crossover. If you get a custom 4-into-1 for your RV, it will be better than a Powerflow they make today, not withstanding their claim of entering the experimental market in the future. The price would be a question, since they are use to getting almost $4,000.00 for exahust!
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The 4 into 1 gives some other advantages besides hp. More even scavenging evens out egt's and allows more agressive leaning/lower fuel burn. All 4 into 1's give you that. And don't forget that part of that $4000 comes from STC paperwork and certification, something not applicable to the RV market.
"They're about 4 or 5 major makers of off the shelf and custom exhaust for RV's. Vetterman is the default choice and popular. Aircraft Technologies is my choice. There is also Custom Aircraft Parts and another in Colorado.
Leave the Powerflow for the Cessna guys, we have better and cheaper alternatives. The quality of our exhaust is every bit as good as any factory plane. Until Powerflow builds a custom exhaust for a RV's there current systems will not fit anyway, unless may be you want pipes sticking out the front of the cowl like a Cessna (ugly and draggy).
** Pipe technology - There are usually two pipes in an exhaust system, the primary and secondary. The primary is the tube that goes from the cylinder to where it joins another pipe or pipes. The secondary is the pipe that collects one or more pipes and exits the exhaust out the cowl. There are ideal lengths, and it's not top secreat. Bottom line the secondary should ideally be 20"-30" long. If you look at Powerflow you see the long secondary exhaust pipe on their Cessna systems. You will note some (not all) 4-into-1 systems on RV's are shorter, like 10"-12". That is a compromise for looks and external drag. It is not ideal as a longer pipe but looses little performance. Even powerflow now offers a "SHORT STACK" version. People did not like that long secondary hanging out in the breeze."
Any improvement to the standard system will involve compromises. Cost vs. performance will always enter into the equation. Looks vs drag might be a factor, but that can surely be addressed. What should really happen is an honest look at the FWF system AS A SYSTEM. Talks are underway. Gamut Services is addressing this as their next major project. Powerflow may be a part of the solution. Costs (when done as a part of the building process) should not be a major issue, although a modification to an existing aircraft could be expensive. There is no target date at present, but having an aircraft as Oshkosh 2007 would be nice. So would having a cowl without thousands of pinholes to fill!
Bob Kelly
