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Power Flow

Posted: Thu Aug 27, 2015 3:08 am
by Enrouk
I guys,

im consideirng to put a poer flow systeme on my da-40, they have a great specail at this time if i put the order before the 31 august,

do you have good or bad coment about this STC.....

Re: Power Flow

Posted: Thu Aug 27, 2015 9:53 am
by DavidS
Mine came OEM with that exhaust, but I have flown non PF exhausted DA40s as well. I believe that there's a notable difference.

So, that said, I think it's worth it, but make sure you don't need a new different prop to make it happen.

Re: Power Flow

Posted: Thu Aug 27, 2015 11:55 am
by h20wrks
I have a DA40 fixed pitch, I added both the Power Flow Exhaust and fine wire plugs. It was an amazing improvement. It climbs so much better and at altitude I can fly 141 knots on 9 GPH. My initial installation was incorrect and the exhaust damaged my cowling. I took the plane to Darren at Power Flow and he corrected the installation and he was a pleasure to deal with. I kept the stock aluminum prop and at altitude you cannot use full power without over revving the engine. Overall I have found better climb rates, higher cruise speeds at lower fuel flow and a smoother running engine.

Jim Villa

Re: Power Flow

Posted: Thu Aug 27, 2015 5:33 pm
by rwtucker
At one time, I saw the power curves on these tuned exhaust systems. As I recall, the HP boost occurs over a narrow RPM range. The parts of the curve that you don't generally see published show net HP across the typical useful range of RPMs (i.e., 2200-2700 RPM). That picture is not as positive.

A tuned exhaust's contribution to engine efficiency comes from reduced back-pressure (atmospheric pressure) at the exhaust port and the exhaust line. The pressure reduction is achieved largely by adjusting the length of each cylinder's exhaust pipe so that one cylinder's exhaust is just passing the entry point for the next firing cylinder's exhaust, thereby reducing the local back-pressure for that cylinder. The length of each exhaust pipe is adjusted so that all four cylinders are, in turn, lowering the back-pressure of the next cylinder to fire. Obviously, this timing can occur perfectly at only one RPM, which is 2,450 for the Power Flow (I think).

This is the benefit of a tuned exhaust. The drawback is that engine efficiency declines on either side of the design frequency. As you move away from the design frequency, you eventually reach a point where each cylinder's exhaust increases the back-pressure on the next cylinder resulting in efficiency that is less than with a straight port untuned exhaust.

As one physics major explained it:
Each time a power stroke occurs and an exhaust valve opens, a positive pressure occurs in the exhaust manifold. A negative pressure occurs in the exhaust manifolds between the positive pressure pulses, especially at lower engine speeds. Some exhaust headers are tuned so the exhaust pulses enter the exhaust manifold between the exhaust pulses from other cylinders, preventing interference between the exhaust pulses. If the exhaust pressure pulses interfere with each other, the exhaust flow is slowed, causing a decrease in volumetric efficiency (and thus decrease in horsepower). Proper exhaust manifold/header tuning actually creates a vacuum, which helps to draw exhaust out of the cylinders and improve volumetric efficiency, resulting in an increase in horsepower. Source: http://www.enginebasics.com/Engine%20Ba ... 0Flow.html
It also occurs to me that anything that affects the velocity of the exhaust will alter the RPM/HP boost curve. I'm guessing but a long duration electronic ignition might alter exhaust velocity when compared with standard magnetos. Density altitude will have an effect. Was the Power Flow system tuned for magnetos or for electronic ignition? What DA? I don't know.

Your ROI decision may come down your flying preferences. If you spend most of your time flying point A to point B at 75-80%, the tuned exhaust will benefit you in terms of engine efficiency. If you are a low RPM sightseer or you spend a lot of your time canyon flying with a notch of flaps at 90-110 kts., you might be better off with a straight port exhaust. You will certainly have more money in the bank

Re: Power Flow

Posted: Thu Aug 27, 2015 7:48 pm
by DavidS
The powerflow guys, when asked at trade shows across multiple years, deny the peak efficiency at 2450RPM point.

Re: Power Flow

Posted: Thu Aug 27, 2015 8:23 pm
by rwtucker
DavidS wrote:The powerflow guys, when asked at trade shows across multiple years, deny the peak efficiency at 2450RPM point.
I heard that as well. AFAIK, there is only one point at which the velocity of the exhaust gasses align with the spacing between the exhaust cycles to arrive at a time to create reduced or negative pressure for the next piston. In other words, PE (and/or peak HP) has to occur at some RPM and there has to be an off-peak curve. There may be other curves of interest, such as peak HP (which might be different than PE across MP). For practical reasons (real world needs for length of pipe, etc.), the sources I consulted say that these systems tend to optimize at lower RPMs, which is ideal for we piston aviation folks. There may be techniques to flatten the efficiency or HP curve a bit but you can't avoid the fundamental pulse pressure cycle time, velocity (accounting for acceleration), and pipe length. PowerFlow could put this discussion to rest by sharing independent dynamometer tests by RPM with MP, ignition, and atmospheric conditions specified. Better still, show the family of curves. I signed up for their e-book. Maybe the curves are in there.

Re: Power Flow

Posted: Thu Aug 27, 2015 8:58 pm
by Rich
Where the benefit shows up most starkly is Climb performance. There is simply no question. Installing Powerflow added 350 FPM to best ROC at reasonably heavy weights. At modest weights and low DA I'll hit 2000 FPM. Best-case 75% cruise (for my 2002) went from 137 to 142 knots.

As far as cost goes, you do need to check the propeller restrictions. Some DA40's were delivered with a particular prop that Hartzell would not certify for use with the PF. All MT's are OK, and you can get Hartzell's that are OK.

I' assume there is a curve for how efficient this system is at different RPM's. But any drop-off doesn't seem to be significant in the rather limited range in which we operate.

Re: Power Flow

Posted: Thu Aug 27, 2015 9:53 pm
by Colin
I've never thought about this before.

Is there a reason there isn't a SuperCharger that works on the exhaust? It seems like it would be a tiny amount of engine power to turn a fan that created a negative-pressure in the exhaust system. With a simple gate so that it didn't run as too powerful a vacuum. If it is worth tuning the exhaust, wouldn't it be worth turning it into a non-issue?

Re: Power Flow

Posted: Thu Aug 27, 2015 10:15 pm
by rwtucker
Colin wrote:I've never thought about this before. Is there a reason there isn't a SuperCharger that works on the exhaust? It seems like it would be a tiny amount of engine power to turn a fan that created a negative-pressure in the exhaust system. With a simple gate so that it didn't run as too powerful a vacuum. If it is worth tuning the exhaust, wouldn't it be worth turning it into a non-issue?
Really interesting idea Colin!

It prompted me to search for articles related to it. I found this article that covers the main issue we are discussing. http://www.epi-eng.com/piston_engine_te ... nology.htm but I couldn't find anything directly on your point. I wonder if a supercharger could have two isolated vanes; one for intake, a smaller one for exhaust.

Re: Power Flow

Posted: Fri Aug 28, 2015 12:50 am
by Enrouk
Wow, interesting reading here, thanks for this great information, so im looking everything in a positive way to proceed to my order, and i have the aluminuim hartzell F7497, so im good to go, and i will let a autorized powerflow shop to put it on my aircraft,

and at the moment they have great discount until the 31 august.... if someone else need one.

Very great comunity here, :P

Merci !!!