N447MR;n1124325 said:
So I made it through the whole thread finally, but I don't think I saw any thrust numbers really. I recently did a static thrust on my stock 130 HP Subaru 2.2 on a hot day to adjust prop and get rpms up a bit.
I have a 68" Warp Drive and saw ~550#s. I'd have to believe that the YG4 would do better, right? I was thinking performance gross payload gain would come from increased horsepower, but I guess it would come from decreased weight instead?
I just realized after going back thru this thread that Teal did post thrust numbers significantly lower than we are getting, but my post above said everyone should be getting the same thing. I don't understand the difference, maybe intake and exhaust? Hard to say.
I think he was trying to figure out if different exhaust headers made an impact on top WOT thrust. He didn't make mention of any thrust numbers in the midrange, which is where the advantage of longer headers is.
1-25-2016
teal;n901288 said:
Header testing
I spent the last couple days doing some header testing. I tested 3 headers. The first was a tri y or what's commonly called a 4-2-1. The next was a short a 4-1 I use on the trikes and the last was a basically the same as the 2nd except with 7 inches added to the primarys. I was using clamp on K&N filters with 170 main jets at 1000 ft msl. (Do not try to run stock jets without the airbox). The prop was a prince p-tip 74 inch dia. 37 inch pitch. The first set of pulls was done with a 3.47:1 ratio gear box and I pulled 550 to 570 lbs with all 3 headers. The long 4-2-1 header seemed a little smoother as it came up to full power so I image the torque was probably Better in the low end. It may have pulled a bit more at wot. Prop rpm went to a little over 2500 and engine rpm went to around 10,300. The next set of pulls was with a 3:1 ratio box, it pulled 510-530 lbs. The prop went to about 2400 and engine was around 8500 rpm. Same results with the 4-2-1 sounding the smoothest it came up on the power but the top end was almost the same.
Before I get to discussing the results from Jenkins test let’s start out by first comparing apples-to-apples.
Jenkins: Prince P-tip (3 blade?) 74" 3.47:1 10,300 ERPM/
2500 PRPM 560 lbf +/- 10
Mills: Warp Drive 3-blade 66" 3.0:1 10,300 ERPM/
2885 PRPM 600 lbf +/- 10
Jenkins: Prince P-tip (3 blade?) 74" 3.0:1 8500 ERPM/
2400 PRPM 520 lbf +/- 10
Mills: Warp Drive 3-blade 68" 2.58:1 8700 ERPM/
2835 PRPM 530 lbf +/- 10
Mills: Warp Drive 3-blade 68" 2.72:1 9200 ERPM/
2840PRPM 560 lbf +/- 10
My setup with smaller, cheaper prop, lower gear ratios, higher humidity, higher prop speeds produced significantly higher thrust/ power. It wasn’t a fluke.
Some of the Georgia results were repeated to within 1 lbf at different times, different airports, done by different people using different measuring instruments at comparable air density.
The Georgia tests were done at same altitude as Arizona test, 1100 MSL, and though probably similar temps (Jenkins’ test was done in January ‘16) the GA humidity is much higher than AZ.
A Prince P Tip Prop costs at least three times as much as a Warp Drive, with the selling point to justify cost being that it produces more thrust. Or so I would imagine.
Next, a bigger prop turning a slower speed is far more efficient than a small one turning a faster speed.
Let talk a little bit about the exhaust, and start out by clarifying terminology so we don't get mixed up down the road:
- "Header(s)" is the section of the exhaust system that includes everything from the head (hence the word "headers") to the first collector.
- "Collector" is the junction of two or more pipes, reducing the number of pipes downstream.
- "Exhaust pipes" is the second section of pipes after the first collector, even if it includes another collector, X crossover, etc.
- "Muffler" is the last thing added to quiet things down so we don't all wind up with tinnitus.
- "Exhaust system", more often called just "the exhaust", is the whole she-bang.
- "WOT" = wide open throttle, and for this discussion only I use it to indicate the engine's peak power RPM, not prop-pitch-limited RPM as it is used to mean in virtually every other discussion regarding aircraft power plants. This is important later on in the discussion so keep this difference in mind.
Three things affect power output when it comes to the exhaust: back-pressure, turbulence, and length of pipe.
First, a little back-pressure is a good thing for fuel consumption, valve and valve seat life, and even maximizing HP. Too much back-pressure results in poor air flow and loss of power.
Second, turbulence reduces air flow. Think of a ball of cotton in a plastic tube. You can blow slowly through it just fine, but when you suddenly try to blow hard it seems like you just plugged it up completely. That's what happens with turbulent air flow.
Third, surface friction and thus turbulence of air flow through the pipe is obviously increased by length of pipe.
Length of
header affects midrange power greatly. A very short
header can reduce back-pressure, resulting in some top-end WOT gains. In an engine that is meant to run WOT only like a dragster this is the best way to go. In an engine that is run most of the time at midrange power below WOT this is not such a good thing, because the 10-20% power loss due to improper free length of
headers results in excessive fuel consumption due to a need to turn up the wick unnecessarily, and excessive engine wear due to running at higher RPM than should be necessary. It also results in a frustrating "peaky" throttle, where you smoothly continue to add throttle and there is no response, until you hit WOT and the engine jumps to full power suddenly.
Resistance to airflow and expanding gases is a function of pipe ID and length, resulting in back-pressure. In some cases of a long
header it helps to gradually increase pipe ID as you progress downstream to the
collector, to decrease back-pressure. In some well-tuned
headers these stepped systems can wind up at very near to top-end power seen with the shorty "dragster" style pipes.
Length of
exhaust pipe AFTER the first
collector affects overall power as backpressure is increased due to longer pipe length. This can affect power across the power band, not just midrange or WOT.
So why is a long header better for midrange power than a shorty? That goes to turbulent back-pressure created by adjoining a second flow to the first. The piston needs to be able to smoothly push out all of its expanding gases from its cylinder through one full stroke w/o interference. The length of pipe required before any such turbulent junction, or
collector, should come as close as possible to a volume equal to that of one piston stroke of the one engine cylinder. You can cheat a lit at the end of the stroke, but the more you cheat the more loss of power you will see.
a 1000cc 4-cyl is 250 cc per stroke, so you can calculate ideal length of header from
250cc =
pi x R[SUP]2[/SUP] X L.
Our pipe is D = 3.28 cm, R = 1.64. We find that the ideal
header length is then 250/2.67 = 93.63 cm = 36.8", less the length of space in the head after the valve.
Like I said, you can cheat. The tuned headers I have built are usually about 21” long, and the length after to the muffler is 7-8”.
It would be a great idea to check midrange thrust numbers as a way to tell what is going on with the power lost due to length there. The trouble is, no one ever does that. Funny, since 90% of everyday flying is done at 75% power setting…
According to my numbers, 1 HP converts to ~4.25 lbf thrust. It is easy enough to interpolate HP given RPM and thrust number, and that tells the tale of lost HP due to insignificant length of headers.