Effect of nitro % on Tuned Pipe length - RC Groups
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Sep 12, 2009, 12:53 PM
Faster than You (in my dreams)

Effect of nitro % on Tuned Pipe length

I'd like to start a discussion on the effect of nitro % on tuned pipe length.

I have an engine/airframe/fuel/prop/pipe length combination that is set up and has been performing very well. I arrived at the combination entirely experimentally through many small changes made one at a time.

I'm now thinking that I would like to change from the 20% nitro fuel I've been running to a lower percentage nitro; probably 10%. I'm expecting that I will have to settle for a little less peak horsepower which will result in slightly lower tuned RPM (using the same prop) and this, of course, requires a longer pipe length.

There will also be the change in combustion products density associated with the change in nitro % and I don't know which direction this pushes the pipe length. My reasoning is that since the combustion products density decreases with a decrease in nitro % that this will reduce the wave propagation speed and that this will require a shorter pipe length to keep the frequency (RPM) the same.

Any ideas? Any experience? Whats the conventional wisdom?
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Sep 12, 2009, 01:17 PM
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Here is a good interactive site on pipes.

As most pipes have a "band" that they resonate in, the change in nitro will not effect the tuned length if the load is changed to stay in the same RPM band. I recall that the flame speed for nirto is lower than that of alcohol. I do find that I need to go with a larger volume pipe if I go from FAI fuel to nitro fuels. So I assume the revers is true going from high to low nitro. The real issue is can I maintain the torque needed to push the prop to the point that the pipe resonates. You might need to lower the sleeve (timing) and adjust the combustion chamber volume to allow the engine to spin a good size prop without pipe assist to the point that the pipe comes on.

What is your application?

Friends don't let friends fly nickel,
Last edited by Konrad; Sep 12, 2009 at 02:19 PM.
Sep 12, 2009, 03:04 PM
Faster than You (in my dreams)
Thanks for the suggestion, Konrad. That's an interesting site. I'll have to look at that a little more closely.

My application is an old Midwest Super Hots (very thick wing) and an OPS 65 with the stock pipe and header turning an 11x7.

I'm pretty sure the OPS came from the factory designed to run FAI fuel. I made a couple of familiarization runs with 5% and no pipe when it was new. When I went to the pipe and 20% I found it ran much better with the head shimmed a little. Other than that, I haven't touched the timing or the combustion chamber.

When you say "I do find that I need to go with a larger volume pipe if I go from FAI fuel to nitro fuels. So I assume the revers is true going from high to low nitro." am I correct in assuming you get the extra volume by lengthening the pipe as opposed to using a larger diameter pipe or header? How much difference did you find between the two fuels?
Sep 12, 2009, 03:36 PM
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No, length effects frequency. Diameter effect flow and cylinder pressure. If the stinger of the pipe has thick enough walls you can machine it to take inserts. This is often the way we change the characteristics of the pipe without needing to change the whole pipe. So if the stinger is now 8-mm in diameter and runs well on 20% fuel. You can trick the pipe into thinking it is a smaller volume pipe by going to a 7 mm diameter stinger for 10% nitro. FYI this is just a SWAG on my part.
Now this sounds like a sport setup. so I would not be concerned with the peak power. But rather the transition from non-pipe to pipe operation. High nitro fuels makes this a lot easier (it masks all sorts of tuning issues). I would experiment with head shims and stinger outlet diameters to get the best transition.
I assume it is a low piped timed sleeve 160 t0 170 exhaust timing. Which OPS pipe? I like the OPS, Picco, CMB, and Rossi engines as they are set up to respond well to a pipe. For the most part the Asian makes are sport engines. Some can be made to respond well to the pipe if they are constructed of proper materials.

I find lots of differences in the performance between FAI and mild nitro 15% fuels. If I go more than say 15% nitro I have to redesign the engine as I think the changes go to more than just tuning modifications.

I see by your low post count you are new to this site (never mind the 2007 date). I have to warn you I'm not a sport flier or sport engine builder. So my experience is towards the high end of the performance scale and is often at odds with what is "Common Knowledge" for sport applications. What I recommend will often make a sport application work that much better it just that most sport flier don't bother or want to know how to make their systems perform to it's peak potential.

Friends don't let friends fly nickel,
Last edited by Konrad; Sep 13, 2009 at 07:31 AM. Reason: grammar; wrong their
Sep 12, 2009, 03:53 PM
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As I don't know the pipe you have, this might not be a good example. Most sport pipes have a straight section between the diverging and converging cones. The wider this straight section is the broader the response band is. This is at the cost of peak power. Also when looking at pipes you want one that allows for the shortest header. You want the diverging cone to start as soon as possible after the exhaust port. Also you want cones that are parabolic in shape not straight walled cones. None of this is critical.
Now for sport application the reflective wall (washer) works very well in place of the converging cone.

All the best,

Stay way from nickel
Last edited by Konrad; Sep 13, 2009 at 07:33 AM. Reason: add:None of this is critical.

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