Glow Engines Discuss RC glow engines

nitro & pipe length tuning

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Old 12-08-2008, 06:53 PM
  #1
brjensen71
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Default nitro & pipe length tuning

I am tuning in the header length for a tuned pipe on an engine that may ran on both 30% & 10% nitro. So far I get a 300 RPM gain from the 30% nitro over the 10%.

Should I tune with 30% or 10%?

Are there any draw backs to just using the 30% for setting up the pipe tune?
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Old 12-09-2008, 12:42 AM
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Default RE: nitro & pipe length tuning

Mr Jensen,


The same engine/pipe setup cannot be used both for 10% and for 30% nitro!

30% nitro fuel will need a significantly lower compression ratio than 10%... Shimming the head up with 0.2-0.3 mm shims is the minimum effort proposition...


The exhaust gas temperature is different as well, making the tuned length for the pipe different, even for the same RPM...

-

Choose your setup first and then tune and optimize the engine, prop, glow-plug and the exhaust length for that choice.

And you must understand that if you try using the other fuel, the setup will not work right!


'Unisex', or in other words, 'one size fits all', does not work in the real world of model engines.


And take another note; 30% nitro fuel is not only much more expensive per gallon. It is also used at a much higher rate than 10% nitro fuel, so your fuel costs will skyrocket if you choose the higher nitro.

It is unnecessary for anything but competition.


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Old 12-09-2008, 01:01 AM
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Default RE: nitro & pipe length tuning

Hi Dar,

I was into racing warbirds at one time. I settled on the P-39 as my airplane. I used a YS 45 RE for engine. Talked to the YS guys about how to break in and run it. The fuel I settled on was 15% nitro 18% castor and the rest methanol (tnx Dar). Broke the engine in w/broad band pipe and 10x6 Zinger Racing prop. Flew it racing, w/10x7 racing splinter Zinger. It was very fast.

That little setup got on the pipe as soon as you got past half throttle. When it got on the pipe, you had to go close to WOT to keep it from leaning out.

Funny, Eh?

CR
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Old 12-09-2008, 01:18 AM
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Default RE: nitro & pipe length tuning

Quote:
ORIGINAL: brjensen71

I am tuning in the header length for a tuned pipe on an engine that may ran on both 30% & 10% nitro. So far I get a 300 RPM gain from the 30% nitro over the 10%.

Should I tune with 30% or 10%?
This is an interesting question. I have to infer that you're in a racing league that allows more nitro in one class than the other.

My approach would be to to tune the pipe for the lower nitro class and concentrate on being running at the finish. Eff all that power, being running at the drop of the checkered flag is lots of the deal.
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Old 12-09-2008, 01:21 AM
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Default RE: nitro & pipe length tuning

It is common for engines to be lean at mid throttle with a pipe if the carb was designed to work well with mufflers. Try richening your low needle until idle is compromised and see how it does at half throttle as the pipe starts working. A three needle carb or an active mixture carb is very handy for adapting various exhaust systems.
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Old 12-09-2008, 01:37 AM
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Default RE: nitro & pipe length tuning


Quote:
ORIGINAL: Charley

...That little setup got on the pipe as soon as you got past half throttle. When it got on the pipe, you had to go close to WOT to keep it from leaning out.
Charley,


This is quite understandable.

When the engine get 'on the pipe', its RPM and inflow of air increase significantly, even at the same throttle setting.


I envision the YS carburettor, with its variable orifice size mid-range fuel control...

Increasing the intake air-flow through any engine's carburettor venturi, will not increase the fuel flow by the same amount.


With YS, and its 'non air-flow dependent' duel delivery, the engine will get roughly the same amount of fuel (no increase in fuel flow), for a significantly higher inflow of air and thus fuel requirement...


This is no surprise!


Read what Ken wrote too.
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Old 12-09-2008, 01:39 AM
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Default RE: nitro & pipe length tuning


Quote:
ORIGINAL: Kweasel

It is common for engines to be lean at mid throttle with a pipe if the carb was designed to work well with mufflers. Try richening your low needle until idle is compromised and see how it does at half throttle as the pipe starts working. A three needle carb or an active mixture carb is very handy for adapting various exhaust systems.
Point taken. But we had what we had. That particular YS .45 turned over 18Kwith a short pipe on a friend's racer. He had to disable the pressure system and use straight muffler pressure to get it to run that fast, the pump system couldn't keep up. He won the Warbird races that year at Mesa, AZ.

His model was a Sureflite foamy P-39 with a thin wing from the Ace kit. It was very fast. Grrrr.....

Ever flown any of those Sureflites?

CR
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Old 12-09-2008, 01:53 AM
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Default RE: nitro & pipe length tuning


[quote]ORIGINAL: DarZeelon

[color=#000099]
Quote:
ORIGINAL: Charley

...That little setup got on the pipe as soon as you got past half throttle. When it got on the pipe, you had to go close to WOT to keep it fr

This is quite understandable.

When the engine get 'on the pipe', its RPM and inflow of air increase significantly, even at the same throttle setting.


I envision the YS carburettor, with its variable orifice size mid-range fuel control...

Increasing the intake air-flow through any engine's carburettor venturi, will not increase the fuel flow by the same amount.


With YS, and its 'non air-flow dependent' duel delivery, the engine will get roughly the same amount of fuel (no increase in fuel flow), for a significantly higher inflow of air and thus fuel requirement...


This is no surprise!

You guys are exactly right as to fuel flow characteristics on the YS metering system. I tuned the system for 16K with a broad band pipe and got my speed from pitch. We used a flying start, y'see, so you acclerated by diving for the starting line. The trick with high pitch was to not stall it. Different strokes......

CR
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Old 12-09-2008, 12:49 PM
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Default RE: nitro & pipe length tuning

You're trying a similar setup to what I am on my Rossi 45. The answer is that you should try to set the compression ratio up so that the motor doesn't detonate on the 30% but set the pipe up on the 10% so that you can get it to stage when you're running 10%. It won't nescessarily be optimized for either setup but a best compromise for each.

In my testing I found that the exhust temperature actually decreased with higher nitro using the same prop and pipe. This means that the pipe's tuned frequency decreases with more nitro. You can probably pull a larger prop with the 30% than the 10% but at a similar rpm.
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Old 12-09-2008, 12:57 PM
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Default RE: nitro & pipe length tuning

A factor possibly more inportant than the temperature decrease is that the exhaust gas should be denser with more nitro.
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Old 12-10-2008, 12:04 AM
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Default RE: nitro & pipe length tuning


Quote:
ORIGINAL: ChrisAttebery

You're trying a similar setup to what I am on my Rossi 45. The answer is that you should try to set the compression ratio up so that the motor doesn't detonate on the 30% but set the pipe up on the 10% so that you can get it to stage when you're running 10%. It won't nescessarily be optimized for either setup but a best compromise for each.

In my testing I found that the exhust temperature actually decreased with higher nitro using the same prop and pipe. This means that the pipe's tuned frequency decreases with more nitro. You can probably pull a larger prop with the 30% than the 10% but at a similar rpm.
This will work OK but the engine will over-run the pipes tuned length on 30%. This is why the temp goes down and power increases are minimal. The easiest way to deal with various volumes of nitro is by changing the pipes length. I once had a piped .61 optimised for 25% that made very good power. This setup would not work on 5% until the pipe was lengthened 1", after that it ran well but made less power.
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Old 12-10-2008, 01:01 AM
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Default RE: nitro & pipe length tuning


Quote:
ORIGINAL: Kweasel

This will work OK but the engine will over-run the pipes tuned length on 30%. This is why the temp goes down and power increases are minimal. The easiest way to deal with various volumes of nitro is by changing the pipes length. I once had a piped .61 optimised for 25% that made very good power. This setup would not work on 5% until the pipe was lengthened 1", after that it ran well but made less power.
Or in short, Ken, one pipe-length will serve one setup combination...


If you optimize a setup for 10% nitro, with a given prop and got a certain RPM value with that setup... and then you change the nitro to 30%... You will need to change another value; probably the size of the prop and the setup can again be optimized, but since a tuning criterion has changed, on account of the higher nitro (different exhaust temperature/density/SoS...), the RPM of this setup will be different...


And, if the engine had a compression ratio to run on 10% nitro, it will need to be 'decompressed' to run on 30%...

...And you were thinking your problems are over...
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Old 12-10-2008, 08:21 AM
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Default RE: nitro & pipe length tuning

Quote:
This is why the temp goes down
The temp goes down because you are burning a richer denser fuel at a lower temperature.
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