Pipe Tuning 101

[Mike Larson]

Hey guys! Need some input on pipe length/tuning etc. Have a spt 40 with an older picco in it. Switched through different pipes/lengths with marginal performance, pipes always run smokin hot, any combination, or length. ( prop?) Ive arrived at a macs long quiet pipe ( exit exhaust from boat without muffler extension) I can tune the boat to keep it on the pipe with the 3rd needle resulting in a lean run, and heat. My thought on pipe theory ( correct?) longer= torque shorter= rpm? will lengthening or shortening work to keep the boat on the pipe and allow a better needle setting? Before i start cuttin the pipe thought id look at opinions..... Theres no "weld" to measure to, hence im runnin it stock length for now. Boat pushes an 450/3 or 2 blade as well, fair speed. I realize the search for the prop/pipe/needle is relentless, Ive a 45 rigger picco/ABparabolic drilled, at 10" that runs consistently well. The AB is too short to get the exhaust outta the spt 40 without an additional muffler, hence the long pipe. The macs has a long middle section and indistinct cones, any measuring techniques? Test the boat? Once you cut, youre cut, so, throw the input if you would.... its gettin wet again tommorow..... Thanks mike

 

[Dan_Cousin]

You are right, The long pipe = low end torque (sacrifice RPM) Short pipe = high RPM (sacrifice low end torque). Typically you want the pipe as short is it can go and still drive the course and mill with a prop that gets you the speed you want.

How hot is your motor? if you can hold your hand on it after a run it is too cold and may be why its not turning on until you lean it way out. Try restricting the water a little at a time while running it rich until it turns on. you might like the result.

Pipes get hot under the cowling if there is not enough ventilation. With the outrigger, the pipe is outin the air. You might want to consider a header cooler.

Good starting place is 10.5" fron the center of the cylinder to the first max diameter on the divergent cone. This should leave plenty of room for shrotening later. If this is too short, something else is wrong.

Hows your motor?

Head clearance? Rotor clearance? Lapped your rotor lately? These two can also make a big difference in the performace of the boat.

As always, good reading are Marty Davis' technical notes at http://rcboat.com/past.htm

They cover a wide variety of topics and are very helpful.

 

[Jerry Wyss]

I think Dan has it pretty much all right, I'm not a Pro on this stuff but I've fought thru mostly on my own and come up with some pretty good moves.

A long pipe and the 450 prop is not going to go anywhere fast, a longer pipe should require more prop,I use the prather 240 pitched a little as a "intermediate" prop or go all the way up to a 1455 to load the engine some more with a long pipe.

The 1455 may be a hard prop to turn for engines with less torque,,they run better with the Picco disc rotor when the engines are fresh and weather conditions are cool. Disc rotor clearance, headspace , piston liner and bearings must be in good shape in order to get anywhere with a prop/pipe and strut depth set-up. A good running motor will make all this stuff alot easier. A hot pipe is really what you want and the engine hot enough where you can grab the head and hold on for 3 or 4 seconds, anyway that's the way I run mine,, opinion's vary.

Only place where I would deviate from Dan's instruction is the "divergent" cone,,most measurements are taken from the begining of the "convergence"( may have spelled that wrong) cone if the pipe has a constant dia center section. Some pipes have no center section like I described,, the two "cones" just meet together,,

probably was what Dan was talking about,, most Mac Muffled and quiet pipe's are that way. I like the Mac pipes that have a "straight" center section,, they will allow the engine to run Ok even if you off a little on the length. Phew! Good Luck

 

[El Zorro]

Hi Fred,

Glad to hear that you are still around! I haven't run an R/C boat for two years........... I started racing "big" boats. I have a C Stock Outboard hydroplane that I race. I'm a rookie but I'm learning! Email me at "[email protected]" and fill me in on things.

Dick

 

[El Zorro]

Hi Fred,    Glad to hear that you are still around! I haven't run an R/C boat for two years........... I started racing "big" boats. I have a C Stock Outboard hydroplane that I race. I'm a rookie but I'm learning! Email me at "[email protected]" and fill me in on things.

            Dick

103769[/snapback]

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Sorry fellas, messed up trying to answer someone's message.

Dick

 

[Dan_Cousin]

Jerry is right I was talking of the pipes the divergent and convergent cones come together with no flat band. If there is a space between I usually spit the difference.

Remember this is all just getting you to a starting point. From there you should tweak.

Try a stock 1455 and see if you can get that running. The folks down here at sea level are running them with a little cup so stock shouldnt bee to bad unless you hull has a lot of drag or weight.

10.5" is plenty long for a .45 tuned pipe. My rigger runs at 9 3/4.

 

[Mike Larson]

Thanks Dan,i moved the pipe shorter, and the boat really turned on..... got a buch of reliable runs in..... problem is, later today had a coflict with a gas boat on my channel and totally demolished the hull...... im sick,.... attempts at repair would be futile,... so,.... appreciate the replies guys, thanks mike

 

[Dan_Cousin]

You might want to talk to Jeff Baham (Hornet Hydro) on this board. He has a sport 40 wood over foam hull that he builds that is definately getting attention.