js3

My Feedback: (3)

Hello speed demons,

Will a 10 CC pipe work on a 15 cc motor? I plan to spin smaller sized props like 11 X 6 to maybe 11 X 9.

Will this work?

Thanks,

evan-RCU

A pipe for a .61 on a .91... sure but you won't have optimal power....

js3

My Feedback: (3)

Right. So that's what I want to know. What will be the consequences?

evan-RCU

More back pressure than normal will help fuel draw but it will be harder to tune the correct pipe length and you won't get as many rpms as the correct pipe will.

YUKONFLYR

If the pipe does not allow the proper amount of flow it could cause the engine to build up heat.
Engines do not like heat.

js3

My Feedback: (3)

All right. Very good points. Thanks for you responses.

I am curious if the volume of the pipe has to do with the displacement of the motor, or if it is more a matter of prop size (load) and target RPM.

To wit: See this chart taken from Macs products site...

Size Length Weight Price Stock #
2.5cc (.10-.15) 12" 2.0 oz 59.95 1210
3.5cc (.20-.36) 13" 2.4 oz 60.95 1220
6.5cc (.35-.45) 14¼" 3.4 oz 69.95 1230
7.5cc (.40-.50) 15¼" 3.6 oz 70.95 1240
8.5cc (.45-.60) 16¾" 4.2 oz 80.95 1250
10cc (.60-.75) 19" 4.8 oz 81.95 1260
15cc (.90-1.2) 21 1/8" 5.0 oz 90.95 1290

Notice that the 8.5cc pipe is for .45 to .60 motors, the 10cc is for .60 to .75 and the 15cc pipe is for .91 to 1.2s. It seems to me that using a pipe made for a .75 (the 10cc) on a .91 is quite a bit closer match than using a 15cc pipe on a 1.2.

As I wish to turn a smaller prop at higher RPM, I'm trying to find out which pipe to select.

Thanks very much!

cncswiss1

the diameter affects the strength of the back pulse, the length (to the high spot) affects the intended RPM band. One is trying to creat a tuned exhaust restriction that creates back pulses to jam more mixture into the cylinder just before it closes the exhaust port. so, if you have a giant pipe diameter on a small engine but with the right length the "pulse" of the engine will "get lost" in that giant ole pipe and have little effect. using too small of a pipe will have a hyper effective pulse coming back and more back pressure (again think of the exhause pulses like a slinkee toy laying on the floor with one end being "pumped, no giggling!). I think the smaller pipe will work, but will have a bit of restriction. think of pipe sizes as a function of the area of a circle and how that relates to displacement and the sizes will make more sence. I'd use the 15cc pipe and cut it to the length you want, if it's ultimate rpm you want look at the noisy pipes or the marine ones.

js3

My Feedback: (3)

Great! Thanks for the reply. I'm learning a lot.

BTW, what makes the marine pipes different? I've seen on Macs site that there is a 13cc unmuffled marine pipe intended for a .80 to .91 size motor. This may be the ticket.

cncswiss1

the marine pipes are for flat out top end only, with no intent to transition smoothly. they are very short and are for very high rpm, they may be too short for any aircraft application.

here is some reading
http://members.bellatlantic.net/~vze...es/engine4.htm
http://rcboat.com/myst4.htm
http://www.mh-aerotools.de/airfoils/javapipe_en.htm

js3

My Feedback: (3)

Thanks for the links. I'm acquainted with MH's site. I just haven't visited it recently.

I'll peruse the others as well.

cncswiss1

basically stick a degree whel on that motor, get a header, run it open header and see what rpm band you are talking about, then choose a pipe.

js3

My Feedback: (3)

Again, thank you.

I've done some reading on the Macs site that talks about tuning the system. See [link=http://www.macspro.com/tuning.asp]Tuning Information[/link]. I've always assumed (perhaps incorrectly) that you need to attach the whole system (header and pipe) to fine tune the setup. But Macs' instructions are unclear as to whether you should run the header AND pipe or just the header.

My assumption was that you first run the motor with open exhaust and note the RPM. Then you add the header and pipe and run again noting the RPM. Then gradually cut off sections of the header until max RPM is reached.

Are you saying to do this with just the header? Or is running the header open first just to determine the RPM range then add the pipe and continue to adjust for max tuning from that point?

Thanks again.