hi all.

ive been messing around with this spare 32cc mac and now i want a pipe for it to replace the stock tin box exhaust.
I cant afford to buy a tuned pipe so i was planning on trying to make my own.

I have worked out all my port timings and are as such -
Intake - 117 degrees
Exhaust - 147 degrees
Transfer port - 104 degrees

I have been playing around with this site here --> www.mh-aerotools.de/airfoils/javapipe_en.htm
which will give me dimensions for a tuned pipe to suit this motor.

all is good but one item i'm a little unsure of, the Velocity of Rotation (rpm).
i'm unsure as to what figure i should input here as i have no way to measure the rpm.
i guess what i'm trying to find out is, is this figure the motors current max rpm? or is it its new max rpm with the designed pipe?

any tips / suggestions?

by the java pipe designer I have here, you'll want to put your max rpm in there.
Not sure for the mac half crank motors but with other half cranks you dont want to over rev them. Homies are around 13,500 rpm max.
The mac 32 I have here has a much beefier crank, pin etc. than my homies so maybe 15,000rpm max??

Cheers oz!
I chucked in the numbers, put the rpm at 15000 and it gave me a pipe with these figures -
Header - 75mm (2.95")
diffuser - 179mm (7.05")
reflector - 106mm (4.17")
stinger - 43mm (1.69")
Total length of 403mm (15.86")

While i was hunting around the shed i stumbled upon an old pipe from an old nitro boat.
Its measurements are very close to what javapipe generated.
The only difference i can se is the length of the diffuser section and stinger.
The pipe i found has a 34mm shorter diffuser and 9mm shorter stinger.
Everything else is identical, pipe diameters (internal) and all

what effect would a shorter diffuser and stinger have on performance?
Could i use this pipe and would there be any gains over the standard tin box exhaust?

Heres the pipe i found after i cleaned it up. Has a hole in it too where i suspect it may have been water fed

It may work fine, you might just need a longer header pipe to get it in tune.
the hole I suspect would have been for the pressure line to the fuel tank being nitro.
No reason you cant put a barb in there and run water through it for cooling.
Remember that you are using the stock port timing, so your designed pipe will be different to race ported engines pipe.

Ah cool, will give it a go! The header will end up longer because i will be using my custom 90 degree adaptor
which will lengthen the header by approx 35-40mm
when i used javapipe, i put in the stock port timings which is what gave me the specs for this pipe which is close to the one i found.
Me uncle dropped off another 2 donor weed wackers. One is a 28cc mac which is near on identical to the 32 except the cc's (even uses the same wt-231carb)
The other weedy is a gmc branded 25cc with some strange unkown carb.
But i had both of them running after 10min of fiddling, more spares!

No worrries, take a look at the GMC snipper. I gave one away a while ago, it was the same as the clone motor that I now have.

ah k, when you say "clone", a clone of what?
i have stripped the gmc down and have found it has a 3 bolt head where the bolts come up thru the crankcase. and the exhaust bolts onto the head as opposed to the macs where they are retained with that damn spring.

i will post some pics later on

I was referring to the engines they have in the cheap RTR's.
Yours doesnt sound anything like the one I have though.

You mentioned about the spring that holds the McCulloch exhaust on.Major Smackdown has a setup for the McCulloch so you can use a header and tuned pipe. He also has headers and pipes that work pretty well on a weedy engine. www.majorsmackdown.com/mcculloch.html

Thanks ridgerunner, i had checked out that site before, they have some good stuff but a cheap ass and like to to save cash and make parts myself.
I ended up making the exhaust port edge flat so i could drill and tap threads to be able to bolt my homemade header on
Check the attached pics for what i did.
Holds the header with pipe attached quite securely, even made a gasket to suit.
Just have to make some motor mounts to bolt er in me mono

i also made a header for the 25cc mac already in the mono for the pipe i have.
This time i had to make it so it could be retained by the springs as there isnt enough meat to drill n tap.
I have tested this motor and seems to run ok but wont know for sure til i hit the water.
Unfortunately that wont be for alittle while yet as the weather here is very dismal, cold wet and windy