flybyjohn

My Feedback: (2)

I recently bought a tower hobbies .75 engine that needed agood cleaning but was still relatively tight at the top end. It did not have a muffler and mufflers forthat engine are hard to come by now unless you buy a whole 61 – 75 tower or gmsengine with muffler.I decided to make aheader pipe and mousse can muffler.Theheader pipe was easy but took a bit of time.It turned out good.I found twodifferent sizes of aluminum cans that I thought would be the right size.I modified them slightly and the differencesare shown in the picture attached.Theyare both the same volume but give very different rpm numbers.I was told that the size of the can was notthat important but perhaps shape makes all the difference.
The rpm numbers are listed below. I added a short piece of aluminum tubing on the short fat pipe tomake the header length the same on both setups which actually lowered the rpmsslightly more.What do you think is the main reason for the800 rpm difference in these two cans are.I am leaning towards the taper ofthe tubing coming into the can and the taper of the tubing going out of thecan.I guess I can add a tapered tubingonto the short fat can and see if I see any improvement and then put a taper onthe rear to see if that makes any difference also.Maybe it is the short fat compared to theskinny long can that makes the difference. I also did try to slide the short fat can up the header with no real difference.
Tower 75 engine 15% nitro 18% oil (50/50 castor syn blend)48 degrees F 3900 ft elev.
Apc 12x8 sport prop
11,400 peak - open face exhaust
11,500 peak-short fat can
12,300 -12,450 peak – long skinny can



I have seen that the bud light aluminum bottles that are 16fl oz work well with the 91 size engines. they have a tapered neck on the bottles.