os 61fx & tuned pipe
01-06-2010, 08:15 PM
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os 61fx & tuned pipe
i have a o.s. 61fx with a tuned pipe and i was wondering do i need to shorten the header pipe to get motor tuned in?.......if so how do i know when its tuned right ?
01-06-2010, 10:01 PM
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RE: os 61fx & tuned pipe
Some of the Macs systems are supposedly "pretuned" (although you might still need to do some cutting) But yes, you will most likely have to cut, especially if it's not a pretuned set. I think when I had my .61 FX the header length, measured from the front of the header mount flange to the end of the header pipe section was around 4.5". You need a tach to tune this correctly, but what you're looking for is an RPM peak, and keep cutting segments of the header off until you get a drop, like 1/4" at time. Then you got it. To regain some of the drop, you can pull the pipe back a tad in the coupler to the previous point where the highest RPM was observed. If you don't cut enough, you won't get the power you're looking for, cut too short and now you start blowing plugs, then eventually snap the connecting rod. Ask me how I know. Also, you will need to open the high end needle more with a pipe than with the stock muffler, it will burn more gas too. But you need a tach, no way around that one.
01-07-2010, 10:24 AM
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RE: os 61fx & tuned pipe
The critical length is the distance from the combustion chamber to the restricted area outlet at the tail of the pipe, not the length of the header. Most of the manufacturers do make their header sufficient in length to allow two or three inchs beyond what will be ultimately needed with an appropriate tunned pipe. Also different brands and types of full wave pipes will affect this header length also.
The worst problem is when you buy a used header its probably already been cut many times and you don,t know, you may even need to extend it. With a used header start with two or three inchs in added length then do the testing for you peak by gradually shortening the silicon hose and or the header as described by Tommygun.
It is not neccessary for the header and pipe to completely touch each other You can always extend the length.
Also frequently you do not need hose clamps or tie wraps on the header and pipe if you use the appropriate size silicon hose. This greatly facultates the testing process.
During this testing you need to use the intended propellor since all the testing will change if you pick another size for flight. Keep in mind and plan for a typically 1500 to 2500 incrase in static full throttle rpm and pick a propellor accordingly.
John
The worst problem is when you buy a used header its probably already been cut many times and you don,t know, you may even need to extend it. With a used header start with two or three inchs in added length then do the testing for you peak by gradually shortening the silicon hose and or the header as described by Tommygun.
It is not neccessary for the header and pipe to completely touch each other You can always extend the length.
Also frequently you do not need hose clamps or tie wraps on the header and pipe if you use the appropriate size silicon hose. This greatly facultates the testing process.
During this testing you need to use the intended propellor since all the testing will change if you pick another size for flight. Keep in mind and plan for a typically 1500 to 2500 incrase in static full throttle rpm and pick a propellor accordingly.
John
