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Author Topic: Boost Ports? Blocking them or it  (Read 2881 times)

Offline Charlie Pate

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Boost Ports? Blocking them or it
« on: July 28, 2006, 11:34:49 AM »
When one talks of boost ports in a 2 cycle engine, which port or ports  is /are
the boost port.
For example:
On an OS FR or FP engine, there are 4 ports.
One is the exhaust port Obvious; thats the one the exhaust comes out of.
Directly across from the exhaust port is another port(Bypass)
At  both sides of the exhaust port (next to each side) there is a port(bypass)
Of the three ports , which  is/ are considered the boost port.
 Now ; when you block the port?s , which get blocked off?
How is this accomplished?
What is the desired result?
                                              Thanks
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Alan Hahn

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Re: Boost Ports? Blocking them or it
« Reply #1 on: July 28, 2006, 12:49:29 PM »
The boost is opposite the exhaust, and on the engines I've looked into, opens a little later than the other two ports. From what I remember, the original schnuerle engine did not have a boot port. It was a later improvement.
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Offline Steve Scott

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Re: Boost Ports? Blocking them or it
« Reply #2 on: July 28, 2006, 01:07:30 PM »
The LA series of OS engines also do not have a boost port.  The one which is missing is the one 180º away from the exhaust port.  The two bypass ports are 90º of the exhaust port.

Some pundits say the boost port was just never cut into the sleeve.  On my LA 40, there is no transfer milled into the crankcase.

Haven't looked at my LA 46 yet but agree with most folks who say it is a very sweet running stunt engine.
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Offline phil c

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Re: Boost Ports? Blocking them or it
« Reply #3 on: July 31, 2006, 01:54:33 PM »
From what I've read, the boost port was added to the Schneurle system to improve horsepower at higher rpm.  A friend of mine was able to get significantly more rpm out of the FP25 by increasing the size of the boost port(its the middle of the three transfers).  All the high rpm engines use the maximum amount of cylinder wall for the transfer ports for high rpm scavenging.  At stunt speeds(8-11,000 rpm) there is no need for the boost port.  A few years ago one of our English friends published some results with the LA 40.  He had both an 3 bypass version and the later one without the boost port.  Mixing and matching the pistons/liners between both engines showed that they got exactly the same rpm on the same stunt props.

The simplest way to block the port is to squirt a small amount of silicon rubber caulk up into the port from the bottom, with the sleeve in the engine.  If everything is oily it won't stick well and you'll still be able to remove the sleeve easily.  It can't slide up and if by some chance it trys to slide down the bottom of the boost channel will stop it.

Or you can disassemble the engine and fill the channel with JB weld or similar epoxy.  Just make sure it doesn't stick up enough to rub on the sleeve and distort it.  That will be a pretty permanent fix and the filler may be hard to remove if you ever want to.
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phil Cartier

Offline Charlie Pate

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Re: Boost Ports? Blocking them or it
« Reply #4 on: July 31, 2006, 08:08:33 PM »
Phil
 do I assume correctly,that if the boost bypass is filled ,the crankcase compression
 will be higher and result in slightly better fuel draw?
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Offline RandySmith

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Re: Boost Ports? Blocking them or it
« Reply #5 on: July 31, 2006, 09:26:54 PM »
Quote from: Charlie Pate on July 31, 2006, 08:08:33 PM
Phil
 do I assume correctly,that if the boost bypass is filled ,the crankcase compression
 will be higher and result in slightly better fuel draw?

No Charlie  you can't assume  that and  I have not found that to be  the case

Regards
Randy
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Offline Charlie Pate

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Re: Boost Ports? Blocking them or it
« Reply #6 on: August 01, 2006, 05:21:47 PM »
Randy
 Would lack of fuel draw increase be due to the fact Intake port inlet area has been reduced by 1/3rd(assuming all three ports are the same).? ???
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Offline phil c

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Re: Boost Ports? Blocking them or it
« Reply #7 on: August 01, 2006, 06:26:58 PM »
At stunt speeds the engine isn't flowing enough air to need the third port.  Blocking the boost port sends all the air through the other ports, at a slightly higher speed.  Even in the LA 40 the two main bypasses are probably way larger than actually needed.  A couple of mine have a bit of the profile burp that a Fox stunt with a larger than average bypass has.

Crankcase compression will only increase a percent or so.  A little glob of silicon, maybe 1/8c1/8x1/8 in has no real effect on the crankcase compression.  What the plug does do is prevent the boost port from actually boosting power and rpm if the motor does try and run away.

blocking the boost port shouldn't have any effect on fuel draw either.  The motor can flow all the air it can handle at lower rpms through the main ports.  You can get whatever fuel draw you want by changing the venturi size and shape.
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phil Cartier

Offline RandySmith

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Re: Boost Ports? Blocking them or it
« Reply #8 on: August 01, 2006, 07:23:41 PM »
Quote from: Charlie Pate on August 01, 2006, 05:21:47 PM
Randy
 Would lack of fuel draw increase be due to the fact Intake port inlet area has been reduced by 1/3rd(assuming all three ports are the same).? ???

Hi Charlie

Couple of things I have found, most engines have an abundance of fuel draw, if they have a good  piston sleeve fit, I have seen no beneficial  increase in th emotors I have checked  and  the differance is  most  ALL  boost ports  are  not  that big, and most all are  smaller  than the main scherle  ports.  A full circle  crank has been used on several engines  to increase the fuel draw  and  pumping of the engines  I have  tried  many engine  with and  without   with almost  ZERO noticable  differance.
If you have  enough  fuel suction at the venturie  adding  more  doesn't  seem  to help  anything
I know the PA  engines  will draw  fuel  from  as much  as  10 inches  behind the  motor.

AS the  old  racer's  saying  goes "if enough is good  too much is  better"   it  doesn't  seem to be  true  in this  case

Regards
Randy
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Offline Charlie Pate

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Re: Boost Ports? Blocking them or it
« Reply #9 on: August 02, 2006, 08:44:24 AM »
Nothing like hands on and lots of testing.
              Thanks Randy
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Offline fred krueger

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Re: Boost Ports? Blocking them or it
« Reply #10 on: August 14, 2006, 12:18:19 PM »
I have blocked the boost bypass on several engines, to get rid of a power surge.  The last 2 were a Magnum 36XLS (latest version) and a Magnum 28XL.  The 36 is in a ARF Nobler.  This engine has been used (stock, I believe) by Phil Granderson to power a much larger airplane.  I tried 10x4 and 11x4 props with 5% and 10% fuel (20% oil).  When the engine would speed up during manuevers, it tended to surge suddenly, gaining maybe another 1000 rpm.  It would come back but the surge was not desireable for a nice pattern.  I blocked the boost bypass and the surge went away.  Further tuning with head shims (+20 thou), venturi size and muffler outlet, made the engine very consistent.

The Magnum 28 is on an ARF Primary Force.  When I initially experienced the same problem, I just duplicated the work from the 36 and I've got a nice running stunt engine.  BTW, both run in a rich 2-stroke.  The 36 swings a 11x4, the 28 a 10x4.

To block the boost bypass, I disassemble the engine; clean the crankcase in hot antifreeze (crockpot); flush and dry thoroughly; scuff the bypass wall in the case with a Dremel tool; and fill the bypass with JB Weld (sufficient to block it).  Try to get the level close before it cures, though excess can be carefully ground off after curing.

Fred
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