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Converting 4 Stroke Glows to Petrol

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Peter Jenkins30/08/2009 23:29:51
1602 forum posts
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Has anybody any experience of using the Just Engines spark ignition kit to convert a Saito (100 upwards) to run on petrol?  Do you need to change the carb to the Saito pumped version?  How difficult is is to install the magmet in the prop driver and what sort of shimming is needed to reduce compression?
 
ken anderson.31/08/2009 09:07:54
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hello peter-there is an on going thread somewhere on here about the very subject-get searching before you spend your/her indoor's dosh.........
 
 
      ken anderson........
Peter Jenkins31/08/2009 23:22:56
1602 forum posts
266 photos
Ken, thanks for the steer but I have to say that having read all three threads I am none the wiser!  An article by the WOO on doing a conversion would be great.  Tantalisingly, his latest review of the Saito SG20 states that it is a standard Saito 120 glow engine but then doesn't go on to say what the changes are to compression ratio.  I note that it has a new pumped carb although WOO says that the needle valve is standard Saito! 
Simon Chaddock06/09/2009 23:35:25
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Peter
I think you will find that most glow engines use a similar compression ratio to petrol so I suspect shimming the head may not be required.
The air/fuel mixture is however very different. In a nitro glow it can be as low as 4:1 but for petrol it needs to be pretty close to 12.5:1. Such a difference may not be within the carbs capability to control the mixture accurately over the full throttle range. 
Former Member12/09/2009 09:21:23

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thomas oliver 117/09/2009 20:13:50
96 forum posts
23 photos
Simon, Are you sure you havent got those ratios the wrong way round?
Brian Parker17/09/2009 20:36:28
538 forum posts
The chemically correct air/fuel mixture for complete combustion is just under 15:1 but because  of uneven distribution within the cylinder Simon’s 12.5:1 is the practical ratio.
Glow fuel carries its own Oxygen in the Nitro and therefore needs less air.
Bert18/09/2009 06:57:03
514 forum posts
10 photos
Posted by Brian Parker on 17/09/2009 20:36:28:
The chemically correct air/fuel mixture for complete combustion is just under 15:1 but because  of uneven distribution within the cylinder Simon’s 12.5:1 is the practical ratio.
Glow fuel carries its own Oxygen in the Nitro and therefore needs less air.
 
Except there ain't no nitro in straight fuel
 
Bert
Steve Hargreaves - Moderator18/09/2009 10:13:44
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The reason for the different air/fuel ratios is that petrol has a much higher calorific value than methanol.....you get a bigger bang with less fuel......
 
For really high power engines methanol is often used as you can cram more of it into a cylinder & compress it more before pre-ignition. Many dragsters use methanol & have no cooling systems because a) the engines only run for a short time b) they are practically liquid cooled by the amount of (liquid) fuel being poured into them!!!!!

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