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HelicopterEngines Plugs Mufflers Fuel › OS 91 SX-H with Pump VS OS 91 SZ-H
11-22-2006 03:34 AM  11 years agoPost 1
OsiViper

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Farmington, NM

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Whats the difference between the two - besides the pump

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11-22-2006 03:54 AM  11 years agoPost 2
Jerry Sudimick

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Connecticut

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SX-H with Pump

I'm assuming you are talking about the OS91 SX-H Cspec with Pump?

This is the previous 91 variant that OS introduced a couple years ago. It includes a perry pump mounted in the backplate. This pump feeds the 60M carb (10.5mm bore), which does not have a midrange needle, but instead routes un-used fuel back to the tank or fuel system. This engine worked well for some, and not so well for others. Common fixes were cateye modifications and or the addition of a 3rd needle to modulate return flow, thus augmenting total flow into carb.

SZ-H (Im assuming you mean without pump, because there is a pumped version)

This is the latest 91 variant from OS. It features a beefed up crankcase, updated cylinder head, different timing and porting, and a 3 needle carburetor with 10.6mm bore. The piston and ring have been optimized a bit more over the Cspec as well. This engine utilizes standard plumbing with muffler pressure. Not as consistent from top of tank to bottom when compared to pump system, but most owners are very happy with this engine in it's stock form.

Jerry

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11-22-2006 04:15 AM  11 years agoPost 3
OsiViper

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Farmington, NM

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But what is the difference between SZ and SX

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11-22-2006 05:04 AM  11 years agoPost 4
Jerry Sudimick

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Connecticut

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I edited the above post to avoid confusion.

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11-22-2006 05:11 AM  11 years agoPost 5
OsiViper

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Farmington, NM

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Are they close in power and smooth running?
Or is one better than the other -- tryin to decide on which one to get

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11-22-2006 05:20 AM  11 years agoPost 6
Jerry Sudimick

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Connecticut

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They are similar in power output and 'smoothness.'

The SZ is the more durable of the two. When comparing their track records you may want to consider the un-pumped SZ further. The piston to liner tolerance is slightly increased. The piston also features a groove below the compression ring to aid in lubrication.

THe Cspec when run at it's peak output could easily be pushed over the edge and damaged. The piston to liner tolerance was less than the SZ. As such, when the piston became very hot, it would swell. The area just below the ring would start to ride the liner, and if pushed just a little harder, was easily convinced to score the piston/ring/liner and damage the engine.

By increasing this tolerance and adding the groove in a new piston, the SZ is more durable. The case is also beefier for whatever that is worth to you (and thus the engine is a little heavier than the Cspec)

Not to mention the un-pumped SZ does not have the complexity of the Cspec PS's fuel delivery system. It also does not have a track record of requiring modifications from various end users.

Jerry

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11-22-2006 05:50 AM  11 years agoPost 7
Chris.C

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Hong Kong

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Not as consistent from top of tank to bottom when compared to pump system, but most owners are very happy with this engine in it's stock form.
For sure I am one of the happy owners. If you use OS engines with 60 series carb, I suggest you to take a look at the write up by Jerry on the carb setting. It is a very good info.

For the non-pumped SZ, don't forget to put header tank just to be on safe side.

Cheers,

Chris

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HelicopterEngines Plugs Mufflers Fuel › OS 91 SX-H with Pump VS OS 91 SZ-H
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