Question on LA .46

A few years ago I bought a LA .46 off ebay. I bought others new from Tower. All 4 .46's use the APC 12.25 x 3.75 and 3% nitro 24% oil. Why would one .46 launch at 8200 rpm and give me 5.4 sec, laps, 63 and 64 foot lines and plenty of power through the pattern and the other 3 launch at 9400 for the same performance? Similiar size planes and no engine unloading after takeoff to speak of. The 8200 engine has a stock head the others have an opened combustion chamber. .282 venturi and .156 ST NVA. The 8200 is in a Pat Johnston Ringmaster 576 and the others are in a Pathfinder profile and two scratch built full fuselage planes. My next move is to compare cylinder liners.

Could the one from ebay be an RC converted to C/L? I think the timing is different from the C/L version.

>Could the one from ebay be an RC converted to C/L? I think
>the timing is different from the C/L version.


Dick,
They are exactly the same timing.I had one with the same symptoms. The cure was for my case so simple it eluded us for awhile. Without going into details it boiled down to getting rid of the Thunderbolt plug and putting in a medium length plug, in this case it was a rRo-Jett plug but an A-3 will do the same. It now runs like the proverbial watch. Timing of the engine is controlled by the heat of the plug and the length. In this case the long plug was fast forwarding the timing. It's the only solution that I can think of as i had done all of the other things mentioned. All I can say is it worked in this instance and it is a relatively painless solution to try on this example. It just might work here also.
dennis

Could be the boost port? I believe most of the newer LA engines do not have a boost port while the Tower is a copy of the FP series which has the boost port.

They have different combustion chambers and in different airplanes? How can you compare anything with so many variables?

MM

Kind of trying to compare a Fox Stunt .35 & a McCoy .35 [photo not recovered: sipcof.gif] Similar but so different?

> A few years ago I bought a LA .46 off ebay. I bought
>others new from Tower. All 4 .46's use the APC 12.25 x 3.75
>and 3% nitro 24% oil. Why would one .46 launch at 8200 rpm and
>give me 5.4 sec, laps, 63 and 64 foot lines and plenty of
>power through the pattern and the other 3 launch at 9400 for
>the same performance? Similiar size planes and no engine
>unloading after takeoff to speak of. The 8200 engine has a
>stock head the others have an opened combustion chamber. .282
>venturi and .156 ST NVA. The 8200 is in a Pat Johnston
>Ringmaster 576 and the others are in a Pathfinder profile and
>two scratch built full fuselage planes. My next move is to
>compare cylinder liners.

You have answered your own question with your description of the engines. First off, you need to know that OS provided many different sizes of venturis on their "stock" LA 46 engines. So, just because you have a "stock" LA 46, does not mean it is comparable to another. There are also manufacturing differences that result in differences of timing and compression. Most are "close" to one another, but occasionally one slips through with a good deal of variation from blueprint. HOEVER:

After saying all of the above, you have changed the head shape on three of the engines. But, even worse, you have fitted three of your engines (the faster running engines at launch) with .282 venturis AND ST needle valve assemblies. These are FAR too small for this engine! The ST needle alone shrinks the venturi to an effective .262 size. This might be OK for a .25 FP.

The later LA engines had venturis of around .300 in size (.295, .305, etc) with a .135 OS needle valve. Check the size of the venturi in the engine that launches at the lower RPM. Then increase the size of your other venturis to .020 larger, to compensate for the larger needle. This will bring things back closer to "par".

Engines are fun to experiment with, but any change you make will affect performance--good or bad. Don't change the size of the needle valve (or anything else) without expecting some change in performance. This is a mistake way too many people make.