>I have two ST .51 engines. One has a front ball bearing and
>one has a plain bearing. The plain bearing one has bigger
>fins and a bigger head. Here's my question. When I bought
>the ball bearing one, it had a delrin venturi that was huge.
> It has the ST nva, off to the rear of the venturi and a
>spraybar. It wouldn't run right and I dropped it off at Lew
>Woolards. He replace the venturi and did other magical
>stuff to it and now it runs exceptional. The venturi on
>that measures .200 id.
>
>Well, its cold and I just got the plain bearing engine so I
>haven't started it. But the venturi is HUGE. Unlike the
>other engine, it has a factory ST nva that runs through the
>center of the venturi, rather than the rear/spraybar setup.
>But it still, the inner diameter measures .354. I could
>almost hide my car keys in it. Is it too big?
>
>And why would BOTH of these engines be equipped with a giant
>venturi when I bought them? Is there some reason they
>were(are) set up that way.
>
>Thanks
First off, you have two different engines. They will react differently. If they have been modified, they will undoubtedly react even more differently.
Now, you measure venturi size by choke area. The one with the restrictor venturi (needle going through the center) will be reduced by the size (length times width) of the needle going through the middle. So these will always be larger in inside diameter. That said, it still sounds way too big for stunt. Questions: Was it modified in any way? Is this a factory venturi or after market venturi? We know the Delrin venturi was not factory. How about the other one.
Next: unless you are buying a stunt engine from a person or company that knows stunt, do not EVER assume that the venturi supplied on the engine will work for stunt. NEVER assume that. The manufacturers are supplying you with a CL version of their engine, not a stunt tuned engine. For that matter, we might even call it a non-RC engine because the same type of venturi would be used for free flight applications, or rat racing or even speed (not a 51, but you get the idea).
Witness in point: the earliest 35 FP engines were supplied with a .287 venturi. Way too large. They then went to .282, 278, .272, and finally settled on .265 for both the 35 and 40 engines at the end. When the LA engine came out, it came out in 40 and 46 versions with the same .265 venturi. Yes, they were using the same venturi on the 35 FP and the 46 LA! The 46 LA's venturi was later changed to (yup) .272, .277, etc. until the latest ones are .295! Now there is a pretty significant difference between a .265 venturi and a .295 venturi. Yet this was the range of the size of the venturis supplied on their engines, box stock, from the factory, depending on when and where you got them. There is NO SUCH THING as running these things out of the box and copying what someone else did. You need to check the size of the venturi. And then, if you change to a different size needle, you are changing it again.
Now, did ST do this? Did someone else put this venturi on it? Did someone else modify it? On some of the cross scavenged engines you will sometimes see people killing the compression and then opening up the venturi to compensate. This was the approach taken by George Aldrich, so you will see it on some of his engines.
As a comparison of size the .201 ventri with the needle not passing through the center would be the rough equivilent of a .323 venturi with the needle passing through the middle. So the .354 that you have is a large venturi, even for a restrictor type--much larger than what would normally be considered to be correct.
Still, if someone killed the compression or did some other modifications...?? If you are planning on using it on something, give it a try. Venturis are very easy to change after the fact.