I would appreciate some feedback from those that have experience with an OS 25LA-s engine on a profile model, and where the fuel tank height ended up in relation to engine centerline for best equal lap times.

More specifics:
-Metal uniflow fuel tank
-Fuel pressurization used
-Stock OS 25LA-s with remote needle valve assembly

I know that on the older engines like the Fox 35 and OS 30s for example (profile mount), the tank needs to be around 1/4" to 5/16" higher than the engine centerline for equal lap times. Do others see the same thing on the OS 25LA on a profile model?

Thanks in advance.

   The 1/4" or so should be a good starting point for the pick up on the tank for the OS.LA.25. I have never had a Fox .35 that close to center line. My target for that is to have the pick up about in line with the bypass port which would be 5'8" or so above centerline.  This will vary if you are using a plastic tank for a metal tank, in my experience.
   Type at you later,
  Dan McEntee

Thx Dan and Aki for the feedback.

I am on the middle of a SIG Skyray build (w/ OS 25LA) and plan to be able to adjust the height of the tank.

Colin, there's a way to  find a "close" setting before first flight...

Using a height-adjustable tank mount... Start the engine. Keep the fuselage centerline level. Roll the wings outboard tip down. Set an easy run. Tach (or listen carefully) and rotate the fuselage to "45°" either way. (... fuse centerline horizontal.) Then 'roll' it to the other side to the same angle  You might - usually will - hear a change in RPM.

If 'richer' rolled toward upright, the tank is too high. Leaner indicates too low.
Stop the engine - nose slightly down, roll out board wingtip up - quick and safe..

Adjust tank height to oppose a rich or lean condition. Run your check again, shut down and adjust the tank again as needed. Repeat until there's little, if any, difference between "45°" rolled attitudes.

Why? The rolled attitudes simulate 'g' effects on the fuel in upright and level flight.. Pull (about 3g) and  gravity (1g) act like a single force aimed along the diagonal of a 1 by 3 rectangle. It should indicate too rich (fuel pouring "downhill" to the needle) or too lean (fuel drawn "uphill" to the needle.

Our stunt pattern has inside and outside, moderate and sharp maneuvering 'g' loads.This wig-wag process should at least put the tank height near the mid point between the extremes. Final tuning can only be done based on in-flight observations.

The 1/4" high tank, ASIR, was most useful for side-mounted Fox Stunt 35s. Many other engines, not  as fussy, ran well with the tank centered on the shaft centerline.

Oops! In my reply #5, the paragraph beginning, "Why?", I left out the word "inverted" before "... level flight."

Low level flight both ways has the same g-loads conditions, except that  the 'slant' of the  theoretical fuel surface reverses. (The Gravity component always points toward the ground.)The Lift component is always perpendicular to the wing chord line. ... except, of course for dihedral or gull wings when the net 'upward' lift, not the lift of each section, is used.