Cody,

if the exhaust is pre-routed and this is a second or third hand pattern model, there's a good chance that the builder and/or previous pilot set the engine up properly and the exhaust is tuned. Length of exhaust is dictated by RPM band of the engine which in turn is dictated by engine design and prop size.

The OS 61VF (front intake), which is what I presume you have (the VR, rear intake, is a different engine), is a short stroke classic engine which has a design power band in the upper RPM range - roughly 13,500-15,000 RPM. To turn those RPM's you will want an 11x7-11x9 prop. Before changing the exhaust length, you should run the engine with these props and record the RPM's. If the exhaust is long, it's possible that the previous owner(s) was running it in "long stroke" fashion with props like 12x8-12x10's. It was doable but this engine wasn't optimal on those props.

If you can get the engine to turn in the upper 14K RPM on a wood 11x7, the exhaust should be roughly right. If it doesn't make it to 13K, it likely was setup for the larger props.

In the end, what prop you use will depend on how you want the model to fly.

David