#1850-T (X15 Turbo Pipe)
Will give best power for your .21 powered 1/10th scale.
Since your car is only 1/10th, it will not accept .21 size pipes, as they are too large, and will not even fit.
You will most likely have to angle the X15 pipe upward a bit (use longer retainer wire) to get the pipe to clear the front left wheel.
You cannot use any of the .15/.12 size manifolds on the included .21 engine.
In the future if you decide to replace the engine, look for a big block .15. Lighter, and a more even power-to-weight ratio.