The norm for 3d is a Throttle pipe.
Thats what i use to good effect in another of my models with an irvine 53 powering it.
Goes well,throttles just as good as with a conventional silencer.
Its the weston throttle pipe.

When i was running a 12x6 it purred quietly, but now it has too little prop when using a 13x4, so its loud.