411 (Series 3-5) Exhaust Tailpipes

[John Keighley]

Could somebody please let me know how far the exhaust tailpipes extend past the rear bumper on the Series 3-5 411s? I have changes the outlets to my 410 to tidy up the overall appearance of the rear of the car and while the result is far better than it originally looked, I am getting two problems:

1. When the windows are opened on the move at any speed, fumes make their way into the cabin.
2. The rear bumper has exhaust stains after the car has been run.

Obviously there is a degree of some low pressure areas around the rear of the car which is causing fumes to be sucked forwards. I suspect that because the outlets terminate directly under the bumper, the disturbance to the air around the bumper is directing exhaust in other directions instead of straight behind. Extending the tailpipes is obviously the solution but I'm not sure how far this should be. As can be seen, the exhaust stains on the bumper are quite evident.
John Keighley.