I have changed my exhaust from a 3 inch down pipe with a mid silencer and then in to two 3inch rear boxes (FC) Although it was fairly quiet from the outside it had a really bad drone at cruising speeds on the insde.
When i put the new in engine in I made a new down pipe which is 3.5 inch and put a straight through 3.5 exhaust all the way down the spit at the back of the car with no mid silencer and although its a tiny bit louder outside at the back of the car its actually quiter inside and doesnt drone anywhere near as much.
Doesnt make sense to me :Hammer I have tried for years to get rid of that drone!
4inch next time then Tony. I\'ve found with mine in car noise levels are better when you can get the gasses out the back without restriction. What you where probably hearing before as a drone was the turbulent air in the mid boxes.
Kevin
Been doing some heavy reading lately (Scientific design of Exhaust and intake systems) and got to thinking about kevs system that his 5in downpipe is action like a silencer in itself allowing the pressure of the gas to reduce and expand as soon as it comes from the engine !
Plus when you go to bigger pipe your increasing the surface area of the pipe too so the damping effect off it can change alot and alot of the top engine note of the exhaust is cropped much more.
Shaun
so the downpipe is acting like diffuser in effect ?
What info have you found Shaun? owt usefull for making a DIY exhaust?
You may also find that the exhaust pipe acts like a tuned port - much like on a reflex speaker cabinet. Changing the length or diameter is going to change the resonant frequency, and may just push it outside the natural resonances of the exhaust gases moving through it.
I bought this book a couple of weeks back :
ISBN10: 0-8376-0309-9
ISBN13: 978-0-8376-0309-4
Cheapest was off the Times book site.
Not completely sure if it any help though. Some heavy reading for DIY. Plus its based on piston engines, not rotories. Somebody on rx7club swears by it.
Shaun
mine now has a nice smooth sweeping downpipe in 3.5 inch straight from the back of the turbo into n 1.2 meter long straight tube in 3.5inch where it Y\'s to two 3 inch. Ive been doing some mileage on the motorway this weekend and its definately got less drone.
Considering i am using the the same turbo but a 1.15 a/r exhuast instead of a 0.96, i cant notice any difference in lag, maybe 100 rpm if that which is nice, hopefully it will flow more at top end and give me the 50bhp that i\'m looking for. Find out next saturday :)