View Full Version : Underslung exhaust

11-06-05, 08:47 AM
Hi Guy's,

Following my previous thread (min ground clearance)...is it possible to 'over silence' the exhaust....the rear boxes which have been made are very big (oval) with offset 'in and out'...so it is baffled well...in fact the car is very quiet!!!....question is...have i gone OTT on the size of the boxes...and will it 'restrict' the performance of the engine????

thanks to all for looking and helping out.......cheers...steve

Happy Jim
11-06-05, 09:01 AM
The short answer is yes, too much backpressure will strangle the engine...I'm sure there is a scientific answer waiting to be typed about min/max gasflow but that's not my strongpoint!



neil still building it
11-06-05, 09:39 AM
hi steve
if you want the deffinitive answer ....you can drill a small hloe near the 'box' attach a pipe feed this into the car -to a pressure gauge 0-15 psi should do it!,and go for a short blast 'round the block' (-you need to get near full throttle and high-ish revs too to get some sort of real indication of back pressure) you really want fairly low .....up to say 5 psi is probably ok?
look this up in a vizard book if you want more info/figures
i suspect you 'll find that unless you're 'cammed up' you'll get a quiet exhaust ...i ran a corvette in saudi without any form of boxes whatever !!!! straight through pipe front to back and only a deep rumble/burble ....!totally stock 77 vette.....

11-06-05, 01:01 PM
Don't worry about the exhaust being "too quiet" until AFTER the SVA!

On my car, only a "diver" bomb thing (straight through silencer) on the end of the exhaust runs - managed to get it through SVA with the help of some wire wool stuffing.

11-06-05, 01:16 PM
In my brief experience, being quiet is not a problem if you're talking about driving experience. I know nowt about the other stuff but strangling definitely not a good thing.

No matter how quiet you think it is, will always make the right roar when you're gunning it!

11-06-05, 01:46 PM
Thanx guy's....will carry on with the boxes i have then...can always take them off after SVA!!!...cheers..steve

11-06-05, 02:27 PM
How quiet they are is not the problem nor is the physical size of the boxes.
The airflow through them is what could cause a problem.
If the bore of the silencers is too small you will (First of all) have trouble starting your car as the back pressure will stop fresh fuel/ air getting into the engine.
If you has straight through silencers (Meaning you could thread a hose pipe through them) and they have a bore of at least 2" for a Rover and 2.25" for a small block and 2.5" for a Big block (Just playing on the safe side)then you should be OK.:thumb:

11-06-05, 06:09 PM
Thanks Dave.....the back boxes i am using are 450mm (18" ) long...but all pipework i am using is 50mm (2")....rear boxes have offsetinlet/outlet....and have now managed to get a MIN of 100mm (4") ground clearance below lowest point...so i ham hoping all will be ok.

Have also included a 'balance' pipe between the two main pipes to equalise the pressure before it gets to rear boxes.

Thanks again for all who have helped....cheers...steve

12-06-05, 08:20 PM
427sc: Your system sounds good, I'd certainly stick with it, too many people seem to believe that loud noise = good noise. A quiet exhaust with a nice sort of muted roar when you nail it is great, and so much nicer to drive. Some of the big bore 'turbo mufflers' flow more than straight though mufflers like Cherry Bombs. Unless you have a very revvy motor, or the boxes are particularly restrictive inside, two 2" boxes should be fine. The balance pipe makes a big difference two, both to the sound and to the back pressure.