13 replies to this topic

[The-Womble]

Model: Sprite 1275 Carb
Year: 1993
Description of problem (please be as in depth as possible):
Not sure if this is the right section for this, so feel free to move it!

Ok, I'm competing in a navigational rally on Saturday. Did it last year no problem, but since then I've upgraded from the stock peashooter exhaust to a Maniflow single box system. It's a fair bit louder, and the MSA only allow a max noise of 98dB at 0.5m from the tailpipe at 4500rpm. I'm going to try and borrow a decibel meter early next week to check this, but if my car is louder than that, is there anything I can do to temporarily reduce the noise level?

I read that a traditional remedy is to shove a Brillo pad up the exhaust as it doesn't restrict gasflow (or not much anyway), but does send burning wire wool and smoke out the exhaust for the first few miles! Didn't really want to have to try that. Is there anything I could wrap the exhaust in or put over the end that wouldn't be detrimental to the engine?

Any ideas appreciated. Thanks!


Any non-standard parts that might be involved with the problem?
Maniflow Single Box exhaust + Stage 2 Maniflow LCB

[Dan]

Various motorsport suppliers sell bolt on tailpipe sileners to deal with tight restrictions at certain tracks.

[Broomer]

You might get away with it. ive got a maniflow single box system ( with the baffling cut out ) which was on my fairly well tuned 1293, and it made 97DB at 4500 rpm. so you should be alright.

itll be close either way.

[rawky]

Chris just remembered I read that directing the tailpipes down kills alot of the sound. Maybe get some flexi exhaust tubing and add an extension of sorts?

[bmcecosse]

I suspect it may take a whole packet of Brillos - not just one. Try it!!

[The-Womble]

Various motorsport suppliers sell bolt on tailpipe sileners to deal with tight restrictions at certain tracks.

Thanks Dan. Might be able to get something like this from CBS or even just fabricate something like THIS at work (if it's as simple as it looks?). Just as long as it doesn't create too much back pressure!

[Cerberus]

At a trackday that my Dad and I went to last year at Oulton park, there was a corvette going through the noise tests. Obviously failed first time, and came back the second time with what looked like a sheet of brillo pad material, just wrapped over the end of the exhausts.

This obviously enabled it to pass the test, as it was out on track, didn't take long for the stuff to fall off though, it must have only lasted 2 laps!

[The_Mistro]

no need for any of that, just alter the tachometer! theres a little screw you can adjust it with! or if you havent got a tachometer, how are they going to know what rpm your doing!?

Edited by The_Mistro, 13 March 2011 - 07:15 PM.

[Cooperman]

With a twin-box Maniflow I just scraped through the noise tests, but after 4 events even the tewin-box system was pushing the upper limits of what's allowed. The best thing would be to get a Maniflow 'Cherry-Bomb' centre box and fit that. I now run a twin-box RC40, with skids welded onto each end of each box. It's quiet enough, but not so well made as the 'Maniflow' systems.

[The-Womble]

Thanks for all the replies guys

Tested it today at ~4000-5000rpm. Came out at around 92/93dB so I should be fine

Tested my mate's Mini while we were at it, as he is also entered into the rally - his came out at 97; Cutting it pretty fine!

[davejf]

Our car came out at 96 with a rough test off an iphone app. i think i need to try and get hold of a proper meter to check it as the new engine went in this weekend and it's a bit louder than the previous one.
Got to say, 4500rpm seems a bit high, castle combe only check it at 4000 or 2/3rds max rpm.

Fingers crossed were all fine!

[The-Womble]

We tried with my mate's free iphone app, and it was pretty indecisive. Varied between ~90 and 110dB.

You can get some fairly cheap sound meters on Ebay (£15 ish). How accurate they'll be though, I'm not sure, and whether they'll come in time is another thing!

[davejf]

if we haven't got one at work then i think i'll be popping to maplins tomorrow night. they have one for £30 which is cheaper that the entry fee, hotel and cost of fuel for the 450 mile round trip just to get turned down!!!

[The-Womble]

Yeah, fair enough! Just get the revs constant before you take the measurement.

If it's close, you might want to get in contact with Bernie Surtees (the MSA Scrutineer for the event) and see if anything can be done or see how lenient he might be!! His number is on the website.