Lotus Elan and Lotus Elan Plus 2 Forums

Has anyone recently had a noise test with the 'standard' twin box silencer ? I've a very strong suspicion that mine is over 100dB A when you get up to 4000rpm so a fail for most track days. I'm interested to know if sub 100 is possible with this type of silencer and if not, would a sprint back box achieve it?

2tmike wrote:Has anyone recently had a noise test with the 'standard' twin box silencer ? I've a very strong suspicion that mine is over 100dB A when you get up to 4000rpm so a fail for most track days. I'm interested to know if sub 100 is possible with this type of silencer and if not, would a sprint back box achieve it?

I would be surprised if it was over 100dB(A). My competition car is 104dB(A) with a TTR system, which drops to 98dB(A) with an additional back box. My standard Sprint is subjectively about as noisy as my competition car with the extra box.

£50 on eBay will buy you a SPL meter if you want to be sure.

Hi Andy, mine is also a subjective assessment !. My GT4 has recorded 98-99 on past noise tests and the Elan sounds much louder!!! It could be that the Elan silencer is carboned up and I don't mind buying a new like for like replacement if I can be sure it will come in below 100dB A

My twin box silencer has always been very noisy, particularly when the engine is cold. I've assumed it's probably because any sound absorbing wadding has long since vanished but I don't think it was ever quiet. It was even criticised for noise back when Motor tested an S4SE back in 1970. No numbers back then of course but my iphone noise level app now reads 93-5 db inside the car at around 3000rpm. Whether that's realistic I've no idea but it's ear plugs level on a long run.

That sounds familiar ! I'll try the app inside the car tomorrow for comparison (don't have anyone available for an outside test by the exhaust) but from your reply and other comments I've read the twin box may not be an option for a track day. Shame as the sound up to 3-4000 is just right for a sports car. I was hoping that a modern copy of mine or even better getting an exhaust builder to open it up and repack it might achieve the right level.

2tmike wrote: I was hoping that a modern copy of mine or even better getting an exhaust builder to open it up and repack it might achieve the right level.

I have considered doing that but it's two silencers to cut through and a lot of welding / rivetting etc to put it all back together again. As my car is road use only track noise limits are not an issue for me so I've tended to leave sleeping dogs lie but it would be nice to have it slightly quieter inside.

I'd also be interested to know if anybody has a solution to this. My mild steel S4 twin box silencer is louder than I would like. And was from new. Needless to say, it sounds wonderful at low revs/speed.

Has anybody tried fitting a glass pack in the intermediate pipe under the chassis?

When I purchased the car it had a non-original Sprint style stainless steel box which sounded tinny and dreadful.

Nick

Just now measured my car in a similar way to that used on track day tests (500mm from the exhaust at 45deg angle) I recorded 102dB A at 3200 RPM with the aid of a phone app and a length of wood holding the throttle open. God knows what it is at 4000....

This is a useful thread....

https://lotuselan.net/forums/viewtopic.php?f=39&t=36438&start=

Regards
Richard

It is Richard, I was thinking that way but getting a fabricator to cut and reweld the boxes. Mine's a Falcon as in the thread and although its really well made I bet there's nothing left inside it.

I am sure the standard (transverse) silencers never had packing in them and doubt later ones did. My present mild steel one has not changed character in 24 years and 80,000 miles, and peering through the splits (I've patched) it is an empty box. Fitting a 4-branch in place of the expansion box made little difference.

Did the later silincers really enhance the power? The transverse box is just a pipe with holes in it so pretty free flowing.

my car had a newer stainless twin can that was quite load. I was given the original mild steel version with the car and find it's quieter. I prefer it. My experience is that stainless muffler are louder than mild steel.

Just a little update... after looking at options I bought a MS Jetex mid box just to see what effect it would have. It's 80mm OD, 400mm long for the can and has 45mm ID connections. Using an iphone app I measured the car at 4000 rpm stationary as follows :
Before fitting
In the drivers seat 91.6 dB A
At the exhaust, 0.5m and 45deg angle 107.8 dB A
After fitting
In the car 86.5 dB A
At the exhaust 99.2 dB A

The Jetex box fits nicely just before the standard silencer boxes with good clearance to the chassis and still allows reasonable ground clearance. On the road the difference is dramatic. with the cacophony of sound above 4000rpm very much reduced. The only downside, for me anyway, is that the low rpm rasp and crackle is also reduced.
Now if I just knew if the phone app was accurate I could take the car to Castle Combe....

Excellent!

Could you post a photo? And perhaps the model/part number of the Jetex?

Nick

Part number is U404500 for the box and UA45-42 for the 2 spacers used to reduce the silencer pipe ID from 45 to 42mm to give a good fit over the intermediate pipe which is 41mm OD. I bought these in mild steel for £50 delivered in the UK really just as a trial. As it was successful I might go for their stainless version later on and have it welded to the intermediate pipe rather than clamped.