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Exhaust. Can it be quiet AND efficient?
cliftyhanger - 21/10/15 at 10:18 AM

Background. 1800 zetec in a spitfire, and dunnel (jenvey) TB's and ECU.
Currently fitted with a "twin sports" exhaust. Basically straight through to a y piece and a pair of oval silencers (straight through type)

At brands recently the tester reckoned 89/91db, and at donnington it didn't upset the drive-by meters. However, it is just too loud especially on a long motorway stint (we recently did 10 countries, 5 days 2 1/2k miles in europe, so fair distances covered)

I have had teh following thoughts:
1. Add a long, maybe 2foot, thin silencer between the chassis rails. I can get 3 1/2" in there just. 4" will rattle.
2. Possibly a short, 8" box close to the manifold.
3. Cut open and repack silencers (not keen, even new systems like this one are renowned for being loud)
4. Fit an efficient but quiet silencer.

I think the centre silencer may be a good place to start?

Bearing in mind my budget won't stretch to a full custom system, I did wonder about using std motorbike cans at the tail end.
If the cans are restrictive, would 2 in parallel reduce the restriction or would it also increase noise?

I did pick up a BMW K1200s exhaust locally to find it has a cat fitted, and peering in the outlet it certainly isn't straight through. But has anybody chopped bike cans open? Can they be OK? Any suggestions?

Ta!

MikeRJ - 21/10/15 at 10:53 AM

quote:
Originally posted by cliftyhanger
I did pick up a BMW K1200s exhaust locally to find it has a cat fitted, and peering in the outlet it certainly isn't straight through. But has anybody chopped bike cans open? Can they be OK? Any suggestions?


OEM bike cans for big four strokes are never straight through, it's only the (noisy) after-market cans that are like this. If you take an original bike can apart you'll find they can be amazingly complicated inside with multiple chambers connected by various length tubes (helmholtz resonators).

e.g. inside a Hayabusa can:
Inside Haybusa can
Inside Haybusa can

and the very complex internals of a KTM RC8
KTM RC8 Silencer internals
KTM RC8 Silencer internals

It definitely possible to have a reasonably quiet straight through exhaust however, you simply need enough silencer volume in the system. On a friends Civic VTi rally car he has three straight through absorption type silencers (one small, two large) and it's almost as quiet as the original system, usualy gives mid 80s on sound testing.

[Edited on 21/10/15 by MikeRJ]

[Edited on 21/10/15 by MikeRJ]

cliftyhanger - 21/10/15 at 10:56 AM

Wow! they are undoubtedly designed to be quiet by the looks of it. I wonder how much power they sap?

MikeRJ - 21/10/15 at 11:07 AM

quote:
Originally posted by cliftyhanger
Wow! they are undoubtedly designed to be quiet by the looks of it. I wonder how much power they sap?


They are invariably a little restrictive which is one of the reasons companies like Akropovic have made a lot of money.

This shows the gains to be had on a Hayabusa. However, on a bike the can is effectively almost right at the end of manifold secondary so restrictions tend to have a larger effect.

cliftyhanger - 21/10/15 at 11:33 AM

That is really helpful. Thank you. It is good to hear from people who know their stuff!
Now, If I grabbed a pair of cans, in parallel, the restrictions should be halved (more or less) and hopefully still quiet.
I don't think the BMW can will make it, after something a bit smaller! And ideally cheap.

Mr Whippy - 21/10/15 at 11:36 AM

Tbh I’d not have bothered with so called sports silencers at the back and just fitted a pair of standard baffled ones of a suitable production car, there’s no way two full size silencers with say a 2 inch pipe would offer any kind of restriction and at the same time would make the car a pleasant place to be in on longer journeys. These days I hate being in load cars as they just suck and you can’t even listen to the radio.

cliftyhanger - 21/10/15 at 11:49 AM

I was gifted the sports system, so can't complain.
But yes, a pair of std bike can may be worth a try.
Bearing in mind a 1000cc bike produces 150-170 bhp, broadly similar to my car, a pair of them should be good enough and not sap too much of that expensive power.
And yes, I agree. Loud is not a good thing.

I am also ditching the K+N's and fitting an airbox with a cone filter to reduce induction noise. Will mean I can fit longer trumpets, so should balance the exhaust losses.

russbost - 21/10/15 at 01:09 PM

An exhaust can certainly be quiet & still be efficient, it's all about allowing room for gases to expand thro' sound deadening material but without restricting (significantly) the flow of gas, hence why an expansion box near the front of the system can help a lot simply because it allows gases to expand & slow down somewhat before travelling the rest of the system length. Fitting "standard" type silencers as fitted on your average tintop may be too restrictive, but choosing silencers designed for a much larger/more powerful engine could certainly work well.

If you have K & N's on it they will make a MUCH more annoying noise in the cockpit on long distances than the exhaust will with the sort of dB output you say you currently have

SJ - 21/10/15 at 01:27 PM

I think your problem is two parallel silencers. Just get one decent silencer and noise will be massively reduced.

cliftyhanger - 21/10/15 at 01:56 PM

Thank you gents.

So, long thin one of these
2" (50mm) 3.5x26" inch Round Universal Stainless Steel ProBomb Exhaust Silencer
fitted early on should help a fair bit. Then I could fit the big old RS turbo scorpion box I have, that is straight through but worked OK IIRC.

Hmm, this won't happen for a month or so. I am also getting the bits to fit a scooby diff in the car so it makes sense to do it all in one go.

Dingz - 22/10/15 at 11:00 AM

Just a thought as the sound levels from the exhaust is fairly low, could it be the exhaust mounts making the noise resonate in the car?