Decat = Induction noise (Maybe)

[Lazzer]

First tech post, new owner, first Caterham.. apologies now if not up to scratch with terms etc.
Been lurking on the forum while I made up my mind on purchase, so many thanks for wealth of advice, information and experiences..
Bought a Roadsport 150 from S&C in Feb, absolutely love the car but didn't take long to start tinkering and fitted a decat pipe a few weeks back,  largely for aesthetics but also for a little more noise, maybe power and to save the Cat. 
Looks better imho, bit better sound but can't say it's changed performance significantly. Maybe mid range but as said, I'm new..
Anyway, now the but..
After fitting, went out for a drive and the noise from the engine, I think induction (certianly not exhaust), was alarming. Thought a big end/s had gone, loud banging (like hammering wood), but only on hard acceleration and focused at 2800+ and with second resonance at around 4000/4500.
So loud I refitted the Cat, went out again and noise gone although I can still hear a similar background tone, same revs but way quieter. 
Happy that the engine was ok, I've refitted the Cat (which now takes 10 mins), and the banging is back.
I'm almost convinced it's induction and saw a description of "Induction hammer" which it kind of matches, but wondered if it was normal following a decat fit?
Have to say the sound is impressive, so long as it's not damaging..
I'm kind of thinking the removal of the Cat has improved airflow through the TB's and attenuated their sound? Maybe running lean now? 
Bit out of my comfort zone on knowledge so could do with some advice or others experience please.
Thanks
Lawrence
 

[Lazzer]

Sry, should have said its a Sigma 1600, 150, 2008. 'Caterham' TB's with low profile Piper cross foam filter.

I believe it has had the map upgrade, it doesn't exhibit and slow drive, in traffic difficulties ive heard of, in fact at low revs its as easy to drive as a tin top.

L

[John Vine]

First of all, welcome!

I'm a K and Duratec man, and know next to nothing about the Sigma.  However, your symptoms do seem very odd and I'm struggling to reconcile them with a simple swap of cat for bypass pipe. 

So, before the Sigma experts arrive, does your cat have a threaded boss for a lambda sensor?  If so, does the bypass pipe have a similar boss for this sensor?

Have you consulted S&C?  Do you have some sort of warranty?

JV 

[ChrisC]

So I think Johns question about the lambda sensor is valid.   The sensor on a standard 150 is attached to the cat, so a de-cat will either mean your moving the sensor from the cat to de-cat, or using a different sensor.    That said lambda sensor faults are very easy to spot once the car is warm and has been running for 60 seconds.     Also warranty should be considered. 

I don't remember any significant noise change when I put the de-cat pipe on my 150, a little harsher sound maybe but nothing significant.    Although I do remember taking the cat with me on my first de-cat track day just in case I needed it for noise test purpose, like you said simple 10 min change.   I was using the CC de-cat pipe, I assume you are to.

Indiction noise through the TB's is normal and the 150 makes a loverly sound when pushed. 

Any chance you could post a video of the difference?

The only think I could think of is maybe the cat is broken up inside, causing more restriction than normal, you could just look down the inside and see the honeycombed to prove that wrong.

[ChrisC]

Also get you hands on a Syncrometer, your going to need one come service time anyway and make sure the balance is spot on.   The number of 150s I have seen that are out of balance is shocking.  

[Lazzer]

Thanks guys, it's a cc decat pipe, one they recommend as replacement. The Lambda sensor boss is on the engine side of the Cat, so assume unaffected by a change. I simply unscrewed the sensor and refitted it into the decat boss which is just after the 4in1 joint. Same angle and position so straight swap.

Haven't got a syncrometer, tbh wouldn't know what/how at the moment but will investigate.

Thought about asking S&C but car runs really well, no running problems with/without decat, just really loud without.

Will try to capture on video and link to YouTube.

Thanks again.

[tomwood]

Intriguing,

Appreciate t's not the same spec as yours but I've just switched to a decat pipe on my Duratec which, like yours, has throttle bodies on it. I've got to be honest, I couldn't really tell much of a difference noise wise. It was bloody loud before I switched it over and it continues to be :)

I can't see how fitting a decat pipe could create a bang really, except perhaps in that unburnt fuel might get further down the exhaust before igniting. Could it be that? Where's the bang coming from?

Tom

[SM25T]

I would talk to S&C and visit if deemed appropriate. Don't leave it unreported.

[Lazzer]

Trying to capture the sound on a go pro but rain intervened..

Isn't it difficult describing sound..

It's really a hammering, best description I found was induction hammer, kind of like hitting wood but only on hard accl/load and at certain revs. Not evident without the decat pipe (ie with cat). Will give S&C a call but as stock, everything fine. Will try cc as well.

Still convinced its induction but like some of the responses, can't fathom why.

Cat was clear BTW.

Thanks guys.

[Tom_Arundel]

My 150 set up does that with standard filter, I also run an airbox which is a lot quieter and gives more torque, so `noise is induction...but I like it.

[ChrisC]

It turns out that Lawrence, and his loverly Roadsport 150 SV, live less than 15 mins from me, so we meet up this evening for a bit of tuning and testing.  So first a compliment, this is the best setup 150 I have ever seen, the only thing I found was the idle was a bit low (but only a tad).   

‘The noise, well it’s exactly the noise I remember (and miss) from my 150.  It may be harsher/ louder with the de-cat, but it’s nothing to worry about.   It’s that old school Weber noise :-)

Hope to see you on the next Essex meet / blat ;-)

Chris

 

[Lazzer]

Thanks to Chris for his time and knowledge. A wealth of info and learnt more about my 7 in a couple of hours than days of Forum lurking..

Mod of idle speed made quite a difference driving home and 'played' with the induction noise all the way...

[John Vine]

‘The noise, well it’s exactly the noise I remember (and miss) from my 150.  It may be harsher/ louder with the de-cat, but it’s nothing to worry about.   It’s that old school Weber noise.

Interesting, but I'm still puzzled.  Where does it come from?  Induction? Exhaust? And if induction, why is it louder with the bypass pipe?

JV

[Lazzer]

Hi John, definitely induction side, the decat pipe seems to attenuate the noise the TB's make at certain revs (2 resonances).

I could hear the same tone (same rev range), with the Cat fitted but way more pronounced without.

Can't determine why but my guess is its pressure wave related increasing sound as air flows into the trumpets, guess the Cat restricted a little and now better flowing.

 

But.. My fluid dynamics is a little rusty so I may be talking complete little round pink hairy things here....

 

Whatever it is, it's not mechanical, so I'm more relaxed, it sounds good and runs a bit better without the Cat, more so after Chris's resetting.

Will still try to capture a sound clip at some point.

[John Vine]

Thanks for that.  Intriguing.

JV