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Crossflow Noise
Knockout - 26/5/18 at 09:51 AM

Hi everyone, I'm just after a bit of advice please...

My 1.7 crossflow-powered Sylva Striker failed its IVA the other week on a few minor issues (a couple of edges needing rounding and side repeater position) but it was also about 1dB to loud, so failed on noise. I did a bit reading on here and thought that the best way to go might be to fit a do killer. I've just tried it out and it had the opposite effect and increased the noise by about 25dB, which I thought was quite impressive! So I am wondering what I have done wrong?

The silencer is the original re-packable Sylva item. I'm running twin dellorto 40's which only have filter socks fitted currently. The db killer noticeably increased induction and general engine bay noise, as well as a high picth whistle at the exhaust outlet as the air accelerates through the killer.

The killer itself had to be shortened to around 140mm but still has plenty of baffles. It also came with some packing material which I fitted.

Any thoughts on what I've done wrong or what to try next would be most welcome.

Thanks


Knockout

Sam_68 - 26/5/18 at 04:38 PM

I can't help with a solution, but I have to say that if you've increased the noise level by 25dB, it's very impressive indeed: when you figure it out, patent it - there's every chance that it could be used as a military weapon!

If I remember correctly from university, on the logarithmic scale, an increase of 6dB equates to a doubling of the sound energy, so you've somehow managed to more than quadruple the amount of energy that is being converted to noise!

Knockout - 26/5/18 at 07:40 PM

I was only using an app on my phone to take the measurement so was never going to be that scientific, but even without looking at the results I knew that it wasn’t going to be good! I might be on to something!

martin500 - 26/5/18 at 10:17 PM

quote:
Originally posted by Knockout
Hi everyone, I'm just after a bit of advice please...

My 1.7 crossflow-powered Sylva Striker failed its IVA the other week on a few minor issues (a couple of edges needing rounding and side repeater position) but it was also about 1dB to loud, so failed on noise. I did a bit reading on here and thought that the best way to go might be to fit a do killer. I've just tried it out and it had the opposite effect and increased the noise by about 25dB, which I thought was quite impressive! So I am wondering what I have done wrong?

The silencer is the original re-packable Sylva item. I'm running twin dellorto 40's which only have filter socks fitted currently. The db killer noticeably increased induction and general engine bay noise, as well as a high picth whistle at the exhaust outlet as the air accelerates through the killer.

The killer itself had to be shortened to around 140mm but still has plenty of baffles. It also came with some packing material which I fitted.

Any thoughts on what I've done wrong or what to try next would be most welcome.

Thanks


Knockout


Hi,

Where are you measuring the db levels?

Fitting a db killer will silence the exhaust, forget about increased engine noise because the test is the exhaust outlet. There are some good apps, I use SPLnFFT which is very accurate. On my car I was reading 115 db @ 8,000 rpm and repacked the exhaust and am now down to 99db, but my readings are being influenced by induction noise, with a cross flow the induction is the opposite side of the exhaust so shouldn't cause a problem. My engine is a bike engine with the carbs stuck out of the top of the bonnet so influencing the readings.

I would suggest that you repack the silencer with acousta-fil and run it as per instructions to get the acousta-fil up to temperature and expanded then remeasure the db level, I don't know what your silencer is packed with currently but acusta-fil is so much more efficient than standard fibreglass based packing. I would be very surprised if this did not fix your problems, if not try the db killer but make sure that you test as per the IVA test.

Martin

joneh - 27/5/18 at 06:26 AM

quote:
Originally posted by Sam_68
I can't help with a solution, but I have to say that if you've increased the noise level by 25dB, it's very impressive indeed: when you figure it out, patent it - there's every chance that it could be used as a military weapon!

If I remember correctly from university, on the logarithmic scale, an increase of 6dB equates to a doubling of the sound energy, so you've somehow managed to more than quadruple the amount of energy that is being converted to noise!


Close, 3dB is doubling the sound energy.

If the OP is now at around 125dB, in a work place, exposure to this would be regarded safe for no more than 3 seconds...

Knockout - 27/5/18 at 10:13 PM

Thanks for the advice, I'll try repacking if I can get the silencer apart.

I was measuring at the exhaust outlet.

Anyway, thanks again

907 - 28/5/18 at 02:50 AM

quote:
Originally posted by Knockout
Thanks for the advice, I'll try repacking if I can get the silencer apart.

I was measuring at the exhaust outlet.

Anyway, thanks again



This is a cut & Paste from the IVA Manual.

HTH
Paul G
(Note that with a phone you will need to face the front of the phone to the car if that's where the mike is.)



IVA M1 Inspection Manual
Document Uncontrolled when Printed
Method of Inspection
Required Standard


With the sound meter horizontal, set the microphone height so that it is at the height of the exhaust outlet or 200mm from the ground, whichever is higher.
a. At an angle of 45 degrees to the exhaust outlet in the direction which gives the greatest distance between it and the vehicle contour

b. At a distance of 500mm from the exhaust outlet.
With the vehicle at normal operating temperature, run the engine at 3/4 of its ‘maximum power’ speed, and note the sound level reading obtained.





[Edited on 28/5/18 by 907]

Knockout - 28/5/18 at 09:36 AM

Thanks

I had my phone sat on an axle stand in approximately the correct test position (as they had at the IVA test) with the microphone towards the exhaust.

It did occur to me that maybe I had the killer fitted the wrong way round. I've got the widest section down-stream?

I will have to re-pack the exhaust I think. In the meantime I've moved the killer to the section of pipe between the manifold and silencer to see if it makes any difference.

adithorp - 28/5/18 at 07:36 PM

I'd repack with accoustafil as it's far superior to anything else. Use the online calculator (just google it) but only use 2/3 of the amount it recommends as it's been found to give best silencing at that. Once you've repacked run it up to temp to melt the quilting threads and allow it to expand before measuring level.

mark chandler - 28/5/18 at 07:44 PM

Easy way to reduce noise is to just turn the outlet it so it points at the ground or under the car, once tested put it back where you want it.

I have a similar problem that is not that easy to sort as I sprint so have to be under 105db tested at every event, I am now looking at making a spiral baffle as I cannot fit any more cans into the system

[Edited on 28/5/18 by mark chandler]

Knockout - 1/6/18 at 01:47 PM

Thanks for everyone's advice on this...just a little update...

I was struggling to get my silencer apart (due to a welded bracket!) so I thought I'd give the db killer one last try, this time just after the down pipes and upstream of the silence in a section on straight pipe. This seemed to have a much greater impact (in the positive direction) lowering the measured noise by 4-6 db. So hopefully that will do the trick for now! I just hope it doesn't affect the drivability too much).

In its previous position it was fitted just after a 45 degree bend (immediately prior to the exit) so I guess the air didn't flow into the killer very smoothly.

Thanks again everyone.

ttalps2000 - 1/6/18 at 08:18 PM

re-check your emissions before IVA. I put my DB killer in the same location and it backed up all the gases and made it run richer! Just scraped through on my re-test because of it.

Knockout - 2/6/18 at 07:57 AM

Interesting point, I didn't realise that they would re-test the emissions. Is that done automatically as I've made a change to the exhaust? I thought that they would only re-test the fail items. Is there any other areas that will get another look? Thanks

ttalps2000 - 2/6/18 at 08:10 AM

They shouldnt, but will depend what side of the bed they got out of that morning!