More IVA prep! Noise and self centering
 

I whipped the car out of the garage yesterday to do some IVA prep.

Tested the noise level and I'm getting 105 dBa under IVA conditions (2/3 max RPM, 45 degree, 500mm away). This is using an expensive properly calibrated meter from work, so the reading should be spot on.
Anyone got any quick and dirty ideas how to get it down by 6 or 7 dB? :eek:

My other issue is that I don't appear to have a shred of self centering :rolleyes:
I'm not sure how IVA man tests this, but I couldn't notice any driving up and down my (private of course ;) ) road.
Does anyone know how it's actually tested, and possible fixes? Preferably ones that don't involve making new wishbones!

The IVA man drives it round the test center.

Adjust your tracking out slightly and see if it improves.

TT

To get my noise down I put a aluminium ring / donut in my exhaust, before the silencer. Ruins performance but eats noise.

Hi John ....if your silencer is removable, make another decibel killer and put it in before the silencer, same as what I did on mine but you will have to use much smaller tube but it will kill some decibels perhaps enough to get you below the threshold.

Bang on TT!
I went from a couple of degrees of toe-in to a couple of degrees of toe-out and I've suddenly got a massive amount of self centering!!
It goes from full lock to almost straight ahead all by itself now :D

I've also had a bit of a play with the exhaust.
Without the dB killer it's 107dB
With the dB killer it's 105dB
With the dB killer wrapped in wire wool it's 96dB!

Strangely I think it actually sounds nicer like that, it possibly idles better too.
I suspect I may have lost a few horses at the top end though!

No problem...

TT

On a slight tangent here but bare with me...
When it comes to exhausts every body says 'oh you need it to be 100% free flowing' to get the best power, and for racing you need the engine to hit max rpm in every gear. On the road in the real world it's all about low down torque and drivability and this is where you need a little back pressure in the exhaust to give the pistons some thing to push against which gives you torque. The reason the car idles better is that the engine is not trying to run away with it's self, as it now has some thing to push against ie the little bit of back pressure your making it work for a living and controls the rpm better.

a rise in tyre pressure will also help more with self centering ontop of toe out.

as for iva it says
With the vehicle at normal operating temperature, run the engine at ¾ of its ‘maximum power’ speed, and note the sound level reading obtained.
Note 1: Manufacturers drain holes are permitted in silencers
Note 2: The exhaust may have several outlets up to 300mm apart connected to the same silencer. In this instance the microphone must face the outlet closest to the vehicle contour or highest from the ground. In all other cases, separate measurements must be taken for each exhaust outlet; the highest value obtained is the test value.
Note 3: Where the engine maximum power speed is not available, the test may be carried out at 2/3 of engine maximum design speed.

i stated my engines max power speed in my iva application this speed is 5500rpm for my exact zetec model not 6800-7000 max design speed

For the MX5 engine I think max power RPM is about 6500, and max RPM is about 7100.

3/4 of 6500 = 4875
2/3 of 7100 = 4728

So not a lot in it really!