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IVA Noise Test Fail due to Carbs?
mark.silcock - 4/7/21 at 10:01 AM

Has anyone had any issues with the noise test picking up the induction noise from the carbs?

Mine failed the test at 103db after repacking the silencer, I have since added another small silencer and a db killer in the end of the tail pipe. The exhaust now is very quiet and all the noise is from the twin 45 weber carbs.

I used my phone to test the noise difference before and after adding the additional silencer and db killer and it was pretty minimal so wondered if the carbs were the main noise being registered.

Just dont want to fail again so am wondering if I should change the filters to quieten them down as the only have filters on the ends of the ram tubes.

https://rimmerbros.com/Item--i-RX2135

Deckman001 - 4/7/21 at 10:39 AM

Hopefully the inlet noise is on the other side of the exhaust so shouldn't be an issue, the testers doing mine allowed the engine revs to slowly grow up to where they are tested, you may also be able to over rev it then allow it to drop down to the correct revs?

Jason

cliftyhanger - 4/7/21 at 11:38 AM

You really do not want those poxy mesh things on the trumpets. The kill power and still let stuff in (but do stop stones and bigger getting in)

Best is an airbox with ducting and a cone filer, or a sausage filter. The airbox will be quieter by some margin.

[Edited on 4/7/21 by cliftyhanger]

Westy1994 - 4/7/21 at 12:09 PM

As above, either build or buy an airbox, I built one about 4 years ago, nothing to do with IVA but in my aging years I just wanted a quieter car overall, I fitted a second silencer as a rear exit , but then, just as you have found out the induction was now the loudest sound , I had the normal K&N type chrome filters, and as pretty as they were they just had to go.

Even if you get one just to get through IVA it can removed afterwards and back to your original setup, but you may get to like the muted tones of webers rather more than the rasp they once did.

mark.silcock - 5/7/21 at 08:33 AM

quote:
Originally posted by cliftyhanger
You really do not want those poxy mesh things on the trumpets. The kill power and still let stuff in (but do stop stones and bigger getting in)

Best is an airbox with ducting and a cone filer, or a sausage filter. The airbox will be quieter by some margin.

[Edited on 4/7/21 by cliftyhanger]


I know all about filters killing power, see rolling road where RamAir foam filters killed 55HP and wouldn't allow the engine to rev above 3500rpm!

The dull yellow line is with the filters on, the bright yellow line is with no filters.

https://photos.app.goo.gl/3uxe6zp7wdwNwBfm6

The mesh filters were recommended by the rolling road operator and technically if they are letting things in like you say then they cant be strangling the carbs.

gingerprince - 5/7/21 at 08:39 AM

quote:
Originally posted by mark.silcock
I know all about filters killing power, see rolling road where RamAir foam filters killed 55HP and wouldn't allow the engine to rev above 3500rpm!


If it didn't rev past 3500 then your jetting was wrong - filters would have changed the AFR. You need to have the right jets for the right air setup (inlet and exhaust). If you change either, then you need to re-jet accordingly. If you don't, you'll get what you saw.

mark.silcock - 5/7/21 at 08:52 AM

quote:
Originally posted by gingerprince
quote:
Originally posted by mark.silcock
I know all about filters killing power, see rolling road where RamAir foam filters killed 55HP and wouldn't allow the engine to rev above 3500rpm!


If it didn't rev past 3500 then your jetting was wrong - filters would have changed the AFR. You need to have the right jets for the right air setup (inlet and exhaust). If you change either, then you need to re-jet accordingly. If you don't, you'll get what you saw.


Sorry you dont understand, the rolling road operator had finished tuning and jetting the carbs which is the bright yellow line. He then put the filters on in case they needed a minor tweak but was shocked to see the car not rev past 3500 and be 55HP down.

pigeondave - 5/7/21 at 09:29 AM

The issue (I believe) is how close the filters are to the ends of the trumpets and the volume of air within the filtered area.

The same can be said with air boxes.

There's a rule, something like 1.5x diameter of the inlets away frim the face of the trumpet.

One of the guys on here changed his Fury homemade airbox to one which had more volume due to an increased height above the inlets.
He saw a massive change in the AFR's to the point he gained horsepower and was running a dangerous AFR.

The way I see it, the 55bhp loss you saw, I think is due to a poorly spec'd filter which may have been dirty, hence the restrictions.

There's a reason why they make different heights of filter, its not only to get over long trumpets, its to increase volume and surface area of the filter material.

You don't have to do a lot of googling/YouTubing to see how much plenum chamber volume affects performance.

mark.silcock - 5/7/21 at 09:34 AM

Anywhere this was not my original question but these threads always seem to get sidetracked by an opinion or two

Does anyone think the carbs can affect the IVA noise test even though they are on the other side of the car pointing away from the tester?

David Jenkins - 5/7/21 at 10:03 AM

I doubt if they will - the sound meter is placed quite close to the exhaust (1 metre?) and a set number of degrees to the side (so it isn't getting direct pulses of exhaust). The body of the car will be between the meter and the carb intakes.

There's nothing in the IVA that says that they must test intake roar - just make sure that the tester doesn't park next to a wall on the intake side, to ensure that reflected sound doesn't influence the result. I believe the manual says that the test must be performed in open space, and this certainly was the case for my car's SVA. My car was out in the middle of the car park, shortly after he'd done the self-centering test. However, I don't know if there's scope for the tester to comment on the noise...

It's something you'll have to sort out if you ever want to do track days, as they will check the total noise coming from the car.

cliftyhanger - 5/7/21 at 10:42 AM

quote:
Originally posted by mark.silcock

The mesh filters were recommended by the rolling road operator and technically if they are letting things in like you say then they cant be strangling the carbs.


On a proper filter either foam or "corrugated" cotton (eg K+N) the area of the filter is huge. On the mesh ones you have, the area is a tiny fraction. So the mesh is eating a huge proportion of the intake, and messing up the airflow with turbulence at a critical stage.

It is the hole size that lets small particles, eg sand etc, in. A proper filter will prevent that. OK if it is a race car with very limited life expectancy, they usually run open trumpets.

pigeondave - 5/7/21 at 11:02 AM

quote:
Originally posted by mark.silcock
Anywhere this was not my original question but these threads always seem to get sidetracked by an opinion or two

Does anyone think the carbs can affect the IVA noise test even though they are on the other side of the car pointing away from the tester?


Yep, when I was running carbs with a big ITG foam filter the car was very loud.

You've done your own testing and seen that sorting the exhaust has made marginal differences. It's the only thing left making noise.

gingerprince - 6/7/21 at 07:43 AM

quote:
Originally posted by mark.silcock
Sorry you dont understand, the rolling road operator had finished tuning and jetting the carbs which is the bright yellow line. He then put the filters on in case they needed a minor tweak but was shocked to see the car not rev past 3500 and be 55HP down.


Again not the original topic, but you're saying he tuned it without the filters on, and then put the filters on and it impacted the performance?

I stand by my original statement. If you want to run it with filters on, you tune it with the filters on...

mark.silcock - 6/7/21 at 08:12 AM

quote:
Originally posted by gingerprince
quote:
Originally posted by mark.silcock
Sorry you dont understand, the rolling road operator had finished tuning and jetting the carbs which is the bright yellow line. He then put the filters on in case they needed a minor tweak but was shocked to see the car not rev past 3500 and be 55HP down.


Again not the original topic, but you're saying he tuned it without the filters on, and then put the filters on and it impacted the performance?

I stand by my original statement. If you want to run it with filters on, you tune it with the filters on...


Clearly that's what he was about to do but no filter should take 55hp out of an engine that only has 135. The tune after putting the filters on should be pretty minor.

mark.silcock - 6/7/21 at 08:42 AM

You can check my thread here on everything we tried to cure the engine cutting out at 3500.

http://www.locostbuilders.co.uk/viewthread.php?tid=215122

Like you we thought it was a fueling issue, then an ignition problem.

It was a air starvation problem caused by the bloody filters.

Deckman001 - 6/7/21 at 03:54 PM

Hi Mark, at least with the filters on, you will have a quieter engine and with less power, it will then be a bit quieter too. So when you get your car tested at the IVA, the noise will be ok and as long as the emissions are sorted with the filter you should be ok, then after the test you can then get it sorted at your leisure.

Jason ( not that I talk from experience )