air box's

[judgie]

running 48 mm throttle bodys with 120mm trumpets on my race engine, makes 149bhp@rear wheels.
need to make a air box to try and reduce the intake roar, moving the silencer oulet to infront of the right hand side rear wheel. i have to reduce the nosie to meet the sound regs i have in the class i race in. currently is 105db@3/.4 max rpm would like to get it down to 100db.
what i need is some info on airbox volumes and the boxes that will go over the trumpets.
was thinking of some tube over the trumpets like you see on most turbo set ups then running a large bore tube to a central airbox with the air intake being a scoope. inlet being outside the engine bay between the deck lid hinges, triming the deck lid so it still fits in the normal way.
any ideas on dimensions for the airbox and "logs" on top of the trumpets?
another option is seperate boxes on each throttle body with a intake in the scoopes in the rear 1/4 panel, empi style 1/4 vents.
want a cold air feed to these as at the moment i have just the open trumpets taking air from the warm engine bay.
cheers rob

[Humble]

What are you running for a muffler? I was able to get my race car in spec with just a muffler change. I'm using a borla 2.5" straight through 5"x10" round can packed with fiberglass and really quiets down the motor.

[judgie]

its running a 400mm long x150 dia 2.5" bore straight through box. problem i have is the sound meter is help .5m from the tail pipe so the meter is picking up all the background engine sound as well. striaght cut cam gears, porsche fan and the intake roar. hence why i'm moving the exhaust exit to in front of the rear wheel, gets the sound meter further away from the engine so hopefully should make a bit of a differance.

[Steve C]

Hi

Do they give an allowance for the motor being air cooled and rear mounted, they should.

A friend had a similar issue and he put some sound insulation on his engine lid which really helped.

Steve

[evilC]

Hi Rob,
I understand the problem as I had a similar one on my Imp road rally car that was marginally over. I used to solve that one in one of two ways 1) reset the tacho so that the revs always read higher than the test limit i.e. at the 5k rpm test point on the tacho the engine was only revving to 4.5k rpm and 2) To always stand over the test point that added some absorption that was usually worth 2dBA.
I think Steve's idea of sound insulation under the deck lid is valid, although weight inducing.
On airbox volumes I thought that the airbox should be 1.5x the swept volume served but I will be corrected on that one. As intake noise will be adding to the woes it would be well worth insulating the airboxes and the connecting pipework that will reduce noise and air temp absorption. Also, it would be worth considering the direction of flow and creating as smooth a flow as possible (pipes dumping into a can ontop of the carbs ought not to flow properly). Air filters can help in reducing noise and smoothing out the flow if you haven't got them already.

Clive

[judgie]

i did think about just conecting the trumpets to pipes running to the air box but had decided that would alter the intake length and as we spent quite some time on the dyno playing with trumpet lengths to get the best torque i would rather not mess with that.
deck lid wise its going to be replacecx with a fiberglass one pretty soon which might help a bit but it has no rear valance so prob not that much.
think i'm just going to have to see what will fit in there and hope it works.

[judgie]

opps double post

[evilC]

I take your point about intake length but the issue of turbulence before the trumpet is a valid one. If you look at EFI inlet plenums the airflow into the plenum box is as smooth as possible so that the intake runners can get the cleanest flow possible. The classic aircooled box bolted ontop of the carb, within which the trumpets sit and where the intake air is a pipe fixed on the side of the airbox just looks all wrong from a clean airfow point of view. With open stacks at least the effective plenum is the engine bay so the carbs can expect a reasonable laminar flow. Put a can around the stack with only a single entry point that is usually below and to the side of the stacks and you cann see that the air has got to be pretty mixed up before it gets to the entry point of the stack.