I need to quieten the Beast

[Zuffen]

I had the engineer out on Friday t certify the miriad modification to my car so I can drive around the streets and not fear the Police.

He picked a couple of minor easily fixed issues but noted that 104 decibels was too loud for the exhaust. I've been concious of the exhaust since I first had the supercharged engine running and went from straight through 3" mufflers with 3" resonator down to 2 1/2" mufflers and no resonators to soften the noise a little.

The exhaust system is 4 into 1 headers > cat converters > tripple pass 21/2 inch mufflers > up and over the axle and out the back. All piping is 3" mandrell bent.

The resonators were deleted as the monkey who built the exhaust arranged them so they would hit the rear diff housing went the suspension dropped. Good move.

I don't have any cross over or balance pipe on the car.

I need to get rid of 8 decibels but try and retain the power I have.

I'd thought about reducing the diameter of the pipes after the mufflers to 2 1/2" but I don't feel this will have enough effect.

It would be possible to squeeze another 2 mufflers under the rear of the car but they would only be 200mm (8") long!

I'm happy to instal a balance pipe if that would help.

So everyone knows what I'm dealing with the engine runs 21.5psi boost via a twin screw and is fully built to handle it and when finally tuned should turn out 450-500hp at the wheels. I think I'll die of old age beforre I ever see it but that's what all the experts say.

I'm open to suggestions.

 

[cribbj]

Rod, the SuperTrapp discs are popular with the GT40 crowd when they have to meet noise regs, then they're simple and quick to remove when they don't.

http://www.supertrapp.com/product_sections/cat.asp?CatID=27

 

[Zuffen]

Unfortunately I don't have either the length or the diameter to fit a Supertrap.

Looking at the figures a 5" unit would be the closest to my needs but stops a few hundred horsepower under my target figure.

I don't think I can squeeze any more muffler (by having larger or more of them) under my car.

I feel pipe diameter will be the answer plus a cross over pipe but I'm no exhaust expert so I'm only guessing.

 

[JustenGT8]

A decent cross over setup will help. Pipe dia not so much. Well designed mufflers should do the job so might need to play with some other options?

Can you get a larger muffler mounted across the rear? If you run the pipes into this and across you kill two birds with one stone...more muffler and acts as balance pipe...admittedly a ways back from where it's needed for flow but will help mellow out the note. The sharper note from an unbalanced twin system will be adding quite a few dB.

 

[Zuffen]

I spent half an hour this morning laying under the car. My wife thinks I've lost it.

I can fit a cross over/balance pipe just in front of the existing mufflers so this may help. I'll take it up to my exhaust man and see what he thinks. I hope he doesn't want the pipes removed fromt he car as I need to remove the transmission to get one out. That or I couls remove the body.

Justen,

I can't fit any larger mufflers under there.

The large single rear muffler isn't impossible to do. I could take the driver's side pipe across to the passenger's side and then feed them into a single 3" muffler.

Let's see what tomorrow brings.

By the way, my engineer is John who did your car. He was a client of mine for about 15 years and I know him through his XJ13 project. Doens't meam he's making it easy for me!

 

[JustenGT8]

I need to line John up again for my V8 Hilux.....no rush though.

 

[JBrady]

With catalysts AND 2.5" 3 pass turbo style mufflers I am surprised it is still too loud. Is the measurement made at full throttle? Depending on where the measurement is made it may be including blower whine AND intake noise. Intake noise can be very loud by itself and a muffled air filter system is found on nearly all production cars to control this component.

Back to the exhaust: Definitely add a cross over pipe as that alone may do more than expected depending on its location. I doubt that going with 2.5" pipe from the mufflers back will hurt power as the mufflers will be the choke point and the smaller pipe will reduce sound slightly. The smaller pipe may allow you to fit some straight through resonators like the ones below.

 

[Zuffen]

The balance pipe goes in tomorrow.

The noise was checked without the charger whine and the intake is well muffled as it's a pod in a box that breathes through a large intake tract built into the bonnet that takes air from above the bonnet and at the front of the bonnet.

I simply don't have room for any more mufflers unless I go to a single rear pipe (which is near impossible to fit under the car) and run a muffler mounter across the rear of the car.

My next step will be to drop to 2.5" pipes after the mufflers if the balance pipe doesn't have any appreciable effect.

The car is loud enough that I don't bother with the radio as it has to be so loud it's embarrasing when you pull up at the traffic lights!

 

[WDoherty]

i have dropped pipe size after the muffler before and it reduced resonance greatly

 

[Quadcob]

I had mufflers made up for my Cobra that used about 3.5 inch tube with an auger (that is used in wheat silos) welded in. In the picture they are the four pipes after the x pipe. They reduce the noise to about 97db. If you can fit similar style mufflers in along with you other mufflers it could help reduce you noise to the required level. This style of mufflers has little or no restriction.

Warren

 

[JBrady]

That is beautiful craftsmanship. I have not seen anything like your design. I take it the insides of each of the "football" shaped tubes has a spiral insert of some sort your "auger". What model Cobra is this? In the states Cobra can be Mustang and or Shelby. What engine combination? Racing? Very nice floorpan. Did you do a before and after dyno or performance comparison?

 

[JustenGT8]

Interesting. That concept has been around for years and was tested on a local rally car and ditched as it was quite restrictive...they played around with a few blade pitches and internal tube dia but anything quiet was restrictive....they didn't use a double set of pipes like you have done though to get the flow back. Nice work if you have the room.

 

[Zuffen]

The pipe is in and the exhaust is certainly quieter but still clearly above 96dba.

The auger mufflers sound interesting. Never seen it done before but it makes sense.

We'd toyed with putting spirals in the short 8" rear section of the pipes hoping that would have an effect by distrbing the pressure waves. Funny how we were thinking that way allready.

I'll twist up some 3" flat and see if can get the noise down with that. I figure if we make it to fit in each rear straight section it may just soften the noise enough to see us out of the woods.

 

[Quadcob]

Justen
The exhaust pipe shop that did my exhaust uses this technique for mufflers where the need for ground clearence is required.


JBrady
The inside of the football shaped pipe is were the auger is attached.
The car is a Cobra Concept replica. The motor is a quad cam 5.4 ltr Ford with a T56 gearbox. No before dyno runs as the exhaust was custom made for the car. The car will be used for street only(registered yesterday).

 

[Zuffen]

That's the most professional looking interior I've ever seen in a "Kit car".

What's it out of?

 

[Quadcob]

Rod,
The donor car for the build is a BA/BF XR8 or GT Falcon. The dash, consol and arm rests are from the Falcon and the seats are SAAS recovered in leather to match the colour of the dash.

Warren

 

[spf_lexus]

Since you said you do not run a balance pipe, you might consider running an X pipe. They knock out a large amount of noise, and cost you no hp. It may even make a few hp. I run rush 2.5" long tube tri y's, no cats, no resonators, mandrel bent at 2.25" that dump into 2 magnaflow straight throughs. At idle its almost silent and very quiet until 3500 and up. Frankly im looking for more dB, so we sorta have opposite goals

 

[Zuffen]

An X pipe would be easy as I have one laying around doing nothing from a previous incantation of the car.

The attached photo shows the severe lack of room. You can see the balance pipe under the handbrake drum on the rear of the trans.

I plan on re-installing the 12" resonators mounted across the rear of the car to see if that hepls.

 

[JustenGT8]

Got a shot of the rear post diff?

 

[Zuffen]

Here's a couple of photos.

One from the front and one from the rear.

As you can see there's less than 300mm behind the diff.

At this stage we're going to try and re-instal the resonators that were mounted across the rear of the car. Unfortunately when the system was built the fabricator (this is one of the few jobs I didn't do myself) had it arranged that the diff hit the resonators when the suspension rose.