Electronic Bi-Modal Valves

[RevNev]

Hey @RevNev, do you have any more photos of what you did to the internals? Currently in the middle of changing my exhaust around and looking for some out of the box ideas. Currently testing out HM headers, Manta centre section with DPE bi-modal mufflers. Previously had DPE centre section in place.

I think most aftermarket rear mufflers are too small and generally speaking in most sports exhaust systems, you need the "biggest" rear mufflers that'll fit in the car and the stock muffler can is a good size. Alternatively if starting from scratch, I'd probably roll up a round muffler can at least 8" in diameter and 14" long.

The design I call the "Lukey open chamber", is a design that Lukey Mufflers (VIC) pioneered in the 1960's using an open centre chamber in a straight through perforated tube fibreglass packed muffler to introduce a Helmholtz resonator to further quieten the exhaust system. If you imagine welding two 12" hot dogs together with 3" inches of straight 3.5" pipe in the middle, you'd end up with a hot dog 27" long with an open centre section and no perforated tube and fibreglass packing. Lukey also did their single inlet/dual outlet mufflers the same way and that's how I modified the stock bimodals.

The inlet end of the muffler is a 2.25" perforated tube fibreglass packed section and 3" of open chamber in the centre. The outer side of the muffler can, has a twin 2" perforated tube fibreglass packed section and the inner tailpipe has the bimodal valve attached. When closed, the exhaust enters at 2.25" and reaches the open chamber in the centre (Helmholtz resonator) then steps down to exit in one 2" outlet. Open of course, it exits via both 2" outlets and flows more than the 2.25" inlet. Twin 2" flows the equivalent of a 2 5/8" single pipe.

Ultimately, we're reducing the muffler flow and noise level by at least 50% with the valves closed.

In relation to a VF LS3, I'm testing the HSV 340 headers, 1 cell cats (use your imagination), and the 2.5" HSV R8 midpipe. I've changed the two 12" hot dogs to 12" long 4" diameter 2.5" perorated tube fibreglass packed resonators as the factory hot dogs step down to 2.25" I think is too early to step down from 2.5" to 2.25". With only an X-pipe and the 2 resonators in the midpipe with the modified bimodals, it's about as loud as your typical 2.5" bolt on with the valves open, loudish but not over the top loud to attract police attention. In fact, it's quieter than I expected to be honest.

Why I'm not jam packed with midpipe mufflers/resonators, is the size of the rear mufflers the aftermarket never use. They go too small in the rear mufflers then try and quieten the exhaust with midpipe muffling like DPE with their 3 foot long hot dogs (crazy!)

Here's the midpipe that isn't overly muffled compared with the aftermarket when using "big enough" rear mufflers they tend to avoid.

 

[hjtrbo]

@RevNev got up and close with Harrop (DPE) bi-modal rear muffler today. No wonder the thing booms with the flaps open on idle. They've got it set up pretty close to how I crudely drew it. 3 odd inch through the large diameter path (bi-modal open path) and roughly 1-3/4" through the narrow path (bi-modal shut path).

Been doing some further reading on the subject and I can see the design you have chosen to go with is good with that centre resonator chamber. What I'm thinking is I already have j-pipes on the inlet to my mufflers to cancel the drone I was getting at around 1800rpm.

So to me it seems the ultimate sports muffler would be of the straight through glass pack type with a resonator chamber (as you have constructed) and then for my specific case, the addition of a tuned helmholtz chamber integral to the muffler to specifically target the drone frequency.

 

[I Wish]

It seems like that this is DPE's standard design that they use for Harrop and Walkinshaw as well. My Manta's are obviously the same. On my 2.5" catback, drone is at 1900 rpm just before it changes up to the next gear. I was told that the original WA Manta mufflers were different internally. So am trying to track down a pair to try out.

 

[RevNev]

got up and close with Harrop (DPE) bi-modal rear muffler today. No wonder the thing booms with the flaps open on idle. They've got it set up pretty close to how I crudely drew it. 3 odd inch through the large diameter path (bi-modal open path) and roughly 1-3/4" through the narrow path (bi-modal shut path).

The DPE design is wasting a lot of muffler capacity with the internal bimodal Y pipe where the Lukey (Helmholtz) design, maximises the muffler's noise reduction potential. Years ago, Lukey had a resonator the size of a 12" hotdog they called an "echo box" to quieten noisy exhausts and the only difference internally was the centre (Helmholtz) chamber. They're were a lot quieter and deeper sounding than a hotdog for the same external size and took an 18" long hotdog to match the sound reduction of Lukey's 12" echo box. The open centre chamber (Helmholtz) design can create turbulence and reduce flow, so they're not ideal close to the engine with an exhaust on the small side, but at the end of the exhaust where the gasses have cooled and lost velocity, they work perfectly restriction free with maximum noise reduction for a "straight through" muffler.

The Harrop (DPE) is likely too small in outer diameter and for drone reduction or "anti drone" design, it's best to use the "biggest" rear muffler that'll fit under the car particularly with a 3" exhaust. The stock muffler canister is a good size to re-work into an effective bimodal.

 

[hjtrbo]

@RevNev
Grabbed the picture from one of your earlier posts.
My stock HSV bi-modal cans arrived. I haven't got the grinder out yet but I just wanted to check with you the proposed mods are what you have done.

 

[RevNev]

@RevNev
Grabbed the picture from one of your earlier posts.
My stock HSV bi-modal cans arrived. I haven't got the grinder out yet but I just wanted to check with you the proposed mods are what you have done.
View attachment 227605

Yes, that's correct!

 

[hjtrbo]

@RevNev wanted to get your opinion on the guts. Currently blown with overdrive pulleys. 1-7/8 long tubes into manta 'quiet' centre section with single cell cats. Thinking going 3" into the chamber then 2 x 2.5" from chamber out to the back. Thoughts? Keep sizing as is?

 

[RevNev]

@RevNev wanted to get your opinion on the guts. Currently blown with overdrive pulleys. 1-7/8 long tubes into manta 'quiet' centre section with single cell cats. Thinking going 3" into the chamber then 2 x 2.5" from chamber out to the back. Thoughts? Keep sizing as is?

Twin 2.25" flows more than single 3", twin 2.5" is equivalent to single 3.5". I'd go twin 2.25" outlet with a 3" inlet. Twin 2.5" will just be louder with no flow or performance gain. If the inlet is 2.5", I'd go twin 2" outlets as I did with mine. The stock bimodal inlet and section of perforated tube in the rear chamber is 2.25" I think is likely too small for a supercharged application and the minimum size I think is perhaps 2.75", There's the cooling effect at the rear of the exhaust to get away with a flow reduction but it's something you'd need to confirm on a dyno, it's hard to predict the ideal size. I'm wondering if anyone has dropped the rear mufflers with an LSA engine and stock HSV exhaust and dyno tested it?

 

[hjtrbo]

Awesome. 3" and 2 x 2-1/4 it is. Will send through progress pics as the job progresses so others can see what's involved. Just picked up a plasma cutter so keen as to test it out.

 

[RevNev]

Awesome. 3" and 2 x 2-1/4 it is. Will send through progress pics as the job progresses so others can see what's involved. Just picked up a plasma cutter so keen as to test it out.

The muffler can is double skinned and a mongrel to weld, I ended up using a pulse MIG and cleaned it up with flap disc, looked ok with coat of paint. I cut mine through with a long blade reciprocating saw 200mm from the inlet end, around the centre of the open chamber.