Exhaust Rapping??

[max57]

So had a Stock LBZ with twin turbos and 60 over injectors. vvt turbo in valley with s475.No rapping from Y exhaust to two 4" 6' pipes at all.
Bought a Long block with built bottom (Mahl pistons .020) ,Rods, alternative Fire Cam, Ported heads, put a S366 in valley and S480 atmosphere twins, 60% injectors , free flow manifolds and ups. Now have a exhaust Rap that is scary. Only above 3000 rpm and hard load, if I let off throttle a little it stops. I do not mind the sound but afraid something is wrong cause never heard any duramax sound like this, What do you think.
https://www.youtube.com/watch?v=UAwGcN3S11w
https://youtu.be/r66wgStzejE
https://youtu.be/o9J1afR4iqA

 

[Chevy1925]

Sounds like the “dmax cackle” to me. When you go from the vgt to the fixed vane, that’s when the cackle becomes noticeable.

What muffler/size exhaust are you running?

 

[max57]

No Mufflers, 5'' out of turbo to Y , TWO 3'' PIPES INTO TWO 4'' SIDE PIPES.

 

[slowlml]

Yeah thats normal. You never heard it before because you had a vgt in the valley. What's this setup in?

 

[darkness]

I don’t think that sounds right for a compound setup .

 

[OregonDMAX]

Sounds normal to me, atleast for a fixed vane

 

[zakkb787]

Fixed vanes are raspy. I put a Flo Pro Twister resonator on my truck and love it. It’s only 12” long. Another problem is your exhaust. 5” into 3” will be way more raspy than a straight 5” IMO

 

[slowlml]

It's in a 57 chevy wagon I'd reckon thats the reason for the smaller piping. I agree thought the smaller piping probabaly does make it more raspy.

 

[zakkb787]

It's in a 57 chevy wagon I'd reckon thats the reason for the smaller piping. I agree thought the smaller piping probabaly does make it more raspy.

Get one of the resonators I mentioned. They can be had for $70 ish.

 

[slowlml]

I dont think he was worried about the sound just making sure it was normal as hes never heard it.

 

[1TRIKHD]

Sounds pretty normal to me as well.

 

[max57]

Thanks guys, my take is it is normal. My exhaust actually is 5'' to a 4''Y split to the two 4'' pipes. I hear LB7's Rap a little with single pipes but not quite like this so I was a little concerned maybe a valve open when it should not be,or a firing cylinder leaking into another. but it probably would have atesomething up by now. Thanks

 

[DAVe3283]

My built LB7 sounds very similar above 2600 RPM and under load. Fixed single turbo on mine. I think a high-lift cam and some decent cylinder pressure leads to the exhaust "rasp", which is why it tracks with load, not just RPM. A VGT would mask that quite a bit.

Yours sounds a bit louder / more extreme than mine, but that's probably the piping used, as everyone is mentioning. Nothing wrong, so either enjoy the sound or tweak your exhaust setup to minimize it. :thumb:

 

[Digmax]

Is your 366 wasted gated? might be when it opens?

Sent from my SM-G935V using Tapatalk

 

[Chevy1925]

it wouldnt hurt to pull the exhaust and leave just he down pipe (if nothing will be harmed running down pipe only) and go for another spirited run to see if the noise/tone changes.

 

[JoshH]

Is your 366 wasted gated? might be when it opens?

Sent from my SM-G935V using Tapatalk

That's a good theory.

 

[darkness]

I’m still not convinced it’s normal. My lb7 never sounded like that with the compounds. The second turbo eliminates it. I think there’s a leak somewhere. I’d look for a leak between the high and low pressure exhaust pipe.

 

[max57]

The noise is definitely come out the chrome pipes , not from the turbo cross over pipe or there would be smoke. And no I have no waste Gate to leak . Good thought though.

 

[JoshH]

Could be pilot injection or timing changes that happen at certain load levels that causes a drastic change in exhaust tone.

 

[max57]

Found the exhaust rap was not the Case. It was a Injector leaking past copper washer only when turbos kicked in. I noticed exhaust soot on my valve cover is how we seen it. What a noise.She is normal sounding now. Thanks guys.