Boomy exhaust

[dlgis]

Hi all, my exhaust is very boomy at certain speeds and i’d like to reduce the noise a bit ( must be getting old ). The exhaust is a bit of a parts mix:

Nearly new stainless steel downpipes.
New Jaguar Classic mild steel silencer
Nearly new stainless steel connecting pipes
Old standard resonators.

I thought when i fitted the new silencer it would be quieter at all speeds, but like I say it is boomy at certain revs.

Would changing the stainless steel parts for mild steel help?

Thanks.

[MarekH]

At every joint where you have pipes joining each other, you have a small box which has a higher volume or cross section than the joining pipe. This creates a low pressure region and depending on the distance between these low pressure pockets, you'll set up resonant or standing waves between them at certain frequencies. That's likely what causes your exhaust to sound non standard at certain rpm - the geometry which has changed has changed that.

Added to that, using a different material will also alter the sound overall - a trumpet made of wood will vibrate differently to one made of brass.

It's not obvious to me how one would disentangle all of that,but I'm guessing that the prettier stainless headers are the most "un-alike" part.

kind regards
Marek

[Heuer]

Dump the s/s downpipes as they are not only noisier but radiate high underbonnet temps that can bubble the paint on the frames or indeed the bonnet. More importantly they will rob the car of torque between 2,250 and 2,750 rpm which, on the E-Type, is the point where you need most torque for overtaking. The cast manifolds were designed specifically for the XK engine to alleviate that problem.

Booming could be caused by the system being too solidly connected to the car - there should be rubber hangers for the silencers and resonators. The type of steel used for the system will dictate the sound with s/s sounding much harsher. However most of the exhaust tone is determined by the resonators so if yours are mild steel (check with a magnet) it should sound great.

Start with the manifolds!

[dlgis]

Just to clarify, i have standard cast manifolds with the stainless downpipes connecting them to the Jaguar Classic silencer. All parts have standard fittings to connect to the car. I agree that my problem probably lies with the stainless parts but having just checked, Jaguar Classic are out of stock on mild steel replacements 🫤

[MarekH]

OK, so this a thread about noisy stainless steel exhausts.

As an aside, appended is some information about improving the torque band by biasing it down to where road cars can most profitably use it.

https://aj6engineering.co.uk/tt-systems-explained/

Noticeable is that the v12 is just twin six cylinder engines fired 60' apart. What XK engined owners with standard manifolds can do is measure the length of the tubing down to where the TT tubes join about 3-4ft away and replace the middle silencers with a joining pipe there. If the tubing is the same diameter as the TT system, it ought to be a neat and cheap way to get better overtaking performance on the road.

By the same token, fitting "fast road cams" that "assist" at higher rpm is counterproductive. What you actually want on the road is a camshaft that delivers maximum torque down at 1700-3500rpm, because that's the rpm you are probably doing when you overtake or accelerate. That'll be a camshaft with a lower duration, not higher. Paradoxically, that'll deliver a lower overall horsepower than standard, but it'll appear to be faster than the traditional "performance" option.

kind regards
Marek

[Mich7920]

Thanks for the link. It's very interesting !!
Mich