View Full Version : Exhausts in the plenum

tonybroad - n/a
28-Aug-07, 11:41 AM
I'm contemplating running my exhausts and some improved silencers inside the plenum mainly in an attempt to reduce noise

apart from maintenance access and local heat affected area issues, does anyone anticipate any problems with plenum pressure affecting exhaust pressure

thanks in advance

Tony Broad

peterd51 - n/a
28-Aug-07, 12:36 PM

it could be a problem on a 2-stroke.

From my motorcycling days I vaguely recall that the whole fuel-exhuast line is a tuned system and the induction relies partly on the spent gasses going out the exhaust creating a partial vacuum to help suck the new mixture through.

It's similar in a 4-stroke but not as critical.

Also, if it's 2-stroke, doesn't the exhaust gasses include a certain amount of oil in vapour form?

You may need to consider how to clean it out if it condenses back to liquid.

What sort of effect would it have on the skirt material?

It sounds a good idea though.



profqwerty - n/a
28-Aug-07, 04:16 PM
I would think the hull pressure is a small % of the exhaust pressure though.

Would the exhaust be released inside the general space of the plenum, or through multiple skirt feed holes?

This would reduce the number of segments that need cleaning, but it might no provide enough pressure perhaps?

Ian Brooks - n/a
28-Aug-07, 05:40 PM
The problem is likely to be exhaust gas coming over the front of the craft and getting into your face! That is what happened on my Osprey - it was most unpleasant (maybe dangerous?) and I had to modify the exhaust accordingly...


Keith Oakley - n/a
28-Aug-07, 06:00 PM
Historically the practise was banned after a couple of cases in the 70s of exhaust gases/unburnt fuel exploding under the craft. However it was a very effective quick fix on Les Brands craft a couple of years ago to solve the noise prob.

Looking at the NoiseCam analysis of your craft at the beginning of this season the splitter plate noise (rhs looking from front) was 2 db higher than the exhaust so you need to sort that (as discussed on another thread) or you won't get any benefit from exhaust changes. We didn't take an overall dba reading then so I can't comment on your 'ranking' but you might want to take a look at Rupert Baker and Nick Longs craft both of which are quietest TZRs. I'm hoping to test an 'exhaust tester' at the next 2 meetings which might give us quantitive data on what works and what doesn't.

hover t - n/a
28-Aug-07, 07:01 PM

This is the conversion I did last year to try and get the noise down

This pic is the original setup in my craft not to good as the silencer sits right in the middle of the plenum and restricted the air flow on the RHS also its almost a straight through


I got another silencer and bolted the two together, made up hanging brackets and mounted it along the back of the craft



This seems to have worked rather well the two bends in the exhaust plus the double silencer have made it a good bit quieter


tonybroad - n/a
29-Aug-07, 10:26 AM
Thanks Keith - splitter design and noise is going to be a big focus for me next year - Ruperts TZR has the exhausts in the plenum and the silencers exit at the back of the craft but not into the plenum

I think i know why Nick is quieter than most (and quicker) even though he uses swarbrick silencers which are usually top end of the noise level

Is sub 90dB an achievable target to aim for for a TZR ?


Keith Oakley - n/a
30-Aug-07, 07:09 PM
I think sub 90dba for a TZR is an achievable target. Nicks was logged static at 90dba and I thought Ruperts was sub 90 but I haven't got all the data at present - it's on the North Sea on its way to Stockholm.