TVR V8 Exhaust power issues.....Help

[mikeb]

I managed to spin my 450SE 'Tasmin' TVR into the kerb backwards. Which was nice!

Apart from my pride the only other damaged was the rear lower exhaust box directly under the rear downforce spoiler.

Now the engine in my car has had a re-build and blueprint job some years ago by a previous owner. At the same time a bespoke exhaust system was also put on in stainless by the engine re-build people.

Having looked at other 400SE / 450SE TVR's they have a straight pipe where my back 'box' is.

Having taken the car down to Longlife exhausts and having the back box replaced with a straight through pipe (as other SE cars) the power seems to have diminished considerably and the consumption of petrol also seems to have gone up - it may be my imagination though!

Question.

By changing the back box for a pipe would this lack of back pressure cause the power loss issues?

Also the noise is now way toooo loud - even for my seasoned ears! so I have to get something done about this also.

Is there some sort of calculation you can do to determine the required amount of resistance and therefore diameter of baffle required??

Thoughts??

MikeB

[350matt]

On the whole the less back pressure the better, however with some camshafts the manufacturers ( Range rover for example) have used the system back pressure to extend the overlap with the high back pressure keeping the charge in the cylinder. Might be an idea to dig out the camshaft specs and speak to someone like V8 developments

Matt

[jchase]

According to "Performance tuning in theory and practice" the location of the baffle sets the resonance scavenging frequency. Front boxes on alfas for example are about 4500 rpm, middle boxes 3000rpm and rears 1000-2000.Something to do with supersonic shock waves in the exhaust which is a different kettle of fish to sub sonic flow. So I would expect your torque to be reduced between 1000-2000 rpm by removing the rear box, but it should not have any effect above 2000rpm, or if anything make a little bit more power over 2000rpm due to lack of resistance.

Assuming the author of the book knows his onions.

-Jim

[trackcar]

I think that sort of tuning only aplies to large changes o vlume .. ie if you have an expansion chamber type box rather than the absorption silencer type box you would normally find on a peformance silencing application. if you don't have a large volume change in the box it behaves essentially as a straight pipe.

[GreenV8S]

I would have thought the subjective effects of the exhaust note change would have more impact than changing the back box. If the change really is that drastic, I'd wonder whether the impact has disturbed something on the engine management system.

[mikeb]

The cylindrical 'box' they took off had a perforated pipe running through it.

It sounds like its blowing - but it ain't, it also now pops and bangs from fourth to third which normally I quite like but these are quite loud.

The whole note of the exhaust has gone from a glorious vicious RV8 roar to a big chavesque waffle, waffle hollow sounding nasty resonant tone - plus the power loss.

Really pissed off about it.

Mike

[stevieturbo]

I imagine any proper exhaust place will replace, as much like for like as they can..


Veering off on tangents about cams, scavenging etc IMO, is just getting carried away.

It sounds like there is something wrong somewhere, rather than an exhaust causing it.

That said, pops and bangs, can be indicative of an air leak in the exhaust, somwhere up front.

[Pigeon]

mikeb said:

The cylindrical 'box' they took off had a perforated pipe running through it.

It sounds like its blowing - but it ain't, it also now pops and bangs from fourth to third which normally I quite like but these are quite loud.

The whole note of the exhaust has gone from a glorious vicious RV8 roar to a big chavesque waffle, waffle hollow sounding nasty resonant tone - plus the power loss.

Really pissed off about it.

Mike

Pigeon's speculation:

You now don't have a reflector/absorber at a point where you had one before that was having some significant effect. Hence the buggered-up exhaust note. You are also missing a pressure pulse reflected back to the ports in time to counteract excessive mixture loss due to valve overlap at low to mid revs, or perhaps you have gained a suction pulse reflection at this timing. Hence the power loss and the pops and bangs.

[mikeb]

Holy Moly! Sounds very technical.

I am gonna take it back and get them to fabricate something akin to what was damaged - I cannot be driving about with what sounds like being followed by a Phantom Jet!

If the car is still 'down' then I guess I will have to start pulliong bits to see what else it might be.

MikeyB

[GreenV8S]

Pigeon said:

Pigeon's speculation:

You now don't have a reflector/absorber at a point where you had one before that was having some significant effect. Hence the buggered-up exhaust note. You are also missing a pressure pulse reflected back to the ports in time to counteract excessive mixture loss due to valve overlap at low to mid revs, or perhaps you have gained a suction pulse reflection at this timing. Hence the power loss and the pops and bangs.

The RV8 doesn't seem to respond to conventional exhaust tuning, probably imo because the uneven firing order which gives us that glorious burble prevents consistent secondary pulses. If the engine is down on power and sounding flat and hollow I wonder whether there is a restriction in the exhaust. It may just be something forcing the gases to go past a sharp edge or round a tight corner causing separation and acting as a choke. All the gas flowing issues that people pay so much attention to in the induction matter in the exhaust too. It's a shame you don't have lambda sensors otherwise you could measure the back pressure very easily. As it is, perhaps the easiest way of doing it in situ would be to drill a hole that fits a pressure tap, and then strap a patch over it or weld it up afterwards. If the exhaust flow characteristics have changed this much then I expect the fuelling could be all over the place too.