Why no exhaust wrap

[AMMO]

Just spoke to the nice chap at Emerald about booking a dyno session on a 2 litre Duratec race engine and had a quick chat about lagging exhausts. Dave Walker's actual words were: "Good if you want to reduce the under bonnet temperatures and burn up your silencers". That was 15 minutes ago.

 

I am not aware of any performance gains apart from the fact that you may reduce the induction temperature and might gain that way. If you have a filter under the bonnet OK. As my throttle bodies stick outside the bonnet I'm pretty sure that it is not relevant.

 

So if you lag your headers you might keep your feet cool but you can damage your headers and your silencer. I've seen silencers burn out their packing and the titanium turn blue. In one extreme case the core was badly damaged and had started to break up too.

 

I just don't think it is a great idea. Only guessing but it could also be that the exhaust valves and seats could suffer if you can't get the heat away.

 

For the people who have had positive experiences, great. Good for you. For the others that have had problems then maybe the best bet is to stop using exhaust wrap?

 

 

Casbar

 

Wouldn't mind seeing the article if you still have it. Ta

 

Ammo

Raceco.com

[Neal Worth]

If you're able too, I know that not all engines installs allow it, you could fit a through the bonnet manifold. Would certainly cut down the temperature side of things and save the alternator.

 

A bit like this one here.

 

Been looking for a picture for ages and finally found one.

 

Hope you don't mind me linking it Steve?

 

Neal.

[AMMO]

😬

 

Funnily enough Neal the new Duratec race exhaust I designed exits via the bonnet.

 

 

 

Ammo

Raceco.com

[Colin Mill]

Back to Basic Physics, which i can't really remember 'cos it was a long time ago, but doesn't an increase in temp increase gas velocity, ergo... wrapping exhausts should increase exhaust gas temp and speed which should increase flow through the engine, which should result in increased induction flow ...which I thought was the factor you are always trying to improve ....or am I wrong ...

---

 

The velocity of sound in a gas increases as the square root of the absolute temperature.

 

So, if you have a resonant exhaust such as the classic twin cone 2-stroke exhaust the rpm at which resonance is reached increases as the temperature of the gasses in the exhaust increase and this increases slightly as the wall temperature of the exhaust increases. However, the residence time of the gasses in the exhaust is usually short enough that the gasses do not get anywhere near equilibrium with the walls and the gas temperature is significantly higher than the exhaust walls. This rather complicates matters when it comes to designing the exhaust.

 

As for changes in the speed of flow of the gasses from the exhaust there are two other factors:-

 

The volume of gas increases linearly with the absolute temperature. This rises more rapidly than the velocity of sound so, all other things being equal, the higher the gas temperature the longer it will take to expel the charge.

 

Although of much less importance in the flow regime (Reynolds number) in an exhaust, the viscosity of gasses increases with temperature - quite the reverse of the behaviour of liquids! - and this will very slightly inhibit the exhaust.

 

Another way of looking at the problem is via Bernoulli's equation which is appropriate for subsonic flow.

 

Pressure = 0.5 density x speed squared.

 

Now if we increase the temperature of the gasses flowing down a given pipe we reduce the density (inversely as the absolute temperature). However, if the mass of gas flowing per second is kept constant the speed must increase to keep density x speed the same. If we plug this back into Bernoulli's equation we see that the pressure needed to drive this flow has increased linearly with speed.

 

Edited by - Colin Mill on 10 Jun 2008 08:58:56

[Casbar]

Ammo, I'll have a look for the article, but to be honest, I chucked most of the Circuit Drivers out. But it was written by Dave Walker, and I remember reading about the positive benefits of exhaust wrap, because I pointed it out to a mate

[Paul McKenzie]

Yeah but.......the volume of gas _can't increase as it's volume is governed by the exhaust dimensions, so instead it squirts out of the exhaust faster

 

Paul

[Colin Mill]

Hi Paul

 

Sorry, I was adding a bit more to the above when you posted. As you can see, you are right - if the exhaust dimensions confine the flow then, for a given mass flow (dictated by the intake conditions etc of the engine) then the flow speed increases and thus the pressure at the cylinder end of the exhaust.

 

While one can get increases in resonant exhaust performance by playing around with increasing the pressure (because the resonant exhaust reflects back pressure waves as rarefaction pulses that help empty the cylinder) this is usually only of benefit where the engine is run virtually at a single speed such is the narrow rpm range over which such exhausts work. (this is OK for us model helicopter fanatics as we run the engines governed to a fixed speed so the exhaust can be incredibly peaky so long as the resonance and the running speed are matched)

[Rob Walker]

I have just emptied and repacked my silencer box as I`m on Le Sept next week. I was surprized to see that all the Eglass and stainless steel wool was in almost perfect condition after 4000miles several track days including the Ring. So all the scaremongers out there that advised me that I would burn up my exhaust packing when using wrapped primaries would appear to be wrong. 😬

[Jason Plato]

mines the same Rob, my can weighed 100g less than it did in January.

 

It would appear to me that the distance between the headers/ collector and the silencer is the main influence and not the wrapping of the headers inside the body.

 

 

here is C7 TOP

Taffia rear gunner

 

[Rob Walker]

The whole idea that wrapping the primaries would quickly burn up the exhaust packing has never made any engineering sense to me,in view of the sheer volume and speed of the hot exhaust gases passing down the pipe its is unlikely that much heat as a percentage of the total hot gases would be lost/ radiated from the 18" directly after the head.

 

I still have concernes however that the exhaust wrap may be contributing to speed up the degredation of the thin 16SWG stainless steel primary tubing ?????????

 

Rob

[Jason Plato]

or maybe its the cheap packing material used in some silencers that burns off quicker ?

 

arf 😬

 

here is C7 TOP

Taffia rear gunner

 

[ECR]

I have the same experience as Rob and Dave...Very little continuous e glass is needed for a repack after substantial use.

[Rob Walker]

Roger,

 

Do you use ss wool with your long strand eglass or just the eglass on its lonesome ???

 

I am now wondering if the ss wool is necessary ???

 

See you a Le Mans

 

Rob

[Jason Plato]

ss woll wrapped around my perf tube prolongs the life of the e glass for sure

 

here is C7 TOP

Taffia rear gunner

 

[ECR]

I don't use steel wool because with my Raceco there is a stainless gauze wrapping to the perforated tube which stops a lot of the fill blowing out.

 

Bon Voyage !

[Doug]

As I've mentioned in a couple of posts before, I've been toying with wrapping the primaries on my Raceline Zetec conversion (former x-flow). The problem I appear to be having is that the primaries are only 6 or so inches from the rear of the alternator.

 

I use a X-flow type alternator with the electronic gubbins on the back, which cooks in heavy traffic when the underbonnet temp rises significantly.

 

I too am convinced that wrapping is not the solution and will eventually lead to other problems but I've got to do something about isolating the alternator from all that heat. At the moment I'm thinking about making a shield to go over and round the back, keeping hot air from the exhaust primaries well away from the apparently heat sensitive diodes at the back of the alt.

 

Any other ideas anyone - this must have been a common problem in the past?

 

'Have you any idea what it's like to have the wind rushing through your hair!' (Quote:Sq Cdr the Lord Flashheart)

[djg]

Doug

 

With regards the heat sheild I do remember that Mk 3 Escort I had, had a heat sheild mounted across the back of the altermator, it would be the right shape and being Ford may even fit !! The exhaust came within cm's of the back of the altermator on that and never suffered, although I know we are talking a lot less heat power etc etc.

 

May even be worth looking under the bonnet of a Zetec engined tin top and find one that does fit straight away

 

Doug

 

SL #57

 

[Jason Plato]

I too use metal gauze sleeve around the perf tube *and* then wrap the gauze in stainless wool, then the e glass around the ouside, wrapped in clingfilm and then over with the zorst outer sleeve.

 

The stainless wool is available from Milner offroad

 

here is C7 TOP

Taffia rear gunner

 

[metal mickey]

Doug, mount your alternator on the other side of the engine.

[Doug]

Moving it to the other side is an option I've considered already. I think I can get a kit from Raceline but it's expensive. I'd rather see if there is a viable solution without going to the trouble. After all, it cannot be a new problem with Caterhams!

[CageyH]

That is why the kit exists

 

Only dead fish go with the flow....!

[metal mickey]

*arrowup*correct obsevation Mr Cagey

[I.Mupferit]

Doug, I used to have a Zetec powered 7 with the Raceline alternator mount under the exhaust primaries and mine suffered from heat soak cooking the alternator as well. I eventually solved this by fabricating a shield from aluminium cut into a square and then getting a cheap soldering mat from the plumbing section of B & Q and fixing a double thickness of the mat to one face.

 

The mat was held in place by folding up the edges of the ali approx 10mm to form a tray, sit the soldering mat cut to size, in the tray and then flatten the folded sides down onto the mat to clamp it in position IYSWIM. Mitre the corners of the upturned edges beforehand to make a neat picture frame sort of effect.

 

The shield was then held onto the nearest offending primary by drilling 4 small holes in the aluminium and passing stainless steel wire through these, behind the mat and round the pipe, twisting the ends tight rather like wiring of boltheads and trimming off the excess. I eventually changed this to fix it in place by cutting a couple of slots in the ali, adjacent to each other and looping a stainless jubilee clip through these and then around the primary. This held the shield more firmly and prevented it slipping.

 

The soldering mat side faced the alternator and proved to be a very effective shield as it still allowed air to flow around the end of the alternator while shielding it from most of the radiated heat. As a rudimentary test, after a long hot run I could still touch the mat face of the shield without it burning me so it was definitely effective.

 

Use the biggest piece of aluminium you can get in there. I think mine was approximately 125mm square but did the job and I never had any further alternator problems after that. It was a cheap solution too, costing no more than a fiver.

 

 

Edited by - Brent Chiswick on 20 Jun 2008 23:23:13