Lagging exhuasts

[Adam43]

As I said on the previous page we did this on our Jags and have now gone for it on the MGB. I can't see a downside, so worse case is no gain. We put it on at the weekend:

The engine and gearbox have just gone back in and it was the ideal time to do it. Of course we forgot and just as I finished tightening the prop shaft bolts engine mounts and I said "you know what we didn't do...". Cue scrapped knuckles. It would have been so easy when the engine was out!

Still wondering whether to have a go at Gordon's cooling inlet manifold suggestion.

[tristancliffe]

I can't remember which Lotus is was that tried it - might have been the Esprit - bug lagging the exhausts caused the metal to melt! Plus it was down on power as the gasses weren't allowed to cool quickly - and you do want them to cool quickly. As I said before, professional teams, be it F1, GP2, DTM etc don't use it because they want more power.

At a push I'd consider ceramic coating, not to improve performance, but to stop bodywork/wiring melting and paint blistering.

TomBE - hot gasses have a higher pressure which means they travel more slowly. Low pressure = fast flow velocity

[Adam43]

Lagging an F1 or DTM exhuast is a little different. These are made of exotic materials and run at different temperatures anyway. GP2 is a spec. series. I dare say that in some applications it is unwise, but for our MGB, I'm pretty confident there isn't a down side. There is plenty of exhuast under the car to lose heat; I'd prefer it to do it there than in the engine bay between my Webers.

[Chris Y]

I wrapped my exhausts a few years back - thinking there weren't any downsides.

Then, after two engine rebuilds (different engines) where the exhaust manifolds had warped and wouldn't fit back onto the heads, I stopped doing it! These were cast iron manifolds though. I think the headers you have there Adam are more suitable.

[BertMk2]

One thing to watch out for with exhaust wrapping - if you have any oil flying about under the bonnet the wrapping soaks it up nicely and eventually starts a small but persistent fire. It takes ages to cut off charred oil soaked wrapping too.

Not that this has ever happened to anyone of course

[terence]

Or indeed fuel dripping onto it,just after the race while the car is in parc ferme.

[dtype38]

Originally Posted by BertMk2

One thing to watch out for with exhaust wrapping - if you have any oil flying about under the bonnet the wrapping soaks it up nicely and eventually starts a small but persistent fire. It takes ages to cut off charred oil soaked wrapping too.

Not that this has ever happened to anyone of course

There's no need for it to start a fire - you can also do the first half of practice in a cloud of white smoke!! Not that this has ever happened to anyone I know either.

As I think Adam will confirm, keeping the oil in a Jaguar engine is like trying to stop the tide coming in.

[Adam43]

Indeed, the time to worry is when it stops leaking oil...

Thanks tristancliffe, Chris Y, BertMk2 and dtype38 for things to look out for.
I'll let you know.

[dtype38]

Oh, just thought of another. If you have a bonnet vent over the manifold, give some thought to a rain cover over the pipes. I didn't. Then one rainy race day morning the wrapping got soaking wet and when I drove down to scrutineering I nearly disappeared in a cloud of steam!

[Belatti]

Hi, I would like to know if any of you have got experience with the use of exhaust wraps in sport or touring racing cars.

I understand that there can be a performance gain due to the decrease of under hood temp, but I wonder if the thermal isolated pipe would suffer because its temp increase.

I race in a series where the rulebook states 150HP to the wheel, measured by the series organizer. Can the wrap affect my engine HP measurement?

Thank you!

[Al Weyman]

I am skeptical on the claims myself and a lot of the yankee sites I go on recon they can rot your headers a lot quicker and are a waste of time and money however I do have them on one of my cars and its nice not to burn yourself if doing a quick plug change and keeps the plug leads nice but I doubt I would bother again. Maybe for under hood and intake temps you would be better if allowed to to have a cold air ram system fitted to your air intake as it would only come into effect when the car is in motion and will give BHP increases with the cold air charge but would not be measurable when the car is static or on rollers.

[Belatti]

Originally Posted by Al Weyman

they can rot your headers a lot quicker

Thanks Al!

Are you talking about corrotion here or just that the pipe thermal loads make them crack ??

[phoenix]

OK - basic physics.......

Heat is energy. Keeping energy 'in' a system rather than 'losing it' is a good thing - unless it causes damage. An internal combustion engine without cooling doesn't exist for long.......

Because the internal combustion engine produces a lot of waste heat, how to capture that heat and use it to as an advantage (i.e. to add to the total power output) has fascinated many engineers for over a century.

Example 1:

A turbo is driven by the energy contained in the exhaust gasses. Most of that energy is heat - not 'volume flow' or 'pressure' but velocity. Reduce the heat of the exhaust (by allowing heat to 'exit' the system, to only heat up the engine bay etc) reduces the power that a turbo can collect from the otherwise waste exhaust.

Example 2:

In a normally aspirated engine the exhaust gasses are similarly full of energy.

The aspect of the energy in the exhaust that has the most potential for tuning in a normally aspirated engine is also velocity. The velocity can be used to drag the last remnants of combusted gasses out of the cylinder, and to start drawing in a fresh air/fuel mixture BEFORE the piston starts going down the bore. This improves volumetric efficiency. Allowing heat to escape from the exhaust reduces the speed of the exiting gasses - literally wasting energy - as exhaust gasses flowing at a slower speed will be less effective in a) emptying the cylinder of combustion waste and b) filling the cylinder with fresh air and fuel.

My vote goes for keeping heat in the exhaust system - particularly the manifold. F1 teams don't use lagging (like us poor people) - they use ceramic coating. The RIGHT ceramic coating applied to the correct areas - and that can include piston tops, combustion chambers and exhaust ports, as well as the more obvious exhaust manifold - will give an engine the potential to produce more power.

[Al Weyman]

I would say the corrosion is more likely when the wrap gets wet and holds in the moisture not sure maybe not so good on a daily driver or a rally car which will get plenty of that but I do know header manufacturers will void the warranty if found to be fitted. Someone I know did a back to back on a Capri engine and he told me it made sod all difference so what can I say, sometimes theory is all well and good but in practice in a real life situation its a different matter especially on a low spec engine (compared to F1 and the like) that the majority of us are using.

[Chris Y]

Personally I wouldn't bother. I've run engines with and without manifold wraps, and they provide little benefit on most engines. I can see the advantage on turbos, but for Mr. Joe Average club racer with a mildly tuned normally aspirated car, there isn't much point. And there isn't much point if your series is limited by BHP.

Plus, the extra heat in the system can sometimes cause the manifold to warp, and if the steel isn't good enough or has been bent incorrectly, can actually blow holes in the manifold or downpipes.

[Belatti]

Originally Posted by Chris Y

Personally I wouldn't bother. I've run engines with and without manifold wraps, and they provide little benefit on most engines. I can see the advantage on turbos, but for Mr. Joe Average club racer with a mildly tuned normally aspirated car, there isn't much point. And there isn't much point if your series is limited by BHP.

Plus, the extra heat in the system can sometimes cause the manifold to warp, and if the steel isn't good enough or has been bent incorrectly, can actually blow holes in the manifold or downpipes.

OK, I was not thinking about BHP anyways, but more about underhood overheating. I dont want to open ventilation holes because it means drag and with limited 150 BHP, any little drag means a lot. These cars are lightweight and can reach 250Kmh so I dont want any drag

I guess Ill test them only in really hot days if the driver and mechanics are suffering it.

Thank you all guys!

[phoenix]

Originally Posted by Chris Y

Personally I wouldn't bother. I've run engines with and without manifold wraps, and they provide little benefit on most engines. I can see the advantage on turbos, but for Mr. Joe Average club racer with a mildly tuned normally aspirated car, there isn't much point. And there isn't much point if your series is limited by BHP.

Plus, the extra heat in the system can sometimes cause the manifold to warp, and if the steel isn't good enough or has been bent incorrectly, can actually blow holes in the manifold or downpipes.

Yes, to get the scavenging advantage on a N/A engine you need to be running cams which have reasonable lift on both inlet and exhaust at TDC, which tends to mean long duration. Also the tuned length of the manifold will need to be re-calculated for the maximum advantage, as the speed of sound increases as gas temperature increases.

We run EGTs of about 950-990 C at the turbo - which is nearly a metre from the exhaust valve. Without wrapping that would be down to around 700-750 C.

You do need a decent material for wrapped exhausts - ours is proper spec stainless steel which has a lower coefficient of expansion than mild steel and so the manifold is not trying to tear itself apart with heat distortion. Internal ceramic coating is a better option, as the manifold is actually protected from the high exhaust temperatures by the ceramic coating, but it is beyound my meagre budget.

[Cameron Winton]

Worth commenting that in an internal combustion engine, effective energy recovery is all to do with mechanical work. The combustion releases energy by expanding the gasses (Increased pressure (potential energy) and velocity (kinetic energy) and heat. All your engine torque and power is through harnessing the potential & kinetic energy in the gasses. The heat energy is in effect unrecoverable losses. All the turbine on a turbocharger does is recover more of the potential/kinetic energy in the gasses than in a normally aspirated engine. Where exhaust wrap may have a minor benefit is in reducing underbonnet temperatures giving more dense inlet air.

[tristancliffe]

I doubt it's the density that would help, but the cooler temperatures. You can then use a bit more ignition advance, a bit more compression or some other means of getting more power.

The 'denser air gives more power' argument is, I suspect, a layman's simplification.

[phoenix]

Originally Posted by Cameron Winton

Worth commenting that in an internal combustion engine, effective energy recovery is all to do with mechanical work. The combustion releases energy by expanding the gasses (Increased pressure (potential energy) and velocity (kinetic energy) and heat. All your engine torque and power is through harnessing the potential & kinetic energy in the gasses. The heat energy is in effect unrecoverable losses. All the turbine on a turbocharger does is recover more of the potential/kinetic energy in the gasses than in a normally aspirated engine. Where exhaust wrap may have a minor benefit is in reducing underbonnet temperatures giving more dense inlet air.

Partly true - but some of the kinetic energy in the exhaust gas is lost as heat is lost from it, because the gas velocity falls as a result. Heat lost out of the manifold before the turbo is therefore wasted energy, not harnessed. So lagging or ceramic coating helps with that.

The kinetic energy in the exhaust gas can also be employed to improve scavenging on N/A engines. Reducing the velocity of the gasses in the manifold reduces scavenging capability. Just as too large a cross sectional area of the primaries can reduce gas speed and impair scavenging as a result. In a normally aspirated engine no energy is recovered in this way, but the kinetic energy is used to improve overall engine efficiency by increasing volumetric efficiency.

[dtype38]

Originally Posted by phoenix

...... Reducing the velocity of the gasses in the manifold reduces scavenging capability. Just as too large a cross sectional area of the primaries can reduce gas speed and impair scavenging as a result. In a normally aspirated engine no energy is recovered in this way, but the kinetic energy is used to improve overall engine efficiency by increasing volumetric efficiency.

I understand that the velocity controls the momentum of the exhaust gas and why this needs to be kept up in the manifold, but I'm a bit confused as to its relationship to the pressure losses through the rest of the exhaust system. Intuitively it would seem that the reduced velocity achieved by allowing the exhaust gas to cool will reduce frictional losses around bends and through the silencer. Indeed, if the exhaust could be cooled rapidly enough the reduction in volume would actually improve the extraction from the manifold. Is it just a case of which one is a first order and which one a second order effect?

[tristancliffe]

I prefer to allow the exhausts to cool as dtype says, and 'tune' the primary pipe lengths to work best with that cooler charge. It saves carrying the weight of lagging, and it works for me. I'm not sure any (many?) of us could numbers to a particular system that answers all the questions for a given application.

Having said all of that, we did ceramic coat the outside of our exhausts on the current car because it go so damn close to some GRP/Carbon bodywork that it wouldn't have lasted long. In addition we used heat-reflective sheilding on the floor of the car. So, in effect, I've gone against my normal practice for this particular car as the requirements were different. If melting stuff wasn't a problem, I wouldn't bother with the cost or weight of lagging.

[phoenix]

Originally Posted by dtype38

I understand that the velocity controls the momentum of the exhaust gas and why this needs to be kept up in the manifold, but I'm a bit confused as to its relationship to the pressure losses through the rest of the exhaust system. Intuitively it would seem that the reduced velocity achieved by allowing the exhaust gas to cool will reduce frictional losses around bends and through the silencer. Indeed, if the exhaust could be cooled rapidly enough the reduction in volume would actually improve the extraction from the manifold. Is it just a case of which one is a first order and which one a second order effect?

May I refer you to some further reading, as this is not the place to go into too much depth?

Advanced Engine Technology, Heinz Heisler, ISBN0 340 56822 4

In particular Chapter 5 sections 14-15 (pages 272 - 286)

The minimal disadvantages of frictional losses compared to the major advantages of maintaining kinetic energy are fairly clear and understandable.

There is another disadvanatge to slowing exhaust gasses. As they slow, so does the pressure increase and back pressure is rarely a good thing.

[dtype38]

Originally Posted by phoenix

There is another disadvanatge to slowing exhaust gasses. As they slow, so does the pressure increase and back pressure is rarely a good thing.

The back pressure was my concern. I simply thought that cooling the gasses and hence reducing their velocity would help reduce static pressure, not increase it. Are you saying that this is offset by an equivalent increase in dynamic pressure and serves no useful purpose? Oh heck, where's Bernoulli when you need him. Just can't remember what's squared and what isn't these days.

Tell you what... I'll just take your word for it and crawl back under my rock

PS thanks for the reference.

[Notso Swift]

Originally Posted by tristancliffe

I can't remember which Lotus is was that tried it - might have been the Esprit - bug lagging the exhausts caused the metal to melt! Plus it was down on power as the gasses weren't allowed to cool quickly - and you do want them to cool quickly. As I said before, professional teams, be it F1, GP2, DTM etc don't use it because they want more power.

But WRC do and I would suggest that is more relevent to saloon cars than F1

My car is turbo so it isn't relevent to most of you but my exhaust manifold is lagged, my turbo (exhaust housing) is blanket wrapped, the first 12 inchs of the dump pipe is also wrapped (although that is mainly for under bonent temptretures) and while my dyno run power is basically the same (up a couple of HP, but that may be just the day variation) what is not is spooling 300 rpm sooner on the same tune, or repeatability between runs (that is a heat soak thing).