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# Insulation of Car Exhaust2

## Insulation of Car Exhaust

(OP)

Insulation of Car Exhaust

I am out of my element here but I was hoping for some help.  I just bought an underpowered import.  I love the car, love the gas mileage, and the true cost of the car was one of the cheapest there was.  I am wondering if there are any cheap methods of modifying the car to increase gas mileage and power.

Reviewing the internet, I have come to the conclusion that the only way to modify a car cheaply to increase power and gas mileage without radical revisions to the car itself is to change the muffler and or insulate the exhaust pipe.  I will start with the exhaust insulation question:

The common explanation for the reason for hot exhaust gas being better than colder exhaust gas is that hot gas is easier to push out than cold gas.  That explanation did not make sense to me so I did a quick pipe pressure drop calculation assuming 130 lbs/hr of flow from the car exhaust and a 2.5 “ tail pipe, I found that hot gas had a higher pressure drop exiting the engine than cold gas.  Viscosity of the exhausts increases with increasing temperature which would also make the gas harder to push out.

I believe that pressure of the gas exiting the muffler would be lower due to Bernoulli and therefore there would be less residual exhaust in the piston chamber making the car more efficient, but I am not sure I am really understanding it right.

So I guess my first question is two parts:

1.Does insulating the exhaust help the gas mileage and fuel economy?
2.Would this practice invalidate the warranty?

### RE: Insulation of Car Exhaust

I guess you must think the factory engineers are so stupid that they overlook cheap ways to increase both fuel economy and power. I expect they are generally quite smart and competent.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

### RE: Insulation of Car Exhaust

If you increase power you will generally decrease fuel efficiency.

Pat -- good one by the way, yep us engineering professionals always miss the easy things, its the hard stuff we're really good at.

Nick
I love materials science!

### RE: Insulation of Car Exhaust

I would think if you got a gain at all, it might be more like 0.1 MPG.

Increasing exhaust gas scavenging during TDC overlap increases power, but decreases fuel efficiency by drawing more unburnt fuel into the exhaust system.

Decreasing scavenging increases fuel efficiency, but decreases power.

Wrapping exhausts in the interests of hot rodding is an attempt to increase power. There are two theories, one I believe and one I doubt.

Theory one. The wrapping insulates the exhaust and keeps under bonnet temperatures lower, thereby preventing vapour lock and other heat related damage and thereby keeping inlet air charge cooler, thus increasing density and thereby making more power.

Theory two. The wrapping insulates the exhaust, thus keeping exhaust gas temperatures higher, and thereby increasing volume and therefore velocity. This increased velocity, somehow makes more power, but increasing velocity by using a smaller exhaust pipe somehow decreases power.

Either way, you considerably increase corrosion in the exhaust.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

### RE: Insulation of Car Exhaust

I would think cooling,the exhuast flow at the exh ports would improve scavenging, which would increase HP but decrease MPG, not sure what the torque would do.

but it would be like a condenser on a gas turbine...

thoughts? ( just me thinking outloud)

### RE: Insulation of Car Exhaust

Finally, making sure the car is properly tuned, filters clean, tires properly inflated, wheels aligned and accelerating gently and coasting up to red lights, etc. will increase the mpg more than all the "re-engineering" in the world.  I do use a K&N filter and synthetic oil.  By doing the above, I routinely exceed the EPA highway rating (19)of my F150 on long trips and approach it during rush hour commuting and I will drive at 65-70 mph.

### RE: Insulation of Car Exhaust

Minimalist

One reason your theory (using Bernoulli’s theorem) is not representing the real condition, is exhaust flow is not easy to calculate, As you have expansion, contraction and reversion, all going on in the same pipe, at the same time.

Like the velocities from the valve start at 285fps to 1100fps, some where down the line. Then at some collector or muffler point there is a reversion wave back to the valve.

I invite the real automotive engineers to help me out here as I may be a savant' that can't add or play the piano.

Cheers

I don't know anything but the people that do.

### RE: Insulation of Car Exhaust

Pipe wraps mean higher exhaust temps, lower density, and thus more velocity in the pipe.  More velocity means more frictional pressure loss in the pipes.  I'm assuming here that the engine & intake are operating at a constant speed and temperature/pressure condition, thus mass flow into and out of the engine are constant.

The pipe loss equation looks like:

dp = (K/2)*(density)*V^2

where dp is the pressure loss due to friction, and V is the mean gas velocity.  For constant mass flow, the decrease in density causes a proportional increase in velocity (mdot = rho*A*V); the net result is friction losses are inversely proportional to density (as temperature rises and density drops, pressure losses increase).

The only place the pressures are the same between the wrapped and unwrapped condition, is at the air inlet and at the outlet of the tailpipe (atmospheric pressure).  Everywhere else, the wrapped engine must therefore be at higher pressure, per above (although I'm ignoring pat's idea of decreased heat rejection to the intake air).  The cylinders therefore are less, not more, scavenged, and thus wrapping the tailpipe would actually decrease the intake charge due to higher back pressure during exhaust stroke, all else being held equal.

### RE: Insulation of Car Exhaust

if you have tuned-length headers, changing the gas temp will change the tuning (speed of sound).  Not all changes are for the better.

### RE: Insulation of Car Exhaust

Many years back I was racing in a rules restricted class that only permitted stock cast iron exhaust manifolds. I fabricated a water jacket to surround the manifolds with engine coolant. Picked up torque noticebly from 3,000 rpm to 6,500. Gained 15 hp at peak. This was a sb chev 350 that was in the 400-450 hp range. The purpose of the modification was to increase power but I also kept track of BSFC. In this case the #'s were favorable to suggest an improvement in fuel economy. Based on that I think finding some method of cooling the exhaust would have benificial effect on fuel economy in a normally aspirated engine. Although what we are discussing here is a dynamic process if you take an instant snapshot of what's happening logic would suggest that (PV=NRT Boyles law) by lowering the temperature you also decrease in this case back pressure in the exhaust. I never tested the engine refferred to at part throttle operation but at WOT there is something to be gained fuel economy wize.----------Phil

### RE: Insulation of Car Exhaust

Not quite on topic. But there are a few very cheap ways to increase your mileage by increasing your tire pressure to 35 psi or even more, don't use a roof rack and always empty your car. (Don't drive around with things you don't need on a daily basis).
And I believe the cheapest ways to increase mileage is by changing the way you drive. Maybe you know this already but most people don't realize that they can increase mileage without driving slower. (As there are a lot of competent engineers there are only a few competent drivers).
If you have a manual gearbox you should try to drive always in the highest gear even at very low speeds and when you accelerate, accelerate always in a high gear but close to full throttle to reduce pumping losses. And when you shift, shift from 1st to 2nd or 3rd directly to 5th.
And try to avoid breaking, which is as long as you look well ahead pretty doable.

Last but not least: These guys showed how you can make your econobox fast for no money: http://forums.vwvortex.com/zerothread?id=776885
The same measures will also increase your mileage. :)

btrueblood, if the exhaust gases accelerate and decelerate, which they always do in a piston engine, then you can at least increase peak exhaust velocity without increasing total mass flow. So, by increasing exhaust gas temperature you can increase exhaust gas velocity and the amplitude of an exhaust pulse, which might or might not beneficial to power.

### RE: Insulation of Car Exhaust

There is some good information for you above about scavenging, flow calculations, trade offs etc.  You need to decide exactly how far you want to go.

No, the engineers are not stupid but they are forced to operate on a budget and inside the space given.  On many cars the exhaust system is almost an afterthought, a rippled bent mangled mess of thin walled tubing with cruthces to try to achieve a goal in whatever space is left over after everything else is finalized.

It is possible to increase power and fuel efficiency with performance oriented exhaust.  The best place for any type of insulation is in the engine compartment (great metallic-ceramic coatings are available for headers and tubes).  Reducing underhood temperatures will increase the longevity of other components (especially plastic parts) as well as help your car run cooler.  The fact that the ducted air intake is running through a cooler enviroment is also a plus because a cooler air intake charge will make more power without hurting your fuel economy. Reduce as many bends and turns as you can and those that must stay should be mandrel bent (no kinks) to avoid additional turbulence.  The diameter of the pipe itself should be calculated to achieve your goal and remember that the system does not have to be the same diameter the entire length.

First determine how much effort and money you are willing to put towards this project and what kind of returns you are looking for. Then we can get more information about your particular automoblie and point you in the right direction or tell you to look elsewhere in the system (car) for improvements.

### RE: Insulation of Car Exhaust

If one were to take cooling the exhaust gasses to the extreme, given that cooling reduces volume, pressure and friction, assuming reasonable airflow, why not manufacture the rear 1/2 to 2/3s of the system with aluminium pipe?  much better thermal dissipation. Maybe even some finned heat sinks to help remove the heat, and to prevent the pipes from melting.

Ken, "i always think of the strangest ideas"

### RE: Insulation of Car Exhaust

Finned aluminum cat back exhaust? Hmmmm... Watch out for speed bumps! Hehe.  You would also have an oxidation problem with the tubes.  Thin wall would not likely last long with the heat and moisture.  It would be a bit extreme and makes sense but I doubt you would get any degree of measurable improvement.  Any improvement would probably be from weight savings.

### RE: Insulation of Car Exhaust

Cooling fins might amplify some sound waves also

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

### RE: Insulation of Car Exhaust

Years ago a friend and I fitted an aluminum exhaust pipe to a Triumph Spitfire sports car.  It was incredibly loud -inside the car - compared to the same system with a steel pipe.  The only explanation we could come up with was that the much lower modulus of elasticity of the aluminum was allowing the internal pressure pulses - the muffler was behind the rather thin alum. pipe - to radiate through the tubing wall.  Sounded a bit far fetched to us, but it was the only thing we could think of.  It came back off the car and we said "So that's why cars never have aluminum exhaust pipes!"

### RE: Insulation of Car Exhaust

Minimalist, the problem with engineers is they are expected to know everything.  Not wanting to let anyone onto the fact that they don't, they like to lash out at anyone who dares to ask a question.  If they don't know the answer, they lash out with proportionately more zeal.  How dare you ask a question and on an engineering tips forum to boot!

You should have asked this question on a purely automotive forum where people who have been tuning cars for years could tell you exactly how they have achieved what engineers haven't.

If you had asked this question in the 1950's someone like patprimmer would have probably had pretty much the same answer, yet here we are in an age where never-before have horsepower, fuel economy and low emissions ever lived so happily together.

I do not know the answer to your specific question, but I think the spirit of your question is, are there ways to improve my fuel economy without sacrificing (or while improving) my performance.

The answer is yes.  Most of the easiest and cheapest ways have been touched on here in this forum.  I can name many others and I do not consider myself an expert by any means.

As a recap, those already mentioned directly or indirectly:
1.  Drive more fuel efficiently.  On Top Gear (a British car show) a journalist drove 800 miles on a single tank of gas in a turbo diesel V8 Audi.  It was about 40 mpg.
2.  Keep your car in tune and tires well inflated.
3.  This is the absolute best way to save on your fuel bill:  lose weight!  There are many ways to lighten a car without significantly effecting it's overall character.  And if you need to lose a few pounds yourself, that wouldn't hurt.  As a benefit, you will not only get better mileage but your car will accelerate faster, stop quicker handle better and the women will take note of your new physique!
4. Have the ecu which is designed to meet a wide variety of driving conditions in a reliable fashion, retuned or replaced so that your engine will maximize economy.

The problem with any modification is that there is always an associated cost involved.  Even filling your tires with extra pressure will stiffen your highway ride and it may increase your stopping distance.  So we really need to know more about how far you are willing to go and what you consider an acceptable dollar amount.  After all you could buy more gas for the cost of that header wrap.

When you say you don't want to do anything radical, do you mean you don't want to have to do engine work or do you mean you don't want to do a full up F1 style redevelopment of your car?  Since I don't know what "radical" means for you, I will just mention a few other ways that you can improve your economy with an eye toward improving (or not decreasing) performance.

5.  Increase your compression ratio.  It helps performance and economy.  Unfortunately your engine will run hotter and endanger it's survival.  This can be mitigated with a water injection  system (instead of added fuel)which can be pretty cheap or pretty expensive depending on how elaborate you go.  If you are willing to do most of the work yourself, the cost to have the head shaved might be worth it.

6.  If you have an automatic, you can switch to a manual.

7.  If you have a manual, switch to a 6 speed manual.  There are companies that specialize in matching nearly any transmission to any car.

8.  Install underdrive pulleys.  These cause other systems that run off the engine (altenator, a/c, water pump, etc.)to run at their minimum acceptable levels, thus decreasing the losses.

9.  Remove unnecessary systems. Power steering can be eliminated.  Do you have a power antenna?  Nav system? Power adjustable seats? Maybe you don't listen to the radio.  It could go.  Do you live in a cool climate?  Remove the A/C.  It's all a matter of personal preference of course.

10.  Improve your aerodynamic efficiency.  This is hard, but perhaps you have an unnecessary wing on the trunk lid.  You can also add a front chin spoiler.  Apparently the reduced drag from eliminating air under the car overcomes the increased frontal area (Not sure I believe this one, so maybe an aeronautical engineer in the forum can shed some light here).

11.  Reclaim some energy.  There is no reason that the principle of regenerative brakes on a hybrid vehicle cannot be applied to a normal car.  Although I guess developing something like this would fall under the "radical" category.  Before you flame me on this one, let it be known that BMW is going to do just this in next years car (I think it was the 3 series).

12.  If you do live in a warm climate, tint your windows.  The less the a/c works, the less your engine has to work.

13.  Go solar!  There are a few systems on a car that can be powered or partially powered by the sun.

14.  Cover your car or park in a garage.  Same principle as the window tint, but for your whole car.

15.  Turn off the engine if you are going to sit longer than about 2 minutes.  Shorter and you will consume more fuel upon starting.

16.  Consider an "on-demand" power adder like nitrous oxide that you only use when you need that performance.

17.  Swap engines.  Toyota has a 1.3 liter turbocharged engine that was used in the Paseo overseas that gets excellent gas milage due to its small size, but still puts out 135 HP.  Perhaps your car has such a Japanese equivalent.  Radcal?  Maybe.

18.  Wash your car.  Yup, wash your car.  UPS washes its trucks every time they pull into the hub at the end of the day.  The primary reason is for image, but they also know that a clean truck reduces maintenance costs.  Don't know if or how much this improves performance, but I doubt it will hurt performance and it's not expensive.

Again, there are associated costs with all of these and I'm sure that an engineer such as yourself can figure out what they are, but there is a lot you can do.  You just have to decide how much money, time and inconvenience you can tolerate.  And anyone who has tried to reduce the weight of a car can tell you that you are not likely to lose it all in one chunk.  You have to shave it off in lots of places and I think that the same can be said for your economy issue.  But I wouldn't expect too much.  Peformance and economy are definitely at odds with one another.

So now that I've said that, I still don't know.  Keeping heat in the exhaust:  good or bad?

### RE: Insulation of Car Exhaust

The best way to lose weight and save gas is to keep the car in the garage and ride a bicycle to work.

No, of course I'm not going to do that - it's 45 miles to work and I often need to carry a suitcase!

However, I've insulated (wrapped) the 4 branch exhaust manifold on my 10 foot long home built car because the battery and the alternator need protection from the heat, also the inlet manifold and carburettor sit directly above the exhaust.

See: http://groups.msn.com/liegecars/liegebimini.msnw?action=ShowPhoto&PhotoID=934

### RE: Insulation of Car Exhaust

I have no problems with questions. It's stupid questions I don't like.

I have no problems with new ideas. I have a problem with ideas that have been tried and tested for 100 years or more being suggested as new.

I have a problem with people who pre-empt their answer with a comment like "I am not an expert, but" then go on to contradict people who obviously do not need that qualifier (because they are experts in that field).

Anyone has a right to an opinion, but the only ones who have a right to a correct opinion are those who are in fact correct.

No one will seriously dispute most of your 18 points except to say, THEY ARE OFF TOPIC. That is not what he asked. We actually answered the OPs question. You simply restated it with a question mark. Most of your points can actually be found in owners manuals and automobile association publications, or at least the points no one will argue can be found there.

To follow your rational, he could go one further and modify the car until it becomes a Moped and make even greater savings.

I once had a 1963 VW Beetle. Raising the compression from about 6:1 up to 10.3:1 had a profound effect on both performance and economy. That is because back in 1963, an engine designed before WW11 had a lot of scope for improvement and because available fuel had a lot higher octane than that reliably available in Germany during WW11.

I now have a 1999 Honda CRV engine in my 1989 Honda  Integra. I will not be increasing the compression on it as it is already optimised, or very close to optimised for fuel currently available.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

### RE: Insulation of Car Exhaust

Exhaust wrap was discussed on a MG Forum some time ago and I found these comments from Peter Burgess interesting.....

I am not convinced of the effectiveness of heat wrap...Keeping the exhaust isolated from the inlet manifold/induction system is great....use a physical barrier. Keeping the heat in the exhaust system could lead to extra heat being trapped in the cylinder head, reducing efficiency...in the same way that I understand stainless steel exhaust manifolds tend to keep the heat in the head. I have had two negative experiences with heat wrap....

1) MGB standard race car with heat wrap....after the end of the season the engine was replaced with next years' engine...only the owner rang up to say the new head was faulty as the studs didn't line up with the manifold ( standard CI)....after much head scratching ( ) it transpired that the manifold had warped by some 1/4 of an inch or so.....we wondered where the extra heat 'lived' under running conditions.No heat wrap from then on!

2) Ford Capri 2.1 Pinto with twin 45 Webers, hairy cam and monster cylinder head.....belonging to a friend...we relayed the story about the MGB ex manifold....bravely, our friend ripped off the wrap (about £30 worth!)....result 4 bhp increase, which seems to be attributed to removing the heat wrap. The engine is a x-flow so no heat was being saved from direct contact with the inlet manifold as in the MGB scenario, the benefit of having a cooler underbonnet area was more than wiped out by whatever was happening inside the head with the reduced heat rejection.

Overall, I get the feeling we can cause more problems than we cure with using heat wrap.

### RE: Insulation of Car Exhaust

One of the car mags, Hot Rod or Car Craft, just did a test on all the heat coatings available for various parts of the engine.

Coating the headers did lower external measured exhaust manifold temps by 200deg, but did not increase dyno HP at all, as I recall.

### RE: Insulation of Car Exhaust

I have used exhaust wrap on volvo b20 engine and I noticed some decreasing in fuel consumtion.
One of my friends who had more tuned B20 reported that he had ~2L/100km decrease in fuel usage with exhaust wrap.

But this engine was similar as the one in picture above so that intake and exhaust manfolds crossed each others and allmost touched each others.

I have also seen some decreases in fuel usage with engines that take air from engine compartment, but I bet that there would have been similar effects with ram air intake systems..

### RE: Insulation of Car Exhaust

Get thee a JC Whitney catalog and for a modest expenditure you can get an intake turbulator, fuel ionizer, miracle fuel additives and negative coefficient of friction oil.  50% fuel economy improvement or more.  And you can violate several laws of thermodynamics without risk of incarceration.

### RE: Insulation of Car Exhaust

You forgot the magnets.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

### RE: Insulation of Car Exhaust

#### Quote (Profozone):

You should have asked this question on a purely automotive forum where people who have been tuning cars for years could tell you exactly how they have achieved what engineers haven't."
LOL!
Yeah,  that must be why garage mechanics are in such high demand to design new vehicles!

The simple answer to the original question is this: increasing exhaust temperature increases the volumetric flow rate,  thus increasing the exhaust backpressure.
This only rarely produces an improvement in fuel economy.

I especially got a laugh out of the aero engineer's comment that " I am positive if your focus was on fuel economy that you could make improvements. "
Uh,  that would be the U.S. auto industry since the mid-70s;  feel free to jump in and market any revolutionary improvements that thousands of engineers have overlooked over the past 30 years!

### RE: Insulation of Car Exhaust

One our large industrial NA engines had an upgrade to increase horsepower.  The upgrade was a water cooled exhaust manifold.  The increase was from 750 HP to 800 HP.  So you are correct on the cooler exhaust.

A larger more efficient air intake CAI or shart arm, again less back pressure on the intake increases torque.

As most have said, driving habits are your best bet for milage.

### RE: Insulation of Car Exhaust

dcasto, I'd be interested to learn the exact mechanism for the power increase.  Also the magnitude of the cooling load increase, and resulting increased parasitic losses (e.g. larger fan, larger waterpump), if any.  What heatsink is used for cooling (I understand it is probably jacket water that is cooling the exhaust manifolds, but where/how is jacket water heat rejected?)?

### RE: Insulation of Car Exhaust

One option would be to lean out the system. I have read about many tuners that do this. Most people use them to increase their horse power injecting more fuel, but several have had tunes created so that the car injects less fuel than the factory programed it
Depending on what year the car is and the make there may be a variety of tuning options available. Mainly by chipping the factory ECU and creating your own fuel table's

Try and find an automotive forum that has many members with the same make of car. They would probably be the most helpful.

### RE: Insulation of Car Exhaust

The engine is a Cooper Energy -White Superior.  The RAM exhaust manifold uses engine jacket water to cool the exhaust and I'm not familar with the whole process, but the cooler exhaust allows the exhaust to "scavenage" out better and allowed for more load.  The RAM air intake I think was just better porting and larger throttles (it has two throttles on an inline 8 cylinder).  I don't know the magnitute exactly, but we ended up putting on a larger cooler, say about 15% bigger.

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