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This is something that puzzles me again and again. 4WD owners wanting to increase the power and / or torque are keen to spend heaps of money to get a replacement exhaust with a bigger diameter.
And there are “experts” who will swear on it and who will explain all the significant positive impacts, that it will reduce the back pressure thus allowing the engine to breath more freely and therefore this modification will gain 10% … 25% more torque and power and will even reduce fuel consumption … no matter whether it is an older engine type, a newer one, diesel, petrol, turbo charged etc.... Hmmmm…
O.k., no matter what engine type, they have something in common: an exhausts directs the combustion gases from “A” to “B”, providing also some back pressure that indeed will have an impact on power and torque. What needs to be considered is that every exhaust system is designed for the maximum power of an engine, because this is the scenario when the gas volume as a result of maximum fuel consumption and combustion process peaks.
More power needs more fuel, more fuel results in a higher gas volume and this gas has to leave the system via the exhaust system.
…a fool and his money are soon parted…
More fuel to burn needs an optimised filling of the combustion chamber / cylinder and a suitable exhaust system has to be designed to maintain the ideal back pressure - especially for engines without a turbo - also at lower RPM when the exhaust gas flow rate is lower without restricting the much higher gas flow rate at maximum RPM.
Some notes about “back pressure”: you can also read statements like …”back pressure is not required and will always cause power losses…”. So that means the easiest way to a powerful engine would be to omit the exhaust totally as this would result in the biggest gain wrt of minimizing the resistance/back pressure?
Beside the fact that it would be a little bit noisy I also believe that it doesn't work like this; there might be only very rare scenarios and engine types where this statement is true, and people who experienced - while driving a non turbo-charged car - a broken exhaust with the unavoidable power loss at normal RPMs maybe able to confirm this.
Valve timing, camshaft degrees and overlapping play a major role in an optimised filling of the combustion chamber and not enough back pressure will often compromise the filling and therefore the power output. A turbo will mitigate the impact of the back pressure caused by the exhaust on the filling, hence will also minimize potential losses anyway.
In former times we tried to increase the horse power of our motorbikes by replacing the camshaft with a more aggressive camshaft to allow more overlapping. At high RPMs and in conjunction with a good tuned exhaust that facilitated the right back pressure significant higher speed / power was achievable, but the trade off was always the loss of torque at lower RPMs. And this is exactly what most of 4WD owner don’t want.
Most 4WD owners driving diesel engines will try to stay away from high RPMs as wear and tear accelerates exponential with higher revs and one of the reasons for diesel engines is the torque they already provide at lower RPMs.
But also drivers of 4WDs with a petrol engine will likely prefer to use lower and middle RPM ranges most of the time as driving at maximum RPMs will not only cause more wear and tear, but will also result in extreme fuel consumption.
Lets use my Terracan as an example. The highest exhaust gas flow rate will occur at maximum RPM; that’s is the physics for all combustion engines. For the Terracan the red range starts at 4,500 RPM and while higher RPMs are possible most people will agree that it is wise not to exceed this threshold, exceptions are sport cars and racing scenarios, but we talking here about 4WDs.
Nevertheless, the exhaust system is designed also for the higher RPMs exceeding the threshold where the red RPM range starts, respectively for maximum exhaust gas flow rates.
To keep it simple I assume that the gas flow rate for my Terracan at 2,250 RPM will be just half of the flow rate at 4,500 RPM - that’s not totally correct and it needs to be considered that gas is compressible and relevant calculations are much more complicated compared to fluids.
But as the gas volume will also depend on the temperature - the higher the temperature the bigger the volume, or the pressure according to the gas equation - it is a conservative approach and the volume increase at higher RPMs will likely be even higher due to a likely increased exhaust gas temperature.
According to my assumption at 2,250 RPMs the engine sends distinct less than 50% of the volume down the exhaust system that is designed for 100%.
So what will be the gain for the lower RPM range when increasing the diameter of the exhaust, which is already sized for a much bigger gas volume produced at 4,500 and even higher RPMs?
But there are more frustrating facts for 4WD owners who spent already $1,000 and more for a new exhaust system. The increase in power and torque achievable for a modern engine by optimising the exhaust is realistically anyway just 2% - 5% at maximum RPMs, no matter whether it is a diesel or a petrol engine. The 10% to 25% some “experts” states are bare nonsense, at least for modern engines. And to make things worse this gain at maximum RPMs comes even likely with some losses at lower RPMs.
So while gaining a little bit extra power at maximum RPMs the engine power will be likely compromised for 99% of the time you drive the vehicle - how often do you really use respectively need the maximum RPMs of your 4WD?…as long as you don’t tow a horse trailer with 160 km/h at 5,000 RPMs over the German Autobahn all the time such a modification is just a waste of money.
Exhaust modifications make sense for sport car engines aimed to operate around the maximum RPMs that cause maximum gas volumes most of the time - which is the case only in racing competitions.
For a 4WD it is a waste of money; for the sake of longevity and reliability the engine should run at lower RPMs, making use of the gears when cruising and of course using also low range in off road scenarios.
Therefore the money is better spent - if a power increase is really required - for measures that will increase power and torque at lower revs - and that is something that can’t be achieved with increasing the diameter of the exhaust system.
A few weeks ago (present older article revised 04-12-2012) I found a typical example for a misleading advertisement in the December edition of the OVERLANDER 4WD mag; unfortunately this journal was recently overtaken by the “action guys”, so likely it won’t take long and it will be on the same level like their other mags… I will miss Dr Karl’s contributions…
On page 44 of this edition you find the following “expert advice”:
quote: …company xx recommends to get the most from your Isuzu D-Max LST with the 3.0L turbo-diesel engine is to do the following: ‘built a dump pipe right off the back of the turbo with a full 2.75in system incorporating a high-flow catalytic converter with a straight-through muffler. This will let the turbo spool up much faster and will give you an additional 8-10% better fuel economy and 10-12% more power’… end quote.
More power and less fuel at moderate revs??? … keep dreaming.
I sent an enquiry to this company, telling them that I don’t want to prepare my D-Max for ‘Paris-Dakar’, but would appreciate the more power and even more the min. 8% fuel savings. I also told them that I would expect that they would warrant their claims as it won’t be a cheap investment …needless to say that I knew their answers with respect to guarantees already…
However, to be fair I have to give this company also some credit, they responses were prompt and the prices they ask for are absolute reasonable; with respect to finish and quality their product is likely far better than the original stock part. If I would need a replacement for my car I might even consider their products, just for the quality, but of course not to increase torque and to drop fuel consumption as this is obviously a myth - at least for a modern engine. I understand that it is a tough market and that every product needs to outperform the product of a competitor - it is a pity that just good craftsmanship isn’t good enough anymore…