While fitting a new exhaust is a common mod to cars varying in stature from budget Hyundais to the current model Nissan 200SX, not too many do it to exotic cars like a Porsche twin turbo. However, there's no reason why a power gain shouldn't result from this modification on any car - irrespective of its price range.
And in this case, careful measurement showed that making some exhaust changes gave excellent results.
Exhausts & Mufflers
Exhausts - or more usually, the mufflers and cat converter(s) - always cause backpressure on the engine or turbo(s). Typical factory muffler designs incorporate baffles and/or pipes with small diameter holes through which the exhaust gases are forced to pass. OE engineers do this not because they like restricting performance - they do it to get the required level of noise and emissions outputs (a small cat improves light-off time) within an appropriate budget.
However, for an enthusiast, where a bit more exhaust noise is tolerable (or even desired!), and absolutely strict emission compliance isn't of necessarily of such a major concern, a free-flow exhaust is likely to give a power gain at relatively low cost.
Testing that AutoSpeed has carried out ["Giant Muffler Comparison - Summary of Results"] has clearly shown that the type of muffler with the lowest exhaust restriction is a straight-through design. That is, the exhaust gases pass through a perforated pipe that is surrounded with stainless steel packing within the body of the muffler. And, contrary to expectations, this type of muffler can also be very quiet - with two caveats. One is that the volume of the straight-thru mufflers employed need to be large - a 'hot dog' resonator isn't going to do a lot on its own. The second caveat is related to the first - often multiple straight-thru designs will need to be used if the lower frequencies are to be reduced sufficiently in sound pressure level. This is because straight-thru designs tend to work better at quelling high-pitched noises - so more are needed to control low frequencies.
Neither of these poses much of a problem in most modern car designs, though. For example, my LS400 Lexus has no less than five standard mufflers - plus two cat converters (cats act as small resonators in exhaust systems, doing their bit to drop noise in addition to reducing emissions). Fitting five aftermarket mufflers - even of the straight-thru variety - is likely to give a system that drops exhaust noise down to a level that an enthusiast would be quite happy with, and with much less than factory flow restriction.
But the Lexus is a large front-engined car with lots of room under it - what about the Porsche, which has the engine stuck out behind the rear wheels? Certainly no space for five mufflers in that design!
The Porsche
The factory exhaust system uses twin catalytic converters (each with an oxygen sensor positioned either side of them), twin mufflers and an elaborate heat exchanger system to feed the climate control. Given its complexity, it was decided the initial step should concentrate on making changes to only the two mufflers. (Note that in most cars, it is the cat converters that cause the greatest exhaust restriction, so the approach shown here isn't the one that you'd always take.)
But how good was the standard exhaust system anyway? There are two ways in which this can be approached.
(And why even consider how good the standard exhaust is? Well, how much money do you want to waste if in fact it proves that the system is pretty good? In some cases - especially when each section of the system is considered individually - parts of the standard system can be fine. Like a Suzuki Swift GTi's exhaust manifold, for instance.)
The first approach is to measure exhaust backpressure. The easiest way is to unscrew the oxygen sensor (if the car has multiple oxygen sensors, then unscrew the one closest to the cylinder head) and replace it with a temporary pressure-sensing fitting. Run this to a normal boost gauge and you can easily read the peak exhaust backpressure reading. The higher it is, the worse the exhaust flow. Easy!
The other way is to cut open the standard mufflers and have a look at their internal construction. In the case of the Porsche, AutoSpeed reader 'Q' - the owner of this car - had been assured that the standard mufflers were in fact very restrictive, so he chose to jump the backpressure measurement step and have a look inside the mufflers for himself.
The inside view can be seen in this diagram. Each muffler uses a perforated pipe and a small bypass tube. The major portion of the exhaust gases is required to pass through the perforations into a long curved outlet pipe, prior to exiting. We'd assume that the bypass tube is to drop light-load (eg cruising) exhaust backpressure to improve fuel economy, without adversely affecting the noise suppression at high gas volumes. But basically, it's a primitive muffler.
'Before' Testing
Before any exhaust modifications were carried out, extensive noise and power testing was undertaken.
| |
Condition |
Standard SPL
(dBA) |
| Outside cabin |
Ambient |
46 |
| Start up |
70 |
| Idle |
63 |
| Constant no load 3000 rpm |
76 |
| Constant no load 5000 rpm |
93 |
| Constant no load 6500 rpm |
96 |
| Drive-off |
90 |
| Drive-by, 60 km/h, 4th gear |
72 |
| Drive-by, 100 km/h, 6th gear |
88 |
| Inside Cabin |
Ambient |
42 |
| Idle |
58 |
| Constant 60 km/h, 4th gear |
60 |
| Constant 100 km/h, 6th gear |
64 |
| 6500 rpm, 2nd gear |
70 |
Sound recordings were also taken, with several of the standard car available here:
| Standard - idling |
|
| Standard - reving from idle |
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| Standard - drive-by at 100 km/h |
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| Standard - 3000 rpm, no load |
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The car was also run on the Rigoli Performance 4WD Dyno Dynamics dyno. This type of dyno has a large tyre/roller interface power loss, and so the figures achieved here should not be compared with, say, the power figures gained from a DynoJet machine. However, as in all of these sorts of tests, it's the 'before' and 'after' comparison which is of greatest relevance.
And the 'before' figure? The standard car had 246hp at all four wheels. However, it also had a 15hp power dip at about 110 km/h in 4th gear - ie at about 3800 rpm.
The Muffler Modifications
Hmmm, so what to replace the standard restrictive mufflers with? Rather than buy off the shelf, it was decided to fabricate new internals for the standard mufflers. The factory mufflers were gutted and a simple straight-thru replacement design used. This consisted of 2.5-inch perforated tube for each straight section of the flow path, with non-perforated 90-degree bends used to connect these bits. The mufflers were re-packed with stainless steel and welded back up.
The Results
With the modified mufflers fitted, it was immediately noticeable that the exhaust noise at idle was louder. And, given that there is a turbo positioned in front of each muffler, the exhaust note was odd - much more 'naturally aspirated' than 'turbo'. The owner described it as alternatively "angry sounding... or holes in the mufflers - depending on your point of view!" However, there was no crackling on the over-run - a positive.
And the measured noise levels?
| |
Condition |
Standard SPL
(dBA) |
Modified SPL
(dBA) |
| Outside Cabin |
Ambient |
46 |
46 |
| Start up |
70 |
74 |
| Idle |
63 |
70 |
| Constant no load 3000 rpm |
76 |
78 |
| Constant no load 5000 rpm |
93 |
95 |
| Constant no load 6500 rpm |
96 |
96 |
| Drive-off |
90 |
90 |
| Drive-by, 60 km/h, 4th gear |
72 |
74 |
| Drive-by, 100 km/h, 6th gear |
88 |
92 |
| Inside Cabin |
Ambient |
42 |
42 |
| Idle |
58 |
58 |
| Constant 60 km/h, 4th gear |
60 |
62 |
| Constant 100 km/h, 6th gear |
64 |
64 |
| 6500 rpm, 2nd gear |
70 |
70 |
Looking at the 'Inside' figures first, it can be seen that cabin noise levels are largely unchanged - just a 2dB increase at a constant 60 km/h. Importantly, the inside idle noise level remained the same, as did the SPL at full load and in 100 km/h cruise.
And outside? Here noise levels were up - in some cases fairly substantially. Start-up increased by 4db(A) and idle by a very substantial 7dB(A). Curiously, though, driving away is no louder and the noise level also isn't increased at no-load max rpm. (The Porsche has quite a lot of mechanical noise coming from its air-cooled engine - this is the dominant source of noise when revving it hard, rather than the exhaust.)
However, the overall noise outcome is very positive - especially given that the noise levels in the cabin are largely unchanged.
And what about power? Here the result was startlingly good - the modified mufflers found no less than 18hp, or a gain in peak power of just over 7 per cent.
Just from changing the mufflers!
While typically modified exhaust power gains of around ten per cent can be found on many cars, this usually involves fitting a completely new system - pipe diameter, cat converter(s), mufflers... the lot. So 7 per cent from just the mufflers is pretty damn good.
But how was the car on the road?
"The first difference is the sharper throttle response," says the car's owner. "Off-idle response is dramatically improved and bottom end power is noticeably better."
Notice from the dyno graph how the turbos spool up much faster - the 'modified' line is well to the left of the 'standard' line. In fact, at 3500 rpm (ie 100 km/h in 4th gear) during a full-load acceleration run on the dyno, the modified car produced no less than 43 per cent more power! (With the car standard, as the engine accelerated past 3500 rpm it was developing 141hp at the wheels; with the mufflers modified, that jumped to 201.) This gives an indication of how much faster the turbos are coming onto boost - no wonder there's sharper throttle response... (Note that the peak boost level remained unchanged with the muffler alteration.)
Adds the owner, "Secondly, the in-gear acceleration is smoother. It appears to be more linear. The power gain is obvious, the noise level is acceptable."
And the sound of the modified system? Here are some 'after' recordings so that you can judge for yourself:
| Mufflers modified - idling |
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| Mufflers modified - reving from idle |
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| Mufflers modified - drive-by at 100 km/h |
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| Mufflers modified - 3000 rpm, no load |
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However, note that the power dip in the curve still remained after the mod had been carried out. Given that it is not noticeable on the road, it may be caused by overly high intake air temps, or a too rich an air/fuel ratio - both conditions occurring only on the dyno.
Conclusion
With the cost of the modifications very low - approximately AUS$600 exchange for the mufflers (ie cut, gutted, pipes installed, re-packed, re-welded) with another AUS$150 for the fabrication of the new inner pipes - the cost/benefit equation is heavily weighted on the side of benefit.
And this story also shows an approach that may well be able to be taken with cars other than Porsches - after all, factory mufflers are these days almost always large, fit perfectly in the floor recess, and are very often stainless steel.
So if the factory mufflers can be easily and cheaply modified to give results this good, a new and viable approach to wholesale replacement is - er, literally - opened up.
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