Spin Cycle

The owner of this Porsche twin turbo would like to remain anonymous, but take it from us he's a guy that likes playing with his cars. From the brand new BMW X5 to the classic 356A Porsche, he's got the need for modified speed. Even on the twin turbo that cost over $300,000 new, he wanted to try a bunch of different mufflers. The aim was better power, a great exhaust note and a quicker spooling-up of the twin turbos.

Under the bum of the Porsche you'll find a complex place: there are twin cats, twin heat exchangers for the cabin heater, and twin mufflers. It's all very squeezy for room, making modifications that much more challenging. In fact, rather than opting for replacement mufflers, the owner decided initially to get another pair of the standard mufflers (internals shown here), gut them and then rebuild them for higher flow.

But before that occurred, a 'before' dyno figure was run - it resulted in a peak power of 204kW at the rear wheels (the normally all-wheel drive Porsche was set to just rear wheel drive via a temporary gearbox mod). However, even more important than the power peak was the shape of the power curve at high revs: by 6200 rpm the power had dropped way down to just 153kW. Ouch!

Next up, the standard mufflers were replaced with the modified 'uns. As shown here, the new internals consisted of three lengths of perforated tube joined with non-perforated bends. The mufflers were packed with stainless steel wool - a great design.

And how did the Porsche respond on the dyn? All the way up to around 4000 rpm there was no change in the power curve at all, while from 4000 to about 5500 rpm power was down a little. But it was at the top end where the real gain came - power was up by 8kW! The owner was ecstatic.

"The first difference is the sharper throttle response," he said. "Off-idle response is dramatically improved and bottom end power is noticeably better.

"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 cost was also more than just acceptable: 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. That made the cost/benefit equation heavily weighted on the side of benefit.

The next pair of mufflers to be tried were the Mille Miglia designs. Expensive - as you'll find all quality mufflers - at $3000 for the pair, they're made from stainless and are obviously of high quality.

Again there was little to separate the designs over the first half of the rev range, but at the top end there was a huge difference: an extra 18kW was unleashed!

So which of the three sets of mufflers would be the pick? Well, that depends on the sound quality as well - and we all liked the Mille Miglias - so which way you chose to go is really up to you. But one thing's for certain, modifying the mufflers can sure make a real difference!

Despite the Porsche 993 twin turbo originally costing over $300,000, these days its engine is really showing its age. Even with the twin puffers, the car is slow to come up on boost and runs out of revs at just over 6500. Peak power is at a miserable 5000 rpm - so the power band is incredibly narrow for a twin turbo. Compared with a modern turbo engine like a Supra sequential twin turbo (or even the now quite old Skyline GT-R mill), the Porsche - despite its massive 300kW power output - is often struggling in day to day use.

The owner of this car - who likes to remain anonymous - thought so too; he decided to trial some different mufflers in the hope of boosting power and response. Having been told that the standard mufflers were quite restrictive, he began with the easiest and cheapest approach - modifying them. With space incredibly tight under the rear-engined car, custom fabrication needed to be avoided - few people, for example, know much about re-working the cabin heat exchangers which are built into the exhaust.

Inside the standard mufflers the picture looks something like this. 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. However, Porsche isn't a company known for lousy designs, so there may well be more to it than first appears.

The new design was kept as simple as possible, in order that build cost could be kept down. Also, a KISS muffler is more likely to work than something really fancy. Inside, it looked like this: 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.

A pair of commercially available Mille Miglia mufflers were also put to one side - the comparison would involve all three sets of mufflers (standard, modified standard and Mille Miglia) tested on three consecutive days. The Mille Miglias cost simply stupid money (how can $3000 be justified on a pair of mufflers?) but they look pretty. Pity you won't see them normally anyway.

Onto the dyno the car went, temporarily modified to run in two-wheel drive. Of course, without the extra driveline drag of the front wheels, the dyno test was immediately not replicating real-world conditions; further, as with any turbo car, unless the dyno was set to give the ramp speeds appropriate for the gear in which it was being tested, the power curve on the road is likely to be quite different. (And even worse, most dyno runs start with the engine already under load - and so with the turbos already spooling. The same thing simply doesn't happen when you're driving along in second gear at a constant throttle, and then floor it.)

In standard form, the car (tested on a lossy Dyno Dynamics machine) achieved a peak power of 204kW. The power curve was reasonably smooth until about 6250 rpm, where there was a slight jump. However, with peak power over 1200 rpm earlier in the rev band, what happens up this end doesn't matter a great deal: the driver will already have changed gear and be way back down the rev band.

With the modified standard mufflers fitted, from 4750 to 5750 rpm power was down over standard - in fact the peak dropped a little from 204kW to 199kW (but since that's only 2 per cent, it's of little importance), and at 5250 rpm it was down by over 4 per cent - still a tiny variation. Above 6000 rpm the modified mufflers performed a little worse, but there was very little in it. So on the dyno, the modification to the mufflers made almost no difference - and if anything, were overall worse performers than the standard designs.

With the Mille Miglia mufflers fitted, the results were even worse - despite making a good gain at the extreme top end, the average power across the rev band was almost certainly down over standard - power was below the standard output from 4000 - 5300 rpm, and really only gave a clear-cut advantage (ie more than few percent difference) from 6200 rpm onwards. Given that peak power occurred 1200 rpm earlier, picking up a bit of power at the extreme top end doesn't help much.

The bottom line? Changing the mufflers makes close to zero difference to the overall dyno power figures, unless you're often using rpm above 6250 (way past peak power).

(We assume that by now you know which sets of mufflers were fitted, so we won't go back over all that again!)

The Porsche is a car where dyno-proving anything is very difficult indeed. The difference in performance with the modified mufflers was quite clear on the road - not (as the dyno shows) at the extreme top end of the rev band, but in the way in which the car came on boost. It was both earlier and the transition to boost stronger with the non-standard units in place.

Being an air-cooled engine and with the intercooler located at the rear of the car, getting adequate cooling in a dyno room is near-impossible. The enclosed room at ChipTorque provides demonstrably better car cooling than the open-workshop dyno location of the (now defunct) Gold Coast Rigoli workshop used in our previous story, but even so ChipTorque dyno operator Mathew Spry had great difficulty in getting consistent engine oil temps between runs. (Sometimes he waited a good 5 minutes for the temp to come back down - and this on a cool 20-degree day.)

The way in which the dyno graph is read can give results as good or bad as the interpreter wishes. The bottom line is that from 2750 rpm to 3800 rpm there is no real difference in power output (the shown variation is easily explained by day-to-day variations); from 3800 to 5400 rpm the standard mufflers were best, and from 5400 to 6000 rpm there was almost nothing in it. Above 6000 rpm the Mille Miglia units gave better results - but with this engine, way too late for normal driving.

And isn't that a different assessment from the other 'spins'?

The owner's thoughts?

"The Mille Miglias were very expensive - in fact too expensive - but since aftermarket Porsche exhausts can sound anything from woeful through to brilliant, I'll keep them on the car - the Milles are at the brilliant end of that field.

"Oh yes, and the on-road performance is better than the standard mufflers."

Our previous story on the standard and modified mufflers concentrated as heavily on noise levels and sound quality as performance. As we then commented, those dyno curves showed a bad dip around 3800 rpm, something which is retrospect was probably caused by the knock sensor pulling back timing as intake air temp and/or engine oil temp rose.

The 'before' and 'after' dyno curves that appeared in that story seemed to match what was found on the road - the car coming up on boost faster and power peaking higher - so we were confident in including them in that article. But now, having performed so many more dyno runs in a cooler dyno room complete with better airflow, we wonder if that difference wasn't more the case of the ambient conditions differing sufficiently for the engine to alter in its output.