Our Peugeot Diesel - Part 1 - The Exhaust

It was Simon, he of Simon’s Car Clinic, who made the radical suggestion. With the Peugeot 405 diesel turbo up on his ramps, I was measuring the size of the standard exhaust. At 2.25 inches, and with no cat converter and no particulate filter, it looked very free-flowing for the 67kW flywheel power. In fact, the pipe size and bends looked fine – there was just the rear muffler as the major flow impediment.

“Take it off!” said Simon, gesturing at the sole muffler.

“Pardon?” I asked.

“Take it off,” he repeated. “We had a diesel ex-ambulance in here that was sluggish-as. When it was converted to an ambulance they’d taken out the middle muffler, and I took off the back muffler. You could barely hear the difference but it went much better.”

So here I was, lying under the back of the Peugeot, undoing the clamp that holds the rear muffler on.

What would the car be like with no mufflers at all?

The answer was quite intriguing.

Firstly, the noise was far less than you’d expect, which makes sense. Most of the noise of an exhaust is caused by the rapid expansion of the hot gasses as they exit the tail-pipe. In the same way as lightning creates very rapidly expanding hot air than in turn causes thunder, the temperature of the exiting exhaust gases will help determine the noise level that’s created. And in the case of a diesel, exhaust gas temperatures are much lower than in petrol engines.

Secondly, as with any turbo car (petrol or diesel), the turbo helps muffle the exhaust – again by reducing its temperature (there’s always a temp drop across the turbine which is indicative of the energy removed from the exhaust stream). In addition, the turbo homogenises the individual pulses.

So, how noisy was the muffler-less Peugeot? Well, probably too noisy for me, but not so noisy that I didn’t do my daily run to the post office box – and there’s police station in the same street as the post office!

But what really blew me away were the changed power characteristics. The turbo felt like it was boosting just so much earlier!

In the standard car you really need to change up gears at 2500 or even 3000 rpm – change at revs below that and in the next gear the engine falls into an off-boost hole. Drive off from a standstill and the turbo can be felt to come on boost only at about 2500 rpm. And since the performance over 4000 rpm starts to die away, that gives just a 1500 rpm power band. (It’s not as bad as it sounds because gearing and the shape of the torque curve also come into it.)

But the rev range between idle and 2000 rpm is basically useless and performance is flat until 2500 rpm.

But without any muffling, the turbo felt like it was effectively boosting just off idle!

I put a boost gauge on the car and went for a drive. This showed that 7 psi boost was being achieved 800rpm earlier in the rev range! Peak boost at 13 psi was still achieved at 3000 rpm, but the boost curve below that was so much stronger that in real world driveability, the car was transformed.

So, this muffler-less performance was seriously good.

But the car was a bit noisy. In fact, when I drove off from my house during testing, my 3 year old son apparently thought the garbage truck had arrived.....

Mufflers

As we’ve written previously in AutoSpeed, if you have a car with a predicted flywheel output of less than 150kW, it’s madness to buy new aftermarket mufflers for it. Why? Well, at least here in Australia, there are so many discarded exhausts from the local sixes and eights lying around that it’s dead easy to pick up dirt cheap mufflers with good silencing characteristics and easily enough flow. So when I decided that the diesel really did need to have a muffler of some sort, I didn’t head for the local muffler shop containing shiny mufflers; instead, I went to my home workshop and took a look at what I’d previously salvaged.

And there it was, the complete after-cat exhaust off a V6 Holden Commodore. I don’t know what model – around VR I guess – but it used 2.25 inch pipe and had two mufflers – a central straight-through resonator and a larger baffled rear muffler. And, both were stainless steeI. I’d bought the system about two years previously from the shop at the local tip – I happened to have my trailer there and the bargain ($20) was too good to pass up.

I cut the mufflers from the rest of the pipe work – both looked fine inside. (Always check secondhand mufflers by shining a strong light into them and making sure they’re not obviously rotted. Shaking and listening for any rattles is also recommended.)

Then, one at a time, I held them up to the truncated Peugeot exhaust while another person revved the engine. The straight-through muffler knocked a bit off the noise – I think I’d have gone for that one. However, my wife thought it was still too loud (and she’ll be mostly driving the car). The baffled muffler was as quiet as the standard muffler. (Note: these days, ‘baffled’ doesn’t have all the negative connotations it used to have. In fact, looking in each end of the Holden rear muffler, it appeared it was closer in design to a reverse flow than a traditional baffled muffler. But you’d need to cut it open to find out for sure.)

And I wasn’t cutting it open – instead I went over it with an angle-grinder wielding a twisted wire brush to remove the surface rust (yes, this grade of stainless will slowly rust) and then painted it with a spray can.

Then it was off to my local muffler shop for fitting. A bit of pipework, the making of a bracket and sixty dollars later, it was on the car.

Results

Over having no muffler at all, the performance has come back a little but it’s still much better than standard. I can get 5.5 psi boost at 2000 rpm in 4^th gear – with the car standard, the dyno graph shows it took until 2700 rpm to do that. So, the car is coming on appreciable boost 700 rpm earlier! (Than compares with 800 rpm earlier with no muffler at all.)

While certainly still no torque monster under 2000 rpm, you can now trickle around in fourth and fifth gears between 1500- 2000 rpm and have enough response to climb small hills (4^th) or happily go along on the flat (5^th). In many situations you can now be a gear higher than before. That should make a noticeable difference to fuel economy as well as making driving much more enjoyable.

And full throttle performance? The car is clearly stronger at 3000 rpm and revs more freely to 4500 rpm. However, above that, the rev limiter starts to impose a dramatic power reduction (yes, even though the redline is at 5000 rpm).

We haven’t done another dyno run at this stage – we’ll do one when a few more very cheap mods have been done.

Interestingly, engine braking down hills has lessened – which makes sense if you think of the diesel always pumping a lot of air, air that is now less resisted in its passage.

Conclusion

At its simplest, all this article says is that we changed the rear muffler. However, to put this another way:

• Inspection revealed the exhaust pipe diameter and bends to be fine and the only major restriction to be the single muffler.

• On-road testing showed that a better flowing exhaust made a major difference to performance, especially the rpm at which effective boost could be developed.

• A very low cost Original Equipment stainless steel muffler was selected, with that selection based on the power of the original donor car and tested noise outputs.

• The total cost of the modification was $80.

• From start to finish, the complete process took just 1 day.

Diesel Exhausts So when thinking about petrol engine and diesel engine exhausts, what are some of the differences to be kept in mind? Depending on the age of the diesel car, the exhaust might contain multiple mufflers, cat converter(s) and a particulate filter. If all these components are present, upgrading the exhaust to a genuinely higher flow design might be very expensive. In fact in some cases it might be impossible, especially if emissions are to remain legal. Despite the lower diesel engine exhaust temperature and different combustion behaviour, diesel and petrol engine mufflers are interchangeable – diesels don’t have unique mufflers. Diesels have very high gas flows all the time. In a turbo diesel, the gas flows will increase with load, as is the case with a petrol engine. But when off-boost, eg in light load cruise at 1500 rpm, the diesel engine will be inhaling and exhaling the same amount of air as a petrol engine of the same capacity would be at a full throttle 1500rpm! Therefore, the average mass airflow is much higher in a diesel engine than a petrol engine. As a result of this, a restrictive exhaust is likely to cause significant pumping losses (where the pistons have to push against exhaust back-pressure) much more often in a diesel than in a petrol engine. So while maximum exhaust flow of a 2 litre turbo diesel running 15 psi boost is no greater than of a petrol 2 litre turbo running 15 psi boost, on the road any backpressure will be felt more often by the diesel. Bottom line: in diesels, exhaust flow is very important!

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