Our Peugeot Diesel - Part 3 - Fuelling Adjustments

The new rear muffler made a huge on-road difference. The new intake made a further, not-huge-but-still-noticeable improvement. Now it was time to add a bit more fuel and then head back to the dyno. But on the rollers, the results were not at all as expected...

More Fuel

Completely unlike an electronic system, the overall rate of full-power fuelling can be adjusted by simply turning a screw. The procedure is to add fuel until there is a touch of black smoke (ie unburned particulate matter) from the exhaust and then to wind the screw back out a little until the smoke is sufficiently reduced.

The fuel distributor pump (newly fitted by the previous owner) had lock-wires and paint tell-tales all over it, but some judicious use with pliers soon removed the wires. The full-load fuel adjuster consists of a 13mm lock nut and a slotted stud. Turning the stud clockwise increases fuel. The screw is very sensitive – as one writer put it, it’s easy to make these cars into black smoke belching monsters!

I found a flat, empty road and measured the rolling time for a 60-90 km/h split in 3^rd gear. This would become my tuning benchmark. The initial position of the screw was at ‘2 o’clock’ (ie the slot was a little past vertical) and this resulted in a 60-90 time of 6.8 seconds. I then pulled over, lifted the bonnet, released the lock nut and turned the screw to ‘3 o’clock’. The 60-90 time dropped to 6.7 seconds. Adjustment was then made to ‘5 o’clock’, which dropped the time to 6.3 seconds. This last setting also caused a little black smoke to be emitted.

Whether the ‘5 o’clock’ setting was too rich (ie there was too much black smoke) was hard to tell from the driver’s seat so I decided I’d leave the setting there until I could get someone to observe the car being driven at full throttle.

Since this tuning was being carried out not far from ChipTorque, I ventured down the road to the company to see if they could slot in a quick dyno run. They could – and so the car went on the rollers.

Dyno

I was pretty expectant that the results would be brilliant – the car was driving in a completely transformed way, with boost coming up much earlier and stronger response from 2000 – 4000 rpm (redline is 5000, and the car starts dropping power from about 4000). In fact, the bottom-end was now so much better that hills, that at 2000 rpm in 4^th gear previously required a down-change to 3^rd, could now be taken at less than full power in 4^th!

But, as shown here, this bottom-end improvement was simply invisible on the dyno. Yes, peak power was up by 9 per cent and power was lifted from 2500 to 4750 rpm. But at 2000 rpm the ‘standard’ and ‘modified’ power outputs were identical! And I simply know that this isn’t the case on the road... Another oddity was that boost was increased across the rev range, but my gauge shows that on the road, there’s been no change in peak value.

Hmmm. There are a few factors at work here. Firstly, the dyno run is made with the accelerator flat to the floor, so any part-throttle gains (or losses) are invisible. Secondly, the ramp rate of the dyno (that is, how quickly it lets the engine accelerate) does not match this car’s acceleration in 4^th gear (the gear in which the dyno run was taken). The dyno is much quicker, not allowing the turbo to build boost early the way it does on the road.

I voiced my concerns to Lachlan Riddel of ChipTorque and he did another dyno run at a slower ramp speed. As seen here, that brought up boost earlier – but peak power was down, probably because the slower ramp gave more time for the underbonnet intercooler to get hot!

And the intercooler brings us to the next point – with the temp of the day higher, the intercooler got noticeably hotter on these runs than it did on the original dyno run of the standard car.

Finally, note that even with the slow ramp speed, the boost curve still does not match what I measure on the road in 4^th gear – it lags by about 250 rpm. (250 rpm, you’re saying – who cares? Well, the performance of this engine is all about boost. Off-boost, it’s terrible. So the on-road difference between having 3 psi boost at 2000 rpm, or 6 psi boost at 2000 rpm, is enough to make the difference between pulling up a hill in 4^th or needing a down-change to 3^rd.)

So yes, the dyno shows a gain but I think it’s completely unrepresentative of what’s actually experienced on the road. (And I don’t know why the dyno showed an increased boost level, either.) Which leads us to the next point –how well does the car go on the road? But first, another tuning change.

Raising the Rev Limit

One thing the dyno graphs showed is that until the rev limiter (‘governor’ in diesel-speak!) starts to cut fuel, the Peugeot engine with exhaust and intake mods was actually heading upwards in power development. This strongly implies that lifting the point at which fuel starts being cut will result in more power.

As with the adjustment to the full-load fuelling, changing the rev limiter is simple: you just turn another screw – unwinding it to increase the limit... (I find it hard to get my head around how easy all this is: so much different to trying to lift the rev limiter on an electronically injected car!) I made the adjustment until the engine would freely rev to near the 5000 rpm redline - you can now feel a fuel drop-off starting to occur only at about 4800 rpm. (The driver needs to watch the redline and not exceed it; something the engine is now capable of.)

Then it was time to head back to my empty road and do some stopwatch testing.

On-Road Performance

Back in January 1994, Wheels magazine tested a Peugeot 405 SRDT. The figures they got included:

Standing start times:

0-100 km/h - 15.3

Rolling split times in 3rd, 4th, 5th

60 - 90 km/h - 5.7, 8.1, 14.9
80 - 110 km/h - 15.1, 10.7, 14.1

Incidentally, the standing start times are quite slow, but the rolling times, while certainly no performance car times, are quite respectable for the era – in some cases, better than the petrol engine equivalent.

So what times would my car do? Would in fact the on-road difference in feel be clearly shown by the stopwatch – or was it all like the dyno... apparently my imagination? Well, the rolling 60-90 km/h times I’d already done when tuning the full-load fuel were not encouraging – I’d got that down to 6.3 seconds in 3^rd gear, versus the original test’s 5.7 seconds.... Ouch.

This time, testing on a new day and the with the fuel pulled back a little to stop black smoke, the modified 60-90 km/h times in 3rd, 4th, 5^th were:

	Wheels test	intake, exhaust, fuel
60-90 km/h 3rd	5.7	5.8
60-90 km/h 4th	8.1	8.9
60-90 km/h 5th	14.9	13.3

As can be seen, there’s certainly no great improvement – two of the times are a little slower and one a bit faster. So maybe the performance gains I could feel weren’t really there? So what did the 80 – 110 km/h splits show?

	Wheels test	intake, exhaust, fuel
80-110 km/h 3rd	15.1	5.9
80-110 km/h 4th	10.7	7.3
80-110 km/h 5th	14.1	13.3

Ah, this is more like it! As can be seen, the 80-110 km/h time in 3^rd gear fell massively, from 15.1 seconds to just 5.9 seconds. In 4^th and 5^th gears there were also improvements, albeit decreasing in size as the gear ratio went up.

And the 0-100 km/h time? It improved, falling from 15.3 seconds to 14.0 seconds.

Now clearly you can juggle figures to prove nearly anything – and certainly the lack of appreciable gain in the 60-90 splits is surprising – but the rest of the road performance data seems to make sense. Where the top-end is needed, as in the 0-100 km/h time, the small power improvement shown on the dyno results in a small performance increase. But where the performance split can make use of the earlier and stronger boosting around 2000 rpm (that’s the improvement invisible on the dyno!), the times tumble.

And you need to remember that peak turbo boost hasn’t yet been changed – there’s no fiddling been done of the wastegate.

Conclusion

Some aspects of this story look pretty weak – excuses for a lack of gain on the dyno and then excuses for the car not being much faster everywhere on the road. You’ll just have to take my word for some of the benefits that have so far been achieved: the car now comes on boost earlier, feels infinitely more responsive, has much better part-load torque, and has appreciably better on-road performance. In fact, no matter what the figures might show, for a total expenditure so far of $120 the difference is just stunning.

The dyno runs were courtesy of ChipTorque.

Another Diesel Dyno One of the few turbo diesel cars we’ve ever had on the dyno is the 405 diesel’s younger brother – the 406 HDi. It may only be one model later but the 406 uses fully electronic injection and the 2-litre engine itself is completely different. So how does the later model engine compare on the dyno with the 405’s engine? It’s an intriguing comparison because the 406’s diesel develops very nearly the same peak power – 60kW at the wheels versus our 405’s 57kW (standard) or 62kW (exhaust, intake and fuel). But peak power isn’t the figure to look at. Instead, watch the rpm at which maximum torque is developed. In the case of the 406, that’s at 1750 rpm, while the 405 has peak torque at about 3200 rpm. Since power is effectively torque multiplied by revs, the much lower revs at which max torque is developed massively balloons out the 406’s power curve. At 1750 rpm, the 406 has 44kW – at the same revs the 405 has something like 15kW... We covered the technology of the 406’s diesel engine at Big Bang Theory: Direct Injection Diesel Tech. Perhaps these lines from that story are rather significant: Exhaust gas is channelled through the turbo, with turbo boost controlled by an ECU-controlled wastegate. Exhaust gas recirculation occurs via another ECU-controlled valve, with the vacuum needed to operate both the EGR and wastegate controls coming from a special vane-type vacuum pump (no throttle = no engine vacuum!). Slight turbo over-boost is permitted to aid acceleration, while at a steady speed between 2500 - 3000 rpm boost is cut to 0.7 bar (10 psi), a move that improves fuel economy by 4 per cent. Hmmm, better boost control on the 405, anyone?