Supercharging a Diesel

[Ferrer]

And the smallest the engine is the worse the problem. VAG's old 1.9 litre diesel on the other hand is remarkable in that aspect. It gives the performance you simply wouldn't believe from a 100bhp engine and considering the output it pulls quite strongly throught the whole rev range.

But that happens also with petrol engines. Outside town, if you want to make progress rapidly the smallest the engine the more stressed and unsuitable it's going to be. And fuel consumption will suffer accordingly.

[Clivey]

And the smallest the engine is the worse the problem. VAG's old 1.9 litre diesel on the other hand is remarkable in that aspect. It gives the performance you simply wouldn't believe from a 100bhp engine and considering the output it pulls quite strongly throught the whole rev range.

But that happens also with petrol engines. Outside town, if you want to make progress rapidly the smallest the engine the more stressed and unsuitable it's going to be. And fuel consumption will suffer accordingly.

Albert; I've driven an A3 with the old VAG 1.9TDI in 105 PS guise. It was:

• Less "laggy" than my car
• But slower than my HDI flat-out
• The gearbox was just as reluctant to change down to 1st or 2nd gears.
• And the engine was very unrefined - NVH was significantly higher than the HDI.

My car is nicely satisfying to drive if you can keep it in the powerband - the shove of torque gives it real world performance to equal most NA 1.8-litre petrols...but it's still nowhere near as satisfying to drive as a good petrol when you stray out of the powerband or are driving flat-out.

Here's the rub though: If I did buy a petrol instead, I'd be spending a third extra on running costs and still only have the same level of performance as with my HDI. Also, I spend a lot of time driving in urban areas or stuck behind other traffic in places where overtaking would be stupid - any extra performance (if I bought a bigger-engined petrol) cannot be used anyway. Overall, I still think the C4 is the best option for a daily driver on my budget (I can't afford a 123d! ).

[henk4]

Overall, I still think the C4 is the best option for a daily driver on my budget (I can't afford a 123d! ).

a two year old C5 with the twin turbo engine would probably fall within your budget, assuming that the second hand price may now have halved...

[Ferrer]

Albert; I've driven an A3 with the old VAG 1.9TDI in 105 PS guise. It was:

• Less "laggy" than my car
• But slower than my HDI flat-out
• The gearbox was just as reluctant to change down to 1st or 2nd gears.
• And the engine was very unrefined - NVH was significantly higher than the HDI.

I don't have experience with point 1-3, but I definitely agree with point 4. However that's common problem to all PD VAG diesels. However if you only look at the performance/fuel consuptiom ratio they are quite impressive. I've driven the 1.9 TDI 100PS in an Ibiza and the performance wasn't what you'd expect from a car with a 100bhp. Even the little 1.4 litre the cylinder was amazing. Though noisy and harsh it picked up speed like 70bhp car shouldn't. The ease with which it got up to triple figure speeds was astonishing.

My car is nicely satisfying to drive if you can keep it in the powerband - the shove of torque gives it real world performance to equal most NA 1.8-litre petrols...but it's still nowhere near as satisfying to drive as a good petrol when you stray out of the powerband or are driving flat-out.

Here's the rub though: If I did buy a petrol instead, I'd be spending a third extra on running costs and still only have the same level of performance as with my HDI. Also, I spend a lot of time driving in urban areas or stuck behind other traffic in places where overtaking would be stupid - any extra performance (if I bought a bigger-engined petrol) cannot be used anyway. Overall, I still think the C4 is the best option for a daily driver on my budget (I can't afford a 123d! ).

It's a trade off. Driving enjoyment or fuel consumption. You (and I) voted with our feet. But having had the experience I now know what my next choices will be.

[LeonOfTheDead]

I remember we had a similar issue with both gearbox on the Phedras (by PSA), both were reluctant to engage the first gear.
Something I noticed on that car as well (2.2 liter, 128 and 136 bhp), was the presence of a quite large flywheel. That would explain the lag general too.

About the power band, yes, it's narrow, at best 1.750 rom to 4.000 (more likely 3.500 in many cars), but it is also covered much more slowly than on a petrol engine, so it's not that narrow and still 50-75% of the whole range available, which is more than what you get on the average petrol car.
Ok, take a direct injection, VGT turbo petrol engine...

[Clivey]

a two year old C5 with the twin turbo engine would probably fall within your budget, assuming that the second hand price may now have halved...

True enough, but if I had one of those I'd be concerned about everyone thinking I was a crazy Dutch guy with a very large camera...

I don't have experience with point 1-3, but I definitely agree with point 4. However that's common problem to all PD VAG diesels. However if you only look at the performance/fuel consuptiom ratio they are quite impressive.

Precisely. And as a result, it takes an enthusiast to justify buying a petrol instead.

Even the little 1.4 litre the cylinder was amazing. Though noisy and harsh it picked up speed like 70bhp car shouldn't. The ease with which it got up to triple figure speeds was astonishing.

Really, you thought so? The reason I ask is because they are still quite a lot slower than the 1.4 Clios, Corsas and Puntos (all around 90BHP) that were designed at the same time.

Sophie's Corsa (my old one) flies when it's above 4,000rpm (twin cam). The close ratio gearbox helps a little, too!

It's a trade off. Driving enjoyment or fuel consumption. You (and I) voted with our feet. But having had the experience I now know what my next choices will be.

Exactly the same feelings here!

- But I have to point-out that my circumstances were different when I bought the C4 and due to things like insurance costs at the time (I was 19), a decent petrol engine would have crippled me.

I remember we had a similar issue with both gearbox on the Phedras (by PSA), both were reluctant to engage the first gear.
Something I noticed on that car as well (2.2 liter, 128 and 136 bhp), was the presence of a quite large flywheel. That would explain the lag general too.

Veeery interesting.

*Scratches chin like a bad guy in a cheap action film*

So: A smaller flywheel could potentially help a lot then?

About the power band, yes, it's narrow, at best 1.750 rom to 4.000 (more likely 3.500 in many cars), but it is also covered much more slowly than on a petrol engine, so it's not that narrow and still 50-75% of the whole range available, which is more than what you get on the average petrol car.
Ok, take a direct injection, VGT turbo petrol engine...

Let me put it this way:

My car's rev limit is around 5k rpm and idles at around 700. So that's 4,300 possible revs. However, you only have a decent amount of power from 2,000 to 4,000 rpm (less than half of the rev range).

Compare that to the 1.6 (non turbo) petrol of the same power, and that will happily pull anywhere from idle to the redline, just like most petrols. They are just so much more flexible than a single turbo diesel.

[Ferrer]

Regarding the first gear reluctancy it might also have to do with the the gearing. I mean, in the Mini downshifting into first while moving was basically impossible because of the very short gearing. In the Lancia you can do it if you want and it's smooth. In the BMW only below 10km/h.

Really, you thought so? The reason I ask is because they are still quite a lot slower than the 1.4 Clios, Corsas and Puntos (all around 90BHP) that were designed at the same time.

Sophie's Corsa (my old one) flies when it's above 4,000rpm (twin cam). The close ratio gearbox helps a little, too!

I was refering to the diesel. I don't have much experience with petrol VAGs.

[LeonOfTheDead]

Veeery interesting.

*Scratches chin like a bad guy in a cheap action film*

So: A smaller flywheel could potentially help a lot then?

It would reduce the general lag (not the turbo lag of course) but would also create a more noisy engine, less prone to maintain a constant speed (rmp) and possibly it would even affect the reliability. It would be a pretty nervous engine if they flywheel was too small.
A large flywheel is usually adopted for not particularly refined and well designed engine but I don't think this is the case, Henk can confirm his C5 was running just fine.
I could understand that on the Phedra though: being an heavy and not particularly aerodynamic car, having a large flywheel gives you this result: when you lift your foot from the throttle, the engine keeps running at the previous rpm for a moment or two. That helps you particularly in small clombs when the heavy car would try to slow down too much. On the other hand it isn't a source of "unintended accelerations" (as Toyota American customers would be happy to call it, and the sue Toyota), as the quantity of energy kept by the flywheel and then released is still somehow minimal.

Exceeding in reducing the flywheel wouldn't allow you to even aproach a climb, too.

Let me put it this way:

My car's rev limit is around 5k rpm and idles at around 700. So that's 4,300 possible revs. However, you only have a decent amount of power from 2,000 to 4,000 rpm (less than half of the rev range).

Compare that to the 1.6 (non turbo) petrol of the same power, and that will happily pull anywhere from idle to the redline, just like most petrols. They are just so much more flexible than a single turbo diesel.

Even if the petrol engine reacts quickly during almost all of its rpm range , the portion of this range during which a certain percentage of power is available is higher on a diesel engine.
Like (random figures), on a petrol engine you generally have the 90% of the maximum power available for about the 10& of the whole range of revs. On a diesel it could be like 90% of power for 25/30% of revs.

Talking about the torque curve is even worse (for the petrol engine), but that's quite influenced by the turbo, and therefore is the power too.

It just ins't possible to have a direct comparison until the two engines aren't using the same technologies too.

[henk4]

True enough, but if I had one of those I'd be concerned about everyone thinking I was a crazy Dutch guy with a very large camera...

I am glad to hear that my car has earned that reputation on your island....it is indeed a regular visitor there.

[henk4]

Like (random figures), on a petrol engine you generally have the 90% of the maximum power available for about the 10& of the whole range of revs. On a diesel it could be like 90% of power for 25/30% of revs.

if you look at the graph Clivey produced about 1.6 Hdi, the "90% power" in terms of torque ranges from 33 to 35 (with and without overboost). That's why you don't need 7 or 8 speed gearboxes to deal with this "narrow power" band. I have now 6 but I tend to skip 4th and 5th because low end power is good enough to keep moving with traffic.

[LeonOfTheDead]

Mine were indeed random figures to show as an diesel engine still produce more power during his rev range.

Let's put it this way:

Having both a naturally aspirated petrol engine and a turbocharged diesel engine, both with the same level of power and even torque (which is a bit absurd I know).
So their power and torque graphs will reach the same level in both cases, but the diesel's ones will be overall "taller", having a larger surface beneath them than the petrol's.
That could be simply related to the diesel engine being a bit more efficient, the diesel itself carrying more energy and the presence of the turbo of course.

[Ferrer]

Petrol engines can be flexible too, though.

I remember in the Mini you could use 6th from 50km/h all the way to top speed.

[LeonOfTheDead]

Petrol engines can be flexible too, though.

I remember in the Mini you could use 6th from 50km/h all the way to top speed.

that had a supercharger though.
I didn't say petrol engines aren't good. Indeed even if the percentile power produced could be slower, they also require less energy to accelerate (and therefore moving the car) as the diesel combustion offers an inner resistance to change the configuration, trough the very lag in the combustion, injection and turbo too.

Petrol engines rev (so they change the situation, or condition if you prefer) more easily and quicker than diesel, therefore they compensate a lower percentile power with an higher speed of acceleration trough the revs, even without a turbo.

Talking about the flexibility, when the revs are too low and the engine can't keep on sustaining the movement and dies, it is much more violent and sudden on a diesel than on a petrol engine, but that has also to do with the higher temps and pressures, and the heavier components of the former.

[Clivey]

Regarding the first gear reluctancy it might also have to do with the the gearing. I mean, in the Mini downshifting into first while moving was basically impossible because of the very short gearing. In the Lancia you can do it if you want and it's smooth. In the BMW only below 10km/h.

Really? In the Mini One D (Toyota-sourced 1.4 turbo diesel with a BMW 6-speed manual 'box) and the new Cooper D (same 1.6 common rail diesel as my car but again with a BMW 6-speed 'box), you can select 1st anywhere below about 10mph with zero reluctance. They're brilliant gearboxes and I wish I had one in my car.

In fact, I had a test drive in a C4 diesel with the EGS paddleshift gearbox and it was much better to use in everyday driving than my manual. I think auto/DSG style gearboxes suit modern diesels better anyway - the good ones use electronic trickery to keep the engine revs where they need to be - there was no issue with lag in the DSG or VAG 'boxes I sampled.

I was refering to the diesel. I don't have much experience with petrol VAGs.

Ah. I now understand where you're coming from. It's the "m4D t0rkz" from the diesel in a small car.

[Waugh-terfall]

I don't have experience with point 1-3, but I definitely agree with point 4. However that's common problem to all PD VAG diesels. However if you only look at the performance/fuel consuptiom ratio they are quite impressive. I've driven the 1.9 TDI 100PS in an Ibiza and the performance wasn't what you'd expect from a car with a 100bhp. Even the little 1.4 litre the cylinder was amazing. Though noisy and harsh it picked up speed like 70bhp car shouldn't. The ease with which it got up to triple figure speeds was astonishing.


It's a trade off. Driving enjoyment or fuel consumption. You (and I) voted with our feet. But having had the experience I now know what my next choices will be.

I've driven my friends '99 Ibiza 1.9TDi (I think it's a 90bhp unit) and it picks its self up really well, still at 150,000mi. It definately feels alot more brisk than my car and there is a noticeable shove in any gear when floored, something that's not always felt in my car, it will also pull itself up big hills in 4th where my car would need a little bit of a run up in 3rd. The engine is very rough, noisey and wobbles the car though