35i vs 35d engines

[Steved]

Right then, exhaustive comparison completed. I used the 3-series to perform a comparison between the 35i and 35d engines and help me decide which to go for on the X6.

I had on loan today a 335d MSport Touring and a 335i MSport Convertible. The 335d had 7500 miles, whereas the 335i was new with just 330 miles on the odometer. I drove the 335d first for 3 hours, then swapped over to the 335i for another 3 hours covering a variety of roads A, B, and Motorway with some city driving through the outskirts of Birmingham.

I covered around 200 miles in each, the 335d averaging just over 40 mph with a consumption of 26.6 mpg, whilst the 335i averaged 55.9 mph with an overall consumption of 21.7 mpg. Both were fully extended as their makers intended and I didn't spare the horses, so these are definitely 'real world' economy figures although I covered slightly more motorway miles in the 335i than the 335d.

Now for my impressions. I was comparing the two engines on the basis of four criteria; actual performance, ease of delivering usable performance, aural enjoyment, engine character.

In terms of 'actual performance' there was little of any note between them, they were both equally as quick but they achieve their speeds in entirely different ways. The 335d is not fitted with a temperature gauge (that I could find), but I noticed its performance tailing off after the first 2 hours of driving, not massively so, but the edge was lost and it seemed to be shy of some torque. The 335i was more consistent, but I am not sure if that was symptomatic of just these cars I drove or the models themselves.

I greatly preferred the 335i though at low revs. The 335d really languishes at low revs, feeling more like an un-boosted diesel for those first few seconds before the turbos kick in, this is nowhere near as noticeable as a 2.0td or even the single turbo 3.0d, but it ‘is’ noticeable and makes it feel initially stodgy. The 335i by comparison has torque from the get-go and pulls cleanly and crisply away from junctions. So, for driving around town the 335i was the one I would have chosen.

In terms of ‘usable performance’ the 335d scored the highest. There is strong torque from around 2000 rpm through to 4000 rpm which means that most of the time it has an answer available for whatever you need, without requiring a change of gear. The 335i I drove seemed slower than the one I’d driven before, but then it was still shy of a few miles and was also carrying the 200kg heavier body shell of the Convertible. It behaved much more like a NA engine needing revs to deliver its performance, although as the miles increased its acceleration felt stronger. Whereas the steering wheel paddles felt pointless in the 335d, in the 335i there was reason to change gear and optimise its performance. Whether that’s a strength or a weakness will depend on what you want from a car, but it certainly made the 335i the more ‘interesting’ engine to drive.

In terms of ‘sounds’ the 335i won my favour without even breaking into a sweat. This is actually quite a significant criteria when comparing these two engines for a number of reasons. Firstly, I have never liked the diesel engine’s death-rattle at low revs and whilst the 335d is a smooth diesel it still sounds industrial and functional at low revs and under light-load before the turbos kick in. Driving around town and through narrow side streets this was a sound that I tried to minimise in the 335d, whereas the 335i sounds considerably better at low revs, delivering a quite exotic 'yowl' from its exhaust and continuing with similar tonal changes as it rises through the revs. So, I found myself indulging in the noises made by the 335i but trying to avoid the ones made by the 335d. For a car with sporting pretensions that’s a significant point to note. The 335d is actually quieter than the 335i under cruising conditions, and even under hard acceleration the diesel is the quieter of the two, however I never found anything actually endearing about the sound of the 335d. it’s one of those engines that ‘does-what-it-says-on-the-tin’, again perhaps betraying its more functional origins.

So, overall the 335d was at its best aurally when at its quietest, whereas the 335i appealed for the orchestra of sounds that it made. This again heightens the reference for a sporting car, which is surely characterised by how good it should sound.

Finally in terms of the ‘character’ of each engine; the 335d impresses for its locomotive shove in the back which then continues on as if it will never stop, but when it does stop its like the lights being switched on at a New Year’s party. Games are over, everybody go home. What seems unstoppable suddenly comes to a halt and for a moment (until the next gear engages and boost is re-engaged) you wonder if you imagined its strength. This pause in acceleration was more noticeable in the 335d, whereas the 335i showed that familiar petrol engined theme of the rev limiter stopping an engine that could have carried on for another few hundred revs. I found myself consciously changing up before the rev limit in the 335d just to avoid the disappointment. Also for me the 335d was a one trick pony, it had great strength from mid-range onwards but that’s all it had. I respected its strength but I never found myself endeared to it. The 335i although feeling weaker, countered with bucket-loads more character, a far more endearing sound and a genuinely sporty (and exotic) feel.

When I brought each car back I felt I had got to grips with the 335d, felt it would make a good companion on a journey but did not feel any further need to drive it. Whereas I was still playing with the 335i, learning more about its torque curve and which gear to be in to maximise its performance, whilst enjoying bouncing the sounds of its exhaust against buildings and parked cars. I did however find myself tiring (slightly) of revving the 335i and the unwelcome increase in decibels this produced, just a little more torque would make this engine near perfect.

So, which would I have chosen for a 3-series? I would have chosen the 335i. It had more character, and a sporty character at that, was quick enough and sounded just how a sporty car should. The 335d just felt too functional by comparison. I found myself weighing up ‘effortless torque’ against ‘character’ and preferred character, and also thought that the 335i would be the more endearing car to own over a longer time-period (where most of us suffer from boredom and pangs of vehicular promiscuity). But in an X5 I would probably choose the 35d engine, because such a functional locomotive quality would be both more necessary and in keeping with its utility nature.

But I’ll admit that after 400 miles or so of driving I’m not sure I’m any closer to deciding what would work best on an X6. I would prefer the 35i engine with more torque, which is perhaps what would be achieved with an ECU mod, and whilst I am sure the 35d engine would work fine I am quite concerned that I would become bored of its singular personality quite quickly.