Si4 or SD4? That is the question

(14-12-2013 12:43pm)berlin2011 Wrote:  Had both myself, first one was Si4 in which I did a tad over 30k miles before switching to the SD4 in which I've now done just over 11k miles. MPG and cost aside, I actually find the SD4 a nicer car to drive all round. Sure it's not quite as nippy as the Si4, but just a more relaxed ride, in heavy traffic and motorway cruising I enjoy it more - surprised me a little but if I had to choose now I'd go SD4 again.

That too, i agree!. More relaxed and effortless,that was the other reason i chose it.....just seemed a more relaxing ride overall for just cruising around.

I did enjoy the way my SD4 just seemed to leap away once it was rolling, it was only pulling away that it had that annoying pause to think and that might have been a quirk of the automatic gear box rather than the engine. The 9 speeder also appears to have the engine almost at tickover in 9th on the motorway (well around 1200 rpm) and with it quite large dollop of torque there was never any delay in it putting on a sprint after cruising for a while. Like all my previous Range Rovers except the BMW engined TD6 which was a wheezy and rattly thing, especially from cold, but also when warmed up, I found the Evoque's TD well up to it's job and quiet on the inside of the car (where it matters, I don't care what people on the outside hear!). I have asked Hadley Green to do the sums and check if changing would sett back the build date. My biggest worry after running diesels since 1999 would be filling it up at the smelly pump by mistake!

As far as I know petrol nozzles are thinner so it is very difficult to put diesel in a petrol unlike the other way around.

Yes they are narrower

Let's be clear - if performance is your goal, the only thing which gets a car up to speed is power from the engine; power is the product of torque * engine speed (if you want to be technical, power in watts = torque in Newton Metres * 2 * PI * Engine Revs per Second).

Compared to petrol engines, diesel engines achieve higher torque because of the higher compression ratio and lower maximum RPM because of the higher moving mass. You can therefore characterise petrol engines as having relatively low torque but able to be revved higher to obtain maximum power; diesel engines have higher torque but quickly run out of steam as they hit the rev limiter. Petrol engines are rather lighter than diesel engines because diesel engines need to be constructed to withstand higher internal pressures.

In both cases, you only achieve maximum power by revving the engine towards the rev limiter. Multi-ratio gearboxes improve performance by keeping the engine running in the maximum power sweet spot for longer, as did close-ratio boxes of old. "S" mode in the transmission improves performance by delaying up-shifts to higher in the rev range, meaning the average power delivered by the engine is higher.

The Si4 engine has nearly 50 bhp more than the SD4, achieved by much higher maximum RPM to compensate for the lower torque. If you are not prepared to rev it, the Si4 engine will be disappointing. What you have to learn to do is use S mode all the time so that you take advantage of the higher power the engine then creates. That might make for a more raucous drive but the car certainly has a turn of speed when you do.

So, the Si4 offers a more sporting drive and will be rather faster in the real world than the lumpen SD4. The only proviso is that you need to run it in S mode when you need the performance and you will be more inclined to use the shift paddles to control it when, for example, overtaking.

My Si4 is fully run-in now and delivers a decent turn of speed; both the horsepower and performance figures speak for themselves.

The main hit on running an Si4 is not going to be the fuel cost but the depreciation in a market which believes diesel is better. I am not among them and petrol was my immediate choice for my RRS.

We also were unsure when ordering our evoque as to have petrol or diesel, we initially thought about the diesel but since we have always had petrol cars and also fancied having that extra more power we decided to go for petrol. It all boils down to what you are seeking ( a car with better performance ) or (fuel costs, consumption ) etc
Paul

No DPF is enough to swing it for me, though to be fair I've never had a problem with any if mine, but I'm always conscious of it as I don't do massive mileage. I'm hopeing the baby disco is offered with a petrol engine and not just for the topper.

Like berlin I, certainly find the SD4 a relaxing drive, compared to my petrol car, no revving hi to find the power, which suits the car in my opinion, its a high SUV and theres no hiding that with any trick suspension. Also the pull away diesel torque especially in the manual blows a petrol into the weeds and on the motorway it just cruises fabulously with oodles of torque and power when required, but the petrol will give a more responsive and exciting ride and as a fun driving experience I probably get more from my petrol car which is similar power and weight, and is tuned well for good torque too, albeit needing revs to get passed the 2000rpm dead for economy/emissions band. It is of course a car not a SUV (I also care much less for it since its value is fractions of the Evoque, and that adds to the driving experience, a bit like your fastest car is the one you hire)

Some great posts here guys, many thanks. Mark_N great explanation too thanks.
After an email to Jeremy at Hadley Green where there is some doubt (only a tiny doubt) that the LR Vista system might not like the engine change. The fact that I prefer a more relaxed type of driving than running in S mode (last car only ever went into S for overtaking lines of traffic on local A roads, rest of the time it stayed in D with the paddles used for swift down-changes to overtake ) means I have left the order as it is with the SD4 engine. Being a fairly frequent changer of cars (well every 2-2.5 years) and depreciation/ residuals are not a huge concern because I loose out anyway changing so soon, the fact that Si4 looses even more than a very high spec SD4 in a n odd colour with unusual interior trim has more influence than normally I would bother about.

Please continue to air your views as I'm sure there are plenty of European and Australian/South Africa members who like us have the option of the Sd or Si derivatives
who will find this thread useful in helping them make up their minds.

With the 9 speed Auto that is keeping Revs low and consistent for economy, the SD4 is probably the way to go, I read at 70mph it sits in 8th at 1200rpm, that's gonna save fuel over the 2000rpm mine does in 6th at 70mph. The petrol may not be ideal for minimizing rev changes, but I do look forward to the feedback from someone with a Petrol 9 speed.