Turbo diesel or Turbo Petrol?
Discussion
On the face of it, this topic has been done to death. We've endured the endless thread about how a tuned 535d is quicker than Concorde and why diesels can never be considered sporty because they do not rev like a petrol.
So please, let's not resurrect those arguments.
What I would like to debate is where the marketplace is going now that BMW have begun applying the same kind of twin-turbo principles from its award winning diesels to the new petrol engines. (twin-turbo 3 litre 6, and now twin-turbo 4.4 litre V8)
I am personally debating the choice between 35i (as in the 335i) and 35d (as in the 535d) engines within the X6 (again, please lets not go off topic debating the merits of the X6. It's a marmite thing!)
The 35i engine as standard has 306bhp/400Nm, whereas the 35d engine has 286bhp/580Nm. In the X6 this results in the 35i being quicker by 2mph, 0.2 sec to 60mph and around the same margin for the standing km. So, they're on a par performance wise.
Tuning wise both can be tuned with the 35d engine reaping another 50bhp and 100Nm, whilst the 35i engine has been tuned by some specialists to liberate another 100bhp and 160NM. Let's assume though for the sake of argument that both can be tuned equally.
Fuel economy wise there is a claimed difference of 7-8 mpg between them, according to BMW's published figures, but I've never managed to get a 35d engined X5 above 23 mpg, so I suspect that the margin is less.
When comparing turbo diesel to petrol, the diesel makes a case for itself with its stonking mid-range torque, but..(and this is important), you cannot compare peak torque figures for diesel and petrol engines because its torque applied to the wheels that is important rather than the flywheel.
In the 35i engine it produces peak torque at 1300rpm and then maintains a totally flat torque curve all the way to 5000rpm before descending over the last 2000rpm. The 35d engine however produces a higher torque peak some 500rpm later, but this only lasts for a few hundred rpm before descending rapidly over the next few thousand rpm. This would be a real pain to drive if BMW had used the same gearing, but instead they fit longer gearing that spreads the torque over a wider speed range and gives the impression of being closer in feel to a petrol engined car. However by using longer gearing the torque applied to the wheels is reduced.
So. Apologies for the long preamble, but since I have not driven 35i and 35d engines in the X6 yet it is difficult to conclude which would be better. Both are known for being torquey engines, and I would so much more prefer the sound of the petrol engine.
Two questions;
1. Can anyone with good mechanical engineering nous calculate what the actual torque to the wheels would be like?, so that we can compare apples with apples.
2. There has been discussion recently that these turbo petrol engines may halt the growth of diesel engines, since they negate much of the diesel's performance/driveability advantages and in some cases offer similar on-the-road fuel economy. What do you think?
p.s. the gearing of both cars are as follows;
35i:
1st 4,171
2nd 2,340
3rd 1,521
4th 1,143
5th 0,867
6th 0,691
Final Drive 3,909
35d:
Main gearing as above.
Final Drive 3,636
Wheels: 9J x 19 with 255/50 R19
So please, let's not resurrect those arguments.
What I would like to debate is where the marketplace is going now that BMW have begun applying the same kind of twin-turbo principles from its award winning diesels to the new petrol engines. (twin-turbo 3 litre 6, and now twin-turbo 4.4 litre V8)
I am personally debating the choice between 35i (as in the 335i) and 35d (as in the 535d) engines within the X6 (again, please lets not go off topic debating the merits of the X6. It's a marmite thing!)
The 35i engine as standard has 306bhp/400Nm, whereas the 35d engine has 286bhp/580Nm. In the X6 this results in the 35i being quicker by 2mph, 0.2 sec to 60mph and around the same margin for the standing km. So, they're on a par performance wise.
Tuning wise both can be tuned with the 35d engine reaping another 50bhp and 100Nm, whilst the 35i engine has been tuned by some specialists to liberate another 100bhp and 160NM. Let's assume though for the sake of argument that both can be tuned equally.
Fuel economy wise there is a claimed difference of 7-8 mpg between them, according to BMW's published figures, but I've never managed to get a 35d engined X5 above 23 mpg, so I suspect that the margin is less.
When comparing turbo diesel to petrol, the diesel makes a case for itself with its stonking mid-range torque, but..(and this is important), you cannot compare peak torque figures for diesel and petrol engines because its torque applied to the wheels that is important rather than the flywheel.
In the 35i engine it produces peak torque at 1300rpm and then maintains a totally flat torque curve all the way to 5000rpm before descending over the last 2000rpm. The 35d engine however produces a higher torque peak some 500rpm later, but this only lasts for a few hundred rpm before descending rapidly over the next few thousand rpm. This would be a real pain to drive if BMW had used the same gearing, but instead they fit longer gearing that spreads the torque over a wider speed range and gives the impression of being closer in feel to a petrol engined car. However by using longer gearing the torque applied to the wheels is reduced.
So. Apologies for the long preamble, but since I have not driven 35i and 35d engines in the X6 yet it is difficult to conclude which would be better. Both are known for being torquey engines, and I would so much more prefer the sound of the petrol engine.
Two questions;
1. Can anyone with good mechanical engineering nous calculate what the actual torque to the wheels would be like?, so that we can compare apples with apples.
2. There has been discussion recently that these turbo petrol engines may halt the growth of diesel engines, since they negate much of the diesel's performance/driveability advantages and in some cases offer similar on-the-road fuel economy. What do you think?
p.s. the gearing of both cars are as follows;
35i:
1st 4,171
2nd 2,340
3rd 1,521
4th 1,143
5th 0,867
6th 0,691
Final Drive 3,909
35d:
Main gearing as above.
Final Drive 3,636
Wheels: 9J x 19 with 255/50 R19
Edited by DoctorD on Sunday 6th January 13:51
"Fuel economy wise there is a claimed difference of 7-8 mpg between them, according to BMW's published figures, but I've never managed to get a 35d engined X5 above 23 mpg, so I suspect that the margin is less."
I could easily get my remapped LCI 535d to do 40mpg on long runs.
Horses for courses, really, and in most cases you dont have a direct choice.
For example, there is no 535i, so a 535d is the only choice there. Neither is available in the 7 series. Only the 35i in the 1 series. Only the 35d in the x5.
So we're into which you would chose in a 3 series. Personally, i'd drive both for a day or two and see which i preferred, weighing everything up.
I could easily get my remapped LCI 535d to do 40mpg on long runs.
Horses for courses, really, and in most cases you dont have a direct choice.
For example, there is no 535i, so a 535d is the only choice there. Neither is available in the 7 series. Only the 35i in the 1 series. Only the 35d in the x5.
So we're into which you would chose in a 3 series. Personally, i'd drive both for a day or two and see which i preferred, weighing everything up.
There is now a 535i. It sells in the US, but there is no 535d in that market. It would be interesting to compare these two engines in a heavier car, since that's where torque at the wheels would be most noticeable.
I've driven these engines in the 335i and 335d and would prefer the 335i, but I've not driven them both in tuned form, nor have I compared them both over similar routes to assess real-world fuel economy.
If I apply a laymans approach to the effect of the final drive ratios for both these engines, then the torque appears to be only reduced by 10% in the 35d engine, which would still equate to around 530NM over the 400NM in the 35i. Of course you then need to take into account the higher rpm that the petrol engine can achieve.
I am probably more interested in what people's views are on the choices now available, given that twin-turbo petrol engines offer a more effective foil to the performance advantages of the turbo diesel format.
I've driven these engines in the 335i and 335d and would prefer the 335i, but I've not driven them both in tuned form, nor have I compared them both over similar routes to assess real-world fuel economy.
If I apply a laymans approach to the effect of the final drive ratios for both these engines, then the torque appears to be only reduced by 10% in the 35d engine, which would still equate to around 530NM over the 400NM in the 35i. Of course you then need to take into account the higher rpm that the petrol engine can achieve.
I am probably more interested in what people's views are on the choices now available, given that twin-turbo petrol engines offer a more effective foil to the performance advantages of the turbo diesel format.
DoctorD said:
There is now a 535i. It sells in the US, but there is no 535d in that market. It would be interesting to compare these two engines in a heavier car, since that's where torque at the wheels would be most noticeable.
I've driven these engines in the 335i and 335d and would prefer the 335i, but I've not driven them both in tuned form, nor have I compared them both over similar routes to assess real-world fuel economy.
If I apply a laymans approach to the effect of the final drive ratios for both these engines, then the torque appears to be only reduced by 10% in the 35d engine, which would still equate to around 530NM over the 400NM in the 35i. Of course you then need to take into account the higher rpm that the petrol engine can achieve.
I am probably more interested in what people's views are on the choices now available, given that twin-turbo petrol engines offer a more effective foil to the performance advantages of the turbo diesel format.
Yes, i was referring to the uk market when i said there was no 535i. Maybe they will bring it in with the new 5 series in 09/10?I've driven these engines in the 335i and 335d and would prefer the 335i, but I've not driven them both in tuned form, nor have I compared them both over similar routes to assess real-world fuel economy.
If I apply a laymans approach to the effect of the final drive ratios for both these engines, then the torque appears to be only reduced by 10% in the 35d engine, which would still equate to around 530NM over the 400NM in the 35i. Of course you then need to take into account the higher rpm that the petrol engine can achieve.
I am probably more interested in what people's views are on the choices now available, given that twin-turbo petrol engines offer a more effective foil to the performance advantages of the turbo diesel format.
Well, with regards to your last paragraph, i'm a big turbo diesel fan - i've now a new x5 3.0d. I wouldnt have even considered the 3.0i.
BUT if i were to be in a position to want to drive a five series again, AND a 535i were to exist, i would find it hard to walk past one, yet i never even considered a 540i / 550i instead of my 535d. Particularly with when using Xede on the 35i you can get nearly an extra 100BHP. Couple that with better fuel economy than the previous petrols and it could be hard to resist.
So, will the new twin turbo petrols help to swing people back to petrols, then for me, personally, i'd have to say yes - and thats a die hard TD fan speaking.
pgilc1 said:
So, will the new twin turbo petrols help to swing people back to petrols, then for me, personally, i'd have to say yes - and thats a die hard TD fan speaking.
That's exactly what I've been musing. Knowing my luck, after years of bucking the trend by owning big-capacity V8s SUVs, just when I decide to join the TD club the market will move back to petrol... 
One of the things I love about the V8 petrol models I've owned is that starting them up is such a sense of occasion. I've not tried to start up a TDi from cold in my garage but I suspect I will be very disappointed.
Edited by DoctorD on Sunday 6th January 16:03
pgilc1 said:
BUT if i were to be in a position to want to drive a five series again, AND a 535i were to exist, i would find it hard to walk past one, yet i never even considered a 540i / 550i instead of my 535d. Particularly with when using Xede on the 35i you can get nearly an extra 100BHP. Couple that with better fuel economy than the previous petrols and it could be hard to resist.
Just thinking a little further on this. There are two or three reasons why you would not perhaps consider a 540i/550i against a 535d (although there are probably more). The obvious ones such as future residual value and fuel running costs, but I also imagine you would have been seduced by the 535d's much cheaper purchase price. That latter argument does not apply with the 35i engine, which is actually £1500 cheaper than the 35d. I wonder whether that will influence the decision when both engines are available in the same car (as in the X6 and 3-er)
Edited by DoctorD on Sunday 6th January 16:10
DoctorD said:
pgilc1 said:
One of the things I love about the V8 petrol models I've owned is that starting them up is such a sense of occasion. I've not tried to start up a TDi from cold in my garage but I suspect I will be very disappointed.
Filling the V8 petrols is NOT so much fun though.Edited by DoctorD on Sunday 6th January 16:03
I have stepped out of the A8 4.2TDI sport, superchipped too 385BHP, into the new RS4 V8 petrol.
Mid range the TDI eats the RS, and never gave less than 29 MPG.
The RS as we all know could NEVER match that.
Conclusions................. Diesel all the way for every day UK driving.
Edited by RS4john on Sunday 6th January 19:49
Yep, I worked out that comparison and to justify the increased purchase price of a 35d over a 35i, based purely on fuel consumed, you would have to cover 37,500 miles before the £1500 higher purchase price was paid back.
The RS4 vs A8 Tdi comparison doesn't quite fit the discussion, although I understand the point being made.
I am talking about whether twin-turbo petrols will create a case for themselves against twin-turbo diesels. Turbo-diesels have been the default choice for large saloons/SUVs in recent years, but I'm wondering if these twin-turbo petrols will buck that trend.
The RS4 vs A8 Tdi comparison doesn't quite fit the discussion, although I understand the point being made.
I am talking about whether twin-turbo petrols will create a case for themselves against twin-turbo diesels. Turbo-diesels have been the default choice for large saloons/SUVs in recent years, but I'm wondering if these twin-turbo petrols will buck that trend.
DoctorD said:
The obvious ones such as future residual value and fuel running costs, but I also imagine you would have been seduced by the 535d's much cheaper purchase price.
Yes, down the line the big petrols would have dropped further in price, and fueling them would be no fun. Interestingly the 540i is actually slightly cheaper than the 535d. The BMW salesman i bought the 535d off doesnt remember the last time he sold a new 550i/540i.Edited by DoctorD on Sunday 6th January 16:10
I have actually never had much of a problem fueling my ML63, despite its 6.2 litre engine it averages 18-19 mpg on a longer trip and goes 500 miles or so before re-fuelling (93 litre tank). In a car with a smaller tank such as my RS4, fuel stops can be every 200-250 miles, which is far less entertaining.. 
DoctorD said:
That latter argument does not apply with the 35i engine, which is actually £1500 cheaper than the 35d.
flattotheboards said:
I would personally go for the 35i because it makes a nicer sounds and it doesnt use that much more fuel and it will be cheaper to buy.
mogul said:
I understood the cost of a diesel car when combined with the extra fuel cost only made sense when you drove above average mileage?
The 335d costs £1900 or so more than the 335i, but the 335i is a manual and the 335d is an auto. If you were to buy the auto 335i as i would suspect most buyers will, then the difference is down to < £400.Plus a lot of people prefer turbo diesels because of their driving characteristics, and their 'tunability'
DoctorD said:
1. Can anyone with good mechanical engineering nous calculate what the actual torque to the wheels would be like?, so that we can compare apples with apples.
If they both have the same drivetrain then the % difference from fly to wheels will be the same. It's around a 24% loss on the auto 35d.Edited by DoctorD on Sunday 6th January 13:51
I'm biased but would take the 35d. Fuel economy and motorway driving dictated the decision for me. My 35d returns an average of 38mpg, this was over 40 until the last couple of months where I have donme limited m/way driving. A colleagues 35i averages 24mpg.....enough said. His is not remapped, mine is and the D leaves it for dead in all comparisons, if his was remapped then obviously this would be different. Mine will probably hold it's value better and this is also an important consideration.
Jules
I had the choice (dont do big miles) and bought the D and got it mapped straight away. fuel wise swmbo on the school run will average 29mpg, get it on the motorway and not hanging about its straight into the 40"s. worst ever fuel consumption I get is towing my race shuttle with a car in the back and it returns 320miles on the tank (22mpg) pretty shit for a diesel but good for the performance the car has, Its quicker than my pals Rs6 and M3. Ive driven both there cars and the power delivery/torque from my car makes it seem much easier to drive plus they dont get the mpg I can.
residuals are looking to be better on the d too.
2006 335d glass"s trade £27.4k
2006 335i auto glass"s trade is £26k manual is £24k.
residuals are looking to be better on the d too.
2006 335d glass"s trade £27.4k
2006 335i auto glass"s trade is £26k manual is £24k.
Might it also be worth considering running costs other than fuel?
If buying new and not keeping for ever I guess it's not a factor but once the warranty runs out I would much rather be in the depreciated petrol than then diesel as a 2nd hand purchaser. I do not do many miles and I do resent the economy I get out of large cars in town but the long term reliability/maintenance of the TD's is a niggle that wont go away for me. Is that unfair criticism?
If buying new and not keeping for ever I guess it's not a factor but once the warranty runs out I would much rather be in the depreciated petrol than then diesel as a 2nd hand purchaser. I do not do many miles and I do resent the economy I get out of large cars in town but the long term reliability/maintenance of the TD's is a niggle that wont go away for me. Is that unfair criticism?
JamesK said:
Might it also be worth considering running costs other than fuel?
If buying new and not keeping for ever I guess it's not a factor but once the warranty runs out I would much rather be in the depreciated petrol than then diesel as a 2nd hand purchaser. I do not do many miles and I do resent the economy I get out of large cars in town but the long term reliability/maintenance of the TD's is a niggle that wont go away for me. Is that unfair criticism?
What makes you think a twin turbo diesel is likely to be any less reliable than a higher revving twin turbo petrol?If buying new and not keeping for ever I guess it's not a factor but once the warranty runs out I would much rather be in the depreciated petrol than then diesel as a 2nd hand purchaser. I do not do many miles and I do resent the economy I get out of large cars in town but the long term reliability/maintenance of the TD's is a niggle that wont go away for me. Is that unfair criticism?
JamesK said:
Might it also be worth considering running costs other than fuel?
If buying new and not keeping for ever I guess it's not a factor but once the warranty runs out I would much rather be in the depreciated petrol than then diesel as a 2nd hand purchaser. I do not do many miles and I do resent the economy I get out of large cars in town but the long term reliability/maintenance of the TD's is a niggle that wont go away for me. Is that unfair criticism?
That's an interesting point, is there a reliability issue longer term with turbo diesels or is this just a factor of the high proportion of engines which have been remapped? If buying new and not keeping for ever I guess it's not a factor but once the warranty runs out I would much rather be in the depreciated petrol than then diesel as a 2nd hand purchaser. I do not do many miles and I do resent the economy I get out of large cars in town but the long term reliability/maintenance of the TD's is a niggle that wont go away for me. Is that unfair criticism?
I was reminded today of another option, which might offer a more effective challenge to BMW's TD engines in the near future, namely their new 'Hybrid Petrol' engines. This is a V8 with an electric engine, where the electric engine can be used not just in town but also to boost the performance of the petrol engine under acceleration. BMW are claiming a 20% improvement in economy compared to the equivalent petrol-only engine. Maybe that's the future for those of us who prefer the sound of a petrol, but still want the economy and residual value benefits of a diesel.
DoctorD said:
That's an interesting point, is there a reliability issue longer term with turbo diesels or is this just a factor of the high proportion of engines which have been remapped?
Modern turbo diesels have all sort of reliability issues when they get older, but the main issues are with the ancillaries rather than with the basic engine. It depends on the engine but turbos, injectors, MAF's, fuel pumps, and EGR valves all give trouble. Oh and by 80k miles the inlet manifold can be so full of carbon that it blocks 50% of the air flow.Heard about the problems some people are having with 320d's now they're getting older?
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