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      09-06-2010, 09:59 AM   #1
RAV
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Turbo Charging Fuel Economy ?

Many think tubo charging is the Holy Grail of increasing fuel ecomomy, but let me propose and idea why it is not. The below example is all within BMW's
present technologies.
The 3.0 N53 in Europe is rated at 272 HP and gets a fuel economy of 38.4 mpg on the EU combined cycle ( subtract 16% to get US highway equivilent ), the coversion to the US standard yields a highway milage of 32.2 MPG. Now lets add valvetronic that will increase horspower and MPG by a conservative 5%. This new engine is now 285 HP with a highway milage of 33.8. One final step would be to increase the displacement to 3.5 liters. Keeping with the 95 BHP per liter, this engine would now put out 332 HP and would most likley deliver the same gas milage ( Honda, Toyota etc. did not see a milage decline when they went from 3 to 3.5 liter V6's ) This proposed engine would now put out 335I HP ( underrated at 300 HP) and deliver 20% better fuel economy. I would also doubt that 4 Cylinder turbo would deliver this type of fuel economy.

The ? then becomes why would they not do this. Could it be space in the engine bay, I doubt this since the E46 M3 at 3.25 liters fit in the smaller body than the present 3 series. I think it is pure cost savings, much easier to slap on a turbo charger and different software to increase power. Turbos do put out a lot of torque, but it drops like a stone after 5500 RPM. The 328 at 6500has lost only 9% of its torque, whereas the 335 has lost 24%. This torque fall off greatly deminshes the High RPM smooth pull that the NA 6 will give you.

I hope I'm missing something here, I would like to be wrong. Any thoughts or comments.
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      09-08-2010, 06:26 PM   #2
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This topic has been discussed in numerous threads and on several boards. One poster did quite an analysis of existing turbo engines in today's cars, few of which produced impressive mileage numbers and one car mag (don't recall which) had a piece which also said don't hold your breath on big mpg numbers. That said, others argue just as hard that turbos will be our salvation with big mpg and big performance.

My take, after two Audi turbos, is that yes they can produce decent power, but not also good mpgs at the same time. When you get on the boost your mileage drops and since most of these engines are smaller anyway, you are going to be on the boost - especially in a BMW. I think when we see the new F30 turbo fours, BMW will be able to post some good numbers in their EPA and European driving cycles because they will be driven "off boost" in those tests; however, I also bet that few enthusiast drivers will ever see them.
Guess we will find out soon enough. I know I wish BMW had done more to wring better mileage out of their I-6, AND to get weight out of the 3er.
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      09-08-2010, 06:41 PM   #3
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Quote:
Originally Posted by RAV View Post
Many think tubo charging is the Holy Grail of increasing fuel ecomomy, but let me propose and idea why it is not. The below example is all within BMW's
present technologies.
The 3.0 N53 in Europe is rated at 272 HP and gets a fuel economy of 38.4 mpg on the EU combined cycle ( subtract 16% to get US highway equivilent ), the coversion to the US standard yields a highway milage of 32.2 MPG. Now lets add valvetronic that will increase horspower and MPG by a conservative 5%. This new engine is now 285 HP with a highway milage of 33.8. One final step would be to increase the displacement to 3.5 liters. Keeping with the 95 BHP per liter, this engine would now put out 332 HP and would most likley deliver the same gas milage ( Honda, Toyota etc. did not see a milage decline when they went from 3 to 3.5 liter V6's ) This proposed engine would now put out 335I HP ( underrated at 300 HP) and deliver 20% better fuel economy. I would also doubt that 4 Cylinder turbo would deliver this type of fuel economy.

The ? then becomes why would they not do this. Could it be space in the engine bay, I doubt this since the E46 M3 at 3.25 liters fit in the smaller body than the present 3 series. I think it is pure cost savings, much easier to slap on a turbo charger and different software to increase power. Turbos do put out a lot of torque, but it drops like a stone after 5500 RPM. The 328 at 6500has lost only 9% of its torque, whereas the 335 has lost 24%. This torque fall off greatly deminshes the High RPM smooth pull that the NA 6 will give you.

I hope I'm missing something here, I would like to be wrong. Any thoughts or comments.

wow there is just soo much more to it than you think...
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      09-08-2010, 09:46 PM   #4
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Quote:
Originally Posted by JonLerd View Post
wow there is just soo much more to it than you think...
There definitely is more to it. I believe you. But the OP is clearly saying he hopes he is wrong and is asking for those that know better to shine some light on the issue. You're just saying you know about it, but aren't really adding any more
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      09-08-2010, 09:49 PM   #5
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Quote:
Originally Posted by MikeTerp View Post
This topic has been discussed in numerous threads and on several boards. One poster did quite an analysis of existing turbo engines in today's cars, few of which produced impressive mileage numbers and one car mag (don't recall which) had a piece which also said don't hold your breath on big mpg numbers. That said, others argue just as hard that turbos will be our salvation with big mpg and big performance.

My take, after two Audi turbos, is that yes they can produce decent power, but not also good mpgs at the same time. When you get on the boost your mileage drops and since most of these engines are smaller anyway, you are going to be on the boost - especially in a BMW. I think when we see the new F30 turbo fours, BMW will be able to post some good numbers in their EPA and European driving cycles because they will be driven "off boost" in those tests; however, I also bet that few enthusiast drivers will ever see them.
Guess we will find out soon enough. I know I wish BMW had done more to wring better mileage out of their I-6, AND to get weight out of the 3er.
That's gold. Simply put, they only shine on paper. They may be marginally better than NA engines for economy while delivery the same or better performance, but I don't think they are going to deliver the drastic improvement we're hoping for. At least not for those that use the power.
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      09-11-2010, 09:05 AM   #6
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I owned 3 turbo cars and only one was doing better than its NA competitors.

Current examples;

EPA mpgUS
2010 AWD RDX Turbo 240HP 17/22 - requires Premium
2010 AWD Tiguan Turbo 200HP 18/24 - recommends Premium
2010 AWD Santa Fe V6 276HP 20/26 - runs on Regular

2011 FWD A4 CVT 211HP 22/25 (combined) /30
2011 RWD 528i 240HP 22/25/32


Turbo = overhyped most of the time.
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      09-13-2010, 09:38 AM   #7
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The biggest problem for modern car's fuel economy...WEIGHT. By downsizing to 4 cyl turbo they save some weight versus the 3.5L 6 cyl NA in your example while producing similar performance numbers on paper.
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      09-13-2010, 10:57 AM   #8
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I've often thought the same thing. My conclusion... which of course was the result of nearly zero research..... turbo's are more fuel efficient when you compare the power under the curve to a N/a counterpart. In otherwords, you get more hp and tq downlow in a turbo car say 1500-3500 rpm than a N/A equivalent... this is where most of the world drives 95% of the time. So if you don't measure peak figures but how much power/tq is actually driven, you get more power/tq for you dollar out of a turbo car. If you only look at peak hp numbers... then it seems to be really close.

Take the 272hp bmw I6 the op mentions , if you took how much hp and tq it makes between 1500-3500 rpm, it's pathetic compared to the 335i with n54 or n55. That's what we drive.
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      09-13-2010, 05:37 PM   #9
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Originally Posted by Cornhusker View Post
Take the 272hp bmw I6 the op mentions , if you took how much hp and tq it makes between 1500-3500 rpm, it's pathetic compared to the 335i with n54 or n55. That's what we drive.
Bingo.
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      09-14-2010, 08:00 AM   #10
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Bingo.
You are absoulutly correct about HP and torque down low and in the mid range. The problem is that people that buy performamce cars like to take them to redline. The turbos torque is starting to fall at 5000 and drops like a stone at 5500. I do not understand why someone would be interested in a performance car if they just cared about low to mid power ( this is what trucks and SUVs are tuned for.) But then again, why would someone buy a performance car with slush box. Lets say you take your 335i to redline in first gear, when you shift to second your RPM's drop to 4700. You are right at where the torque is just about to fall. This is why the turbo puts out tons more power in the mid range and not so much more on top. The 335i puts out 50% more torque than the 328 but only 30% more HP.
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      09-14-2010, 10:57 PM   #11
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Quote:
Originally Posted by RAV View Post
You are absoulutly correct about HP and torque down low and in the mid range. The problem is that people that buy performamce cars like to take them to redline. The turbos torque is starting to fall at 5000 and drops like a stone at 5500. I do not understand why someone would be interested in a performance car if they just cared about low to mid power ( this is what trucks and SUVs are tuned for.) But then again, why would someone buy a performance car with slush box. Lets say you take your 335i to redline in first gear, when you shift to second your RPM's drop to 4700. You are right at where the torque is just about to fall. This is why the turbo puts out tons more power in the mid range and not so much more on top. The 335i puts out 50% more torque than the 328 but only 30% more HP.
First of all, the 335i is underrated, so compare the rwhp ratings, and you will see more than a 30% advantage. Secondly, you do not drive around redline all that much. And if you did, you would most likely be getting less than 6mpg. So honestly, if you are getting better than 6mpg, you aren't that kind of "performance" driver to begin with.

Secondly, there are plenty of torquey sporty cars that don't rev that high, or have maximum torque around redline. Just look at a Porsche GT2RS Turbo. Doesn't even rev that high compared to a GT3, but with over 600hp, it will leave the GT3 for dead in any kind of acceleration contest. So feel happy revving the normally aspirated version, but I would take the faster version any day .
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