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inline 6 vs V6

54K views 121 replies 50 participants last post by  hondo402000 
#1 ·
On this forum when other cars are compared to 3-series, I see frequent comments that the inline 6 is superior to a V6, in more ways than one. Is there some inherent advantage in the configuration? Or is it just the long experience and engineering that has gone into the BMW engine? Car manufacturers with V6's certainly aren't reticent about promoting them.
 
#79 ·
As BMW seems to realize now with the N54...

and the V6 has twin cylinder banks, which = more heat loss. More heat loss = power loss.
Disagree - in normal operation the problem is getting rid of the heat, not keeping it in. That's why all engines have a cooling system.

You cannot really say which engine is better at dissipating heat - since that greatly depends on the configuration of the engine compartment and location of the engine. A short V6 may not get under the bulkhead like an I6, so cooling is probably more even across the cylinders. But because it is wider, it's possible that not enough air circulates around the sides to cool it.
 
#3 ·
Upside: Typically more efficient and smoother. Efficient because of less frictional losses (more valve gears and camshafts for an equivalent V6) and the V6 has twin cylinder banks, which = more heat loss. More heat loss = power loss.
Smoother because the inline-6 is inherently balanced (vertical and transverse forces generated from individual cylinders are completely canceled out by the other cylinders). Not so with the V6.

Downside: Space. They're loooong. Thus damn difficult (don't know if anyone's ever done it) to package with a front-drive, for example. This also makes for a large engine bay (long hood).
 
#14 ·
Downside: Space. They're loooong. Thus damn difficult (don't know if anyone's ever done it) to package with a front-drive, for example. This also makes for a large engine bay (long hood).
Mercedes-Benz dropped their inline-6, and so did Lexus, in the interest of space (packaging under the hood).

Volvo shoehorns an inline-6 transversely under their hoods. In fact, the V8 in my XC90 sits transversely as well (see pic below)! Volvo is nuts, but they claim it adds "crush space" in an accident.

(37,000 miles on the Volvo engine. Yes, I keep my engines clean - that shot was taken on Sunday.)
 

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#4 ·
The I6 is referred to as Newton's engine.. I think a reference to it's smoothness and balance
What Galahad said is spot on..
 
#5 ·
Downside: space.
Means you can, say, pack 3.0-3.2L inline-6 into the bay and get 255-270hp from it.
Or you can pack 3.5-3.7L V6 into the same bay and get 300-330hp.
That's with all other things being equal - N/A and civil redline on both.

BMW was a long-time advocate of N/A engines. And for a good reasons. But when V6 competition pushed the HP envelope, BMW was forced to go turbo route. Sad.
 
#6 ·
I think you forgot the M3 3.2L at 333HP and the 2003 M3 CSL at 360HP from 3.2L BMW can also make big HP without Turbos. It's just cheaper to do with Turbos. They save the big power naturally aspirated engines for the M's.

Red
 
#8 ·
Another couple of advantages to the I6 are
1) The straight-6 is smoother than engines with a fewer number of cylinders because the power strokes of pistons partially overlap. Since each power stroke lasts 180 degrees of crankshaft rotation, while a new piston starts its power stroke every 120 degrees, there are 60 degrees of overlap on each stroke in which one piston is finishing while the next is starting. (plagiarism alert - I copied the above from wikipedia)
2) Aforementioned minimized frictional/power-sapping losses from two valvetrains,
3) Balance advantages from all cylinders moving in the same plane

Good article here http://en.wikipedia.org/wiki/Straight-6

I believe another reason BMW has finally gone the route of forced induction is that you can bring to market a vehicle that will have the fuel efficiency of the NA vehicle while still delivering the power needed to be competitive (seemingly 300+ hp now). With good turbo/ecm management the engine can receive minimal boost in 'normal' driving, saving fuel.
 
#11 ·
I believe another reason BMW has finally gone the route of forced induction is that you can bring to market a vehicle that will have the fuel efficiency of the NA vehicle while still delivering the power needed to be competitive (seemingly 300+ hp now). With good turbo/ecm management the engine can receive minimal boost in 'normal' driving, saving fuel.
Plus for me - having just driven a few M3s - the thing I love about my 335 is that if you want to blast around you can.. but if I am in the mood to cruise around quietly .. the 335 if not in your face like M3's can be. Of course there is a attaction to the M3's in your face at any speed.. but I have gotten to old to want that all the time.

I believe dual personality ability is because of the turbo, ... Gas milage? ...on a freeway stretch I can see 32 mpg on the computer just show you how little the engine has to work to do 75mph.

craig
 
#10 ·
All things being equal, it's much more expensive to bring to market a high-revving engine (S54B32). The engineering, development, manufacturing, and warranty costs all go up at a much faster rate than does the horsepower. Also, on high-revving engines like these, the peak horsepower and torque comes fairly high in the rpm range of the vehicle. So while BMW certainly can development high horsepower normally-aspirated I6's, it seems the route going forward will be forced induction which they are quite good at too, see the 1986 Formula One engines they developed 1.5 liter, 4-cyl, 5.5 bar turbos making >1300bhp.
 
#18 ·
I6's are magical. I suppose part of that is how I like long hood lines and the roar of an I6. Too many V6 engines sound like coffee grinders.

There is a 3.8L I6 sitting in a jumble of parts pretending to be a Jaguar E-type in my garage. Rumor has it the real output was around 220 HP so considering a 3.0L e90 is supposed to make 255 HP that is quite an advance.
 
#23 ·
Just a note: Several american companies have made inline six cylinders (I'm embarassed to admit but I BELIEVE my 97 Jeep Wrangler Sahara 4.0 was an inline six)...

I don't remember it being a model of newtonian goodness, although it was an awesome engine for around town driving :)

Joe
 
#24 ·
#25 ·
I read an article where BMW considered 3.5L -3.7L for their inline 6 to make the requisite 300+hp NA. But the increased size of the pistons (with the elevated weight) compromised the inherent primary balance which meant counter-weight balancers, etc. Basically leading engineering down the path of over complexity in an originally perfectly balanced concept.

Another area that BMW has used to their advantage is placement of the engine to optimize handling. If one is already "stuck" with a long hood and rear wheel drive style, then why not push the front wheels all the to front, move the bulk of the engine behind the front axle centerline, thus promoting mass centralization. And keeping the 50/50 handling that BMW markets.

The Japanese have mostly stayed with the V-6 due to their extensive experience in packaging that engine configuration in front wheel drive platforms.
 
#36 ·
Another area that BMW has used to their advantage is placement of the engine to optimize handling. If one is already "stuck" with a long hood and rear wheel drive style, then why not push the front wheels all the to front, move the bulk of the engine behind the front axle centerline, thus promoting mass centralization. And keeping the 50/50 handling that BMW markets.

The Japanese have mostly stayed with the V-6 due to their extensive experience in packaging that engine configuration in front wheel drive platforms.
That is exactly right, the length of the e4ngine allows is to aid in the 50/50 distribution of the cars. Most automakers packe a V6 in the front, along with front wheel drive and a battery all under the hood making for a completely unbalanced car.
 
#27 ·
I don't see BMW offering a larger version of their inline-6 in the future. It makes 300+ horses without drama, and it can easily be bumped to 350 hp without too much concern. At that point, there are several V8's than can pick it where the six leaves off...
 
#33 ·
Don't forget the Chrysler slant-6 engine. My family had two Darts when I was growing up. I later had a Duster. All three had the 225 slant-6 and all went well past 100k. One Dart we sold at 190k. Very reliable and a very easy engine to work on.
 
#44 · (Edited)
yeah, that is the reason, V-8 allows a more balanced piston firing order than a V-6. The advantage of a straight 6 over a V-6 is the balanced piston firing order that is mentioned in an earlier post. The straight 8 doesn't carry a significant advantage over a v-8 due to 4 pistons per cylinder bank. The V-8 is so much better balanced and compact over a straight 8 that the straight 8 is antiquated. Straight 8s and 10s ad 12s were used on old receprecating aircraft engines.
 
#35 ·
I believe BMW is getting 272hp from their latest iteration of the N-series engine (N53B30 available in EU). While 300hp would still be a significant, 10%, increase, it goes to show you how much experience/know-how they have with the 3.0l I6.

What it really makes me wish for is a 335/330 lineup with the the 335 boosted to a (apparently) feasible 340hp and a 272hp 330. Or maybe a 335/330/325 lineup? I know the additional models would add complexity costs, but hey, I'm a consumer and I like options...
 
#69 ·
Yep. I don't remember the proper name. "Not cross-plane" doesn't exactly roll off the tongue. :p
 
#70 ·
Good observation. I hadn't considered that. Taxation is also the reason that Japanese 4-pots tend to be 2 liters.
 
#49 ·
The inline 6 is inherently smoother than the V-6 - less vibration modes. Years and years ago, my brother in law had a 1953 Buick straight 8. Very smooth running, but physically a monstrosity under the hood. The disadvantage of an inline block is its physical length. I had a Volvo S60R, which had all wheel drive and a transverse inline 5 engine. The shortcoming of that design was that, with the long engine and oversized wheels and brakes, there was so much stuff between the wheels that the turning circle was severely compromised. 5 and 6 cylnder inline engines create bad packaging problems if mounted transversely, but as we know, BMW 6's run fore and aft.
If you look under the E90 hood, the front axle is roughly in between cylinders 3 and 4. Th engine is far enough back that the car avoids being front heavy, at the expense of a slightly long hood. I also have a Honda S2000. In that car, the inline 4 is completely behind the front axle, which gives almost perfect 50/50 front to rear weight balance, but the hood is incredibly long for an otherwise tiny car. The E90 ends up with a weight balance around 51/49, which ain't that bad.
 
#53 ·
Those are pretty good cross-section drawings.

Infiniti G35 is still a bit more nose-heavy when you look at the numbers on a scale. Infiniti tosses in an aluminum hood to help keep the weight down in the front end (at least they used to). However, I still feel the G37 has more oversteer (rear weight bias feel) than the 335i on the track. I've said before, the G37 - in stock configuration - is a track star.

Hey void.crusader, what Nissan do you drive?

- Mike
 
#55 · (Edited)
void.crusader said:
What octane fuel?
Is it certified for US?
Is it on sale now?

Btw, one way to estimate engine efficiency is ~ (peak hp) / (peak rpm * liters).
.
Both on RON 98 (Euro equivalent to (MON+RON)/2 AKI 94). As you quoted the spec for the Euro GT3, the comparison was with a Euro M3 CSL. M3 CSL is not on sale at the moment as it is an E46 model, but it was a 911 competitor (basically a model that is just out of production with a stronger engine for it's capacity than the current Porsche). Really just an illustration that specific output for BMW engines is just as good as for Porsche.

BTW, another way (of many ways) to estimate engine output efficiency (or perhaps capability is a better word here) is (torque under curve) / litres. Or better expressed as (average torque * RPM range) / litres. This measurement will take into account any non-linearity of the power curve that peak measurements don't take into account.
 
#63 ·
Both on RON 98 (Euro equivalent to (MON+RON)/2 AKI 94). As you quoted the spec for the Euro GT3, the comparison was with a Euro M3 CSL. M3 CSL is not on sale at the moment as it is an E46 model, but it was a 911 competitor (basically a model that is just out of production with a stronger engine for it's capacity than the current Porsche). Really just an illustration that specific output for BMW engines is just as good as for Porsche.
I think I quoted US-spec GT3 with hp (SAE) rating. In euro horses: 415 bhp @ 7600 rpm, 299 lb-ft @ 5500 rpm.

http://www.caranddriver.com/reviews...orsche_911_gt3_first_drive_review+page-2.html

"Owing to the small production run and the complications of clearing DOT and EPA standards for the North American market, BMW never exported the CSL to the United States, although some parts from it were later made available on the regular M3 as part of an optional Competition Package."
http://en.wikipedia.org/wiki/BMW_M3

Looks like E46 CSL was never cleared for the states? Anyway

3246cc, 360 bhp @ 7900, 274 lb-ft @ 4900

CSL hp/rpm*l = 360/(7900*3.246) = 0.01404

GT3 hp/rpm*l = 415/(7600*3.600) = 0.01517

http://www.rsportscars.com/bmw/2003-bmw-m3-csl/

Not sure specs for CSL are correct on this website, but can't find anything better.

BTW, another way (of many ways) to estimate engine output efficiency (or perhaps capability is a better word here) is (torque under curve) / litres. Or better expressed as (average torque * RPM range) / litres. This measurement will take into account any non-linearity of the power curve that peak measurements don't take into account.
It's good, I agree, the only problem is we need curves for this.

Penforhire said:
I don't think we should get hung up on specific output (that GT3 comparison) as ANY measure of goodness. The original 2L I4 Honda S2000 motor made 120 HP/L, naturally aspirated. BFD, because 240 HP was on the weak side.
S2K (F20C) hp/rpm*l = 240/(8300*1.997) = 0.01448
 
#61 ·
I don't think we should get hung up on specific output (that GT3 comparison) as ANY measure of goodness. The original 2L I4 Honda S2000 motor made 120 HP/L, naturally aspirated. BFD, because 240 HP was on the weak side. I got BETTER highway mileage from a 5.7 V8 (C5 Z06). Specific output is a masturbatory bragging right, not much more.
 
#62 ·
+1 that original Honda 2000 was the HP/L king. But had such bad low end toqure Honda increased the CC's of the engine.
 
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