What's the advantage/disadvantage of a high revving turbo engine?

[RPiM5]

Gents,

I've been studying high performance engines for a long time and I wanted to get the communities take of turbo engines with high redlines. I only bring up this topic because the new M4 S55 engine has a pretty high redline for a Turbo Engine, 7,600rpm I think. There's very few cars that come to mind that come from the factory with a high redline and is turbo charged. The Mclaren 12C is one, which is 8,500rpm. For those of us who have driven a 12C before, people say that it sounds very very good inside.

So I'm wondering, what's the limiting factors or advantages/disadvantages of high revving turbo charged engines? Why does the torque in a turbocharged engine die off so drastically at a certain rpm? I am thinking a smaller displacement engine like a 3.0L S55 engine will have a lower rotational mass and would be easier to have a higher redline than a 4.4L V8. But I've also heard from some car magazines and auto journalists that after a certain rpm there's very little need to go all the way up to 7,600rpm in the new M4, because the torque is just not there. Would a higher redline correspond to more unusable powerband in a turbocharged engine?

Here's my thinking. Would it be possible for ///M to develop a 4.0L Twin Turbo V8 engine that revs up to 8,000rpm? Or how about a 3.0L Twin Turbo V10 that revs to 8,300rpm for the next M5? Does a higher rpm necessarily correspond to worse fuel economy also?

For us lovers of both the old high revving V10 of the yesterday's E60 M5 and who equally love today's turbo charged V8 engine in the F10 M5, I'm just wondering, is it possible to have an engine that is reliable and allow us to enjoy the best of both worlds, or in other words, to have our cake and eat it too? Or is this something that wouldn't even be practical?

Just thought this would be an interesting discussion.

[GoodandEvil]

As far as I know, three methods to increase power output are increasing displacement (which has came to a dead end for manufactures), increasing engine rpm (high cost for development and building, stability and reliability issue), and finally supercharging (effective and low cost). M5's engine is BMW's answer for high revving big engine with tons of available torque at lower rpm. I think Honda is the manufacture that we can hopefully expect to build a truly high revving (above 8000) small V6(Honda likes compact engines) with large turbo chargers. We'll see once they release the new NSX in 2016 and new S2000 in 2019

[Boss330]

Your questions are good and poses many interesting scenarios. A few of my thoughts on this:

-Current F1 engines are 1,6l V6 turbo with a redline at 15.000rpm (but hardly revs above 12k due to fuel flow limits). So, yes it's not impossible to have a high rpm turbo engine.

-High rpm = more HP. Since higher rpm means more work is done per second (or any other time unit)

-High rpm means higher piston speeds which again means going the oversquare route (stroke is smaller than bore) like McLaren's engine is. Fully doable and not that big trade offs.

-High rpm means even more exhaust flow, which might necessitate larger turbos which means lag might be more of an issue... (that's where the hybrids "torque fill" is brilliant)

-Since HP and Torque are related to each other, the torque curve reflects how the turbo boost is regulated so that the HP doesn't "go through the roof" towards the redline... (it's not high HP that's the main concern but combustion pressure, loads etc.)
For instance, if the S55 had made the same 406ft.lbs at 7300rpm as it does at 5500rpm, the engine would have made 565hp @ 7000rpm ... and 587hp @ 7600rpm... That's clearly beyond their target and would have meant new and stronger engine internals. Hence why they need to drop that enormous mid range torque towards the higher parts of the rev range...


Basically, on a NA engine you need RPM to make more power (since the higher the rpm, the more work is done per time unit). This also applies to a turbo engine of course. But there is a much easier and cheaper way of making more power on a turbo engine. Crank up the boost and force more air into the engine (effectively increasing the engines displacement).

There simply are less incentives to do a high rpm turbo engine. Keep it at a fairly decent rpm and you can maintain a long stroke and keep piston speeds in check while making plenty of power by "just" adding more boost. Go high rpm and you need to deal with all the related issues connected with high RPMs and high piston speeds... Especially when the same can be achieved much easier with more boost.

But, a 3l turbo with 1 bar boost should make more hp at 9000rpm than it does at 7000rpm (provided that you have large enough turbos to feed the engines air supply need at 9000rpm). But then again, the same engine could also make the same hp at 7000rpm, that the other did at 9000rpm, with simply increasing boost to 1,5 bar

[RPiM5]

Originally Posted by Boss330

Your questions are good and poses many interesting scenarios. A few of my thoughts on this:

-Current F1 engines are 1,6l V6 turbo with a redline at 15.000rpm (but hardly revs above 12k due to fuel flow limits). So, yes it's not impossible to have a high rpm turbo engine.

-High rpm = more HP. Since higher rpm means more work is done per second (or any other time unit)

-High rpm means higher piston speeds which again means going the oversquare route (stroke is smaller than bore) like McLaren's engine is. Fully doable and not that big trade offs.

-High rpm means even more exhaust flow, which might necessitate larger turbos which means lag might be more of an issue... (that's where the hybrids "torque fill" is brilliant)

-Since HP and Torque are related to each other, the torque curve reflects how the turbo boost is regulated so that the HP doesn't "go through the roof" towards the redline... (it's not high HP that's the main concern but combustion pressure, loads etc.)
For instance, if the S55 had made the same 406ft.lbs at 7300rpm as it does at 5500rpm, the engine would have made 565hp @ 7000rpm ... and 587hp @ 7600rpm... That's clearly beyond their target and would have meant new and stronger engine internals. Hence why they need to drop that enormous mid range torque towards the higher parts of the rev range...


Basically, on a NA engine you need RPM to make more power (since the higher the rpm, the more work is done per time unit). This also applies to a turbo engine of course. But there is a much easier and cheaper way of making more power on a turbo engine. Crank up the boost and force more air into the engine (effectively increasing the engines displacement).

There simply are less incentives to do a high rpm turbo engine. Keep it at a fairly decent rpm and you can maintain a long stroke and keep piston speeds in check while making plenty of power by "just" adding more boost. Go high rpm and you need to deal with all the related issues connected with high RPMs and high piston speeds... Especially when the same can be achieved much easier with more boost.

But, a 3l turbo with 1 bar boost should make more hp at 9000rpm than it does at 7000rpm (provided that you have large enough turbos to feed the engines air supply need at 9000rpm). But then again, the same engine could also make the same hp at 7000rpm, that the other did at 9000rpm, with simply increasing boost to 1,5 bar

Boss, they don't call you the Boss for nothing man.

Interesting assessment on the S55 engine. If I do recall correctly, I think they did make the Bore bigger on the S55.

I see your points. Why increase RPM for more HP when you can just increase boost to create the same result. As you stated in your F1 engine example, high revving turbo engines are totally possible, and I guess some of the drawbacks to designing a high revving turbo engine would be reliability and possibly fuel economy for high volume mass production engines meant to last for hundreds of thousands of miles.

My hope is that ///M would try to create an engine just a little more bespoke than what the competition has, as they have always done before. I can see Hybrid Assist playing a role in the future too. The technology may be matured and costs lowered in 4 or 5 years.

[Tom C]

I think a lot of it has to do with a car manufacturer "right sizing" the turbo to the engine's displacement and ideal power curve for that application. If you want an engine that is turbocharged to make its peak power at a high redline, one method (cheaper) is probably to go with bigger turbos. The bigger the turbo, the more likely you are going to experience turbo-lag down low, unless you start using lighter weight compressor wheels and/or more efficient turbos. Another method would be to keep relatively small turbos but increase the boost/keep higher boost for longer duration higher into the RPM band. This would necessitate bringing in heavier duty intercooling to help with the additional heat generated by the higher boost. The design of the S63TU with the turbos mounted into the valley of the V helps cut down on the turbo lag and the twin intercooler cores at the front of the engine are monsterous compared to other applications. On my old CL65, the intercooler cores were poorly designed (mounted directly on top of the rear of the engine) and you couldn't touch them without burning your fingers for at least 45 minutes of cooldown time. The M5's intercooler cores are never even hot when popping the hood immediately after spirited driving.

I think most manufacturers who run turbo applications are not looking to generate "unnecessary" R&D and production costs to have a high RPM turbo engine. I think McLaren found it to be a necessity because it is matched up directly with the 458 Italia and Gallardo (now Huracan). It needed to have a more "epic" power delivery and sensation that the other cars in its class have in abundances. Similarly, the M division needed to have a more free revving engine to keep in line with it's M tradition of free revving engines. Although not perfect, it is definitely noticeable compared to its AMG competition.

So to answer your question, the torque dies off in a turbo engine because the turbo's are usually producing their peak boost in the low to mid-range RPM and boost tapers off later into the redline because of the concerns mentioned above for intercooling efficiency. Most manufacturers are using the turbos to help give a car that torquey feel down low. The more you rely on the turbos up high to produce power the more inconsistent the powerplant becomes (i.e. heat soak). As mentioned above, there are ways around this, but they may not be cost effective to the manufacturer.

[Boss330]

And BTW, turbo flow charts and compressor maps are a science in itself...

The point you mentioned about the torque curves falling towards the end (and as explained need to fall unless you have a desire to make 600hp in the S55 at redline). You could also look at it a different way. Think of it as the torque curve actually being boosted above the normal NA torque in the mid range and then as you reach the high end of the rev range it falls back to a normal level again.

Even though it falls dramatically, it still makes more torque at 7600rpm than the S65 does at 7600rpm...

If you had huge turbos that not only can keep the boost, but more importantly can flow enough air, then you could see a torque curve that remained at 406ft.lbs even at redline. But that would yield 600hp, and a very much less responsive engine in the lower and mid end of the rev range (due to the need for larger compressor side that could flow more air, meaning it would also be less efficient at lower air volumes).

[greentrbo95gst]

Boss330

[RPiM5]

Originally Posted by Boss330

And BTW, turbo flow charts and compressor maps are a science in itself...

The point you mentioned about the torque curves falling towards the end (and as explained need to fall unless you have a desire to make 600hp in the S55 at redline). You could also look at it a different way. Think of it as the torque curve actually being boosted above the normal NA torque in the mid range and then as you reach the high end of the rev range it falls back to a normal level again.

Even though it falls dramatically, it still makes more torque at 7600rpm than the S65 does at 7600rpm...

If you had huge turbos that not only can keep the boost, but more importantly can flow enough air, then you could see a torque curve that remained at 406ft.lbs even at redline. But that would yield 600hp, and a very much less responsive engine in the lower and mid end of the rev range (due to the need for larger compressor side that could flow more air, meaning it would also be less efficient at lower air volumes).

I've heard of designs where an engine would have one small turbo for the lower rev range and one large turbo for the high rev range. Thereby covering a large torque band for almost the entire rev range. Also, I wonder how a Tri-Turbo setup would work. Let's say 2 small turbos and 1 large turbo. I'm sure having a Tri-Turbo setup would be much more complicated for the ECU to manage.

Over in the M4 section, there's new talk from ///M Division about the next M3/M4 incorporating a 4 cylinder turbo engine. I wonder what kind of boost levels would have to be present in order to make more power than the current 425hp in the S55. Probably a lot. Would a tri-turbo setup on a 4 cylinder engine work better for producing more power? And what about rpms and redline for a 4 cylinder turbo engine? Currently Mercedes makes the highest horsepower 4 cylinder turbo from the factory in a production car, with the new CLA 45 AMG. I would say 4 cylinder turbo engines probably won't sound very good, but the Fiat Abarth is one of the best sounding cars I've ever heard.

Anyways, good discussion.

[RPiM5]

Originally Posted by greentrbo95gst

Boss330

Here here for Boss330!

[Dionysus]

Originally Posted by RPiM5

Currently Mercedes makes the highest horsepower 4 cylinder turbo from the factory in a production car, with the new CLA 45 AMG.

VW have confirmed plans for 395 (hp/bhp?) from 2.0l petrol...

http://www.autocar.co.uk/car-news/be...ing-motor-show

We'll probably soon be seeing M5 vs. VW Golf drag races

[RPiM5]

Originally Posted by Dionysus

VW have confirmed plans for 395 (hp/bhp?) from 2.0l petrol...

http://www.autocar.co.uk/car-news/be...ing-motor-show

We'll probably soon be seeing M5 vs. VW Golf drag races

You mean, M4 vs VW Golf drag races.

[Boss330]

Originally Posted by RPiM5

I've heard of designs where an engine would have one small turbo for the lower rev range and one large turbo for the high rev range. Thereby covering a large torque band for almost the entire rev range. Also, I wonder how a Tri-Turbo setup would work. Let's say 2 small turbos and 1 large turbo. I'm sure having a Tri-Turbo setup would be much more complicated for the ECU to manage.

Over in the M4 section, there's new talk from ///M Division about the next M3/M4 incorporating a 4 cylinder turbo engine. I wonder what kind of boost levels would have to be present in order to make more power than the current 425hp in the S55. Probably a lot. Would a tri-turbo setup on a 4 cylinder engine work better for producing more power? And what about rpms and redline for a 4 cylinder turbo engine? Currently Mercedes makes the highest horsepower 4 cylinder turbo from the factory in a production car, with the new CLA 45 AMG. I would say 4 cylinder turbo engines probably won't sound very good, but the Fiat Abarth is one of the best sounding cars I've ever heard.

Anyways, good discussion.

Yes, there are setups with one small and one large turbo that covers exctly what you mention. BMW allready has a triturbo engine... The M550d diesel 3l is a triturbo on a straight six. So that can work as well!

The S55 actually makes it's power at a lower boost than what it's capable of, same as on the S63Tü. If you live in a close to seal level and normal temperature climate, you would see that the boost would never reach the stated max boost, but instead be at around 17psi I believe (on a S63Tü engined car). The same is true for the S55.

Why?

Because BMW have left a substantial "boost reserve" for those situations where you are at the top of Mount Everest in humid weather and 140deg F temperature... BMW wants to make sure that the engine makes 560hp (or "a bit" more) even in those not so optimal conditions. It compensates for altitude, humidity and temperature etc by having a boost reserve that is used to achieve the same performance as it does at sea level in cool climate.

So in fact the S55 (and S63Tü) makes it's stated power at less than the stated max boost

Which I guess is why at least 5 German tuners expect to see 500+hp from a piggyback tune on the new M3/4. They might not even go above the BMW max boost level, they just enable the engine to use that max boost also in ideal circumstances...

[Boss330]

Originally Posted by greentrbo95gst

Boss330

Originally Posted by RPiM5

Here here for Boss330!

You guys make me blush

[Actor]

Originally Posted by Boss330

You guys make me blush

[richard in NC]

Originally Posted by RPiM5

I've heard of designs where an engine would have one small turbo for the lower rev range and one large turbo for the high rev range. Thereby covering a large torque band for almost the entire rev range. Also, I wonder how a Tri-Turbo setup would work. Let's say 2 small turbos and 1 large turbo. I'm sure having a Tri-Turbo setup would be much more complicated for the ECU to manage.

Anyways, good discussion.

Hard to add to these very good explanations but I'd think the ///M division spent a lot of time refining the responsiveness of the S63Tu and S55. Hence the totally unique cross flow intakes, etc. Managing sequential or parallel turbos of different sizes could result in very uneven throttle response, etc. Especially given that M's live on street and track. The new ones work incredibly well at both. As discussed, increasing boost is easy. Keeping driveability and durability across the wide spectrum of use across the life of a car is not so easy.

[RPiM5]

So with what's been stated I wonder if we'll see a Tri-Turbo setup in the next M5. I still think it's anyone's guess as to the engine in the next M5 though.

I mean who here would be opposed to a higher revving turbo engine? I would love it. I still miss the S85 sometimes.

[Hockey Guy]

Great discussion. So let's see:

We want more HP (600+). Really who wouldn't want more...

Sub-4,000 lb weight (no increase in 5-series car size pls and thx)

Potentially more torque to include low end hybrid assist? Halo cars have gone hybrid in various forms so why not BMW's next-gen flagship?

Higher rev's although this will be difficult it seems with the need for fuel efficiency gains (yes even Ms seek this as the M5/4/3 show...and damn impressive gains at that!)

Optional AWD for those who want off-the-line speed, live in winter climes and who simply want to see how Mercedes and Porsche etc can pull off AWD and good handling (remember that our Main Man Freddie N said that the M5 was "suffering" which usually means Sales are not what they hoped...this has to be taken seriously by RWD purists like me ...like it or not)

Oh and did I mention more raw sound from that trick, no turbo lag set-up we are all getting used to...

How is that for raising the bar for the next great M supercar? (And yes I will love and cherish mine while I wait!)

Get to work Freddie!

[clar]

The S55 is definitely not performing at its peak efficiency even tho the size of its turbos are rather small. BMW needs to maintain some performance gap with the M5/M6. In addition, there will be the inevitable comp pack coming up in less than 2 years time, which should bring along another lousy 20 bhp or so. I have always thot why not add an electric motor to the turbos to help it spool up faster to prevent lag. That way, you can have bigger turbos for more high end performance. I was half expecting something similar happening in the S55, but was disappointed that it din happen. The M engineer I spoke with at Sepang promised me something "cool" in the S55, but i guess that translates to the anti-lag feature in the Sport-plus throttle mode...

[M6-Coupe]

Originally Posted by RPiM5

So I'm wondering, what's the limiting factors or advantages/disadvantages of high revving turbo charged engines?

It was a great discussion. I think beside the reasons other members referred to as far as cost or boost compensation, another reason is the difficulty of driving a high revving turbo charged car as a DD car for people who want to drive these cars in normal driving condition like streets and safety is the main concern.

I found this article and I thought you may be interested to read:

Torque is the rotational version of force. The more torque an engine produces, the more force it can exert at the rim of a flywheel of a given radius.

Power is force multiplied by speed. The more power an engine generates, the more work it can do in a given time.

Torque increases as rotational speed increase from idle to a certain figure and then falls as the rotational speed increase above this figure. Acceleration is proportional to the amount of force pushing the vehicle forward; so maximum acceleration in a given gear is obtained when maximum torque is obtained.

Power is force (~torque) multiplied by speed (~rotational speed) so power increases with rotational speed up to and past the point of maximum torque. However, at still higher rotational speed the engine starts to be limited by the amount of air that it can draw in ; and torque then decreases more rapidly than the rotational speed increase, and therefore power also decreases. Maximum acceleration is obtained by having maximum propulsive force at the wheels. Use of a low gear ratio multiplies the engine torque at the wheel (at the price of having the engine rotate more quickly), maximum acceleration at a given speed is obtained by having the engine operate at maximum power.

Driving a car is easier and more relaxed if it has a flexible engine. Flexibility even becomes a safety issue in four-wheel drive vehicles. Maximum torque is obtained at a certain rotational speed, and maximum power at a higher rotational speed. An engine is flexible if these maximums occur at widely different rotational speeds.

A vehicle engine operating at a rotational speed above its maximum-torque point is in a "stable" speed regime. If it slows down by a small amount (due to the vehicle encountering an incline, head-wind, etc.) engine torque will tend to increase and resist the slowing. Conversely, if it speeds up by a small amount, torque will tend to decrease and discourage a further increase in speed.

A vehicle engine operating at a rotational speed below its maximum-torque point is in an “unstable speed” regime. If it slows down by a small amount, the torque decreases and its speed will fall further. The driver can compensate by opening the throttle. Conversely, if the speed increases then the torque increases and the speed increases even more. The driver can compensate by closing the throttle. The driver has to actively compensate for these.

The driver cannot correct for a fall in engine rotational speed and loss of torque if the throttle is already wide open, except by changing into a lower gear. This can be a safety matter on a steep road. If you start up a steep hill in too high a gear, or have to slow down due to obstacles, for example, the engine may fall below its maximum-torque point and be unable to recover from a downward speed spiral. Changing gears causes a loss of more speed and, additionally, traction may be lost as the clutch engages. Inaction on the part of the driver may lead to a stalled engine and a forced restart on a dangerous slope. These risks are minimized if maximum engine torque is designed to occur at low engine rotational speed.

So, if you are into racing, particularly Formula One racing, you want an engine with the highest possible power output and this is best achieved by generating maximum power at very high rpm. Such engines also tend to generate maximum torque at very high rpm and are inflexible and difficult to drive.

If you are into four wheel driving or drag racing, you want a very flexible engine that generates maximum torque, and lots of it, at low rotational speed and generates maximum power at a higher rotational speed.