Hi guys,

Have a question after reading about the TwinTurbo in the 335i: so in order to shorten turbo lag, they chose to use 2 small turbos and it certainly helped in the front end of the RPM curve. But, when the revving goes really high, my understanding is that there will "breathing" problem for small turbos at high RPM's. Isn't that the tradeoff between small and large turbos? Large turbos have turbo lag at the frontend (i.e. low RPM) but has no problems at high RPMs.

Anyone know how BMW overcome this technical issue?

 

Just because it has 2 small turbos doesn't mean that it will run out of air up top. It might though depending on how they sized it. They definitely design around their needs. If you try to raise boost to higher levels, it may run out of air up top. The higher you try to boost it, the faster you will see it max out in the rpm range and start to fall off. Turbos as with anything else need to be sized based on certain compromises. Do you want high end power or do you want fast spool and instant low end power? Of course the new VTG or VATN (variable turbos) try to change the rules in this regards so that you can have the best of both worlds. They too are devices based on compromise. Their tolerance range is just much wider.

If someone wanted more top end power than what the BMW turbos could provide, they'd need to change out the turbos. Either to a larger single turbo or 2 larger twins. Depending on what type of turbos BMW uses, you may or may not lose spool properties. Turbo technology has gotten much better and it's easy to find turbos with very good bearings that spin easily. ON the RX-7s it was possible to upgrade to a larger turbo and get faster spool at the same time since the factory turbo didn't spin as well.

Opel took a very different approach to twin turbo setups with their diesel engine. It uses twins in series and parallel through the use of a couple of simple valves. Basically it's a small turbo on the low end and then transitions to a large turbo on the top end. It completely hands off to the other one rather than running them at the same time. Neat idea but plumbing is an issue.

It's hard to give any direct answer as to what BMW can do to overcome the issue. There are always going to be tradeoffs. I guess the simplest thing to do would be to use a variable geometry turbo if they need more top end but quick low end spool. It would simplify plumbing as well.

 

I'll bite.

Your understanding about the difference between small and large turbos seems to be on the right track, but not quite.

BMW simply chose a set of turbos that moves enough ft^3/min of air to sustain a positive charge for the powerband that they wanted. If BMW wanted a higher output profile, they probably would have chosen a larger turbine.

:|

 

Thanks guys, for your opinions. I enjoyed reading them.

 

Despite the claims of many owners, even BMW is subject to the laws of physics when it comes to forced induction. The 335i is an awesome car but the power delivery has its limitations. Check out the dyno plot below. I've seen a couple now for stock 335i's and they all look pretty similair. Torque comes on strong VERY early in the power range for a turbocharged car, as does peak horsepower. However the numbers drop off after about 5.7k RPM's. This engine makes power just like the old LS1 motors in the WS6/SS F-bodies. Actually it's eerily similair when you compare the plots.

 

In other words, the RX-8 is faster because it makes peak horsepower at 8500 rpms!

 

The price of the 335Ci gives me breathing problems.

 

 

Looks like a Mazda 2.3L DISI Turbo Dyno Plot.... same thing.... small turbo, big torque down low.... dead by 6K....

 

Very true. Nowadays manufacturers seem more obsessed with dispelling turbo stereotypes (terrible lag, unreliable, etc) than actually making good power. With an inline 6 you're inherently going to have good torque, off the boost, down low. It's not a small engine. I think twin turbocharging the engine was overkill but neccesary for the target demographic. I'd love to have seen one big snail on there. I'm anxious to see what the E92 can withstand when people start going that route.

 

Looks like BMW did a great job with this engine. With just an ECU piggyback and exhaust this beast is putting down 350+ HP and 350+ torque at the wheels!!! For once, the price on a BMW seems worthwhile... Is this the new Supra? only time will tell.

 

Yep, I actually kinda like turbo lag. Why have a turbo car if you don't get that big rush of power that makes turbo cars so fun. If I wanted something that felt like a NA car with more displacement I would have bought and NA car with more displacement...

 

I think it's a pretty decent alternative setup though. For one thing, wouldn't a turbo setup like this to simulate a big displacement NA weigh less than its equivalent NA counterpart? I'm not sure. My knowledge in turbocharging is limited.

 

Twin-turbo has small inertia and has very little turbo lag but has difficulties breathing when the RPM goes up.

On the other hand, single large turbos have lag at low RPM but performs fine at high RPM.


Variable Geometry turbos (like the Porsche 911 Turbo, made by BorgWarren) configs itself as a small turbo at low RPM and reconfigs itself as a large turbo at high RPM, so to speak. Interesting, nice adaptive design.

More interesting to note: that turbo in the 335i is made by Mitsubishi ---- huh ... an Evo in a BMW's clothing?

http://www.caranddriver.com/roadtest...ecs-page4.html