Why does TQ drop like a stone after ~ 5,000 RPM?

[iminhell1]

I've watched about every video on Youtube, looked at everything I can find from performance companies and the like. It's always the same thing, after about 5,000 RPM TQ just falls fast.
It's not turbo related, as I've seen mentioned in posts from years back. There are dyno's with significantly larger turbo's flowing much more air up top that do the same thing. Infact it's not turbo related at all because both the N54 and N55 are nearly identical.
It's not fuel. People running pump gas, race fuel and alcohol experience the same things.

I don't know BMW engines well at all, so forgive some of my ignorance. But looking over different dyno's from various BMW I6's, they all look very similar with TQ dropping fast in the upper RPM range.



So my thought/question is this:
If it's a choke problem that is preventing higher RPM flow and that can be fixed through headwork which would cost similar to a turbo upgrade,
is a turbo/s upgrade worth the money?


I do understand that RPM isn't everything. But if you've never had the power there you don't understand how the math really compounds the gains.

[Pissoff]

The n54 and n55 are both turbo engines.

[edisapimp]

It's turbo related.

The stock head has flowed 30psi+ on upgraded turbos.

You're watching the wrong videos.

[iminhell1]

Neither of you understand what I'm saying, with respect.

The ability to flow pressure does not equate volume. Actually pressure is resistance to flow (volume). So 30 PSI would indicate a restriction. Or in other words, more pressure is not more air.

[edisapimp]

With respect, you don't understand how turbocharged combustion engines work.

I understand what you're saying, and I'm trying to tell you that you're wrong - that it isn't the stock head that isn't flowing above 5500rpm, it's the inefficiency of the stock turbos to supply appropriate boost to the stock heads above 5500rpm.

I read what you wrote; it wasn't Einsteinian - your argument is that flowing your head (read: getting the head ported and polished) would be similar to a turbo upgrade in terms of performance. The answer is no. Not only would it cost astronomically more than a simple turbo upgrade, but you would STILL be limited by the stock turbos insofar as the CFM they can flow.

Stop trying to out-physics me. I know how Boyle's law works. What I was trying to explain to you is that GIVEN TURBOCHARGERS CAPABLE OF PRODUCING 30PSI OF BOOST (read: additional manifold absolute pressure greater than the atmosphere), the stock head on the n54 is not a limiting factor at all.

[BEAR-AvHistory]

The nod goes to edisapimp, the stock turbos are tapped out at the top end. With big turbos N54's have run 700WHP+ on pure stock heads.

[Ginger_Extract]

Look up Vargas Turbos, or VTT dynos. Stock housing turbos pushing 600+whp, and are very happy making power up to, or past 7000rpm. The stock turbos are just small, and can't be spun that fast, and still be efficient.

[PA135i]

My head is spinning faster than the stock turbos reading this thread.

[Jim621]

I'm not taking sides here, but you could both be right.

For arguments sake, assume the valvetronic system is not working correctly and only opening the valves up half way at 5000rpm - you could be pressurising the inlet air up to the turbo side of the intake valve....and that pressure not making its way into the cylinder.
This would exhibit symptoms of full boost pressure, but the engine would be down on power since its not getting the full volume of air it needs.
Steve

[iminhell1]

Source: http://www.e90post.com/forums/showthread.php?t=1082739

I'm seeing TQ drop about 150 lb/ft from peak (~4,800 RPM) to 7,000 RPM

I would call that rather significant.
But I'm not sure on what the map looks like for the 15T wheels ... I've never found a reliable map for a machined/hybrid turbo; often the housing is the restriction with oversized wheels.

Also, it seems the boost control method plays a part. Anyone happen to have an updated dyno of this setup that has a better boost control map? Curious how much that plays in.
Or a map where boost doesn't taper, regardless of pressure.


There are just too many different websites to search ...

http://www.e90post.com/forums/showthread.php?t=1110928

[BrokenVert]

Originally Posted by Jim621

I'm not taking sides here, but you could both be right.

For arguments sake, assume the valvetronic system is not working correctly and only opening the valves up half way at 5000rpm - you could be pressurising the inlet air up to the turbo side of the intake valve....and that pressure not making its way into the cylinder.
This would exhibit symptoms of full boost pressure, but the engine would be down on power since its not getting the full volume of air it needs.
Steve

N54 doesnt have valvetronic.


The limiting factor is the small turbos. Theyre designed for quick spooling and low RPM operation. They simply are not large enough to supply the engine above 5500 - 6K

[rrinker]

We aren't driving Hondas, our cars make lots of grunt down low. They don't have to be revved past 5K just to get power. It's a different world. We have relatively large 3 liter I6 motors that make a decent amount of power by themselves, plus a turbo (or turbos) for even more low end grunt. Completely different from a 1.8L 4 cylinder that must be thrashed to within an inch of its life to make power.

[ShocknAwe]

You can optimize the stock snails by improving flow (changing cold and hot side piping). Reduces rpms and heat necessary for a given output value. But then you turn up the boost, and run into the same problem.