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BMW Z4 Forums (G29) BMW Z4 (G29) General Forum New Z4: Same torque, higher HP...huh?

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      12-18-2018, 08:06 AM   #1
vwcrusher
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New Z4: Same torque, higher HP...huh?

Maybe I am missing something but in reading the new Roundel magazine, the new Z4 looks great, but the M40 version states it has the same torque as my M240i, but 40 more HP?

I do not understand how that can be as both numbers are related to each other.

thanks
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      12-18-2018, 08:21 AM   #2
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It probably starts with the transmission. Transmission are torque limited. With today's electronic engine controls engines can be mapped to limit torque and protect the transmission. That mapping affects lower rpm ranges than where max horsepower is produced, so limited torque and more hp can coexist.
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      12-18-2018, 08:32 AM   #3
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Keep in mind that the Z4 M40i comes stock with the same M-like 100% locking torque-vectoring LSD.

It would have better traction than the one-wheel peel 240i even with the optional mechanical locking 30% diff.

I'm sure that also factored in when BMW decided to unleashed more power from the B58 in the new Z4.

More power is no fun if you just going to have wheel spin or hop and can't get that extra torque down properly to the ground.
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      12-18-2018, 09:43 AM   #4
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Thanks guys; that makes a bit more sense...I was thinking old school. Also, I read somewhere that they understated the M240 HP as they didn't want to encroach on the M2....but who knows if that is accurate.
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      12-18-2018, 11:30 AM   #5
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With the B58 and AFAIK all recent BMW turbo engines the maximum torque is not developed at a specific RPM value, it reaches maximum at approximately 2000RPM and stays as a plateau until about 5000RPM by design. Allowing the engine to breath better, using a modified cam or increasing the rev limit could all cause an increase in the maximum power developed above 5000RPM, but don't have to affect the engine management limited torque plateau. Besides limiting the torque due to transmission constraints, it is also beneficial to engine life to limit the boost pressure, especially for large throttle openings at lower RPMs, effectively creating the torque plateau.
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      12-18-2018, 12:51 PM   #6
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HP varies across markets, due largely to emissions regulations, which are stricter in Europe. A particulate filtering system there restricts maximum output. As others have noted, though, very few people ever reach maximum HP, which is always very peaky, while maximum torque is remarkably flat in modern engines over a very wide range. So OP should be consoled a bit: drivers of the Z4 and even more so of the M4 are highly unlikely to ever get to use that extra HP.
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      12-18-2018, 01:33 PM   #7
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Quote:
Originally Posted by vwcrusher View Post
Maybe I am missing something but in reading the new Roundel magazine, the new Z4 looks great, but the M40 version states it has the same torque as my M240i, but 40 more HP?

I do not understand how that can be as both numbers are related to each other.

thanks
The numbers are related, but the other variable is RPM.

If different engines produce the same torque at the same RPM, they make the same HP at that RPM. If not, someone is lying.
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      12-19-2018, 11:44 AM   #8
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How hard is it to understand? Max torque may stay the same, but if that same max torque is still present at higher RPMs, then the same torque rating engine makes more max power, period.

Attached are two sample engine dyno graphs I made up to illustrate the point. Assuming BOTH engine makes the same max torque of 300 ft-lbs, but one makes it at the 3,000 RPM mark, the other shifts the max torque of 300 to the 5,000 RPM mark, theoretically the engine that make that max torque later will make significantly MORE HP towards the top end.



300 ft-lbs max torque, 300 hp engine.



300 ft-bls max torque, 370 hp engine.

It's not hard to understand, on a force induction engine the torque curve can easily be manipulated by using a number of techniques, such as multiple turbos of different sizes, determining when the second turbo spools up, cam lift and duration...etc. You sacrifice a small amount of low-end torque, for a moderate gain at the high end of the rev range, and you see significant power increase.

These type of torque curve manipulation has been in engine builder's bag of tricks for decades. Combine it with an 8 or 10 speed transmission you can make the narrower upper torque band work magics without sacrificing actual daily usability in losing the low end torque.
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      12-19-2018, 09:52 PM   #9
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H.P. is equal to the rate of work being done. Torque does no work at all unless combined with RPM. Turbo motors can create the torque component at a steady rate and at lower revs. The H.P curve will go up with the revs until the torque falls off to the point where the H.P. peaks. The Turbo thus creates a nice wide power band without extreme RPM. Some believe that naturally aspirated engines are more enjoyable and engaging to drive, probably because the revs need to be kept high and the shifter "rowed" actively to get the same result.
Bottom line to the OP is that if both engines have the same torque curve, the only way for one to make more H.P. is for it to be able to rev higher. But I suspect the actual torque curves are not the same either.
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