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06-23-2013, 12:58 PM | #69 | |
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My personal understanding of lag, or turbo lag, is the delay between throttle pedal movement and power output change. A high revving NA engine with individual throttle bodies will have a very sharp response to throttle pedal movements. Such a characteristic makes the car very enjoyable and easy to control and balance at the limit. On the other hand, a turbocharged engine will have inherent delay between the throttle pedal application and the power increase. Some operating conditions will make this lag worse. For example going from a fully closed throttle position to a fully opened position, regardless of RPM. When the throttle is closed, very little gases pass through the engine and the turbo spools down. When the throttle is re-opened, there is a time delay for the lower pressure intake charge to enter the engine, go through the combustion cycle, travel trough the exhaust and spool up the turbine; and then for the compressor to push more air in the intake, for the higher intake charge to reach the cylinder and finally for the engine to make more power. With your background, I am sure you are familiar with the various methods used to minimize or counter this delay, like using smaller turbos or exhaust fuel injection to keep the turbo spooling (those lovely pops that rally cars do on rundown) just to name two. The engineers at ///M seem to have to done a pretty good job to minimize this on the S63Tu, but the lag is still there to some extent according to most reviewers. I am very curious about what technology the ///M engineers will use to minimize the lag on the S55 (electric turbo ??). I am sure they will be very creative about it Last edited by CanAutM3; 06-23-2013 at 01:54 PM.. |
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06-23-2013, 01:29 PM | #70 |
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the F10 M5 should be able to beat a E60.
Even with little turbo Lag. The lag on a 100% stock car is not going to be that bad. I think its a "feel" of the engine that people don't like as much. People coming from a N/A engine might feel it more so, than people driving FI engines. |
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06-23-2013, 03:25 PM | #71 | |
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The other FI trait, is it is less dependant on inlet restrictions, which means throttle modulation is de-sensitised. However on modern systems this is programmed out to an extent. |
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06-23-2013, 03:55 PM | #72 | |||
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Good post
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No I am not going to change the mind of those that religiously believe in the 80's turbo lag phenomenon (carburettor application has a lot to pay for), but I have personally seen turbo's on 2.6 litre straight sixes make too much boost on WOT, so much so that it caused the turbo to go into 'surge', a condition where the engine can't flow the volume of air coming from the turbo. I am very curious about what technology the ///M engineers will use to minimize the lag on the S55 (electric turbo ??). I am sure they will be very creative about it [/QUOTE] On a theoretical engine as swamp mentioned, an NA M4 with 3.0l displacement vs. a turbo variant of the same. The FI version doesn't need to be as 'cammy' to produce the power. Therefore the low rev cam can be profiled to suit low engine speeds, increasing like for like un-boosted torque (outside VANOS limits). I'm not sure the 'anti-lag' you describe would be an adopted solution for road use, but even launch control holding WOT does get turbo's spooled. (clearly demonstrated in the Nissan GTR, the LC mode obliterates the non LC launch, purely because of boost at launch. On a RWD, this would be too much. One thing for sure, unless BMW have integrated a 'flat shift' facility, a MT is going to be slower than DCT by a bigger margin. Plus side is the DCT will feel manic going from full boost to full boost straight away. Last edited by NISFAN; 06-23-2013 at 04:52 PM.. |
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06-23-2013, 04:57 PM | #74 |
is nonexistent with N/A
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06-23-2013, 09:47 PM | #75 |
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^^ All FI set up are mod "friendly".
because ESS perchargers are aweful expensive. NA tunes only yield minium gains and awefully expensive on E90 M. vs. FI are easy to put small $ and gain lots of hp. i.e exhaust and tune on evo yields around 50whp gain on a 2liter engine, vs E90 exhaust and tune if you are lucky to get 15~20whp. meaning on a v6 biturbo platform, exhaust and tune with slight increase in boost may yield massive gains. Remeber supra TT (I6 biturbo)?? go look them up see how mod friendly they were. |
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06-23-2013, 09:55 PM | #76 | |||
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You bring interesting points
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Fuelling lag is pretty much a non-issue on the S65 since it relies mostly on RPM and throttle position to establish the required fuel schedule. So as soon as the throttles open, the right amount of fuel is sent to the injectors for that given RPM. BTW, even on older carburetor engines, more sophisticated units had an "acceleration pump" to push an extra burst of fuel when the throttles were abruptly opened. While I agree that NA engines do have some lag, the configuration of the engine has a big impact on this lag. The S65 with high revving (good breathing) capability and individual throttle bodies positioned very close to the intake valves greatly minimize the lag to almost nothing. You might be confusing absolute pressure with total pressure. Manifold pressure is a static pressure measurement. While an intake tract has some inherent pressure loss, the main reason the static pressure drops is because of the velocity of the intake charge. The total pressure in the manifold still remains close to 1 bar. To repeat our previous scenario with the S65, with fully closed throttles, the static pressure in the manifold is at about 1 bar (no flow). When the throttles open, the ECU instantly adjusts the fuel flow and the full (max) intake charge only has a few inches to travel to reach the cylinder to produce power. Clearly much faster than the turbo scenario. So not sure myth is "busted" here . Quote:
(I work with aircraft turbine engines, so I very well know what a surge is: ) Quote:
I am not concerned about lag on initial acceleration from a standing start or from a roll. It is more the precise control of the power delivery that I am referring to. I understand that not everyone tracks their M3, but I do. I have always enjoyed the very precise and predictable nature of the power delivery of my M3s. When cornering at the limit, I can precisely and smoothly adjust the cornering attitude of the car by modulating the throttle. It is this behaviour I hope will be preserved in a FI F80/F82 ///M. Agreed. The DCT will definitely contribute to make this new power plant shine. The quick shifts will keep the engine on boost and if the gear ratios are well chosen, the acceleration curve will be very linear going from gear to gear. Last edited by CanAutM3; 06-23-2013 at 11:02 PM.. |
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06-23-2013, 10:18 PM | #77 | ||||||||
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Neither of us is a drivetrain nor automotive engineer and again our education nor titles don't really matter either. Everyone on the planet including automotive engineers agree on what turbo lag is.
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Of course I agree about the importance of hp over torque. Quote:
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-The new M3/4 will be lighter than the existing model. -The new M3/4 will have more power than the existing model. -The new M3/4 will be faster than the existing model. Well now that the sarcasm is done here is the last point -The new engine in the M3/4 will be substantially less appealing in character and also likely in throttle response (as compared to the S65). Wasting a lot of breath here...
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06-23-2013, 11:26 PM | #78 |
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Hey Swampy, stop reading BS journalists and come for a ride in my F10. You need to experience the lack of lag personally.
The new M4 could be very special indeed. Drop 300lbs and add real torque from 1500 to 8 grand. Honestly, my biggest concerns are steering (which they appear to be working out with F10/12 LCI) and turbos overheating (limp mode be gone!) I will miss the NA noise, but the S65 just isn't torquey enough.
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06-24-2013, 07:34 AM | #80 |
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swamp, I don't recall seeing that comment about the S63Tu vs the S63 before. I have to wonder about how the addition of Valvetronic would effect turbo lag itself. I'm sure turbo lag is present in some form or another in any current turbocharged engine, so I won't argue that point. But the attribution of the increase in lag to the Valvrtronic throttle control seems like it could be debated. It seems to me that this would more likely point toward throttle lag. Perhaps the software in this case was not fully baked yet and needed tweaking. You and I know this type of situation well, what with the early DCT software woes. Speaking of which - there's one more place in the system that lag can rear its ugly head - gear ratio changes. Of course, we'd like to think BMW has that all handled by now. Still, it is curious that the X5 M also uses a different transmission from the M5, which somewhat interferes with the idea of comparing their ability to respond to driver inputs.
I believe that some day we'll be driving turbocharged cars with lag-less character. But I don't necessarily think BMW is going to usher us into that era with the S55. However, they may be prepared to make a significant breakthrough at least. I for one am excited to see what, if anything truly revolutionary, they've come up with. I think that is one thing we all anticipate here, and can all get behind. It could be the single most exciting trait about this engine if even some of the current rumors pan out. |
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06-24-2013, 08:30 AM | #81 | |
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06-24-2013, 10:55 AM | #82 | |
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In practice it hasn't been done yet to my knowledge, at least not in a production car. The problem is, it takes a significant amount of energy to keep turbos spinning at high RPM and boost pressure up. A KERS type system could work, but adds weight and cost. There's no free lunch. In any case a separate electric turbo (so-called tri-turbo) seems like an inefficient solution. Instead, you can add electric assist to the existing turbos, eliminating some hardware and complicated manifold routing. |
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06-24-2013, 02:06 PM | #83 | |
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I mostly agree with all of your other comments. However, I don't think the M3 (with it very large torque multiplication through gearing) needs much more low end torque.
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06-24-2013, 02:44 PM | #85 | |
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But I'm sure (or hope) that ///M works its magic - all the cars are pretty special. I hope the torque just doesn't drop like a cliff at high revs. Last edited by pkimM3r; 06-24-2013 at 03:42 PM.. |
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06-24-2013, 04:55 PM | #86 | |||||
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Post #5 was your best in this thread. Cheers.
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06-24-2013, 06:49 PM | #87 |
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06-28-2013, 02:10 PM | #88 | |
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But it is quick. Quicker. And with more finesse, excitement, and sheer speed that the 335i could ever muster, no matter how good a car she was. And now that the V8 growl sings to me every day, it's beautiful music. More violin-like, than drums 'n bass thuggery of the N54. Not as eager to kick my coccyx into the seatback and spin the rears at 1500rpm, but a scalpel around the bends, and surging crisp upshifts all the way up the speedo; never wheezy above 6000. |
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