Bmw 435i test drive experience
Discussion
Below I’ve described the various types of delay you can experience in a car. The 435i that you test drove would have been suffering from Drive By Wire lag and Automatic Gearbox Lag (the latter two below). The 435i is a turbo, but it’s unlikely that turbo lag is what you’re describing. If you go for a manual 435i, you’ll remove the auto lag, but the DBW lag will still be there. My advice would be to try a manual car before deciding against the 435i.
Throttle Response: This applies to all internal combustion engined cars and describes the laziness with which the engine speed increases when you increase the throttle opening. This is can be caused by a number of things, the main ones being the fuelling method (carb, injection etc) and the rotational inertia of the engine (flywheel mass and diameter, piston mass etc). Put simply, a 1 litre Panda is likely to have a better throttle response than a 5 litre V12 Mercedes S Class.
Turbo lag: This applies to turbocharged engines only. A turbo works by forcing more air into the engine, which in turn means more fuel can be added, and this increased mass of fuel/air mixture increases the intensity of the combustion and the torque and power that this generates. The turbo is spun by an impeller in the engine’s exhaust gas flow. Because the turbo needs a build up of exhaust gas pressure to function properly, there is often a lag before the turbo’s assistance is felt. Modern engines get around this lag by various means, but in certain conditions you can usually feel it. Many old school turbo cars have quite an alarming delay before full power kicks in (930, F40 etc), although you can still use the engine “off boost” until this time, so mid corner control doesn’t normally suffer, but corner exit often does. The 435i you drove is turbocharged, but it’s unlikely that the turbo lag would have been prominent enough for you to feel it regularly and for it to annoy you – with most well engineered modern road cars like the 435i or 420d etc, experiencing the turbo lag is quite rare and requires the driver to do a number of unusual things in the right order.
Drive by Wire Throttle Lag: Traditional engines put a cable between the throttle pedal and the engine (exactly like the brake cable on a bicycle), so the two are connected together and move in direct synchrony. For greater control over the engine though, about 15-20 years ago most modern production cars started to instead use a potentiometer on the throttle pedal which sends a signal to the ECU that then decides to open the throttle (for example – the BMW 5 series got this in 1997 and the BMW 3 series in 2000). Most modern full production (i.e. not SVA) road cars have an associated drive by wire throttle lag. I’ve heard various explanations, but the most plausible seem to be: a) that the ECU is waiting for optimal combustion conditions in the engine before delivering a spark and b) that the delay smooths out a bad driver’s jittery right foot. Note that DBW doesn’t have to give a lag – many racing cars run DBW throttles with no issues, as do many SVA road cars (e.g. the Lotus 2-Eleven that I owned until recently). The reason I’m mentioned twice early on this thread is that I’m quite a vocal critic of DBW lag, so it was inevitable I’d post with a big rant! My reason for disliking DBW lag so much is that unlike the aforementioned throttle response or turbo lag, with DBW lag the throttle pedal appears completely dead for a while, so you’re left coasting. 90% of my enjoyment of driving a car is the cornering, and for many cars, the lag is such that you can’t use the throttle to control the car through most corners – instead you just coast whilst the ECU dithers about what to do. If you do the maths then for the average DBW delay in the average corner you’ll see that the driver frequently has no control over a cornering car with the throttle at all. For example, a half second delay at 40mph equates to 9 metres coasting, which is a significant proportion of the corner, or possibly even all of the corner. I’ve tried to own two cars with a lag, but gave up and sold them. I now drive a diesel with no DBW lag and a cable throttled single seater. The diesel might seem like an odd choice (esp given it’s a turbo, but see above), but because 90% of my enjoyment of driving is cornering, I’m willing to sacrifice the other 10% that includes enjoyment of a decent engine and the performance it gives in order to get satisfaction from the 90%. Many PHers can’t accept this preference of mine and that can cause some long arguments, which is why I’ve become associated with DBW lag on PH and was mentioned early on in this thread.
Automatic Gearbox Lag: Automatic gearboxes use throttle input as one of many triggers to change gear, and associated with this processing is a lag and also there’s the time taken to change gears and re-engage drive. The worst I’ve ever experienced was an early X3 3.0 auto, which had an absolutely huge lag if you suddenly floored the throttle – around 3-4 seconds, during which time the revs increased gradually but the car went nowhere. I had the car on loan for two weeks and it was hilarious – it still amused me even after ten days of driving it. BMWs now use a much better automatic gearbox by ZF.
ETA: As mentioned on page one, BMW do seem particularly guilty with respect to control lag. I guess they just have a higher tolerance for delays in their engineering development and acceptance. Sony are a good analogy - everything Sony I own has a huge delay: 30 seconds to eject a Blu-Ray from my BR player, 30 seconds before you can change channels after turning the TV on. Sony devices have always done it - same with BMW I guess. The annoying thing for me is that my preference for cornering enjoyment in cars results in me only really liking rear drive and manual gearboxes, which narrows my choices somewhat for my daily driver (which must have a tow bar, load space etc).
Edited by RobM77 on Monday 30th November 12:36
I believe it depends partly on what kind of driver you are, whether or not you are much affected by DBW lag.
From what I've heard, DBW copes ok with transient throttle responses - normal throttle modulation when driving along - but is at it's worst when a positive throttle opening from zero is requested. I'm sure Max Torque or Rob could give a more accurate explanation, but I think rapid throttle openings from zero are more likely to see rich, polluting fuel mixtures, and so the ECU overrides these request for a more gradual throttle ramp and sweeter emissions.
If you're a heel n' toer, DBW lag is pretty annoying as every throttle blip is met with an ECU pause, forcing you to wait for the car to catch up. Similarly if you make a positive transfer from brake to throttle, such as when balancing a car in the corner entry phase, you can also encounter a pause as the ECU overrides what you want to do.
However, if you're a clutch-slipper you probably rarely transfer positively to the throttle with the clutch fully engaged, so you rarely encounter DBW at it's worst.
From what I've heard, DBW copes ok with transient throttle responses - normal throttle modulation when driving along - but is at it's worst when a positive throttle opening from zero is requested. I'm sure Max Torque or Rob could give a more accurate explanation, but I think rapid throttle openings from zero are more likely to see rich, polluting fuel mixtures, and so the ECU overrides these request for a more gradual throttle ramp and sweeter emissions.
If you're a heel n' toer, DBW lag is pretty annoying as every throttle blip is met with an ECU pause, forcing you to wait for the car to catch up. Similarly if you make a positive transfer from brake to throttle, such as when balancing a car in the corner entry phase, you can also encounter a pause as the ECU overrides what you want to do.
However, if you're a clutch-slipper you probably rarely transfer positively to the throttle with the clutch fully engaged, so you rarely encounter DBW at it's worst.
I've not noticed any throttle or turbo lag in my 235. But you can confuse the autobox resulting in a good 2 second lag.
I find that if you press the throttle down not quite quickly enough, then the box goes "I'll drop a gear, erm, i'll drop another one... ok how about this gear that puts you 200 revs away from the redline?".
I find that if you press the throttle down not quite quickly enough, then the box goes "I'll drop a gear, erm, i'll drop another one... ok how about this gear that puts you 200 revs away from the redline?".
NDNDNDND said:
I believe it depends partly on what kind of driver you are, whether or not you are much affected by DBW lag.
From what I've heard, DBW copes ok with transient throttle responses - normal throttle modulation when driving along - but is at it's worst when a positive throttle opening from zero is requested. I'm sure Max Torque or Rob could give a more accurate explanation, but I think rapid throttle openings from zero are more likely to see rich, polluting fuel mixtures, and so the ECU overrides these request for a more gradual throttle ramp and sweeter emissions.
If you're a heel n' toer, DBW lag is pretty annoying as every throttle blip is met with an ECU pause, forcing you to wait for the car to catch up. Similarly if you make a positive transfer from brake to throttle, such as when balancing a car in the corner entry phase, you can also encounter a pause as the ECU overrides what you want to do.
However, if you're a clutch-slipper you probably rarely transfer positively to the throttle with the clutch fully engaged, so you rarely encounter DBW at it's worst.
I'm not aware of the reason (although what you say sounds plausible and matches up with what I describe above), but yes, it's 0% to n% that causes a lag. I rev-match with every downchange, but I can just time that differently, so that doesn't really annoy me much (it just makes it a bit harder), but as you describe, it's coming onto a balancing throttle after braking for a corner that really grates. Typically, you're left coasting for 0.5-1 second after turn in, after which you get a sudden jerk as the ECU responds to the amount of throttle you requested. The coast is obviously annoying (because it wrecks any dynamic stability you've created by the way you've come off the brake pedal), but even worse is when the throttle comes in, as with no instant response you've no idea how much throttle you've applied (the brain/control feedback loop for throttle control is typically governed by response, not input) until you suddenly get it delivered all in one go.From what I've heard, DBW copes ok with transient throttle responses - normal throttle modulation when driving along - but is at it's worst when a positive throttle opening from zero is requested. I'm sure Max Torque or Rob could give a more accurate explanation, but I think rapid throttle openings from zero are more likely to see rich, polluting fuel mixtures, and so the ECU overrides these request for a more gradual throttle ramp and sweeter emissions.
If you're a heel n' toer, DBW lag is pretty annoying as every throttle blip is met with an ECU pause, forcing you to wait for the car to catch up. Similarly if you make a positive transfer from brake to throttle, such as when balancing a car in the corner entry phase, you can also encounter a pause as the ECU overrides what you want to do.
However, if you're a clutch-slipper you probably rarely transfer positively to the throttle with the clutch fully engaged, so you rarely encounter DBW at it's worst.
As I said above, most of my enjoyment of a car is cornering, so this sadly makes some otherwise superb driver's cars all but useless for me; for example, the E46 M3, Cayman S and Evora S all do it. No matter how good the engine and straight line performance of those cars is, it'll only satisfy the part of me that enjoys those aspects of a car - the majority of my enjoyment in driving is from controlling the car in a corner, and that majority is left seriously wanting by their laggy throttle response.
Edited by RobM77 on Monday 30th November 14:20
stumpage said:
IceBoy said:
Must be on all BMW's !
I have a 2015 520d Msport auto on loan and although in sport mode it is better, in comfort, there is a distinct 1 second lapse before anything happens?!
Iceboy
Oh yes....put it in Ecopro mode that makes matters even worse.I have a 2015 520d Msport auto on loan and although in sport mode it is better, in comfort, there is a distinct 1 second lapse before anything happens?!
Iceboy
I cant believe anyone can actually drive a BMW in that mode. It pretty much disconnects the throttle pedal from the engine. If you are driving around in EcoPro to save fuel, you would be better of buying a bicycle or just giving up and staying at home.
Thanks to Rob for his long and good-natured post. As ever, all good stuff
Re h&t driving gods, the DBW delay has to be pretty bad before it creates much a problem. The engine in my 997 responds quickly and predictably to little prods for rev matching (although it is much better with the sport button pressed than not
) whereas my wife's 320i largely ignores quick changes to pedal position. A little bit of DBW delay just manifests as slight lethargy, whereas bad DBW delay pretty much rules out good and fast road driving in a manual car.Gassing Station | General Gassing | Top of Page | What's New | My Stuff