[TreeSkier]

I have a base 35i, no DHP or SAP and am not finding the engine breaking very powerful at higher speeds for use controlling speed when driving down steep mountain rodes, although it works very well at low speed (< 15 mph). I think that putting the transmission in DS mode gives the best engine braking but that manually downshifting to a lower gear does not provide much engine breaking, the engine revs pretty freely and RPMs run high. This requires much more breaking than I would like driving down mountain rodes. Any one else have any suggestions or different experience.

[sparoz]

I noticed this today too, but as it's an 8 speed box, may be we just have to down shift a bit more to get the required engine braking.

[vrrooom]

I have a nice 1 mile 7% grade hill down to my lake house. I set the cruise conrtol to 55, and the system downshifts and uses engine braking to maintain the set speed. In fact as I near the bottom of the hill, I use the steering wheel togle to reduce the speed to 40 for the hair pin at the bottom. Not everyones cup of tea I am sure, but very interesting use of systems in my 3.5.

[Lotus7]

Originally Posted by vrrooom

I have a nice 1 mile 7% grade hill down to my lake house. I set the cruise conrtol to 55, and the system downshifts and uses engine braking to maintain the set speed. In fact as I near the bottom of the hill, I use the steering wheel togle to reduce the speed to 40 for the hair pin at the bottom. Not everyones cup of tea I am sure, but very interesting use of systems in my 3.5.

The X3 (F25) Dynamic Cruise Control is able to apply the vehicle brakes if the vehicle speed is more than 5 mph above the cruse setting. For small adjustments of a few mph or less, speed is controlled by electronic throttle modulation. For larger differences (5mph), the brakes are applied.

You can easily confirm the automatic brake operation by cruising at a fixed speed, say 50 mph. If you press the cruise control "on/off" button, the cruise control will disengage and the car will slow gradually using only engine braking. However, if you keep the cruise control engaged and quickly drop the speed setting from 50 mph to 40 mph using the speed-set toggle, the car will slow down quickly. The deceleration will be via electronic application of the rear wheel brakes.

The braking is actually done via the "high-speed" electric servo brakes on the rear axle that are part of the Stability Control System.

There are actually three ways to apply the rear brakes:

(1) Via the conventional dual-circuit, foot operated hydraulic system.

(2) Via the console E-Brake pull toggle switch which operates a remote motor and cable "low speed" on-off mechanical brake.

(3) Via the "high speed servo" variable brake actuators that are controlled electronically by the Stability Control system and the Dynamic Cruise Control.

All three systems actually apply braking torque through the rear brake pads, but use different means to "push" the pads.

The "HDC" Hill Descent Control also uses the rear wheel servo actuators to maintain a constant speed on steep downhill gradients at speeds of 22 to 3 mph (speeds below the range of the cruise control). The HDC software monitors the amount of use and will disengage the HDC if it predicts rear brake overheating (the HDC display icon will change from green to orange).

For long descents on VERY steep grades, it's actually better to downshift AND to use the brake pedal to apply braking force to all (4) brakes to dissipate the energy absorption (brake heating) over all (4) brakes, rather than to let the automatic system use only the rear brakes and not the fronts.

[Beyu]

Since the 35i is my first car with a turbo is it bad for the turbocharger to engine brake? I notice that when I downshift around the 2k-ish range and it revs up to around 3k rpm the car actually speeds up a bit because the turbo spools up so I in turn have to brake harder or downshift again and rev the engine higher than 3k rpm for a larger engine braking effect.

All of my previous cars had NA engines and I've never experienced this issue so I'm not sure if the problem arises from having a turbocharger or if it's from the X3/8-speed.

[paul386]

I would think that in a modern sophisticated powertrain downshifting in an attempt to secure more engine braking might not be effective... especially on a car which has such extensive control over the valves...

I had a previous gen Jetta rental a few weeks back and it had a sport mode with manual gear selection. There was a warning that no engine braking was achieved by downshifting into a low gear and maintaining high rpm's.

[Lotus7]

As paul386 correctly states, the N55 VALVETRONIC system essentially closes the intake and exhaust valves during closed throttle operation (during engine braking) that results in lowered pumping losses increasing efficiency, but gives nowhere near the engine braking of an engine with fully opening intake valves pumping against an already evacuated intake manifold. The X3 (F25) does provide some additional "engine braking" via the BER- Brake energy recovery system, at closed throttle, the super-high output alternator switches to a high charge condition and adds an additional load of up to 3.3 HP to the engine through the front accessory drive belt. 3.3 HP is not a tremendous amount of power, but does aid in slowing the vehicle down a little.

Turbo wear: Although N55 vehicles without automated dual clutch transmissions do not use a throttle "blipping" program, they may open the throttle slightly on downshifts as well as slightly close the throttle during upshifts to smooth out the shifts. During closed throttle, engine braking situations, the intake pressure will never begin to go positive, so even if the turbo spins up a little, it is far, far from being "worked " enough to create any significant additional wear. Essentially all the turbo "wear and tear" is during boosted conditions when the turbo really spins up to 150,000 rpm and pressurizes the intercooler and intake manifold with 10psi of hot, compressed intake air.

The only real way to know what the turbo is actually doing, and to get some idea of how hard you're working it, is with proper instrumentation, which for a turbocharged vehicle means a boost gauge (and if you're REALLY serious, an exhaust temperature gauge). I've had boost gauges (either factory equipped, or ones I've installed myself) on every boosted vehicle I've ever driven. I'm still looking for a way to install one on the X3, but it's not easy because of several factors.

[richardew]

If you look at the torque curve for the 35i, it is almost at peak torque by 1500rpm's. When you downshift you still are at almost peak torque and therefore there isn't a lot of braking from the engine as the torque developed maintains the vehicle's speed. With a normally aspirated engine there is a much steeper torque curve. Torque is what causes the vehicle to accelerate/deaccelerate.

[Lotus7]

[QUOTE=richardew;10707936... Torque is what causes the vehicle to accelerate/deaccelerate.[/QUOTE]

The ability to GENERATE rotational torque over a certain rpm band is certainly what causes a vehicle to accelerate. A torque curve is measured engine OUTPUT at full throttle with the engine loaded. It has nothing to do with how much torque is required from the vehicle's stored kinetic energy (forward velocity X mass) to rotate the engine in an "engine braking" situation.

Engine braking is influenced by frictional losses, accessory drive loading and internal pumping losses (which are influenced by valve timing). It bears no relationship with the published or measured torque curve. The torque curve represents engine output (at open throttle), not the ability of the engine to absorb energy (at closed throttle) during coasting in gear (engine braking).

Measured [full-throttle] torque of the N55 peaks at about 1700 rpm (dark blue curve below). I haven't ever seen a published "drag" curve for the N55 which would indicate the amount of available engine braking at each rpm. However, for most engines, a typical drag curve is a pretty linear relationship of drag torque vs. rpm with more drag generated at higher rpms. Drag curves (or "coast-down"curves) are often measured on chassis dynos to get an idea of drive-line losses and engine drag when race tuning a car, and to improve the accuracy of engine power measurments.

[TreeSkier]

Originally Posted by Lotus7

The X3 (F25) Dynamic Cruise Control is able to apply the vehicle brakes if the vehicle speed is more than 5 mph above the cruse setting. For small adjustments of a few mph or less, speed is controlled by electronic throttle modulation. For larger differences (5mph), the brakes are applied.

The deceleration will be via electronic application of the rear wheel brakes.

For long descents on VERY steep grades, it's actually better to downshift AND to use the brake pedal to apply braking force to all (4) brakes to dissipate the energy absorption (brake heating) over all (4) brakes, rather than to let the automatic system use only the rear brakes and not the fronts.

Thanks for the info. I was thinking of using the cruise control for downhill breaking but was wondering if that was the best thing to do. With rear wheel braking only, it probably isn't the best for extended steep downhill sections but might be a good option for less steep sections.

[Lotus7]

Originally Posted by jdavid2

Thanks for the info. I was thinking of using the cruise control for downhill breaking but was wondering if that was the best thing to do. With rear wheel braking only, it probably isn't the best for extended steep downhill sections but might be a good option for less steep sections.

Yep, that's exactly how it's intended to be used.