Q45 Gearing???

[zinkie13]

I have always wondered why the Q45 is geared so horribly for highway driving. I realize its about average to run 2000 rpms at 60 mph. However, it seems like most other cars are running a lot lower rpms at that speed. I would imagine it would be more efficient running at 1500 rpms, but that's just my guess. Even now, with the newest model of the Q45 they tacked on an extra gear to make it a five speed automatic. However, it still runs the same rpms at cruising speeds.

For instance, my XK runs 2000 rpms at about 70 mph. A guy I used to work with on a summer job said his C5 ran a little over 1500 rpms at 75 mph! Now maybe I'm just ignorant, but if Infiniti is adding an extra gear to make the distance between shifts shorter, why not make the last gear more efficient?

[DrewQ45]

Hmmm.... are you talking about a 97Q? My G50 (90) cruises at very low RPMs on the highway.... Only time I ever noticed any high speed gear hunting was when I switched to 93 tcu for 1st gear start... but even that seems much better now since changing trannys. Next time on highway I'll take note of the RPMs and post my numbers.

[bmwkiller]

...I switched to 93 tcu for 1st gear start...

I have a '93, but I'm looking at a '91, which I presume has the 2nd gear start. Whats the advantage of one over the other?Thanks.

[texasoil]

The Q45 gearing is,like all transmissions & axle combinations, a compromise. Acceleration, fuel economy, vibration modes, noise, etc are all considerations, with many conflicting characteristics. The US CAFE rules demand best fuel economy at 55 MPH, while 'real world' argues for 70 MPH. In similare fashion, the EPA 'city cycle' argues for high fuel efficiency in a driving cycle that is far removed from reality.

Most of us realize the first and second generation Q45 body/exhaust/driveline has a characteristic resonate frequency around 2250-2400 RPM's--which is right in the most common highway cruising range. For that reason, my '94 seems to prefer78-79 as an 'ideal' cruising speed--completely smooth and silent. Thermodynamics argues for low RPM, near wide open throttle for maximum engine efficiency at cruise, but that requires a sensitive transmission that downshifts at most any need for more power--slight grades, overpasses,etc--which is hard on transmissions (and sensitive drivers).

One also has to consider all the engine driven 'accessories' in the complex evaluation--things like drive belt harmonic vibrations caused by AC compressors, power steering pumps, etc. Finaliz ing 'the whole package' can be a very difficult and tedious effort for a performance oriented 'luxury' car like the Q45--whose owners are likely to be demanding for the best of all worlds--quiet, smooth high and low speed cruise, extremely responsive to manual throttle input yet very stable under cruise control, enough sensory inputs when 'romped on' to satisfy the desire for the hard to quantify 'feel and sound of power', yet totally predictable and linear response to those inputs. Not an easy task--but the DID do a great job.

[DrewQ45]

I have a '93, but I'm looking at a '91, which I presume has the 2nd gear start. Whats the advantage of one over the other?Thanks.

Well..just picture a manual trans.... imagine the feeling of moving off in 2nd versus 1st gear. Pretty much the same difference... a very noticeable difference too. Nissan had the early Qs start out in 2nd for added smoothness. With the first gear start, it's quicker with a less labored feel off the line. You may get a harsher 1-2 shift but only under hard accelleration. Are you selling the 93? You may want to try swopping them before you do. TCU is located under drivers side kick panel on the left of where your left ankle rests.
Modified by DrewQ45 at 10:08 AM 11/27/2005

[Q45tech]

In 1988 did mpg matter to the wealthiest top 10% - the US market for the Q.When you wrote that check for $38,000 plus tax [like $66K plus today].

It only matters when used [poorer] buyers get involved today.

Check Lexus out the current LS430 [4.3 liter vs our 4.5] still turns 1800 rpms at 60 mph.............not much progress in 15 years of the best designers in the world. BMW745~1800, Audi A8 ~1800.

10% reduction in 15 years wow. Most if that is allowed by much improve torque convertor and transmission efficiency and lower rolling resistance tires and aerodynamics requiring a few less HP to move at 60mph.

The following is interesting but far removed from cars - however!http://www.everytime.cummins.c...r.pdf

[97Q45t]

It is also a compromise between economy and the feel of sportiness in the highway. If everytime you want to pass on the highway, even with moderate speed, the car would have to downshift if the engine RPM is too low. Passing without downshifting is much more satisfying, especially in a car with automatic transmission.

In comparison, Q45's engine speed at 60 MPH is not high at all (in RPM):

Y33: 1972 '02 Q45: 1741'03-up Q45: 1973'02 540iA: 2032'02 A6 4.2: 1761'02 S-Type 4.0: 1903'02 GS430: 1939'02 E430: 1865
Modified by 97Q45t at 1:28 PM 11/27/2005

[Rex]

The last gear covers from ~60 @ 2k rpm to >140 @ ~6k rpms. It may not provide the most fuel efficiency, but as Q45Tech mentioned, that wasn't Nis/Inf's target in design.

My only thing is why a "city/rural" driving mode wasn't incorperated to negate the need to manually place it in 3rd?

[zinkie13]

I understand all of the arguments. It's a compromise between performance and smooth driving and everything else. I'm just saying, if they are going to tack another gear on the end that is essentially for better efficiency why not make it more efficient. That last gear of the car is not used in a performance situation until you are around 120-130 mph!

And personally when it comes to passing (maybe its just me) but I get it done as quickly as possible. I love the feel of it shifting down into 2nd when I slam the petal to the floor. Then hitting 100 before I even get to the car sometimes is a great feeling. Passing a car is one of the most fun things about driving because that speed on any Q is where it performs the best.

[elwesso]

I agree with all the points above...... another thing to consider is when you are in 4th gear at lower speeds, the car would definitely lug under higher load......

Like texasoil said, they did a HELL of a job with a good compromise.

[bmwkiller]

Well..just picture a manual trans.... imagine the feeling of moving off in 2nd versus 1st gear. Pretty much the same difference... ...earlyQs start out in 2nd for added smoothness. Modified by DrewQ45 at 10:08 AM 11/27/2005

I had in mind the manual transmission analogy. I was thinking it would bog, but I guess with 300 hp you can start in any gear you want. I assume the control module is programed to account for load, throttle input, etc, and will start in first when these factors call for it, or do you only get first when you select it with the shifter?Thanks

[elwesso]

Youll get 1st only if you floor it or select 1st with the shifter.