Q45 w/ 4.08 diff curious

[66mgb]

I'm curious to hear about how the 4.08 diff changes the Q.

I'm sure that driving around town isn't much too different w/ regards to RPM's. I'm also sure that it rev's much easier.

What are the RPM's on the highway (70 - 75mph).

What mod's do you have to do to the transmission for the 4.08 gearing?i.e. speedometer error correction.

Are there problems w/ TCS? (other than having to turn TCS off for smokey burnouts)

[Q45tech]

4.083/3.538= 15.4% increase in rpm for the same speed. The tire/road torque will be increased by the same pecentage.

The cube root of the 15.4% = 2.5% increase in accceleration

If it takes 14 seconds to accelerate to 90 mph stock expect it to take 13.65 seconds with a 4.083 gear.

[Rex]

I can't make a "true" comparison, as one of my Q 's has it (along with headers and a 90-93 geared transmission) and the other is stock. The car with the 4.08 seems SO much more responsive during normal driving. I've noticed no issues with road trips (been from Miami to Cincy once, and from Cincy to South GA, and back once).

I have not replaced the "littel gear" to fix the speedo error, the J30 is supposed to reduce it to 5% from the current 15%. Rob tried the "little Yellow Box" for speedo error correction, but had some issue with getting it to work.

I currently get the TCS/SLIP lights due to a errant sensor. I attempted to replace it, but the drivers side would have required me to drop the rear or exhaust and I just wasn't up for that the day I was under there.

Overall, it brings a feeling of strength/speed to the Q at lower levels, as we all know it's a beast from 70 up. It feels much faster/stronger from ~60 to 100.

In short, if you have the means, I strongly recommend picking one up .

[superuber]

Overall, it brings a feeling of strength/speed to the Q at lower levels, as we all know it's a beast from 70 up. It feels much faster/stronger from ~60 to 100.

Rex is so right on here! When I was doing my diff I had some issueswith seals. I pulled it out to service and replaced the "doggy" stockgears.....WOW is was much less responsive esp. over 60. The nextday threw my 4:08 back in and noticed a big difference! I built the gears myself and have not had any problem so far. I used synthetic gear oil and I run a 100 shot NOS and so far.....so good!I just got a 150 shot I am installing this week. I'll let you know what blowsfirst, the diff, the transmission, or the motor! (just kidding, I hope!)

http://community.webshots.com/user/neheadsuperuber

[FarFetched]

At cruise rpms are about 2500-2600 at ~70Mph. Burn outs are very hard to achieve even with 4.08 rear end. 4.08 is mainly for city driving. It gives a bit more get up and go. There are no reported problems (I didn't have problem) with either TCS or ABS when swapping 4.08. Cheers!

[Q45tech]

The stock 333 rpm per 10 mph [depending on the brand and size of the nominal 215/65/15 tire] [ inflation and temperature] gets upped to 333 x 1.154 or 384 rpm per 10 mph x 7= 2690 rpm at 70 mph.

Unfortunately the same multiplication limits 2nd gear to 80-82 mph at 6900 rpm redline..........nominal shift around 75-76 mph.

Think carefully about the situation when you are on a 2 lane road facing a semi whether you want a shift [or a rev limiter hit] to occur in a pass?

The shift time delay and the diminished acceleration from 76 to 90 mph being in 3rd gear vs 2nd unmodified. The torque drop is 25% in this speed range vs stock diff gear! THAT'S a 2.9% decrease in acceleration ~~ 0.1 PLUS 0.5 -1.0 seconds for the shift and full power return vs STOCK.

Raising the stock redline from 6900 to 7300 allows you to not hit the 2nd gear redline until 85 mph...........improving emergency passing safety.

Everything has it's ying and yang!

[66mgb]

LOVE your scenario! LOL!

I plan to install the NICO ECU (whenever it gets here...). That would allow a higher redline so that all gears can rev further.

It's too bad that the TCU can't be flashed to allow customization of the shift points.

I think I'm going to go with the 4.08. I was concerned with complete loss of any MPG on the highway. Seriously, this isn't an econo car to begin with, so I'm treating it as a car to have fun with and as Wes says, "drive it like it's supposed to be driven".

[maxnix]

Unfortunately the same multiplication limits 2nd gear to 80-82 mph at 6900 rpm redline..........nominal shift around 75-76 mph.

Think carefully about the situation when you are on a 2 lane road facing a semi whether you want a shift [or a rev limiter hit] to occur in a pass?

The shift time delay and the diminished acceleration from 76 to 90 mph being in 3rd gear vs 2nd unmodified. The torque drop is 25% in this speed range vs stock diff gear! THAT'S a 2.9% decrease in acceleration ~~ 0.1 PLUS 0.5 -1.0 seconds for the shift and full power return vs STOCK.

Raising the stock redline from 6900 to 7300 allows you to not hit the 2nd gear redline until 85 mph...........improving emergency passing safety.

Everything has it's ying and yang!

Since I don't drag much on the streets, but do pass alot at highway speeds and cruise at 2500rpm, and I like my 17+mpg mixed mileage, without a 5 speed auto or 6 speed manual, the 4.08 is too much of a penalty for my everyday driver.

I just love being able to blow by everything in the 55-95 range, and still get 20+ at high speed cruise. Launch just isn't that big a deal for me.

[Rex]

I don't do much highway driving during my "normal commuting" and feel the lower (higher numeric) rear gear really adds to the day to day driving experience of a Q45. My fuel mileage is ~15-16, but I don't calculate it each fill up. I have some weird "opinions" on gas - I buy the best I can get, when I need it and drive as much or how I want ... if 10 cents or 20 miles worth of driving is going to influence gas purchases, a Q45 (or any V8) is the wrong car.

I did however track my drive back from Miami which included some serious "Spirited Driving" (ie Miami, FL to Valdosta, GA in 5 hours, including a pit stop) and saw 17.5MPH (see details below).

I'll have to do some "real world" testing when weather, road and traffic conditions allow, as I do not feel the 2nd to 3rd shift is pulled very far down into the MPH range.

Now for some specs ...

1268 miles71 gallons17.5 mpg18 hrs (total including stops & 2 BAD traffic jams)70.5 mph

If you do the math, it seems nearly impossible to avg that kind of speed ... but it's not .