3.45 Differential install

[elc32955]

Maurice, welcome to the 3.45 club. I HIGHLY recommend this upgrade for all V8 Caprices. Key is finding the 3.45 pumpkin at a good price and having the right facility & tools to work on the changeout. And, thanks for the excellent write-up with the detailed pics!

Eric

[95_9C1]

What's a good price on one of these used, low mile differentials?

Also, what happens if you don't tune the the PCM to accommodate for the 3.45 gears?

[Gerome]

I would also like to know the same thing . I have seen them for as low as 500 and as high as a 1000 . So that is a really good question .

[elc32955]

It depends on where you're sourcing them from and the miles on the clock. I paid $900 or so for mine but it had 1,800 miles on it. Friends with salvage yards are invaluable, just sayin

Basically if you don't tell the PCM you have 3.45's, the computer will still think you have 2.92's and try to run the car accordingly. Your speedo will be off, your transmission shift points will be screwed up probably and a host of other smaller details including your top speed limiter being off, yada yada... I've not tried the experiment.

I keep three PCM programs on-tap for my car using HP tuners. #1 is my bone-stock original which reflects 2.92's. That's basically just a "hail mary" archive. #2 is the same as #1 with the exception of 3.45 gears, this is the one you want loaded if you have to make a dealer visit for any reason (or in my case if my wife yells at me on a road trip about the firmness of the shifts.... sigh...). And, #3 is my street tune. I can swap those in and out as needed, takes less then 3 minutes from the time you connect the laptop to the car if things are booted & ready to go.

[95_9C1]

What about a 3.70 diff from a Pontiac G8 GXP?

If a 3.45 is good, wouldn't a 3.70 be better?

Too bad 3.70's aren't as prevalent as the Camaro gears ...

Currently, there are three 3.45 diffs on Ebay for about $1K each

JHP has new 3.70 diffs for $2.2K

Eric, are you doing your own programming with HPTuners?

[95_9C1]

Found this:

viewtopic.php?f=17&t=83

[Hendo]

I used a gearing calculator http://www.grimmjeeper.com/gears.html" onclick="window.open(this.href);return false; and ran some numbers. Stock with 235/50 18s @ 6000 RPMs with 2.92s max speed(theoretically) in each gear is 41mph in 1st, 71mph in 2nd, 109mph in 3rd, 145mph in 4th, 196mph in 5th and 249mph in 6th. With the 3.45s @ 6000 RPMs 35mph in 1st, 60mph in 2nd, 92mph in 3rd, 123mph in 4th,166mph in 5th and 211mph in 6th. This is my hypothesis on the effects of 3.45s with stock hp or mild bolt ons and/or a performance tune. A dramatic decrease in the 0-60 time, the 1/8th mile and at least a 166mph top speed in 5th. It does look like the 1/4 mile may be slightly hampered with 3.45s considering that with 2.92s 109mph comes at the top of 3rd gear, with 3.45s the transmission would have to shift into 4th after 92mph and 4th tops at 123mph. I worry that the engine would fall out of the power band (after the 3-4 shift the rpms would drop to 4501@ 92mph and would be @ 5332rpms if it trapped 109mph ) and the 1/4 mile time might not be much better than with 2.92s. I would like love to see what the effects would be in real world testing. I believe 100% that the 3.45s would be more lively on the street and at the sort of speeds people drive on the street. Might not be the best choice for 1/4 mile drag racing though. I would also be interested in seeing the effects on 70 mph highway cruising mpg between stock @ 1689rpms with AFM and with 3.45s @ 1995rpms and AFM turned off or deleted. I am guessing that 24mpg (EPA rating) for example could drop to 20mpg. Doesn't sound so bad but, that would mean spending 16.66% more on fuel to travel on long trips. Over 300,000 miles getting 24mpg compared to 20mpg @ $4 a gallon would cost you $10,000 more...something to think about although obviously not a huge priority to muscle car aficionados. I am not trying to be negative at all, I think what you did is AWESOME. I just wanted to share some things that started rattling around in my head when I saw this and considered doing it myself.

[Pursuit]

Hendo, no worries on the comments, sir. This has been a great group of guys and we appreciate any input, from anyone! You are correct about the fuel economy. Just drove from Omaha, NE to Lafayette, IN. 580 miles at speeds of 77-82 mph. Trip computer stated 69.5 average mph at 19.5 mpg. I can also say that at that speed/RPM the AFM doesn't get activated. Too much TPS% and RPM. I would also agree with your 1/4 mile observations. 3.45's do put the car at a weird spot in the power band. If the car was more of drag car, I would consider modding the motor, deeper gears etc. Right now the car is a blast to drive. The car sees 99% of it's life in the real world on the street. I have raced the PPV in the 1/8th mile, 9.0 at 80 mph. With the 3.45's the tires just spin on the 1-2 and 2-3 shifts. But, I probably won't go buy drag radials for what may be another 10-20 passes down the strip. My racing for a time slip is me competing against me, just to validate any changes I made. Next change will be mufflers and then I will probably hold off on anything else for awhile.

Maurice

[elc32955]

Just a few quick notes on your computed figures....

Real world with my 3.45's, 70 MPH equals 2150 RPM. With my tune, the max speed AFM will engage at is set to 72 MPH so that's generally where I run down the Interstate under cruise. With the tune in, the 5-6 shift with WOT takes place at 150 MPH. I'd have to take a look at the stock calibration to see what the 5-6 shift point recalculated to with HP Tuners based on the OEM (non-tuned) settings, shift points are one of the parameters that I believe are automatically recalculated when you change ratios. But yeah, unless you are purpose-built for racing and trailer the car, 95 +% or more of the time you're looking at zero to 90 MPH for max street range of speed daily driving. The 3.45's get you noticeable improvements there without too much impact on fuel economy. I still see 22 MPG average with mixed driving.

It's so very important to run a tune in these computers after you start playing with the ratios to get the most out of your car. GM does traditionally tend to be somewhat conservative, even on a PPV calibration. Ask the guys down at Redline Motorsports in Pompano, Howard and his crew down there have had very good luck tweaking more power with the PC when he does special projects for GM or custom builds for his company's stuff. But, most times you couple tunes with hardware mods anyway so you play with A, you change B.

Oh, I didn't see the earlier question but a little bit of the tweaking is mine, most of it is Chris Henry's tune which is his proprietary sold product. He's the guy we need for this discussion

[greasychevy]

Pulled the trigger on a 3.27 camaro dif yesterday. $620 to my door. 3k miles. I would have gone 3.45 but i think that would be over kill for my car, actually the 3.27 will prolly be borderline. Going to have to do some suspension mods eventually anyway though! If im feeling saucy i think i can get it it next weekend and have one chance at a track day before it closes. with the weather and the diff and my new lighter wheels..... I'm hoping to see 10.80s-10.90s.