Seat of the pants experience by changing ratios?

DocDirk · #1 (**permalink**) 10-18-2009, 12:36 PM

I'm currently running a built C6 with a 2.75 open rear differential. This winter I plan to switch to an AOD to enjoy more leisurely cruising rpms. In choosing the ratio I want for the new positraction rear it seems I could easily go 3.55 or 3.73. Since it'll still be an automatic (ankles prohibit a manual) I'm wondering what kind of actual driving experience changes I should expect. Will it just run through the gears up to third in a shorter time? Is there wisdom in adjusting the tranny shift point upwards to a higher rpm point in order to feel more of a quick launch experience? My car is a West Coast so it's one of the heavier cobras
but with 475 RWHP I would think (hope) there's a way to get it to feel a lot zippier.
It has a Ford 9" btw.

Thanks for any answers and advice!

Dirk

bobcowan · #2 (**permalink**) 10-18-2009, 05:17 PM

You need to look at where you're going to do most of your crusing. Is it back roads at 55-60mph? Or is it highway at 75-80mph? Or is it city streets at 35-45mph? Select your rear gear based on that, and you'll have a pretty good ride.

rustyBob · #3 (**permalink**) 10-18-2009, 05:41 PM

i say use the 3.73 with the AOD, plenty of zip around town and with the OD it will cruise fine down HWY 99 or I-5.

Mohuska · #4 (**permalink**) 10-18-2009, 05:50 PM

Doc,

There is a very good website that will help you with gearing. It has several utility calculators that you can feed in tire size and rear end and it will give you such things as rpms @ a certain speed. This will help you compare to choose the best ratio for your car and the type of driving you do.

The website is www.smokemup.com

Try it out....

DocDirk · #5 (**permalink**) 10-18-2009, 07:56 PM

Thanks for your feedback. I understand what you're saying about comfortable cruising speed, I'll think about it, because we do a lot of all kinds of roads, but prefer backroads at 45-50. I was wondering if I'll feel like it's a peppier car with the new gearing or not...this is a first for me, and I'd like a bit more of a launch potential, you know, kind of a whoa! experience in town as well as a right cruising speed rpm. Yeah, it's an individual thing, I can believe that and maybe there isn't any more than a generalized answer

rustyBob you sound like you've lived in these parts - I love the small town experience of foothill Hwy 49 travel, but also we like going to Monterey or North to Shasta, and those are weary trips with high rpm highway cruising. I've heard of "quickchange" rear gears, would that be an easy final fix for the never-good-enough in me or is "quick" merely in comparison with regular gear change?

My best regards,

Dirk

xlr8or · #6 (**permalink**) 10-18-2009, 09:28 PM

With that 533" motor you will be smoking a lot of tires with a 3.73 rear. I'd go with 3.55 or even a 3.40

Coming from a 2.75 gear you will definitely notice a difference.

rustyBob · #7 (**permalink**) 10-18-2009, 09:44 PM

Doc.... i am a former Sacto resident and have inlaws in Atwater and Winton areas so i know my way around the central valley pretty well.I have attended the Knotts show a few times and have been to the bash by the bay a couple of times also and if things dont perk up i may be back in Sacto the first of the year.........a quick change would be nice to have, but would it really be enough?

Gerry K · #8 (**permalink**) 10-19-2009, 05:27 AM

I agree with "xlr8or". I have a 533 cu/in BB Ford in a West Coast Cobra with
an AOD transmission. It has a 3.55 Posi rear end. I can smoke the tires any
time I want. The RPM @ 70 MPH is about 1900 in overdrive with a 3500 RPM
stall converter.

Gerry

Cobrabill · #9 (**permalink**) 10-19-2009, 08:56 AM

If your cruising speed is BELOW you timing curve(total advance)you are losing MPG's AND driviability.
You need to look at the WHOLE equation.

kitcarbp · #10 (**permalink**) 10-19-2009, 09:17 AM

Dirk, If you are really set on removing the C6 and putting in the AOD, then my vote is for a set-up similar to Gerry K's in his ride. The beauty of the AOD is that low crusing rpm. Before Gerry K posted his set-up I would have recommended a rear ratio between 3.25-3.55. The 3.55 may give you the best seat of the pants tire smoking "WHoa !!" you're looking for. You'll need sticky tires to run with that combo for maximum enjoyment, like a set of drag radials.

DocDirk · #11 (**permalink**) 10-19-2009, 04:50 PM

Wow, THAT's the kind of real world hands-on personal experience feedback I was hoping to find! Gerry K thanks for the ideal experience shared, I'm going with the 3.55 - I understand that my hardish Cooper Cobra 295 / 15's will have to give way to something like a lot stickier, but WHAT FUN! Maybe I'll finally have the buttkicker fun launch as well as the comfy cruiser - I know it's nigh impossible to have the best of both worlds.

Thanks again, this takes a load off my mind. I'll be sure and post my experience with the new setup after it's in.

Regards!

Dirk

FWB · #12 (**permalink**) 10-20-2009, 09:41 AM

dirk, i have the aod in mine. i currently have 3.73's and for cruising it likes to be shifted into OD at the 45-50 mph range unfortunately it is at the speed in which most cruising is done. so i get in the in-between feeling where i'm at a too high revs in 3rd gear and too low feeling in 4th. i am currently switching to a 3.50-3.55 ratio to put my shift point at a higher mph to better overall drivability. i would recomend highly to go with the 3.55 so the secondary road cruising will be in 3rd and highway driving will use 4th.

fred