OK, I'm a nerd... But what do you think re: actual RPMs vs expected

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I agree get the new trans in and drive it for a while to see how well you like the existing setup. Focus on how it feels accelerating off the line and how well it accelerates from a rolling 60 to 70 mph, and from a rolling 70 to 90 mph without downshifting. Your looking for how much accelerating power it has in overdrive.

Also see how it feels/how much throttle it takes the car from that rolling 70 to downshift to 3rd and accelerate up to 90 mph or so.

Report back your findings on these three areas. First and top gear (overdrive) are the two areas where you'll have smiles or frowns.
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Yeah - I'm not convinced that data is correct (VERY slow start like a slug).

Why? Because I remember seeing a couple of videos of Doc's car (FMX w/either 2.75s or 3.00s - can't remember which) accelerating unbelievably from zero-100 in about 8-9 seconds, while not even catching 'D' (i.e. only 1st & 2nd). Or was I imagining things?
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I found Doc's video thread. He has a 351C 2V, FMX and 3.50 rear gears. His first gear rato is 8.40:1. It's very similar to CZ's current 9.23:1 first gear ratio with his new wide ratio AOD. Remember, a 9.5 to 10:1 fist gear ratio is optimal IMHO.

http://www.7173mustangs.com/thread-celebrating-the-new-73vertproject

Here's the vid and I still can't get the grin off my face -

Keep in mind that the 3.50 affects speedo calibration, but it still felt the same as the crappy videoing looks... YAHOO!!!

Many of the people around here know what I've been doing, and understand when someone like me lets his hair down (OK, years ago I let my hair down, too, but I've kept it up pretty well for quite a while now :D ). Just saying that the way it sounds on the vid is that way it sounds.

EDIT - OK, looking back at the videos, I apologize for the crappy camera work, but the rest felt good. VERY GOOD. Our cars are as awesome as my friends in the late '70s said they were. Cheers to 7173 Mustangs!



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CZ, how about a video of a 0 to 100 mph full throttle run? Both you guys have a 351C 2V engines so this is turning into some really good research. :)

Also please do the rolling 70 - 80 mph acceleration test to determine the pulling power / lack of power with the 3.25 gear setup.

 
OK... I finally picked up the car from the shop. At first I was very disappointed. The car shifted into higher gears too quickly, and wouldn't kick down into passing gear... it turned out that the set screw on the Lokar TV cable slipped, and left me with about an eighth of an inch of slack at idle. I re-adjusted to remove the slack, and now it shifts as it should.

I am very happy with first gear performance... squealed the wheels for a good 2-3 seconds when floored from a stop.

And at 70 MPH i am right at 2,000 RPM... further suggesting a 3.25 rear end.

I will see what I can do for a 0-100 MPH video... not sure where to do that yet, but will noodle on it.

I will also report back about going from a rolling 70 to 80 "stomp on it" test.

 
why didn't you just look at the speedometer drive gear? If the speedometer is right, it's color/or number of teeth would tell you what you have in the car and it is a single 7/16ths bolt to pull and inspect-a 2 minute job. Instead you keep assuming. Why?

 
Because the car was at the shop. It now has a 23 tooth speedo gear, anticipating the 3.89 rear end going in at some point.


Also... I can add that while traveling at 70 MPH, confirmed by GPS, my speedo was reading 55, and the tach was at 2,000

 
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what is your tire size?
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225x70 R14.... so 26.4"

I don't know the exat calculations, but I'm getting real close to being convinced I have 3.25 rear end:

a 3.89 rear end wants a 23.75 tooth, while a 3.25 wants a 19.8 tooth speedo gear. 19.8/23.75 = 83%. 83% of 70 MPH = 58 MPH... which is right about what my speedometer was reading when I was traveling 70 MPH

 
I ran the numbers, but without factoring in the number of drive teeth as I don't know whether your driven gear is 7 or 8 teeth.

at 70 in OD which IIRC is a .67 overdrive, a 3.25 rear will be around 1950 rpm and a 3.50 ratio will be right at 2100.

Your car should have a 9" rear end and there are numerous ratios available.

here is a fairly complete chart http://www.oldmanengineering.com/9inch.htm

It does look likely to be a 3.25

 
I ran the numbers, but without factoring in the number of drive teeth as I don't know whether your driven gear is 7 or 8 teeth.

at 70 in OD which IIRC is a .67 overdrive, a 3.25 rear will be around 1950 rpm and a 3.50 ratio will be right at 2100.

Your car should have a 9" rear end and there are numerous ratios available.

here is a fairly complete chart http://www.oldmanengineering.com/9inch.htm

It does look likely to be a 3.25
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Jeff, that's a handy little chart. Did you see the spreadsheet I added to our wikisection? Hmm, maybe I should add pinion/ring tooth count data and speedo gear ratios to the spreadsheet for each trans & rear end combo.

http://www.7173mustangs.com/thread-trans-gear-vs-rear-gear-vs-rpm-vs-mph

 
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Here are some interesting stats (assuming I do have 3.25s...):

FMX w/ 3.25: (this is what I started with)

First Gear = 7.80

Top Gear = 3.25 (2,897 RPM at 70 MPH)

WR AOD w/ 3.25: (this is what I have now)

First Gear = 9.23

Top Gear = 2.28 (2,028 RPM at 70 MPH)

WR AOD w/ 3.89: (this is where I'm heading)

First Gear = 11.05

Top gear = 2.72 (2,427 RPM at 70 MPH)

So... by the time I swap in the 3.89 rear end, I will have taken first gear from 7.80 to 11.05... a 41.7% improvement in gear ratio.

And at 70 MPH, my RPMs should improve by 470 (from 2,897 to 2,427), or 16.2% improvement.

And if I can squeal tires now, going from 9.23 first gear to a 11.05 should really let me light things up.

 
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I run 3.89's and an overdrive. If I were doing it again I'd opt for a 3.70 as the deep 1st gear makes traction a mythical creature.
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Like a dump truck winding out instantly? What's your first gear ratio? I don't remember which trans you have. I bet it's close to 12:1 or higher.


One other piece of the equation is drone. On my car it happens between 2100 and 2400 rpm. If you do a lot of highway driving you may want to add that as a factor to keep the highway rpm out of the drone area for your car. I opt for above that rpm to keep the engine in the power area. Jeff mentioned the rpms being too low on his car in high gear at 70mph as being a problem area in another post on another thread.

 
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My car ran just below 1900 rpm at 70 when I had the 3.50 gear in it. ( I was also running 28" tall back tires at the time) This led to a lack of power at cruising speed and a need to downshift to get onto my powerband. I'm running a converted T-45 and it has a deep 1st gear of 3.37, so with the 3.89 gear and 26 inch tires, (if I had a running engine) traction truly was a mythical beast. I don't expect that adding another 150 horsepower is going to do anything to cure that.

So Don the calculation is a bit over 13- a lot of fun, but not much use.

As to drone . . . a good exhaust shop can fix that. Or run open headers. ;)

 
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Ok, I finally made an appointment to have my rear end gears swapped out.

I recently installed a wide ration AOD transmission, and have cometo the conclusion that I currently have a 3.25 rear end, which the PO must have installed since teh Marti report says it left teh assmebly line with a 2.75.

I can easily state, that with an OD gear in the transmission, a 3.25 is not enough gear.... so I'm happy I chose a 3.89 trac-lock to go into the 9" rear end.

The transmission I have is using a custom made torqu converter that locks up in 3rd and 4th gear. So cruising along at 40MPH in 3rd gear, or 60MPH in 4th, and I'm south of 2,000 RPMs, when I'd rather be north of 2,000RPMs.

This gear swap should get me tuned right in. :)

 
I think the 3.89s will put an s in your miles.

Question though. How does your first gear currently feel with the 3.25 gears?

 
Right now, the first gear feels a little steeper than my VW Tuareg which has an 8-speed automatic. I can also feel a nice surge when it shifts into 2nd with a nice swift shift.

 
Sounds like you got 3:25's too me...that is the same my car runs at 70 mph..almost 3,000 rpms...and i have original 14 inch rims with original c6 trans.

And im sure your speedo is right if you checked it with your gps..those things are close ..My freind has a tom tom gps in a new mustang and it was on the money all the way...I was amazing how close it was.

 
I dropped of the car at 8:00 this morning at IPS Automotive in Columbus, OH. I have heard good things about them. They came highly recommended by a couple people I know.

They quoted me 4 hours to rebuild the rear end (putting in a trac lock carrier and the 3.89 gears) and to install an H-pipe in the exhaust.

I'm really looking forward to getting it back later today (fingers crossed) so I can feel the difference.

 

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