3.27 rear ratio vs. 3.73

FastBird91

Registered User
Assuming my automatic 91' SC modified does 145 mph top end with the rear ratio at 3.27, what do you guys think ill be at with a 3.73 rear ratio top end? I want faster 1/4 miles but i definately dont want to lose top end.
 
You'll definitely lose top end. It makes the car feel like 500 lb's lighter though. You should jump around 1/2 sec in the 1/4 if you use the 100/10th moto. ... You'll also be revving about 5-600 rpm's higher than normal in the higher speeds. I was always use to doing anywhere from 80-100mph on the way to work. With the 3.73's it's way to many rpm's to be cruising nicely at those speeds now. ... Great mod though for that low end punch.

I'll admit I'm glad I tried them. But I think of all the mods I have done, that is one that I think I am going to undo. I like to cruise down the turnpike with the flow (that's about 80-90 around here) without having the car at like 3000+ rpm's just going to work.

Now, I'm not saying it's not worth it. There are plenty of people that are going to chime in & say it's the best ratio for an auto. So, if you plan on doing it yourself & your 3.27's work well, get another pumpkin & put the 3.73's in there incase you don't like it, then you can swap back & sell the 3.73 as a pumpkin for more that it cost you to build. ... Wish I took that route. Now I'm waiting to stumble across a stock pumpkin so I can swap in and out as race season permits.

Good Luck,
Anthony
 
Thanks Anthony, as much i like to slingshot when the light turns green ill probably leave the pumpkin stock. I probably like that 90 mph even more especially when i punch it at those speeds. I know it wont be the same and i dont want to cruise at over 3000 rpms when im doing 90 mph, i like it right were it is at 2600 rpms. And that boost if i floor it when im on the freeway.......i definately dont want to mess with that! Thanks Again!
 
i just ordered a 3.73 diff/housing, alot of ppl will tell you its the best bang for the buck mod, I can see if your commute was in-town and no freeway driving that the gears wouldn't matter but cruisin high RPM's for extended periods on freeways wouldn't work (for me atleast) i would have to put in a tranny cooler and an engine oil cooler, the reason i went with a whole housing was so i could swap back to my original 3.27 diff for road trips.
 
FastBird91 said:
Assuming my automatic 91' SC modified does 145 mph top end with the rear ratio at 3.27, what do you guys think ill be at with a 3.73 rear ratio top end? I want faster 1/4 miles but i definately dont want to lose top end.

I don't think your going to loose any top end speed, you'll just need to rev about 500 rpm higher to do the same speed in a given gear. Assuming you have adequate traction you will also be quicker on the track and your MPH won't suffer.

Even with a slippy non locking converter, smaller than stock dia. tires and 3.73 gears, I can easily reach 120 in 3rd. So you should have no problem going 145 in overdrive. The 3.73s will make it easier to accelerate at any speed and IMO are one of the best mods you can do on an AOD equipped SC.

David

PS: Anthony doesn't need the gearing with the 2.3 whipple, he needs more RPMs in 3rd.
 
PS: Anthony doesn't need the gearing with the 2.3 whipple, he needs more RPMs in 3rd.

Yeah. I'm going to need those right along with new tires too. .. I haven't really had the chance to drive the whipple around though. Damn trans having too much trouble. Most of what I said was with my M90. I have about 8k miles on the M90 & 3.73's & only about 50 miles on the Whipple & 3.73's. Believe me, I'd take 4.11's & the Whipple if it meant I could drive it without the trans overheating! Even if I could never go over 40mph. I would sure hit 130mph real quick!

When I answered the original post, I was gearing (ha) more towards traveling around more than the track times. I liked what 92silverlx said about having the two pumpkins. Because it sure is nice to have the 3.73's for the track. But you definitely lose on drivability because of that 500 rpm difference. And not to question you Dave, but in 4th gear with the lose of 500rpm's wouldn't that potentially mean that he is going to lose TMPH? I've always contended that if he redline's 500rpm sooner than usual, those rpm's would pretty valuable in OD.

Although it's a great track mod. It's not real fun for fast driving. Fast driving - going down the turnpike at 100mph for 40 miles.

And if he's hitting 145 in the 1/4, then I want two of what he has! :D

Good Luck FastBird91, it's a touch choice.
Anthony
 
And not to question you Dave, but in 4th gear with the lose of 500rpm's wouldn't that potentially mean that he is going to lose TMPH? I've always contended that if he redline's 500rpm sooner than usual, those rpm's would pretty valuable in OD.

No...because top speed will be determined by a speed limiter and not the rev limiter and both of those can be disabled with a chip. I haven't seen an SC yet that has hit redline in top gear. We just don't have enough HP to overcome the wind resistance at those speeds so it's not an issue.

However, if your thing is more about cruising at 100 mph on the open road than 0-60 and 1/4 mile performance, you should probably keep the stock gears.


Examples: Assume stock tire diameter of 26.63", locking converter and .67:1.00 overdrive ratio.

75 mph w/3.27=2075 rpms
75 mph w/3.73=2367 rpms

143 mph w/3.27=3956 rpms
143 mph w/3.73=4513 rpms

Max speed excluding wind resistance @ 6200 rpms

6200 rpms w/3.27=224 mph
6200 rpms w/3.73=197 mph


David
 
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If thats not good info i dont know what is.....i think i probably wont lose any top end speed, only be about 500 rpms higher on the freeway cruising speed.
 
With those calculations, it IS safe to say that he will lose MPH in the upper RPM's. Unless he is chipless and limited.

And weird as it is, when I am at 80 I am just over the 3000 line. That's when I'm in the gas to maintain it. But when I let off of the throttle (still at 80) the RPM's drop about 500 rpm's. Perhaps it's the non-locking. I don't know. .. But either way to maintain 80 I'm just over 3000 with the 3.73's.

Not many people rev out to 6200. Unless their crazy! Especially with a stock bottom end ;)

Anthony
 
hm...

i think u guys might be wrong, i have 3.73's in my 5spd, and when i mentioned it to bill at SCP like 2 yrs ago he even said now my top speed would be about 110 (guessing), u definetly loose top speed, my custom speedo hasnt worked yet since i installed it so i'm not sure how fast i go, i tried to calabrate it the way it says, but i'm sure i couldnt go the speeds i could of before with my stock gears on the highway. i git about 135or 140 before with my SC and i backed off because i got afraid liek 3 yrs ago. i'd have to have my friend in his 00 lightning tell me how fast i get in 5th,and have him follow me but i dont think its what i used to hit, now i'm in 5th all the time on the highway. maybe i'll see sometime soon. and get back on it. Alex L
 
The stock ratio on the 5 speed is 2.73. You now have a 3.73?? Thats a hell of a difference thats why your guessing 110mph and even then id say your closer to 120mph top speed.
 
i got a 91 sc automatic and i have a 3.73 and i love it.it wakes your car up big time.car feels a million pounds lighter.if you go with 245/50/16 tires you need a 23 tooth speedo gear.put mine needle right on the money.or 245/45/17 it's right on the money there too.that is what i have now the 17" wheels.also where are you gonna do 140+ at anyways?not in the 1/4 or any highway really...
 
There is plenty of places i can go 140mph+ around here even in so. Calif. So how fast can your SC go top speed Sho?
 
Can't wait to install my new 3.73 posi pumpkin

Greetings,
Ya, I bought one of those Motorsport aluminum-housed 3.73 positraction pumpkins (like what's on the Cobra/Mustang) for my '95 SC automatic. I am going to do the whole IRS rebuild ala Rich Thomson, with new Konis, poly bushings, etc. I am just waiting for delivery of the shocks. I also plan to rebuild the front end with new Moog parts.

I just had the transmission overhauled (at 121k), and put in an Art Carr heavy duty rebuild kit and hardened intermediate shaft. Some time next year (around 175k-I drive it alot) I plan to do the engine and have Steiegmeire do the heads and cam. I talked to him on the phone a month or so ago, and he was telling me about the availability of the new Comp Cams billet cam for our cars, so he will use a new cam instead of regrinding an old one.

Long live the MN-12 SC and Cougars!

Don Klitzke
Prospect, TN
 
SilverCasket said:
With those calculations, it IS safe to say that he will lose MPH in the upper RPM's. Unless he is chipless and limited.

And weird as it is, when I am at 80 I am just over the 3000 line. That's when I'm in the gas to maintain it. But when I let off of the throttle (still at 80) the RPM's drop about 500 rpm's. Perhaps it's the non-locking. I don't know. .. But either way to maintain 80 I'm just over 3000 with the 3.73's.

Not many people rev out to 6200. Unless their crazy! Especially with a stock bottom end ;)

Anthony

Anthony,

You must have a non locking converter if your reving that high. My converter stalls at about 2700 rpm and at 75 mph on level ground I'm showing 2700-2900 on the tach. The warmer the trans fluid the higher it revs. Going up a slight to moderate grade requires 2800-3000 to maintain the same speed. So it sounds like yours is pretty close to mine or may even have a higher stall setting.

Do you have a trans temp gauge ? How big is the cooler ?

6200 isn't that high. I try to shift between 6000 & 6200 at the track and usually hit the 6500 rev limiter just doing a burnout and sometimes on the 1-2 shift. I had to raise the rev limiter to 6800 on the nitrous program to avoid hitting it on the 1-2 shift and getting a nitrous backfire.

David
 
6200 isn't that high. I try to shift between 6000 & 6200 at the track and usually hit the 6500 rev limiter just doing a burnout and sometimes on the 1-2 shift. I had to raise the rev limiter to 6800 on the nitrous program to avoid hitting it on the 1-2 shift and getting a nitrous backfire.

Good Lord that's A Lot! I only wish I had a trans that would allow those shifts. .... Well... I don't have a trans cooler yet. That's a good part of my trans issue. Level Ten didn't believe I would need a cooler. Boy were they wrong. And they said that trans temp gauge's all leak. So I didn't put one in. Apparently they didn't want me to have a gauge, because then I would see how terrible their valve body was flowing. .. Sure wish I knew more about trans's when I was looking to buy. I got a $3k paperweight. It's killing a LOT of performance out of the car on the street. .. Going to the track tonight though for the first time since before I even put on the heads/cam.

In St. Louis, when Phil drove the car (what looked like in reverse), that was the only time it was too the track since I dropped the new Stock bottom end and the heads/cam/ etc. . And the last it was there as well.

At least I know that I will be able to tell a difference with the 3.73's vs. 3.27's. Because shortly after this, I am going to swap back and see if there is a difference. (always like to bring a thread back where it belongs)

Anthony
 
With air tools, jackstands, floorjacks and the experince of doing it a few times, it took me Chris Wise about 4 or 5 hours (including beer breaks) the last time we changed mine.

IMO, it's a major pain in the rear, neck, shoulders ect.. and not the kind of thing your going to do, just when you want to go to the track.

David
 
David Neibert said:
Anthony,

You must have a non locking converter if your reving that high. My converter stalls at about 2700 rpm and at 75 mph on level ground I'm showing 2700-2900 on the tach. The warmer the trans fluid the higher it revs. Going up a slight to moderate grade requires 2800-3000 to maintain the same speed. So it sounds like yours is pretty close to mine or may even have a higher stall setting.

Do you have a trans temp gauge ? How big is the cooler ?

6200 isn't that high. I try to shift between 6000 & 6200 at the track and usually hit the 6500 rev limiter just doing a burnout and sometimes on the 1-2 shift. I had to raise the rev limiter to 6800 on the nitrous program to avoid hitting it on the 1-2 shift and getting a nitrous backfire.

David

That is awesome!!!
 
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