CR toploader w/ 3.08 or 3.31 rear diff?

[NKOTB]

Hi all,

I've been pondering the pro's & cons of a Souther Automotive's Super FE (Stroker 390 w. 500+ hp & torque) with a close ratio toploader, & but I am still waffling between a 3.08 and a 3.31 rear diff gear... Any sage advice? (Rear tires will be 295/50/15's w/ some highway driving)



Thanks,

David

[jdean]

Go with the 3.31s unless you plan on putting a lot of miles on it. 3.31s are not too bad on the highway either.

[FUNFER2]

331's or higher, if you'll just be on the street. I live 10 miles from the city so, my 331's are great. It runs 2,000 rpm @ 60mph with a Tremec in 5th gear.

[427SnakeSC]

I read on another thread here, that someone who was running 3.31 final drives loved them. His only complaint was it was a little difficult trying to idle through parking lots and heavy traffic. I am going to run the CR toploader in the new car when it arrives, but I have decided to go with the 3.54 final drive. I don't feel I need a top end greater than 160 mph!

[NKOTB]

Thanks ya'll. Good point on the parking lot creeps... I wonder if the 3.08 would make me constantly slip/ride the clutch in slow moving?...

Funfer2, my question for you is how would you feel about driving on the highway for extended periods in 4th, which is what would be equivalent to my toploader's 1:1... I'm thinking you'd be turning over 2700 rpm's for a while... is this too high in the rpm range for long term durability?

On the other hand, would the 3.08 rear sap the TL's 1st & 2nd gear acceration too much, even considering I'll have a little over 500 ft/lbs of torque to work with... Hmmm????

Please keep your thoughts a comin'.
Thanks,
David

[427SnakeSC]

With the CR toploader 4 spd and a final drive of 3.54. 1st gear, still gets you 60 mph. IMHO, I am looking for those catapult hole shots. With 500+ lbs. of torque, you will have a problem getting traction out of the hole. The 3.31 or 3.08 will help with the traction problems. With a 3.08, you will still get a pretty good acceleration, but it will be aggravating in parking lots and traffic, not to mention the wear and tear on your clutch. Just my opinion.
http://www.secondstrike.com/Technical/GearCalc.asp http://www.368s.com/tools_rpm.php
There are a couple of websites to help you out. Hope that makes your choices a little less complicated. Here is some original specs of the 427 Cobra, 3.54 final drive and a CR toploader. These look pretty good to me!

[FUNFER2]

Yes, I believe 2,700 rpm is too much plus, it would be real loud. I don't think your rpm would be that high though. With my Pro-Street Camaro, 468 cu., four speed and 456 gears, it turned 2,600 on the highway. You might check out the web-sites above. You could always get a different gear for fourth, to lower the rpm's. Think I would still go 331's. This is why I went with the Tremec. You should think about it because you can actually go with a steeper gear than 331 for the street and still have a over drive for the highway.

[427SnakeSC]

Here is another performance result test. The Corvette is equipped with a 3.36 final drive and quite a bit more weight. It still comes up with some pretty impressive performance stats!

I want to change my 4th gear from 1.1 to .82, but I have been told that isn't possible with a toploader. The .82 will drop the cruising rpm to a comfortable zone and with that 3.54 final drive, still have lots of zap in 4th gear. SPF runs a 3.46 final drive as standard in their cars. I have heard good things about this final drive ratio. The owner of the local SPF dealership swears by it!

[NKOTB]

great info!

I think the 3.31 is looking right for me....

[Anthony]

1st gear in the vette is 2.2:1, with the CL muncie. Assuming similar tire diameters, utilizing the CL toploadwer with a 2.32:1 ratio, an equivalent rear in the cobra would be 3.19 rears.

Being that the vette is 600lbs heavier, I would think that 3.08's would still be fine, but 1st gear would not be as good for parades, putting through cruises. The best tranny for that is tremec 3550 with a 3.27 1st and with 3.73 rears.

It's a tough call to use 3.54's, 3.31's, or even 3.08's with a bigblock, as all I think would be fine, with the edge for crusing economy for the 3.08's, and 1st gear idling speed going for the 3.54's. On a track, I think it wouldn't matter which rear you chose. It's give and take.

Actually, I have 3.08's in my cobra now, with a wide ratio TL (2.73 1st), and am contemplating switching to a CL toploader, keeping the 3.08 rears.

[Tim Brewer]

Unless you're going to do a LOT of hiway driving, you will absoutly HATE a 3:08, 3:31 and even a 3:50. All of those gears will take away most of your fun factor for driving around town. With my CR toploader with 3:50's, I couldn't cruise through any parking lot at all without hopping and lunging all over the place. I now run a 3:73 and it works pretty good. I'm even thinking of possibly a 4:11. Check out www.secondstrike.com/technical/gearcalc.asp
Tim

[xlr8or]

With the big block the 3.31 rear and a TKO600 makes a great combination. The gear spreads are ideal to me in this trans with the 3.31 BB combo. I would recommend if you do that you get the .82 overdrive or 5th will be useless until you up to 80 MPH.

[Ant]

It all depends on tire size and where your engine power is, a friend of mine has a Cobra with 450bhp 390 cu in engine, and the CR Toploader, going to a 3.08 rear end, and then 26.5" diameter Toyo tires. My recommendation was to go to a wide space Toploader, as first gear on the CR is sort of good for over 60mph @6500rpm maximum. He went for a good strong paddle type dual lining clutch, so time will tell on how the clutch will handle the gearing, the cars are reasonably light and he is doing mainly road cruising!

On the other hand I have a 675bhp 373 canted valve Ford engine, in a very lightweight car with five speed Jerico, first gear is 2.906, which I chose to give me a bit of take off driveability both on the road and track, fifth gear is 1.1, going to a 3.27 on 25.6" diameter rubber, this gives me 60mph at around 2500rpm, and about 6600rpm at the end of the straights on my local race tracks, as this gives me a reasonable cruising speed, I may have to go to a 3.50 rear end depending on how the car behaves around the circuit, gearing a car for road and race is difficult, I thought by having five speeds that will help but trial and error with tell the story. With the speed limit in NZ and the amount of cars on the road I will go for more acceleration than top speed.

[427SnakeSC]

Originally Posted by Tim Brewer

Unless you're going to do a LOT of hiway driving, you will absoutly HATE a 3:08, 3:31 and even a 3:50. All of those gears will take away most of your fun factor for driving around town. With my CR toploader with 3:50's, I couldn't cruise through any parking lot at all without hopping and lunging all over the place. I now run a 3:73 and it works pretty good. I'm even thinking of possibly a 4:11. Check out www.secondstrike.com/technical/gearcalc.asp
Tim

I run a 5 spd and a 4.11 final drive, but just a punched SB. I find 1st gear practically useless and sometime annoying! I've gotten to the point where I start off in 2nd gear now, just to get some "legs". I will be running a CR 4spd., toploader with a final drive of 3.54 in the new car when it gets here. I figure 60 mph in 1st and a top speed somewhere north of 150 is enough for me! The only concern I do have is the 2500+ rpm at highway speeds. I was out yesterday in my Classic and I ran it in 4th to get a feel for the noise at that rpm. IMHO, it was music! Love the sound of a great motor at rev!

[Clois Harlan]

Well here is another point you might consider. I started out with a 3.70 : 1 behind my toploader and I couldn't drive the freeways and expressways below 4000 RPM. I switched to a 3.50 and now I can run 70 mph at about 3200 RPM. A good friend of mine just went down to the 3.31 :1 rear gears and loves it. He cruises at 2500 RPM and has no problem going through a parking lot or school zone with loping or slipping his clutch.

These FE engines have tremendous torque and if you get your gears to low then all you are going to do is spin your tires (not a good thing on a road course coming out of a tight turn). As for drag racing I can turn the mid 11's with my set up as long as I don't go crazy coming off the line. It's easy to make smoke from your tires with any of the gears mentioned but I think our engines will live longer running either 3.31 or IDEALLY a 3.42 : 1 ratio. I would go for the 3.31 gears.

Clois

[427SnakeSC]

Clois, I am having a difficult time deciding between the 3.54 and the 3.31. Right now I am too lazy to look it up, but was the 3.31 final drive available as an optional rear end with the original '60's version? Being "correct" with my new car is a very important issue with me.

[427SnakeSC]

Clois, that's okay. I just looked it up, and the 3.31 final drive was offered as a final drive in the '60's car. Most of my driving takes place, just cruising around town. I have a few quick jonts on the highway, but for the most of it, I try to stay off the major highways! You take your life in your own hands when you enter onto our 400 series highways! I do make a few trips north of the city for car meets, but really not to often. I am wondering if the 3.54 would work for me? Thanks.

[decooney]

I've run 3.08 with close ratio TL and 3.31 with a custom wide ratio TL, and prefer the 3.31 diff / wide ratio setup; ran 295/50s for rear tires as described. 3.08s in either close/wide trans works, and it launches well for quick takeoffs with a torquey big-block for sure, BUT for mild bumbling when caught in traffic, the 3.31 diff / wide ratio trans is hard to beat - especially if you are running a performance cam and need just a tad deeper gear. Anything lower like 3.54 or 3.73 is just more tire spin. These light weight cars with more torque/horsepower motors these days love the taller gears for overall street driving IMO. Good luck.

[Cobrabill]

The early SPF cars came with an advertized 3.73 gear.Mine had 3.55s but i didn't find this out untill i bought the 3.27s and pulled everything apart.In went the 3.27s behind a CR NASCAR T/L.As some have mentioned,the car was a bit jumpy in traffic or parking lots.Especially with a aluminum flywheel.

The T/L cam out and went down to David Kee who changed it to a W/R among other things.
The 2.68 first gear transformed the car.No more low speed twitch.

[427SnakeSC]

I have read other member's comments on the 3.31 final drive, and some have complained about the lurching in parking lots. I do think that the 3.54 will work for me and my usage. Thanks.