350SBC/700R4 which Rear End Ratio??

[danc]

Guys:

I have an 83ZX SBC, 700R4 and a 390:1 rear end. The power/torque curve (Engine Torque 430 @4000 Rpms (Horsepower 430 at 5800 Rpms)) is definitely not optimum with the big first gear in the 700R4. I'm thinking 354:1, but have no formulary reference. So lacking that, I'm not sure if a 336:1 would be better or why?? I've heard a couple discussions on an optimum rear-end ratio for a 700R4 and heard it expressed in numbers like 11 and 13?? The car is used in fair weather one the street. Any input from you guys out there with a similar application would be appreciated.

Thanx

Dan

[fl327]

3.54 works good since the z is very light and the American transmissions are geared to pull a much heavier car off the line. The mix of the american trans ratios and the 3.54 rear gearing makes for some hairy acceleration.

[John Scott]

What's your cam duration? Your hp/tq peaks sound like a healthy bumpstick. Another factor to consider is your cruise rpm in 4th. Driving along at 80+mph, just to keep the engine from bucking isn't always a good idea. If your engine pulls ok from the 1800-2000 range, then the 3.55s should be fine. I'd guess the taller 3.36 gears would make using your 4th impossible at least at legal hiway speeds. I had a 700r4 and loved that short 1st gear! The WOT shifts were too hairy so I've since gone 6 speed. My 244@.050 duration is borderline at 75 in 6th.

John

[jt1]

When I had a 700R4 in my car, I cruised at 70, turning a little over 2000 rpm, with 3.54 gear and 225/60/15's, which were 24" tall. A very nice combo for the highway.

 

Tirerack has a calculator on their site to determine your rpm with different tire & gear combos.

 

John

[Phantom]

Let me put it in perspective. I have 3.70:1 gears behind my LS1 with stock diameter tires.

4th gear, 1:1 @60mph = 3,000 rpm

5th gear, .75:1@ 60mph =2,250 rpm

6th gear, .50:1@60 rpm = 1,500 rpm

a 3..54:1 differential would have you turning slightly slower = 3.54/3.70 = .956 Assuming the 4th gear overdrive on the R700 is about .62:1 then the engine rpm at 60 mph would be 3,000 x .956 x .62 = 1,780 rpm.

a 3.36:1 would have you down to 1780 x 3.36/3.54 = 1,690 rpm.

 

My LS1 cruises fine at 1,500 rpm and actually does pretty well as low as 1,200 rpm but my cam profile is much different than yours. If either of these rpms is in your "smooth idle" range then I'd opt for the lowest one that would work.

[danc]

Guys:

Appreciate the thoughts and responses. John....The cam (hydraulic roller) set up is: lift .474/.510 and Duration @.050,.208/221. It (Gm Performance Parts LT-4 "HOt Cam" pulls hard and very quickly in the lower RPM ranges and really all the way thru the low 5k horsepower range. The street tires are 25.6 tall. The 700R4 is a Bowtie Overdrive Level 2 with a 2400 RPM stall convertor. I'm not happy with the Bowtie Overdrive people but the transmission is great after I spent some more $s on it, that's another story. As we all know first gear is almost non-existant with the 3:90. There's no traction from a standing start and when I roll out in first the shift light is the only thing that I have time to look at in the nano-seconds before shifting to second. The car is not on the hightway much, and I have no desire to see what the top end would be so I'm primarily trying to expand the time between 1st and 3rd to make better use of the torque/horsepower range. Now it's sort of like "johnny we hardly knew ye". It's no doubt fun to mash-the-motor (however briefly in first and second) and it seemed to me that a 3:36 would be going too far that other way in terms of even finding someplace to go thru the gears without having to go on a highway between "burbs" to do it. The 4th gear O/D is .070. Again, appreicate the input and any other thoughts would be helpful..

[Phantom]

If your fourth gear is .70 rather than .62 then the rpm would be higher and I would definitely try to find a 3.36. I know Lone Star 1, who was a supercharged LS1 (420 HP and torque) and the 4L60E, has opted for that ratio. I saw him run before he supercharged the motor and he was running close to 12 flat. Car was impressively quick.

[BrandonsZ]

think about 200-4r (search on it) it's first gear is better for handeling the power and it's 4th gear is higher so your hyway rpm is a little better .67:1

 

Could the 200-4r handle 430hp?

 

Not sure anyone?

[danc]

Guys:

Again, appreciate the responses. I'm aware of the differences between the 700 and 200 transmissions and I don't think my HP/torque would be a problem for a 200. In a perfect world a 200 would have been better, but I'm already down the road on the 700 investement and would have to modify the trans mount as well as get a new driveshaft due to a difference in length. I am not disappointed in the 700, and a 3.90:1 rear end is way too much gear running a 25" tire anyway. Appreciate the input on the 3.36 too. I'll hopefully be able to continue sampling the experience of guys lke you on the forum to make the decision between a 3.36 and a 3.54. Am having big fun with the car today, but I just want to get the max out of the combination I have.. I read a comparison between the 700R4 first gears (3.06:1) and a TH350 (2.52) that was pretty graphic. A 700R4 running a 3.73 is close to a 350 running a 4.56.