I have an S4 clutch type diff in my S5 turbo II... I was wondering if anybody knew what these diffs are capable of top speed wise? I have personally hit 178mph according to my datalog (car had much more in it but I backed off.)

Just wondering if anybody knew

car is a heads cam LS1 swap btw... 5th gear tops at about 170mph so i just kissed 6th gear on that run

thanks,
-Jeff

 

Other than horsepower required (weight/drag), horsepower available, and traction, the limiting factor on speed is the engine redline given the drive ratio of the transmission/differential/tire system.

Just a note, I hope you have the proper upgraded suspension, front air dam, and speed rated tires for what you are doing, because you are dangerously close to killing yourself at that speed.

 

Yeah I hope you have a cage, helmet, harnesses and some damn good tires to be driving that fast. The diff shouldn't have any affect on top speed though.

 

As Evil Aviator noted, the diff isn't the limiting factor on top speed, gearing (redline), and simply being able to handle the power are - you are putting a lot more power and torque through it than stock. I'd keep a close eye on the condition of the lube.

Assuming the accuracy of your datalogger (how does it record the speed?), that is insane fast - I hope you've got the safety gear 1sweet7 mentions, and effective aero at both ends to generate some downforce and keep it glued down.

 

Just take the ratio's of your transmission and compute it with the 4.1:1 ratio of the diff to find your top speed.

 

lol just because I have an american v8 in my car does not make me a domestic guy... I am an import guy at heart and my previous cars were a 16g 95 talon and a K20A-R swapped 92 civic hatch.

And as for the car being capable of the speeds... has a fc2000 front end (built in air dam) with fins on the sides, a rear GT wing and carbon fiber diffuser underneath... Full coilovers all around, EVERY SINGLE bushing in the car has been replaced with polyurethane, prothane or new OEM if aftermarket was not available. All tires have the max speed rating of (Y) which is 186mph+. running 275/40R17 rears and 255/40R17 fronts... brakes are all upgraded with powerslot rotors and ebc red brake pads.

car has cage/rollbar inside along with both front and rear strut tower bars. also a extra subframe bar bolted inbetween the frame rails halfway down the car for added stability

datalogger's speed was calibrated when the car was being tuned on the dyno.. it is bang on with my gps

a new diff with an aftermarket LSD will be put in when this one pops... axles are already upgraded ones from driveshaft shop.

I basically wanted to know if it was safe to spin the RX7 differential up that high seeing as i have WAY more gear with my 6 speed tranny than the stock 5 speed fc tranny has... I didn't want to over spin it and have it let go... that is why I was asking the original question.

I only hit that speed once... IT was more of a just lets see what this thing is really capable of kinda thing and you guys can't tell me you have never tried topping out your cars... every guy does that haha... I will never be able to actually top out the car with this tranny. There is just way way too much gear in 6th as its a 0.5 overdrive... what I did was fast enough and I backed off

 

I wasn't looking to find out what my top speed was... i have done the math and its something ridiculous i cant remember... may have been 420... I will NEVER hit that... don't have the power to overcome the drag anyway... Just wanted to know if the diff would handle the speed as its the only part of my car i have not upgraded as of yet

 

With typical rear tires, a 0.50 6th gear ratio, and a 4.10 differential, the engine would need to run at 11,650 rpm in order to reach a speed of 420mph. That's one heck of a piston engine you have there.

Just wondering, which brand of Y-rated tires did you choose?

Also, who built your roll cage? The excellent shop that built mine is no longer in business.

 

[QUOTE=Evil Aviator;9581826]With typical rear tires, a 0.50 6th gear ratio, and a 4.10 differential, the engine would need to run at 11,650 rpm in order to reach a speed of 420mph. That's one heck of a piston engine you have there.

Sorry haha I am from Canada... when I wrote 420 i meant 420km/h... I didn't think before writing it

With my tires, the 4.1 rear and my 6 speed tranny... @ 7500 rpm its tops out at just shy of 280mph... which the car will never actually get close to haha

and as for tires in the front I am running 255/40R17 Toyo Proxes T1R 98Y and rears are 275/40R17 BFG G-Force T/A KD 98Y

And a local shop here in the city I live did my cage

 

280? You just need power and a salt flat to leg it stretch its legs.

 

The only thing I would be worried about at those speed is the temp of the diff. I could see that much speed cause quite a bit of heat buildup in it.

 

time to install a diff cooler I guess haha

 

280 is probably a lot harder to hit then you would think. Racing Beat went 238mph on the flats, but I don't know what all they did to hit those numbers.

Aerodynamics probably plays a huge roll on not just being able to get up to 280, but also being able to do it without a terrible accident.

 

I know some guys that set a record in an Alfa Spider a few years back. Got the 2liter to make over 400hp and made it up to 203 mph. Aero has everything to do with it.

To use that 6th gear all the way to top rpm would probably take 1000 hp. That gear is strictly to keep the revs down for fuel mileage on the street.