Open spool or posi-traction rear ends??

[Avanti Kid]

Hi Guys and gals, I've been racing my Studebaker powered Avanti now for 20 years and set records that still stand at Bonneville, El Mirage and Muroc, but my question is: in your opinion which is better for racing at Bonneville, open spool or posi-type rear end. I only have used the open spool rear end but always wondered if the posi- rear end would give better traction on the salt thus a higher top end speed, what do you think?? thanks Dave

[Sumner]

...I've ... set records that still stand at Bonneville, El Mirage and Muroc,..... I only have used the open spool rear ....

I'm a little confused by "open spool rear".  Is it an open rear-end with spider gears or a spool?

I'll jump in and say there is no set answer.  In your case running in the low 200's on the 5 mile course I'd imagine you can reach your top speed regardless of the rearend and that the engine is probably the limiting factor.  Now if you are still accelerating in the final mile and you didn't get there faster since you were spinning the tires getting there then the options you mentioned need to be explored.

Myself if your times are traction limited I'd go with the GM 7.5 with the Torsen center ...



http://www.torsen.com/products/T-2.htm

...as it will not only limit slippage for the wheel with no traction but transfer power to the wheel that has traction.  They aren't expensive as they came stock in some late 90's-early 2000's Camaros and can be had in a number of ratios and should be able to handle your power.  

You will have to install C-clip eliminators and they come in two different series.  One for Series 2 axles (2.73, 2.93, and 3.08 rear axle ratios) and the other Series 3 axles (3.23, 3.42, 3.73, and 4.11 rear axle ratios).  So if you go looking for one on e-bay make sure you get the one that will work with the final gear ratios you will be using.  You can find them for under $150 and they use a 28 spline axle vs. the more common 26 spline found in other Camaro rearends,

Sum

[Avanti Kid]

Hi Sumner, thanks for your response and it is very useful and interesting. At Bonneville my top speed is only 216mph at 4 mile because we reduced the turbo boost to 20 lbs and set the rev limiter down to only 6300 rpm, we also run a Ford 9" with a ratio of 2:47 to put a big load on the single big turbo, we thought we tuned the car for 212mph but got 216 instead, by open spool we only have power to one wheel, some say that good for going straight where if you have power to both wheels like posi or locker rear end, it some times makes the car fight back and forth because of different traction on both tires, but I really don't know which is best. We took the 2:75 ratio out and installed the 2:47 and we did better for top speed, some day if we race again we will move the rev limiter up to 7,000 rpm and give it more boost and more fuel and see if we can do around 225. Again thanks for your inputs, take care, Dave

[jdincau]

Your question is confusing because an open rear end and a spool are the opposite ends of the choices. "Open" means an un regulated differential, "Spool" refers to a non differential rear end with both axles tied together with a solid spool instead of a differential carrier.

[Avanti Kid]

Thanks for your correction on rear ends, so I guess mine is an Open rear end and was wondering if a Spool would be better! take care, Dave

[SPARKY]

loose that 9" put in a 7.5  ( will be good for an increase of thrust effort of over 4%)  with 2.41s  for a net pick up of 2% thrust and a drop of 2% or 130 in Rs

[kiwi belly tank]

Now that JD has schooled you on the terminology, you need to look at how they all work & what you're trying to achieve.
LSR tires are skinny, hard as a piece of wood & a pair are never the same size at the same psi stationary or at speed. In this application a spool is obviously not desirable due to the tire rollout being different. A locker gives the same result, it just comes on like a sledge hammer under power & you can imagine what happens when you have to back-pedal the thing.
An open carrier should really be called an unlimited slip.
An LSD unit will allow a small difference in wheel speed to occur but contain an excessive difference therefore accommodating the individual tire requirements.
There are salt racers that swear by every available option, this is just the fact's Mam!
  Sid.

[jimmy six]

My theory has been a simple one over 37 years of LSR. If your car spins and you have one take it out. Next and most important. True your wheels to within .002" on the place your going run them and mark accordingly. After tire is mounted on the wheel true it. I use a belt sander and it takes a lot of patience. Remember you have 2 large flywheels out back and 2 more in front.......how far out would you allow your engine flywheel.

Long wheelbase cars don't have the same problems as cars under or near 115"........good luck.

[Avanti Kid]

Hi guys, those are all very good comments! but what do the rest of you guys chose between "Open & Spool" rear ends, so far do I understand Sid likes Open better? From my experience if I need more traction with my "Open system" I just add more ballast, Dave 

[jdincau]

On the 270 MPH C/BFMR, we run a spool.

On the 160 MPH F/STR, we run an open differential. I have installed a speed sensor on each rear wheel on it and we will see in May whether we are spinning one wheel or both at El Mirage.

[kiwi belly tank]

Hi guys, those are all very good comments! but what do the rest of you guys chose between "Open & Spool" rear ends, so far do I understand Sid likes Open better? From my experience if I need more traction with my "Open system" I just add more ballast, Dave  

My opinion... No, I don't like an open carrier (unlimited slip), one wheel burnouts are counter productive, I prefer LSD.
If you can true a steel wheel to .002 runout, you are having an exceptional day.
There is not enough rubber on a Mickey to true it up with a belt sander, you will destroy it.
  Sid.

[Stan Back]

My experience at Bonneville . . .

111" wheelbase roadster -- open rear end -- five Bonneville records up to 206.  Tach reads EXACTLY what speed should be.

Same car -- one run with positraction -- spin.

Next day -- same ratio, open -- record at 210.

[DallasV]

Hi Sumner, thanks for your response and it is very useful and interesting. At Bonneville my top speed is only 216mph at 4 mile because we reduced the turbo boost to 20 lbs and set the rev limiter down to only 6300 rpm, we also run a Ford 9" with a ratio of 2:47 to put a big load on the single big turbo, we thought we tuned the car for 212mph but got 216 instead, by open spool we only have power to one wheel, some say that good for going straight where if you have power to both wheels like posi or locker rear end, it some times makes the car fight back and forth because of different traction on both tires, but I really don't know which is best. We took the 2:75 ratio out and installed the 2:47 and we did better for top speed, some day if we race again we will move the rev limiter up to 7,000 rpm and give it more boost and more fuel and see if we can do around 225. Again thanks for your inputs, take care, Dave

We run the Ford 9" spool with the 2:47 gear in our roadster. The car has been 250 with a blown small block. We have been running the car for 35 years and it has never spun. we stay with the 9" over the 7.5 because the 9" is bullet proof and we have had better luck finding the gear ratios we need.

[Sumner]

Hooley's Studebaker has always run the Ford 9 inch with the spool.  Break the tires loose and it will jump sideways just like a jeep will with a locked rearend on a side hill when both tires break loose at the same time.  The rear is going to go down the hill.  Same Jeep with the rear unlocked on a side hill and one wheel will break loose, but the back won't go down the hill since 1 back tire is still biting it.  The same can hold true on the salt.  The only problem that I see and can't back this up is that it is going to take more power to break both tires loose with the spool vs. break one tire loose with the open rear.  If that is true then with a lower HP car the open might work just fine.  With a high HP car the spool might stay hooked up longer before you spin both tires  .

I'm going with the 7 1/2 with the Torsen in the lakester as I was impressed how Sparky could accelerate with one in his old lakester...

http://www.youtube.com/watch?v=PtGmGTHK6qc

.... it was about 2/3's the weight of the Stude and probably 100 HP more and he could accelerate much faster than the Stude without the wheel spin we get in the lower gears.  With the Stude you just couldn't give it that much throttle in 1st and 2nd and not break the tires loose.  We were running a 2.47 rear with a 1.9 1st and a 1.34 2nd so were geared lower than he was with his 2 speed powerglide and I think 2.14 rear gears so that probably made a difference also.

We were going to try the 7 1/2 in the Stude with the Torsen since we needed taller gears than the 2.47's but now we are going to try the 2.26 8.8 Ford rear and since there isn't a 'cheap' Torsen for that will probably go with a full spool like Hooley is running now.  If that doesn't work well then maybe spend the money on the Torsen, not actually that expensive at $570 new.

There is no 'one right' answer to this as we have seen from the responses above.  It depends on the car, the HP/Torque, the speeds, the cars weight, etc..  Search for what works for you and use it.  In Avanti's case since he is adding weight to take care of the problem I'd be tempted first to try the 7 1/2 with the Torsen.  Adding weight, especially if it is behind the axle, can lead to other problems once the tires do spin and the back of the car wants to pass the front due to the center of pressure/center of gravity deal.  We've added weight to the back ahead of the axle, but have also then added a lot more weight forward to help with the CP/CG and that weight is just hurting acceleration,

Sum

[Avanti Kid]

Guys, your giving very good comments and ideas about what rear end system to use, I found it interesting that Stan Back who used a "Open" one time and a "Spool" another time on his roadster and did better and higher speed with the Open system, at least he is one racer who tried both system, thanks everyone some of these comments from you guys may help some of the new racers at Bonneville get some good information, take care, Dave