GUYS GUYS! thats my ex.......she is 41 not bad eh? I am dating a 23 year old red head now LOL

 

LOL, hopefully it was windy.

 

Heck of a thread I started!!!! Thanks everyone, I basically knew it would be more responsive off the line , but all the expert knowledge on the ratio science has been very interesting.
One other thing how much does the LSD really help? Does an Open Diff make it less performance oriented?

Jason

 

Auto or 5 speed?

 

LSD is good for you and your tires. (see Bill Ball's sig line... without LSD)

Jim, he has a 5-speed.

 

The biggest difference is probably how the car comes off a tight or 90 degree corner. With no slip the inside unloaded rear wheel easily does Bill Ball Burnouts impressive but not especially quick. With a limited slip both rear wheels get power to some extent(depending on the slip used) and you get more drive off the corner however it increases the possibility of throttle oversteer which can be great fun (think drifting) but not necessarily the best way around a corner. Drag racing start the slip is much better. Overall I have to say the slip does make a difference which is why I put one in the very brown 1980 and confirmed with better lap times at the Streets of Willow.

 

I wonder how easy it is going to be for a boosted 5 speed (LSD)to beat a boosted auto (no LSD) in the 1/4? I think the LSD can hurt you here. The auto can spin the tires and hook up, with some techinique, and do well. Breaking loose with the LSD 5 speed is brutal from the wheel hop and forces you to lift. I am convinced the best option is the SC (TS)/LSD/Auto

 

Go watch the bracket racers at the drags and the vast majority run automatics simply because it is far easier to be consistent when you remove a couple of human variables like moving your left foot and right hand . The 2.20 auto is running a 8.14 first gear , the 2.37 and 2.538 are 1 st gears of 9.1216 and 9.822

 

What about multiplication from the torque convertor?

 

From Wikiped.."A torque converter differs from a simple fluid coupling by the addition of a stator, a disc with fan-like blades connected to the transmission via a fixed shaft with a one-way clutch that allows it to rotate only in the opposite direction of the fluid's radial motion. Without the stator, fluid leaving the turbine would strike the impeller with a radial motion opposite its rotation, causing a braking effect. With the stator, the returning fluid strikes the stator blades, which reverses the radial direction of the fluid's motion so that it is moving the same direction as the impeller when it reenters the impeller chambers. This reversal of direction greatly increases the efficiency of the impeller, and the force of the fluid striking the stator blades also exerts torque on the turbine output shaft, providing additional torque multiplication equivalent to a higher numerical gear ratio.

As engine speed increases the speed of the impeller and the turbine become nearly the same (reaching their point of minimum slippage). Because the turbine is spinning faster than the fluid can exit its radial chambers, the net angular momentum of the exiting fluid is in the same direction as the turbine's rotation, rather than opposite it. As the impeller approaches this speed, the torque multiplication provided by the stator decreases. At that critical speed (the converter's stall speed) the fluid strikes the back of the stator blades, causing the stator to freewheel so that it will not interfere with the return flow of fluid.

The maximum amount of torque multiplication provided by the stator depends on the angle and design of its blades. Typical torque multiplication ranges from 1.8 to 2.5:1 for most automotive applications, up to 5.0:1 or more for static industrial applications or heavy maritime propulsion systems. The blade angle and shape also affects the stall speed of the stator (although actual stall speed is also a function of the engine's input torque; an engine with less torque will stall the stator at lower rpm).

While stator multiplication increases the torque delivered to the turbine output shaft, it also increases the slippage within the converter, raising the temperature of the fluid and reducing overall efficiency. For this reason, the characteristics of the torque converter must be matched to the torque curve of the power source and the intended application. For example, drag racing transmissions often use converters with high stall speeds to improve off-the-line torque because converter efficiency at cruising speeds is not significant....." pretty good expalination if you ask me , so 2-2.5x multiplication of torque BUT decreaeses as yo approach stall speed and 928 3 speed automatics stall range from around 2,300 USA to 2600 +,- 200 for Euro three speeds while S-4 2:20 is 1650-2050 RPM and the 2.53 is 1750-2150. So when the RPMs hit stall speed the big torque multiplication is OVER..... which explains why dedicated drag cars may run 5,000 RPM stall convertors

 

Sure.

What I was getting at was a ballpark 2x multiplication at launch so the auto has a much higher gear ratio off the line. That'd be at the expense of applied power, but at launch that's not an issue.

 

Understood but only at very low engine RPM which is very low engine torque as well since the torque multiplication basicaly ends at stall speed..."As the impeller approaches this speed, the torque multiplication provided by the stator decreases. At that critical speed (the converter's stall speed) the fluid strikes the back of the stator blades, causing the stator to freewheel so that it will not interfere with the return flow of fluid " .So the torque convertor gives you a very low low gear but only at low RPM where torque is also quite low. When you are on the gas hard you hit stall speed in a heartbeat and have to live with the gearing !

 

I guess I have some experience with this. My car had a 2.20 gearbox that blew up so it was replaced with another, which turned out to be a 2.64.

I track my car a lot and for that purpose I do not think there is an advantage to one ratio over another another. Each is merely a different compromise. I can say 2.64 is better at Thunderhill, but 2.20 is better at Laguna Seca. At Sears Point, it's a wash. It all depends on where the shiftpoints come at various corners. Once you're out of 1st gear, it doesn't matter. The shiftpoint to 2nd gear still kinda feels unnaturally early to me.

One thing I'll mention though, the flexibility of 3rd gear with the 2.20 is amazing. Our cars have enough torque that you can leave it in that gear from 25 to 110 mph. I was just instructing Rich S at the track in his 2.20-equipped 928S4, and we could leave it in 3rd for the ENTIRE lap. No need to up or down shift, which is a HUGE advantage when you're trying to learn the track layout.

OK another thing where I have some experience. I had an open diff on my car, then had the standard 40% LSD installed. After I wore that LSD out, it was modded to a 80% (maybe full locking) diff. I just blew that diff up so I'm planning on going back to 40%. Again, I'll relate my views as seen through the prism of track use.

0% - freaking annoying. Coming out of slow corners, you'd plant the throttle and the inside wheel would SPINNNNNNNN. Revs would soar and you'd leave one long black line. Wheelspin until you do a deliberate and decisive lift.

40% - Much better. Get on the power, rear end would step out, but if you timed it right at the exit, you could stay on the power and the wheelspin would subside on its own. Understeer not too noticeable on entry.

80% - Interesting on track, not good at all for street. Very sensitive to power at mid-corner, rear end wants to step out big time. Good grip exiting so you could go full throttle earlier but you lost some mid corner speed due to that throttle sensitivity. Kind of a wash speed-wise. Noticeable understeer on corner entry at slow speed. Massive understeer if it's wet. On the street, the rear tires would hop and skip any time the steering wheel was turned more than 90 deg. People looked at me like I was an alien when I would try to manuever in a parking lot. In any event, don't do this mod because your diff will blow up like mine.

 

Wow, I never realized that was you in your avatar.

 

Dennis..."I was just instructing Rich S at the track in his 2.20-equipped 928S4, and we could leave it in 3rd for the ENTIRE lap. No need to up or down shift, which is a HUGE advantage when you're trying to learn the track layout." Correct much less noise and fury far less to concentrate on but definately slower than using the the gears but not as much as most would think. My student last weekend 6 spd 03 Vette also did the "torque it around" exercise for a while until he picked up more of the line !