Oz, that would be a factor. Mongo thought the shorter final overall gearing would help his accelaration, but in truth only first gear (with identical boxes) would give a lower overall ratio. It's probable that the overall ratio in first gear with the taller geared diff would be shorter than second gear with the shorter geared diff, so you can see how the ratios between the two start to overlap after the shift to second gear with the short geared diff.
My comment about the S4's acceleration times remaining the same I think proves your point. The '87-'88 S4's had the same HP rating as the '89-'91's, but their accelaration specs remained the same. The '85-'86 32v S also had the same 2.20 ratio (as the '87-'88 S4) but was factory rated at about 30hp less (is this what your refering to?), and had slower accelaration specs - also proving your point.

 

I have the 2.73:1 ration in my '81 3 speed auto 4.5L
You can do all the legal speeds at less than 3000RPM so it's half redline speed to cruise. For The Northern Territory at 130Km/Hr I think the engine was over 3000 by a little.
From memory at 5200RPM the car travells at around 225Km/Hr, thats 1000RPM till redline!

Now when I get the 240HP 4.5L to have close to 300 it will certainly get moving a bit quicker.

Glenn
'81 928
Melbourne Australia

 

Not sure what my RPM are at 130 Glenn....still locked up in the garage trying to sort out my lean problem afta bolting a twin screw on it.

 

I held back long enough

As we have talked about, gearing doesnt create HP, it only makes more effective the HP you have based on a desired operating speed range. Gear spacing is MUCH more important. However all of our gear boxes have near the same gear spacing, so changing rear ends ONLY changes the sweet spots you might operate in. Lower and lower doesnt nessarily gain even "out of the hole" advantages as Jim says, due to limitations in grip. this can be seen on any drag simulator where you can input the grip factor.
For example, an S4 has a speed ranges of :
0-50
50-80
80-118
118-150mph in the 1st 4 gears. on street tires, most all will be burnning rubber in 1st, so it is less important than most make it out to be. the key thing is having the ability to have the engine operating near Max Hp for most of the time.

after all Acceleration = power/(mass x velocity) This means POWER determines the acceleration at any vehicle speed.

As a note, just changing the final drive ratios does some crazy things to the total ratios, as the gear boxes themselves have different ratios too. in otherwords, most of the difference in the total ratios is less than 10% from a GTS box to a 2.2. BUT, if you take 2.72/2.2 that is a much higher change. You can see what that can do to the total ratios if just a rear end change is done.

the 2.2 is pretty optimal for most of the tracks I see. (i use all of 3 gears evenly, 2nd through 4th, as you dont use more than 3 unless you get into racing gear box spacing) as an added advantage, i get to cruise home with the engine lugging along the 2000rpm mark to get 20+mpg even with a stroker!

mk

 

the 2.73 early 3 speed auto gears do not directly interchange into the later 4 speed differential housing.The big issue is how the units keep the gear lube and trans fluid seperated.

 

I have an 89 2.64 5-speed box in my 87. It is a totally different animal, and I mean totally. I used to run strong against other 2.2's here locally, but when I went to the 2.64 I noticed a pretty clear advantage in acceleration. Not off the line necessarily though it does pick up revs a lot more quickly in 1st ... but, from 2nd onwards, I see a marked increase in ability to outrun other Porsches going through the gears. The legs aren't as long so you don't get that loooooooonnnngggggg freeway pass with speed coming on and on and on like a freight train. What you rather have, is a more interactive experience.

I've noticed that with this setup, if I'm in the wrong gear, I'm toast because my optimal torque band lasts a lot less time in every gear, whereas the 2.2 used to stay in optimal torque a lot longer. But conversely, when I'm in the right gear and choose properly to shift when needed, up or down, I see quite clear improvement. Acceleration through the torque band is improved.

And yes, freeway cruise is at around 3,000RPM at 80. More or less, from memory. On a long road trip (which I rarely take) this would be irritating.

 

"Totally Different Animal"? The actual gear ratio differences are less than 10%. Much of what you feel is perception. You have to realize, I can compare this on a race track every month for hours of full throttle operation against, scot, who for a long time, had only 15hp less than i did, yet I would beat him on every straight, out of every turn, etc. the main reason for this was my BROADER HP curve, not his "Totally Different Animal" 2.75:1 gear ratio, nor did others that visited the track with their 2.75:1 gear boxes. Sure, if you are looking or or sensing a gear to gear acceleration feeling, you are absolutely right. This perception is really incorrect, as fun as it is, because in each gear you are going 10% slower. the reason that the lower gear ratio CANNOT in all speed ranges improve acceleration, is because acceleration is proportional to HP, not gear ratios. So, what you feel as an advantage in one gear, will be a huge disavantage just after you shift up to the next gear, where the comparative difference would be near 30%!! (for a short time). So, your 10% adantage for any comparative gear, is offset with the disadvantage of even less torque at the rear tires for a shorter time. in the end, they cancel out, until a target speed has been reached , and that can be any speed. In otherwords, a comparison of two gear boxes with different final drive ratios BUT the same gear spacings, will have advantages each their own for any given speed range.
1st is debatable based on traction, but if traction is not looked at (silly not to), then, 1st would always have an advantage. After that, there are trade offs and it depends on speed. Forget about talking in terms of rear end ratio, talk in terms of final ratios and speed ranges.

again, in terms of real life measureable differnces, ive raced 2.75 and 2.2 cars and have not seen one bit of difference in final lap times, and neither one feels any faster than the other because I truely understand the trade offs at any speed. (scots, my 84, andersons, Fans, holbert, and chuck's, who's car i raced one time with ONLY 1 gear at laguna, 4th for a couple of hours and 1:45 lap time with one gear ) . The only real diffence I noticed was that shift points were changed. Some at awkward points on the track vs my 2.2 at Laguna for example.

mk

 

Mark I really don't give a flying f___ man. I posted my findings. Just shut up already ... Can you let others post their experience without attacking it everytime? You are wrong. But I am not going to address that. All I'm saying is, shut up a little and let people say what they experience, EVEN IF YOU THINK THEY'RE WRONG.

 

Settle down. What I am saying nothing but the truth. You cant argue with newton, but you are sure giving the old college try. It is not my interpretation, but a complete assessment of the concept.
You dont have to agree with me, but agree with Newton! A=F/m and A=power/Momentum

Im not going to argue feelings, that is your game.
I have experienced what you are talking about. It does feel cool to gear everything down, but in the end, you are not really doing much for overall acceleration. I used to do that with my motorcycle when i was a kid. Think of it as a optimization tool so that you change the shift points where they are best suited for your range of speed. This allows you to have max acceleration where you will spend most of your time, you know, that kind of stuff.

You also mention that you have greater or less time in particular gears. This is a factor of gear spacing, for which the 2.64 doesnt change anything. you said "I've noticed that with this setup, if I'm in the wrong gear, I'm toast because my optimal torque band lasts a lot less time in every gear, whereas the 2.2 used to stay in optimal torque a lot longer " Because the gear spacing is the same in both gear boxes, it is impossible for this to occur

Dont blast me. Im just the messenger here!

Now, instead of saying that I am wrong, why dont you provide ONE shred of proof to back it up if you are posting a "finding"? I can do this so very easily. Ill show you the force at the rear tires at ANY vehicle speed, AND the time you spend in any range of speeds and gears to show average or peak accelerations.

Can you ??

So, either show some proof, or you take some of you own advice and stay with the FACT that you have some "Feelings" about your findings.

Just trying to help someone that might be spending cash to do something that might not work in reality, but could provide those feelings.!
Hey, we are all trying to help here and provide perspective.



mk

 

Somehow I think I just did the wrong thing posting this thread up...

 

 

Mark,

My question though is how do you address the acceleration bonus of a higher torque multiplication by having higher final drive gears? I understand your argument about how gearing wise it just depends on how long you spend in your powerband, and I can even understand how along a racetrack where your average speed is much higher than normal driving how it could not have as drastic of an effect, but if we are talking about off-the-line acceleration, the higher gears will give the effect of more "umph" and your 0-60 will be faster because you will have more torque at the wheels via the magic of math.

 

That's easy. out of the hole , or starting out, since accleration is proportional to power, the closer you can get to max HP from a stand still, would be the greatest gear reduction. however, even it has its limits due to traction. If you know the coefficient of friction of the tire and surface, optimal slip percentages, you can get some number for any speed from 0 onward. once you have launched, in whatever 1st gear you have chosen, the rest of the gears have to follow with near the same spacing. So, ignoring the launch (and we all agree this is critical to drag racing), after that there are trade offs with torque at the wheels and vehicle speeds for all shifts of the gear ratios up or down. (assuming the rear end ratio is the only thing that is changed).

If you go to a drag calculator you can see these trade offs at work. Even 0-60mph. If you have a very low 1st gear, lets say 20% lower (like 2.2 / 2.75), so instead of accelerating out of the hole at X lbs of force, you accelerate at X +25% lbs of force, but you do this untl -20% less speed. (assuming 1st gear goes to a very tall 60mph at redline for the sake of argument). lets say 1st gear to 60mph took 5 seconds. Assuming no traction differences (unlikely), 0-48mph does now have 25% more force. BUT, 48 to 60mph, where the time is going to be the most concentrated, will have 30% less force for the longest time for near 3 of the (10) .5seconds 5mph segments. You can see a ball park of advantage of less than the gear change % gain for the 0-60 run . now, factor in grip issues from 0-30mph and you can see depending on the tires, and grip, there might not be any difference at all.

Now, start from speed ranges where there are no grip issues and you are shifting. The trade offs of staying in lower gears to a lower speed and shifting earlier, speed-wise has its trade offs. theoretically, depending on the speed range, there might be equal trade offs with no gains. No surprises there. you dont get power by changing gears. you better , more effectively utilize the power available with the proper gears for the desired speed ranges.

when you start getting the ratios closer together, you start taking better advantage of the power you do have. In our 928s, the gear spacing is pretty close to the same accoss the gear boxes. this is why when there was talk of using a "3.09" on a GTS gear box, we did the quick math and found that it created a 4 speed, with near the same gears as an S4 2.2, but gave a super 1st gear that might or might not have been able to get any grip to make a drag difference. Certainly after 35mph, there was no differecne as the spacing and gear ratios were near identical. (however the 3.09 didnt have any "5th "gear now, only its effectively, 4th as a top gear. (i.e. 3.1 :1 ratio which is near the normal 4th gear of 3.2 :1 on a stock S4 2.2 box)

 

forget all the arguing. Its not religion or something! Here is the simulator. Just plug in your different ratios for a given HP/torque curve and see what happens. this is a good simulator, because it incorporates slip off the line. set it to whatever you think is realistic. watch the trade offs. It gives segment times. you can see any range of speed, what is optimal for rear end or total ratios.
It holds all the answers Ive been talking about.


http://www.nightrider.com/biketech/accel_sim.htm

Edit:

I just for fun, plugged in a 2.75 vs a 2.2 with gears of 1,2,3,4,5:1 (5th through 1st)

Put the weigh at 3000lbs, 100" wheel base, our sized tires, and a torque curve that was 400ftlbs from2500rpm to 4000rpm and then falling off 10ftlbs per 100rpm

so, the inputs were random and I had no idea what I would end up with.

All metrics were to the advange of the 2.2. 0-60mph, even 60ft mark, and 1/8th mile as well as the 1/4 mile running at near 13 seconds vs 12.7 for the 2.2:1 ratio. So the 2.2 vs 2.75 ended up killing the 2.75

I'm not saying that the 2.2 is better, what im saying is that it depends. Go ahead, find your optimal gear ratios for your optimal torque curve and I can do the same thing with changing the amount of torque, rear end, etc to give better results.

Every car will react differently to a change of rear ends. Its just plane phyisics.
What works for someone, will not work for someone else, if you change speed ranges, weights, hp, etc. Whats nice about this simulator is that it takes into account clutch slip, traction control for the start. Im sure you can plug in the ratios for the stock S4 2.2 and then just change rear end to a 2.64 and see what happens. I dont know what it will turn out like. The end results will depend on a lot of factors. take a look at speed ranges too. Lots of trade offs!

Have fun!

mk

 

Hmm twin screw hey, Why did we not drop by your way when we were out in the Northern Territory?? Next time
If you have the 2.2:1 rear in your S4, I remember the RPM being somewhere around 2500 @ 130Km/Hr.

Interesting reading Mark, your comments are Einstein like and a little newton flavour thrown in, I like it.

I dont know what the final outcome or advantage would be to change the 2.2:1 to a 2.73:1 rear in an S4 - maybe just to drop the overall speed of the car?!

Have done a lot of drag racing in my time and with one particular car I had, I found I was shifting into top gear just before the traps. Not Ideal at all. So a diff gear change was made after sitting down and working out the maths to get the engine just short of it's peak horsepower through the traps (almost redline). Not only did it almost send the car skyward off the line but the top speed dropped to only 104MPH! - just an example rant guys.

Some experience with my 928.
3 speed Auto 2.73:1 rear axle ratio 240HP 4.5L 1530Kg
1st gear can hold up to 60mph
2nd holds from 60 up to 100mph
3rd holds up to 140mph - need more horsepower to get past this, although engine speed is well below redline. This is what I do in my car, forget what the book says, it's happy with those speeds.

2nd gear is just fantastic between 50mph and 100mph, on a mountain road just hovering the engine speed between 2800 and 5000 RPM where the peak torque is at 3500RPM it's a real thrill. It gets the whole car working, the rear suspension works, the brakes work, the steering and agility of the car really shines, who said the 928 was fat and lazy!. The torque converter always keeps the engine above 2500RPM when you are travelling slower than what it's gear to be (Stall speed is 2500RPM) Try it someday if you dare
Actuall the M28/10 4.5L engine has more torque than the S 4.7 300HP engine below 3500RPM. But they soon dissapear when givin the space to use that extra 60HP !
Some 928 driving experience from me.

Glenn
AU '81 928