IcemanG17

How much of a difference does the 89+ 2.54 axle ratio make in terms of acceleration vs the 87-88 2.20 axle ratio (automatic)? My 2.20 seems fairly strong, the 2.54 must really fly!

mark kibort

0 , none, nothing. Gearing has little to do with acceleration for street applications. its an effeciency factor. however, it can effect acceleration for a target speed or target range. gearing doesnt produce hp, but it does determine how efficient the application of hp is over an operational range.

Folks have chimed in about how much faster certain gear boxes were when tested unscientifically, and some even had changed gears so much that they created the exact same ratios. (just with different numbers attached to them, like 2nd becoming 1st, or 4th becoming 3rd, etc)

the way to look at gears is trade offs when it comes to the rear end especially. with a larger numerical rear end, you will have less torque in the 1st gear you start at, but when you change in tho the next gear, it will have proportionately more torque in that gear than the lower numerical rear end for a longer period of time. In general terms, its a trade off, and why wouldnt it be. you are not creating more acceleration power, just redistributing it. the only way gearing really effects accleration , is when you make the gears closer together, and right now, most all 928s up to 4th gear have the same spacing. (or if you select a target speed that makes one of the gears hit it at redline. example, if your goal is to run 0-60 and nothing else matters, you want a 928 gear box and rear end that allows for redline at 60mph in 2nd gear. this is probably the GTS, GT or 79 gear box . However, any other speed target, all bets are off for which is the best. all about trade offs)

MK

BC

I have an 89 with the 2.54. I can't say that I have driven a 5spd 2.2, so I really have no idea.

mark kibort

anderson has raced both the 2.2 and the 2.72 and has never posted a time faster with either due to just the gear box. some tracks he will shift more, but in actuality, each one of these gear boxes will be better at some tracks than others. I can clearly say that, depending on the Hp, the 2.2 is better for laguna and the 2.72 is better for sears point. bottomline, this is a big range of gear ratio changes. (actually less, and the actual final gear ratios are closer than the 20%+ difference of the final drive differences. its more like a net diff of around 10%, excluding 5th on the 2.2)
do the final gear ratios of the automatic change with the 2.54 vs the 2.2 to the same extent?

mk

drnick

mark that sounds contradictory, but i agree with the part where you say that gearing CAN affect acceleration... making the gears closer together has the result that the highway speeds seen at a given engine speed are less different. that seems to be what happens when the final drive ratio is increased, albeit this effect is achieved through restricting the operational range!

mark kibort

what part of it sounds contradictory?

"highway speed seen at a given engine speed are less different" ? explain what you mean.

the operational range is not really effected with a larger final drive when it comes to acceleraton over the range that 300 to 400hp would be. (ie 0-155mph in 1st to 4th gears in a 2.2) in fact, if your range was targeting a 155mph top speed on a road course, the 2.2 set up would work best and be most efficient . (also depending on other road course lower speed straights)

mk

Tony

Now Im scratching my head?
In simple terms then.
I know your a little "girly man" You get your 16 speed road bicycle and i will get mine. You in 16th gear me in 1st gear.

Who will acclerate quicker?


maybe the "target speed" thing is confusing me ??

kuwona

If the race starts while both bikes are already moving at 10 mph, I think the guy in 16th will accelerate faster.

Jim bailey - 928 International

I hope none of the American Iron or Ricer drag racers stumble on to this topic they will be rolling on the floor hysterical !! While the gearing makes much less difference in road racing where you go from 50mph to 150mph. The initial launch zero-20 mph low gears work IF you have sufficient traction. Otherwise as Tony points out you would only need top gear if there were no difference

mark kibort

no question that if you are limited to a single gear, we are not fully using the Hp of the engine (or girly man) . In other words, the 16 gear start would be very very slow, but eventually, it would exceed the speed of the 1st gear and then what would you be able to do? bad example, but the point is, with an infinitely variable gear box, your engine would run up to max hp and stay there. in reality, we use our engines at redline,( just above max hp) so that our average hp used by the gears is maximized. this is why gear closeness is important. now, getting to a more specific example related to your example. to 16 speeds, one with slightly differnt ratios in each gear. depending on the top speed target or the top speed of any straight, one of the bikes would have an advantage. their is absolutely no power gained by gearing, but the gearing gives efficiency of that power applied to the wheels.

two 928s race. one with a 2.75 and one with a 2.2 rear end. 0-60 may be faster with the 2.75, but 0-75 may be faster with the 2.2. 60-100may be faster with the 2.75, but 75-110 would be faster with the 2.2. speeds in between would be equal. these are the trade offs.

is that now as clear as mud now?? sorry, its simple stuff,but it can be hard to explain on the list

Mk

mark kibort

actually, many of the drag racers are lost here too. launch is important, but most of the high powered cars are just burning rubber in 1st gear anyway. an early shift can mean lost time for questionable gains by a lower final drive. however, where most of the time can be lost, would be in the 3rd gear range, where most of the time would be spent. if you spend that time at only 80% ave of the max hp while the guy next to you is at 90% max hp ave, its like having more hp for that entire 3rd gear. But your right on one point as well, launch can be the difference of winning and loosing right out of the hole. a big jump by reation or traction is impossible to make up even by a car with 10-20% more power!!

anyway, we are just talking about street and road racing here.

however, i know that even with my 400 hp and certainly mark A's 550hp, on our standing starts in speedGT, all i got was wheel spin and certainly was only in the gear for a second or so.

Mk

Ispeed

I have spent some time trying to know the difference myself, and from what I know, 4th gear is lower. The gears were not all the same with 2.20 and 2.54. The biggest functional difference is that in "D" the engine runs at higher revs and has more power without downshifting, and this comes into play probably from 40 mph on up. I have had cars with super tall overdrive and I hate it.
What I do know for sure is that I love how the 2.54 is geared, and wouldn't change it. On the highway there is good acceleration at 70-90 mph without downshifting. When you do get 3rd, it really hauls ***. Also I think it has a different shift program than previous years. The GTS a/t with the same gear ratios must be an animal with more torque and mid power.

Jim bailey - 928 International

Mark 1st gear is good till about 70 kph last I looked which is what 40 mph or so. I think you were in first a bit longer than you might remember.

6.0-928S

Boy Jim you got that right! My Hemi Dart picked up .58 seconds & 6 mph in the quarter mile going from a 4:10 to a 4:88 rear axle gear. I believe the laws of physics would qualify that as improving the acceleration!
Hammer
Also - no tire spin is involved. Very hard to spin the rear tires when you're carrying the front ones!!

mark kibort

so, going from a 4.88 to a 5.81 will gain another 6 mph and half second???? you see how silly the discussion gets when you attribute the gains to the rear end mechanical advantage?? IN fact, most race gear boxes have very close to this kind of spacing. MEANING that, in this case, 1st gear could be rendered usless and the rest of the gears could end up EXACTLY the same. either way, you are shaving hairs. another data point is that 6mph of top speed by going to this 119% reducton means that if he did gain speed, it made more efficient the hp he was producing that day. meaning, if your Dart ran 120mph the first time, and now was at 126mph, it means that the prior gear box didnt allow for redline in that last gear,and you were using more of the hp with the lower rear end the second time. All folks in this game understand the results of changing rear ends. its fine tuning and a back to back run with your old gear box on that same day with the exact same conditions may yield a much closer result.

this is not rocket science here, its just simple multiplication. (and a little calculus or easier still, use of averages)

Dont take my word for it, there is a great chapter in the Carrol "Tune To Win" book that goes over some of these points.

by just saying you changed your gear box from a 4:10 to a 4.88 and you gained acceleration is only a part of the story. If in fact you did gain, then the difference would be what you gained on launch and what you optimized in the final gear of your run.

The laws of physics are never wrong, but sometimes our use of them can be.

Mk