mark kibort

It would depend on what tracks I was going to visit and how much hp I would have . That determines what rear end you would want, and mostly, ONLY those two factors. (of couse set up, driver talent, and tire siize would also come in to play too )

by the way, i am probably the only one that has frequented a track and has run two idential cars with only this change. the difference was not in time at all, it was in being a little busier shifting. But then again, gear boxes dont make HP, they only optimize or un-optimze the hp you have for a given range of operating speeds.
Go check out my videos of me racing scots 5 liter part euro with the 2.75 vs my 2.2 with the exact same powerplant back in the day of my '84 5 liter. Also, scot just did the test too, because its HIS car and he now has the 2.2. no difference in time and he likes the optimized gearing a little better for Laguna. Might be different for other tracks, but its better for him at this one.

Lizard928

Cliff,

This is not a way i would recommend for everyone.

And i am afraid i only have pictures of the underside of the car right now.

But yes this means you have to do less cutting. And it is far easier to install that the factory parts. And significantly easier to bleed.

mark kibort

We with 2.2s have useless 5th gears, except for long cruises, so its not that bad. he will still have a 170mph top speed if he needs it.

1st gear will just be a launch spring for him

dprantl

5th gear is not useless with 500rwhp

Dan
'91 928GT S/C 475hp/460lb.ft

mark kibort

thats true if you plan on going faster than 155mph with a 2.2.

Mike Simard

I wish I had a 2.73. 5th gear is useless on a racer with 2.20. I did once shift into 5th at 170ish and it didn't feel like anything was going to happen, not that I had much time anyway.
Back when I had an early 2.73 box, 4th and 5th were a nice combination and easy to shift back and forth.
Either way the car is effectively a four speed. I did like being able to putt around the paddock with the too low first gear in the 2.73 box.

dprantl

Interesting perspectives. I guess for me with a daily driver, I would prefer the longer 1st gear and 5th gear too. But I can see for a racecar the choice could be different.

Dan
'91 928GT S/C 475hp/460lb.ft

blown 87

And were off!

blown 87

Why did you change gear boxes Mike?
(with as much power as Mike makes, I suspect I know the answer to that)
And for folks that have never heard Mikes car, I have never heard a engine on gasoline that had the throttle response his car has, frikken unbelieveable.

mark kibort

how did you shift into 5th with a 2.2 at 170mph? 155mph with 26" tires is 6600rpm. did you shift at 7200rpm???

anyway, certainly its when a 2.75 gear box is needed, AND you have over 375rwhp to get and race at speeds in the plus 155 range. below that, its not needed. effectively, our cars are 3 speeds sometimes 4 speeds. I remember using 1st gear out of turn 11 a handful of times in the heat of a major battle , but dont know if it really bought me anything as it was less than a second of in gear time, and the shifts cost a tenth or two.

as trying to be optimal for racing comfort, its better to have the straight shot to use 2nd to 3rd most often, rather than 4-5th of a 2.75. 3-4th is a pretty easy shift at the easier to match speeds. (most critical and hard shifts are at the lower speeds, from a human perforamance perspective)

around the paddock, i must just drive 10% faster to get the same RPM sound. 11mph vs your 10mph.

robot808

So Greg, how's that car comin?

hacker-pschorr

+1

On top of that, Jim and Todd thought it was a good idea to spend as much time in 5th as possible not only due to the 1:1 ratio but the way the 928 box is configured.
As Jim over simplified this for me by explaining you could essentially remove 1st - 4th and still have a fully functioning 5th gear. So why spend all that time in 4th dropping down the the "lower row" of gears when you can be in the straight through portion of 5th?

I'm probably not explaining this correctly, sure you get the gist of it.

mark kibort

Its not longer,its just taller in 1st. On the track, its all optimizing HP for each of the most often traveled straights. I doesnt matter if its a 2.75 or a 2.2 or a 4.10. its all about space of gears and the speeds you are going. think of MPH in each gear and forget about what they are numbered. (i.e. 1st , 2nd..etc)

2.2;1 is: 50, 80, 118, 155mph and a usless 5th and rarely used 1st
2.75:1 is: 55, 73, 105, 130mph, and a rarely used 5th and never used 1st.

Find what YOU would do on each of the straights on a track near you , or that you frequent, and select the gear box that allows for the most HP/seconds to the ground!

mark kibort

yes, you explained it well. you do gain efficiency, but the problem is, you spend most of your time on a track in 3rd gear, so it really doesnt matter that much. Now, at a fast track, thats not the case, you could gain that few hp that being in the direct drive gear would provide. you certainly woldnt want to do it, and sacrafice optimization at a track like laguna if you have under 300 or over 550hp. far more gains are to be made by optimizing the HP of the engine, rather than the efficiency of the gear box.

blown 87

We just started pulling the airbox off to get the auto out, had a couple of other folks cars to attend to.