Rear end gearing change 86, options
07-06-2007, 06:38 PM
#91
Chronic Tool Dropper
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Originally Posted by TopDownL79
Does that mean that I should find a way to slip a Powerglide into the 928? 
The auto trans is another issue completely since there is little or no loss of acceleration while shifting. A powerglide would be a good choice, especially if the gear spacing is really tight and you have carefully selected the final drive ratio for the conditions in which you drive.
It wasn't that long ago (20 years now...) that I was modeling ideal gearsets for the box in my little Yamaha-powered sports racer. It had a power band that started around 9000 and was over by 11000, so keeping the car on the pipe was the only way to go fast in it. Lots of smoke and clutch slipping to get launched of course. Final tuning was easy-- change a sprocket up or down. But getting the right spacing for the engine characteristics was always fun. Higher altitudes and lower air densities on hot days mean lower power. Bigger axle sprocket. Sea level, cool day, a little humidity? Smaller axle sprocket to take advantage of the extra power. No long straightaways or no room to go 170? Larger sprocket to get as much as we could out of the engine within the speed range available. Then fine tune individual gearsets to give best exit speeds and launches to the next corner. And yes Mark, spending more time at the top of the power band made a HUGE difference in the real world.
07-06-2007, 07:18 PM
#93
Rennlist Member
No Jim, we are all friends here and i respect your opinions and wouldnt think that you would intentially mislead anyone. Let me be very clear about that.
in the bolt on performance world, folks get all hopped up about all sorts of things. Heck, im as guilty as the next guy. However, i was only trying to add some science to the decision making process about what actually happens wen the rubber meets the road. Lower gears are always not "better", there are trade offs. thats my only point.
for street driving, having driven a 2.75, it is kind of cool to run around at the lower speeds at some higher rpms. I like shifting, and having reasons to shift.
mk
in the bolt on performance world, folks get all hopped up about all sorts of things. Heck, im as guilty as the next guy. However, i was only trying to add some science to the decision making process about what actually happens wen the rubber meets the road. Lower gears are always not "better", there are trade offs. thats my only point.
for street driving, having driven a 2.75, it is kind of cool to run around at the lower speeds at some higher rpms. I like shifting, and having reasons to shift.
mk
Originally Posted by Jim bailey - 928 International
Your comment "you are being completey misleading to a guy that is considering a 2.7 rear end bolt on to a 2.2, as it may not do any of the things you guarantee with your position." might be considered a comment on my ethics/ honesty / integrity, I hope that was not your intent.
07-06-2007, 07:22 PM
#94
Rennlist Member
And you know i agree with this, more than you know!
I still have arguments with my BMW racer buddies about this topic. I still win races over them, partly because they dont get the most out of their engines. they still think when the torque is falling, its time to shift! If they listened, as it is all about maximizing HP, they might end up ahead of me! However they would need to give up the torque curve analysis and look at the HP curve!
mk
I still have arguments with my BMW racer buddies about this topic. I still win races over them, partly because they dont get the most out of their engines. they still think when the torque is falling, its time to shift! If they listened, as it is all about maximizing HP, they might end up ahead of me! However they would need to give up the torque curve analysis and look at the HP curve!
mk
Originally Posted by dr bob
And yes Mark, spending more time at the top of the power band made a HUGE difference in the real world.
Last edited by mark kibort; 07-06-2007 at 08:20 PM.
07-06-2007, 08:37 PM
#96
Rennlist Member
mph closer, yes, but thats not the point or the correct terminology definition. Percentage is what we are talking about. Gear spacing doesnt change with the rear end change. you could have all gears max out at 90mph but the spacing would still be the same. however you would be shifting a lot in the beginning, but the spacing where you care about it would be the same (ie 44mph, 63mph 93mph, etc)
The 2.75 on an S4, is a very interesting gear box for me as my car sits now.
most of my fastest straights are near 125mph. But, because most of my time is spent in 3rd gear in the 90 to 115 rang, and many of the curves need a strong exit speed from 80 to 100mph, the 2.2 mix of geas work better. this would change if I had 500hp, as those kind of variables determine the match effectiveness for a particular track layout.
i spend the lion share of the time on the race track, in 3rd gear, especially near the top of 3rd gear. If i had to use 4th, i would be at a huge disavantage in areas where i spend most of my time. more things to look at before making a desision. I would hope, if i competed with anyone in a 928, that they had a 2.72 ring and pinion. they would be easy pickings at Laguna and probably a wash at Sears.
mk
The 2.75 on an S4, is a very interesting gear box for me as my car sits now.
most of my fastest straights are near 125mph. But, because most of my time is spent in 3rd gear in the 90 to 115 rang, and many of the curves need a strong exit speed from 80 to 100mph, the 2.2 mix of geas work better. this would change if I had 500hp, as those kind of variables determine the match effectiveness for a particular track layout.
i spend the lion share of the time on the race track, in 3rd gear, especially near the top of 3rd gear. If i had to use 4th, i would be at a huge disavantage in areas where i spend most of my time. more things to look at before making a desision. I would hope, if i competed with anyone in a 928, that they had a 2.72 ring and pinion. they would be easy pickings at Laguna and probably a wash at Sears.
mk
Originally Posted by Shane
They do:
Using FlyingDogs .xls spreadsheet here are the results, which clearly show that 1-4 is indeed closer together:
So for my level of driving around where I drive and at the HP level my cars have, it makes all my gears more useable, then the 2.20 does.
I mean I could top out fourth gear now at 123 mph without really risking a lengthy jail sentence, where as if I top out fourth gear in a 2.20 I'd be doing 152mph and I could totally forget about getting anywhere near redline with only 300rwhp.
Using FlyingDogs .xls spreadsheet here are the results, which clearly show that 1-4 is indeed closer together:
So for my level of driving around where I drive and at the HP level my cars have, it makes all my gears more useable, then the 2.20 does.
I mean I could top out fourth gear now at 123 mph without really risking a lengthy jail sentence, where as if I top out fourth gear in a 2.20 I'd be doing 152mph and I could totally forget about getting anywhere near redline with only 300rwhp.
06-13-2008, 08:23 PM
#97
Burning Brakes
Join Date: Jun 2008
Location: NY
Posts: 909
Likes: 0
Received 0 Likes
on
0 Posts
Mark when I drag raced my 416 cu in small block street car(was 4inch stroker) stock stroke was 3.31)340 mopar motor stroked to 416cu in.
60foot times were best of 1.53
and 1/8mile was best of 7.06
car was only setup for 1/4mile top speed of 125mph.
Formula for standard shift was
(rpm*tire height)/340*rear gear ratio
To use same formula you can do it for each gear as
Formula for standard shift was
(rpm*tire height)/340*rear gear ratio*(each transmission gear)
example 6200rpm*24.5 tire/355*rear gear ratio
fixed number (355 for automatic car) (340 for manual shift cars)(some books say 336 instead of the 340 number)
so looking at a stock s4 lets say auto car with 2.20 rear ratio and a 1st gear of 3.68
(6200rpm*24.5")/(355*2.20*3.68) 3.68 if being 1st gear in an auto car?
That would be 151900/2874.08=52.85mph 1st gear
2nd gear in auto car if being 2.41
(6200rpm*24.5"tire)/(355*2.20*2.41)=80.70mph
3rd gear in auto car if being 1.44
(6200rpm*24.5"tire)/(355*2.20*1.44)=135mph
4th gear in auto car if being 1.00
(5500rpm*24.5"tire)/(355*2.20*1.00)=172mph
I put in around 5500rpm as 4th gear in an automatic car would be hard to pull 6200rpm
You can put in numbers for your tire height and gear ratios to see what you need etc.
You can take tape measure and go around the tire for circumfence and divide by 3.1416
for the tire height in the formula.
Not sure if this will help you may have used it already its just what I have used in the past.
Mark
60foot times were best of 1.53
and 1/8mile was best of 7.06
car was only setup for 1/4mile top speed of 125mph.
Formula for standard shift was
(rpm*tire height)/340*rear gear ratio
To use same formula you can do it for each gear as
Formula for standard shift was
(rpm*tire height)/340*rear gear ratio*(each transmission gear)
example 6200rpm*24.5 tire/355*rear gear ratio
fixed number (355 for automatic car) (340 for manual shift cars)(some books say 336 instead of the 340 number)
so looking at a stock s4 lets say auto car with 2.20 rear ratio and a 1st gear of 3.68
(6200rpm*24.5")/(355*2.20*3.68) 3.68 if being 1st gear in an auto car?
That would be 151900/2874.08=52.85mph 1st gear
2nd gear in auto car if being 2.41
(6200rpm*24.5"tire)/(355*2.20*2.41)=80.70mph
3rd gear in auto car if being 1.44
(6200rpm*24.5"tire)/(355*2.20*1.44)=135mph
4th gear in auto car if being 1.00
(5500rpm*24.5"tire)/(355*2.20*1.00)=172mph
I put in around 5500rpm as 4th gear in an automatic car would be hard to pull 6200rpm
You can put in numbers for your tire height and gear ratios to see what you need etc.
You can take tape measure and go around the tire for circumfence and divide by 3.1416
for the tire height in the formula.
Not sure if this will help you may have used it already its just what I have used in the past.
Mark
