6spd conversion?
10-14-2005, 02:56 PM
#46
"if you are talking about a stock S4 box, or even a GTS 2.75, another gear on top of the one that gets the car to 185mph , wouldnt do much for top end, as the GTS would barely get to 185mph on its best day."
I had no particular 928 model in mind, it doesn't make any difference. Add an overdrive gear to any 928, and it will have a positive and significant effect on mileage, and depending on the engine power curve of the 928 in question, might improve top end as well.
"...as long as it didnt bog too much crusing at 1500rpm at 65mph may just cause cam wear and build up in the cylinders."
Obviously an overdrive for a 928 would not be utilized at 65mph. At 100mph the fuel savings would be significant though.
I had no particular 928 model in mind, it doesn't make any difference. Add an overdrive gear to any 928, and it will have a positive and significant effect on mileage, and depending on the engine power curve of the 928 in question, might improve top end as well.
"...as long as it didnt bog too much crusing at 1500rpm at 65mph may just cause cam wear and build up in the cylinders."
Obviously an overdrive for a 928 would not be utilized at 65mph. At 100mph the fuel savings would be significant though.
10-14-2005, 02:59 PM
#47
Originally Posted by SharkSkin
Note that usually there is no difference in rolling radius. My 17's are within an insignificant RCH of the rolling radius I had with 16's. IIRC, DR's 18" wheelsets also have the same rolling radius, and are lighter to boot -- which should improve acceleration.
All other things being equal you'd need to add approx 50hp to a 928 to get that much of a performance gain. So if you can do it just by a trans swap, well, that's a damned good mod.
10-14-2005, 03:01 PM
#48
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Originally Posted by mark kibort
yes, the rolling radius is pretty close in most wheel changes, however, any weigth savings is insignificant in HP savings. (ie acceleration difference potential)
i often change back and forth between 5 lb ligher tires (toyo vs S03 hoosiers) on the dyno , there is no differnce,and why would there be. acceleration time range is too long to effect HP . 10lbs for both 17-18" diameter masses, accelerating from 700rpm to 1800rpm (ie 4th gear 3000rpm to redline) over 7-8 seconds requires less than 1hp. however, this number goes up significantly in 2nd and 1st gears, to a level of around 5-10hp . remember the reflected inertia is reduced to the engine through the gear box . on a flywheel , that amount of weight , say 5lbs, becauses it is rolling is like having 7lbs in the car in 4th , but more like 50lbs in 1st gear as far as acceleration effects. on the wheel, a rule of thumb is that for every pound of wheel weight, double it for its effects of acceleration as if it was in the car. 5lbs wheel weight acts like 10lbs if it was in the car. 100lbs, in the car for example, acts like 10hp for most cars in our weight/hp range.
Mk
i often change back and forth between 5 lb ligher tires (toyo vs S03 hoosiers) on the dyno , there is no differnce,and why would there be. acceleration time range is too long to effect HP . 10lbs for both 17-18" diameter masses, accelerating from 700rpm to 1800rpm (ie 4th gear 3000rpm to redline) over 7-8 seconds requires less than 1hp. however, this number goes up significantly in 2nd and 1st gears, to a level of around 5-10hp . remember the reflected inertia is reduced to the engine through the gear box . on a flywheel , that amount of weight , say 5lbs, becauses it is rolling is like having 7lbs in the car in 4th , but more like 50lbs in 1st gear as far as acceleration effects. on the wheel, a rule of thumb is that for every pound of wheel weight, double it for its effects of acceleration as if it was in the car. 5lbs wheel weight acts like 10lbs if it was in the car. 100lbs, in the car for example, acts like 10hp for most cars in our weight/hp range.
Mk
10-14-2005, 03:23 PM
#49
Rennlist Member
who said .5 second a lap or .5second reducton on a 1/4 mile. those kind of time changes are for big mods, think of gearing as fine tuning type changes. However, in a 1/4 mile, some big changes on launch can change ets much more than on a race course. actually, ive seen .5 second on a race course change with 20hp (flappy vs no flappy), also 20hp with cam timing changes, however, there have NEVER been this kind of change going from 25 to 30lb tires weights, or 5-10% changes in gear ratios.
Mk
Mk
Originally Posted by m21sniper
.5 second per lap, or a .5 second reduction in 1/4 mile ET is HUGE.
All other things being equal you'd need to add approx 50hp to a 928 to get that much of a performance gain. So if you can do it just by a trans swap, well, that's a damned good mod.
All other things being equal you'd need to add approx 50hp to a 928 to get that much of a performance gain. So if you can do it just by a trans swap, well, that's a damned good mod.
10-14-2005, 03:27 PM
#50
Rennlist Member
at 100mph, its hard to say what ratio would be most efficient as far as fuel consumption. sometimes the lowest rpm is not the most efficient range to operate. the combustion process can rely on higher rpms to get more out of the combusion. Point is, lowest is not nessarily best as far as economy, as the HP required at 100mph is substantially greater than at 65mph. semantics i know, but i think the main spirit of the discussion is about effects on performance of the different gear boxes on our cars.
MK
MK
Originally Posted by m21sniper
"if you are talking about a stock S4 box, or even a GTS 2.75, another gear on top of the one that gets the car to 185mph , wouldnt do much for top end, as the GTS would barely get to 185mph on its best day."
I had no particular 928 model in mind, it doesn't make any difference. Add an overdrive gear to any 928, and it will have a positive and significant effect on mileage, and depending on the engine power curve of the 928 in question, might improve top end as well.
"...as long as it didnt bog too much crusing at 1500rpm at 65mph may just cause cam wear and build up in the cylinders."
Obviously an overdrive for a 928 would not be utilized at 65mph. At 100mph the fuel savings would be significant though.
I had no particular 928 model in mind, it doesn't make any difference. Add an overdrive gear to any 928, and it will have a positive and significant effect on mileage, and depending on the engine power curve of the 928 in question, might improve top end as well.
"...as long as it didnt bog too much crusing at 1500rpm at 65mph may just cause cam wear and build up in the cylinders."
Obviously an overdrive for a 928 would not be utilized at 65mph. At 100mph the fuel savings would be significant though.
10-14-2005, 03:27 PM
#51
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On the fuel mileage comment, it depends on the load of the engine.. You can have the engine turning at a lower RPM and be under more load and consume more fuel.. Mileage is not a linear function of RPM..
The OD of my 18 inch wheels is the same as the OD on my 16 inch wheels. In addition, the 18 inchers are lighter..
The 6 speed moving the gearset closer so the max HP can be applied would be better. However, I have to agree with MK on the physics and the math.. The "feels" faster just doesn't cut it.. It would be an amazing feat to "feel" 5hp...
It has been rumored that the internals of the Mercedes 722 %speed Auto can fit into a 928.. If anyone has further info on this please post it.. A well as the AMG 4 spd that can take the 1000 HP/TQ..
Thanks,
The OD of my 18 inch wheels is the same as the OD on my 16 inch wheels. In addition, the 18 inchers are lighter..
The 6 speed moving the gearset closer so the max HP can be applied would be better. However, I have to agree with MK on the physics and the math.. The "feels" faster just doesn't cut it.. It would be an amazing feat to "feel" 5hp...
It has been rumored that the internals of the Mercedes 722 %speed Auto can fit into a 928.. If anyone has further info on this please post it.. A well as the AMG 4 spd that can take the 1000 HP/TQ..
Thanks,
10-14-2005, 03:28 PM
#52
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Correction,
It has been rumored that the internals of the Mercedes 722 5 speed Auto can fit into a 928.. If anyone has further info on this please post it.. A well as the AMG 4 spd that can take the 1000 HP/TQ..
It has been rumored that the internals of the Mercedes 722 5 speed Auto can fit into a 928.. If anyone has further info on this please post it.. A well as the AMG 4 spd that can take the 1000 HP/TQ..
10-14-2005, 03:37 PM
#53
Originally Posted by mark kibort
who said .5 second a lap or .5second reducton on a 1/4 mile. those kind of time changes are for big mods, think of gearing as fine tuning type changes. However, in a 1/4 mile, some big changes on launch can change ets much more than on a race course. actually, ive seen .5 second on a race course change with 20hp (flappy vs no flappy), also 20hp with cam timing changes, however, there have NEVER been this kind of change going from 25 to 30lb tires weights, or 5-10% changes in gear ratios.
Mk
Mk
10-14-2005, 03:39 PM
#54
Originally Posted by blau928
I have to agree with MK on the physics and the math.. The "feels" faster just doesn't cut it.. It would be an amazing feat to "feel" 5hp...
10-14-2005, 03:43 PM
#55
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my laps are pretty close. no more than .2 seconds variation when im hitting my marks. to be .5 seconds faster or .5 seconds slower, would be due to something major. If you are hitting your marks, there is not much else going on.
I agree that on drag cars, that the launch is probably making much of this differnence, or the top speed hitting redline can be a big deal, especially for a 2.73 to 3.73 change . this is a HUGE change in gear ratios. actually, this would be like having an extra gear before 1st! but all other gears would be the same, so a launch gain, right!? gear spaciing is around .72 and thats the difference of a 2.73 to a 3.73. you dont make the gears closer, you just add a extra starting gear!. however, changes in between can effect the last 20mph of acceleration. you want to be hitting redline as you hit the finish (ideally)
Im not a big drag race guy, but this is the general idea.
Mk
I agree that on drag cars, that the launch is probably making much of this differnence, or the top speed hitting redline can be a big deal, especially for a 2.73 to 3.73 change . this is a HUGE change in gear ratios. actually, this would be like having an extra gear before 1st! but all other gears would be the same, so a launch gain, right!? gear spaciing is around .72 and thats the difference of a 2.73 to a 3.73. you dont make the gears closer, you just add a extra starting gear!. however, changes in between can effect the last 20mph of acceleration. you want to be hitting redline as you hit the finish (ideally)
Im not a big drag race guy, but this is the general idea.
Mk
Originally Posted by m21sniper
A road course is a lot harder to go by because there's so much at play. Miss one corner or nail one better than usual, and that can cause a 1 sec or more diff. But yes, i have seen many, many, many drag cars shave .5 sec and more via gearbox changes, without touching anything else. The mustang retards always swap up from the stock 2.73s(or 3.08s or even 3.23s) to the 3.56 or 3.73s, and it always makes a nice difference in ET.
10-15-2005, 04:00 PM
#57
Originally Posted by mark kibort
my laps are pretty close. no more than .2 seconds variation when im hitting my marks. to be .5 seconds faster or .5 seconds slower, would be due to something major. If you are hitting your marks, there is not much else going on.
Originally Posted by mark kibort
I agree that on drag cars, that the launch is probably making much of this differnence, or the top speed hitting redline can be a big deal, especially for a 2.73 to 3.73 change . this is a HUGE change in gear ratios. actually, this would be like having an extra gear before 1st! but all other gears would be the same, so a launch gain, right!?
What you're doing is shifting the entire gear range down one full point by going from 2.73 to 3.73.
When you say gear ratio changes don't make a difference you have to realize that often we are talking about a HUGE change in final drive(almost invariably lower), and that it really does transform the performance of the car for the type of performance THEY'RE shooting for.
. however, changes in between can effect the last 20mph of acceleration. you
"gear spaciing is around .72 and thats the difference of a 2.73 to a 3.73. you dont make the gears closer, you just add a extra starting gear!"
Again, what you're doing is shifting the entire gear set down one full point. 5th now pulls about as hard as 3d did before the swap, but at a much lower speed. You're getting all your gearing at lower speeds, making the effective closeness of the gears much, much better for standing start 0-100 type acceleration(ie such as is desired in a drag car).
10-16-2005, 01:55 AM
#59
Rennlist Member
Thanks!
Re-read the last part of your post. when you do a .73 rear end change, you do shift all the gears down one point, but you DONT change the spacing. So, as i said, you now have a .5 of 1st gear, then your 2-5th all become exactly the same as 1st through 4th. (and you loose 5th) so, not really very much is changed, except in drag racing, you could have a much better launch, but everything after that would be identical. dont get caught up in the numerical designators of the gears. look at the absolute final ratios. since most all of our gears are about .72 spacing, you can see how this kind of a rear end change doesnt do very much, and certainly doesnt change the "closeness of the gears" that is fixed by the transmission ratios.
MK
Re-read the last part of your post. when you do a .73 rear end change, you do shift all the gears down one point, but you DONT change the spacing. So, as i said, you now have a .5 of 1st gear, then your 2-5th all become exactly the same as 1st through 4th. (and you loose 5th) so, not really very much is changed, except in drag racing, you could have a much better launch, but everything after that would be identical. dont get caught up in the numerical designators of the gears. look at the absolute final ratios. since most all of our gears are about .72 spacing, you can see how this kind of a rear end change doesnt do very much, and certainly doesnt change the "closeness of the gears" that is fixed by the transmission ratios.
MK
Originally Posted by m21sniper
You're not exactly the average race bear either...
Actually all the gears would be a full point lower, 1st straight up through 5th. Essentially 2d becomes first(roughly), straight up through the line.
What you're doing is shifting the entire gear range down one full point by going from 2.73 to 3.73.
When you say gear ratio changes don't make a difference you have to realize that often we are talking about a HUGE change in final drive(almost invariably lower), and that it really does transform the performance of the car for the type of performance THEY'RE shooting for.
. however, changes in between can effect the last 20mph of acceleration. you
"gear spaciing is around .72 and thats the difference of a 2.73 to a 3.73. you dont make the gears closer, you just add a extra starting gear!"
Again, what you're doing is shifting the entire gear set down one full point. 5th now pulls about as hard as 3d did before the swap, but at a much lower speed. You're getting all your gearing at lower speeds, making the effective closeness of the gears much, much better for standing start 0-100 type acceleration(ie such as is desired in a drag car).
Actually all the gears would be a full point lower, 1st straight up through 5th. Essentially 2d becomes first(roughly), straight up through the line.
What you're doing is shifting the entire gear range down one full point by going from 2.73 to 3.73.
When you say gear ratio changes don't make a difference you have to realize that often we are talking about a HUGE change in final drive(almost invariably lower), and that it really does transform the performance of the car for the type of performance THEY'RE shooting for.
. however, changes in between can effect the last 20mph of acceleration. you
"gear spaciing is around .72 and thats the difference of a 2.73 to a 3.73. you dont make the gears closer, you just add a extra starting gear!"
Again, what you're doing is shifting the entire gear set down one full point. 5th now pulls about as hard as 3d did before the swap, but at a much lower speed. You're getting all your gearing at lower speeds, making the effective closeness of the gears much, much better for standing start 0-100 type acceleration(ie such as is desired in a drag car).
10-17-2005, 02:35 PM
#60
"So, as i said, you now have a .5 of 1st gear, then your 2-5th all become exactly the same as 1st through 4th. (and you loose 5th)"
Precisely.
"so, not really very much is changed, except in drag racing, you could have a much better launch, but everything after that would be identical."
Well a lot is changed, 2d now acts as 1st did, and 3d is as 2d was, etc.
So at any speed up to about 100mph you're going to get much harder acceleration gear for gear(ie 2d in the 3.73 car vs 2d in the 2.73 car). It doesn't just work for first, but for all the gears.
Of course all that wonderful acceleration comes at the cost of a lot of top end, but drag racers dont really care about that sort of thing.
Getting to redline in top gear by the end of the 1/4 is all that matters.
Precisely.
"so, not really very much is changed, except in drag racing, you could have a much better launch, but everything after that would be identical."
Well a lot is changed, 2d now acts as 1st did, and 3d is as 2d was, etc.
So at any speed up to about 100mph you're going to get much harder acceleration gear for gear(ie 2d in the 3.73 car vs 2d in the 2.73 car). It doesn't just work for first, but for all the gears.
Of course all that wonderful acceleration comes at the cost of a lot of top end, but drag racers dont really care about that sort of thing.
Getting to redline in top gear by the end of the 1/4 is all that matters.