Would 944 5th Gears Fit a 951 Transmission
02-11-2003, 08:13 PM
#1
Three Wheelin'
Thread Starter
Would 944 5th Gears Fit a 951 Transmission
I recently developed a spreadsheet that integrates dynamometer torque curves as a method of comparing the performance of different cars. One of the things I found is that stock 951/951S cars are drag limited, but the bigger horsepower modified cars are rpm limited. Looking at the acceleration curves by gear, it appears that part of the problem is that the 951 5th gear ratio is too close to 4th gear (0.829 VS 1.034) A taller 5th gear allows the 4th/5th shift to be closer to the engine's torque peak, not slowing the car significantly but increasing the top end.
Then I remembered that the US model 944 NA got a "fuel efficient" 5th gear between '85.5 and '87 (016K transmissions) that they didn't really have the torque to use well. The gears are 37:27 (0.73 ratio), and would complement a modified 951 very well. This would allow a true 190 mph at the stock 6,450 redline, which any well-modified 951 with a Garrett or KKK27 turbo should be able to hit. Even a stock 951 with a Guru Stage 2 MAP kit should be drag limited at about 175 mph, rather than rev limited at 166 mph.
Does anyone know if these gears would fit a 951 transmission? Would they be strong enough? The 5th gears from a 944S or S2 would be another option at 0.78:1, iirc. This is all hypothetical for me, but may be of interest to those of you with big torque (HP) cars.
Standard Safety Disclaimer: I would not encourage anyone to go 190 mph in a car with street tires. Obey all speed limits. Brush your teeth after every meal. <img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" />
Then I remembered that the US model 944 NA got a "fuel efficient" 5th gear between '85.5 and '87 (016K transmissions) that they didn't really have the torque to use well. The gears are 37:27 (0.73 ratio), and would complement a modified 951 very well. This would allow a true 190 mph at the stock 6,450 redline, which any well-modified 951 with a Garrett or KKK27 turbo should be able to hit. Even a stock 951 with a Guru Stage 2 MAP kit should be drag limited at about 175 mph, rather than rev limited at 166 mph.
Does anyone know if these gears would fit a 951 transmission? Would they be strong enough? The 5th gears from a 944S or S2 would be another option at 0.78:1, iirc. This is all hypothetical for me, but may be of interest to those of you with big torque (HP) cars.
Standard Safety Disclaimer: I would not encourage anyone to go 190 mph in a car with street tires. Obey all speed limits. Brush your teeth after every meal. <img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" />
02-11-2003, 09:02 PM
#2
Burning Brakes
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It seems to me that there are alot of shops out there that will do custom hear ratios for porsche trannies. (I remember seeing ads in Panorama, etc.) I'm sure one of them would have the appropriatly strong gear for the 951. If you're going to go through the trouble of rebuilding the your turbo tranny, you might as well pay a little extra and find a gear from someone that does custom ones regularly. It can't cost TOO much, can it?
02-11-2003, 10:03 PM
#3
RL Community Team
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<a href="http://www.weissach.net/924-944-968_Suppliers.html#TransmissionsGearsLSD" target="_blank">http://www.weissach.net/924-944-968_Suppliers.html#TransmissionsGearsLSD</a>
02-12-2003, 12:04 AM
#5
Race Director
Takes about 2 hours to swap out the 5th gear for the NA one. Since the torque-multiplication is less in the taller gears, you don't have to worry about strength (except for 1st & 2nd).
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... don't lose those springs.... <img border="0" title="" alt="[Wink]" src="wink.gif" />
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02-12-2003, 02:27 PM
#9
Three Wheelin'
Thread Starter
Since my car is mostly stock, this is only a concept for me. I haven't seen sites offering custom gearing for the 944/951 transmissions, so it is nice to know that this could be done using stock Porsche gears from the N/A transmissions.
Luke, there is no way that I would take a car to 190 mph, period. However, I wouldn't do many of the things that people on this board have done (or at least have claimed to ....)
Nerf, one of the problems with this idea is that you would have to invest in an accurate speedometer before even trying it; cost more than modifying the transmission! As Danno will tell you, you can bury the needle of a 170 mph or 180 mph speedometer with very little in the way of modifications using stock gearing, due to the 7% or 8% optimism they have built into them.
Raising the rev limit to 7,000 rpm would get you to a true 180 mph with stock gearing. However, most of the torque curves I have seen for turbos with 2 valve heads are dropping before 7,000 rpm no matter how big the turbo. Taller gearing would probably get you a bit more speed (for what, I do not know; maybe our German brethren could use it on the Autobahn.) A true 190 mph is realistic, if scary. Ian Manning's car, for example, is running a Garrett T04E and has attention paid to its breathing. At 18 psi he pulled over 320 hp and 320 ft-lbs. Using his torque curve, he could hit a true 180 mph at 7,000 rpm. Using a 0.73:1 5th gear, he would be drag limited at 187 mph true speed at 6,350 rpm. (He has opened up his exhaust system since that dyno run, so could probably hit 190 mph now with taller gears.)
Luke, there is no way that I would take a car to 190 mph, period. However, I wouldn't do many of the things that people on this board have done (or at least have claimed to ....)
Nerf, one of the problems with this idea is that you would have to invest in an accurate speedometer before even trying it; cost more than modifying the transmission! As Danno will tell you, you can bury the needle of a 170 mph or 180 mph speedometer with very little in the way of modifications using stock gearing, due to the 7% or 8% optimism they have built into them.
Raising the rev limit to 7,000 rpm would get you to a true 180 mph with stock gearing. However, most of the torque curves I have seen for turbos with 2 valve heads are dropping before 7,000 rpm no matter how big the turbo. Taller gearing would probably get you a bit more speed (for what, I do not know; maybe our German brethren could use it on the Autobahn.) A true 190 mph is realistic, if scary. Ian Manning's car, for example, is running a Garrett T04E and has attention paid to its breathing. At 18 psi he pulled over 320 hp and 320 ft-lbs. Using his torque curve, he could hit a true 180 mph at 7,000 rpm. Using a 0.73:1 5th gear, he would be drag limited at 187 mph true speed at 6,350 rpm. (He has opened up his exhaust system since that dyno run, so could probably hit 190 mph now with taller gears.)
02-13-2003, 03:59 AM
#10
Race Director
"Taller gearing would probably get you a bit more speed"
Most likely not, unless you have sufficient power to drive that taller 5th gear. Here is Ian's chart with a curve I drew in that could be known as "power-required" for the stock 5th gear:
<img src="http://www.gururacing.net/ImagesMisc/DynoIanManningTopSpeed.gif" alt=" - " />
Listed at 500rpm increments are the speeds in top gear along with the power required to achieve that speed. Air resistance goes up with the square of velocity and the power-required to overcome that resistance increase to the cube power. Meaning, if you want to go twice as fast, you'll need 8x the power!
It's power, not torque that determines top-speed and Ian's isn't quite sufficient to redline top-gear at 7000rpm. Due to the dropping power-curve, it's power at redline, not peak that counts. He might be able to squeeze 180mph @ 6900rpm.
If you add the taller 5th gear from a 944NA, you'll have to raise blue "power-required" curve by 33%. So in order to hit that same 180mph as before, you have to generate that same 305rwhp at only 6100rpm now. In this case, it works in Ian's favor because he'll have excess power at that RPM.
Top-speed with that taller 5th will be 210mph @ 7000rpm. In order to actually get that top-speed, you'll need a power-curve that generates at least 460rwhp @ 7000rpm. Accounting for the dropping HP-curve, that means around 490-525rwhp @ the power-peak.
Most likely not, unless you have sufficient power to drive that taller 5th gear. Here is Ian's chart with a curve I drew in that could be known as "power-required" for the stock 5th gear:
<img src="http://www.gururacing.net/ImagesMisc/DynoIanManningTopSpeed.gif" alt=" - " />
Listed at 500rpm increments are the speeds in top gear along with the power required to achieve that speed. Air resistance goes up with the square of velocity and the power-required to overcome that resistance increase to the cube power. Meaning, if you want to go twice as fast, you'll need 8x the power!
It's power, not torque that determines top-speed and Ian's isn't quite sufficient to redline top-gear at 7000rpm. Due to the dropping power-curve, it's power at redline, not peak that counts. He might be able to squeeze 180mph @ 6900rpm.
If you add the taller 5th gear from a 944NA, you'll have to raise blue "power-required" curve by 33%. So in order to hit that same 180mph as before, you have to generate that same 305rwhp at only 6100rpm now. In this case, it works in Ian's favor because he'll have excess power at that RPM.
Top-speed with that taller 5th will be 210mph @ 7000rpm. In order to actually get that top-speed, you'll need a power-curve that generates at least 460rwhp @ 7000rpm. Accounting for the dropping HP-curve, that means around 490-525rwhp @ the power-peak.
02-13-2003, 02:29 PM
#11
Three Wheelin'
Thread Starter
Danno, I didn't mean to imply that Ian would have the power to drive a taller 5th gear to redline; that would take too much horsepower, as you pointed out. However, since his HP curve peaks just over 6,000 rpm he does get some benefit from taller gearing that gets him nearer this power peak at his maximum speed.
Since his dyno curve stops at 6,800 rpm, I assumed that would be his redline (matter of choice on his part.) With his stock gearing, his acceleration by gear looks like this:
<img src="http://boards.rennlist.com/upload/IanM-Stock5th.jpg" alt=" - " />
First gear is useless and second gear is marginal at full boost. Fourth and fifth are close together, with top speed limited to 177 mph by the assumed 6,800 rpm redline.
With the 0.73:1 fifth gear ratio from the N/A, his gearing looks like this:
<img src="http://boards.rennlist.com/upload/IanM-0.73_5th.jpg" alt=" - " />
He doesn't have the power to reach redline in fifth gear, but he does reach 187 mph before drag limits him. He still shifts to 5th above his peak torque, so acceleration in 5th is not much less than with the lower gearing.
I plugged in a number of gear ratios for fifth:
0.83:1 (stock) 177 mph@6,800 rpm, rev limited
0.78:1 (944S/S2) 186 mph@6,733 rpm, drag limited
0.73:1 (US944NA) 187 mph@6350 rpm, drag limited
The top speed does go up 1 mph with 0.73:1 gears over 0.78:1 gears, since his RWHP is slowly dropping in this rpm range. The effect is slight, however, and at the power level he had at the time of his dyno run, the 0.78:1 ratio would probably be his best choice. For cars running truly big horsepower (400+ rwhp), the taller gears would likely be the better choice, depending on the shape of their power curve at high rpm. Of course, most of those cars are built for track use, so top speed has lttle appeal for them.
Since his dyno curve stops at 6,800 rpm, I assumed that would be his redline (matter of choice on his part.) With his stock gearing, his acceleration by gear looks like this:
<img src="http://boards.rennlist.com/upload/IanM-Stock5th.jpg" alt=" - " />
First gear is useless and second gear is marginal at full boost. Fourth and fifth are close together, with top speed limited to 177 mph by the assumed 6,800 rpm redline.
With the 0.73:1 fifth gear ratio from the N/A, his gearing looks like this:
<img src="http://boards.rennlist.com/upload/IanM-0.73_5th.jpg" alt=" - " />
He doesn't have the power to reach redline in fifth gear, but he does reach 187 mph before drag limits him. He still shifts to 5th above his peak torque, so acceleration in 5th is not much less than with the lower gearing.
I plugged in a number of gear ratios for fifth:
0.83:1 (stock) 177 mph@6,800 rpm, rev limited
0.78:1 (944S/S2) 186 mph@6,733 rpm, drag limited
0.73:1 (US944NA) 187 mph@6350 rpm, drag limited
The top speed does go up 1 mph with 0.73:1 gears over 0.78:1 gears, since his RWHP is slowly dropping in this rpm range. The effect is slight, however, and at the power level he had at the time of his dyno run, the 0.78:1 ratio would probably be his best choice. For cars running truly big horsepower (400+ rwhp), the taller gears would likely be the better choice, depending on the shape of their power curve at high rpm. Of course, most of those cars are built for track use, so top speed has lttle appeal for them.
02-13-2003, 02:38 PM
#12
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</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by Waterguy:
<strong>I recently developed a spreadsheet that integrates dynamometer torque curves as a method of comparing the performance of different cars. One of the things I found is that stock 951/951S cars are drag limited, but the bigger horsepower modified cars are rpm limited. Looking at the acceleration curves by gear, it appears that part of the problem is that the 951 5th gear ratio is too close to 4th gear (0.829 VS 1.034) A taller 5th gear allows the 4th/5th shift to be closer to the engine's torque peak, not slowing the car significantly but increasing the top end.
Then I remembered that the US model 944 NA got a "fuel efficient" 5th gear between '85.5 and '87 (016K transmissions) that they didn't really have the torque to use well. The gears are 37:27 (0.73 ratio), and would complement a modified 951 very well. This would allow a true 190 mph at the stock 6,450 redline, which any well-modified 951 with a Garrett or KKK27 turbo should be able to hit. Even a stock 951 with a Guru Stage 2 MAP kit should be drag limited at about 175 mph, rather than rev limited at 166 mph.
Does anyone know if these gears would fit a 951 transmission? Would they be strong enough? The 5th gears from a 944S or S2 would be another option at 0.78:1, iirc. This is all hypothetical for me, but may be of interest to those of you with big torque (HP) cars.
Standard Safety Disclaimer: I would not encourage anyone to go 190 mph in a car with street tires. Obey all speed limits. Brush your teeth after every meal. <img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" /> </strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">At the risk of being cantankerous...who the hell cares...are you running the thing on the banking at Daytona?
<strong>I recently developed a spreadsheet that integrates dynamometer torque curves as a method of comparing the performance of different cars. One of the things I found is that stock 951/951S cars are drag limited, but the bigger horsepower modified cars are rpm limited. Looking at the acceleration curves by gear, it appears that part of the problem is that the 951 5th gear ratio is too close to 4th gear (0.829 VS 1.034) A taller 5th gear allows the 4th/5th shift to be closer to the engine's torque peak, not slowing the car significantly but increasing the top end.
Then I remembered that the US model 944 NA got a "fuel efficient" 5th gear between '85.5 and '87 (016K transmissions) that they didn't really have the torque to use well. The gears are 37:27 (0.73 ratio), and would complement a modified 951 very well. This would allow a true 190 mph at the stock 6,450 redline, which any well-modified 951 with a Garrett or KKK27 turbo should be able to hit. Even a stock 951 with a Guru Stage 2 MAP kit should be drag limited at about 175 mph, rather than rev limited at 166 mph.
Does anyone know if these gears would fit a 951 transmission? Would they be strong enough? The 5th gears from a 944S or S2 would be another option at 0.78:1, iirc. This is all hypothetical for me, but may be of interest to those of you with big torque (HP) cars.
Standard Safety Disclaimer: I would not encourage anyone to go 190 mph in a car with street tires. Obey all speed limits. Brush your teeth after every meal. <img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" /> </strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">At the risk of being cantankerous...who the hell cares...are you running the thing on the banking at Daytona?
