Winter Drag loss project
12-01-2007, 01:51 PM
#31
Rennlist Member
so, your question is how do you get more acceleration in your 5th gear, as rigth now you are pulling hard to 6800rpm in 4th? you are at 6000rpm in 4th at 120mph and , as you say, you pull away from a 996? so, what's the problem. just keep it in 4th to 6800rpm and you maximize you acceleration. 136mph would be achieved at that point. Hey, you need to shift, and aero will not do anything noticable here, besides some stability and grip if you increase downforce. the costs would not be that much as i have mentioned in the earlier post.
If your objective is to be able to "short shift" into 5th , then the only thing you can do is lower the overall gearing, or use a smaller diameter tire. those gains would be slight and have trade offs as do most all gear changes. might be good (or slightly better) for VIR, but bad for RA. its all about maximizing time spent near max HP. if a lap at VIR would have too much time spent in the lower rpms of 5th, then you might need to change the rear end to shift all the gears down, or smaller rear tires for a subtle change, or find a way to make 5th closer to 4th. (and that usually is cost prohibitive.)
Best advice is to make a splitter set up that fits your look expectations, and add some downforce in the rear. dont worry about drag and speed costs, you wont find any. I have gone from a stock body, have slowly added downforce devices and by using telemetry data, my top speeds have not changed one MPH, yet handling has improved. again, based on the data collected on the drag forces of the wing for example (GT3cup wing), its cost is near 2ft-lbs of engine torque at 100mph. barely a rounding error!
MK
you said that
If your objective is to be able to "short shift" into 5th , then the only thing you can do is lower the overall gearing, or use a smaller diameter tire. those gains would be slight and have trade offs as do most all gear changes. might be good (or slightly better) for VIR, but bad for RA. its all about maximizing time spent near max HP. if a lap at VIR would have too much time spent in the lower rpms of 5th, then you might need to change the rear end to shift all the gears down, or smaller rear tires for a subtle change, or find a way to make 5th closer to 4th. (and that usually is cost prohibitive.)
Best advice is to make a splitter set up that fits your look expectations, and add some downforce in the rear. dont worry about drag and speed costs, you wont find any. I have gone from a stock body, have slowly added downforce devices and by using telemetry data, my top speeds have not changed one MPH, yet handling has improved. again, based on the data collected on the drag forces of the wing for example (GT3cup wing), its cost is near 2ft-lbs of engine torque at 100mph. barely a rounding error!
MK
you said that
A number of us were discussing the top speed of a 911 - specifically 74 through 89 body. https://rennlist.com/forums/showthread.php?t=393305
My speedometer is fairly accurate now, due in a large part to taller then standard rear tires (255/40-17). At VIR the car will pull strongly out of Oak Tree and will pull away from a 996 until about 120, 6000 in fourth. It will continue to pull strongly to 6800, shifting into fifth produces little that could be considered acceleration. Fifth gear equivalent rpm is about 5500. Shifting at lower rpm results in lower speed, suggesting that fifth gear is too high.
Looking for means to raise this limit. Unfortunately I would also like to reduce lift or increase downforce. The car feels too light above 125. (not a normal SC, 95 993 engine and other mods)
My speedometer is fairly accurate now, due in a large part to taller then standard rear tires (255/40-17). At VIR the car will pull strongly out of Oak Tree and will pull away from a 996 until about 120, 6000 in fourth. It will continue to pull strongly to 6800, shifting into fifth produces little that could be considered acceleration. Fifth gear equivalent rpm is about 5500. Shifting at lower rpm results in lower speed, suggesting that fifth gear is too high.
Looking for means to raise this limit. Unfortunately I would also like to reduce lift or increase downforce. The car feels too light above 125. (not a normal SC, 95 993 engine and other mods)
Last edited by mark kibort; 12-01-2007 at 02:09 PM.
12-01-2007, 02:26 PM
#32
mark, it sounds like what you're saying is that you've improved handling and grip but not straightaway speed. To me that means you have increased drag since top speed for a given straightaway (not ultimate top speed for the car) should climb as corner exit speed goes up...unless it's a long straight where you reach the ultimate top speed of the car. In that case you should reach peak speed sooner.
12-01-2007, 07:27 PM
#33
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Thread Starter
Excellent information!! Mark thanks for all the research on drag. Very helpful. Combined with the suggestions for lower ratios. Although, I feel the car would go faster if it dropped into fifth higher on the power curve. Currently shifting into fifth does not result in acceleration. Still looking for recommendations on the gear stack. My old speedster had what were called airport gears and acceleration was dramatically with minimal impact on top speed. Current gear set leaves over 1000 rpm on the table; but, is fine for touring with a theoretical 170 top speed.
12-01-2007, 07:37 PM
#34
Race Car
There's no free lunch. To keep the motor in an optimal rev range, you need to close up the rpm drop that comes with each shift. Shortening the gears isn't the goal, really, so much as shortening the gaps when you upshift. The only way to do this is with a lower top speed. Some guys tighten up 2-4 and leave 5th as an overdrive-type gear. This is only good for really short tracks or autocross courses.
If you click on this Willow Springs lap video, you'll see a track that only uses gears 3-5, average speed is just over 100 mph for the complete lap. Slowest point is about 55, highest point is 133 or so. The close-ratio gearing gives me an advantage even at tracks where I briefly run out of gearing and bounce off the limiter in 5th.
If track use is what matters, then you need to look at what's going to contribute to lower lap times more than what sort of top speed boast you're going to be able to make in the paddock. Steve Weiner will no doubt be helpful in choosing the right gear sets for the tracks you drive.
If you click on this Willow Springs lap video, you'll see a track that only uses gears 3-5, average speed is just over 100 mph for the complete lap. Slowest point is about 55, highest point is 133 or so. The close-ratio gearing gives me an advantage even at tracks where I briefly run out of gearing and bounce off the limiter in 5th.
If track use is what matters, then you need to look at what's going to contribute to lower lap times more than what sort of top speed boast you're going to be able to make in the paddock. Steve Weiner will no doubt be helpful in choosing the right gear sets for the tracks you drive.
12-01-2007, 10:05 PM
#35
Something doesn't sound right. A well sorted SC can pull 5th gear. Not very strong but it pulls it nonetheless. You have a Euro gearbox which has a slightly taller 5th but that should be more than made up for and then some by the +90 hp you have over a stock SC. I would expect your car to pull the Euro 5th pretty well. Sure, a shorter 5th may pull better but your claim that your car cannot pull 5th at all may indicate some other issue. It would be interesting to see what a USA SC 5th gear would pull like.
12-01-2007, 11:28 PM
#36
RL Technical Advisor
Hi Alan:
There is some really good information and input here,..
I'll talk to to you this week in detail, but I see two separate subjects here:
1) Aerodynamic management (lift & drag)
2) Gearing to optimize acceleration on the track
Aero changes are made for both drag reduction and lift cancellation/downforce generation. Reducing lift makes the car more stable as well as generating increases in speed. Downforce generation can increase drag, but it usually results in reduced lap times due to higher corner entry & exit speeds.
Close-ratio gears simply keep the engine producing maximum accelerative effort by maintaining RPM between peak torque and peak power production. In almost all cases, optimal gear choices become a compromise unless one has the luxury of affording several transmissions for all the tracks you run.
Gear ratios are chosen by factoring the engine's dyno charts, rear tire diameters, and the tracks that you run for desired top speeds. There is an excellent compromise 915 stack on my website for a 25" tire and a stock-type engine. Race motors with high RPM cams and/or 26" tires need a different solution.
Call me this week,.....
There is some really good information and input here,..
I'll talk to to you this week in detail, but I see two separate subjects here:
1) Aerodynamic management (lift & drag)
2) Gearing to optimize acceleration on the track
Aero changes are made for both drag reduction and lift cancellation/downforce generation. Reducing lift makes the car more stable as well as generating increases in speed. Downforce generation can increase drag, but it usually results in reduced lap times due to higher corner entry & exit speeds.
Close-ratio gears simply keep the engine producing maximum accelerative effort by maintaining RPM between peak torque and peak power production. In almost all cases, optimal gear choices become a compromise unless one has the luxury of affording several transmissions for all the tracks you run.
Gear ratios are chosen by factoring the engine's dyno charts, rear tire diameters, and the tracks that you run for desired top speeds. There is an excellent compromise 915 stack on my website for a 25" tire and a stock-type engine. Race motors with high RPM cams and/or 26" tires need a different solution.
Call me this week,.....
12-02-2007, 02:16 AM
#37
Rennlist Member
They maximize accelerative forces by allowing you to keep rpms as close to max HP as possible. with most cars its between redline and some point lower. Max torque point is not really a data point that is used.
So, yes, matching gear boxes to the tracks is what an unlimited budget could afford you . the rest of us try and pick one that generally hits the marks for most tracks.
Mk
So, yes, matching gear boxes to the tracks is what an unlimited budget could afford you . the rest of us try and pick one that generally hits the marks for most tracks.
Mk
12-02-2007, 02:24 AM
#38
Rennlist Member
good catch. However, the handling hasnt so much improved for entrance, cornering and exit speeds. However, it has made those areas easier to drive.
one thing that i didnt mention is that im missing about 20hp as the dyno says now as well. that could be a reason my top speeds have not increased.
Im a little suspect on the dyno run done last time. The lap times have dropped maybe .5 seconds with the wing and splitter set up. part could be driver over the last couple of years, and some could be due to the improvements of the aero changes. the changes are subtle, and i would be willing to bet that my on edge driving that seems to be controlled better now, results in the same corner speeds, but less tire wear as im not sliding around as much.
bottomline, we are talking about 2ft-lbs of torque lost to my extra 80lbs of downforce at 100mph.
I remember phil McClure at Sears Point World challenge practice, while in the Vet he had done tests with and without the required restrictor plate . They put on a 40% restrictor, it dynoed down about 25hp and his radared turn 1 speed was only down 2mph. thats 25hp folks!!
mk
one thing that i didnt mention is that im missing about 20hp as the dyno says now as well. that could be a reason my top speeds have not increased.
Im a little suspect on the dyno run done last time. The lap times have dropped maybe .5 seconds with the wing and splitter set up. part could be driver over the last couple of years, and some could be due to the improvements of the aero changes. the changes are subtle, and i would be willing to bet that my on edge driving that seems to be controlled better now, results in the same corner speeds, but less tire wear as im not sliding around as much.
bottomline, we are talking about 2ft-lbs of torque lost to my extra 80lbs of downforce at 100mph.
I remember phil McClure at Sears Point World challenge practice, while in the Vet he had done tests with and without the required restrictor plate . They put on a 40% restrictor, it dynoed down about 25hp and his radared turn 1 speed was only down 2mph. thats 25hp folks!!
mk
mark, it sounds like what you're saying is that you've improved handling and grip but not straightaway speed. To me that means you have increased drag since top speed for a given straightaway (not ultimate top speed for the car) should climb as corner exit speed goes up...unless it's a long straight where you reach the ultimate top speed of the car. In that case you should reach peak speed sooner.
