Planet-9 Porsche Forum banner

1 - 17 of 17 Posts

·
Registered
Joined
·
1,758 Posts
Discussion Starter #1 (Edited)
I took a 2.9 for a test drive. It seemed to really shine in 2nd gear from 40mph to 60mph, and in 3rd gear from 70mph to 90mph.

Can you guys chime in and give me some idea of the conversion to mph, for each engine's sweet spot in the RPM's range?

I was just curious where the real pep is in each engine, as it relates to daily driving... in 1st, 2nd, 3rd, 4th gear (4th gear is already way too fast).

How do you guys rate the Cayman and Cayman S in gearing, and being in the right RPM to pull off a quick pass on the highway (55-75mph speed limit) vs driving in town (35-45mph range) ... did Porsche get the gearing and rpm's right, for fun on the public road?

2.7-
2.9-
3.4-
3.4 gen. II -
 

·
Registered
Joined
·
182 Posts
I'm no expert but I like being above 3800rpm when passing. It's a good rpm in any gear for quick throttle response. Mine is a gen 1, 06CS with Softronics remap and other mods though; not stock.
 

·
Registered
Joined
·
1,758 Posts
Discussion Starter #3 (Edited)
I'm no expert but I like being above 3800rpm when passing.
So above 3800rpm's in 2nd or 3rd gear equates to how many mph on a 3.4 2006?

Everyone talks about HP, torque, and rpm's.... I'm trying to convert this into mph. (for example... and this is just a guess?)

2nd gear in a...
2.7 hits its best acceleration between 40mph and 55mph
2.9 hits its best acceleration range between 45mph and 60mph
3.4 hits its best acceleration range between 55mph and 65mph
3.4 gen II best acceleration range between 65mph and 75mph

3rd gear...
2.7
2.9
3.4
3.4 gen II
 

·
Premium Member
Joined
·
311 Posts
There is a simple rule for maximizing acceleration at a given mph speed. Choose the gear that operates the engine as close as possible to its horsepower peak. Everything else follows from that.
 

·
Registered
Joined
·
1,758 Posts
Discussion Starter #5 (Edited)
There is a simple rule for maximizing acceleration at a given mph speed. Choose the gear that operates the engine as close as possible to its horsepower peak. Everything else follows from that.
I guess the reason for the question... needs better explanation.

Why don't car companies sell cars, with information that makes it easier to pick which engine is best for your personal driving needs?

I've looked at a new 2.9, used 2007 S, and still wondering if I need to spend the extra money to go for the 2010 S.

If I had a 2nd and 3rd gear chart. It would be kind of fun to look at where the car offers the most thrill, and how far above the speed limits I'd be on my favorite roads. Like most folks.. I want to maximize my fun with this car... and not buy something that is over-kill or under powered.

I just thought this might be fun, helpful and interesting? :gossip:
I'd love to have a good chart... and hear what experienced owners thought about how well their car fits in with the roads they actually drive on. (would anyone ever admit, a car with less HP might be more fun because its not over-kill for the speed limits in their local area?)

3rd gear (I'm looking for something like this? I'm just guessing?)
2.7 Cayman has the most "wow factor" from 60-80mph
2.9 Cayman has the most "fun factor" from 70mph-90mph
3.4 Cayman give you the biggest kick in the pants from 80-100mph
3.4 (320hp) has a sweet spot in 3rd gear at 90-110mph

Maybe as a non-track guy... I'm asking a question which is so simple, that I'm embarrassing myself?

I've done a few test drives. And you always get the instant rush of having a new car for a short time, and everything is exciting! I was hoping to get longer-term opinions of what the car is like after the owner has had time to settle in and live with it.

Maybe I should have started a poll to construct my data chart? By simply asking, what engine do you have and what range of speed (in mph) is most fun in 3rd gear, 2nd gear?
 

·
Registered
Joined
·
182 Posts
Sorry but I'm no help. i'd normally be more than happy to go out and write up some numbers for you but I'm out of the drivers seat with a foot injury right now. All I can say is that 3rd & 4th get a pretty good work out in my CS. 5th & 6th are really unnecessary except for those intent on on getting maximum mpg. You can break any posted speed limit in your area in third.
 

·
Premium Member
Joined
·
311 Posts
I guess the reason for the question... needs better explanation.
It's all basic physics. Maximum acceleration is produced when the wheel torque at the driving wheels is maximized. Do a little math and you'll find out that at any given mph speed this occurs when you select the gear that operates the engine as close as possible to its horsepower peak. This is universally true for all engines. In the Cayman S this is around 6000-6500 rpm, so you want to be in the gear that puts you in this rpm range for best acceleration.

To put it another way, you want to take as much advantage as possible of the torque multiplication provided by the gear box. The lower the gear, the greater the multiplicative factor. This generally means using the lowest possible gear, short of redlining the engine.
 

·
Registered
Joined
·
55 Posts
I took a 2.9 for a test drive. It seemed to really shine in 2nd gear from 40mph to 60mph, and in 3rd gear from 70mph to 90mph.

Can you guys chime in and give me some idea of the conversion to mph, for each engine's sweet spot in the RPM's range?

I was just curious where the real pep is in each engine, as it relates to daily driving... in 1st, 2nd, 3rd, 4th gear (4th gear is already way too fast).

How do you guys rate the Cayman and Cayman S in gearing, and being in the right RPM to pull off a quick pass on the highway (55-75mph speed limit) vs driving in town (35-45mph range) ... did Porsche get the gearing and rpm's right, for fun on the public road?

2.7-
2.9-
3.4-
3.4 gen. II -
The attached charts of accelerating force (lb.) versus speed (mph) in gear should answer your questions. Enjoy!

I calculated the curves based on Porsche's published stock specifications for torque vs. rpm, gear ratios, final drive ratio, and tire size for the: 2007 987 (2.7L), 2007 987S (3.4L), 2009 987 II (2.9L), 2009 987S II (3.4L), and 2009 987S II (3.4L) PDK. These idealized, straightforward calculations assume that the torque specified at the flywheel is conveyed to the rear wheels without loss (friction) in the drive train and also ignore aerodynamic drag.

As I could not find a published value for the gear ratio of 6th gear for the Gen II manual gearbox, I took a rough guess. Based on the ratio for 5th gear (0.84), 6th gear for the Gen II is an extreme overdrive.
 

·
Premium Member
Joined
·
311 Posts
The attached charts of accelerating force (lb.) versus speed (mph) in gear should answer your questions.
Good job on doing the math on this. Your charts very nicely demonstrate the point I was making, namely that lower gears produce stronger acceleration. For maximum acceleration you want to hold it in the lowest gear possible, up to redline in that gear. When you upshift you lose significant torque multiplication from the gear box. BTW I think the vertical axis on the charts should be lb-ft, not lb.
 

·
Registered
Joined
·
55 Posts
Good job on doing the math on this. Your charts very nicely demonstrate the point I was making, namely that lower gears produce stronger acceleration. For maximum acceleration you want to hold it in the lowest gear possible, up to redline in that gear. When you upshift you lose significant torque multiplication from the gear box. BTW I think the vertical axis on the charts should be lb-ft, not lb.
Thanks.

Regarding the units of the vertical axis, no, the units are correct as stated, i.e. lb., for the accelerating force (let's call it F_A), which is the total force at the tire contact patchs that results in acceleration. The accelerating force is the product of the torque (ft-lb) at the rear wheel divided by the radius (ft) of the rear tire. Hence, it has units of lb.

Why plot accelerating force instead of torque at the rear wheel? Because accelerating force is directly proportional to the acceleration, which is the quantitative measure of the "fun factor" that the original poster is interested in.

The acceleration of the car can be calculated from F_A using Newton's 2nd Law (Newton's laws of motion - Wikipedia, the free encyclopedia) F_A = m x A, given the mass of the car. Conveniently, the mass of the can be calculated from its weight again using Newton's Law, but applied to the normal to the road surface, namely mass (slug) = weight (lb) / gravitational acceleration (ft/sec/sec) = weight (lb.) / g (ft/sec/sec), where g is the gravitational constant at the surface of the earth - the "g" in the familiar "g's".

So, for the car, F_A = (weight/g) x A, and so solving for A we can write, A = [F_A / weight (lb) ] x g. As an example, if the weight of the 987 is about 3,000 lbs., and the accelerating force in 1st gear is about 3,000 lb., the acceleration is A = (3,000/3,000) x g = 1 g, which the driver :), and passenger:eek:, will certainly feel.

In principle from A we can also calculate time to speed, however, in practice it would be necessary to account for clutch and tire slip until the car is hooked up. However, some folks ( Modified Car Owners Club UK • View topic - 0-60 mph Calculator ) have done some studies of measurements of 0-60 mph times for a variety of cars and have found that the empirical relationship

time (0-60 mph, sec) ~= (mass of car in kilograms) / (horsepower in bhp x 0.9)

works amazing well. (At the surface of the earth 1 kg <--> 2.20 lb.)

So, for a 3,000 lb 987 with 300 bhp, the approximate 0-60 mph time is

t_0-60mph ~= (3,000/2.2)/(300 x 0.9) sec ~= 5.05 sec :cheers:
 

·
Registered
Joined
·
1,758 Posts
Discussion Starter #13
Sorry but I'm no help. i'd normally be more than happy to go out and write up some numbers for you but I'm out of the drivers seat with a foot injury right now. All I can say is that 3rd & 4th get a pretty good work out in my CS. 5th & 6th are really unnecessary except for those intent on on getting maximum mpg. You can break any posted speed limit in your area in third.
Thanks for offering to help. Hope your injury gets better fast, so you can get back in the drivers seat ;)
 

·
Registered
Joined
·
55 Posts
Good work.
So..peak acceleration occurs at peak Torque then, not peak horsepower as stated above - correct?
Both statements are correct; the latter is almost aways true, but there can be exceptions.

First, the two statements refer to slightly different things. Yes, it's always true that "peak acceleration occurs at peak Torque" in a given gear. However, when comparing two gears for a given car, say second and third, the acceleration possible in second gear at any rpm may be higher than the acceleration that's possible in third gear. In particular, as can be seen in the charts for the various 987's, the acceleration possible in second gear within the allowed rpm range is always greater than the acceleration that's possible in 3rd gear. In fact, examination of all the curves for all the 987's and all gears indicates this is almost always the case. This is why jake said, and I'm paraphrasing, that for maximum acceleration (meaning time to speed or distance) it's best to run the engine out to red-line before upshifting.

However, while running out to red-line almost always results in the fastest time to distance, it's not universally true. In general it depends upon the shape of the torque curve and the particular gearing. For example, see the acceleration curves for 4th and 5th gear for the 2007 987S. These curves intersect at about 120 mph, which is a couple of hundred rpm below red-line in this particular case. So, in theory optimal time to distance would be achieved by shifting just below red-line. (That assumes the driver has infinitesimal reaction time.)

To get back to your original question, I think you can see from the charts that Porsche has chosen gearings, including tire sizes, that result in placing the broadest, higher parts of the torque curves for a given gear in similar speed (mph) ranges. For example consider and compare the curves for all cars and a given gear. Look at 2nd gear and also 3rd, which is a very versatile gear on the street and at the track, as was pointed out in the above thread. The broad part of the torque curves all occur in nearly the same speed (mph) range. Note too though that the accelerating force is almost always higher for a given gear and speed (mph) range for the larger engine. (The exception is for the automatic.) That means for whatever speed range (mph) you are comfortable with, eg. obeying the speed limits, Porsche has designed the gearing such that you are able to accelerate faster the larger the engine is.

So, how much fun do you want to have at 50 mph? 0.5g? 0.33g?

Note, drivers have been known to receive tickets for having too much fun, i.e. accelerating too quickly - aka careless driving, even while obeying the speed limit.

My daily driver is a 2007 Cayman S. It's way fun - both on the street for my daily commute, and also on the track. See 2009 Motor Trend Best Driver's Car - Best driver's car competition - Motor Trend

Buy what you're comfortable with. A certified pre-owned 2007 987S is a great value and great car. However, things almost always get better, so I imagine the 2009's and 2010's also provide wonderful driving experiences. Enjoy the process of being able to select one.
 

·
Premium Member
Joined
·
311 Posts
Good work.
So..peak acceleration occurs at peak Torque then, not peak horsepower as stated above - correct?
What I said is that at a specified vehicle speed in mph, it is horsepower that determines how rapidly the vehicle can accelerate from that speed. This is a mathematically true statement and corresponds to what happens in reality on the street or on the race track. If you want maximum acceleration from whatever vehicle speed you are at, then select the gear that operates the engine as close as possible to the horsepower peak, not the torque peak. This is true of every engine. In practice, this almost always works out to using the lowest gear possible, and this is illustrated in topdownride's charts, because this almost always puts the engine the closest to its horsepower peak.

Note that this is NOT the same thing as saying that the maximum acceleration in a FIXED gear occurs at the horsepower peak This is because if the gear is fixed and not the vehicle speed, then allowing the vehicle speed to vary will result in peak acceleration occurring at whatever engine rpm corresponds to peak torque. This is the difference between allowing the gears to be changed to maximize torque multiplication to the rear wheels (and thereby maximizing the wheel torque), and fixing the gear where the multiplicative factor is not allowed to change.

If you can understand these statements, then you have grasped a basic concept about vehicle physics that eludes 98% of the drivers out there.
 

·
Registered
Joined
·
55 Posts
There is a simple rule for maximizing acceleration at a given mph speed. Choose the gear that operates the engine as close as possible to its horsepower peak. Everything else follows from that.
Jake/John, thanks for reiterating. You are absolutely correct. As Jeff said, we're all on the same page.

For completeness, it's probably worth noting that the handy rule pointed out by Jake/John follows from the fact that the power, P, expended to accelerate the car traveling at speed, S, can be expressed as the product of the accelerating force, F, and the speed, i.e.

P = F x S

So, if we consider two gears, gear a and gear b, that provide accelerating forces F_a and F_b at speed, S, then

P_a = F_a x S and P_b = F_b x S

Consequently, because the speed is taken to be the same for both gears these two relations imply

F_a / F_b = P_a / P_b,

which is precisely what Jake/John said - to increase the acceleration, which is proportional to the force, at a given speed, it suffices to operate at a higher power output.

If the power output is not a monotonic function of rpm, i.e. the power output has a peak at some rpm below the rev-limit, then given some particular gearbox (set of gear ratios) and some speed, s, of the car, it may not immediately be obvious whether downshifting or upshifting will necessarily increase the power output, and therefore the accelerating force. The ability to quickly decide whether to remain in the current gear or downshift or upshift can only come from experience with the particular car (horsepower curve and gear ratios) and an ability to assess the potential changes in rpm on the fly. ... Hence the utility of the charts.

Thanks again all for the informative posts!
 
1 - 17 of 17 Posts
Top