Hornet

I've noticed in Car and Driver they do a particular test that is of some interest for everyday driving. That is the top gear acceleration from 30-50mph and 50-70mph (I don't really get the 30-50 in top gear though).

My question is, why don't any of the magazines do an optimal gear acceleration test at those same speeds? I know if I have to squeeze in while merging on the freeway usually it requires a downshift to the gear that will give the quickest acceleration for smoother merging into traffic.

Anyone have a clue as to why none of the magazines do this?

Magic8

Precedence.

Old habit die hard.

Change sux.

Gord96BRG

They do, but you just have to do a little math. Want to know how long it takes to go 60-80 in the optimal gear? Take their 0-80 time, subtract their 0-60 time. voila! 60-80 time. Same thing 30-50 - take 0-50, subtract 0-30, you have 30-50.

(The ideal of 30-50 in top gear is to test tractability at low rpm and wide throttle openings, a potential problem back in the days of carburetors. Also, it gives a very good indication of low-rpm torque output of an engine)

Regards,
Gordon

Hornet

Strangely, I don't think that method would work. Why? Because by the same example the difference of 0-60 and 5-60 should work out by the same formula but it rarely does. Part of it probably works out to wind resitance slowing the acceleration since the momentum of the initial launch isn't there (I know it's not much at 5mph but imagine 30 or 50). I do understand that there are different launches which make the difference in the 0-60 of some cars but not all cars require the special launch technique and that formula doesn't work for them.

Here's an example from a '01 Corvette Z06

0-60: 4.3s
5-60: 4.9s

This is where I found this info:

http://www.caranddriver.com/article....&page_number=4

Lawerence

You cant just subtract the 0-60 from the 0-80 and get the correct time.

Smoker

No no, this is different. This is rolling start aceleration test (5-60)VS a standing start (0-60). This is totally two different test because weight of the drivetrain, weight of the vechicle, tires etc can vary the two test time dramatically.

Gord96BRG

Yes, you can. You will get the time it took to accelerate from 60-80. What the hell else do you think you will get?

You do know that the car mags don't do separate runs for 0-30, 0-40, 0-60, 0-80, etc., don't you? To simplify (because they do multiple runs, different directions, and average the results) - they do a single 0-120 or so run, and datalog it. Then they pick off the times to the different speeds and report those. So, we see a subset of the data generated, but can use that data just as if we had the original datalog from the 0-120 run and measured the time between 60 mph and 80 mph.

Hornet, regarding 0-60 vs. 5-60 - those are different tests. The 5-60 test has a different purpose, to quantify the effect of low rpm torque from a rolling start, not maximum acceleration. You asked about 30-50 times at maximum acceleration in the optimal gear vs in top gear - you CAN interpolate that time from the various 0-XX results presented.

Regards,
Gordon

Hornet

Honestly I don't see exactly what the difference is from the 5-60 and the 30-50 and 50-70 when testing (other than the obvious). The 5-60 more than likely is done in it's optimal gear which is 1st and also the same gear as the 0-60. When you look at the test results they are obviously different and I'm sure they are both done WOT. If you look at the 30-50 or 50-70 in the way I'm seeing it you would think along the lines of; you are cruising at 30 or 50 in the optimal gear (meaning the engine is not WOT) then almost all of a sudden you give it full throttle then the car must make adjustments such as adding more fuel and overcoming wind resistance. Really it isn't the same as charging all the way from 0 in WOT.

Gord96BRG

0-60 test - they rev the engine, drop the clutch to generate some optimal wheelspin, and away they go. 5-60 test - they're driving along in first gear, then floor the throttle. Usually, the 0-60 time is quicker than the 5-60 because of the optimal launch to keep the engine in the power band.

The 30-50 and 50-70 tests are NOT done in the optimal gear - they use the top gear, cruising at the starting speed, then floor the throttle. If an automatic, the transmission can downshift. If a manual trans, they don't downshift, the trans is left in top gear. THAT is why to calculate the maximum acceleration in the optimal gear for 50-70 you need to subtract the 0-50 from the 0-70 times.

Regards,
Gordon

Hornet

I still don't think you understand why I doubt that method. I also did mention I take into account the optimal launch (even though on some cars it doesn't make much of a difference). I am aware of how they do the top gear tests and I am really asking why they don't do an optimal gear test just like that. As far as your reasoning for the 0-60 vs 5-60 here is something else from the Dec '03 issue of C&D. There is a short take about the Grand Prix GT2 equipped with a 4spd Auto Trans (and of course it is FWD). 0-60 is 8.1sec and 5-60 is 8.7sec! Explain the .6 difference in this auto trans FWD car please.

BTW Gord, don't take any of this as disrespect it's just a good ol' debate!

RENESIS

The Pontiac example can be explained by "brake standing" automatics for the 0-60 test. The driver stands on the brake with his left foot, and presses the gas with the right foot until the RPMs stop increasing usually around 2000 RPM. Then he realease the brake launching the car. It has the same effect as dropping the clutch on a manual transmission because the car starts accelerating with more torque than at idle. Don't brake stand your own car unless you want to buy a transmission though. Its very bad.

For this reason, the 5-60 times in magazines are the time you can expect in the real world since the magazines don't need to pay for clutches and transmissions.

Gordon is right about the other stuff.

GiN

If you're talking about continuous acceleration from 0-80 then the speed vs time would look something like this graph


but if you're talking about rolling at 60 and then accelerating to 80, then it gets tricky (to figure it out by math) because you'd have to calculate the car's momentum, instantaneous acceleration, and moment of inertia. In any case, { T[0-80]-T[0-60] } < T[60-80] assuming there is zero acceleration while rolling at constant 60mph.

babylou

Moment of inertia? Come on! We are not talking about bending the car or spinning it.

Momentum? The momentum of a mass is directly related to velocity. Acceleration ain't got nuttin to do with it.

This reminds me of the time I was watching Bugs Bunny and someone said to him that he could not do some sort of stunt because the stunt violated the laws of physics. Ole Bugs went ahead and did the stunt and said "I don't obey the laws of physics because I never studied law"

GiN

Sorry it's been 10 years since I've had a formal mechanical physics class. BUT, [whatever the term is that relates how a body at rest can resist a change in velocity] and [the term that describes how the product of mass and velocity are conserved unless there is an outside force to change its velocity or mass] are required to figure out the difference in the two scenarios because they do not behave the same way in both scenarios.

Reeko

My take...

The reason why it doesn't work 5-60 is that if you are doing a 0-60 run, you probably are still spinning the tires well past the 5mph point, this means the revs are staying higher = more HP.

Once you reach the point where the tires are no longer spinning in the 0-60 run, I am pretty sure that the acceleration from then on is the same no matter if you did a rolling start.

So, I think that the 50-70 mph times will be the same if you are doing a rolling start OR a hard dump.

Hornet

The scenario of rolling at 60 is exactly the type of situation I am talking about.

babylou

Tada we have a winner.

gusmahler

To answer the original question, I don't know.

But some motorcycle mags actually test 60 to 80 in all the gears.

As for Car and Driver, I have no idea why they do those tests. Automatics have an incredibly unfair advantage in those tests (they automatically downshift to the optimum gear.) Plus, 30-50 in top gear is stupid for some cars. If I'm going 30, you can bet that my 8 won't be in 6th gear. Road & Track does 60-80 which is much more useful.

As for why you can't just subtract 0-60 from 0-80, it's not what they are testing. When they go 0-80, they instantaneously pass through 60. The way I envision the test is a simulation of what happens when you're stuck in 60 mph traffic and the lane opens up--you go from partial throttle to WOT. If you just subtract 0-60 from 0-80, you're WOT all the time.

army_rx8

i thought you were talking about 30-50 and 50-70 or whatever in top gear (i.e. 6th)...so you couldn't use the 0-60 & 0-80 times. or are you guys interchanging words (top and optimum) when you shouldn't? just curious.

Hornet

Wow, my topic came back! :D

My thought was more along the lines of cruising at about 65-70 mph in 3rd gear. While you might be in the best gear the rate of acceleration from 65 or 70 mph to say about 90 might be different than accelerating from 0-90 mph at WOT and subtracting the difference of 65 or 70 and 90! As you mentioned gusmahler the motorcycle magazines do it! Is there usually a different number than the difference of 0-60 and 0-80 or does the number of 60-80 remain the same?

I hope people can distinguish what I wrote here! If I didn't write it I don't know if I would!

Gord96BRG

Cool! So, I just read through this again - I'm still right.

Regards,
Gordon

Hornet

Alright it's time for this topic to rise again!

I just found a magazine that does this test that I mentioned! While it's not the most respected of auto mags "MPH" actually does this test! I noticed it in a comparison of the Charger, GTO, and Mustang GT! Now as an example here are the Mustang GT's numbers:

0-50: 4.2sec
0-70: 7.1sec

which as we can see is a 2.9sec difference. But the actual 50-70 test in which I guess the optimum gear might have been 4th shows a 5.2sec time (maybe they needed to be in 3rd instead of 4th).

A possible better example was the Charger!

0-50: 4.6sec
0-70: 7.5sec

Again the 2.9 sec difference!

50-70: 3.5sec

The GTO's numbers were close enough to argue that they just rounded them up 4.3sec difference in the 0-50 and 0-70 and only 4.4sec in 50-70! That's probably mostly due to the huge amount of torque readily available!

I know this isn't the most respected magazine out there but they helped me in establishing that this may in fact be a valid test especially considering that sometimes the ability to pass is an important aspect of everyday driving! Pass them and be done with it right! Just had to make this point! :D

gusmahler

Since this topic is back, I'll expand on my earlier answer.

Here's why you can't just subtract 0-60 time from 0-80 time to get the 60-80 time:

As stated earlier, the mags just go 0-120 and plot out the time for each speed. Thus, it's just instantaenously passing 60. OK, I said this before, but here's the kicker. If you just subtract 0-60 from 0-80, you're Wide-open throttle all the time. There's no throttle response to take into effect. Plus, the gearing is different.

Let's take the real-world RX-8. If you were testing 0-120, you would shift from 2nd to 3rd at about 68 mph.

If you were on the highway at 60mph, and wanted to pass, you would probably downshift to third, not 2nd, then floor it. (At least I would, just so I don't have to bother with an upshift while in the middle of a pass. On the track, it may be different, but we're talking real-world here).

Because subtracting the 0-60 times and 0-80 time includes a shift from 2nd to 3rd, it is not a direct comparison to what Hornet wants to time--downshift to 3rd at 60mph and floor it.

Hornet

Thanks Gusmahler! I'm glad someone saw where I was going with this. I just knew it is never that simple as to hit the throttle and pick up the momentum exactly where you would have left off on a 0-70, 80 or whatever run!