Instantaneous?? you can divide the time to as small as you want, its still an integration (area under the torque or force curve) . No movement, no acceleration. acceleration is the 1st dervative of speed.

balanced forces are not experiencing a net force. its a balanaced force, so there is no acceleration. no instanteanous acceleration , nada.
if you have a weight sitting on a chair, it has a force acting on the chair and the chair has a force acting on the weight. no movement, no NET force, no acceleration.
I know this is the semantic argument we had before, but its just a communication errror , i hope.

if you have a net force through the wheels acting on the mass, it will have an acceleration. as you know, the force degrades with our cars due to gearing and kenetic forces growing at a faster rate than the wheel generated forces decay.

Mk

 

"The result is no "net motion." It's like the difference between stress and strain."
by Glen

A truism. Well put.

 

Hmm. stress, strain. they are kind of the same thing. does a sprinter release his stress and strain after the gun goes off and he starts moving? I tend to think so. cant you strain to try and remove that nut on the harmonic balancer? isnt it a stress on the arms and the nut, crank, etc.

Glen, you are righ that there is no net motion (kind of redundant as a term) if there is no net force. net force is the force that will propel a mass. force without movement is just a force, with no acceleration. for acceleration to occur, there has to be speed change (ie movement), instantaneous or otherwise. I think you are confusing , instantaneous speed changes, which do occur as a part of vibration and resonance. those speed variations, on a occilascope, look like positive and negative forces are acting on the speed measured load, but in actuality, they are is just variations of positive net force. the greater the variations, the more the vibrations or greater the resonance. you can also have resonance sitting in a chair, the speed up and down would be integrated to find the acceleration up or down. sure in this case the time period and would be small, but there is still a movement . without movement, there is no acceleration, and no net force.

Mk


However,

 

I want alot of both. Its more fun.

 

Mark,

"Stress" and "strain" are technical terms equivalent to "force" and "deformation" respectively. These are very different things.

I tried a while back to explain instantaneous acceleration in terms of a limit. And by "limit" I mean the mathematical term. I suppose I can't teach a few chapters of HS calculus this way. (My bad!) No point in going on.

 

I understand, i was just busting your you know whats

when im quoting "net force" thats from year 1 physics. not that hard to grasp.

Ive already forgotten the purpose instantaneous acceleration discussion. what was the message? if you are going to use limits as you do in calculus, there is a range of speeds, times or distances. there is no instaneaous net force or acceleration with someone sitting on a chair no matter how far you divide up the integration. there are two forces, no positive net force, and no acceleration.


Mk

.

 

An Engineering teacher at Cal State LA was conducting a class on high performance internal combustion engine design and building offered an elegantly simple philosophy intended to result in the most sucessful hi po motor: Go for the torque and the horsepower will follow.

The thing I love about the 928 V8 aside from the smoothness is the available torque at all RPMs. You are never left waiting to get on top of the cam for the thing to move. And from a small displacement engine too. Very nice job, Porsche. A great engine design unfortunately born at the most depressing era in motoring history while manufactures fumbled around trying to deal with the fuel shortage and air polution standards. Pop the hood on about any other engine in '78 or '79 and nothing ran like a 928 or 911 for that matter. Remember how pathetic the Ferarri 308 was? Like the BMW M-1, production 928's are underpowered for the the chassis. As far as how torque and HP feel, the big V8s with tons of torque give that pinned back in the seat sensation where the small displacement engines with HP and not as much torque have a kind of "weightless" sensation under hard acceleration.

I've decided I need at least 500HP in my 928. Let's see.... Start with a stroker kit from Devek and then........

 

For what it is worth.

Check out the below composite Gtech graph. The Gtech software won't show RPM and G's on the same graph, but with a little Photoshop work you can see them at the same time.

After the wheels stop spinning, at just after one second, you can see the RPM's in red climing from about 2400 rpm to about 5800 rpm at the 3.6 sec. mark.

During that time check out the g curve in black. The g curve shows my cars torque curve including the flappy switch at about the 2.0 second mark. Fairly cool that you can see the torque curve in the g measurement.

I have dynoed my car and the "DynoComp" torque plots closely match the g curve shown by the Gtech. Both the Gtech and the DynoComp plots show a decline in torque starting at 4500rpm. The Gtech and DynoComp both agree that the flappy dip occurs around 3600 rpm.

The DynoComp shows my HP going up and peaking at about 6100 rpm. Yet the g curve shows acceleration dropping to its post flappy worst just before first to second shift at 5800. The acceleration of your car does NOT reflect the horsepower curve. Instead the g curve follows the torque curve.

When my car shifts, it shifts into the middle of the second torque peak and then the g's steadily decline from there to redline.

What does this mean??? I have noticed that some of my best runs occur when I shift a little later that the standard "in D" mode, and well before redline.

 

True to a certain extent. remember, a stock 6.0 stroker puts out gobs of torque, but only 300ish hp at the wheels with no other mods. so, just going for torque doesnt really do it. (as seen in this example of big torque gains and not a huge amount of HP. ) so, if you want HP, you have to try and get torque at top speed. Again, another case for looking at HP rather than just torque. better said, you want more torque at the higher speeds!
porsche and ferarri both got this, so they both made moderate torque engines, but reved and breathed better!

No question, the 928 was a remarkable feat. just think. 310hp in 1984 out of the 928 in europe. Back then, the vets were only producing 240hp!!! and it wasnt a smooth 240 either .

MK

 

wait a minute. we all know that acceleration in each gear will happen at the max torque. since torque falls off from there, acceleration will fall off . however, running to max hp will maximize your accelleration rates.

I think you failed to see that at every shift, the rate of acceleration in the next gear is slighly lower, even though the next gear is operating at a higher engine torque. maximum overall acceleration will occure when you shift at max rpms. even though in that prior gear, the max rates of acceleration were at the peak engine torque area. this only makes good sense.

we have been all over this before.

I like the graphs, but i think the jumps before each shift are due to your automatic storing and releasing energy between shifts. I also like the little hitch in first gear. is that where the tires finally hooked up in 1st? pretty cool.

if you notice, your average acceleration decays with vehicle speed, as it should

all you need to do is look at you ratios, plot them to your HP curve from the dyno and see where the shift points will be. very unlikely, that you would have ANY advantage in shifting before redline (ie 6400rpm ) think about it, what is the max hp. (or torque at max rpms) x the gear ratio, and then compare that to the peak torque (usually 10-15% higher on the engine) with a 30-35% drop in mechanical advantage in the next gear. What this means, is even though from 5800 to 6400rpm you have 15% less torque than peak torque, if you shift early, you loose 20-5% torque to the wheels by a earlyshift, including the increase of engine torque running at a lower rpm.

If what you are saying, and i think you are, by shifting at 5800rpm you are gaining acceleration by keeping the engine at peak torque, and this is wong. Sorry, you will follow a faster accel shaped line if you shift as high as you can.
If you were right, it would REVOLUTIONIZE racing. sorry, it doesnt.

MK

 

You really need to stop thinking everyone else is dumber than you.

Never suggested that you stop increasing RPM after the torque peak. Staying with the higher torque multiplying gear is best for acceleration. The graphs show that, and yes I have noticed that.

Just making the point that for any one gear, the engines ability to accelerate the car coincides with the torque peak, not the HP peak.

I have done many runs, and taking it out to 6400 is not optimum in my car. I was surprised to find this and I reapeated the experiment several times, but first to second at about 6K is best for me. I dont have enough data for 2 to 3 to tell yet. For a showroom stock S4 I do better than many.

The jumps at the shift points are due to the stored energy, and interestingly enough do not occur over a wide range of shift points. Allowing the car to shift itself will not show these pronounced spikes. Holding the car in 1st and waiting a bit longer will slways produce the spikes. I like spikes.

By the way, manual transmissions are always easy to spot on a GTECH plot, usually a severe drop in G's for around 0.5s. I prefer the spike for 0.5s.

 

Hey, i dont think that at all.

I just want to make sure the the complete truth is understood to avoid the common misconceptions.

YES, for any one gear, the max acceration occures at max torque. we all have said this. However, max acceration for the system (ie the car, your driving) will occure at keeping the engine at near max HP through the gears. this means you have to strattle the peak hp to get this. (ie if you have a peak hp at 5800rpm and your rpm drops are 1200rpm and assuming a even arc of a curve, then you would shift at 6400rpm and really no faster). if you are not seeing this, and you have lower shift points. THEN, go back to your HP torque curves and see if your torque at 6000 vs 6400rpm is less than the torque in the next gear through a 33% less mechanical advantage next gear. since, most all of our engine are between 12 to 17% less engine torque from peak to redline, its hard to imagine that you would have a gain by a short shift. If you do, you have a breathing problem, spark problem, mixture issue, etc. when our cars are running correctly, you shouldnt see that much of a torque /hp fall off at near redline.

Thats the only point. If you have to short shift for max acceleration, you have a more than normal fall off of torque. There could also be another factor, your auto transmissoin, as those losses go up with speed too.

MK

btw, your graphs already prove for some reason, you have a non-linear rate of acceleration after your 1-2nd gear shift (a short shift too) while your 2-3 gear shift seems to be a more linear rate of acceleration . kind of contradicts your observation. however, if you have tested a redline 1-2nd gear shift and it is even worse, then you have an unusual torque fall off at near max rpms.