this is seriously the best thread i've ever seen on tr. so fucking interesting. i'm pretty good at basic calc and physics, and i'm still having a hard time following a lot of this stuff. someday i'll get it... someday.

i see now, for the first time in my life, why they say that guys have more mathematical/scientifically inclined brains than girls. you don't find many females who will bother trying to understand this shit, let alone contemplate and debate it for fun.

 

Top fuel cars have a slipper clutch that is set up to provide a certain amount of slip for the first part of the track and they turn a high rpm at the top end of the track. Also, top fuel cars are just that badassed.


Double edit- don't read this next part, I now think I was wrong.
There's something I'm still confused about- let me describe my understanding of the situation and you can all point out any inconsistencies.

The peak acceleration of the car should be at the peak hp, because hp is a measure of work being done (accelerating the car). The torque peak is the point when the most force is being applied per revolution of the engine. But that doesn't necessarily mean that the torque peak is when the most work is being done. In a hypothetical car engine situation where the engine produces 300 lb/ft of torque at 3000 rpm (171 hp) and 200 lb/ft at 6000 rpm (228 hp) more force is applied each time a plug fires and a piston is pushed down in its bore at the torque peak, but more work is done overall at the power peak because there are more sparks per given time and that makes up for the lesser force per each revolution. Now in a situation where the car makes 150 lb-ft at 6000 (171 hp), the car should still be accelerating just as quickly as it was at 3000 rpm and still has the benefits of torque multiplication through the lower gear, which would make it much faster than shifting at 3k (though that's not what we're really talking about here.)

Here's where I go more out on a limb- the feeling of acceleration you get at torque peak is actually the feeling of rapidly increasing acceleration. You punch the gas right before the torque peak and the car really leans back and goes. The perception that there is less acceleration after torque peak is because accleration has leveled off and we perceive changes in acceleration more than constant acceleration. Like in skydiving (well, what I've heard, never done it)- you feel the lurch as you accelerate downward but when you hit max speed the feeling subsides and you're just floating there. (edit- I just read this and realized that the analogy is not apt, because you stop accelerating completely at that point. just ignore it, haha)

Please comment and criticize, I'm just trying to reach a better understanding.

 

Shadow....Tony is right. MAximum acceleration occurs at peak Tq...always. If you start off in first gear at idle in a manual transmission car....and mash the gas....your acceleration RATE will mimic that of the Tq curve you would see on a dyno print out. On your avaerage NA v8 for instance you would have peak TQ in a mid rpm at say....4000 rpm. At that very point you are under the greatest acceleration.. If max accell was at peak HP it would be glaringly apparent by the seat of the pants meter since ther is hardly any HP made in the lower RPMs of most NA engines. However...some Tq monsters like the L98 will KICK you in the pants at 2500 rpm...even though they are barely making any significant HP at that point.

If you need direct measurable proof.....look at some dyno runs and plot it against time. See where tha max acceleration takes place....it will clearly be where the TQ peaks.

I was just looking at my last dyno run on the Dynojet runviewer. It has some neat options on it where you can select different info to be displayed. I just set it up to show time vs rpm. If I need to I will do a "print screen" with the info if you would like to see. Anyways...if i select a window of one second to view exactly how many rpm the engine accelerated during that second....you can get that info easily.
If I select...on my dyno run the one second period that equals peak TQ from 5 seconds into the run to 6...you will see that I went from 5210 rpm to 5780 rpm in that one second span. Thats a gain of 570 rpm in one second at peak TQ of around 440 ft/lbs and HP was approx 430 rwhp. If you compare this to seconds 8 through 9 where I was making roughly 510 rwhp but less than 400 TQ you will see that I only gained 430 rpm during the time of peak HP.

Again...during exactly one second of peak TQ I gained 570 rpm...during 1 second of peak HP...i gained 430 rpm. Peak TQ accelerates the car faster than peak HP.

I almost never look at HP figures when trying to determine how fast a car will be and how it will behave under acceleration through the gears.

I think people get fixed on the notion that TQ is low end grunt. It is not. Low end grunt is ALOT of TQ in the lower RPMs. Big HP is alot of TQ in the higher RPMS. ITS ALL TQ. You just need to look at WHERE in the rpm range you are making that TQ. If you want to do the math to figure HP then thats fine. But the TQ line is the easiest way to see what an engine combo will do....atleast for me.

Now to gearing and why that may be important..and how it relates to high rpm TQ (HP), shift points and rear wheel TQ.

So for now...forget the HP numbers....to me they are nearly useless. Concentrate on that TQ graph. Again thats going to determine where everything happens.

SO if peak TQ is max accel...why dont we set up our shift points to go just beyond peak TQ as we look at our dyno graphs and shift then. We would be right back in our MAX accel point wouldnt we?
Usually no!

Max acceleration at peak TQ is only true for THAT GEAR YOU ARE IN. Once you shift...its a completely different calculation taking into acct the next gear multiplier you just shifted into and you obviously cannot compare one gear to another. Yes the peak acceleration in first gear will happen the same rpm as peak acceleration in second. But acceleration in the higher rpm...well after peak TQ in first will usually result in more TQ applied at the rear wheels and thus better acceleration than the acceleration rate even at peak TQ in second. You will always accelerate progressivley slower as your gear multiplier decreases. So unless your motor just DIES big time up top (makes no TQ in upper rpm range and falls of sharply) its usally better to shift around peak HP than peak TQ. Thats where the misnomer comes in that HP is more important than TQ....because you willl accelerate slower once you shift gears...even though you may be closer to peak TQ after the shift...and where closer to peak HP before the shift.

This still doesnt take into acct Tq converters and stall speeds, boosted cars, nitrous. It also doesnt fully take into account usable rpm range and ways to broaden it...and then gear accordingly.

Although alot of you guys did remember the first gear issue and how that affects ET.


This is all just my opinion on what I have seen so far. Some may not be exact thats for sure. Its just a very good topic that really gives the melon a workout when you try to take ALL of this crap into acct and what is best.

Cam selection, cubic inches, stroke and piston speed, Rod to stroke ratio, valvtrain and spring selection, intake design, gearing, Tq converter, power adder setup, tranny type will all play into the "best possible" setup. And its damn mind boggling

 

well..alot of talk while I was giving my .02....looks like you guys have it all down pretty good. Its alwasy good to see other POV on this subject because every once in a while someone puts it in such a way to make it more graspable.


Now lets talk about cam selection

 

Wow! That must have taken Al an hour to type..

 

I waited all day to get back to my computer and type a reply but it looks like my points have been made already. Good stuff guys.

 

I think I'm starting to get it, and I think Tony and Al are right. Horsepower is a measure of work an engine can do in a certain time, (torque*revolutions per minute)/5252, torque is a direct measure of the force being put through the drivetrain to accelerate the car at any given rpm point. The point with the most force is the point of most acceleration, most force pushing through the drivetrain to push the car forward. Continuing past that point is just letting gear multiplication increase the torque put to the ground over time (work, hp) because there will still be more torque with the multiplication than there would be even at peak in the next gear. This is how you use gearing to harness hp (torque at high rpm) to do work, but max accelerative force to the ground in any given gear will be the torque peak. Go to the next gear and your multiplication is much less.

HP has always just been a very rough way to estimate the powerband of an engine. If you have a hp peak at a high rpm and a low torque peak figure high rpm, it's a high-winding peaky engine. If you have lower hp at a low rpm and high torque at a low rpm, it's an engine with low-end grunt that doesn't do well at high rpm.

 

hybridSS: you are falling into the same fallacy trap as tony:

peak acceleration only happens in WHATEVER gear has the highest POWER AT THAT SPEED. in first gear there is no lower gear so your acceleration peak will be at the torque peak in that gear. yes. i NEVER DISPUTED THAT.

i said that at ANY given speed (excepting the first gear scenario), your peak acceleration will be at peak HORSEPOWER.

trust me, i am not wrong.

peak acceleration at any given speed is where power is maximized. period. empirical rule. bar none, no arguments. if you disagree, i can prove why you are wrong.

in the case of first gear:
at the torque peak in first gear there is NO gear selection that will give you more power. go beyond that and your acceleration DOES slow, but that's only because there's still no example of a gear where you have more power than first gear.

tell me. which do you think will give you more acceleration at say 60mph?
200ft-lbs @ 1500rpm? or 125ft-lbs @ 10,000 rpm (from the SAME hypothetical engine, in the SAME car)

ill give you a hint:
pick the second one

i NEVER disputed the peak acceleration in a car overall would be at the torque peak in 1st gear. that' the ONLY time you will ge more acceleration from the torque peak than the power peak. in ALL OTHER GEARS if you drop a gear and make more power you WILL get more acceleration. period.

tony is ONLY right in the context of a car with a one-ratio transmission or if you only look at each gear independently.

i made 2 statements earlier (modified here to account for the 1st gear anomoly and to include both discrete ratio transmissions and CVTs).

• at any given speed peak acceleration occurs at whatever available gear ratio maximizes power.
• in any given gear, peak acceleration occurs at maximal torque.

both statements are 100% factually correct. dispute them all you want, if you choose to do so, you are wrong.

the reason why the first statement says at at any given speed (and i have been trying to emphasize this point but no one seems to get it). obviously as speed goes up, acceleration goes down and there are several reasons for this:

1: more air resistance
2: after gear change, less mechanical advantage
3: the power/work the engine does changes the kinetic energy

to further explain point 3:
kinetic energy = 1/2 * m * v^2. as your speed goes up it takes a smaller change in velocity to get the same change in kinetic energy (which is limited by engine horsepower and ultimately what the the engine does AFTER air resistance and rolling friction). less change in velocity in a given time span = less acceleration.

 

I think we've all been saying the same thing, just in different terms.
Tony and Al- in each gear, peak acceleration is at torque peak but revving higher than that takes advantage of gearing and hp to do more work overall
Shawn- at a given speed, peak acceleration is at horsepower peak in most suitable gear