I disagree. Acceleration is the result of a force applied to a mass. F=ma, or in terms of angular quantities T=I*a_{c}
A total change in kinetic energy over a time interval is the result of power applied to a mass over that time interval, which is perhaps what you were trying to articulate, but even that IMO is a sort of butchering of the physics because powers aren't really "applied", at least not in the physics sense as far as I see it. Forces and torques are applied.



The instantaneous value of angular acceleration of a body is most certainly the instantaneous value of of the of the torque curve. Similarly, the instantaneous value of the change in angular acceleration of a body is the instantaneous value of the slope of the torque curve. This is not high-level silliness, it is basic intro mechanics, which I've taught.

This statement I believe to be false. Instantaneous Acceleration can most certainly be considered and measured physically over an infinitesimal time interval, because it is simply proportional to applied force (or torque). Acceleration can occur without an interval or time. Power on the other hand is a calculated (as opposed to directly measured) quantity that requires a non zero time interval over which other physical quantities are measured, and then the values of which are used in a calculation of power.

This is false as stated. It should be restated:
"a 4-cylinder engine that puts out the same average power over an interval as a V8 will on average accelerate the car just as fast if all else is equal."
There is a very high chance that with the V8, there will be sub interval over which the V8 has a greater value of instantaneous acceleration, thanks to its larger values of torque produced.

I disagree with this, and it goes back to our earlier conversation about measured vs calculated quantities. As far as I'm concerned when it comes to causality, determinations of things, and unique solutions to differential equations that specify a particular velocity as a function of time v(t) curve as a solution, power is not what determines changes in acceleration, rather the instantaneous slope of a torque (or force) vs time curve is what determines instantaneous values of change in acceleration. This I believe was greg's "gobbledeegook" point about slopes of torque curves.
I would agree with your statement if you instead had said:
"Calculated average power can be used to calculate average acceleration"

This is not true. The true statement could have been "The car that puts down the most average power over a time interval will have the largest value of average acceleration as calculated over that interval"
The point that I would like to make to you now is that the car that puts down the most average power over time interval might not necessarily be the car that experiences the greatest value of instantaneous acceleration in that interval. That trophy goes to the car with the largest magnitude value in its torque curve, and that is why people like big torque numbers over short RPM ranges. This means a steep torque curve, and larger instantaneous values of acceleration.

I would like to point out here that your language here seems to imply a difference between power and work, and there is no physical difference between those two things.

First I would like to point out that language like "Acceleration is fastest" is really quite poor to use in this sort of technical discussion. Your statement might have been "Acceleration has the greatest instantaneous magnitude when instantaneous power has the greatest magnitude"
But even then, that statement is not necessarily be true. It can be true, but it is not necessarily true. What is necessarily true is "Acceleration has the greatest instantaneous magnitude when instantaneous force/torque is greatest"


First I would like to point out that language like "Acceleration is fastest" and "torque CLIMBS fastest" is really quite poor to use in this sort of technical discussion. Your statement might have been "Acceleration has the greatest instantaneous change to its magnitude when the instantaneous slope of the torque/force curve has its greatest magnitude" and that sort of statement would be entirely and always true, and I think really captures the essence of why people do, should, and ought to care about torque curves.



To sort of bring this all back to reality, I love my L98 car. It wears its "1989C4" vanity plate with pride! Sure I get my doors blasted off on the freeways and interstates by brand new challengers and mustangs, but stoplight to stoplight, onramps, or any other place where the total time interval being considered is rather small, the L98 is able to hang with newer, faster and higher horsepower producing cars thanks to its steep torque curve with large maximum values of torque, which allow the motor (for a given gearing) to propel the car with large instantaneous values in the change in acceleration, as well as large magnitude instantaneous values OF acceleration. Oh did I mention the donuts?

 

Gregg, I already gave you a comparison on a car with similar mass that was rated at 241hp peak, and its acceleration compares favorably to the L98 C4. What more can you possibly want?! There are almost no production cars with engines so damn peaky that their peak power rating promises more than their average power (under the curve during an acceleration run) delivers. The few that come to mind include a couple Honda VTEC models: the S2000 and the Integra R, and a few shitty turbo cars like the SVO Mustangs that didn't come on boost until about 4000rpm and had such shitty heads that they were all done by 5500rpm. I've ridden a couple motorcycles like that, though, and they sucked. These days there are no cars like that. None.

But you seem to be confusing a low peak power RPM with a broad power band, and that is not always the case. In fact, it's often not the case. The L98 is a good example: it's good power range is basically 3000-45000rpm, and if you end up above or below that you're sucking wind. The SVO is another example. The L98 does not have a broad power band, and its torque curve is much peakier and narrower than the LT1's (look at the factory dyno graphs I included in post #53). The LT1 is far easier to keep near its peak power range, and it will have far more power under the curve and require less rowing of the gears to stay in the fat part of that. The L98 is a relatively peaky engine, but it's peak just happens to be at a very low RPM.

Again, I don't know how many times I have to write this, but I was very clear that the L98 C4 was a great car when it came out. Nobody here has said otherwise. Stop trying to read **** into posts that isn't there and start reading the actual content I posted. This will go a lot better for you if you do that.

Nobody attacked you. We are trying to provide accurate content for everybody's sake. When you're trying to predict acceleration, torque doesn't mean ****. That's as simple as I can make it. The predictive metric is average power applied over the acceleration run. Period. jdjenk just provided the most concise explanation of why this is the case: torque isn't a measurement of work.

Yes, it does. If two engines have the same peak torque but one's is at a lower RPM, then it's making less power and will accelerate the car more slowly. Again, the examples of vehicle comparisons clearly demonstrates this! Why do you keep ignoring it?!
You're just confusing yourself with this. You've been deluded into thinking some power matters more than other power. If you would lose the word "torque" from your vocabulary and just focus on power, you'd be much better off for it.

 

If you run out of air with the standard TPI, would a SuperRam help the situation? Anybody on this thread tried one?

 

2. No,
3, LOL, Indeed. Where are the data I asked you for? : You're the guy who goes to the drag track, and is somehow surprised & mortified that you lost to a same hp 4 banger. But indeed, you lost. Using your argument, a 240 hp Cummins should smoke a TPI, right? B/c, I mean, it "does it by" 2000 RPM. Right?

Greg, I'll give ya this: You're doing a HELL of a job, rationalizing your own "beliefs".


.

 

Ah....was THAT what this thread was about!? "Why a L98 was good for it's day".....Hmmmm...nope. That wasn't this thread, that must have been a different thread. I'm pretty sure that this thread was; How were C4's so quick?
Now, I didn't realize that the "TPI"=ALL C4. AFAIK, C4 included L83, L98, LT1, LT4, and LT5. All of which produce about what they should, near the top of their game for their day and none of them have any "magic" in their intakes.

 

At the other end of the spectrum, why is a 2 liter Honda S2000 so slow with respect to an L98 C4? With similar power (240 hp), 9,000 rpm capability and weight under 2800 lbs. the S2000 looks pretty good on paper.

Broken car, or poor example maybe, but definitely slow in this test: https://www.caranddriver.com/reviews...d-test-review/

I acknowledge that the gearing is not at all optimized for low speed acceleration with the 4.10 ratio. Speed in gear @ 8900 rpm with 832 rev/mile tire: 50 mph 1st gear, 77 mph 2nd gear, 106 mph 3rd gear and 135 mph 4th gear. It really begs for a 5.13 gear to make effective use of all 4 gears (for 1/4 mile and to reach 60 mph at the top of 2nd).

 

the S2000 was literally one of the two cars I mentioned that may have an especially poor average power through its gear because it's power band is, in fact, quite peaky. However...even as such, it managed a 96mph trap speed which is in the realm of an L98. I'd suggest it's bigger problem was getting an adequate launch with it's peaky powerband and possibly a tall first gear (to make the transmission a close-ratio unit due to the peaky engine), but it seems that once it got rolling a bit it was able to put down some decent average power. Their 0-60 was an abysmal 6.8s, whereas I've seen a lot of those be 1s faster or more. I've seen quarter-mile times for that car closer to 14.0@99, too. Regardless, I never was interested because it is actual work to keep them in their happy place in the power band. No thanks.

 

Of course F=ma, but we weren't talking about the direct force that accelerates the vehicle but rather what parameter of the engine's output determines the vehicle's acceleration. The direct force (F) that accelerates a car isn't the torque at the crankshaft: it's the tractive force at the driven tires' contact patches, which I've already referred to. That's a linear force, not an angular one. And the only two factors that determine that force, ever, for a wheel-driven vehicle are the vehicle's current road speed (inversely proportional) and the engine's power output at that time (proportional). Ergo, the engine parameter that determines how fast a car accelerates (instantaneously or over some time or distance interval) is the power it puts out (at any given instant or average over the interval, respectively).

Vehicle performance - the topic of this thread - is pretty much always discussed over a time or distance interval. Literally the entire purpose of a car is to move a certain distance, and performance is all about how fast it can do that. Nobody ever talks about instantaneous performance in terms of vehicles. That's just not relevant. The force that's applied to a car to accelerate it (tractive force) is directly proportional to the power the engine is making, not the torque.

No it isn't. It is the instantaneous value of the tractive force curve, but that is a result of the instantaneous power the engine is producing divided by the vehicle's road speed. It is not a product of the engine's torque curve. You are conflating the tractive force and the engine's torque output. They are not at all the same thing.

Gregg was saying that acceleration is directly related to the slope of both the torque and power curves. That's different than the rate of change of acceleration, which is what you're saying. However, again, you're both wrong because you're confusing tractive force with the engine's torque output. They aren't the same thing, and again the tractive force is dependent solely on the engine's power output and the vehicle's road speed. The engine's torque curve has nothing to do with it.

That's fine. Nobody ever discussed vehicle performance in terms of instantaneous acceleration, but this is a more accurate way to state what I was saying.

First of all, if the V8 car has one instant where it's accelerating harder than the 4-cylinder, but they both cross the finish line at the same time, then who cares? They both performed equally well over the interval. Besides, that necessarily also means there was also some time during the interval where the 4-cylinder was accelerating harder than the V8 car. This has to be the case if they both complete the interval exactly the same.

But again, none of that acceleration (instantaneous or average) is determined by the engine's torque output. Regarding engine parameters, acceleration only results from the engine's power output. If you were talking about tractive force instead of crankshaft torque, I'd agree wholeheartedly. But again, tractive effort results from the engine's power output, not its torque. If two cars are otherwise equal and going the same speed, and both are accelerating with an instantaneous 240hp, but one car is making 240hp at 4000rpm with 315lb/ft and the other is making its 240hp at 8000rpm with 157.5lb/ft, they are still both accelerating at the same rate (because the tractive force is the same) even though the first car's engine is making twice the torque of the second car's engine. This is super basic stuff here. As Tom pointed out earlier, if an engine's torque were what determined a car's acceleration, then we'd all be racing Detroit Diesels!

Wrong. Here's the proof:

• A 3000lb car is going 30mph and its engine is making 300lb/ft of torque.
• Ignoring friction and aerodynamic drag, and without knowing RPM or gearing (and thus power), at what rate is it accelerating at that instant?

Or if you want to relate this to the slope of the torque curve:

• A 3000lb car is going 30mph and its engine is gaining 100lb/ft of torque with every 1000rpm increase at this instant.
• Ignoring friction and aerodynamic drag, and without knowing RPM or gearing (and thus power), at what rate is it accelerating at that instant?

The fact is, you can't answer either of those questions because the engine's torque doesn't tell us squat about a car's acceleration. Note that if I substitute power for torque, I can solve the problem without knowing anything about the engine's torque or RPM:

• A 3000lb car is going 30mph and its engine is making 300hp at that instant.
• Ignoring friction and aerodynamic drag, and without knowing torque or gearing, at what rate is it accelerating at that instant?
• The drive tires have 3750lb of tractive force and the car is accelerating at 1.25G or 40.2ft/s^2.

All I need to know is the engine's power output, and it's torque output or curve doesn't matter in the slightest. Knowing the power that reaches the contact patches is both necessary and sufficient to calculate the car's acceleration.

 

Someone way smarter than me posted these gems awhile back to illustrate both hp and torque vs velocity for a few C4 gear combinations for a specific member's car:

https://www.corvetteforum.com/forums...post1563905394
https://www.corvetteforum.com/forums...post1563916454

Sorry if redundant. The tractive force will directly follow the axle torque (divided by the tire radius) vs velocity, given sufficient vertical tire loading and tire to surface friction. Hope this is clear.

Images from links above:







Because racecar (HP*375)/MPH is hard to visualize, but indeed handy.

 

This is a good and interesting graph that demonstrates concretely how road speed affects the tractive force at the wheels. Also, it shows clearly why torque doesn't directly predict your acceleration: if that were true, then you'd shift to maximize torque under the curve instead of power, and if you look at the graph you'll see how much axle torque (and ergo tractive force) you'd be leaving on the table through a run.

I'll just mention that the two other variables not included here are transmission ratios and (assuming it's an automatic because only four gears) torque converter multiplication at low RPMs. That was obviously outside the scope of the discussion for which these graphs were used, but I'm just noting that there are a lot of variables that affect tractive force.

 

I agree with your view. I have one season of OTDs under my belt. So I am experiencing what you are describing. I drive in Michigan at Waterford Hills, Grattan, and once at Gingerman. There were a couple of times at Grattan when I wanted more out of my L98. As in "How did that Carra get down to the end of the straightway so fast." She passed me at the beginning of the straightaway, then instantly was a long way away, as if she had afterburners. lol Where are you driving?

 

I understand your feedback though I suspect the alternate statement you suggest is too wordy for a general audience. Some of what I said didn't land well either. I shouldn't have entered a discussion like this without better examples and well-worded explanation. I think you can see acceleration means the wrong thing to many people. To you, I tried to send a PM but your mailbox is full.

For newbies, I'll suggest a link to a better comparison to a TPI vs LT1 intake. Actually, the power levels are approximately correct -- definitely closer than the L98 Corvette vs LT1 (with it's higher compression and cylinder head cooling.) When you look at the imbedded dyno comparison, try and imagine the 275hp LT intake lowered to 250hp. THAT'S what you'd might be comparing comparing back in the late 1980's.

Comparing a 250hp L98 to another 1980's non-TPI car might look something like the blue vs orange lines below.


Yeah....they orange lines don't cross at 5200 rpms. They aren't real. They are just proposed lines for the purpose of demonstration.

 

Gregg, I don't know what you're trying to say with all this graphed stuff with imaginary lines on it. You still don't get it: the acceleration of a car is not linked directly to the engine's torque output or the slope of the torque curve, but rather to the car's tractive force at the contact patches. Tractive force is the force that accelerates the car, and it is directly determined by the power reaching the wheels and inversely by the car's speed. Period.

 

That’s for this thread, the OP has 4 posts total
and has left this thread 🧵

 

IDK why. Was it the fantasy world of DATSUN's or the fantasy world of TPI TORK monstah's?


For a meaningful demonstration, the data needs to be something that could actually happen in real life. Fantasy data helps nobody.

 

I'm down to learn a thing:
In any given gear, I blieve max acceleration will be the point where the engine is at max tq. NOT where the tq curve slope is steepest, but where it is totally flat; at it's PEAK. The engine is creating the most possible rotational force that it can make....transmitted through the gear ratio, through the diff to the tires it will create the highest tractive force (in that gear).

In this scenario, the acceleration is greatest at 3200 RPM (TPI) or 4000 RPM (LT1) in a given gear....however, the power reaching the wheels would be highest at about 4000 RPM (TPI) or 5100 RPM (LT!). Thus my confusion.
Now if we had a CVT....

 

This is true, 100%. But...

A CVT is the perfect example to think about this. If you have a CVT and it's set to hold the engine at maximum power during WOT, it will accelerate faster than if the CVT were set to hold the engine at max torque instead. That is, the tractive force would be greatest at any particular speed with the engine held at peak power instead of peak torque. Why? torque multiplication through the transmission? For the L98 in the factory graph, the peak power comes at 25% higher RPM than the peak torque, but the torque at 4000rpm hasn't fallen by 25% yet. So at the peak power RPM, there is more force at the driveshaft/wheels, even though the engine's torque output is lower at that RPM. If you never shift, you won't experience this as such: just know that the greater power output is being "soaked up" by the great road speed: remember that tractive force is Power/RoadSpeed, and again, the road speed at peak power RPM vs peak torque RPM has increased more than the torque has decreased.

In the world of conventional transmissions with discrete gear ratios, you'd see this manifest as shifting to maximize "power under the curve" vs "torque under the curve." If you shift just after torque peak at, say, 30mph, then you will have reduced the engine's leverage over the wheels. Whereas if you shift just after the power peak, you keep the engine's greater leverage (numerically higher total ratio) over the tires beyond 30mph. For the L98, you'd still be in the lower gear up to 37.5mph, which is 25% faster, and yet the engine's torque output hasn't fallen by 25% yet. Ergo, the tractive force is higher and you accelerate harder at 37mph than if you'd shifter earlier.

 

Copy. Those two lines were the crux of what I wasn't wrapping my brain around. Using percentages to show the speed vs. tq helped to visualize it more easily, too.

The CVT is an easy way to visualize "perfect world" for sure. We "clutch" snowmobiles to run WOT a little above their peak hp rated RPM....and not their peak tq RPM for the reasons that you stated.

 

I was trying to figure out a way to write that so that it made some kind of sense. Glad it worked! I have to remember for the next time I get into this discussion with someone (and there will surely be a next time).

Now, just to add one more layer to the understanding and kind of bring it full-circle: when you start talking about staying in the lower gear for longer, and how that means less torque but more multiplication leverage to get more force at the wheels, you may also realize that you're also talking about more engine RPMs. Ergo, when you talking about gearing and torque you are really using more/different verbiage to refer to power (because torque*RPM=power=MPH*TractiveForce). And it starts to sink in how the metric of power wraps all this complicated interaction between torque and gearing and MPH into one neat number that fully describes the rate at which the engine can accelerate the vehicle (and/or what top speed it can drive it to).

Industrial, ship, and aviation specs always talk about power exclusively because that's what determines the rate at which an engine can do work. They never squawk about torque unless they need to know how to spec a drivetrain to withstand the motor's force. If you want to know how fast a pump can move a fluid, you need to know the motor's power, not its torque.

 

This is quite the thread. To answer the OP……..The stock L98’s and LT1’s were quick in the day but those days are over. The amount of time spent on this topic is mind boggling.

Go take a C6 or a C7 out for a spin…….Take care!