How were C4s so quick
#61
Drifting
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?
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.
"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"
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.
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"
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?
#62
Le Mans Master
Ugghhhh...That was MY point. ESPECIALLY the first sentence. I said that a few posts ago. and you obviously understand the point. They interpreted by comparison of 250hp engines to mean avg. I meant peak. EVERYONE assumes peak when you reference how much power an engine makes. Unless you specific avg values, of course. THEY are saying any two 250hp engines will be equally fast. That's what I read.
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.
The problem is no one is explaining how a 250 hp TPI wasn't so bad....especially in the late 1980s.
Instead I'm pretty sure I was attacked at chastized for suggesting Torque SHOULDN'T BE EXCLUDED from the explanation.
It doesn't allow explanation of torque existing at lower rpms translates to lower advertized HP.
Differences in torque are how two 250hp PEAK engines can have difference avg HP and different results. Obviously, differences in where the torque occurs makes a difference too.
#64
Team Owner
Pro Mechanic
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".
.
Last edited by Tom400CFI; 12-18-2022 at 02:45 PM.
#65
Team Owner
Pro Mechanic
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.
#66
Melting Slicks
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).
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).
Last edited by tequilaboy; 12-18-2022 at 11:14 PM.
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yakmastermax (12-18-2022)
#67
Le Mans Master
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/
Broken car, or poor example maybe, but definitely slow in this test: https://www.caranddriver.com/reviews...d-test-review/
#68
Le Mans Master
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 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."
"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.
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!
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.
- 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.
Last edited by MatthewMiller; 12-20-2022 at 09:28 PM.
#69
Melting Slicks
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:
Becauseracecar (HP*375)/MPH is hard to visualize, but indeed handy.
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
Last edited by tequilaboy; 12-19-2022 at 09:54 AM.
#70
Le Mans Master
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.
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.
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.
#71
Advanced
I agree.
any chance i get to sing the praises of the L98 on the road track, i take.
most guys never take a car to the track (to my surprise!!!!!its a riot!!) but an L98 in the powerband is completely fine on track. Its up to the driver to keep it there specifically on downshifts, learning rev match shifting (i grin every time i nail a rev match downshift that dumps just inside the powerband).
keep it in the powerband right around 3500 and use the 330ftlbs thats there for u. If you have an LT1, you will be doing the exact same thing but the powerband is higher. You as the driver control where you are using your stick.
If you are on a course with a lonng long straight away you will still be fine on a L98 but u will be in a higher gear at the end of it or u will have fallen on your face at 5000rpm. Learn to shift, learn to be quick with your feet.
Think of it like......Tom Cruise in the new TopGun (which is off the charts good)....he was such a good pilot that he knew how to extract the most out of the outdated Jet of his era.
Ofcourse im not saying that an L98 will run with a modern car. But ive personally seen an L98 have far faster lap times than an LS its all about extracting the juice out of the fruit. Technique.
People say L98’s are awesome street cars because you never get out of second gear and barely into 3rd. A street is like a track with no straight aways longer than the stoplight distance. .... just keep the engine in the powerband....and u can go to a decent straight.
Now COTA etc is another thing your L98 will get dusted. Where i live all road course tracks have didly for long straights.
L98 FTW, i love them. They give the next gen of Corvette owners a chance to get in cheap, they are rediculously easy to work on with home depot toolsets. Ive never owned an LT1 or and LT4 but i *really* look fwd to picking one up and learning that platform too. I went straight to LT5 and im in lllooooovvveeee.
And....4+3 rock. So do ZF’s.
get to the track baby!!!! its an addiction!!
most guys never take a car to the track (to my surprise!!!!!its a riot!!) but an L98 in the powerband is completely fine on track. Its up to the driver to keep it there specifically on downshifts, learning rev match shifting (i grin every time i nail a rev match downshift that dumps just inside the powerband).
keep it in the powerband right around 3500 and use the 330ftlbs thats there for u. If you have an LT1, you will be doing the exact same thing but the powerband is higher. You as the driver control where you are using your stick.
If you are on a course with a lonng long straight away you will still be fine on a L98 but u will be in a higher gear at the end of it or u will have fallen on your face at 5000rpm. Learn to shift, learn to be quick with your feet.
Think of it like......Tom Cruise in the new TopGun (which is off the charts good)....he was such a good pilot that he knew how to extract the most out of the outdated Jet of his era.
Ofcourse im not saying that an L98 will run with a modern car. But ive personally seen an L98 have far faster lap times than an LS its all about extracting the juice out of the fruit. Technique.
People say L98’s are awesome street cars because you never get out of second gear and barely into 3rd. A street is like a track with no straight aways longer than the stoplight distance. .... just keep the engine in the powerband....and u can go to a decent straight.
Now COTA etc is another thing your L98 will get dusted. Where i live all road course tracks have didly for long straights.
L98 FTW, i love them. They give the next gen of Corvette owners a chance to get in cheap, they are rediculously easy to work on with home depot toolsets. Ive never owned an LT1 or and LT4 but i *really* look fwd to picking one up and learning that platform too. I went straight to LT5 and im in lllooooovvveeee.
And....4+3 rock. So do ZF’s.
get to the track baby!!!! its an addiction!!
#72
Race Director
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.
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.
#73
Le Mans Master
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.
Last edited by MatthewMiller; 12-23-2022 at 12:45 PM.
#74
That’s for this thread, the OP has 4 posts total
and has left this thread 🧵
#75
Team Owner
Pro Mechanic
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.
For a meaningful demonstration, the data needs to be something that could actually happen in real life. Fantasy data helps nobody.
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JD'S WHITE 93 (12-23-2022)
#76
Team Owner
Pro Mechanic
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.
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....
#77
Le Mans Master
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 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....
Now if we had a CVT....
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.
#78
Team Owner
Pro Mechanic
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.
#79
Le Mans Master
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.