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QUOTE: Originally posted by Randy Stahl With the latest questions about horse power/ tractive effort ,this thread will answer many questions and provide alot of formulea for the math inclined. Randy
"No soup for you!" - Yev Kassem (from Seinfeld)
Originally posted by Randy Stahl I'd be glad to send any one copies of the EMD charts & graphs for just about any locomotive EMD built, in fact I have one in front of me right now for an sd-45. I still don't see any thing about tractive effort, how ever you are correct in that a wheel slip correction will affect main generator exitation , but when the HP is calibrated on a locomotive, tractive effort is not an issue, the wheel slip system is calibrated by itself. Locomotive horsepower is kilowatts Period. It is correct to say the GP-40 has it's motors wired in parallel, it does not make transition. The AR-10 main alternator has it's limitations therfore with the addition of 2 more traction motors the capacity of the AR10 is exeeded, the locomotive starts in series to reduce current demands on the AR10. You are correct, the GP-40 should kick the SD's butt, however since the kilowatt out put of the AR10 is divided over 4 motors, not 6 ,the horsepower per traction motor is greater causing wheel slip/ lower tractive effort, the GP40 will accelerate very quickly. The comparison I use when I'm holding training classes is to compare the GP and the SD to a corvette and a Jeep. Answer this, suppose you went out to the service tracks ,to a locomotive you were entirely unfamilier with, suppose the locomotive" would not pull" , your job is to repair this engine. Where would you start? Remember you are not familier with the engine, HP , TE is all unknown , all you have is a wiring schematic. The formula for calculating steam locomotive HP is correct, it is well known that steam loco hp is limited only by mechanical technology, had the steam locomotive continued developement we would have seen how limitless they really are, just the developement of poppet valves made a big difference in the valve efficiency. The formula does not lie. take a calculator and try it. It is true that a diesel locomotive HP is infinite you can increase RPM, increas the fuel etc in fact EMD did just that on the DD 40X , they took a regular 16 645 e3 and raised the RPM to 950 instead of 900 and increased the horsepower to 3300 instead of 3000. I would like to raise the HP on my GP-40s to 4000 but the short term results would be undesireable, it could do it for a short time though!!! Here are the formulas we don't need to know or care about. HPx308 divided by speed = TE 308 = constant, coefficient of friction etc. MPH x (TE) divided by 375 = rail HP 375= mile lbs per HP Constant. Adhesion = TE divided by weight on drivers Max TE = weight on drivers (assume 100%) x HP divided by # drivers [/quoteThis is the one. Randy Reply Anonymous Member sinceApril 2003 305,205 posts Posted by Anonymous on Monday, August 2, 2004 7:28 AM QUOTE: Originally posted by adrianspeeder Visit http://www.vcn.com/~alkrug/rrfacts/hp_te.htm That is the best explanation i have ever read. Adrianspeeder This is the most clealy written and concise explanation to all this tarnation yet, whew !!!! Reply Edit dinwitty Member sinceAugust 2004 2,844 posts Posted by dinwitty on Monday, November 15, 2010 11:03 PM old thread but curious. How can a simple thought get more confused sure, power varies at speed and torque, but so many steam engine ratings are noted at...like the nkp 765 64100 lbs tractive effort. All you want is a simple comparison to the diesel horsepower. So lets get a general ballpark figure than a rocket scientist description. Reply Anonymous Member sinceApril 2003 305,205 posts Posted by Anonymous on Tuesday, November 16, 2010 8:06 AM dinwitty old thread but curious. How can a simple thought get more confused sure, power varies at speed and torque, but so many steam engine ratings are noted at...like the nkp 765 64100 lbs tractive effort. All you want is a simple comparison to the diesel horsepower. So lets get a general ballpark figure than a rocket scientist description. Well if the original poster wanted a simple answer, he should not have based his question on comparing an apple to an orange. Both steam and diesel locomotives have tractive effort and horsepower. There is no formula to convert tractive effort alone into horsepower because horsepower is a measure of force and speed, whereas tractive effort is only a measure of force. So the answer to the original poster’s question is no. Perhaps the question could be revised to address what was being sought. There are a variety of ways to compare the performance of locomotives. It is possible to have high horsepower and low tractive effort, and vice versa. For example (if it were not for frictional losses), a lawnmower engine could pull a train, but just not very fast. Reply Edit Deggesty Member sinceAugust 2005 From: At the Crossroads of the West 11,013 posts Posted by Deggesty on Tuesday, November 16, 2010 7:51 PM Modelcar....Would you believe Ed I actually have a real slide rule still in the office here and one that was actually used ...[in the past, at work]...before calcuators. It's a nice big one too. One of our Engineers in our lab used to use a round slide rule...Ever see one of those...? Yes, Quentin, I have seen, and may still have around somewhere, a circular slide rule. I do still have my 10" K&E slide rule, which I used in college when I did not need four or five place logarithms for my calculations. Now, did you ever see a 20" slide rule? one of my college friends had one; its case, which could hung from his belt, resembled a short sword scabbard. Johnny Reply ICLand Member sinceMay 2010 172 posts Posted by ICLand on Tuesday, November 16, 2010 8:00 PM DeggestyNow, did you ever see a 20" slide rule? one of my college friends had one; its case, which could hung from his belt, resembled a short sword scabbard. I've still got mine from college. Reply Paul of Covington Member sinceJuly 2010 From: Louisiana 2,295 posts Posted by Paul of Covington on Tuesday, November 16, 2010 9:49 PM I assume you guys are talking about the K&E Log log duplex decitrig. ( I threw that in to puzzle the youngsters.) I do remember in college seeing a round one and a 20-inch one. _____________ "A stranger's just a friend you ain't met yet." --- Dave Gardner Reply Deggesty Member sinceAugust 2005 From: At the Crossroads of the West 11,013 posts Posted by Deggesty on Tuesday, November 16, 2010 10:05 PM I forgot about the one we had in the math classroom. It was about three feet long, was made of three tongue-and-groove boards--and had only two scales on it. It was used to demonstrate to the students who had never seen a slide rule how it worked for multiplying and dividing (adding and subtracting logarithms). You might have been able to read it to five places. Johnny Reply ICLand Member sinceMay 2010 172 posts Posted by ICLand on Tuesday, November 16, 2010 10:20 PM Deggesty I forgot about the one we had in the math classroom. It was about three feet long, was made of three tongue-and-groove boards--and had only two scales on it. It was used to demonstrate to the students who had never seen a slide rule how it worked for multiplying and dividing (adding and subtracting logarithms). You might have been able to read it to five places. I was in one of those high schools that offered advanced Math/Engineering courses through the University, and we had one of those big "Demonstrators" for those of us with the GPA to participate and to get up to speed with slide rules before we went off to the college courses. The personal slide rule was a hard steel, yellow Pickett Slide rule. High precision and good for self-defense as well ... Reply Semper Vaporo Member sinceApril 2007 From: Iowa 3,293 posts Posted by Semper Vaporo on Tuesday, November 16, 2010 10:28 PM I have a round one... 2nd one I got because I lost the first one. The first one was outstanding, but the one I have now was not well made and 2 x 2 does not equal 4 on it! My brother had a Picket sliderule that he wore in a fancy leather scabbard on his belt. He said he wore it there so no one would "pocket his Picket". Semper Vaporo Pkgs. Reply ICLand Member sinceMay 2010 172 posts Posted by ICLand on Tuesday, November 16, 2010 10:53 PM Semper Vaporo My brother had a Picket sliderule that he wore in a fancy leather scabbard on his belt. He said he wore it there so no one would "pocket his Picket". The steel Picketts came with a fancy hard plastic "scabbard" with both a belt loop through its leather covering, and a loop and clip. I never used the "belt loop" feature, which mimicked a gun holster or a Samurai sword I suppose. Last used that old slide rule on a motive power study in 1974. That Pickett is still here in my desk. And yes, there is an accepted formula that converts tractive effort to horsepower at given speeds and it is the same formula for both Diesel-electric and Steam. Most of the original power curves for steam and diesel utilized slide rules to make those calculations. HP=(Tractive Effort x Speed)/375 Contrary to some of the comments, that "tractive effort" is a "railfans" view, the Tractive Effort is what is measured by the dynamometer car, FROM WHICH HORSEPOWER IS CALCULATED. Tractive Effort is always the primary measure. Horsepower is always the secondary, calculated, measure. Reply Deggesty Member sinceAugust 2005 From: At the Crossroads of the West 11,013 posts Posted by Deggesty on Wednesday, November 17, 2010 12:55 PM Semper Vaporo I have a round one... 2nd one I got because I lost the first one. The first one was outstanding, but the one I have now was not well made and 2 x 2 does not equal 4 on it! The new math? Johnny Reply selector Member sinceFebruary 2005 From: Vancouver Island, BC 23,326 posts Posted by selector on Wednesday, November 17, 2010 1:30 PM Au contraire mon ami...it is a perfectly made 'quantum' physics version which takes into account uncertainty. I mean, can we ever really be certain that one of the 2's is really a 2? Reply Semper Vaporo Member sinceApril 2007 From: Iowa 3,293 posts Posted by Semper Vaporo on Wednesday, November 17, 2010 1:51 PM Gee! I always thought it was just MY uncertainty in using a slide rule in the first place! It must actually be an artifact of my proving the Heisenburg principle wrong... I know both "Where" I was AND "When" I lost the first one. Semper Vaporo Pkgs. Reply feltonhill Member sinceAugust 2003 From: Northern VA 484 posts Posted by feltonhill on Wednesday, November 17, 2010 5:18 PM The above tractive effort formula is correct, as long as you use the proper TE at each speed increment. However, dynamometer cars measure drawbar pull, not TE. TE at speed is calculated using mean effective pressure in the cylinders, which is estimated from indicator diagrams Reply selector Member sinceFebruary 2005 From: Vancouver Island, BC 23,326 posts Posted by selector on Wednesday, November 17, 2010 5:23 PM Can we ever really be sure there is "effective pressure" in the cylinders? Reply dinwitty Member sinceAugust 2004 2,844 posts Posted by dinwitty on Wednesday, November 17, 2010 8:34 PM well then all the Diesel power ratings we see is deceptive. We can't look at the basic ratings and compare just for the general interest without doing some rocket scientist calculations. It almost makes me think this is done on purpose to make the Diesel "look good" vs the steam engine.Just about every steam description I have seen says "Tractive Effort" and any diesel description says "Horsepower". Duh. Really, somebody somewhere decided this is going to be -THE- way to describe Diesel power. Fooey. Somebody fix this. Reply JayPotter Member sinceMay 2002 318 posts Posted by JayPotter on Thursday, November 18, 2010 5:36 AM Dinwitty, I think that some of the explanations you're seeking can be found in the book The Steam Locomotive, by Ralph P. Johnson. Reply Anonymous Member sinceApril 2003 305,205 posts Posted by Anonymous on Thursday, November 18, 2010 10:56 AM dinwitty well then all the Diesel power ratings we see is deceptive. It almost makes me think this is done on purpose to make the Diesel "look good" vs the steam engine.Just about every steam description I have seen says "Tractive Effort" and any diesel description says "Horsepower". Duh. Really, somebody somewhere decided this is going to be -THE- way to describe Diesel power. Fooey. Somebody fix this. There is no conspiracy to make diesels seem better than steam. But it is hard to state one common rating or measure of performance that is the most meaningful. Generally, the early diesels had considerably less horsepower than the modern steamers that they replaced. But diesels had the ability to M.U., so the horsepower disparity could be compensated for. And since M.U. operation adds more drivers, the tractive effort of diesels could rival or exceed that of a steam locomotive of equivalent horsepower. But the greatest fundamental advantage of diesel over steam is the fact that a diesel can develop full horsepower when starting from zero speed, and deliver that horsepower when accelerating from a stop, as well as when lugging at near stall speed. Whereas a steamer cannot develop its full horsepower until it reaches its maximum speed. Fundamentally, a diesel-electric locomotive has a transmission, while a steamer does not. Although one might make the case that the variable cutoff of a steamer amounts to a form of transmission. Reply Edit carnej1 Member sinceNovember 2003 From: Rhode Island 2,289 posts Posted by carnej1 on Thursday, November 18, 2010 11:09 AM adrianspeederI want to know what will win an good ol' tug of war. Big Boy vs. The SD-90. (Da da da. SUNDAY SUNDAY SOMEDAY) Somewhat related: I wanted to know what would win a tug of war with stick vs. auto. That was found out when my buddy got the same exact bronco as mine with the 302 v8, except his was an auto and mine is a five speed. Out to a hard packed field with three big chains we went and the pull was on. In the end there were two broncos that dug themselves down to the axles without moving one way or the other. If the big boy could out tug the sd90, how many sd90s would be needed? Adrianspeeder An SD90MAC-H has a starting tractive effort of 200,000 lbf. A Big Boy had a continouos tractive effort of 135,500 lbf.(no higher at starting). the MAC would pull the Big boy backward. Electric traction motors produce significantly higher low end torque than a steam piston acting on a wheel does... "I Often Dream of Trains"-From the Album of the Same Name by Robyn Hitchcock Reply timz Member sinceFebruary 2005 2,350 posts Posted by timz on Thursday, November 18, 2010 12:34 PM dinwittyIt almost makes me think this is done on purpose to make the Diesel "look good" vs the steam engine.Just about every steam description I have seen says "Tractive Effort" and any diesel description says "Horsepower".Anyone who wants to make the diesel look good will quote its so-called "Tractive Effort", which would make a single F7 look comparable to a smallish 4-8-4. That would indeed be deceptive-- so you shouldn't complain about diesel makers quoting horsepower. Reply Paul_D_North_Jr Member sinceOctober 2006 From: Allentown, PA 9,810 posts Posted by Paul_D_North_Jr on Thursday, November 18, 2010 5:07 PM Bucyrus [snip] And since M.U. operation adds more drivers, the tractive effort of diesels could rival or exceed that of a steam locomotive of equivalent horsepower. And in most cases, all of a diesel's weight is on the drivers - there's no possible tractive weight lost to non-contributing pony or pilot trucks, or trailing trucks and tenders - except for when those are equipped with boosters. Bucyrus But the greatest fundamental advantage of diesel over steam is the fact that a diesel can develop full horsepower when starting from zero speed, and deliver that horsepower when accelerating from a stop, as well as when lugging at near stall speed. [snip] Not quite - the diesel developing full horsepower is subject to/ limited by: 1. Maximum current ratings to keep the motors from overheating, unless for very short time periods - that would effectively de-rate them when lugging for more than 15 or 30 minutes or so. 2. Low speed, even when maximum tractive effort is being put out and the motor current isn't limiting. Generally it's not until the speed gets up to between 12 to 20 MPH that the product of speed x Tractive Effort starts to equal the potential horsepower output of the diesel engine. Below that speed, the engine can usually produce the horsepower, but the resulting motor torque causes the tractive effort to exceed what the adhesion (= coefficient of friction x weight on drivers) can sustain - it's almost an inverse relationship, and the theoretical TE gets very high as the speed gets very low, but it can't be utilized due to the adhesion limitation. As a result, the effective horsepower output is limited by the maximum TE, and essentially varies linearly with speed - HP = TE x V / 375 - until that 12 - 20 MPH speed is reached where all of the HP that the engine is capable of producing can be utilized. It's just like starting your car on wet or icy pavement - if you stomp on the gas, all you're going to do is spin the tires, because the engine is then producing more power and accelerating force than the adhesion and traction of the tires can transmit. But as you start moving, you can apply more power and hence tractive effort as you go faster, and the tires won't spin if you're gentle about it. Finally, after you get up to road speed, then the engine's horsepower output - or the legal speed limit or road conditions, etc. - become the limiting factors, not the traction of the tires. - Paul North. "This Fascinating Railroad Business" (title of 1943 book by Robert Selph Henry of the AAR) Reply 12 Join our Community! 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QUOTE: Originally posted by adrianspeeder Visit http://www.vcn.com/~alkrug/rrfacts/hp_te.htm That is the best explanation i have ever read. Adrianspeeder
old thread but curious. How can a simple thought get more confused
sure, power varies at speed and torque, but so many steam engine ratings are noted at...like the nkp 765 64100 lbs tractive effort. All you want is a simple comparison to the diesel horsepower. So lets get a general ballpark figure than a rocket scientist description.
dinwitty old thread but curious. How can a simple thought get more confused sure, power varies at speed and torque, but so many steam engine ratings are noted at...like the nkp 765 64100 lbs tractive effort. All you want is a simple comparison to the diesel horsepower. So lets get a general ballpark figure than a rocket scientist description.
Well if the original poster wanted a simple answer, he should not have based his question on comparing an apple to an orange. Both steam and diesel locomotives have tractive effort and horsepower.
There is no formula to convert tractive effort alone into horsepower because horsepower is a measure of force and speed, whereas tractive effort is only a measure of force.
So the answer to the original poster’s question is no.
Perhaps the question could be revised to address what was being sought. There are a variety of ways to compare the performance of locomotives.
It is possible to have high horsepower and low tractive effort, and vice versa. For example (if it were not for frictional losses), a lawnmower engine could pull a train, but just not very fast.
Modelcar....Would you believe Ed I actually have a real slide rule still in the office here and one that was actually used ...[in the past, at work]...before calcuators. It's a nice big one too. One of our Engineers in our lab used to use a round slide rule...Ever see one of those...?
Johnny
DeggestyNow, did you ever see a 20" slide rule? one of my college friends had one; its case, which could hung from his belt, resembled a short sword scabbard.
I've still got mine from college.
I assume you guys are talking about the K&E Log log duplex decitrig. ( I threw that in to puzzle the youngsters.) I do remember in college seeing a round one and a 20-inch one.
_____________
"A stranger's just a friend you ain't met yet." --- Dave Gardner
I forgot about the one we had in the math classroom. It was about three feet long, was made of three tongue-and-groove boards--and had only two scales on it. It was used to demonstrate to the students who had never seen a slide rule how it worked for multiplying and dividing (adding and subtracting logarithms). You might have been able to read it to five places.
Deggesty I forgot about the one we had in the math classroom. It was about three feet long, was made of three tongue-and-groove boards--and had only two scales on it. It was used to demonstrate to the students who had never seen a slide rule how it worked for multiplying and dividing (adding and subtracting logarithms). You might have been able to read it to five places.
I was in one of those high schools that offered advanced Math/Engineering courses through the University, and we had one of those big "Demonstrators" for those of us with the GPA to participate and to get up to speed with slide rules before we went off to the college courses.
The personal slide rule was a hard steel, yellow Pickett Slide rule. High precision and good for self-defense as well ...
I have a round one... 2nd one I got because I lost the first one. The first one was outstanding, but the one I have now was not well made and 2 x 2 does not equal 4 on it!
My brother had a Picket sliderule that he wore in a fancy leather scabbard on his belt. He said he wore it there so no one would "pocket his Picket".
Semper Vaporo
Pkgs.
Semper Vaporo My brother had a Picket sliderule that he wore in a fancy leather scabbard on his belt. He said he wore it there so no one would "pocket his Picket".
The steel Picketts came with a fancy hard plastic "scabbard" with both a belt loop through its leather covering, and a loop and clip. I never used the "belt loop" feature, which mimicked a gun holster or a Samurai sword I suppose. Last used that old slide rule on a motive power study in 1974. That Pickett is still here in my desk.
And yes, there is an accepted formula that converts tractive effort to horsepower at given speeds and it is the same formula for both Diesel-electric and Steam.
Most of the original power curves for steam and diesel utilized slide rules to make those calculations.
HP=(Tractive Effort x Speed)/375
Contrary to some of the comments, that "tractive effort" is a "railfans" view, the Tractive Effort is what is measured by the dynamometer car, FROM WHICH HORSEPOWER IS CALCULATED.
Tractive Effort is always the primary measure. Horsepower is always the secondary, calculated, measure.
Semper Vaporo I have a round one... 2nd one I got because I lost the first one. The first one was outstanding, but the one I have now was not well made and 2 x 2 does not equal 4 on it!
Au contraire mon ami...it is a perfectly made 'quantum' physics version which takes into account uncertainty. I mean, can we ever really be certain that one of the 2's is really a 2?
Gee! I always thought it was just MY uncertainty in using a slide rule in the first place! It must actually be an artifact of my proving the Heisenburg principle wrong... I know both "Where" I was AND "When" I lost the first one.
The above tractive effort formula is correct, as long as you use the proper TE at each speed increment. However, dynamometer cars measure drawbar pull, not TE. TE at speed is calculated using mean effective pressure in the cylinders, which is estimated from indicator diagrams
Can we ever really be sure there is "effective pressure" in the cylinders?
well then all the Diesel power ratings we see is deceptive. We can't look at the basic ratings and compare just for the general interest without doing some rocket scientist calculations. It almost makes me think this is done on purpose to make the Diesel "look good" vs the steam engine.Just about every steam description I have seen says "Tractive Effort" and any diesel description says "Horsepower". Duh.
Really, somebody somewhere decided this is going to be -THE- way to describe Diesel power. Fooey. Somebody fix this.
Dinwitty, I think that some of the explanations you're seeking can be found in the book The Steam Locomotive, by Ralph P. Johnson.
dinwitty well then all the Diesel power ratings we see is deceptive. It almost makes me think this is done on purpose to make the Diesel "look good" vs the steam engine.Just about every steam description I have seen says "Tractive Effort" and any diesel description says "Horsepower". Duh. Really, somebody somewhere decided this is going to be -THE- way to describe Diesel power. Fooey. Somebody fix this.
well then all the Diesel power ratings we see is deceptive. It almost makes me think this is done on purpose to make the Diesel "look good" vs the steam engine.Just about every steam description I have seen says "Tractive Effort" and any diesel description says "Horsepower". Duh.
There is no conspiracy to make diesels seem better than steam. But it is hard to state one common rating or measure of performance that is the most meaningful. Generally, the early diesels had considerably less horsepower than the modern steamers that they replaced. But diesels had the ability to M.U., so the horsepower disparity could be compensated for. And since M.U. operation adds more drivers, the tractive effort of diesels could rival or exceed that of a steam locomotive of equivalent horsepower.
But the greatest fundamental advantage of diesel over steam is the fact that a diesel can develop full horsepower when starting from zero speed, and deliver that horsepower when accelerating from a stop, as well as when lugging at near stall speed. Whereas a steamer cannot develop its full horsepower until it reaches its maximum speed.
Fundamentally, a diesel-electric locomotive has a transmission, while a steamer does not. Although one might make the case that the variable cutoff of a steamer amounts to a form of transmission.
adrianspeederI want to know what will win an good ol' tug of war. Big Boy vs. The SD-90. (Da da da. SUNDAY SUNDAY SOMEDAY) Somewhat related: I wanted to know what would win a tug of war with stick vs. auto. That was found out when my buddy got the same exact bronco as mine with the 302 v8, except his was an auto and mine is a five speed. Out to a hard packed field with three big chains we went and the pull was on. In the end there were two broncos that dug themselves down to the axles without moving one way or the other. If the big boy could out tug the sd90, how many sd90s would be needed? Adrianspeeder
An SD90MAC-H has a starting tractive effort of 200,000 lbf. A Big Boy had a continouos tractive effort of 135,500 lbf.(no higher at starting). the MAC would pull the Big boy backward.
Electric traction motors produce significantly higher low end torque than a steam piston acting on a wheel does...
"I Often Dream of Trains"-From the Album of the Same Name by Robyn Hitchcock
dinwittyIt almost makes me think this is done on purpose to make the Diesel "look good" vs the steam engine.Just about every steam description I have seen says "Tractive Effort" and any diesel description says "Horsepower".
Bucyrus [snip] And since M.U. operation adds more drivers, the tractive effort of diesels could rival or exceed that of a steam locomotive of equivalent horsepower.
And in most cases, all of a diesel's weight is on the drivers - there's no possible tractive weight lost to non-contributing pony or pilot trucks, or trailing trucks and tenders - except for when those are equipped with boosters.
Bucyrus But the greatest fundamental advantage of diesel over steam is the fact that a diesel can develop full horsepower when starting from zero speed, and deliver that horsepower when accelerating from a stop, as well as when lugging at near stall speed. [snip]
Not quite - the diesel developing full horsepower is subject to/ limited by:
1. Maximum current ratings to keep the motors from overheating, unless for very short time periods - that would effectively de-rate them when lugging for more than 15 or 30 minutes or so.
2. Low speed, even when maximum tractive effort is being put out and the motor current isn't limiting. Generally it's not until the speed gets up to between 12 to 20 MPH that the product of speed x Tractive Effort starts to equal the potential horsepower output of the diesel engine.
Below that speed, the engine can usually produce the horsepower, but the resulting motor torque causes the tractive effort to exceed what the adhesion (= coefficient of friction x weight on drivers) can sustain - it's almost an inverse relationship, and the theoretical TE gets very high as the speed gets very low, but it can't be utilized due to the adhesion limitation.
As a result, the effective horsepower output is limited by the maximum TE, and essentially varies linearly with speed - HP = TE x V / 375 - until that 12 - 20 MPH speed is reached where all of the HP that the engine is capable of producing can be utilized.
It's just like starting your car on wet or icy pavement - if you stomp on the gas, all you're going to do is spin the tires, because the engine is then producing more power and accelerating force than the adhesion and traction of the tires can transmit. But as you start moving, you can apply more power and hence tractive effort as you go faster, and the tires won't spin if you're gentle about it. Finally, after you get up to road speed, then the engine's horsepower output - or the legal speed limit or road conditions, etc. - become the limiting factors, not the traction of the tires.
- Paul North.
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