Was looking through a book I have . It's called "How to build Big-Inch GM LS -Series engines" . SA Design SA203 Stephen Kim if anyone is interested. It's a very good book with lots of pictures and good explanations of the hows and whys. Thought I would give him credit for what I'm posting so maybe some of you will be interested in getting a copy. Anyway he has the best explanation I've seen on how the number 5252 came about. Thats the point where (on a graph) torque peaks and begins to decline........ It also explains why HP can continue to rise when torque is falling. There are only 2 ways to increase HP. Increase torque OR Increase RPM 5252 is the rpm point where an engine begins to benefit more from RPM than from Torque. But WHY is that the same in all engines ? Here is a simple dyno chart that demonstrates the 5252 principle. Here is the explanation.......... And now ya know............. (Again, an excellent book for learning about building LS engines and which parts you should and should not use) Let me make a point here to clarify something. If you have a specific RPM where you want to increase your HP, then you MUST increase the Torque at that RPM to derive more HP. There are only two variables . Torque and RPM. People rev their motors higher to gain HP..........That simply means if an engine is producing X torque at each revolution......more revolutions will produce that X Torque more times in one minute......so more HP. If you want more HP for your midrange, say at 2500 RPMs. Each rpm level will need more TORQUE at that RPM to increase the resultant HP at that rpm. You can increase Torque at a specific RPM many ways. Using a larger displacement, or adding a super charger or even tweaking the compression or camshaft....but you must increase torque to increase HP at a given rpm.
Yes HP is a function of torque and RPM. Always cracks me up during the torque and HP debates when someone says they would rather have torque versus HP or vice versa. I think the toque crowd really wants a flat torque curve starting at low rpm. At the end of the day or race the car with the most HP will be faster and quicker if said car is set up right. It’s simple physics horse power is work done over time. Dan
The reason the number is 5252 is because James Watt decided that a horse can sustain a rate of energy production of 33,000 lb-ft per minute. If you divide that by 2π, you get the 5252 number Since that number is a constant in the equation to calculate HP from Torque and RPM, torque and HP always have to be equal at 5252 RPM. It has nothing to do with how a particular engine is built, it just has to do with some simple math.
I had a customer whom i had done a proposal for a hydraulic system (Indian engineer) ask me, "what is this 5252?" I had to take him through all the calculations to derive it. I wasn't long out of college at that time and it was pretty easy for me to do. His employer overheard the conversation and later told me, "I knew I was in trouble when he didn't know what 5252 was!"
When I saw the title, my dislecsia thought of the old tune "in the year, 25, 25" I don't race, but on the street I love torque. 5,252 RPM is rarely much of an issue in suburbia.
Also: it depends on what units of measure you use. For instance, 5252rpm is the engine speed at which 0.7456999kW translates to 1.35582Nm. The 1-to-1 correspondences will happen at different speeds for all the various combinations of units. kW = lb.ft @ 7043rpm hp = Nm @ 3874rpm kW = Nm @ 5195rpm This also highlights how peak power and peak torque happen at specific speeds. So when figuring out how a torque converter is going to behave using its K-factor, it is necessary to consider the torque it sees at that engine speed. It isn't seeing peak torque all the time.
2π (2 x 3.14) is 6.28 as mentioned in the article, and yes it is a constant. I think the beauty in the thing was that Watt recognized that using a 1 foot radius and then calculating the distance it travels (the circumference) AND how many times it travels that distance (1 rpm X ? rpms) he could come up with a usable way to accurately calculate the resulting force available. What I wonder about is why he used "lbs/ft" as Scotland (where he lived) used the Metric system. Up to then, I don't know of any other method of comparing output. He was a brilliant guy. Yes, thats the point I'm making ..........but most of us only knew that it happens that way..........but not why it happens that way.
Indeed: https://en.wikipedia.org/wiki/Scottish_units Watt would have lived in the midst of a process of standardizing Imperial units.
He only missed the metric system by about 145 years... James Watt Born: 19 January 1736, Greenock, United Kingdom Died: 25 August 1819 (age 83 years), Heathfield Hall The British Imperial System of weights and measures was used officially in Great Britain from 1824 until the adoption of the metric system beginning in 1965.
Well that explains why he used "foot" instead of "meters" , thanks for the explanation. I know there are historically lots of "systems of measurement" and variations of those systems that have come and gone. The Metric System also has/had variations as well. I remember that while working as a Machinist apprentice at the Naval Ordnance Station we used to get drawings for parts for an Italian weapon, and the drawings and documentation were not always accurate, requiring us to contact the engineering department for explanation. That was 1966 or so. I'm glad Watt chose a horse for his basis to calculate power............otherwise we might not have Hot Rods today because there would be no such thing as "horsepower" Think about it, at the time he invented his equation, there were no engines that turned 5252 rpms. I just think what he was able to assimilate into a usable equation was pretty remarkable for his time. "Imperial System: . The Imperial system was officially adopted in Great Britain in 1824 and defined by the Weights and Measures Act. It includes units like the inch, foot, yard, mile, ounce, pound, and gallon" I was in Scotland in the sixties courtesy of the USN. Only thing I think I remember was that beer was served in pints...........but my memory of that time period is hazy due to consuming that beer. I think I'm still having lingering effects.
Good to see that author use ft.-lbs. instead of modern pounds feet. You can pounds yer feet all you want; it will always be ft.-lbs. to me. I`ve also seen dyno graphs where the two don`t intersect and find that odd.
Now how much of an increase in RPM does it take to make up for the lack of torque to get an equal ET and/or MPH out of the same HP number?
What ever RPM is required to get the HP. Two engines: 1 torque monster makes peak HP of 400 at 4500 RPM 2 high winder makes peak HP of 400 at 7200 RPM Both engines powering the same car should have equal ETs and top speeds if they are set up set up for the power curve, that is gearing etc has to be maximized to take advantage of the curve. The engine producing the HP at lower RPM should be easier to drive and attain its best ET. If you ever rode an old school dirt bike you’ll understand. Dan
Like the old 2 stroke bikes when they’d hit their powerband. I remember riding Honda CRs and how they’d take off once you got the revs up.
The Jalopy Journal
