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View Full Version : Where Does Torque Come From??



grumpyvette
05-03-02, 11:02 AM
no, not the KEBBLER ELVES OR THE STORK, look here!
heres a chart of a typical engine http://www.geocities.com/ljaya6390/analysis.htm
btw for those of you that can,t convert MPa-to-PSI in your head
http://epics.aps.anl.gov/asd/me/UnitConv.html

btw if your still not able to get it 5.402 MPa=783 psi

now thats in about an 8:1 cpr engine you can reach 1200psi in a 11:1-13:1 racing engine or a engine with the correctly tuned cam and intake/exhaust pulse timeing to efficiently fill the cylinders, thats where most of the better torque comes from! also notice that the pressure is only high for about 30 degs of the total 720 deg cycle and its only able to do usefull mechanical work for about 20 degs of the total 720 degs

now if thats a typical smogger 350 chevy.
a 4" bore has about 12.588 sq inchs of surface area x 783 psi /720 degs in the cycle x 20 degs of usefull work= 273 ft lbs of torque applied to the crankshaft every 90 degs, but kick the pressure to 1200psi in a 11:1 engine, and that same 350 makes 12.588sq inches x 1200psi /720degs in the cycle x 20 degs of usefull work= 419.7 ft lbs of torque, now alot depends on the dcr and volumetric efficiency but you should be getting the idea here by now!as long as the cylinders can fill completely you get a good fuel/air burn so you get a good cylinder pressure curve against the piston each time the cylinder fires,THE ENGINES TORQUE CURVE INCREASES WITH THE NUMBER OF EFFECTIVE POWER STROKES PER SECOND, at very low speeds theres not enough air velocity to mix the fuel correctly or produce a effective ram tuneing effect but as the rpms increase the cylinders fill very efficiently untill the rpms reach a point where the cylinders just don,t have the time necessary to flow
enough air through the valves to fill the cylinders , remember a 5000rpm the intake valve out of 720 degs in each cycle opens for about 250degs of effective flow even with a hot roller cam, now thats only about 35% of the time and theres 41.6 intake strokes per second , thats only 1/60th of a second for air to flow into the cylinder, I found this graph that shows the relationship between V.E.(VOLUMETRIC EFFICIENCY) and AN ENGINEs torque CURVE http://www.n2performance.com/lectures/lect1/n2perf5.gif

WHAT THAT GRAPH SHOWS RATHER EFFECTIVELY is that its your engines ability to fill the cylinders that increases your power and the more efficiently you do that the higher the rpm level you can acomplish that at the more power your engine makes, remember the formula for hp is (torque x rpm/ 5252=hp)so moveing the torque curve higher in the rpm range increases hp


keep the rubber side down and the fiberglass off the guard rails

grumpyvette
05-03-02, 04:07 PM
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Ill grant you that I over simplifyed formula greatly so as to not confuze most of the readers (some of which know even more than I do Ill also grant you) but the cylinder pressure is applied over the surface area of the piston but only applies its peak force for about the first 18-23 degs of crankshaft rotation,If we compare a 383 (4.03 bore/3.75" stroke) to a 377(4.155 bore/3.48 stroke) sbc, the longer stroke allows that 18-23 degs of rotation to be applied over a longer distance with more leverage, a 4.155" bore would have 13.56 sq inches of surface area per piston, thats an approximate 7.7% increase in surface area for the pressure to work against for the 18 to 23 degs of rotation durring that peak force period,now the shorter stroke crank covers less distance durring its effective pressure peak,(lets say 20 degs as opposed to 21 degs for a 383 with its longer 3.75" stroke)thats one of the reasons big bores and small strokes work well for racing, not only do larger bores apply more force(although for a shorter distance )they also allow more strokes per minute for any given max piston speed,they allow a greater volume of fuel/air mix to be burned in relation to the cylinders surface area and the have a smaller ratio of pisrton ring surface area in relation to the pistons total top surface where the pressure is applied,(I started to do the math but after 2 pages worth I said ,no ones going to follow this anyway) the formula for hp is (torque x rpm / 5252= hp) that basically reflects the that torque is cylinder pressure during the power stroke times the number of strokes per minute times the leverage(stroke) now if we take a 4000fps max piston speed for both engines and 1000psi cylinder pressure for both engines we find that the 383 has a 6400rpm max piston speed (3200 power strokes) a 12.7sq inch piston area and a 1.875" leverage over about 21 degs of effective peak pressure durring the power stroke,
now with the 377 we find a 6896rpm max piston speed,(3448 power strokes) a 13.56 square inches of piston area and about 20 degs of effective stroke and a 1.74" leverage over about 20 degs of effective peak pressure durring the power stroke,
that the 377 has for an advantage(thats about 3% advantage in work force available for the 377 over the 383 (or about 191 extra power strokes per minute if your useing the 383 power strokes as a base measure, but of course the 383s longer stroke are slightly more effective but thats WHY the 377 is used more than the 383 as a racing engine now its very likely that if you don,t take advantage of that extra higher rpm torque range of the 377 that a 383 geared more to ITS IDEAL TORQUE RANGE will run with or beat the 377. if you take a 3600 lb car with a 3.08 rear gears the 383 is likely to win, but put 4.88 gears in a 2600 lb nova and theres no contest the 377 will win.
BTW you can have some of the advantages of both combos by building a 406 sbc(4.155 bore x 3.75stroke)(3200 x 13.56 x 1000psi with that 1.875 thats the torque of the 383 and the power of the 377 + 1% because of the larger displacement. and after building about 20 383 engines and about a dozen 377s I can tell you a 406 will normall beat either of the smaller sizes.

now as far as how the number of cylinders affects the torque, torque of a 6 cylinder turning the same speed with equal displacement/stroke per cylinder will be slightly less than 3/4 as much due mostly to a greater number of degs between cylinders fireing and a 12 cylinder with the same displacement/stroke per cylinder will give slightly more than 150% of the torque if the cylinders fireing are placed fewer degs apart than 90 degs or if two cylinders fire at the same time.

1985,1992,1996 vettes
keep the rubber side down and the fiberglass off the guard rails

vettepilot
05-03-02, 04:53 PM
I'm glad you included and discussed the effects of stroke in your second post. I was about to start typing a reply to your first post, then I noticed that you were essentially saying what I was going to say. Stroked engines became very popular in the early 70s on through today because they increase the usable torque down in the lower RPM range where most street cars operate. True they are RPM limited because of the rotating mass, but they don't need high RPM to produce the torque needed for launch and acceleration, which is what most people are really looking for.
When you hear somebody saying they wish they had more HP, what they really need is low to mid-range torque. HP is needed at the upper end to overcome wind and rolling resistance (drag), usually not a concern on the street because most don't reach those extreme high speeds on the street. (Now you watch, somebody is going to tell us they do get over 150 MPH, on the way to work no doubt).

Anyway, very good post!!!!! :BOW
vettepilot :bu

RalleyRed
05-03-02, 06:56 PM
Now watch somebody is going to tell us that they do get 150mph and on the way to work no doubt

Only, and I do mean only because I was running late!!:bu


All kidding aside, Grumpy you amaze me with all your tech post's and knowlege.:BOW

Rick

grumpyvette
05-03-02, 07:36 PM
heres a chart showing how far the piston moves per deg of rotation of the crank, http://www.iskycams.com/ncrank1.PDF
and heres the pressure curve per deg chart
http://www.geocities.com/ljaya6390/analysis.htm
notice the pressure peaks at 10 degs past tdc and stays high for about 20 degs, compare that to the other chart with the piston movement down the cylinder, notice almost all the usefull pressure is between 10degs after tdc and 30degs after tdc and that the piston move away from tdc slightly slower with the longer rod allowing a slightly longer effective use of that pressure.
what Im trying to get accross here is that larger bore piston dia. allow more piston surface area for the cylinder pressure to act upon and that longer rods and strokes allow a more effective use of that pressure durring the 20 or so degs that the pressure peaks but shorter strokes allow more power strokes per minute that in some combos more than make up for the lower efficiency by the sheer number of extra less efficient power strokes so a ballance within the engines rpm stress limits must be made and the higher the cylinder pressure and the longer the leverage arm the crank has and the faster you spin it the more torque is available to produce hp.

78SilvAnniv
05-03-02, 10:35 PM
...but I enjoy reading the information Grumpyvette posts. I figure that by reading it, I will at least be familiar with it.
That's half the battle, right?
Silver

vettepilot
05-05-02, 12:37 PM
78SilvAniv,
OK, Maybe you don't understand everything, but if you happen to see or hear this again you will think back about what you read and say... Hey! I know about that! Next thing ya know, you will be writing an article for HOT ROD... LOL :w
vettepilot