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How to get low rpm torque ?

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DougG View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DougG Quote  Post ReplyReply Direct Link To This Post Topic: How to get low rpm torque ?
    Posted: 13 Mar 2019 at 6:03pm
I know on a 301 diesel they run awesome at 4500 rpms, but wondering how to get low end torque say in a 3000 rpm limit class with a 301 and 426 diesel both,,
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DMiller View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMiller Quote  Post ReplyReply Direct Link To This Post Posted: 13 Mar 2019 at 7:48pm
Stroke and flywheel mass. Is how AC made lower HP low RPM torque monsters years ago.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Macon Rounds Quote  Post ReplyReply Direct Link To This Post Posted: 13 Mar 2019 at 8:06pm
Great question....
What is the magic formula for that ?
 
RPM, Horse Power, torque....
Then add the variables of : cubic inches, fuel and ignition timing. 
 
Heavy flywheel for ???
 
Stock flywheel for ???
 
Lighten the flywheel for ???
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DrAllis Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 2019 at 7:20am
More low-end torque is achieved by multiple things. Turbo selection (exhaust A/R, overall wheel sizes, Map ring slots on the compressor, waste gate etc, etc), camshaft grind and proper degreeing/timing of the camshaft, valve and valve runner sizes and shape, compression ratio, injection pump timing and duration of injection, injector hole sizes and spray angles, intercooler (or not), and on and on and on. When you spend enough $$$$$ and time on the dyno you can slowly figure out what works best for YOUR particular engine.  In short, THERE IS NO ONE THING. It is a whole list of things.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brian Jasper co. Ia Quote  Post ReplyReply Direct Link To This Post Posted: 17 Mar 2019 at 8:51pm
If you can add stroke that will help. To give an idea on that my 390 in my Torino has been stroked from 3.78 to 4.25. That brings it up to 447 inches with a .040 overbore. Along with a cam change, an identical built engine done by Survival Motorsports produced 531hp @ 5400, 500ftlbs @ 4000. My 6.0 Powerstroke produces 580ftlbs around 3000. It's definitely a list, but adding stroke is the equivalent of a cheater pipe on a breaker bar...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HudCo Quote  Post ReplyReply Direct Link To This Post Posted: 17 Mar 2019 at 9:02pm
heavey flywheels will keep things spinning
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DaveKamp Quote  Post ReplyReply Direct Link To This Post Posted: 19 Mar 2019 at 10:12pm
Torque is a produced by a function of pressure and leverage.  Increasing combustion pressure (adding boost, increasing fuel) helps. In order for that combustion pressure to do work, it must need surface area to act upon.

Let's do some THEORETICAL MATH here:
Let's say the stock bore is 4.25".  That's a surface area of 2.125*2.125*3.14 =  14.18 square inches.
Let's say you can push that to 4.5"... that's a surface area of 2.25*2.25*3.14 = 15.89 square inches.
Let's estimate that an 'average' production diesel manages a peak pressure of around 2000psi. 
For your smaller bore, that's 2000*14.18= 28360lbs of linear force.
For the larger bore, that's 2000*15.89= 31780lbs of linear force.

Let's say your engine's stroke is 5", and you're considering going to 5.5"...

At 5", With the crank at 90 degrees from TDC, your ratio of force (in foot-lbs) will be 5/12... so torque = piston linear force * 5/12.... or  0.4167
At 5.5", with crank at 90, ratio will be piston linear force *5.5/12... or 0.4583

Let's say you're getting that 2000psi into a 4.25" bore for 28360lbs... that's 28360*0.4167 = 11817 ft-lbs applied to the crank.
Same on a 5.5" crank comes to 28360 * 0..4583= 12997ft-lbs applied to the crank
Doing the same at big bore @ 5" stroke yields 31780*0.4167 = 13242ft-lbs
While big bore @ 5.5" yields 31780*0.4583 = 14654ft-lbs.

Now, increase the combustion pressure to 2500psi:
2500*14.18=35450
2500*15.89=39725

4" bore With 5" stroke =  35450* 0.4167 = 14772
4.5" bore with 5" stroke= 39725*0.4167 =  16553
4" bore with 5.5" stroke = 35450*0.4583  = 16246
4.5" bore with 5.5" stroke = 39725*0.4583 = 18205

Now notice- these are some seemingly astronomical torque figures.  yes, this is NOT what you'll see at the flywheel.  Why?

Several reasons:

First- max cylinder pressures occur somewhere between firing time, but substantially BEFORE the crank hits 90 degrees.

Second- max cylinder pressure hits that PEAK, but because the piston is MOVING, the cylinder volume is increasing, typically at a FASTER RATE than the combustion can proceed, and maintain that pressure.  Effectively, about ONE TENTH of that force results in pressure at the 90 degree mark, and it doesn't last long.  Realize that cylinder pressure does the greatest amount of work from 45 degrees after TDC, to 135 degrees.  Anything still burning by 135 degrees is, for all practical purposes, wasted, and any substantial pressure generated PRIOR to 45 degrees, is simply stressing the block.

Last- the force that DOES occur in the 45-135 degree arc, is being absorbed in greatest amount, by the inertia of the other pistons, the crank, and the flywheel, so the piston's force is being used to 'keep' the motion through another 585 degrees of rotation... that's (four stroke =) 720 degrees, where only 90 degrees (45 to 135) is being substantially 'pushed'... that's 90/720ths or 0.0125 of the four-cycle engine's operation actually doing work.  Take that 18205 from above, and multiply it by 0.0125, and you've got 227ft-lbs from that one piston.  Multiply that by six, and you've got 1365ft-lbs.  Figure that you lose about 25% of that to internal losses (running oil pump, crankshaft flex, etc) and you've got 1000 or so available at the flywheel.

So... why don't we just delay ignition untill 45 degrees AFTER?

Several reasons-
First, a compression ignition engine doesn't get to choose ignition point... not by much, anyway... it's a function of autoignition temperature of the fuel, and the point at which it's injected into the pressurized chamber.  At 45 degrees after, the chamber's pressure is simply too low for autoignition of the fuel to occur.

Second... fuel doesn't burn instantaniously... it burns in a pattern that starts at one point, and moves to another.  Imagine throwing a match at one end of a long, skinny pile of leaves.  it burns itself along that long pile at a certain rate.  Try the same by making a big tall pile, and lighting it right at the top.  It burns down, and outward, but once the top leaves burn, the ashes fall on the lower leaves, slowing their burn.  Now try lighting it around the perimeter, let it burn in... it'll burn faster, but still, there's an ash-covered pile in the middle that doesn't burn.

Now spread out a nice flat pile, light it in the center... it burns outward, it burns fast, and the leftovers are small, and don't obstruct any other leaves from burning.

Any time you have unburned fuel, you see black ash coming out the stack.  Black ash means incomplete combustion... wasted fuel, and consequently, that fuel is absorbing combustion heat, making LESS cylinder pressure.  To fix this, either add more oxygen, or more burn time, or more turbulence in the chamber required to make that a complete burn... or change the shape of the chamber so that the FLAME MOVES FASTER.

Third-  When running at a constant RPM, the piston, at top dead center, has absolutely NO speed.  As the crank rotates, the piston slowly starts to move.  As it approaches 45 degrees, it's moving fairly fast... and at 90, it's reached it's highest velocity downward.  After that, it slows, and by 135, it's going dramatically slower.  At BDC, it's stopped again.

When burning your leaves, if you spread the leaves out DURING the fire, the fire slows down.  Why?  Because the molecules of air and fuel are getting too far apart to help accellerate the chemical reaction.  This means the burn will be incomplete.  A short-stroke engine has a natural advantage in this respect-  the piston speed from 45 to 135 is proportionally slower than a long-stroke engine.  Giving up the leverage, particularly in a slow-burning fuel in a fast-spinning engine, results in a more efficient burn, hence, more 'useful' pressure applied to the piston during the power stroke.

Horsepower is a function of torque and speed.  Obviously, if you can make the same torque at a higher speed, you'll have a higher horsepower number.  If you cannot spin it faster than 3000rpm, then the only way to get more power, is to increase torque at that speed.  Can you make a complete burn occur well in the 45-135 degree arc, then eject the spent charge, clear the chamber, and reload it for the next cycle, over and over again?  These factors are why the answer is 'no one' thing- it's a combination of MANY things, all working in concert.


Edited by DaveKamp - 19 Mar 2019 at 10:14pm
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Big Orange View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Big Orange Quote  Post ReplyReply Direct Link To This Post Posted: 19 May 2019 at 12:46am
The first think to do is have cam ground with very little overlap, and advanced timing 8 to 10 degrees, if the cam grinder can't get that much you can make a offset key. Adjust the injection timing & fuel to get max torque at the rpm needed. Turbo changes will be needed. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tbone95 Quote  Post ReplyReply Direct Link To This Post Posted: 30 May 2019 at 12:07pm
Originally posted by Brian Jasper co. Ia Brian Jasper co. Ia wrote:

If you can add stroke that will help. To give an idea on that my 390 in my Torino has been stroked from 3.78 to 4.25. That brings it up to 447 inches with a .040 overbore. Along with a cam change, an identical built engine done by Survival Motorsports produced 531hp @ 5400, 500ftlbs @ 4000. My 6.0 Powerstroke produces 580ftlbs around 3000. It's definitely a list, but adding stroke is the equivalent of a cheater pipe on a breaker bar...
May I have a ride please?Clap
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