Ling Rod Conversion - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 14 (permalink) Old 12-30-2009, 07:29 PM Thread Starter
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Ling Rod Conversion

Back in the day when I was nuts over the 2.3 turbo ford and my Merkur, many of us put small block Chevy rods in them and pistons with higher pin placement to make the rod longer. The point was to slow the movement at TDC and get a more complete burn, meaning more horsepower. If I understand correctly, one of the major hurdles to power in the 7.3 is Fueling at higher RPM. IF in theory we were to slow the piston down and create a larger window for the injection event would that not allow more room for more fuel, and a more complete burn. Most of us need new rods and pistons if we tear down the motor, so why not??????

What the hell is a ling rod.... sorry for the error. I intended LONG ROD. To many beers already, and it isn't even 3 yet.
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post #2 of 14 (permalink) Old 12-30-2009, 07:57 PM
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Re: Ling Rod Conversion

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Originally Posted by jimdawg185 View Post
Back in the day when I was nuts over the 2.3 turbo ford and my Merkur, many of us put small block Chevy rods in them and pistons with higher pin placement to make the rod longer. The point was to slow the movement at TDC and get a more complete burn, meaning more horsepower. If I understand correctly, one of the major hurdles to power in the 7.3 is Fueling at higher RPM. IF in theory we were to slow the piston down and create a larger window for the injection event would that not allow more room for more fuel, and a more complete burn. Most of us need new rods and pistons if we tear down the motor, so why not??????

What the hell is a ling rod.... sorry for the error. I intended LONG ROD. To many beers already, and it isn't even 3 yet.
Increasing the piston dwell does not give you a longer window for injection. It can help you change the shape of your cylinder pressure curve, because the longer the rod, the longer the dwell and if the dwell is longer it will give you more time for the fuel to burn and build pressure.

A similar concept would be to offset the wrist pin to alter the dwell and accels and decels of the piston.

Honestly I haven't looked in depth at this option because there are more pressing matters when it comes to HP/dollar spent. Is there something to be gained here? Maybe - but I think its marginal in the grand scheme of things. Maybe someone could provide some cylinder pressure versus crank position curves and some estimation can be made without the expense of actually building it. You'll know existing combustion volume and pressure, and then using PV=nRT you can decipher potential cylinder pressures based on the new combustion volumes for the geometry you intend to change - then infer some HP, Torque and cylinder pressure changes from that data....

Sorry - the engineer in me got excited about math..... The redneck farmboy in me says its not a big enough gain to screw with it.... but it does sound like a fun mathematical model to build.

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post #3 of 14 (permalink) Old 12-30-2009, 09:21 PM Thread Starter
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Re: Ling Rod Conversion

No need to be sorry about the math, I love that stuff.

Are you thinking that injection event could not be longer because it happens much sooner than in a gasser, and thus could not be lengthened because the event happens before the dwell would be most evident?

You have a point about the pressure curve, the full value of that had not occurred to me. As you are more than aware of the flow of these heads are not overcome by high boost as in the 12 valve cummins, I may be wrong, but I assume that is part of the "lack of power" equation (the biggest being fuel, I'm sure)

Longer dwell possibly = more time for the cylinder to fill (during intake stroke, it dwells longer @ BDC as well) = more efficient burn (during combustion event) = power.

I am taking some engineering classes as electives, just for fun, I was going to focus on hydraulics, but this fascinates me.
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post #4 of 14 (permalink) Old 12-30-2009, 09:46 PM
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Re: Ling Rod Conversion

He's what rod length effects inside of the motor.....

Piston Acceleration Rates
Connecting Rod Dynamic Loads
Rod Angularity
Cylinder Wall Loads
Piston Speed, (Not to be confused with acceleration rates)
Effective Stroke
Dwell Time

I'd stick with the stock or aftermarket setup......
I'm sure the pulling guys would have some input on this as torque can be increased through Rod Angularity(Leverage). The "Block Strength" I think would be the limiting factor?

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post #5 of 14 (permalink) Old 12-30-2009, 09:48 PM
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Re: Ling Rod Conversion

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Originally Posted by jimdawg185 View Post
No need to be sorry about the math, I love that stuff.

Are you thinking that injection event could not be longer because it happens much sooner than in a gasser, and thus could not be lengthened because the event happens before the dwell would be most evident?

You have a point about the pressure curve, the full value of that had not occurred to me. As you are more than aware of the flow of these heads are not overcome by high boost as in the 12 valve cummins, I may be wrong, but I assume that is part of the "lack of power" equation (the biggest being fuel, I'm sure)

Longer dwell possibly = more time for the cylinder to fill (during intake stroke, it dwells longer @ BDC as well) = more efficient burn (during combustion event) = power.

I am taking some engineering classes as electives, just for fun, I was going to focus on hydraulics, but this fascinates me.
Look at it this way: we do not have a spark plug to control ignition of the fuel - its done entirely on compression (heat). If your engine is spinning at say 4000 RPMs, the time from BDC to TDC has constant, a longer rod, offset wrist pin, whatever, could increase the dwell time. In order for that piston to hang out longer at its extremes the accels and decels need to be steeper, If its spending more time at the extremes - then it will inevitably be igniting that fuel charge earlier (the piston has to arrive earlier/leave later to increase dwell). If it arrives earlier, and hangs out longer, the cylinder pressure will start climing earlier and will continue to increase higher than before - that is where power is being made in a gasser. We don't have issues making enough cylinder pressure to throw parts into orbit - we don't have issues igniting the fuel mixture as early as we want it.

The only real way to fuel at higher RPMs is to increase the injection rate - so you can inject fuel in the required time window, a window that gets narrower as RPMs increase.

Some reading from months past on these issues:

https://powerstrokenation.com/forums/...ad.php?t=18566

https://powerstrokenation.com/forums/...ad.php?t=33513

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post #6 of 14 (permalink) Old 12-31-2009, 03:07 AM
 
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Re: Ling Rod Conversion

Rod Length (and, in turn, Rod Ration) plays many roles in how an engine operates. Theoretically, as Smokey Yunick said, 'Stuff the longest rod you can fit in the engine'. By increasing dwell at TDC, you contain the combustion area for longer in the initial ignition, thus minimizing the risk of abnormal combustion effects and reducing stress on the rotating assembly as cylinder pressure builds. Dwell at BDC allows 'blowdown' which is evacuation of exhaust gases unaided by piston exhaust stroke... and, in turn, less parasytic loss. Also, reducing the angle between the connecting rod and the cylinder bore greatly reduces thrust side-bore load. Less parasytic loss to friction, greater engine longevity. A benefit to gasoline engines that won't effect the Diesel crowd is the increased charge velocity during the intake stroke as well due to higher average piston velocity.

The downside is the shorter compression height of the piston resulting from raised wrist pin location, in extremes even necessitating a spacer in the oil ring.

Why don't you see rotating assembly manipulation in the diesel crowd like you do in the gas crowd? Trade off. For the thousands you'd spend on custom rods (variable rod lengths for gas engines are already mass produced... go price some custom 7.3 rods!) and the other goodies, you could get a lot more benefit from giant injectors, HPOP, turbo, etc... Also, never taken a diesel block down, but with the super high compression I imagine there isn't a lot of room to play with on piston to valve clearance (something effected by piston dwell at TDC)... maybe one of you professionals can chime in there?
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post #7 of 14 (permalink) Old 12-31-2009, 03:35 AM
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Re: Ling Rod Conversion

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Originally Posted by YellerMax View Post
Rod Length (and, in turn, Rod Ration) plays many roles in how an engine operates. Theoretically, as Smokey Yunick said, 'Stuff the longest rod you can fit in the engine'. By increasing dwell at TDC, you contain the combustion area for longer in the initial ignition, thus minimizing the risk of abnormal combustion effects and reducing stress on the rotating assembly as cylinder pressure builds.
Generally when we are talking about engines I would say that if it applies to gas engines, it applies to diesel engines - except in this instance. Gas engines are ignited using a spark plug and there is a flame front propagation that emanates from that point - always. Diesel engines are compression ignition - just compressing (and the resultant heating) will cause ignition - and while there is a burn rate the mixture of burnt fuel and unburnt fuel is much closer to being a homogeneous mixture throughout the combustion area. Increasing dwell on a gas motor - helps the flame front propagate to the entire mixture faster. In diesels the flame front has minimal propagation to be everywhere once it first ignites.

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post #8 of 14 (permalink) Old 12-31-2009, 03:48 AM
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Re: Ling Rod Conversion

The major benefit of a long rod motor is that you can use a short piston which lowers rotating assembly weight. Smokey preached long rods because the heads of the day were small-port/low-flow compared to what we have today, and the faster piston acceleration rate helped air flow into the motor right off BDC. Now with bigger ports and faster cam profiles the long rod motors only consistent advantage is rotating weight. Some generic characteristics are that shorter rod motors have better acceleration rates off a corner, but the longer rod motor will hold peak HP longer. Under boost or in a steady state load condition such as a sled pull the difference will be nil.

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post #9 of 14 (permalink) Old 12-31-2009, 03:27 PM
 
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Re: Ling Rod Conversion

Machinist - don't forget you still have the benefit of lower thrust side-bore load, better blowdown from dwell at BDC, and depending on the properties of ignition, less rotating assembly stress. Regarding the lower benefit to pulling motors? I think you are probably right... all my experience is drag racing gas motors, so I'm a bit out of my element on that one.

O/T, what motor do you have in your 66 stangs? I still remember seeing a red 66 convertible Mustang when I was 12... it was the INSTANT I got hooked on cars

HotRodTractor - I realize that ignition properties are wildly different for diesel motors, but are we still concerned, in diesels, with combustion patterns etc...? My understanding is that injectors play a much larger role on this (and, of course, piston and combustion chamber shape), but if so wouldn't the rod ratio play a role in swirl, etc? After 20 years of gasoline stuff, I'm trying to play 'catch up' on diesel knowledge!
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post #10 of 14 (permalink) Old 12-31-2009, 04:16 PM
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Re: Ling Rod Conversion

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Originally Posted by YellerMax View Post
Machinist - don't forget you still have the benefit of lower thrust side-bore load, better blowdown from dwell at BDC, and depending on the properties of ignition, less rotating assembly stress. Regarding the lower benefit to pulling motors? I think you are probably right... all my experience is drag racing gas motors, so I'm a bit out of my element on that one.

O/T, what motor do you have in your 66 stangs? I still remember seeing a red 66 convertible Mustang when I was 12... it was the INSTANT I got hooked on cars

HotRodTractor - I realize that ignition properties are wildly different for diesel motors, but are we still concerned, in diesels, with combustion patterns etc...? My understanding is that injectors play a much larger role on this (and, of course, piston and combustion chamber shape), but if so wouldn't the rod ratio play a role in swirl, etc? After 20 years of gasoline stuff, I'm trying to play 'catch up' on diesel knowledge!
Injectors add fuel - that is all they do. The begin injecting then compression causes the fuel in the chamber to ignite and the injectors typically continue to feed the fire so to speak. Swirl is important to keep the mixture as homogeneous as possible - but we only really care about it once injection starts. To keep things simple lets look at nice and simple p-pump on a 12 valve Cummins. It is set for static timing of lets say 18*. So no matter where it is in the RPM band it will start injecting at 18* BTDC - that is the start of injection. The start of combustion happens when the conditions are right - ie the temperature is hot enough in the combustion chamber is to set it off. Lets for arguments sake that you just install your longer rod in your motor - all the parts just drop right no no problem. Doing so increases your dwell at TDC - indicating that your piston has to get to TDC earlier. You use that same 18* that you had your pump set at before so your start of injection is the same - BUT your start of combustion has just been bumped a couple degrees further BTDC. So your cylinder pressures begin coming up sooner. Now since your dwelling longer - that also means that your piston is hanging out longer ATDC. So your cylinder pressures that started climbing earlier - are now also reaching higher because the volume is currently smaller than it was before - so the pressure has to be greater.

I'm not saying that there is nothing to be gained by looking at this. I'm not saying that swirl isn't important. I'm saying that we currently do not have issues building cylinder pressure - more than enough to pop a motor. So be cautious about cylinder pressures when thinking about changing the geometry. I am sure there is an ideal geometry for a target RPM as well. But I AM confident that right now efforts can be better spent elsewhere on the motor for bigger gains per dollar spent.

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