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More on rod length

944 Views 12 Replies 4 Participants Last post by  CJ7Taz
Alright you guys, I'm sure some of you are tired of this subject... THEN DON'T READ IT :)
For those of you who are sitting on the edge of their seat, I have more good stuff for you. JJC, I thought long and hard about what you posted, I even did the drawing just to see for myself and contemplate why, if you are right, do long rod engines consistently make more torque? It finally came to me, advance in an engine is to make up for the speed at which the gas vapor ignites (flame propagation). Where then? or should I say when in the crank's rotation is the explosion the most efficient for an engine? Well I don't have the numbers in front of me, but don't you think that the spark advance is designed to take advantage of the time that the rods are closest to the tangent of the crank's arc? - I'd bet on it. In fact, you can see that from 45 deg After Top Dead Center(ATDC) to 135 deg ATDC a long rod's angle is closer to the tangent than a short one. Wanna see it? Try the new page: This makes a short rod better for the first 45 deg ATDC, but there are 135 deg more of push in that stroke... and the middle 90deg (argueably the most important) are taken advantage of better with a longer rod. So I'll give you the point that at 10deg ATDC a short rod will be better, but if you look at the "meat" of the power stroke, remembering that flame propogation is not instant, a long rod will win. As you said, "If we really do a top notch design combustion pressure will be at its max when angle theta is at the optimum for the designed r.p.m." An inline engine (being limited by the RPM) will take advantage of the long rods. I sincerely appreciate the explination you gave as to how rod length affects piston acceleration, but it's working against you - "Conversely if rod length were increased the amount of time the crank has to
slow down, stop, and accelerate increases." - Won't that top out the RPM limit faster, giving shorter rods better rev range? "...shorter rods make more torque, longer rods allow more rpm's" - That seems to be opposite what you posted orginally. I'm listening, and thinking hard, hope to hear from you.
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GA reminded me about the issue of piston cock, this is an area I'm unfamiliar with except in diesel engines... but it appears that the closer the connecting rod gets to 90deg tangent of the crank's arc - the more piston cock you are experiencing in every power stroke. Piston cock is generally looked at as dangerous because of a wear issue not a loss of power issue, so I haven't pushed that yet. But just to please everyone, piston cock will go down with long rods.


Gosh ..I love a good debate.....I can't keep up with this .....but as they say in the Olympics ..."it is not that you've won......but that you came to compete".....well .....spectators can have fun too. In my practical, albeit limited, grasp upon this're saying that initially the short rod, due to it having initial angular advantage over the crank (I reason this by envisioning Conan the Barbarian pushing the grinding stone .....and he is always at a right angle to the crank/wwwthreads_images/icons/wink.gif), is superior only in the first few degrees of rotation ......beyond that..... the excessive angle actually moves it further from the ideal right angle.........the longer rod ...........having less of an angular change ......stays closer to the ideal....(I'm seeing the "tangents" of the longer rod staying in a narrower range)......did I get it????
Now for me to throw the spice into the broth here.....I have to take exception with your term "explosion". The combustion process is ideally a controlled "flash" which starts near the end of the compression cycle (27-30 degrees before TDC) advancing flame front into an ever decreasing area .....if you time it right ....the flame front is almost at a standstill at TDC ......if you start it too early and it expands beyond the pressure/temp limits of the air/fuel vapor's tollerance .....if detonates (explodes)......if you start it too late've wasted energy because the "meat" of the combustion process will continue beyond the "effective stroke duration"......we are, after all, talking about a process in motion. This now makes me look at the "short rod" with its angular advantage early in the rotation......

........and so it continues......

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Sully ......I was just envisioning some "reactive" forces here.......just pointing out that although the piston is pushing "down" is not necessarily pushing toward dead center on the crank due to the piston could assume the greater the angle of the connecting rod .......the higher the lateral forces and the higher the "compulsion" to cock..(try and plot all the stress "vectors" here.....gosh I remember doing a suspension bridge once ....ONCE!)....and yes it is only refered to in a wear issue......hence the policy of most shops for "reversing" the pistons in rebuild.


The power from the combustion has only one place to go, straight down on the piston. Do a vector
analysis for the connecting rod. All the power MUST go into pushing on the rod except that that is lost to
friction. In an ideal system with no friction, short rod, long rod makes no difference. The friction of the
piston against the cylinder wall and the friction of the main journal against the bearing are the only
differences in the real world.

Another thing to keep in mind is that in those few degrees ATDC when the short rod has the advangage, there is very little piston movement relative to crank rotation. The time when the long rod has the advantage, the piston is at max speed in the power stroke (90*ATDC)

Hey Ron,
I have a question for you and please forgive my entry level Physics. On V engines, isn't there a component vector force of the connecting rod that is at an angle to the direction of the entire system? I ask because I seem to remember it has something to do with the weight of the body (in this case, the rod). If this is true, then some of the force generated by combustion is not in line with the direction of travel and less torque is generated than would be if the rod were completely vertical. Let me know what you think. /wwwthreads_images/icons/smile.gif

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Sully, I have been waiting for your response, unfortunately I could not get back to the computer until a late hour, I am going to read your post carefully and respond when my mind is fresh. Do you have any books that cover this subject? if you do I may have it already or can get access to a copy, it might be helpful to be working from a common foundation of knowledge.

Sully and others, good morning! First things first in every human endeavor there is the likely hood of human error,only two kinds of people don't make mistakes dead people and lazy people, the most succesfull people make the most mistakes,
So, if you find some error in my thoughts please post your thoughts on the subject, we will all gain from it
Yes sully the ignition timing advance is used to compensate for the flame propogation rate in the cylinder wich is for the most part a constant, higher rpms require more advance, this also gives the gasoline engine two limiting factors, piston diameter and rpm's, the bigger the piston diameter the further the flame needs to travel in a very limited amount of time.
Next subject, I am going to make an assumption here so please i invite anyone to comment that has the right answer. "the power stroke starts at tdc and ends when the exhaust valve opens to some meaningful amount", I tried to hunt up a cam card to check this but i could not find one so here is where i will make some gueses, stock s.b. chevy 262/265 degrees duration and maybe 30 degrees overlap? if i am guessing right then the exhaust valve opens about 35 degrees before bottom dead center (bdc)? this cuts your power stroke down to 145 degrees.
(note to cjdave detroit diesel 2 cycle engines exhaust valves open at 90degrees after tdc, it really takes a lot of time to scavenge these things)
piston acceleration: i will cover two seperate thoughts here
1. with the longer rod more time is spent lingering around top and bottom dead center, this allows more oppurtunity for the piston to slow down and the reaccelerate at reasonable rates, analogy; think about driving through a residential neighborhood with a stop sign on every block, if we timed your progress over say 10 blocks we would get your average speed, distance/time. Now if you tried to get through in the fastest possible time while still coming to a comlete stop at every stop sign you would soon have a sore neck from jumping from the gas,brake,gas,brake..etc. your average speed would increase slightly, but the abuse to you would be much greater. conclusion: long stroke motors need longer rods to keep the pistons from getting whiplash.
2. One revolution of the crank requires one unit of time to complete. if longer rods take more time at top and bottom dead center then it has to spend less time at some other location? this occurs at 90 and 270 precisely where you would like the meat of the power stroke to be.
piston cock, this can to some degree be compensated for by offsetting the piston pin bore from the piston centerline. many flat top pistons are marked with an arrow to enshure the offset is installed correctly.
In conclusion: there are many variables that must be balanced to create an engine that functions the way we would like, for every change you make you pay a price someplace else.
-increasing stroke creates more torque, and more problems with piston acceleration
-increasing piston diameter limits rpm's due to the further distance the flame needs to travel
-increasing rod length improves angles and acceleration but you pay for it by the time lost in the power stroke
Have i helped any? or just muddied the waters

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Re: Long rods for high RPM

Sully, you only missed the mark on the point about short rods theoretically being better for high-rpm. The higher angle of the short rod causes stress on the cylinder walls at high-rpm (in addition to limiting power output). And equally as important as rod length, is rod-stroke ratio. That's why NASCAR motors use a short stroke & long rod. Contrary to what you'd guess, the 358" Chevrolet NASCAR engine is NOT an over-bored 350. Instead, it is a destroked 400 (running a 3.25" 327 crank) with a .030 overbore & rods in excess of 6". The extremely long rod combined with a very short stroke keep rod angles (and friction) at a minimum & efficiency at a max.


/wwwthreads_images/icons/wink.gif Got Mud?
G.U.M.B.O. Mud Racing
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Re: More on rod length-CJ7Taz

CJ7Taz......I need help with this (and yes, I really want help with this) assert that since the piston has only one way to go that the attitude of the "applied" force is inconsequential............let's for a moment say the instead of a stationary piston/cylinder assembly .....we have a stationary crank that the piston/cylinder rotated around .....the only "twist" that I want to throw in here that the rod/piston/cylinder are manipulated to always be "inline" with no angular deflection .....and (to really make this extreme) are ALWAYS at .001 degrees to the crank...........that is "they only have one place to go" .....up and down ......if I can apply your view on things ......then this setup will have as much .....for lack of a better term.....power than one manipulated to be at 90 degrees in relationship to the crank ........what I think that I've done (I tried to anyway) is make the piston/cylinder/rod (by manipulating ITS postion) always inline .........and always at the same angle to the crank (yes this would be a really strange mechanism ......but ..........I'm trying .....struggling actually to get this out) SO if we put everything back to normal and say that the same rules apply ........(again applying your rules here)...wouldn't we have the same "advantage" over the crank regardless of what angle the rod is to the crank? This does not seem correct...

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I just wanted to share some formulas that could help.

Bore vs. Stroke ratio Stroke / Bore X 100

Chrysler 440 example 3.75 / 4.32 X 100 = 86.8%

94% is the best compromise between friction and horsepower for a bore stroke ratio. That ratio was learned with high performance piston engines of World War II fighter aircraft.

Rod ratio formula: Rod (Center to Center) / Stroke

Chrysler 440 example 6.76 / 3.75 = 1.80

The bigger the rod ratio the longer the piston will dwell at TDC. A rod ratio of 1.80 will stay at TDC longer than a rod ratio of 1.65.

Rod angle formula: Stroke / 2 / rod length = 1

Chrysler 440 example 3.75 / 6.76 = .277366863
(note some advanced math is required) Sin 1 = rod angle in degrees
.277366863 = 16.1 degrees

So what does all this mean? Here are some samples of engines to see what is what.

Engine, Rod Length, Rod Ratio, Rod Angle

Chevy 350, 5.7" 1.638 17.77
Ford 302, 5.156" 1.718 16.91
Dodge 426 Hemi, 6.86 1.83 15.86

My personal opinion: Longer rods can be used to help produce more torque, but many engine formulas must be considered! I hope some of my information helps a long rod engine builder get the most torque they can from the formulas I've provided. Take into consideration camshaft profiles as well. High cylinder pressures are required to develope high torque. A piston that dwells at TDC will need to have a lower compression ratio, based on current pump gas available!

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Re: More on rod length-CJ7Taz


No, I don't think that's what I said. Power applied at the 90 deg would have the torque advantage and
both the short and long piston went through the 90 deg portion of the stroke. Converting reciprocating
force into rotational force is very different from finding the optimum angle for rotary forces.

I do like the idea of keeping the crank stationary though. I have often thought that a Pratt and Whitney
Wasp mounted with the cylinders rotating in a horizontal plane would make a very stable oval track car.
Unfortunately, steering would be difficult since the rest of the car would always tend to rotate in the
opposing direction.
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