Long rod/Short rod



Some people were getting confused as to what length rod increases the piston velocity and so forth so I have done 3 graphs to show, quantitatively, what happens when you change the rod length.

These graphs of the function,

y = -R*(1-cosx+(L/R)*(1-sqrt(1-((sinx)/(L/R))²)))

(which can be verified) and it's first derivative (velocity), and it's second derivative (acceleration), where
R = crank offset (stroke/2)
L = conn rod length.

The black curve represents the specs for a 221 ci engine (rod length 5.13, stroke 3.46);
the blue curve represents rod length 200 inches, stroke 3.46;
the red curve represents rod length 3.46, stroke 3.46;
and the grey curve represents rod length 1.73 (3.46/2; gudgeon pin axis would line up with CS axis at BDC) and stroke 3.46.

Acceleration and velocity graphs are with crank rotational speed 1 radian per second (2pi radians in 360 degrees), and rotational acceleration 0 radians per second.

Position vs CS angle:
http://www.users.bigpond.com/themilnefa ... stposi.gif

Velocity vs CS angle:
http://www.users.bigpond.com/themilnefa ... tvelo1.gif

Acceleration vs CS angle:
http://www.users.bigpond.com/themilnefa ... accel1.gif

It is interesting to notice that the shorter rod length actually means that there are two pulses but less peak acceleration toward BDC, but considerably higher peak acceleration at TDC. You can also see that with the long rod you have lower piston speeds, albeit very marginally; the shorter rod moves the piston further down the cylinder for the same CS angle, meaning better (speculative) torque characteristics.
Man, that was bold and power-full! I'm now looking at what you've said and I haven't got it all yet! I've grabbed what you've said, and tried to get a handle on it. I think I'm paraphrasing your discipline here, but this is how I've done it.

The respective crank stroke to con rod centre ratios are :-

Case 1 Black:- 1.48:1 stock 1969 Aussie or Argentine I6 Falcon

Case 2 Blue:- 57.80:1 (no side loads, case tending to the idealised concept of why the rich and famous change rod length. This would be like a 221cube 1969 Aussie Falcon with a bonnet bulge for a 15 .9 foot or 4.950 meter taller block )

Case 3 Red:- 1:1, the idealised example of another rich and famous person who cut 1.67 inches off the deck and rod length of his 221 engine, and didn't care how much side load he had to encounter to get it)

Case 4 Grey:-0.50:1, the rich and famous guy who was able to rip 3.4 inhes off his 5.13 inch rod and his 8.425 inch block to reduce his block to 5 inches deep from crank centre line to top of the block deck. Wow!

You've clearly kept the crank stroke the same. That you've sat down and worked through this is commendable. That you've covered the "factions" of purist long rod theory verses the drag racers short rod/long stroke philosphy is just brilliant! I've done some print-outs and am looking at this real close !
29 minutes response time, Adam. Made my call 4.28, you replied 4.57. You are da man! :rolflmao:

addo says
XE: try this ... It's just not listed for some reason.

Like I said
Man, that was bold and power-full! I'm now looking at what you've said and I haven't got it all yet!

What do other people think?
addo":29kxr6dg said:
XE: try this ... It's just not listed for some reason.
:oops: I was trying to show the grey (gray) text colour function. What I actually wanted to show was: [ color=gray]...sample...[/color] - just leave out the redundant space in the first square bracket set.
you just DONT let up. Thats one,two,three,four ,five,six,seven, eight minutes. How quick is that.

Addo da Switch!

Grey is a nark, but green upsets some colour/color blind people.
Log rods isn't just about piston speeds or accelerations and sideloads. Dwell at TDC is an important matter.

Reference: Mabie, H. and Ocvirk F. "Mechanics and Dynamics of Machinery". Wiley.

I did it with the 221 because that's what I'm familiar with. I can do some more graphs of specific ratios / specific conn rod and stroke ratios if you want, along with specific data points. I didn't do the graphs from a practical standpoint, rather a theoretical one. I gave some extremes and a middle of the road - the 221.

I too was sceptical when I saw the two pulses at BDC but then I did the grey curve and it all came clearer. The piston would go down until it hit BDC @ CS angle 90*, when it would stop, and the crank and conn rod would swivel around on the same axis until 270*, where (in theory - probably not in reality) the piston would start up the cylinder again.

I know that the 0.5:1 ratio will not work at all as a real, useful engine, but I put it in there as a reference, like the 57.8:1 ratio curve. Calculating side load curves would be a little bit harder.
There's a lot of discussion about the long rod conversion of the 300. Could you please do a set of curves for 3.98 stroke with both a 6.2097" rod and a 6.7947" one. It would help quantify a lot of the BS.

Black curve is 6.2097 with 3.98
Blue curve is 6.7947 with 3.98

(Yes there is both a black curve and a blue one in there!)

http://www.users.bigpond.com/themilnefa ... osi300.gif

Black curve definite integral from x=0 to x=2pi is -13.5254
Blue curve definite inttegral from same is -13.4343

http://www.users.bigpond.com/themilnefa ... elo300.gif

Black curve max speed at x = 1.2806, 4.9971, y = -2.09056, 2.09056 respectively
Blue curve max speed at x = 1.30593, 4.97717, y = -2.07411, 2.07411 respectively

http://www.users.bigpond.com/themilnefa ... cel300.gif

Max acceleration is at TDC,
Black curve -2.62843
Blue curve -2.57286
Much thanks. It looks as though the "long-rod" version doesn't gain you much in terms of piston dynamics. Acc/dec forces will be 97.88% of stock, another one of those invisible in the real world modifications. The side thrust issue is still present, but I suspect the gains may be on the same order of magnitude.
XT500....now you're the switch! A mind sharper than a gin trap!

I have an NZCE in Civil engineering, and am part way to finishing a Diploma in Higway Engineering. My back ground is Civil, not mechanical. And I'm less experienced practically than most guys here on matters of engines. I also trust my instincts, rather than really good text book theroy, so it makes it hard to validate why I make the stements I do.I've only ever gone to a Polytechnic, never university, so you guys can sort out if I've got the right of reply. Just being frank here, if any one wants to ask. I don't have a hang -up about it!

Back to de statement StrangeRanger made...

Much thanks. It looks as though the "long-rod" version doesn't gain you much in terms of piston dynamics. Acc/dec forces will be 97.88% of stock, another one of those invisible in the real world modifications. The side thrust issue is still present, but I suspect the gains may be on the same order of magnitude.

The reduction of side thrust is counter productive when L/R ratios exceed 1.8:1, in my opinion. The assertion I make is that above 1.8:1, you have to have:-

a) A taller block (read more weight/mass)

b) or a shallower (read more expensive) piston

c) IMO, there is a "rob Peter, give to Paul" dynamic in that the recipricating masses increase with rod length increase and piston pin height reduction. You can test this by getting all the masses of similar I6 conrods, and using a standard piston design with different deck heights. Inspect all the recipricating masses of each engine from 144 to 250. The cursory glance I've had at other engines seems to bear this out.

d)Circumstance alter cases, but unless you are absolutely thrashing a specific design for a racing application, exceeding 1.8:1 is counter productive. Most race engines, like DFX Cosworths, run L/R's well in excess of 2:1.

My "opinion"
StrangeRanger, I would still look at the longer rod 300 "Hybrid".Even though the 6.7947 inch 240 rod is heavier than the 6.2097 inch 300 rod, and even though the changes required in piston height will still not offset the increase in the recipricating mass to get an L/R of 1.71:1, I am sure that frictional horspower loss resulting from less side loads would result in an improvement in Hp every where. Cam has to suit the L/R ratio to create an equal footing. David Vizard in 1983 (see his book on A-seires Austin four cylinder engine) qualified this on a 1400 cc verses 1567 cc dyno session where he used two standardised engines and calculated frictional Hp losses. Only thing was, he did use different bore to stroke ratios, so a lot of people would question if the massive increse in frictional hp was related to something other than L/R ratio change. Any way, he found going from 1.8:1 down to 1.62:1 lost 4 hp due to frictional losses. On something like a 300, I'd make an educated extrapolation (hazard a guess, in other words, boffins!) and say 4 hp would become 12 hp. That's not qualified by a mechanical engineering degree, but it's what I'm prepared to say on the record.

Yes, it is a guess. But I am certain that with mathematical intergration using actual measured engine component masses would prove what I'm saying on pints a, b, c and d. At present, my math isn't up to the task!

Things are so interelated in the internal combustion engine!

People often get mixed up with B/S (bore x stroke) ratio, not L/R. An example I can cite is from Aussie Formula 5000 racer Kevin Bartlett. In the 1970's the race team he was associated with used three different engine configerations for there 5000 cc, 302 cubic inch Chevy based race engines. One was a std Z28 4 inch by 3 inch bore x stroke dimension block with 9 inches deck, 5.7 inch rods and tall 1.8 inch piston register. L/R 1.9:1. Then they start looking at 400 Chev blocks with 4.125 inch bores and 2.82 inch strokes created differing low end power curves for different tracks. A case where everthing changes, the heads get unshrouded, the cylinder walls have differing strengths, and any improvement in L/R ratio is lost in the translation!
Let me throw one more variable into your equations.

On most pistons the pin is not dead center, but is offset several hundred thousanths of an inch. this is to reduce the side loads, but also makes a slight difference in the exact timing of TDC and BDC.
I still have that book somewhere (Mabie et al). Along with Shigley & Mischke and a whole buncha other heavy-duty "plan weights".

In fact I can remember the class when Dr Tanner (now a serious grand fromage in mech engineering) dragged us through the calcs. Never thought I'd see them again, but had been contemplating it lately.

Jack: It won't affect the dwell time. But, it would be interesting to refine the graphs to factor this offset in.

We are closer and closer to being able to show an exact moving model (in 2D) for the different engines. Exaggerate the characteristics for clarity, slow the revs down, and it would be interesting stuff.

Sure beats arguing about poverty!

Cheers, Adam.
Indeed. Usually, the pistons wrist pin is offset 1/16 th of an inch to allow a two stage softening "click" into TDC. If there is no offset, what the English call gudgeon pin knock occurs. In an engine that has been in some service, this knock is significant. It may have been Pontiac that pioneered this offset, but my old 58 PA Vauxhall 138 cube six had this. This increased the piston side thrust on compression, so more power was consumed in creating a quieter running engine.

Lessons from actual Documented History:-
In the mid seventies, Ford Australia raced 351 Cleveland Falcon GT Hardtops at Bathurst and the Australian Touring Car Championship, and they reversed the offset of the piston, placing the front indicator tang of there forged flat-top 351 4v HO pistons 180 degrees around, facing the back of the block. There were "measurable" power gains in excess of 3 hp at the flywheel. This offset reduces side thrust at expense of quietness. The side thrust must, by inspection, be even less than a conventional non -offset piston pin.

Related Issue:-
In the mid 1980's an Australian backyard inventor decided to line bore his 138 cube EJ Holden Special six with a massive 10 degree tilt to the cylinder bore. The combustion chamber, crank and pistons were more or less stock with only minor modifications, and dated back to an ancient 1948 design taken off the Stovebolt Six/Blueflame Six. He used the same kind of offset piston pin, and claimed a massive increase in low end torque due to reduced frictional losses acrued to this side thrust vector low L/R ratio piston engines produce. He was able to run a 1:1 top gear of 25 mph per 1000 rpm with 3.08: 1 gears and very tall tires. The drive reports in an Aussie magazine stated that the car had very real low end torque. The car, and designer, have faded into history. Looking back, most of the improvements were likely to be the improved combustion chamber shape from the old perpendicular valve bath tub head. The rest was possibly the related to this side thrust reduction. If it was cost effective, I feel quite sure someone whould have adopted the design by now!
Bl**dy Addo. You bet me by mere seconds. Notice we both dropped our subject at 11:15 pm? Cut me a switch, boy!

Typical Aussie. Smart, well read, and two extra stars :eek:z: for any test

When :NZ: becomes the seventh state of Australia like it was supposed to be, we won't have this rivalry!

Haven't read those books...yet!
Billy Squier? :unsure:

Guess I've just lead a sheltered childhood in a quiet Central Otago town. In the 1970's, our music was ten years behind the rest of the world. We thought a FE 390 powered Pommy Ford Zephyr Zodiac was the worlds best car. When the biker gangs hit town the whole city behaved like the conservatives in "The Wild One". It was banned in NZ until 1977!

Now I'm making up for lost time.

Stroking is fun, de-stroking is expensive, and anybody trying to correct an L/R stuff up from a factory design (like a 250 I6) needs both deap pokets and a great shrink!

Next time, I'll buy a 240 I6, thats all I can say!
Great stuff XT500...thanks! :D

I have been playing with both S/R and L/R engine combos for about 10 years now, and can share some of what I've learned from numerous engines and 5-figures spent in the dyno room.

L/R engines have a *real world* advantage over their more prosaic brethren only at the top end of the usable RPM band, say, the last 1000 RPM. Let me give you an example. The last-but-one Toyota 4AGE engine we built featured the stock rod ratio and developed 232hp at 9200 RPM and 118 lbs-ft at 7600 RPM. Last winter we built a L/R version of the engine that saw a 5-7hp increase with no loss of torque, all above 9000 RPMs. After flogging the engine on the dyno for days, we could not get any additional power out of it below that engine speed. None. Nada. Zip.

My '73 Mustang's drag racing engine is built to a completely different spec. It has a stroked 460 with shorty BBC rods that absolutely kills the rod ratio, but develops 600+ lbs-ft of torque just off idle for head-snapping acceleration. I don't care about the additional wear from the short rod ratio because this is pretty much a pure drag racing engine. It might see a couple thousand mile a year. But you don't have to go this radical to see additional low-end torque. A less radically shortened rod ratio can do great things for a street engine in a heavy car, and is sure a lot cheaper than buying trick racing pieces that will never get a chance to show their stuff.

So my advice to any considering pursuing a L/R engine program is, please be sure you're doing it for the right reasons before spending your hard earned money.

Jus' my dos centavos... ;)
The side thrust on the piston is greatest when the angle between the bore centerline and the conn rod axis is greatest. Ignoring wrist pin offset, this occurs when the crank throw is horizontal.

The side thrust would be proportional to the tangent of that angle. For the standard rod 300 the angle is 18.691 degrees, for the long rod it would be 16.324 degrees. The respective tangents are .3383 and .2929. The long rod would therefore reduce the maximum side thrust to about 86.5% of that experienced with the stock rod.

At TDC and BDC the difference would be zero, rising and falling to that value through each rotation. I don't have a clue as to whether the magnitude of these numbers is enough to make this difference significant within the entire scheme of things, but on the surface a 13.5% reduction in any loss seems worth investigating.

As for rod length vs. torque, working out what happens through the stroke requires more effort than I'm going to spend, but the case of maximum transmission of force to the crank is pretty easy. It occurs when the axis of the conn rod is perpendicular to the crank throw. Ignoring wrist pin offset, this occurs at 72.231 deg ATDC with the standard rod and 73.676 with the long rod. The force transmitted to the crank is proportional to 1/sin for these angles or 1.050 and 1.042 respectively meaning that in the maximum condition, the standard rod transmits 0.77% more force to the crank. Differences at other places in the stroke may well be either less or greater and may even favor the long rod but I doubt they are an order of magnitude different.