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Some things just aren't worth worrying about. Any time you're modifying an engine, building one from scratch, or even bench racing, there are literally hundreds of details, dimensions, material specs and processes you must consider to maximize your performance. What bore and stroke should your engine have, what compression ratio, piston material, valve seat pressure, cam profile...you get the idea.

Often, with so many considerations, the smart thing is simply ignoring some of them. Planning on changing your compression ratio? Fine, you probably don't have to worry about what country your valve spring wire came from.

On the other hand, the whole point of this column is to dwell on stuff you shouldn't worry about, so what the hell, here's something else to consider: Rod ratio.

The rod ratio is simply the length of the connecting rod divided by the stroke. A Subaru WRX's EJ20, for example, has a 75mm stroke and a 130.5mm rod, so the rod ratio is 1.74:1.

So what?

Turns out the length of the connecting rod can affect the internal frictional losses, the acceleration loads on the piston, and even how the engine breathes. The piston loading is easy. Look at what happens to the connecting rod with the crank at half stroke if you have two different rod ratios at opposite ends of the normal range.

With a 1.8:1 rod ratio on the left, the connecting rod will be at a 16.1 degree angle from vertical (that's just trigonometry, trust me on that one). With a 1.43:1 rod ratio, the rod is leaned over 20.5 degrees. The steeper this angle, the harder the piston gets shoved against the cylinder wall as it pushes down on the connecting rod, so a short rod ratio means higher side loading, more friction and more wear.

Now, let's look at speed and breathing. With the shorter rod, the piston accelerates away from top dead center more quickly than with a long rod. Don't believe me? You could always play with this equation and prove it to yourself.

Yeah, I didn't think so.

Let's skip the math and prove it with reason. The highest piston speed occurs right after the connecting rod is at a 90-degree angle to the throw of the crank. The point at which this magical 90-degree angle appears depends on the length of the rod. Did I say no math? How about just a little?

OK, nerd-boy. Get the Hewlett-Packard and for the rest of you who don't remember what sin, cos, and atan stuff is, that's trigonometry. If you had a Hewlett-Packard calculator, there would be buttons for those. With a 1.8:1 rod ratio, this 90-degree mark is 74.5 degrees from top dead center, but with the 1.43:1 rod ratio, it happens at 70.7. Even if you assume this peak speed is the same with both rod ratios (actually it's faster with the short rod, but you'd have to deal with that big equation to know that) the short-rod engine reaches that speed 4 degrees sooner, which means it has to accelerate faster.

That harder acceleration means short-rod engines are happier at low rpm for two reasons. First, the higher acceleration means more stress on the whole reciprocating assembly, and the higher the engine speed, the closer that stress is to blowing things up. Less obviously, though, the more rapid acceleration away from top dead center means a short-rod engine breathes better at low rpm than a long-rod engine. That rapid piston acceleration gets the port velocity up sooner, improving low-rpm cylinder filling, fuel atomization, and ultimately torque output. The ludicrously low 1.43:1 rod ratio of Nissan's new QR25DE is part of the secret to its tire-hating torque band.

As the revs climb, of course, the piston starts pulling away from top dead center drop too fast to really help airflow. At the other end of the rod-ratio spectrum is Nissan's rare, Japan-only SR16VE. This high-revving 1.6-liter monster makes 200 naturally aspirated hp. How? Well, variable valve timing and extensively hand-massaged ports do most of it, but facilitating that high-rpm breathing is a 2.1:1 rod ratio.

If you're starting to think rod ratio is worth worrying about, think again. The fact is, within the confines of a given engine, virtually everything is more important than the rod ratio. For example, taking the SR20VE's 86mm bore, 86mm stroke, and 136.3mm connecting rod, and shortening the stroke to 68.7mm created the SR16. The short stroke made room for an extra-long 144.95mm rod, but the 400cc of lost displacement could have made more power and far more torque than the long rod ever did.

Of course, if you're already replacing the rod and piston, there may be room for a slightly longer rod and an equally shorter piston, but only barely. Jim Wolf Technology has a rod and piston set for the QR25DE, for example, that lengthens the rod to 148mm, giving a slightly more rev-friendly 1.48:1 rod ratio, but this really only drops peak piston speed about 0.4 percent. You have to want the stronger rod and different compression ratio to really justify the switch.

Rod ratio is only significant enough to consider if you're designing an engine from scratch, or building a cost-is-no-object race engine for a class that allows you to make the cylinder block taller to make more rod room. Of course, if that's the case, you probably didn't learn anything by reading this.

Rod/Stroke Ratio - What's Your Angle?
Dip into the intricate world of internal engine geometry and look closely at something you know very little about: rod/stroke ratios. (C'mon, admit it!) Editor Bob demystifies.
By Bob Hernandez

Engine Building Internals

If there's one truth about Honda engines, it's that they like to scream. And Honda enthusiasts like to make them scream. The numbers on the tach reach so high, Honda practically offers the license: Go ahead. Make it sing. It's good at it. So long as you don't miss a shift, all is bliss.

Lightweight components, stronger materials and shorter strokes enable modern four-cylinder engines to spin very fast, yet last longer than ever. A tremendous amount of science goes into engineering and creating these high-spinning machines, most of it rooted in elementary principles of physics and geometry-fundamentals anyone planning to build an engine should know.

Understanding rod/stroke ratio, or the amount that a rod deviates from an imaginary straight line extending from the center of the crank journal to the center of the piston, is key to knowing how these machines deliver power at high rpm.

Determining the Rod/Stroke Ratio
To determine a motor's rod/stroke ratio, divide rod length (distance in millimeters from the center of the big and small ends) by stroke. A B18C1, for example, combines 138mm rods with an 87.2mm stroke for a 1.58:1 ratio.

Most engine builders shoot for a ratio between 1.5:1 and 1.8:1 on a street motor, with 1.75:1 considered ideal, regardless of application. (The most highly developed four-stroke engines in the world-F1 and motorcycle engines-have rod ratios of more than 2:1.)

The rod/stroke ratio affects several engine dynamics, including piston speed and acceleration, piston dwell at top dead center and bottom dead center, piston side loads, cylinder loading and bearing loads. Many of these elements play roles in engine aspiration, combustion and wear.

Generally, a lower ratio means a high rod angle, creating greater potential for accelerated wear to cylinder walls, pistons and rings. A low enough ratio, due to the severity of its rod angle, can drive a piston right into the cylinder wall.

Higher ratio engines, on the other hand, don't have the same friction concerns, but compromise in other areas. Air does not fill the intake ports with the same velocity, and there is less demand for the ports to flow as well since there is more time to fill and scavenge the cylinder (we discuss this phenomenon later). This typically means stagnant airflow at low revs and weaker torque. Hey, you can't have it all.

Lower Ratios-A Honda Characteristic
As the chart on this page indicates, many Honda ratios-designed for economy-fall on the low side. Honda produces compact, short four-cylinder engine blocks that don't require long rods. Most Honda blocks also feature a small bore. When coupled with a short stroke, the rod angle is still harsh, though not as bad as if the piston were larger in diameter.

Some tuners take the geometry into their own hands with longer rods. A longer rod makes more torque with the same piston force, and since it's less angular than a shorter rod, reduces sidewall loading and decreases friction. All of this adds up to more power.

Rod/Stroke Ratio - What's Your Angle?

Engine Building Rod Angle

Longer rods also give the pistons more "dwell," the brief periods of time the piston is at top dead center and bottom dead center. A longer dwell allows for better flow of intake and exhaust gases since the piston moves slower between up- and downstrokes.

Longer dwell also offers more time to fill the cylinders during the intake stroke and more time to scavenge during overlap. And since the piston hangs out at or near TDC longer, the combustion stroke has more time to deliver a thorough release of energy on to the piston.

In a stroked motor, the piston ultimately reaches greater speeds to cover the additional stroke. The speed makes intake, compression and exhaust strokes more turbulent and, consequently, more powerful. It also comes with its price in component wear, something to consider when looking into parts that increase stroke.

With a short stroke and a long rod, however, the piston accelerates more gently from TDC. It picks up its greatest speed further down the bore, at the point where the crank pin relative to the rod angle reaches 90 degrees. Since the pistons move from TDC slower, the entire bottom end absorbs less mechanical stress.

Advancing Toward A Thin Line
Even the short-stroke/long-rod combo has its limits. To accommodate extra rod length, some builders will move the piston pin higher into the slug, or opt for a deck plate. Either method requires an experienced wrench with access to a lot of custom parts.

Longer rods in a stroked motor can act to offset any increase in rod angle, but also requires a shorter piston. The deeper you dig into a piston to shorten it, the greater your odds of cutting into the oil ring groove and wreaking havoc with oil consumption. Most piston companies in the sport compact market engineer pistons with tighter ring packs and bridge rings to help avoid this problem.

Regardless of whether you take the stroker route or just run longer rods, you reach a point where you can no longer shorten a piston any further without compromising dependability.

Friendly Advice
Most engine builders believe longer rods are better, but a fringe of enthusiasts still dig the low-rpm torque that shorter rods can make. We advise builders who want a ratio of less than 1.6:1 to use the strongest aftermarket rods they can find, given the angle. We also recommend aftermarket sleeves to better fend off the lateral stress created by the rod angle.

Here's one last nugget to impress your friends with: a formula for calculating piston speed in feet or meters per second. The equation illustrates the point that the longer the stroke, the faster the piston travels at the same rpm.

Take a B16A2 vs. an H23. At 7000 rpm, the B16 slug moves 18 m/sec. At the same rpm, the H23 piston hauls additional ass-22 m/sec. Simply multiply stroke by rpm, and voil-minutes of endless doodling in class.

Stock Rod/Stroke Ratio Information For Some Popular Honda Engines
Block   Rod length   Stroke   Rod ratio
D16A6, Z6, Y7, Y8   137mm   90mm   1.52:1
B16A1, A2, A3   134.4mm   77.4mm   1.74:1
B17A1   131.9mm   81.4mm   1.62:1
B18A1, B1, B20B4   137mm   89mm   1.54:1
B18C1, C5   138mm   87.2mm   1.58:1
H22A1   143mm   90.7mm   1.58:1
H23A1, A4   141.5mm   95mm   1.49:1
K20A, A2   139mm   86mm   1.62:1
K24A   152mm   99mm   1.54:1

(http://image.hondatuningmagazine.com/f/9329722/0506_ht_02_z+engine_building+rod_angle.jpg)

the rod ratio in a gasoline engine determines the piston dwell at TDC. the longer the rod, the less downwards piston motion will be the result of the rod angle changing.

if the crank rotates 10 dergees, then a very short rod will be at a greater angle than fi the rod is very long. since the rod doesnt change length then some of the pistons downwards motion will be a result of this phenomenon, so the longer the rod, the longer the "dwell" and the longer the dwell the more complete combustion and higher cylinder pressure. The rod length is limited by other engineering conserns, like bloch deck height, piston height, and thus inherrent stability, and resultant wear. and so on.


In most large cubic inch engines it is hard to get a very high ratio. A Stock 350 Chevy has a 1.64:1 RL/S ratio, which is not very good. By increasing the rod length to 6" the ratio increases to 1.72:1, which is much better. You can squeeze a 6.1" rod in a 350 with little trouble, but longer than that requires plugging the piston pin bores after assembly to support the oil rings. It is not worth the extra expense for the little gains, so a 6" rod in a small-block Chevy has become common because everything fits right in. With endurance engines, longer rods are always better. Most endurance engines are using a RL/S ratio of at least 1.9:1 and some as high as 2.2:1. Before you go out and buy longer rods, let me just say that the gains are very small. This debate has been argued for years and will not end anytime soon. In my opinion, if are building an engine and need new rods and pistons, a longer rod will cost about the same. That makes the small benefits worth it. I would not waste my money buying longer rods if you have a good set of rods that you can use. Use that money to make more power elsewhere in the engine.

Rod Angularity

A longer rod reduces the maximum rod angle to the cylinder bore centerline. Less rod angle will reduce piston side loading; there will be less friction and less bore wear. Less rod angle also gives better average leverage on the crank for a longer period of time. A 5.7" rod with a 3.48" stroke (stock Chevy 350), will have a maximum of 17.774° rod angle. Switching to a 6" rod will reduce that to 16.858°, assuming that the wrist pin has no offset. (on a ford small block, installing the pistons backwards will actually gain almost 20 Hp. since the piston acts like it was mounted on a longer rod, as the ofset is reversed, at the price of increased noise and wear)

Piston Pin Height

A higher pin height will reduce piston rock and aid ring seal. please no screaming and name calling about anything about a tighter ring pack, we are talking about pin height and pin height only. Moving the pin closer the the center of gravity of the pistons makes the piston more stable.

Rings

As the compression height is reduced, the space for the ring pack also get reduced. This can be a problem on some engines. It is good for power to have the top ring as close to the top as possible, but this is limited to the strength of the top ring land. As it becomes thin, it becomes weak. High output engines (especially nitrous engines) need a thicker ring land to keep the cylinder pressure from pinching the top ring. In my opinion, if you have to compromise ring location, it would be better to run a slightly shorter rod.

Skirts

Shorter skirts are usually combined with a shorter piston for a longer rod, but they are not really related. There is no reason to reduce the size of the skirts just because the pin location changed. A shorter skirts are used to reduce friction and lighten the piston. The cost is a little less stability, but it is arguable that a lighter piston with a higher pin height does not need the extra stability. For a street car, I would increase the rod length if it meant a reduction is skirt size. Most of the things listed here are for competition motors to gain a few hp, not worth a lot of effort for most street engines.

Piston-to-Valve Clearance

A longer rod decelerates toward TDC and accelerates away from TDC slower than a shorter rod, so piston-to-valve clearances are tighter with a longer rod. This may require deeper valve relief's in the piston (but probably not). A short rod is just the opposite, there is more clearance because the piston decelerates toward and accelerated away for TDC faster.

Piston Velocity

A longer rod reduces peak piston speeds slightly and delays peak piston velocity until the piston is further down the bore, which gives the intake valve more time to open more. Peak piston velocity is usually somewhere around 75° ATDC and since most cams cannot fully open the cam until at least 106° ATDC, it leaves the valve as a major obstacle when airflow demand is at its greatest. By delaying peak piston velocity, even if it's only 1 or 2 crankshaft degrees, it can allow the valve to open another 0.010-0.015", before peak airflow demand is reached. Not a huge help, but a step in the right direction. With a 350 Chevy, switching to 6" rods from 5.7" ones will delay peak piston velocity from 74.5° to 75.5°.

Piston Acceleration / Deceleration

Reducing piston acceleration / deceleration from and toward TCD will reduce tensile loading of the rod, the number 1 cause of rod failure. A Chevy 350 with 5.7" rods will have a peak piston acceleration rate of 101699.636 ft/sec/sec at 7000 rpm. Swapping in 6" rods will reduce that to 100510.406 ft/sec/sec at that same rpm. That is a reduction of 1189.23 ft/sec/sec.

Intake Runner Volume

Since it is easier for an engine to breath with a longer rod, less runner volume is needed. This allows more room for an intake system (this is a very small gain, but is real).

Exhaust Gas Scavenging

A longer rod is moving slower at TDC, which reduces the speed of the exhaust gasses during the overlap period. This reduces the scavenging effect at low rpm and reduces low rpm power slightly (makes the engine run more cammie). A short rod on the other hand moves faster past TDC and increases the scavenging effect and help low rpm power.

Ignition Timing Requirements

Due to the fact that the longer rod moves past TDC slower, it gives the charge a longer time to burn. So you need less timing for peak power. Using less timing also reduces the chance of detonation; so higher compression ratios can be used. Switching from 5.7" to 6" rods on a 350 Chevy can allow as much as 1 full point increase in compression. In other words, if you could only run 9.5:1 with 5.7" rods, you could run 9.6:1 with 6" rods.


Longer Rod Pros

Less rod angularity
Higher wrist pin location
Helps resist detonation
A lighter reciprocating assembly
Reduced piston rock
Better leverage on the crank for a longer time
Less ignition timing is required
Allow slightly more compression to be used before detonation is a problem
Less average and peak piston velocity
Peak piston velocity is later in the down stroke
Less intake runner volume is needed

Longer Rod Cons


Closer Piston-to-valve clearances
Makes the engine run a little more cammie at low rpm
Reduces scavenging at low rpm

Shorter Rod Pros


Increased scavenging effect at low rpm
Helps flow at low valve lifts (a benefit if the heads are ported with this in mind)
Slower piston speeds near BDC
Allows the intake valve to be open longer with less reversion
More piston-to-valve clearance
Can allow for a shorter deck height

Shorter Rod Cons


More rod angularity
Lower piston pin height (if the deck is not shorter)
Taller and heavier pistons are required (again, if the deck height is not reduced)
More ignition timing is required for peak power

some of this must be attributed to grape ape racing...

http://users.erols.com/srweiss/tablersn.htm (http://users.erols.com/srweiss/tablersn.htm)
http://www.stahlheaders.com/Lit_Rod%20Length.htm (http://www.stahlheaders.com/Lit_Rod%20Length.htm)

Evo da pojasnimo ukratko nabolje vrijednost za najoptimalnije performacce je vrijednost koja krece od 1,75 do 1,9.
S ovim omejrom se dobije veci raspon okretnog momenta i fleksibilniji (elasticniji) motor.

Evo kalkulacija za fijat motore, 128 sohc .

kod motora s 55mm hodom, 1,1 i 1,3l sohc  vrijednost je 2.18 sto je jasan ogovor da se radi o formula 1 motoru. pravu snagu i performanse mogu ocekivati tek iznad 5000 rpm..
Nije ni cudo sto se mogu olako vrtiti u 13000 sto i povjest potvrdjuje.
Te motore neki nazivaju ferrari motori.

 63.90mm hod je samo u 1.5l motoru i omejr je 2.0

Za motore s hodom 67,46, 1,4 1,6 (16V) vrijednost iznosi 1,9 sto je pravi izbor za motorsport program.

Da ti nije kalkulator u kvaru ili je programiran da laze ?1,1 ima 120mm C toC i klip 80mm znaci odnos je 1,5
za 1600 je 128,5 i klip 86,4 znaci da je rod ratio 1,487
za 1400 je 1,596...
ST-2000 motori su imali oko 2 :1 da klip ostane sto duze ...a limit je bio ogranicen pravilima na 8500

ILI sam Ja u krivom?

koliko smo shvatili mozda smo u krivu... ide duzna klipnjace djeljeno s hodom.. ??


Stock Rod/Stroke Ratio Information For Some Popular Honda Engines:

Block                Rod length   Stroke     Rod ratio
D16A6, Z6, Y7, Y8    137mm        90mm       1.52:1
B16A1, A2, A3        134.4mm      77.4mm     1.74:1
B17A1                131.9mm      81.4mm     1.62:1
B18A1, B1, B20B4     137mm        89mm       1.54:1
B18C1, C5            138mm        87.2mm     1.58:1
H22A1                143mm        90.7mm     1.58:1
H23A1, A4            141.5mm      95mm       1.49:1
K20A, A2             139mm        86mm       1.62:1
K24A                 152mm   99mm   1.54:1

Znaci ja sam u krivom . ko prizna pola mu se dodaje

samo pravi auti imaju vrijednosti blizu 2. npr. audi iz 70ih. box

evo jos nekih r/s ratio

http://users.erols.com/srweiss/tablersn.htm (http://users.erols.com/srweiss/tablersn.htm)

Peugeot 306 s16 i 405 mi16 phase 2 i T16 imaju 1.76 .

Peugeot 306 gti-6 ima 1.83......i u ovaj auto se da stavit 162mm klipnjaca u odnosu na serijskih 158mm i to bi bilo 1.88.
Takoder i u ove gore se da stavit 162mm klipnjaca naravno uz odgovarajuci klip.

405Mi16 phase1 ima 1.68

svida mi se tih 1.75 kad pratim ti omijeri imaju dosta dobrih rezultata...

inace povrsno razmisljanje, ali primjer da radim 4*4 mislim da bi povoljniji odnos bio 1.6 do 1.7 koji bi se koristio u podrucju do 6500-7000rpm (da neulazim vezano za turbinu i usis), dok neki motor za pogon na jednoj osovini na veci od 1.75 do 1.9...mislim da ima veliku ulog u ozboru o disciplini, primjer neki drift auto isto bi odabiro nesto 1.65 do 1.75 opet ovisno i o zapremini!

za ono sto ja hocu ,a to je 600 ks i da motor sto duze traje i da na sto manjem boostu dodem do tih 600 ks.....
Znaci ja biram sto mogucu duzu klipnjacu mogu bez obzira sto cu nesto izgubit u donjem rezimu rada,ali cu puno vise dobit u gornjem rezimu rada nego sto cu izgubit dolje.
naravno ovo je samo jedno od varijabli vezanih za ovo.

Ne vidim razliku izmedu fwd i 4wd vezanu za rod ratio???
Jedna stvar kojom se vodimu izboru rod ratio-a je namjena motora(znaci snaga/moment ,raspored snage/momenta,trajnost,itd itd)

to je ok slazem se s tobom , gledano sa kompletnog stajalista tvog motora, posto te nezanima doljni dio, a velika snaga, te hoces trajnost motora s manjim kutom (duzom klipnjacom) manje trenje blabla...

a primjer 4wd, ja to vidim ovako...da se radi Delta motor dva ista motora samo da je razlika u ratiu, bolje ce se ponasat manji na 4wd jel bi trebao biti nesto zivlji u doljnjem kraju (puno snage odlazi na pogon), da i negovorim o autu za trke! mislim da bi trebalo tako funkcionirat, u teoriji je veci kut, veca i brzina, bolje punjenje u doljenjem dijelu,, ali ko zna kako se to u praksi pokaze! dok recimo auto na prednji pogon od istog tog deltinog motora bi napravio sa nesto vecim rs ratiom...znaci govorim o nekim manjim snagama, vise nesto za svaki dan!