Rod to Stroke ratio

[tricky]

Yes, it is the mujch anticipated thread, designed to derive an empirical relationship between the length of ones johnson and the required frequency of self-releif

Well not really--

I've been doing a bit of research lately on the effect of rod:stroke ratio on a NA engine, and while there is a lot of information out there, there seems to be a lot of conjecture and hearsay.

Here's a bit of background:

• Short rods
  
  • Higher piston accelleration and peak velocity
    This means the engine will suck harder, and consequently have good low-midrange torque. Larger cams and bigger diameter runners can be used with fewer negative effects. The downside of this is that the forces on the bottom end are greater.
  • Greater NVH.
    Obviously this is a downside...
  • Allows for a shorter block
    This is only really a concern for the factory. There's nothing we can do about this.
  • Due to high velocity away from TDC, the maximum cylinder pressure is delayed.
    Good for forced induction, detonation resistance etc.
  • more ignition advance is required due to geometry
  
  
• Long rods
  
  • Reduced rod angle
    This means reduced friction and allows the use of short skirt pistons (lighter)
  • Allows for increased piston dwell at TDC
    Leads to more complete combustion and higher ATDC cylinder pressures. This also means that they are more knock sensitive...
  • Less homogeneous air/fuel mixtures at low rpm
  • Comparibly crap low-midrange torque due to lower gas velocities.
    Also means that the engine has poorer fuel economy and emissions

In short, a short rod:stroke is good for torque, economy and generally low engine speeds, whereas a long rod:stroke favours higher outright power, but it really needs revs to boogie.

This is fair enough, but how does one decide what rod:stroke ratio is required for a desired application? This is where the guessing comes in. Many quote 1.75:1 as a optimal figure for a performance engine, but what factors affect this? It seems the breathing of the engine is the factor most relevant.

Here, I'd like to bring up a couple of examples:

• 18RG (stock)

The geometry of this engine is undersquare: 88 x 80mm with a rod length of ~138mm for a rod:stroke ratio of 1.73:1 As we all know, this puppy can rev well if it is balanced properly, and we also know that bottom end torque sucks... Fits the trend!

• 18RG (with 22r rods)

This is a theoretical engine, and I don't even know if the block length allows this configuration (I suspect not), but for arguments sake... This gives a rod:stroke ratio of 1.85:1. Long, but still within the range employed by some designs. Presumably, the engine would become a pig to drive, but peak power would be higher, and high revs would be safer. Another negative would be detonation susceptability due to the crap cc design.

• BEAMS 3SGE (stock)

This engine is perfectly square: 86 x 86 mm with a stock rod length of 138mm -->r/s = 1.6:1. This is relatively short. But the engines are quite different. The reciprocating mass is less, the balancing from the factory is significantly better and the head breathes well. Interestingly, peak power is achieved at a similar point in the rev range.

• BEAMS 3SGE (TRD)

The TRD BEAMS 3SGE powerup involves using 146mm rods for a r/s = 1.74:1. Obviously this makes power up a bit higher again, but the r/s is comparable to a stock 18RG.

One thing now I wonder is what the effect of the 'squareness' is on the optimal r/s ratio. In the undersquare case, the larger bore means more volume to fill per degree of crankshaft revolution, so a higher gas velocity would be achieved using the same runners... Similar effect to shorter rods without actually changing the geometry?

I'm thinking that this is less significant than the combination of head flow, port velocity, valve area, cam duration, timing and lift...

Now, really I'm wanting to see if anyone has significant first hand experience in this area or has a good grasp of the theory, and we can nut out some optimal numbers. Any examples or theories would be appreciated (if anyone has 1UZ numbers, that would be interesting too). While I have focused on NA, feel free to add forced induction arguments too!

I'm off to clear the head now

[oldcorollas]

high or low torque is controlled by the cam primarily.
rod ratio affects friction losses, and prodction of torque due to better angle of piston pushing on rod, and lower cylinder wall losses.

This means the engine will suck harder, and consequently have good low-midrange torque. Larger cams and bigger diameter runners can be used with fewer negative effects
Due to high velocity away from TDC, the maximum cylinder pressure is delayed.
Good for forced induction, detonation resistance etc.
Less homogeneous air/fuel mixtures at low rpm
Comparibly crap low-midrange torque due to lower gas velocities.
Also means that the engine has poorer fuel economy and emissions

are you sure on these??

the main thing... is that when peak cylinder pressure is reached, the piston is pushing more down than sideways (relative to short rod).
and there is longer dwell, so that more pressure pushes down on piston for longer.
both = more torque.

the timing of peak pressure depends on design... not just rod/stroke ratio, many things..

saying rod stroke ratio greatly afects gas velocity, or air/fuel homogeneity, or emissions... thats reaching a bit perhaps....
just my drunken 2 yen

[mic*]

I agree wih the first part of Stu's post but i can see the "suck harder/higher velocity" point.

The pistons have the same average velocity, which is determined by the stroke (ignoring revs) but a shorter rod will have a higher instantaneous velocity immediately before & after TDC.

A greater angle occurs between a piston and a short rod as the crank moves laterally away from the perpendicular centre line of the bore (following TDC). This means the piston must be pulled down further by the same travel in the crank.

This (shorter rod) may be more knock resistant but the piston is running away from the combustiuon pressure. Ie, the piston is accelerating more by its own means immediately after TDC meaning more frictional losses and less combustion gain.

It makes me curious of the effects of a shorter vs longer rod in the actual compression of the charge.

[1jzracing]

i was alwayse under the inpression that a longer rod was better for torque and rpm while a short rod was only a result of minimum manufacturing requirements

a longer rod is needed for higher rpm to reduce bore piston and rod loads and peak piston inertia as it passes over tdc (this increases with a short rod limiting the max rpm)

old slow reving torquey engines typically had long rods some later engines which grew in size over the models slowly developed short rods because longer strokes were used in a previously made engine ie chev went from 283 all the way to 350 cu with the same rods then they went to a 400 which used even shorter rods!!! typical bone-head yanks hense everyone puts the 6" rod in

i think there is an optimum ratio for all situations and thats about 1.7 this makes best use of torque vs rpm vs reciprocating mass vs piston thrust load

i played with longer rods in a 2.7 beemer engine once and it went like the clappers but i also had mild cam etc in atmo form it made 130rwkw, i have essentially the same engine but with short rods in the car now and it certainly doesnt like to rev as much and is about 20kw down but its lower compression and different cam so not really a fare comparison

[mic*]

Shorter rod = greater peak piston IMPULSE as it passes over TDC

The larger rod and piston has more INERTIA as it has more mass (and slightly greater peak velocity @ 90 & 270 deg?).

And this greater mass of the longer rod setup is the only downside i can see...

Engineering limitations too... It would be hard to fit a two foot tall block into any car that looked good and could still take a corner.

[kingmick]

ok to take the formula's out of it and look at it in a real world situation.

long rods are only useful in a high reving atmo engine and turbo engine.
the long rods make it much harder to tune because of the vacuum signal{low signal} so they are very hard to tune properly and diffrent atmospheric makes them very hard to keep in tune for carbie's and hard with efi. the slow TDC makes them suck less hard than short rods.

getting the cam timing right with short rods is much easier than long rods.the tuning is far easier with short rods and unless you have the money to take the crank jornal down to 32mm you only get a small HP advantage with higher reving longer rod. the to take advantage of the long rod you want a short skirt and light piston in 260 gram range.

if your have a good budget to to a long rod atmo, fine they work very well but if you dont stick with the short rod length.

im trying to make it easy so every one can get there head around it.

[mic*]

Mick, can you explain how cam timing is more difficult with long rods?

And when you say "take the crank journal right down to 32mm" are you refering to a specific motor?

[3sgte]

If I may dwell on one point....

Short rods

Higher piston accelleration and peak velocity
This means the engine will suck harder, and consequently have good low-midrange torque. Larger cams and bigger diameter runners can be used with fewer negative effects.
The downside of this is that the forces on the bottom end are greater.

This (aside from the conclusion I greyed out) has always been my understanding of the hazard of having short rods as a means of increasing displacement. Peak piston speeds increase, along with the destructive forces.

I have to agree with OldCorollas.
The conclusions listed here about effects of the bore stroke ratio are a bit too far reaching IMHO.

There is a boatload of stuff on the web about this. One advocate of long rods, Smoky Yunick:
http://www.google.ca/search?num=20&h...length&spell=1

Having said that, giving up stroke (ie. displacement), to have longer rods probably isn't the best bet.

This link says that the Ford 4.6l has a 2.5:1 ratio rod:
http://72.14.203.104/search?q=cache:...&ct=clnk&cd=10

[kingmick]

getting the cam timing just right on the long rods takes time because of the slow TDC{ low vacuum}. with the long rods the mix isnt sucked in as hard so it takes awhile to get them set at there optimum.
yes 32mm journal i was talking about the 7a bottom end. this is a new thing that we are trying with a standard crank to see it can take it. if it cant then we will make a crank with 32mm journal size.

[kingmick]

hahaha god bring up smokey! the ledged himself. i spoke to him on the phone around 7 years ago when he rang to speak to the old fella. very nice manner and i spent 10 or so years with hearing at least once a week "this is a new thing smokey said to try or smokey said" lol .
RIP you ledgend.

[tricky]

Thanks lads.

3SGTE, the instances I am talking about keep the bore and stroke constant. It is only the length of the rod that changes, but the links are good!

kingmick, thanks for the real world advice!

1jzracing - also, real world examples are great!

oldcorollas, my retort to the first part, mic* summed up... But my runner velocity/homogeneity claims... Assume the engine is the same other than the modified r/s. Again, the argument stems from the fact that the peak piston velocity is increased. This means that the intake pulse will be stronger, so the instantaneous runner velocity is increased. This is directly proportional to the homogeneity of the fuel/air mix. This is also the reason for short r/s engines having more low end grunt.

mic*, cheers. Good info!

It seems that with modern materials (read light and strong), oversquare engines with relatively short r/s ratios are being produced for the mass market. In modern racing engines, again, it looks like the acceptable r/s is getting shorter than it used to be. I'm thinking that this also stems from heads which flow (velocity AND volume) so much better than old school two valve heads.

[oldcorollas]

peak piston velocity... is controlled by the stroke... when piston is halfway down the bore, does the rod length have that much effect? when piston is halfway down more, and crank is pulling parallel to the piston, then both long and short rods should have same velocity?, ie, the rotational speed of the crank journal...
the sideways movement of the rod at that point is zero... because it has moved away from the centreline, and is starting to move back...... so the sideways velocity of the rod is zero (both at the piston and crank... when piston is halfway down bore)

so i fail to see how short rods increase piston PEAK velocity.

the difference is in the acceleration and deceleration of pistons from zero, to the same peak velocity.... ie the movement is not a perfect sine curve.
short rods will have higher acceleration initially than long rods (i think?) but as to why that has such an effect on cam timing..... perhaps for longer rods, cam duration an overlap need to be less to take advantage of the slower piston near TDC with longer rods?
arrgh, need to draw graphs... have better things to do



interesting stuff tho.

[3sgte]

peak piston velocity... is controlled by the stroke...
arrgh, need to draw graphs... have better things to do



interesting stuff tho.

I think this link can shed some light:
http://e30m3performance.com/tech_art...atio/index.htm

Especially the spreadsheet that someone created (end of page 2).
(no need to make graphs, someone has already done it!)

My understanding is that average piston speed would be the same over the revolution (edit: for a long and short rod motor)
However, because the piston has travelled slower for a period of time, it now has to make up that difference.

The length of time for the engine revolution to occur remains the same for short and long rod.
The piston moves slower for a longer period of time on the short rod motor, meaning it has to travel faster on the "fast" part of its travel to make up for the time at the lower speed.


Props to whoever put the time into that spreadsheet!
(I don't have the math skills to verify the calculations, please report! )

[Celica RA45]

tricky the only reason that TRD increased the rod ratio was so the red line of the motor could be moved up the rev range ,std altezza 3sge 7800 rpm redline with trd rods and pistons 8800 red line as everybody has said ,pistons will dwell up at tdc longer creating more torque and its also better on the bores as well ,my rods are also 146mm long and the new motor is slightly smaller in the rods by 1.2mm shorter in length

[LeeRoy]

I understand the majority of this but how does rod length have any impact on piston accelleration? Wouldn't piston accel be determined by the crank?. Adding a longer or shorter rod will only change the placement of the piston in the bore.

- LeeRoy