Holy crap, where's all that gas going?

[blitz]

Cookie, you're right; 1930's. GM by far had the best, most well-funded R&D department in the 50's through the 70's, and during that period added a lot of authentication to a lot of the previously accepted design theory (e.g. using the see-through quartz cylinder & high-speed movie photography to document abnormal combustion events, etc.).

 

Here's some links on which I base my "stick/ slip", and optimal viscosity commentary:

 

http://www.lubedev.com/articles/friction.htm

http://www.lubedev.com/articles/slipstick.htm

 

All said, the boxer 4 layout has a likeable personality. Like you say, it has a good inherent balance, and what small amount of harmonic that is created it is a very pleasing one. It's got a nice snarl at both the intake and exhaust, sits nice and low in the chassis, etc. IMHO, the best configuration for an 8 cylinder is the V8, the best configuration for a 6 cylinder is an inline 6, and the best configuration for a four cylinder is the boxer 4, all open to debate I 'spose.

 

I've wondered how an uneven number of cylinders would function in boxer form; like an H-5, or H-7? But I'm not sure how the crank throws would be arranged.

[slo5oh]

around WW2 for aero engines if I recall correctly.

I don't know how many, but was under the impression that most small (trick) planes still use a 4 cyl. boxer style engine.

 

Blitz,

Don't motorcycles use an ultra low rod/stroke ratio?

Of course they also have an incredible power to weight ratio.

But the low rod/stroke ratio allows them to rev up into the sky.

[cookie]

and pipers that are the backbone of civil aviation. Blitz has defineately made an interesting question here. I wonder how the rod angle and piston speed of say a Cessna 172 compare here?

[blitz]

Lycoming 0-320-B

 

Bore: 5.125

Stroke: 4.125

Displacement:320 ci (5.25 liters)

Compression Ratio: 8.5:1

Power: 160 hp

Rated Speed: 2700 rpm

 

I couldn't find anything on the connecting rod length. I know that the Lycomings are high torque, low rpm motors (I'm assuming that the "rated speed" of 2700 is the redline). No torque figure was given, but I'll take a stab at 300 ft. lbs.

 

When a Subaru motor is converted for av duty, it's routed through a 2:1 gear reduction. I'd imagine that the overall engine width is a constraint in a small aircraft as well as it is in an auto.

[blitz]

Blitz,

Don't motorcycles use an ultra low rod/stroke ratio?

Of course they also have an incredible power to weight ratio.

But the low rod/stroke ratio allows them to rev up into the sky.

 

I'm honestly not sure about bike rod/ stroke ratios. As far a I've been able to learn on this subject, a short rod can rev high as long as the ports have the ability to build the neccesary maximum velocity, and modern crotch-rocket motors do have those short, straight, carefully worked-out port-shapes that are light-years ahead most of production car engines (deficiencies in port-design are more obvious with a shorter rod). I would think that a dedicated high-rpm design would ideally favor a short stroke in combination with slightly longer rod if space permitted. I'm not an authority on any of this stuff, but there's a lot of white-papers available at the touch of a search button. That's how I pulled up the info to conclude that Subaru trades a bit of fuel efficiency in exchange for overall engine width.

[blitz]

Here's a cut & paste of one of the papers I gleaned from:

 

The ratio between the connecting rod length and the stroke length of a motor greatly affects the way it performs, and how long it lasts. This ratio (normally represented by “n”) can be calculated as follows:

 

Ratio “n” = Rod Length ÷ Stroke

 

The rod’s length is measured (for this purpose) from the center of the piston-pin opening to the center of the big-end bore, not overall. There is a small range of ratios for most conventional piston engines: the rod is between roughly 1.4 and 2.2 times the stroke length. It’s not possible for the rod to be the same length as the stroke, and rods much longer than twice the stroke make the motor very tall, and are not practical for most purposes (although used for racing).

 

The rod angle must not encourage excessive friction at the cylinder wall and piston skirt. A greater angle (smaller value of “n”) will occur by installing a shorter rod or by increasing the stroke. A reduced angle (larger value of “n”) will occur with a longer rod or a shorter stroke.

 

If the rod length is decreased, or the stroke is increased, the “n” ratio value becomes smaller. This has several effects. The most obvious is the mechanical effect. Motors with low values of “n” (proportionately short rods or long strokes) typically exhibit the following characteristics (compared to high “n” motors):

»

 

physically shorter top-to-bottom & left-to-right

»

 

lower block weight

»

 

higher level of vibration

»

 

shorter pistons, measured from the pin center to the bottom of the skirt

»

 

greater wear on piston skirts and cylinder walls

»

 

slightly higher operating temperature & oil temperature due to friction

There are also differences in how the motor breathes:

»

 

intake vacuum rises sooner ATDC, allowing bigger carburetors or intake port runner & plenum volumes to be used without loss of response

»

 

on the negative side, a small or badly designed port will “run out of breath” sooner

»

 

piston motion away from BDC is slower, trapping a higher percentage of cylinder volume, making the motor less sensitive to late intake valve closing (hot cams)

Spark advance is also affected:

»

 

earlier timing (more advance) is required, as the chamber volume is larger (piston is farther from TDC) at the same point of rotation

»

 

the motor may also be less knock-sensitive, as the chamber volume increases more rapidly ATDC, lowering combustion pressure (this is useful for nitrous & supercharged motors)

 

-----------------------------------

 

Effects of Long Rods

 

 

Pro:

»

 

Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque.

»

 

The longer rod will reduce friction within the engine, due to the reduced angle which will place less stress at the thrust surface of the piston during combustion. These rods work well with numerically high gear ratios and lighter vehicles.

»

 

For the same total deck height, a longer rod will use a shorter (and therefore lighter) piston, and generally have a safer maximum RPM.

 

 

 

 

 

Con:

»

 

They do not promote good cylinder filling (volumetric efficiency) at low to moderate engine speeds due to reduced air flow velocity. After the first few degrees beyond TDC piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshaft’s rotation.

»

 

Longer rods have greater interference with the cylinder bottom & water jacket area, pan rails, pan, and camshaft - some combinations of stroke length & rod choice are not practical.

To take advantage of the energy that occurs within the movement of a column of air, it is important to select manifold and port dimensions that will promote high velocity within both the intake and exhaust passages. Long runners and reduced inside diameter air passages work well with long rods.

Camshaft selection must be carefully considered. Long duration cams will reduce the cylinder pressure dramatically during the closing period of the intake cycle.

 

------------------------------

 

Effects of Short Rods

 

 

Pro:

»

 

Provides very good intake and exhaust velocities at low to moderate engine speeds causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. The faster piston movement away from TDC of the intake stroke provides more displacement under the valve at every point of crank rotation, increasing vacuum. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect.

»

 

The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide.

»

 

Cam timing (especially intake valve closing) can be more radical than in a long-rod motor.

 

 

 

 

 

Con:

»

 

Causes an increase in piston speed away from TDC which, at very high RPM, will out-run the flame front, causing a decrease in total cylinder pressure (Brake Mean Effective Pressure) at the end of the combustion cycle.

»

 

Due to the reduced dwell time of the piston at TDC the piston will descend at a faster rate with a reduction in cylinder pressure and temperature as compared to a long-rod motor. This will reduce total combustion.

 

---------------------------------

 

Rod Ratio vs. Intake Efficiency

An “n” value of 1.75 is considered “ideal” by some respected engine builders, if the breathing is optimized for the design. Except for purpose-built racing engines, most other projects are compromises where 1.75 may not produce the best results. There will be instances where the choice of stroke or rod has not been made, but the intake pieces (carburetor, manifold, and head) have been selected. Some discretion exists here for making the rod and/or stroke choice compatible with the existing intake. The “n” value can be used to compensate for less-than-perfect match of intake parts to motor size & speed. The reverse is also possible: the lower end is done, but there are still choices for the top end. Again, the “n” value can be used as a correction factor to better “match” the intake to the lower end.

 

The comments in the following table are not fixed rules, but general tendencies, and may be helpful in limiting the range of choices to those more likely to produce acceptable results. Rather than specify which variable will be changed in the lower end, “n” values will be used. Low “n” numbers (1.45 - 1.75) are produced by short rods in relation to the stroke. High “n” numbers (1.75 - 2.1) are produced by long rods in relation to the stroke.

[cookie]

These engines fell peppy, not revvy to me. I guess that transulates to the fact that you can feel a bit of a push when you stomp on the accellerator, wnere some engines have to rev to the sky before giving you some pull.

It is also interesting that the 2.2 has had a reputation of running for high mileage like a Diesel.

[outback_97]

I read this thread when it was first posted, and I knew it would provoke some interesting discussion. It has long since gone over my head, but I don't think this has been mentioned before: It's generally accepted that the Forester's HP number is underrated, so the car is actually making quite a bit more power at the crank than the stated HP. This is not from personal knowledge, just from what I have read. Also, I think the only way to really know how the cars compare in mileage is with the same driver and same drive, same gas, etc.

 

I do wish the Subies got better mileage, but I honestly can't think of another vehicle that would have done as well what my OB has done for the several years I've owned it.

 

Steve

[slo5oh]

I second that. We've ran this topic way out there... but it's good stuff.

 

Blitz,

That paper you posted confirms everything I've ever seen, heard, and thought about.

 

A mustang mag I used to read religiously did a 351 long rod build up. They built it up to 11:1 and were able to run 87 octane still pushing (I think) 400hp at the crank.

 

Perhaps the horribly short rods we have in our engines were a blessing... now if only we can all put good turbos on....

We need to find someone that's willing to experiment with the gears.... I'd love to know if a taller gear will give decent gains on the MPG by keeping the RPMs down during normal freeway cruising. I think these engines have the TQ to maintain enough power even if cruising RPM were around 2200 to 2500 instad of 3000 to 3500. Damn I miss Gran Turismo...