Actually, it won't. For a given displacement what little change there is in mechanical leverage is offset by the change in piston area (swept area). If you increase stroke, you have to offset it with decreased bore size to maintain the same displacement, and vice-versa.

The major factor in increased torque with increased stroke (vs the stock stroke length on a given engine) is the increase in displacement.

Here are the calcs so you can see for yourself. They are from a post I did on another forum addressing this very issue.


Brad

 

You can do all the calculations you want, but a longer stroke will give you, in general, a broader torque curve than a short stroke. I think that has been empirically proven over time. You can check it out by playing with a desktop dyno. They have some pretty complicated math in those programs and for a given engine combination if you keep the displacement the same, the shorter stroked engine will make more hp higher up in the RPM range and less torque.

 

Engines set up to make more HP up top (higher RPM) do not make that power because of the stroke length, they make it because they are optimized for better air flow management at higher RPM. The stroke is short to keep the rotating mass closer to the crank centerline in order to minimize piston speed and related mechanical stresses. The lower polar moment of inertia also allows the engine to "spool up" faster.

The stroke length myth is always guaranteed to spark a heated debate because it appeals to a basic (if flawed) premise. Unfortunately it does not stand up to some basic scientific scrutiny.

Sorry to have to break it to you, but you're trying to argue with physics. And you're losing.

Brad

 

Yes I know the EM Big Block Comp. is over, but the way it cycles out, 2005 will be another BB Comp. year. Brad Johnson, you are correct, as this checks out in many cases. The fact is, if built correctly, a smaller engine, with less rotating mass(which happens to be the case with a shorter stroke) and correct cam timing, will always out power a long stroke engine. I have found this to be extremely funny, as everyone says it won't work. But have you, BigSnag, ever tried?

 

Yes, it will be a BB in 2005...but it will be a stroker combination...should do more research next time.

It's true that short stroke motors will make more power, but when you consider that the pulls are made 2500-6500rpm and averaged power wins...the longer stroke engines will prevail. Not only because they produce more torque due to the leverage of the crankshaft...but also because the longer stroke elevates piston speed for a given rpm. This means for any given rpm, the air will move with more velocity which is a good thing to a point when the port cannot keep up and becomes turbulent...which is not a concern at relatively low 6500rpm ceiling. You need only look a the top three engines this year and the small blocks from last year to see that huge oversquare engines were not the way to go.

 

Incorrect. Refer to calcs in prior post. For a given displacement, variations in stroke length vs bore size will cancel each other out.

It's not piston speed, it's volume (displacement) and RPM. The piston speed can vary all over the place, but the fact is you still have to fill the same volume in a given amount of time.

Brad

 

For a given rpm, a longer stroke motor will have increased piston speed...correct?

So in order to fill the same displacement cylinder, the air has to be moving at higher velocity...

Higher velocity means less reversion, more cylinder blowdown at overlap and better scavenging...

 

Not that I know much of anything...but Im under the impression that the longer stroke and resultant longer dwell will in fact aid in cylinder filling and scavenging as chilly mentioned. The downside to a long rod that I see is the higher piston speed can creat a bomb at the same rpm that a shorter stroke may live happily. I see it as a give and take. How good of rod, and how light of piston do you run? After you decide that, than consider a long stroke.....or not......based on the realistic expectaions. I would think that a big bore short stroke would take to excessive RPM's where as a smaller bore and longer stroke would be a great set up for a comparitivly lower RPM operating window. You dont see many 571 engines running 8500 rpm, but yet they make fantastic power and work very well using gears to offset the loss of ultra high RPM potential. Take a 500 or 521 and 8500 RPM is not that hard to safely achieve. A bit off topic, but what I think is that there is more to the conversation than just long-vs-short stroke and whats better. It would depend on the expected rpm window and budget involved.....IMHO....

 

Piston Speed
Formula: Piston Speed in fpm (Feet per Minute) =
stroke (in.) x rpm/6

Example: 3.11" x 6000rpm/6 = 3110 fpm

Now to really go away from the topic, but in a supportive way....

The following maximum piston speeds are from the book, Performance Tuning in
Theory and Practice, by A.G.Bell.

Stock Motor - 3,500 fpm (cast crank, stock rods and cast
piston)
Heavy Duty Motor - 4,000 fpm (forged crank, peened rods w/ good
bolts, forged piston)
Drag Racing Motor - 5,000 fpm (forged crank, alum rods, lighweight
pistons, etc.)


Different "stock" Ford engines have different maximum rpms based on piston
speed:
289 - 7,317 rpm max
302/5.0 - 7,000 rpm
351 - 6,000 rpm
390 - 5,556 rpm
400 - 5,250 rpm
428 - 5,276 rpm
460 - 5,455 rpm
(Some high performance "stock" engines have forged cranks and pistons and
could survive higher rpms than listed above, use the heavy duty formula for
those engines)

 

If you fill a given volume in the same period of time, the piston speed is irrelevant in terms of intake charge velocity in the intake runners and at the valve. The piston may be moving faster, but you are still filling the same amount of space in the same amount of time.

Look at it this way...

Let's say you have two containers. Each is 144 cubic inches. One is 1" per side by 144" tall. The other is 12" per side by 1" tall. Both are being fill through a hose that is 2" in diameter. It takes exactly one minute to fill them. So, you are filling 144 cu inches of space in one minute's time. That's 144 cu inches per minute. Agreed?

Since you are filling the same volume in the same period of time it doesn't matter what is happening inside the container, you are still filling them at a rate of 144 cu inches per minute. Also, since you are also flowing the same amount of material through the hose in the same amount of time the velocity of the material will also remain the same.

The stroke length WILL influence airflow dynamics in the cylinder as the physical layout of the cylinder will change as you alter bore / stroke ratios. However, the effect will occur only after the air mass enters the cylinder. It will not alter the volume of material entering the cylinder (which is a function of displacement), nor will it change the time interval (which is a function of engine speed).

Brad

 

Well said.

As the revs rise, engine builders have a critial decision. They can only spin the motor so fast before everything goes "POOF!". In order to increase the revs, they get everything as light as they can, then shorten the stroke as far as possible to get the reciprocating mass as close to the crank centerline as possible. Materials technology has advanced to the point that they can engineer parts to do pretty much anything. They are now up against things that are really head scratchers, like...

- Fuel will only burn so fast. How do they maximize the flame front propagation to make sure they get all the power potential out of the fuel and not have it wasted out the exhaust because the piston is traveling faster than the fuel can be effectively burned.

- Sometimes atmospheric pressure just ain't enough. There comes a point that the 16 or so psi of atmospheric pressure just won't cut it, and you simply can't get the air to move fast enough. Air, as thin and etheric as it is, still has mass and it takes time for that mass to move. Unless you force-feed your engine, there is a point where atmospheric pressure is simply not enough to overcome the resistance of the air mass to movement (or to restrictions in the path).

A lot of the "long-stroke" myth is the American tradition of big, reliable, low-stressed engine with tuning that emphasized low-rpm torque (which was easy given the added displacement). When we began to see smaller, more efficient engines, most of them used engine configurations designed to move their meager torque outpur higher in the RPM range to mazimixe horsepower. Part of the design emphasis was lower reciprocating weight, which was the result of lighter parts swinging in smaller circles.

So... we saw big, long stroke engines making tons of torque but relatively little horsepower, and small, short-stroke engines making less torque but more horsepower. Plus, we all use breaker bars or breakover handles to give us a little added twist when needed, so it seemed applicable that longer stroke equals more leverage. Never mind that the longer stroke had a correspondingly smaller bore, or that the stroke lengths varied by a few percent and not the hundreds of a percent you see with a breaker bar.

Combine a little misperception with not-really-applicable shop wisdom and you get a big, fat myth. Served up hot with a side order of fries. Some long-time engine builders still swear up and down that it's applicable. However, the more knowledgeable (especially those with an engineering backgound) will tell you it's just not the case.

Brad

 

One may also choose to add that you can "cheat" the effect of a long stroke by a tall deck block and a loooonggg rod that offers a better rod inclination angle, and it offers a longer yet dwell time with a less violent direction change of the piston. I see a long rod as slowing the piston down less violently, and also taking off the other direction less violently. This can work to allow a longer than conventional stroke to exist in a window of RPM's that may be considered taboo.

The thing is, all of the scenarios here will work fine, but you have to know up front what your doing and design the port, the crank, rod, and piston around the same idealology. If you confuse the available options you will get an engine that does not meet your expectations.

 

Bingo!

Brad

 

Bottom line: Shorter stroke allows you to rev higher, and therefore make more ulitmate hp. However, in the EM challenge max power is NOT the goal.

Truckpuller,
You never refuted to longer stroke gives more dwell to allow for cylinder filling argument. Care to offer your opinion??

 

Max power may not be the key to winning, but, if the internal components, like the cam, allow a broader torque band, all else, like stroke, becomes moot. The reason for the short stroke is to reduce "felt" weight, much like putting an aluminum hub damper on. While one might think the long rods defeat this, it is the weight at the end of the rod that makes the difference. Think of it like this: As a kid, you were on the playground. You go to the see-saw, and you have a choice of a 50 lb. kid sitting on the end of the 10 ft. plank, or the 100 lb. kid only sitting 5 ft. away. The weight is the same, yet it is easier to move the 50 lb. kid. Correct me if I am wrong.