# Piston Stroke vs Engine RPM



## cfellows (Apr 8, 2013)

There is a generally held notion that higher reving engines should have a shorter stroke, the corollary being that longer stroke engines should not be operated at higher RPM's.

However, do shorter stroke engines operate well at lower RPM's as well?  In other words, if I want to design a slow running engine, does it have to have a longer (than square) stroke?

Chuck


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## Hopper (Apr 9, 2013)

The main reason high rev engines have a shorter stroke is to keep the piston speed down to a rate of feet per minute that will not cause damage/seizure. A longer stroke means more distance travelled by the piston per minute at a set RPM, making its surface speed too high.
Second reason for short stroke is to give a larger piston diameter, which allows the fitting of bigger valves, or four valves instead of two, for high speed performance.

Short stroke motors  in my experience, which is mostly on motorcycles not model engines, will run ok at lower RPMs. But they will not develop the great gobs of  low rev torque of a long-stroke motor. A heavy flywheel helps for low speed running too. 

For example i have a 650cc V-twin Suzuki with a bore of 81mm and stroke of 62mm and it runs just fine from idle at 1,000 rpm up to redline at 14,000rpm or more. It is actually a very torquey motor and gets off the line very quickly thanks to its torque right from low revs. It actually chuffs along at low speed just as sweetly as my very long-stroke vintage Harley, which has a 2.75" bore and 3.80" stroke.


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## Entropy455 (Apr 9, 2013)

A major design consideration for low rpm engines is piston dwell. A long rod-stroke ratio will cause the piston to spend more time towards the top of the stroke. At low rpm, a long rod-stroke ratio engine is more octane-sensitive under load  as cylinder pressures will spike during combustion. For a long rod-stroke ratio engine, increasing rpm or decreasing compression will keep the knock away. In turn, shortening the rod-stroke ratio will reduce octane sensitivity at low rpm, as it reduces piston dwell time. At high rpm, a short rod-stroke ratio engine can outrun the flame front.

Generally, the bore-stroke ratio has some correlation to the rod-stroke ratio of an engine, but not always. 

Heres some interesting tidbits: 

In 2006, a 2.4 liter Toyota F-1 engine made 740 horsepowerat 19,000 rpm. The engine was very over-square with a 98 mm bore, and a 39.8 mmstroke. The engine made approximately 5 horsepower per cubic inch, however engine torque output at this power level was only 205 ft-lbs.

An 8.2 liter top fuel engine is under-square, with a 106.4mm bore, and a 114.3 mm stroke. Power output approaches 20 horsepower per cubic inch. Torque output nears 6000 ft-lbs. This is at under half the rpm of the F-1 engine.


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## lohring (Apr 9, 2013)

High performance internal combustion engines' power is usually limited by breathing.  Piston speed is secondary.  In conventional four strokes with overhead valves that means the cylinder head area needs to be as large as possible for a given displacement.  That's the main reason Formula 1 engines have very short strokes.  It's different with standard two strokes.  When the ports are in the cylinder wall their area is proportional to the bore times the stroke.  Since the displacement increases as the square of the bore, the cylinder volume fed by the ports grows rapidly as you increase the bore.  Because of this, high power, small two strokes usually have a bore close to equal to the stroke.  Piston speed issues don't allow really long strokes except in low rpm marine two strokes.  

Lohring Miller


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## dman (Apr 9, 2013)

there may be some physics to burning gasses that prefer undersquare/long stroke engines as far as torque goes but the factor that is more quanifiable is the rod to stroke ratio. less rod and more stroke is higher peak piston velocity and more mechanical advantage as well as less dwell and better octane tolerance. but it's also higher sidewall and wristpin loading and piston acceleration. 

take some of that with a grain of salt though. if you plot out the range of rod/stroke ratio and piston movement on a graph you would see that unless you go to extremes the plots look indistinguishable for the dimensions used in modern automotive engines (though older designs may use much longer rods) which would be a range of about 1.5:1 to 1.8:1. if you get near 1.4:1 or 2.5:1 things start to look a little different. but as you get higher the differences become less and less as you approach a sine wave. shorter rods also have a positive effect on the "true" compression ratio because the piston is lower in the bore when the valve closes.

some engine builders prefer a longer rod and there is some evidence it may make more top end power but you have to go to extremes and the weight becomes an issue. maybe it worked better in the hayday of nascar where you had to run crappy factory heads with poor cylinder filling and could run the slowest burning leaded gasoline imaginable. but today most stuff is a short as possible before things start to break because less mass means faster acceleration and more velocity means they can run bigger cylinder heads/valves and still get some energy into the intake charge for a good ramming effect for 100+% ve. the reson you don't see shorter rods in hotrod magazine is because the deck height dictates it to some level. if you just ran short rods you'd need tall top heavy pistons. the aftermartket sells what works well with the stock block.

you may be able to make good torque with an over square motor (think ford 302) by using a rod as short as you can without breaking anything (about 1.4 times the stroke  but maybe shorter will be tolerated in model scales) and long (more mass) narrow (more velocity) intake runners. also a late opening intake valve gives the air a good yank and helps to fill the cylinder better but late closing will cause problems with reversion so if torque is the goal short duration (late open + early close) is best.  

there is a common misconception that it is the longer lever of the longer stroke that creates the torque. the truth is that displacement and rod/stroke being equal the volume/degree plot will be identical, while the long stroke will have a longer lever the large piston will have the pressure pushing a larger area in the same proportion. as long as the volume/deg plot is the same the mechanical advantage is the same. the keys to torque have to do with intake and combustion physics, heat retention/dissipation and true compression ratio, too much heat retention and the engine knocks, too little and you lose power. more dissipation and you can cram more air/fuel in or into a smaller space.  once you figure out how much air/fuel you can cram in you can work out the compression ratio and heat control.


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## chrsbrbnk (Apr 9, 2013)

I wonder if you'd get more percentage of blowby on an over square  versus  along stroke  say for a given ring fit and all  sort of tangential to the topic


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## Hopper (Apr 10, 2013)

It depends too on what you plan to use the engine for. If the original question relates to a high-reving model aircraft engine the answer may different than if it relates to a low revving boat engine, or a low revving demonstration engine with no load.


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## Optimus (Apr 10, 2013)

Hi everyone
Just joined this great forum as I just bought a mini lathe & milling machine & would like to get into model engine building. Being a newbie & trying to remember metal class all those years ago I want to start with something maybe not too difficult so I was thinking about a flame eater. I was just thinking on what I would make the bore & stroke when I stumbled upon this topic & I hope it's ok to ask this question here but if I want a slow running engine is there a ratio or rule as to what the bore & stroke should be for such an engine. Sorry if I'm in the wrong place to ask this,still trying to find my way around  & thx for any info Lee


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## Hopper (Apr 11, 2013)

Optimus said:


> Hi everyone
> Just joined this great forum as I just bought a mini lathe & milling machine & would like to get into model engine building. Being a newbie & trying to remember metal class all those years ago I want to start with something maybe not too difficult so I was thinking about a flame eater. I was just thinking on what I would make the bore & stroke when I stumbled upon this topic & I hope it's ok to ask this question here but if I want a slow running engine is there a ratio or rule as to what the bore & stroke should be for such an engine. Sorry if I'm in the wrong place to ask this,still trying to find my way around  & thx for any info Lee



Lee, as a noob you are way better off building your first project(s) to an established design. Jan Ridder's website has a number of plans for flame eaters, which he will email to you for free or with a small optional donation to help pay for his site.
It is here http://ridders.nu/Webpaginas/pagina_overzicht_happermodellen/happeroverzicht_frameset.htm

His designs are well tested and true and have been made by many modellers with great success, so you know it is going to work if you do your part correctly. If nothing else, his plans give you a starting point for bore, stroke, materials etc etc.


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## Optimus (Apr 11, 2013)

Awesome thanks Hopper that's what I just did & your right got all the info there. He seems  super friendly & helpful thx


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