Bore vs Stroke: Which One is Better?

If you were for example engineering an engine designed for race use, it would more sense to use a large bore and short stroke so you could more easily increase the RPM limit to increase power.

If you were engineering an engine designed for a truck, it might make more sense to use a large stroke to increase power while having a low RPM limit, since you wouldn't want a truck to be revving to the moon while towing.

If you were engineering an engine for simple commuter, somewhere inbetween would make the most sense, since you would have acceptable power at low and high RPM.

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[theotherguy]

An undersquare engine doesn’t necessarily make more torque than an oversquare engine if both engines have the same displacement.

Torque = Force x Distance
Force = Pressure x Area

Increasing the stroke improves torque by increasing the distance between the connecting rod and the center of the crankshaft. But increasing the bore also improves torque because the pressure from combustion is exerted onto a larger piston which increases the force.

While both engines make the same peak torque, the undersquare engine will make its peak torque earlier in the rpm range. That’s because the relatively high piston speeds and small intake valves improve air velocity at low rpm which improves air/fuel mixing which improves combustion quality and thus combustion pressure.

The tradeoff, however, is that undersquare engines can’t rev as high which means they make less peak horsepower.

[lastcenturyclassics]

Thanks for explaining Bore vs Stroke, as I never understood HP vs Torque till now lol.

[juancarlossaavedra]

I don't want to bore anyone but the gearbox also plays a relevant role. AUTO, MANUAL or CVT in the stroke/bore ratio. Sheers from Sydney, Australia ( Planet Earth )

[marksmigle]

As a retired Dodge dealer mechanic w all 8 ASE tests (master technician) advanced level engine diadnostics and a machinest test for engine assembly I found this video very interesting and informative. And it explains things in laymen's terms so everyone can understand. Around 2006 I bought an 88 turbo LeBaron conv with an auto trans and 2.2 L turbo 1 engine. It ran good but my goal was to make it a mid 14 sec car. I don't remember the bore which was the same in the 2.2 and 2.5 engine. It was approx 3.5. the 2.2 had a 3.6 inch stroke and the 2.5 had a 4.09 inch stroke. I met my goal with a 14.1 ET at 97.3 MPH. I embaressed a lot of V8 cars. After watching the video I can't believe the engine shifted at 6000 RPM. With a 4.09 in stroke. And nothing broke. The 80s Mopar 4 cylinders were bulletproof. Also the 2.2 and 2.5 was designed by the same person who made the famous slant 6. From 1984 to 2013 I only saw only one 2.5 come apart and was due to driving it with NO OIL (had a hole in the oil pan). I still have a hard time believing an over 4 inch stroke could hit 6000R PM!! The only weak spot was the head gaskets on the turbo engines. And those motors were so easy to work on, a head gasket could be replaced in 3 hours or less. These motors are in my opinion were the best domestic 4 cyls, just like a small block Chevy was the best. And don't forget the origional Ford 302.

[lobstertantrum]

Damn, great video. Simple and straight forward explaination.

[javierhualde]

There must be more to the explanation behind more stroke-> more torque, because if displacement remains constant, the longer stroke engine also has a smaller bore, with a proportionally smaller piston surface area, which means less force. If the pressure inside the cyilinder is the same, you would end with the same torque either with a big bore-short stroke engine and a small bore-long stroke engine. It is true that big bore-short stroke engines can rev up much higher, which make them suitable for sports cars, and small bore-long stroke engines are more efficient, so there's that.

[vincentbourke]

Will u talk about the Mercedes F1 engine and the honda v6 f1 engine i think there both 1.6L v6

[troyridesph]

You can increase fuel efficiency with a big bore cylinder by adding compression say like a domed piston but it will also affect the fuel intake a little

[saqibnawaz]

Concise as gets about bore & stroke functional architecture

[lollipop]

Which is better is an unanswerable loaded question. It's all application, combination and setup specific

[Nightingale]

Hi DRA, recently subscribed to your channel cause I love how you do your videos! I was wondering if you would consider doing videos going over various transmissions and transaxles? I would love to see you go over them in as much detail as you do with the engines.

[Shadowdarknessone]

What if u use custom pistons where the pin sits higher and pistons are shorter. U can use longer rods with the same stroke and bore and reduce angel of the conrod.

[SECX]

That was informative. I like you included so much engineering aspects that influence engine design.

[mrjed]

what if i converted an 848cc Reliant motors I4 into a short stroke from a long stroke. These engine don't like high rpms but are very light and low on capacity.
And even though it has a long stroke it still handles high rpms well which makes me wonder if a Reliant engine could rev up to 10k rpm after the conversion.

[davelowets]

My wife of 25 years now has a large bore... 😟 in BOTH cylinders. 😢


Compression is WAY down today also... The number one hole is shot, there's nothing left. The number 2 hole still has some PSI, but that one doesn't fire very often. It's a shame... 😦

[lilhaxxor]

This was excellent. You only missed the definitions of oversquare and undersquare. You only talked about square (without defining it either I think).

[nhytgtgyuugjmg]

If an engine have long stroke it will have a small piston. A small piston will create a smaller force on the crank so it loose the same amount of torque ending up equal.

[crazyDIYguy]

I'm wondering whether or not to de-stroke a Subaru ej257 from 79mm to 75mm it's turbocharged and I'm thinking it will be much more dependable... Decisions..🙄

[madcat]

How you calculate displacement blew my mind.

I always wondered how they calculated the sizes.

This video was very insightsful and helped me understand bore and stroke better.

So for a (NA) Diesel 4wd it would be better to have a longer stroke to get better pulling power.

Diesel engines also rev a lot lower than petrol so you sidestep a lot of the pitfalls a petrol experiences with a longer stroke. (at least that is my understanding of it)

[bystander]

Do a video on the Toyota u engines, preferably the 4u gse