Bore vs Stroke

[Nick R]

Was doing some reading on the internet tonight, and something I've been curious about. In an engine, would you prefer longer stroke, smaller bore, or vice versa? to use my cruze as an example again, the little 1.4 has a stroke of 3.252", and a bore of 2.854".

The equinox HFV6 has a bore of 3.5", and a stroke of 3.15".

My cruze has a lot of low end torque, a lot of which is attributable to the turbo obviously. But the V6 makes it's torque peak above 5,000RPM, and it's horsepower peak it basically at redline of 7,000.

I wonder what you all think. Longer bore = more low end torque? Or does it not really matter all that much? I'm curious to hear what you think about this.

 

[oilboy123]

It's a trade off for more free revving, or more torque. I'm certainly not versed on this. But you have the basics.

Small block Chevy engines had 3.0, 3.25, 3.5 inch stroke and 4.00 bores to make 302, 327, and 350 V-8 engines. The first two were better at revving than the 350. Of course the 302 was the best for revving and the 350 the best for torque. Of course more cubes = more HP.

 

[mechanicx]

Yes the bore stroke ratio effects the torque curve. You have to select an application's intend use street-driven, race engine etc. and your targets for peak HP, redline etc For a two-valve/cylinder street engine maximum torque under the curve might be at a stroke/bore of 90%, and for a 4 valve engine it might be at 115% or more.

There are a lot of variable but a longer stroke to bore and shorter conecting rod to stroke ratio usually gives better fuel economy and better low speed torque, but limits redline and HP potential. A shorter stroke, longer rod/stroke allows for bigger valves and better breathing and more HP and can rev and run smoother but cost fuel economy. I prefer an engine to be slightly oversquare even if a 4-valve, but you wouldn't expect to see that in a 4 cylinder designed to fuel efficient.

 

[rationull]

Seems to me that most engines anymore are undersquare (stroke bigger than bore). Or maybe it's just the smaller engines that I'm familiar with. It seems like the "trend" would be higher engine speeds for large bore/short stroke and more low-end torque for long stroke/small bore. Kind of makes sense when you consider a larger bore could allow for more airflow (bigger valves) and a longer stroke could allow for a longer duration power stroke (not sure if I'm using the right terminology here).

However in practice I don't think it's quite so straightforward, as there are plenty of revvy undersquare engines and plenty of oversquare engines without stratospheric redlines. Moreoever, there are so many variables that it doesn't seem possible to attribute the characteristics of a given engine to its bore/stroke ratio -- especially when dealing with technologies like turbocharging and variable valve timing.

I will say that my very oversquare turbo Subaru 2.5 (3.92 inch bore to 3.11 inch stroke) revs much faster and smoother than my previous undersquare Honda 1.8 (3.19 inch bore to 3.44 inch stroke). However, we have another car with an undersquare 2.3 I4 which revs just fine, and I previously had an oversquare 2.8 I6 which didn't rev quite as fast (but was plenty smooth).

I'd venture to guess that the decisions made wrt bore vs stroke these days probably have more to do with manufacturing than any innate performance advantages. For example, based on block design (bore centers and wall thickness) there will be some maximum feasible bore for a given engine design. Likewise, packaging (deck height) will influence the stroke decision. E.g. Subaru pretty much has to have a shortish stroke since the flat engine has to sit between the front frame rails. And IIRC I read somewhere that smaller bores are used in small transverse FWD cars also to help limit engine width.

 

[Shannow]

When analysing engines on a dimensional analysis basis (like they did in the old days before calculators), displacement is displacement...

Power is

# cyls * (pi*B*B/4) * S * (RPM*2*pi/60) * BMEP
# cyls * piston area x stroke x rotation speed *BMEP

BMEP is Brake Mean Effective Pressure, and arbitrary number, usually derived from measured power, but it basically tells you how effectively the engine uses each cubic inch of air and fuel.

So in an engine, power increases as:
* linearly with number of cyls
* the square of the bore (so does displacement)
* linearly with stroke (so does displacement)
* linearly with revs (that's why racers are revvers)
* linearly with BMEP (why we want great volumetric efficiency and good combustion, high comp ratios etc...best BMEP is usually torque peak)...BMEP is lower off the cam, and falls after torque peak, giving a typical power curve.

Things to be considered, particularly in the bad old days of so so metallurgy, combustion design, and manufacturing, the compromises were typically around a 2 valve arrangement.

Piston speed (stroke times speed) were "known" to have a maximum limit. You could swap stroke for revs, or revs for stroke...in the UK, they considered for "road tax" that it was fixed.

More cylinders = more things happening, = more power. = smaller bore for a given displacement = lower BMEP

More Bore = more displacement = bigger valves = (also) more prone to detonation, as the flame travels longer = lower BMEP.

Side valves allow smaller bore with whatever size valves you like, but with rubbish combustion = lower BMEP.

So in the UK, when they taxed cars on rated power, they assumed that
stroke x speeed x BMEP
piston speed x BMEP
were simply laws of physics issues that could not be improved on, and made them a constant, and "rated" engines at K*B*B*# cyls.

Metallurgy (higher speeds), airflow (BMEP), reductions in friction meant that many brit engines achieved many times their "rated" power.

But with modern metallurgy, speeds can go up. VVT and multi valves mean that you can rev harder without losing driveability, and increased BMEP at higher revs. Better fuels and chamber design mean that bores can be made bigger, and still burn efficiently.

 

[OVERKILL]

Shannow and mechanicx summed it up well. This was actually bit of an argument between mechanicx and I a while back, if you care to dig it up, you'll probably learn a fair bit.

The argument revolved around two very similar displacement engines:

Chevy 305 (3.73" bore x 3.48" stroke)
Ford 302 (4.00" bore x 3.00" stroke)

The 305 has a reduced bore and longer stroke. The 302 has a larger bore and shorter stroke.

The 305 is much closer to "square" than the 302 is. The 302 should then be more "rev happy" (and a look at the BOSS 302 showed it sure could be) than the 305. And as mechanicx touched on, due to bore size, the 302 also had the ability to run large valve heads, with less shrouding meaning that in theory, it could make more power.

Again in theory, that meant that the 305 SHOULD have made more torque. It didn't however. That may have been cam and induction choice on GM's part however.

This changes up somewhat when dealing with multiple overhead cams, as the issue of valve shrouding and size is significantly reduced.

A few more examples:

BMW S62: 3.7" bore, 3.5" stroke (yes, another way to arrive at ~302ci)
Ford 4.6L: 3.55" bore, 3.54" stroke (yes, it is basically square)
Ford 5.4L: 3.55" bore, 4.17" stroke (and here we go the other way)
Ford 5.0L: 3.63" bore, 3.65" stroke (this is Coyote)
Ford 6.2L: 4.02" bore, 3.75" stroke (this is BOSS)

GM LS1: 3.898" bore, 3.62" stroke
GM 4.8L: 3.78" bore, 3.27" stroke
GM 5.3L: 3.78" bore, 3.62" stroke
GM 6.0L: 4.00" bore, 3.62" stroke
GM 7.0L: 4.125" bore, 4.00" stroke

 

[Klutch9]

Originally Posted By: mechanicx
I prefer an engine to be slightly oversquare even if a 4-valve, but you wouldn't expect to see that in a 4 cylinder designed to fuel efficient.


Sure you could. Look at the new Ford Focus engine's bore and stroke. It is oversquare. And it delivers 40 mpg to boot! In fact, the Duratec 20 is also quite fuel efficient, and also has the exact same bore and stroke as Ford's new GDI 4-pot (same bore spacing as well), but is a totally different engine other than those similarities.

 

[meep]

when we start talking within 15%, it all goes out the window as different gearing, aero drag, intakes, etc start to color the overall, optimized system.

I was curious about this very thing recently, as this 3.8L minivan I've recently picked up gets noticeably better mpg than its 3.3 version i owned years ago. And folks argue bigger/smaller gets better/worse mpg as much as they argue ford/chevy. The 3.8 has a good bit more stroke in it than the 3.3.

when I started digging, it did seem to me that the stroker engines had efficiency advantages.

I personally like the torque-heavy engines that are less rev-oriented, but that's just me. I got along well with the subaru torque curve, which was similar to a V6. The newer honda vtecs come close, especially that civic si.

A gearhead autoX buddy of mine explained that one of the limiting factors is a derivative of "piston speed." a stroked engine has a piston that must move quicker (must cover more distance per revolution), and hence face more acceleration and deceleration forces, than an oversquare engine. It's more complicated than simply that, but if you look at it being a rough equalizer, it demonstrates how a stroked engine redlines lower than a shorter-stroke variant, all other things equal. That also may be why the transmissions are gaining more gears--- to keep that engine loaded up at lower rpms where it is more efficient while still giving wide wheel speed range (how many folks have driven a diesel truck that can't make it above 65 as the diesel just doesn't turn that fast?) The marinized chryco 318 was a great example readline around 3600 rpm, but made serious torque right off idle and felt like a diesel.

I know-- some rambling-- but I've had a lot of disconnected thoughts on this lately and it's a GREAT topic.

M

 

[JHZR2]

Originally Posted By: Shannow
When analysing engines on a dimensional analysis basis (like they did in the old days before calculators), displacement is displacement...

Power is

# cyls * (pi*B*B/4) * S * (RPM*2*pi/60) * BMEP
# cyls * piston area x stroke x rotation speed *BMEP

BMEP is Brake Mean Effective Pressure, and arbitrary number, usually derived from measured power, but it basically tells you how effectively the engine uses each cubic inch of air and fuel.

So in an engine, power increases as:
* linearly with number of cyls
* the square of the bore (so does displacement)
* linearly with stroke (so does displacement)
* linearly with revs (that's why racers are revvers)
* linearly with BMEP (why we want great volumetric efficiency and good combustion, high comp ratios etc...best BMEP is usually torque peak)...BMEP is lower off the cam, and falls after torque peak, giving a typical power curve.

Things to be considered, particularly in the bad old days of so so metallurgy, combustion design, and manufacturing, the compromises were typically around a 2 valve arrangement.

Piston speed (stroke times speed) were "known" to have a maximum limit. You could swap stroke for revs, or revs for stroke...in the UK, they considered for "road tax" that it was fixed.

More cylinders = more things happening, = more power. = smaller bore for a given displacement = lower BMEP

More Bore = more displacement = bigger valves = (also) more prone to detonation, as the flame travels longer = lower BMEP.

Side valves allow smaller bore with whatever size valves you like, but with rubbish combustion = lower BMEP.

So in the UK, when they taxed cars on rated power, they assumed that
stroke x speeed x BMEP
piston speed x BMEP
were simply laws of physics issues that could not be improved on, and made them a constant, and "rated" engines at K*B*B*# cyls.

Metallurgy (higher speeds), airflow (BMEP), reductions in friction meant that many brit engines achieved many times their "rated" power.

But with modern metallurgy, speeds can go up. VVT and multi valves mean that you can rev harder without losing driveability, and increased BMEP at higher revs. Better fuels and chamber design mean that bores can be made bigger, and still burn efficiently.



POTY (post of the year). Very clear, concise, explanatory basis that is most helpful. Thanks!

 

[mechtech2]

When designers play with bore/stroke sizes, they also use rod length , cam timing, # of valves, turbo/no turbo, etc. as factors.
We can't separate any of these when saying what is 'best'.

When street racer guys used to put in a stroker crank, it simply made the engine bigger. This was 90% of the power increase. But the leverage advantage helped some.

 

[OceanDoctor]

Good show, mechtech & Shannow, very pertinent information that basically sums it all up.

To speculate why MFGs choose the geometries they do seems to be based on thermal efficiency first, which drives them to smaller bores/undersquare, as there is less surface area per unit of piston travel to absorb heat on the power stroke while also benefitting from smaller bore spacing for packaging reasons. Unfortunately, the rod length to stroke ratio's seem to be simply a result of consequence- this is one personal consideration I take into account when determining just how "engineered" an engine has been. I prefer the greatest rod length acheivable in an engine. Though, I have seen very FEW manufacturers try to optimise that one, by dropping compression height as much as possible, and even shrinking the pin. Anything less than 1.5:1 is bad engine design IMO and anything over 1.62:1 minimum is commendable, 1.75:1+ is outstanding!

Originally Posted By: Klutch9
Sure you could. Look at the new Ford Focus engine's bore and stroke. It is oversquare.


hehe that engine's been around since 2001. The GDI version having the exact bore, stroke and spacing should also tell you something- most likely minor casting changes + GDI. A GDI Mazda version has been around Europe for many years now. Ford banked on using the 87.5mm bore engines for a long, long time. It's a good engine, sports a nice 1.75:1 rod ratio which is why it revs so smoothly.

 

[subiedriver]

My '04 Subie Legacy with the 2.5 is very torquey for a short stroke motor, its surprising. The torque plateau starts at 2400 rpm, this allows me to pull moderate hills in 5th gear. As the rev's climb, the hp climbs, but the torque stays consistent. I really like this engine!

 

[dwendt44]

Rotating mass (flywheel and damper included) have a lot to do with 'smoothness' and how quick a particular engine 'revs'.
'hop up' can include a 'lightened' flywheel, etc. that promotes quick revving.

 

[SteveSRT8]

^^^^Absolutely agreed.

I have completely changed the personality of a project hot rod by using a special flywheel that was machined even further by my BIL.

Dramatic difference.

 

[Shannow]

Originally Posted By: mechtech2
When designers play with bore/stroke sizes, they also use rod length


Often they are stuck with Rod Length, and just have to take what comes due to the engine geometry.

Holden had a range of 6 cylinder displacements in the same block, with a 3" stroke and 5.25" rod, and they were all sweet. (138, 149, 161, 173, 179), and the best of them all, the 186.

186 was 3.625" bore, 3" stroke, and 5.25 rod.
They "upgraded" it to the 202, 3.625" bore 3.25" stroke, and the same 5.25" rod.

Given all else being equal (they were the same engine bar stroke), the 186 was always preferable to the 202, smoother, revier..and the 202 liked breaking #2 or #5 pistons.

 

[mechanicx]

I agree. Although I don't think a short stroke engine would make less peak torque. In fact it could make more because that is mostly due to volumetric efficiency. But the longer stroke motor of the same displacement will usually make more low speed torque and have slightly better efficiency. Looking at the LSx motors and others the trend is back to smaller bore/longer strok.

 

[SteveSRT8]

Didn't see much in the LS specs to indicate their bore/stroke has changed much. They still favor big bore/short stroke.

Can anyone say camshaft(s)?

The heart and soul of the motor is how the breathing is handled.

Valve gear, intakes, exhaust, etc., have a profound effect on the engines power output and are often used to 'tweak' a design in the desired direction.

There are tons of big bore/short stroke engines that are torquey as heck because the valve timing is shortened at low revs while the intake uses longer runners. Then at higher engine speeds the valve events are lengthened and the intake switches to longer runners to enhance high rpm power.

Honda (and numerous others) has used this strategy for a LONG time as many of their engines are actually long strokers, but they rev well and develop excellent high rpm HP anyway.

 

[artificialist]

Many cars were being built with narrower bores because the combustion flame more easily burns the gasoline in the chamber. In short narrow bores are for reducing HC and CO.

No matter what Ford did with the 5.0, 5.8, and 7.5 V8, they could not get it to pass smog without an air pump. All engines had wide bores. Almost no Ford 4.6, 5.4, or 6.8 uses an air pump. Their bores are far narrower. Their exist other examples with other auto builders, but the is the easiest one for me to remember.

 

[chris67]

Do not forget about rod angularity as it relates to the crank throw as well as the amount of force on the load side of the piston especially with the non-skirted, low drag stuff used today. The longer the rod, the less the angularity. A longer rod does not mean a longer stroke. Stroke is set by the crank throw length times 2.

 

[morris]

gas millage? in the 20s-30s most engines was long stroke, short bore. this pulled the fuel for longer time, and they had flat heads that spins the fuel going in.