Boring / Stroking engines

[Hawk Tuah]

VW aircooled engines are commonly uprated by increasing the bore and crank.

Increasing the bore usually will have a drawback. Making the cylinder walls thinner can weaken the engine, or decrease heat dissipation which can lead to distortion.

Increasing the stroke is a better way of increasing the power. I'd much rather have more torque low dow, but its generally more expensive as the engine has to have more work done to allow the bigger crank.

 

[Dr Who]

You need to brush up on your basic physics. A shorter stroke engine of the same capacity has a larger bore. Force=pressure * area. Torque = force * distance. Do the math as they say in the US.

As I have already stated, all else being equal a long stroke and short stroke engine of the same capacity will produce the same torque. The point is that all else is usually not equal as I have also explained.

We aren't talking about all else being equal, we are talking about modifying the same engine.

When you modify the stroke or bore you change cam timings to suit. You dont need to optimise a cam for the longer stroke engines high RPM, therefore you can use a soft cam which enables them to produce more torque and power lower in the rpm range with less torque drop off.

Because a shorter stroke engine is capable of reving higher, you take advantage of that to increase its work rate (BHP) and without an optimised VTEC style system, they lose lower RPM torque, ie, the force on the piston is lower at lower RPM.

That is the force you are applying as a constant, and it isn't a constant... A long stroke engine will generally be using a higher combustion pressure at low RPM.

This is the beauty of a VTEC system, as well designed it doesnt have that compromise in cam timing.

 

[whitecrook]

VW aircooled engines are commonly uprated by increasing the bore and crank.

Increasing the bore usually will have a drawback. Making the cylinder walls thinner can weaken the engine, or decrease heat dissipation which can lead to distortion.

Increasing the stroke is a better way of increasing the power. I'd much rather have more torque low dow, but its generally more expensive as the engine has to have more work done to allow the bigger crank.

tbh it was these engines I had in mind when thinking about it, it's a very common way to increase performance/power with these things.

But as mentioned above, from the factory there are loads of engines that come bored/stroked. e.g. the 4.2 litres V8s in the range rovers are bored (and stroked?)

 

[Hawk Tuah]

Increasing the bore is a lot easier. Because the barrels are attached to the side and can be exchanged its an easy way of increasing displacement. The only machining needed is to clearance where the barrels fit onto the case. You can't increase it by much though without making compromises both in heat and the fact that the barrels are only so close to each other.

Increasing the stroke takes more work as you have to split the crankcase and clearance the insides for the bigger crank. I've seen how you can increase the stoke using a crank from a later waterboxer and a bit of clearancing work. I might give it a go with a spare engine one day. Could do with a bit more torque in the Baja.

I was thinking. Are elliptical barrels and pistons possible? That could increase bore whist still having the barrels a set distance apart.

 

[Hawk Tuah]

Engine stroking
Phwoar!

Looks more like a diff.

Internet pedant at its best.

 

[Numbnuts]

Engine stroking:

Phwoar!

Isnt that a Diff

 

[Spuderoony]

Quite. But it's what Google delivered and I'll be buggered if I'm going to make my own 'engine stroking' image for the purposes of a quick quip on here!

 

[Dogbreath]

We aren't talking about all else being equal, we are talking about modifying the same engine.

When you modify the stroke or bore you change cam timings to suit. You dont need to optimise a cam for the longer stroke engines high RPM, therefore you can use a soft cam which enables them to produce more torque and power lower in the rpm range with less torque drop off.

But you were suggesting a longer stroke gave more torque due to simple physics, i.e. more leverage on crank. I'm saying this is not the case. Take any engine, increase capacity the same amount by either increasing bore or stroke, and the torque increase will be similar in either case.