Bore/stroke ratio

[MaVac]

Hello!
I am trying to figure out, how the bore/stroke ratio affects the engine stats and the only thing I've found is MTBF. Best is at about 1.11 ratio and it goes down as the ratio decreases. And on long-stroke engine (like 0,75-) the engine starts to fail.
Am I doing something wrong or is this the only one engine statistics, that is affected by bore/stroke ratio?

[Noxan9]

bore and stroke defines the size of the engine.
shorter stroke = it withstands higher rpm and it prolongs it's life.
long stroke = lots and lots of torque, but low rpm

[Daffyflyer]

That's almost true, but with one little thing, longer stroke doesn't actually increase torque output, the main reason for long stroke engines being torquey is that they are tuned as such (via cam timing, header length, intake manifold runner length and port sizing).


It's pretty much like this.

Bigger Bore:

• Larger space for valves, thus better head flow & a little more torque, but heavier valves that are more likely to float/fail.
  
• An all around larger engine, the block has to get wider and taller to fit the larger bores, as does the head. We base overall engine size on bore with a relative limit to how much stroke that engine size will allow.
  
• Shorter distance for the piston to travel (short stroke) means the pistons are traveling more slowly for a given RPM, thus reducing the forces on the bottom end components and making it able to rev harder without exploding
  
• Slightly higher octane requirement, as the large bore kinda.. takes longer for the flame front to cross it, thus giving it less space/time to explode instead of burning properly.
  

[pyrlix]

Uhm yes - the bore and stroke define how large the engine is in length, width and height, aswell as the displacement. The Ratio between Bore and Stroke is more like to define how good a engine can breath. Higher B/S means basically more prone to valve float which can reduce Engine Power and Lifespan but also a better Airflow that allows for more Power - so yes, there is a sweet spot.

The B/S does NOT affect MTBF that much... its your stroke that it affecting it. Sure if you have a Stroke of 100mm and Bore of 100mm you have a B/S of 1. But with a Stroke of 92 and Bore of 102.2 you will have 1.11 B/S - and better MTBF.
More Stroke means that the Pistons and Conrods travel at a higher speed, which results in more Friciton and probably less MTBF, dependent on your Pistons. Lightweight Forged and Forged can be used at engine with way more RPMs then Cast Stuff - but they also cost more.

Edit: Daffy you were a second faster then me

Greets
Pyrlix

[MaVac]

OK guys, i know the theory (thanks, Wikipedia! ) but my question was focused on Automation...I made some tests, where I started with the default setup of Engine designer (86mm x 86mm) and measured how power, torque, economy, RON, MTBF etc. changes, when I change the bore/stroke ratio...and the change was about 1% in power and 1Nm in torque. And as I said the only thing, that changed a lot was MTBF, which was highest at ratio about 1.1 and was falling down with longer stroke...

So my question is, how bore/stroke ratio is affects the calculation of engine stats in Automation.

[Der Bayer]

The bore/stroke-ratio does itself not affect the stats too much, though you can notice that, especially on crappy valve trains the engines with more bore can breathe better and thus make slightly more power. The RON rating for bigger bore engines is higher though. The massive drop in MTBF for long stroke engines depends not on the stroke, but on conrods and pistons which exceed their maximum (average) speed. So if you increase stroke and MTBF drops, just set the RPM limiter lower and you get nearly the same/a bit higher MTBF values than you had before.

[Daffyflyer]

So my question is, how bore/stroke ratio is affects the calculation of engine stats in Automation.

I know, I outlined how it affects the stats in my post

The only reason it'll make reliability worse is if the stroke is longer as an absolute number, as pyrlix says, the RATIO is not important when it comes to reliability, what is important is stroke vs RPM, the longer the stroke the lower RPM or stronger internals you'll have to run to have a reliable engine

[Killrob]

Automation works those things out pretty realistically, all those effects are considered in one way or the other.
Your tests were not conducted in a regime that makes these mechanics apparent though:

- You only use MTBF if you reach the maximum supported piston speed.
- You only gain significant amounts of power/breathing if you are breathing limited in the first place.

Try to see the breathing effects with 2-valve setups, there it is the most obvious for the case when you want to push performance a bit (when revving a bit higher).

Edit: Double-ninjad by Martin and Andrew!

[MaVac]

OK Killrob, so I have to use OHC/SOHC setup with 2 valves and move the cam higher than standard 40 and the effect should be seen in higher revs, so the limiter should be also moved higer than standard 6800, right? I have to try this...

[Killrob]

If you really want to see the effect, yes, that might be a good setup to use. ~7k RPM sounds reasonable though, use DAOHC to get there AND show the effect. Its breathing capabilities are pretty limited.