Boring vs. Stroking an Engine

[BenSkywalker]

Although not relevant to this thread, overboring has a few more great benefits over stroking.

Depending on the application and engine, overboring and destroking can work quite nicely too.

 

[Pacfanweb]

Although not relevant to this thread, overboring has a few more great benefits over stroking.

That depends on the original configuration of the engine, so it's not necessarily true in a general sense if you talk "all engines" and not just "race engines". There are advantages of longer strokes on the street.

Typically, race engines are built "over square", which means the bore is larger than the stroke.
Like the 454 Chevy: 4.25" bore, 4" stroke.
For a larger size, Chevy went to a 4.47" bore and kept the 4" stroke when they made the 502.

Also depends on what you want out of the engine.

But if you want maximum power, you want the largest possible bore.

Great article on this very topic, by drag engine guru David Reyer:

http://findarticles.com/p/articles/mi_qa4078/is_200509/ai_n15614621/

 

[exdeath]

That depends on the original configuration of the engine, so it's not necessarily true in a general sense if you talk "all engines" and not just "race engines". There are advantages of longer strokes on the street.

Typically, race engines are built "over square", which means the bore is larger than the stroke.
Like the 454 Chevy: 4.25" bore, 4" stroke.
For a larger size, Chevy went to a 4.47" bore and kept the 4" stroke when they made the 502.

Also depends on what you want out of the engine.

But if you want maximum power, you want the largest possible bore.

Great article on this very topic, by drag engine guru David Reyer:

http://findarticles.com/p/articles/mi_qa4078/is_200509/ai_n15614621/

The part about cylinder pressure being greatly diminished at the bottom of the stroke changes when you bring up forced induction. Stroking and boost work together quite well. Boost doesn't increase max cylinder pressure, but keeps pressure more consistent on the piston throughout the full stroke; mixture burns at the same rate but there is still mixture burning all the way to BDC.

 

[Pacfanweb]

The part about cylinder pressure being greatly diminished at the bottom of the stroke changes when you bring up forced induction. Stroking and boost work together quite well. Boost doesn't increase max cylinder pressure, but keeps pressure more consistent on the piston throughout the full stroke; mixture burns at the same rate but there is still mixture burning all the way to BDC.

Why would boost have anything to do with pressure being more "consistent"? At BDC, boost is irrelevant. The valves are closed. Cylinder has already been filled, sealed and fired.

On the way down on the power stroke, the engine has no idea whether it' boosted somehow or naturally aspirated.

Boost (whether nitrous or forced induction) only makes a difference when you're filling the cylinder. That's all it does.

In a nutshell, boost is a way of getting more air into an engine than it could suck in on its own....so it essentially makes the engine act like it has more displacement.

I don't see how it would make any difference on the power stroke. Now, on the INTAKE stroke....I could see your point.

 

[exdeath]

Why would boost have anything to do with pressure being more "consistent"? At BDC, boost is irrelevant. The valves are closed. Cylinder has already been filled, sealed and fired.

On the way down on the power stroke, the engine has no idea whether it' boosted somehow or naturally aspirated.

Boost (whether nitrous or forced induction) only makes a difference when you're filling the cylinder. That's all it does.

In a nutshell, boost is a way of getting more air into an engine than it could suck in on its own....so it essentially makes the engine act like it has more displacement.

I don't see how it would make any difference on the power stroke. Now, on the INTAKE stroke....I could see your point.

1) mixture burns at the same speed no matter how much is in there
2) there is more mixture in the cylinder with boost (given 1), it takes longer to burn)
3) 1)+2) there is still mixture burning and creating pressure on the piston at the bottom of the stroke, long after the mixture in a N/A charge has stopped doing any useful work; this is why superchargers/turbochargers make more power, but are safe because they don't increase peak pressure.

If you look at the combustion chamber pressure curve of a N/A engine vs a boosted engine, the boosted engine doesn't drop off sharply after peak ignition pressure, it's pushing on the piston all the way to BDC because it's more mixture burning at the same speed meaning it burns/expands longer. Thus, you sorta double dip when stroking + boosting.