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Boring vs. Stroking an Engine

TehMac

TehMac

Diamond Member
Ok, so the compression ratio thread got me thinking.

If Compression Ratios are determined by cylinder bore and piston length, wouldn't increasing the displacement of the engine (by boring or stroking, or both) increase the compression ratios of the engine as well?
 
TehMac said:
Ok, so the compression ratio thread got me thinking.

If Compression Ratios are determined by cylinder bore and piston length, wouldn't increasing the displacement of the engine (by boring or stroking, or both) increase the compression ratios of the engine as well?
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This is an incorrect assumption.

The Static Compression RATIO is defined as the Volume of the Combustion Chamber when the piston is at the very bottom of it's travel (called "bottom dead center" or BDC) DIVIDED BY the Volume of the Combustion Chamber when the piston is at the very top of it's travel (called "top dead center" or TDC).
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The second value in the ratio is the key. Combustion chamber volume. Think of what the difference would be between a flat topped piston and a domed piston. That flat or domed top determine the size of the combustion chamber at TDC.

Which would provide a greater compression ratio?

Edit: I have misunderstood the OP. I will let this stand, but in the context of his question, it is not relevant.
 
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To answer the question OP wants to know, even though it isn't worded quite right...yes, increasing engine displacement increases compression, if nothing else is changed.

Doesn't matter HOW you increase the displacement, whether by an overbore or increasing the stroke, the compression will increase.
 
boomerang said:
This is an incorrect assumption.



The second value in the ratio is the key. Combustion chamber volume. Think of what the difference would be between a flat topped piston and a domed piston. That flat or domed top determine the size of the combustion chamber at TDC.

Which would provide a greater compression ratio?
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Obviously the dome would, but that's not what OP is asking.
 
PlasmaBomb said:
As I think exdeath mentioned you can change the pistons to maintain the compression ratio.
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There's no need, the gain in compression is negligible.

edit: I guess unless you're already at the absolute edge of detonation already....but typically, even big overbores don't really raise the compression that much.
 
either way, the change in volume of the chamber is nothing compared to the volume of the fuel/air going in, so the ratio is changed very little
 
punjabiplaya said:
either way, the change in volume of the chamber is nothing compared to the volume of the fuel/air going in, so the ratio is changed very little
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What?

There is no change of volume in the chamber when you bore or stroke an engine. There IS a change in volume of fuel/air, since the engine's displacement is increased.

Or did you mean something else?
 
'boring' an engine does little for compression ratio. both the swept volume and combustion chamber volume will get SLIGHTLY bigger.

'slight' being the key word with an overbore. for example, take a SBC that is 350ci stock, and bore it .060 over (a pretty big overbore, probably around the max for that motor, but i'm no chevy guy). what do you get? a lousy 10ci. a 2.8% increase in displacement. there is no reason to 'bore' an engine for performance- you should always remove the minimum amount of material needed to get rid of cylinder taper, out-of-round, or any kind of damage. to do otherwise is a waste of a perfectly good engine block, as you're basically saying you want to condemn it to its final rebuild (unless the next guy wants to sleeve it).

stroking is a whole different matter. compression ratio going to be determined by the kit that you use- remember that in addition to changing rod length, you're changing the location of the wrist pin, which is how you're going to control the change compression.
 
brblx said:
'boring' an engine does little for compression ratio. both the swept volume and combustion chamber volume will get SLIGHTLY bigger.

'slight' being the key word with an overbore. for example, take a SBC that is 350ci stock, and bore it .060 over (a pretty big overbore, probably around the max for that motor, but i'm no chevy guy). what do you get? a lousy 10ci. a 2.8% increase in displacement. there is no reason to 'bore' an engine for performance- you should always remove the minimum amount of material needed to get rid of cylinder taper, out-of-round, or any kind of damage. to do otherwise is a waste of a perfectly good engine block, as you're basically saying you want to condemn it to its final rebuild (unless the next guy wants to sleeve it).

stroking is a whole different matter. compression ratio going to be determined by the kit that you use- remember that in addition to changing rod length, you're changing the location of the wrist pin, which is how you're going to control the change compression.
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With all due respect, you might want to acquire a better knowledge of how engines work before posting something like this again. It's almost completely wrong.

First off....you don't change rod length when you increase stroke. Not necessary at all.

The only thing changing the wrist pin location does is keep the piston at the same TDC...in other words, if you increased stroke and didn't change piston pin height (what it's really called), the piston would stick out the top of the block.

If I change rod length, I have to change my pistons to ones with a different, corresponding pin height. If I change my stroke, same thing. You CAN do both at the same time....but you don't have to. I can install a stroker crank and keep my same rods. I can install longer rods and keep my same crank. Both changes, again, require new pistons, but neither are required with each other.

So no....none of those things has anything to do with compression.

Again....whether you increase your displacement by bore or by stroke....it increases the compression by the same amount per CI increase. e.g., gain 6 CID by bore or by stroke, increases compression by the same amount....and that amount is negligible.
 
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Pacfanweb said:
Or you could just use a thicker head gasket to offset the increased compression. Lots of ways to skin this cat.

You could also open up the combustion chamber a bit.
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While that works and is quite popular, it also decreases your quench clearance and can lead to hot spots and detonation, so it's not the correct way to do it. For most people bolting on turbos to Hondas on a budget for some extra kick or things like that where they aren't seeking 110% performance, it's not a big deal. But if you are really wringing everything out of a high performance build and want precision control and reliability, you practically want your piston touching your head at redline at operating temp (or as close as you can get away with).
 
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As I demonstrated in the other thread, anything you do to add volume that is going to equally affect both TDC and BDC by the same amount is going to occupy a disproportionally larger percentage of the volume at TDC than BDC, so by definition compression ratio is changed.

Sample fraction math: 14/1 + .25/.25 = 14.25 / 1.25 = 11.4/1. This would be like dishing a piston or using a thicker head gasket, different combustion chamber/head, anything that adds the same to both TDC and BDC: the change in volume is equal to both TDC and BDC volumes, compression decrease (the inverse is also true, if you subtract volume with domed pistons or thinner head gasket, compression increases). The smaller denominator is affected more.

Stroking and boring is different and increases compression because they add ZERO volume at TDC and add volume only at BDC (or anything other than TDC). The position of the piston at the top of the cylinder doesn't change at TDC when you stroke, it just moves down more, and at TDC the increased bore gives zero volume because the height of the swept cylinder is zero at TDC. Thus only the numerator changes at anything other than TDC, and you always end up with increased compression ratio.

In reality it's very little. Boring or stroking an engine a moderate amount doesn't add more than .5-1.0 compression. 9.5 to 10.5 isn't going to hurt anything, it will just give you more power associated with the stroking modification. Even if you do get a slight knock on an older car, you just retard the distributor by 1-2 degrees; the increased displacement and compression ratio is still going to give more power than the loss of 1-2 degrees timing.
 
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Pacfanweb said:
With all due respect, you might want to acquire a better knowledge of how engines work before posting something like this again. It's almost completely wrong.

First off....you don't change rod length when you increase stroke. Not necessary at all.

The only thing changing the wrist pin location does is keep the piston at the same TDC...in other words, if you increased stroke and didn't change piston pin height (what it's really called), the piston would stick out the top of the block.

If I change rod length, I have to change my pistons to ones with a different, corresponding pin height. If I change my stroke, same thing. You CAN do both at the same time....but you don't have to. I can install a stroker crank and keep my same rods. I can install longer rods and keep my same crank. Both changes, again, require new pistons, but neither are required with each other.

So no....none of those things has anything to do with compression.

Again....whether you increase your displacement by bore or by stroke....it increases the compression by the same amount per CI increase. e.g., gain 6 CID by bore or by stroke, increases compression by the same amount....and that amount is negligible.
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If you get a new piston to maintain the same Top Dead Center, then compression goes up by whatever percentage you increase displacement... If you stroke a 4.0L Jeep engine to 4.6L (a common mod), that's a 15% increase in compression because 15% more air is compressed to the same volume.
 
Throckmorton said:
If you get a new piston to maintain the same Top Dead Center, then compression goes up by whatever percentage you increase displacement... If you stroke a 4.0L Jeep engine to 4.6L (a common mod), that's a 15% increase in compression because 15% more air is compressed to the same volume.
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Which is precisely what I just said in the post you quoted. Has nothing to do with rod length, which was my point. I can't vouch for your 15% number because I haven't done the math, but my general statement is correct.
 
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I will amend my earlier statement that increasing displacement will result in a negligible increase in compression ratio:

Just from over boring, like going .030 or .060" over....THOSE won't increase compression much.

But with large increases in displacement, you WILL get a significant increase in compression.

Like the Jeep 4.0 mentioned.....15% increase in compression is over a point increase. That is most certainly significant, if you are trying to run on pump gas and not spark knock.

For whatever reason, I was thinking about the increase you get from boring then applying that to any increase in displacement, when I said the increase is negligible, and that was incorrect.

If you take a 454 Chevy, bore it .060 over, and add 1/4" to the stroke, you'll get a 496. And you'll definitely have a "significant" increase in compression. But if you just bore it .060 over, you'll have a 468 and a negligible increase in compression.
 
brblx said:
'boring' an engine does little for compression ratio. both the swept volume and combustion chamber volume will get SLIGHTLY bigger.
Click to expand...

The combustion chamber size doesn't change with bore. The chamber in the head isn't the size of the bore to begin with, most of it is flat around the bore. So since volume added by bore only affects everything but TDC, compression increases, though it is very small amount.

Of course there are variances in head designs, gaskets, etc, and engines like the 409 that have the combustion chamber in the block that can cause all kinds of exceptions, but generally the combustion chamber is independent of cylinder bore.
 
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Pacfanweb said:
Or you could just use a thicker head gasket to offset the increased compression. Lots of ways to skin this cat.

You could also open up the combustion chamber a bit.
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The head gasket thickness is covered in the calc., granted I didn't specifically mention it. Same goes for combustion chamber volume...
 
The combustion chamber size doesn't change with bore.
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Not entirely accurate depending on the engine. A typical small block Chevy has a deck height of 0.20-0.40, boring will also increase crevice volume. In a run of the mill 383 stroker ~75% of the combustion volume is chamber, ~10% gasket, ~9% deck, ~6% valve reliefs. 19% of the total combustion volume is impacted by boring(although not a perfectly linear increase).
 
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