Boring Out

[chris1044]

With the combustion chamber being part of the head (which isn't bored), tell me how it also 'grows' in size with the overbore..............................

Some folks need to do some homework.


Bird

Bird,

I understand exactly what your saying, BUT....

Assuming the piston is not even with the deck at TDC, then the area above the piston is "technically" a portion of the combustion chamber (I guess clearance volume would be a better term). If the piston doesn't travel all the way to the deck, then the area above the piston @ TDC from the top of the piston to the deck also increases with the rest of the bore size. This means that the clearance volume increases as well, which would mean the comp ratio stays the same (if clearance volume changed at the same factor which the bore is changed)


With that said, I think people are missing the point here - compression ratio can be minutely changed by numerous things....and IMO static comp ratio isn't worth much - dynamic is where its at

 

[chris1044]

Bird is (mostly) right. I made a basic error.

The combustion chamber is not bored. That is, the numerator of the combustion ratio (displacement + combustion chamber volume) goes up due to the bore, and the denominator doesn't. So, there is a change in static CR, assuming the piston shape is the same. If it isn't, you can make the CR do just about anything you want.

Now, it also assumes the piston at TDC is level with the deck, which doesn't have to be so. If the piston is below the deck at TDC, the combustion chamber IS larger (assuming for simplicity that the piston has a flat top). The limiting case (all the combustion chamber inside the block) is the only one where the CR doesn't change. You could build a cylinder that way, but it wouldn't work very well. Most real cylinders have a piston height very close to zero. Piston height is not changed by boring.

Beat me to it lol

 

[AllanD]

No. Changes in stroke change the compression ratio. Changes in bore do not.

Any change in volume changes the displacement. And I'd really suggest that anyone with questions about whether to do that should calculate stock and modified displacement. This will lay your questions to rest.

Actually ANY change in DISPLACEMENT relative to the combustion chamber
increases the compression ratio.
It matters not if the increase in displacement is the result
of stroke or bore.

This presumes that the "dish" volume in the new pistons are the same
as the original ones.

Infact you can have a change in C/R with NO change in displacement.
Either a smaller "dish" (or no dish at all) in the pistons or smaller combustion chambers.

though the effects of boring or minor stroke changes have a minimal effect on C/R.

Decking the block or surfacing the cylinder head usually has a greater effect, though doing EITHER on a V engine creates "issues" with intake manifold sealing.

In either case 0.030" over on a 350cid Small block chevy is good for 5cid.
IT is good for proportionally less on a 4.0 engine, 3-ish cubic inches.
3" out of 245cid is trivial.

AD

 

[woodyedmiston]

Before you leave this topic, I'd just like to add my .02 for dynamic compression ratio. One of the things that attracts me to the idea of turbocharging the 4.0 is the large dish in the piston and head. The top of that piston is sort of like an upside down hemispherical combustion chamber and - although Hemi is one of the most over hyped words in the English language - it should allow a great burn over the top of the piston in a turbocharged motor. That is where the dynamic thing comes in - pressurized air/fuel squeezed further is the equivalent of higher compression ratio.

 

[krugford]

From what I've seen, dynamic compression ratio is usually referring to the thermal changes in the engine when it warms up. It's usually good for about half a point on the engines I work with.

What you're talking about would refer to running higher intake pressures to force more air (and more fuel) in the cylinder. It affects bsfc and the smoke limit, but not compression ratio. Compression ratio is taken on a volumetric basis, not a mass basis. Different piston geometries affect in-cylinder swirl and turbulence as well as compression ratio. All three play a large part in torque output.

There are ways to change what I would call "effective" compression ratio if you want to start dealing with valve timing changes. But that's another topic.

 

[Bird]

Bird,

I understand exactly what your saying, BUT....

Assuming the piston is not even with the deck at TDC, then the area above the piston is "technically" a portion of the combustion chamber (I guess clearance volume would be a better term). If the piston doesn't travel all the way to the deck, then the area above the piston @ TDC from the top of the piston to the deck also increases with the rest of the bore size. This means that the clearance volume increases as well, which would mean the comp ratio stays the same (if clearance volume changed at the same factor which the bore is changed)


With that said, I think people are missing the point here - compression ratio can be minutely changed by numerous things....and IMO static comp ratio isn't worth much - dynamic is where its at

But, where you are screwing up in your thinking is that you are assuming that the slight extra volume from the overbore at the top of the stroke is equal to the extra volume from the rest of the stroke of the overbore - do the math.
1/16" of .030 overbore (piston sitting at tdc) does not equal 3" of .030 overbore (extra volume of the entire stroke).

Homework, boys......homework.

FWIW, .030" overbore will only bump the CR of the 4.0 at most .1 (under optimal conditions) - I prefer boost to do it right.....................


Bird

 

[Will]

Dynamic compression ratio just means that the intake valve is still open as the piston begins to come up so not all off the swept volume is actually compressed into the combustion area. It is a matter of valve timing. You have to know how high the piston is in the bore when the intake valve closes.

 

[woodyedmiston]

I'm not talking about the math involved in calculating CR. I'm talking about the effect of CR on a running engine and how to get power out of it relatively easy.

An engine with high static compression ratio has high compression when it is at idle or when it's running flat out. On a diesel that's OK. A gas engine isn't as rugged and isn't as heavy. If you care about those things - the gas engine sort of beats itself to death the higher the compression ratio and needs premium fuel all the time.

Dynamic means change - look it up. Evidently some engineering schools use the term to mean something else. That's not uncommon. I didn't know that! What I'm referring to is using boost to get higher power at the RPM range were you get work done. Something related to higher compression is going on because just about every system out there uses a knock sensor; At 12 pounds of boost an aluminum head lifts right off the block around 7000 RPM; start with high compression and boost it - the pistons melt. An 8 or 9 to one ratio makes your motor like itself a little better sitting at a stop light. At 4000 the effect of more air/fuel makes the motor effectively work like it had a higher CR. The dynamic effect, that is the changing effect, it to have an effectively higher CR when you want it. When I was building turbocharged racing engines we called that dynamic compression ratio.

If the valve is still open when the piston starts down the CR changes - therefore it's dynamic. The swirl effect that krugman mentions is what I'm referring to when I say that the semi-hemi shape in the top of the engine makes it a natural for turbocharging. There should be a good uniform flame over the top of the piston. Therefore you should be able to use a stock piston, 5-7 pounds of boost and do quite well.

I'm suggesting that because the guy who originally posted this question was going to send his engine to the machine shop for an overbore - my ASSumption is that he wants more power. My suggestion is only bore it if he needs to straighten the bore and that turbocharging has some really positive effects on usable horsepower with stock pistons, unmodified heads, etc. etc. Turbocharging can be bolt-on horsepower and most anything else that advertises that is pulling your leg.

 

[chris1044]

But, where you are screwing up in your thinking is that you are assuming that the slight extra volume from the overbore at the top of the stroke is equal to the extra volume from the rest of the stroke of the overbore - do the math.
1/16" of .030 overbore (piston sitting at tdc) does not equal 3" of .030 overbore (extra volume of the entire stroke).

Homework, boys......homework.

FWIW, .030" overbore will only bump the CR of the 4.0 at most .1 (under optimal conditions) - I prefer boost to do it right.....................


Bird

Thats why I said "assuming the clearance volume changes with the same factor the displaced volume is changed"......I haven't done the math on this motor, so I honestly don't know how much it changes the CR

But, as you state above, it only bumps it at most .1 points......while it's an increase, IMO its negligible on a motor you're not trying to get every little HP out of