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View Full Version : What increases after boring an engine?

KYJ10
10-12-2003, 06:59 AM
Just wondering if this increases say a 5.9l to say as an example, a 6.0? My engine is bored out .40 over. Figure it's a small increase, but was wondering if anyone knows for sure how much an increase? Thanks, Dennis
Stuka
10-12-2003, 07:05 AM
It can aid if you have several other mods done, But generallly its not noticable. Maybe a few HP.
Mikel
10-12-2003, 08:08 AM
Boring will also increase compression a tiny amount.
Chevelleguy
10-12-2003, 09:03 AM
.040 over should be worth about 7 cubic inches.
KYJ10
10-12-2003, 09:15 AM
So I have an AMC 367 LOL Dennis
Stuka
10-12-2003, 09:23 AM
Originally posted by Mikel:
Boring will also increase compression a tiny amount.really...

how...
Mikel
10-12-2003, 09:30 AM
Displacement is increased, while the volume of the combustion chamber in the head remains the same smile.gif
Stuka
10-12-2003, 09:32 AM
The piston doesnt seal to the combustion chamber though. So while the displacement gets larger, the total area for the air to be compressed into also increases. There by keeping compression the same.
Mikel
10-12-2003, 09:48 AM
Stuka,
I'm not sure I'm following you. When the cylinder is bored and displacement is increased, when the piston comes to TDC, it tries to force an increased amount of air/fuel into a combustion chamber that hasn't changed (disregarding the neglegible increase in volume from the top of the rings to the head gasket when the piston is at TDC).
That's how it was explained to me.
Stuka
10-12-2003, 09:54 AM
The way I see it is each cylinder (in the case of KY's engine) will suck in an extra 1.5% or so (rounded off the top of my head heh) or air and fuel mix. But, while the cumbustion chamber has not been change, the top area of the cylinder has been by the same percentage. So this would lead me to think that the compression would not be changed. But I could be wrong ;) But thats the way I see it.
Mikel
10-12-2003, 10:18 AM
I think your idea would work if the combustion chamber in the head was increased in the same proportion as the diameter of the cylinder, which is not the case when a block is bored.

What you have is more volume crammed into pretty much* the same combustion chamber, thus increassing compression.

You can try plugging some overbore numbers here and see what it does to compression:

http://www.corral.net/tech/compress.html
http://www.bgsoflex.com/cr.html

Of course, I've been known to be wrong before ;)
Hammer
10-12-2003, 03:01 PM
366.89 CID

As for compression increases, the bases for increasing the compression ratio is founded on a piston that basically comes to the top of the cilnder wall, especailly a piston that is not dished like ours.
Now, to apply this to our engine designs, stock pistons still sit a ways down from the head, so these leaves more volume of the final compression chamber (piston at TDC) that has also been increased. So the effect on compression from overboring engines like ours is very small. Maybe as much as a compression point (8.4 to 8.5). Not enough to make a difference really.
AMX factor
10-12-2003, 04:15 PM
Boring does increase compression but with today's crappy ethanol laced gas you'd never know the difference. You build more compression when you mill your cylinder heads than when you bore the block.