Boring Out

[samsonitesamsonite]

I have a spare 4 oh that I am tearing down and completely rebuilding. I'm almost to the stage to send the block off to the machine shop, would I notice much of a difference if I say, had it bored .030 over?

 

[Twizzler09]

there should be a bit of a difference, or so I would think.

However I'm not sure its safe... as I've heard that the 4.0 block is pretty well maxed already... it was described to me as "its like making a chevy 400 out of a 256 block"

(I think I saw that somewhere here on the forums... or on another forum somewhere..)

As I understand it, theres not that much space left between the cylinders in the first place, but I've never had a 4.0 torn apart before.

 

[MAKG]

No need to guess.

Calculate the volume of a cylinder with diameter = bore and height = stroke. Add .030 to the bore. See the difference.

I'm leaving this as an exercise for the reader.

 

[85_Ranger4x4]

Only way to get much more power by boring out is to put fancier pistons in it when you do it, the selection for the 4.0 isn't that hot.

 

[woodyedmiston]

I've also got a 4.0 down for rebuild - curious about the same question - I just ran out and did a very rough calculation of the thickness of the cylinder of the 4.0 I have torn down. It's roughly .225 on a stock bore. Subtracting .30 = .195 . . .roughly

How thin is too thin.

 

[Bird]

The 4.0 is a 'thin wall' block with minimal support for the cylinders - therefore, you want to bore it as little as needed otherwise cylinder wall flex can show up if you plan to run it up there a bit. .020-.030 is considered safe with .040 as an absolute max unless you have the block sonic checked. The earlier blocks (92,93,94) seem to be a little thicker and I actually have one that sonic checked out much thicker than several others - I bored it out to a 4.0" bore and still appear to have decent wall thickness.

As MAKG said - any increase in 'swept' volume will result in an increase in static compression, so calculate away my friends.


Bird

 

[bottledgt]

.030 over wont really be noticed. youd get more from a cam or other stuff

 

[MAKG]

As MAKG said - any increase in 'swept' volume will result in an increase in static compression, so calculate away my friends.

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.

 

[chris1044]

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.

Assuming that clearance volume stays the same, the compression ratio will increase with a bore.....If clearance volume increases at the same ratio as the swept volume, CR won't change......

typically when you bore an engine the clearance volume does increase as the piston doesn't come right up to the top of the deck, so you don't get a compression increase worth noting

 

[Bird]

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.

MAKG - think about what you just said..........................

Larger bore with the same stroke will mean more swept area to be compressed into the same combustion chamber = higher compression. One of the reasons you can put a 350 crankshaft in a 400 block and with the 350 heads and the same style pistons, the compression ratio will jump an additional point- granted, a .030 overbore on a 4.0 won't change it much, but it truly does happen.

Bird

 

[thegoat4]

MAKG - think about what you just said..........................

Larger bore with the same stroke will mean more swept area to be compressed into the same combustion chamber = higher compression. One of the reasons you can put a 350 crankshaft in a 400 block and with the 350 heads and the same style pistons, the compression ratio will jump an additional point- granted, a .030 overbore on a 4.0 won't change it much, but it truly does happen.

Bird

No. As you say, larger bore means larger swept area. But it also means larger combustion chamber. Unless you have tapered "cylinders."

 

[Sunk]

MAKG - think about what you just said..........................

Larger bore with the same stroke will mean more swept area to be compressed into the same combustion chamber = higher compression. One of the reasons you can put a 350 crankshaft in a 400 block and with the 350 heads and the same style pistons, the compression ratio will jump an additional point- granted, a .030 overbore on a 4.0 won't change it much, but it truly does happen.

Bird

No, your combustion chamber will also increase in size. 0.030 in in diameter to be exact.

 

[chris1044]

No, your combustion chamber will also increase in size. 0.030 in in diameter to be exact.

X2, unless you deck the block/heads or install a piston with a different shape to it

Which is the reason I put what I did - most shops will take a negligible amount off the deck surface to make sure it's even and has the correct surface finish when they do a rebuild...

 

[Bird]

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

 

[MAKG]

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.

Now, these things are not at all hard to calculate, and anyone contemplating boring to raise compression really should calculate the effect. All you need is the bore, stroke, and stock static compression ratio to estimate its effect. You can do without the CR (which often isn't very accurately spec'd) if a fractional (percent) change is enough.