Compression Ratio increase

[Anthony]

All other things being equal compression ratio will increase all be it very slightly.

The static compression ratio is made up of the cylinder volume and the CC volume. If you increase clyinder volume and the CC chamber volume remains static then the ratio increase.

I doubt that the compression ratio will jump .5 points but it will increase.

See the discussion on the FAQ previously

http://www.bmw2002faq.com/component/option,com_forum/Itemid,57/page,viewtopic/t,215138/start,0/postdays,0/postorder,asc/highlight,earth%2Bflat/

[john_a]

.... The static compression ratio is made up of the cylinder volume and the CC volume ....

But wait, there's more!

Of course Anthony is correct, but it is important to understand what really defines those two pieces of the formula for CR.

I prefer to call the first item "swept volume" as that is really the only part that matters. Swept volume can be done in simple math as Bore x Stroke. It is a more definitive term than cylinder volume, because you are focused on a specific space within a cylinder. For consideration of CR, it will help your conceptual understanding if you think of the swept volume as having its diameter & height defined by the top edge of the compression ring on the piston. That ring's top edge will physically locate the diameter, top, and bottom of the actual "cylinder" that is the swept volume (using the full stroke from BDC to TDC.) Again, that will be a number that is simply Bore X Stroke, but understanding WHERE the swept volume is found is necessary to define the other element of CR, the so-called CC volume.

CC = Combustion Chamber volume, but you can't easily calculate this number, nor will you find it by just CC'ing the cylinder head.

The volume you need to find is the empty space EVERYWHERE above the top edge of the piston compression ring at TDC. That's why I said it would be useful to visualize swept volume in the manner mentioned above.

This volume is found in the cylinder block (in the piston side clearance above the ring and often some portion on top of the piston); in the gasket; and of course in the cylinder head chamber.

You can see that the ring placement, piston clearance, compression height, piston profile (dish/dome/reliefs); deck height, gasket volume, all have an impact on the upper volume, and must be considered along with the head's chamber.

You can measure the volume of the head's combustion chamber directly by using fluid dispensed from a graduated burette, then add the calculated volume of the gasket, then another burette job & some math for the rest.

Any portion of the piston that comes above the block at TDC will need to be subtracted from the gasket & chamber volume. Any volume of space below the deck and above the compression ring needs to be added. You can still use the burette to find these quantities by dropping the piston down into the bore a set distance (say 0.5" below TDC), then introduce the fluid with the burette to fill up to the deck height. (Smear a small amount of stiff grease around the compression ring to seal the measuring fluid.) Subtract the "dropped" volume (i.e., 0.5" X bore) from the measured volume dispensed thru the burette, and you will have the number (may be positive or negative) to add together with the gasket & head chamber volumes to get true chamber volume.

Then CR is calculated as SV + CV / CV

SV = pi/4 x bore squared x stroke

Example follows:

For a 90 mm bore with a standard 02's 80 mm stroke, the numbers look like this for cubic centimeters:

0.7853982 x 81 x 8 = 508.938 cc per cylinder

The 1.5 mm thick gasket is compressed a bit and yields a volume of 9.21 cc

Add that to a head with a 61 cc Chamber, and you have 70.21 cc

With a piston factored in, say that volume is reduced by 10.38 to yield 59.83 cc TRUE Chamber Volume.

SV + CV / CV = CR

508.938 + 59.83 / 59.83 = CR

568.768/59.83 = 9.5 Mahle !!

Pop quiz: If you take the sample above, and reduce the block's deck height by .5 mm, what is the new CR?

After so much typing, I only did a half-ass proof read so there might be some glaring errors.. don't take this as concrete numbers, it's just an illustration

[Anthony]

Thanks John,

I love the fact that you took the time to explain it.

Assuming the deck height change doesn't effect the CC volume it lowers the swept volume.

I get 9.45 so compression ratio actually drops 0.05.

505.757+59.83 / 59.83 = 9.4532341

but of course this is not taking into account the volume of the cylinder below the deck height and above the top ring.

[john_a]

.

Assuming the deck height change doesn't effect the CC volume it lowers the swept volume.

I get 9.45 so compression ratio actually drops 0.05.

505.757+59.83 / 59.83 = 9.4532341

but of course this is not taking into account the volume of the cylinder below the deck height and above the top ring.

You got the right volume, but subtracted it from the wrong side.

Maybe I was not clear, but cutting the deck changes only the CV for the formula. Remember, CV is EVERYTHING above the top ring @ TDC.

Swept Volume (SV) is not influenced by anything other than bore or stroke.

Cutting 0.5 mm off the block would take 3.18 cc from a 90 mm bore, but this is ABOVE the piston compression ring.

That changes only the CV

So if the original was like this:

SV + CV / CV = CR

508.938 + 59.83 / 59.83 = CR

568.768 / 59.83 = 9.5

Decking the block 0.5 mm gives:

508.938 + 56.65 / 56.65 =

565.588 / 56.65 = 9.984

That's about .5 point increase in CR

Now, if you were to mill the head by that same 0.5 mm, what happens?

You can't use simple math to solve it exactly, the shape is too complex.

But what direction does CR go, and is this a greater or lesser change than what was just solved for the block? (This is an easier question.)

[Anthony]

Yep I had a momentary lapse in judgement. As an expecting father it is happening quite frequently these days. Totally makes sense and thank you again for explaining it to the board. We should put your post up as an FAQ.

Thanks again. I always enjoy reading your posts.