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Compression Ratio - Can't Confirm!
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Author:  bwhitejr [ Tue Dec 27, 2005 8:55 am ]
Post subject:  Compression Ratio - Can't Confirm!
Everywhere I have seen it listed it says:
Removal of 0.090 from head will raise the compression ratio on a 225 Slant six to 9.5:1.0. I can't make the math work out and get to 9.5:1.0 from 8.4:1.0 by shaving the head 0.090.

Help!

bwhitejr
Author:  SlantSixDan [ Tue Dec 27, 2005 9:56 am ]
Post subject: 
This is one of those pearls of "wisdom" from the Direct Connection/Mopar Performance 6-cylinder engine book...or, stated another way, this is one of those statements that's been repeated and reprinted so often that people believe it, even though it's got little basis in fact.

Factory compression ratio varied from a nominal 8.2 to a nominal 8.5, and in reality it's probably variable from 7.9 through 8.7 or so. So, there's no such a thing as you just remove the amount of metal the book says to remove and arrive at precisely 9.5. It jest don't work that way! Unfortunately, the only way to calculate your real compression ratio and figure out what to do in order to arrive at the ratio you want is to measure your combustion chambers (w/plexiglas, coloured fluid and a burette), measure your piston deck height and crunch the numbers.
Author:  bwhitejr [ Tue Dec 27, 2005 10:40 am ]
Post subject:  Compression Ratio - Can't Confirm!
Thanks SSDan,

I figured I would all kinds of pinging going on with this alleged 9.69 CR.
That is 0.040 oversize pistons and 0.090 shaved off of head. I am guessing that is not even close to 9.69:1! I was running the numbers and couldn't come up with anywhere near the expected CR.

Let see:
Crank centerline to deck = 10.68"
Rod length = 6.699"
Bore = 3.44"
Stroke = 4.125"
Stock head Volume = 54 cc's
0.090 Shaved head Vol = 51 cc's
Compression Ht of piston = 1.74"
Deck Height = -0.175"
Gasket Thickness = 0.020"
Flat Top Piston = 0.00"
Volume space between
piston and piston wall = 3.0 cc's

These are numbers I am using.

Is there a way to tell from the pressure measured from a compression check? It is a new engine.

bwhitejr
Author:  Tom Drake [ Tue Dec 27, 2005 11:13 am ]
Post subject: 
http://www.rbracing-rsr.com/compstaticcalc.html

Try this one and see what it gives you.
Author:  SlantSixDan [ Tue Dec 27, 2005 11:18 am ]
Post subject:  Re: Compression Ratio - Can't Confirm!
Quote:
Crank centerline to deck = 10.68"
Measured? Or assumed?
Quote:
Stock head Volume = 54 cc's
Measured? Or assumed? This is where much of the error will come from...you cannot just rely on a number printed in a book; the manufacturing was not that consistent! I'd even take the trouble to CC each and every combustion chamber. Sometimes you'll see the front chambers a little bigger than the rears (or vice versa), and you can compensate with a slight longitudinal angle mill.
Quote:
0.090 Shaved head Vol = 51 cc's
This number would be contingent upon the actual combustion chamber volume.
Quote:
Compression Ht of piston = 1.74"
Deck Height = -0.175"
Same question: Measured or assumed?

If your deck height is -0.175", which is entirely plausible, and you find your combustion chambers are all nearly identical in volume, you may want to look at increasing your compression ratio by milling the block rather than the head; this gives an advantage of improved "squish".
Quote:
Is there a way to tell from the pressure measured from a compression check?
Nope.
Author:  oldgoat83 [ Tue Dec 27, 2005 1:05 pm ]
Post subject: 
Dan, I'm gathering from what you're saying that the compression ratio isn't necessarily uniform across all cylindars. Is that right?
Author:  bwhitejr [ Tue Dec 27, 2005 1:06 pm ]
Post subject:  Compression Ratio - Can't Confirm!
All measurements are assumed from catalog/book data. No hard measurements here, just straight out of the book.

I realize this is a casting and it could be off some. So, how accurate is 0.0066"/cc of head reduction. That sounds fairly exact to be so sloppy!

If the measurements given in the FSM are any indication of a real motor then you should be able to get close. I see tolereances in the single digit thousands for parts used throughout the engine. It is hard to believe that the you can't get closer than plus or minus one atmosphere. :shock:

I believe the solution is to CC the heads! Just like you said. :)

bwhitejr
Author:  SlantSixDan [ Tue Dec 27, 2005 1:10 pm ]
Post subject: 
Quote:
Dan, I'm gathering from what you're saying that the compression ratio isn't necessarily uniform across all cylindars. Is that right?
That's one thing I'm saying. The other thing I'm saying is that the compression ratio didn't necessarily start out at 8.4.
Author:  SlantSixDan [ Tue Dec 27, 2005 1:14 pm ]
Post subject:  Re: Compression Ratio - Can't Confirm!
Quote:
All measurements are assumed from catalog/book data. No hard measurements here, just straight out of the book.
Thought that might be so. If you can possibly do it, it'll pay dividends to measure these parameters for your engine and work from there.
Quote:
I realize this is a casting and it could be off some. So, how accurate is 0.0066"/cc of head reduction. That sounds fairly exact to be so sloppy!
It's entirely possible that removing 0.0066" means you remove one cc of combustion chamber volume. Or, that figure could be off by 0.3712484%. (See how easy it is to make precise-sounding numbers? All ya need is a small collection of digits that aren't 0 or 5, 'cause those are suspiciously round! ;-) )

Seriously, even if it turns out to be true (or true enough) that removing 0.0066" from the head surface will reduce the combustion chamber volume by 1cc, you're still guessing at how many cc you started with unless you check for yourself.
Quote:
I believe the solution is to CC the heads! Just like you said. :)
Certainly the safest way to do it. Be sure to use the same spark plug type you'll be running.
Author:  DusterIdiot [ Tue Dec 27, 2005 1:51 pm ]
Post subject:  Yep...
Just to confirm Dan's advice...I have a few heads from different "eras"... and no two are similar...(the BL heads are a different CC, than the pre-68 heads, and are different than even the 1974 head I have...by one CC!)...

The "66" number is one Doc has in his article for some estimation toward calcualting things. When building an engine you need to get in there with some instruments and get some numbers...


-D.Idiot
Author:  bwhitejr [ Tue Dec 27, 2005 1:57 pm ]
Post subject:  Compression Ratio - Can't Confirm!
I have the 1980 head with BL spark plugs. How many CC's are those heads?

bwhitejr
Author:  SlantSixDan [ Tue Dec 27, 2005 2:25 pm ]
Post subject:  Re: Compression Ratio - Can't Confirm!
Quote:
I have the 1980 head with BL spark plugs. How many CC's are those heads?
However many you measure...

Image
Author:  slantzilla [ Tue Dec 27, 2005 2:27 pm ]
Post subject: 
You really have to measure. The head that Mike just did for me had bigger chambers at the back than at the front. They had to angle mill the head front to back to get them even. IIRC, the later heads are right at 57CC. :shock:
Author:  slantvaliant [ Tue Dec 27, 2005 2:39 pm ]
Post subject: 
Quote:
right at 57CC.
... plus or minus a good bit, I would imagine. Sand castings tend to have pretty large standard deviations.
Author:  sandy in BC [ Tue Dec 27, 2005 3:18 pm ]
Post subject: 
This is not as difficult as it seems.
When I did my big valve head(sbc 1.72/1.50) I also radiused the chambers to allow more flow around the larger valves. This, of course ,increased chamber volume.

The block was bored .40 over which also changes CR.

I wanted to leave the head deck as close to stock as possible so that the radiusing around the valves would be most effective and so I would not have to notch the bores to "clear" the larger valves. (notching the bores would also increase volume).

When I took my head/block to the shop I asked for a target 9.25 CR to match my cam and planned cylinder pressure targets.

They bored the block, fitted the pistons, measured the deck height.
Then they measured chamber volumes...minimally decked the head.

Then they decked the block to get the correct CR.

On a slant you must know the chamber volume.....cylinder volume (deck height, bore and stroke). To know these things you MUST measure them.

The cost of getting the shop to do it is minimal. In fact they will think you are stupid if you get them to mill without measuring.

So just freekin do it!
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