Milling Head, its effect on compression ratio?

Joined
Jul 25, 2006
Messages
170
Location
California (Corona, Irvine, Anaheim)
When doing the head gasket, if the head is milled say 0.010" or 0.015", what will the resultant compression ratio be?

Thanks!
 
Joined
Mar 27, 2003
Messages
12,519
Location
Somewhere in the foothills...
... what will the resultant compression ratio be?

The resultant compression ratio will be higher. I'm pretty sure you're gonna need to do the math yourself if you are hoping someone will give you a specific number.

-B-
 
Joined
Jul 25, 2006
Messages
170
Location
California (Corona, Irvine, Anaheim)
I guess all I would need to know is the stroke and then I can calculate the change. What is the piston stroke?

Thanks!
 
Joined
Mar 27, 2003
Messages
12,519
Location
Somewhere in the foothills...
I guess all I would need to know is the stroke and then I can calculate the change. What is the piston stroke?

Thanks!

The piston stroke will not change and I do not believe that is where you would start. You need to:

1. Find out the stock compression ratio.
2. Compute the volume of the stock combustion chamber.
3. Compute the volume of the modified combustion chamber.
4. Calculate the percentage change between #2 and #3.
5. Apply the computed % change from step #4 to the stock CR #1.

These will not be trivial calculations. The reliable way to do it would be to use a fluid measure of the stock combustion chamber and the modified combustion chamber. They should be able to do that for you when you have the head milled. I'm pretty sure that race shops do this all the time.

There is a chance that #1 and #2 would be in the NCF manual for the 1996 LX450 or in the 1993 NCF manual. (You can DL using Nate's automated process.) I have also seen a post by SUMOTOY that implied he had access to an ASME (?) design article that described the 1FZ-FE in great detail. PM him and see if you can get this information or get access to the article.

-B-
 
Joined
Jul 25, 2006
Messages
170
Location
California (Corona, Irvine, Anaheim)
Since I already know the stock compression ratio and engine volume, all I need to know is the stroke and then I can calculate the compression ratio change.

I don't want to know the change to 0.001 accuracy, I just want to get a ballpark idea of the change. I'll just use 9th grade geometry/algebra, no need to get into 12th grade calculus.
 
Joined
Mar 27, 2003
Messages
12,519
Location
Somewhere in the foothills...
... all I need to know is the stroke and then I can calculate the compression ratio change.

Enlighten me with an example please.

1FZ-FE Facts:
9.0:1 CR
100mm Bore
95mm Stroke

How do you calculate the new CR after shaving of the head?




(Interesting that Toyota seems to think this engine has a timing belt instead of the timing chain that we know it has.) :D

-B-
Engine Specs - 1FZ-FE.jpg
 
Last edited:
Joined
Jul 25, 2006
Messages
170
Location
California (Corona, Irvine, Anaheim)
I found the bore and stroke from http://www.4wdonline.com/Toyota/80.html

So after a quick and dirty Excel spreadsheet:
Original Compression ratio: 9.0:1

Assuming the combustion chamber has a cylindrical cross section of 100mm diameter (it is actually clover shaped):
Milling head by:-----resultant CR:
0.005"----------------9.11
0.010"----------------9.22
0.015"----------------9.34
0.020"----------------9.45
0.025"----------------9.58
0.030"----------------9.70

Since the combustion chamber is actually somewhat clovered shaped, I would guess that the resultant compression ratio GAIN is actually about 30% less than calculated above. This is based on eyeballing the shape of the projected crosssectional area of the combustion chamber. Here are the results:
0.005"----------------9.08
0.010"----------------9.15
0.015"----------------9.24
0.020"----------------9.32
0.025"----------------9.40
0.030"----------------9.49
 
Last edited:
Joined
Mar 27, 2003
Messages
12,519
Location
Somewhere in the foothills...
So after a quick and dirty Excel spreadsheet:

What formula are you using and how did you determine the 30% difference between the actual shape of the combustion chamber and a sphere?

-B-
 
Joined
Jul 25, 2006
Messages
170
Location
California (Corona, Irvine, Anaheim)
I'll email you the excel spreadsheet. I'll have to clean it up a little so that you know where to enter the variables.

To determine the 30% difference in combustion chamber shape crossectional area, I used a very scientific method called "eyeballing". My eyeballs have not been calibrated so I cannot vouch for their accuracy, haha.
 
Joined
Nov 30, 2006
Messages
783
Location
Traverse City, MI
Website
www.myspace.com
This is an informative thread, please keep the info rollin!

FWIW: IIRC, the most milling safely allowed by our 1FZ gurus is 0.010 before the timing is affected.
 
Joined
Mar 27, 2003
Messages
12,519
Location
Somewhere in the foothills...

landtank

Supporting Vendor
SILVER Star
Joined
Mar 27, 2003
Messages
20,482
Location
Groveland MA
Website
landtankproducts.com
knowing the bore and stroke the cylinder volume is 746.428cc

So a 9:1 compression ratio would give you a head volume of 82.942cc (746.428/9)

working with a round cylinder head each .010" removed would reduce the head volume by approx. .02cc

so a .010 shave would yield a head volume of 82.922cc and a compression ratio of 9.0015:1
 
Joined
Jul 25, 2006
Messages
170
Location
California (Corona, Irvine, Anaheim)
Your calcs look good. I'll have to check my spreadsheet. It looks like I could have gotten a decimal place incorrect.
 
Joined
Jul 25, 2006
Messages
170
Location
California (Corona, Irvine, Anaheim)
I double checked my calcs, I got the original answer. It could easily mean I did the calc wrong both times, haha.

4477------engine size (cubic centimeters)
273.203--engine size (cubic inches)
3.937-----bore (inches)
3.740-----Stroke (inches)
9.0:1-----Stock Compression Ratio

45.534---(cubic inches) per cylinder
5.059-----stock combustion chamber (cubic inches)

12.174----combustion chamber crossectional area inches sq (assuming cylinder)

0.010-----mill head (inches)
0.122-----combustion chamber volume removed (cubic inches)

4.938-----resultant milled combustion chamber volume (cubic inches)
9.222:1---milled compression ratio
 
Last edited:
Joined
Jan 7, 2005
Messages
6,560
Location
The Greatland
Website
www.facebook.com
I'd say rather that this is a mis-informative thread. ;)

You can't eyeball a combustion chamber and guess at it's volume. The comment that all you need to know is the bore and stroke doesn't even make sense. Milling the head doesn't change the stroke. It only changes the CC volume.

The bore and stroke calculation will give you the swept volume. Which you already know because the swept volume of all the cylinders combined is the displacement of the engine. You haven't changed that.



The missing variable is the volume of the combustion chamber. The combustion chamber is not a cylinder nor a hemisphere. No easy calculation and no eyeballing. You also have to factor in the volume of the head gasket. Don't forget that any valve work that removes material from the seats or the mating surface of the valve changes the CC volume too.

Toyota doesn't provide a number for the CC volume. If you want to be accurate you will have to measure the CC yourself. If you are not concerned about absolute accuracy you could determine the surface area of the face of the combustion chamber (you will need to be as accurate as you can on this measurement). Then assume no change in that footprint within a .015 depth. This will give you an approximate number for the reduction in volume you have achieved.

Divide the swept volume of a cylinder by the nominal compression ration. This will give you a (very) approximate number for the original Cc volume. Now you have the original volume and the amount of change. This give you a couple of different ways to determine the new compression ratio.

It won't be accurate for serious engine performance work, but will be enough for what you are looking for.


When all is said and done, removal of 10 - 15 thousandths of an inch of material from the head surface will not create enough change in compression to exceed normal variances in combustion chamber and head gasket volume.


Mark...
 

semlin

curmudgeon
Joined
Mar 27, 2003
Messages
5,237
Location
north of 49
robbie/powederpig has chimed in on this before. shaving .010 is the absolute maximum he recommends without timing issues and he has done this many times. the machine shop i took my head to was very reluctant to go .010 and wanted to take only what they had to based on shaving newer toyota heads in the past and encountering timing issues. as mark says, the compression depends disproportionately on the irregular volume inside the head cavity at the moment of combustion. by reducing this, you are making a much larger change in compression than your calculation shows.
 
Joined
Jul 25, 2006
Messages
170
Location
California (Corona, Irvine, Anaheim)
Mark W,
I emphasized "eyeballing" to make clear the fact that I did not measure it. My purpose was to find the "ORDER OF MAGNITUDE" change that occurs when the head is milled. I found my answer:
Compression ratio is raised by APROXIMATELY 0.22 (not 0.022, not 2.2) for every 0.010" that is milled off.
 
Last edited:
Joined
Jan 7, 2005
Messages
6,560
Location
The Greatland
Website
www.facebook.com
I had always thought the compression ratio was the volume of the cylinder compared to that of the head. I stand corrected.

It is.

But it is just a nominal ratio. You have to include head gasket volume, piston compression height (the amount that the piston remains below the block deck at TDC), and any sort of dome or depression in the top of the piston. Too be real picky you need to include the volume of the area between the piston and the cylinder wall, down to the top ring. But that's getting really really picky and makes virtually difference.

For most uses, swept volume divivded by combustion chamber volume is close enough. Anytime you real any sort of non-technical release from the manufacturer, this is all they use. And it's more often than not rounded some too.


Mark...
 

semlin

curmudgeon
Joined
Mar 27, 2003
Messages
5,237
Location
north of 49
pulse, compression is the ratio between combustion chamber volume at the bottom and top of the stroke.

when you shave the head you are making a greater change to the top stroke than to the bottom stroke because the area shaved all comes out of the much smaller irregularly changed top stroke "chamber". i do not see how your calculation takes this into account. am i missing something?
 

Users who are viewing this thread

Top Bottom