How much to mill off the OZ head to reach target compression

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Anonymous

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OK, gang. the block ('68) is done and the head (OZ2502V) is at the shop now. They are going to CC it for me and once I know what the combustion chamber volume is, I need to be able to tell them how much to mill. I used the compression calculator from the Schjeldahls with these values (bored .020, decked to .010, metal shim gasket on the way so i used .027 has it's compressed value):

Number Of Cylinders 6
Bore Diameter [inches] 3.70
Stroke Length [inches] 3.126
Combustion Chamber Volume [cubic centimeters] 50
Head Gasket Compressed Thickness [inches] .027
Head Gasket Bore Diameter [inches] 3.81
Piston To Deck Clearance [inches] .010
Select Piston Type Dished
Dish/Valve Relief/Dome Volume [positive cubic centimeters] 7
Volumetric Efficiency: 90%
RPM: 5000


Results:
Displacement, [cubic inches] 202
Displacement, [liters] 3.3
Static compression ratio 9.6:1
Cubic Feet per Minute required @ 5000 rpm, [cfm] 263
Estimated Horsepower @ 5000 rpm
Assumes altitude of sealevel, barometric pressure of 14.696 and 60° air supply to carburator 162


How does that all look? Obviously, the combustion chamber size I used is the variable I can change to hit my target of 9.6:1. I used 50 in this calculation to get there. From what I've read, most of the OZ heads run between 54-56...right?

Is there an equation I can use to figure out how much to mill to lower the chamber volume by x? Like:

.010 = 1cc or something?

The "Handbook" says .060 will raise the compression by 1 value. Is that a GOOD general assumption even on the OZ head? The book also states that milling .030 on a '78 head from 60.5 to 54. that means a .030 milling reduced the volume by 6.5 CC's. Does this apply to the OZ head?
 
Are you running a BONE STOCK cam? That is how the calculator you used is figured for HP.

If not, you need to use a proper analyser program. I referred to this when you asked about building up, a few weeks ago. You can still use the Bros Schjeldahl site for static compression calcs if needed.

Adam.
 
I'm not really targeting HP so much as compression. 9.6 was given as a recommended target, so whatever my HP ends up being, I can live with. I just need to hit it with the milling spec.

I'm not running anything stock (well, almost nothing):

cam is from mike: 264/274 – 214/224 - .450/.450 – 110*
 
the shop should be calling any minute with the CC numbers! I need to know how much to have it milled SOON! Where are the engineers and OZ head experts today?! :cry:

milling .0xx reduces chamber CC by y ??
 
Have you talked to your machinist about it? He might make a better guess than any of the rest of us. You can do it mathematically if you can figure the area of the chamber at the surface of the head. Unfortunately, it's not completely round, so you gotta guess a bit.

From my experience, a .060" cut is going to take off about 7-9 CCs
 
Thanks John. I do plan to ask him once I have the numbers but I was hoping to have something as a reference to use when he gives me his opinion.

I think I'm going to end up having him cut it twice. We'll guesstimate the total to remove. cut half of that, re-cc and cut again.

I just have to convince him no to charge me twice!

Anyone else with milling specs to share?
 
When I had mine machined, we measured, made a .010 pass and remeasured. This way we knew what we were working with. Just remember to take into account all the other factors when calculating the CR (deck height, piston dish, compressed gasket, etc.

Mike
 
OK, the head is 53CC's. If I drop it to 50CC's, I'll hit 9.7:1

I need to drop 3CC's = how much to remove?

The Handbook says:

".010 = 2+ CC's"

I just spoke to them. THey said to drop 3CC's to 50CC's they have to take off .021. He said they use a 7:1 ratio: .007 = 1CC
 
What was it before the .010 slice. Take the difference and you'll have your answer.

Mike
 
HP comes with the correct engine build to suit the parts.

Else you are wasting fuel (under-optimised build) or wearing the consequences (aviation fuel only). Surely you aren't doing all this as an exercise in wasting money.

I can't believe nobody else has articulated this. What is the [b]manufacturer's[/b] recommended static compression? That's more significant as a start point than anything else people here might say. You can probably add .2 to this (makers tend to be conservative).

FWIW, 2V heads are typically 56-58cc from my experiences. The larger number is closer to/more likely factory. When uncut, the "wells" cast around the pushrod holes in the gasket face of the head are often approx 7/64" deep. Note also Ford did not cc their heads and the volume can vary 2cc between ends or chambers.

Adam.
 
Personally, I would only mill the head enought to get it true. You might want to target 9.6, but a couple points less isn't going to make that much difference in HP. Check the difference in the compression calculator to see if you can live with it. Remember, taking less off now, means more meat is left for future rebuilds, which I would consider to be the critical factor given the rarity of the OZ heads. Next time you go to rebuild the head, you don't want to wind up with a boat anchor. Unless you like fishing more than driving the droptop. ;)
 
Well, i do like fishing more than driving..but I absolutely see your point. The calculator says that moving from the current 53CC (after a .010 milling) to 50 CC's (by taking an additional .021 off) would move me from 9.3:1 to 9.7:1. Is this a major/noticable difference?
 
at 53CC and leaving the RPM's at 4500 default = 9.3:1 @ 125HP

dropping to 50CC and leaving the RPM's at 4500 default = 9.7:1 @ 130HP
 
I'd be willing to sacrifice 5HP to make sure the head is rebuildable down the road. Just my two cents.
 
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