how much to mill a C9 head?

[Asa]

i want 9.5 CR on this
i have yet to measure it, but my machinist should be able to do it with no problems

i've got the Compression Calculator, but i'm not sure how to use it...
http://falconperformance.sundog.net/compcalculator.asp

and i'm also not sure of the gasket thickness
Engine Seal Gaskets part # 81025
SKU ref # 7916 PT-1
interchanges with
Fel-Pro 7916 PT-1
Detroit 20336CS
McCord 6777 M
Victor 3910 VC


i'd like to get 9.5:1 (91/93 isn't really a problem), so what should the chamber size be? how much would it need to be milled to do that? and is that within the max milling range?

thanks

 

[Asa]

bumpage?

 

[addo]

What's the bore? Plus 20 or 40?

How big are the piston dishes? Use geometry to measure them.

What's your deck height? I allow 38 thou for a compressed gasket but others may vary in this allowance.

Plug in the numbers and juggle your calculations by changing the head volume.

 

[Asa]

it's all stock as far as i can tell...

 

[addo]

You've still got to measure it!

Got verniers and a calculator?

I'm off to bed; someone else can help you.

 

[Anonymous]

asa67_stang,

I'll take a shot at it. Whoever put the calculator together has done most of the work for us.

You have to CC you head. I read in the other thread that you were questioning other possible methods but I've always used the plexi-glass, grease and transmission fluid, with a large syringe method. However you do it you have to know chamber size.

You also need to know the piston to deck measurement. From the top of the piston to the top of the deck at full height of the piston.

Last is whatever your gasket of choice compressed dimension is. I saw a list of gaskets and their compressed measurement somewhere, maybe the FSSP tech section.

Put those three numbers in the calculator and hit go. Unless you have changed anything else from stock that should give you your compression with the deck height, gasket compression and head CC. You can adjust those measurements to reach your desired or you can change to flat top pistions rather than dished to increase compression ratio.

Easiest is gasket choice and then milling the head to acheive desired CC. Figure .010 for every 2 decrease in CC. If your head is currently 52 and you need 50 to achieve your goal have the head milled .010.

Hope this helps some.

GT200Z

 

[Asa]

FelPro 7916 PT-1, New compressed .050"
piston to deck clearance - 3/64th
2 1/8" dish diameter
1/8th dish depth

my math isn't working on the dish cc.... help?

 

[addo]

¶r²h

r=½ dish diameter
h=dish depth
¶=lame attempt at Pi

 

[jamyers]

mmm...

2 1/8" = 2.125"=diam
1.0625=r
r-squared=1.12890625
times pi (3.14)=3.544765625
times depth (.125")=0.443095703125 square inches.

converted to cc at http://www.easysurf.cc/metric/cnver6.htm#ic2 = 7.261037645234376 cc, or 7.3cc

piston is 3/64", or 0.046875" down in hole, used .047 for calculator.

Leaving other settings stock, you'd have 8.1 CR.

You can plug in different gasket numbers, etc and see what the results are.

To get to 9.5, you've got to reduce the chamber to 39cc's. But with a .05 gasket and the piston .047 down in the hole, you've got no quench or squish going on at all - .097 quench distance (piston-to-head clearance on non-valve side). Not the best in terms of detonation-avoidance (but then, the factory engines weren't either...)

[Rant ON]These Fords seem to be like Buicks in that the factory pistons are WAY down in the holes, and to get any decent quench distance (.040 or so) you've got to zero-deck the block and use a thin gasket, but then you run the risk of sending the compression ratio through the roof (so to speak), so sometimes (in Buickland) you have to actually open up the chamber to reduce the CR. Crazy, huh? But then, I know several guys running 87 octane in 455's with 11-1 cr or better, just because they got the quench distance down under .040. [Rant OFF]

 

[Asa]

thanks James, now that i have the completed picture i can play with it myself

looks like a CC of 43 would be 9.1, what would the quench look like there?


of course, i've yet to hear from the machinist... so i have no clue what the actual cc numbers are....

 

[Anonymous]

asa67_stang,

Your "quench" will be the same no matter how much you mill the head. Quench is the distance from the piston to the head. Deck clearance + gasket compression. It is recommended on the 200 to "zero deck" the block. This means to have a machinist mill the block until the piston at full height is at the top of the block.

The dish in your piston combined with the thickness of the gasket gives you valve clearance. In fact I think Mustang Geezer is running flat top pistons with a zero decked block. His compression ratio is in the 10's though.

Decking the block can be expensive and requires you to pull the engine.

Assuming 52 CC heads--Using a NAPA composite Victor #3910, new .046" compressed .044", which is the best most readily available gasket unless you can find a FoMoCo composite you would need to mill your head .060 to achieve your goal.

If it's in your budget zero decking the block you could remove .040 there .020 from your head and it would really improve your quench. The better your quench the lower your octane has to be before detonation. So with better quench the higher compression you can run on pump gas and the more advance timing you can put it.

GT200Z

 

[Asa]

i'm not decking the block at all
this is a cheapo stopgap measure to get the car on the road with slightly better compression than it previously had until something better comes along, IE the Oz 250 2V engine i have coming in march

in the meantime i just want to know if what i have planned will hurt or help performance

 

[Anonymous]

asa67_stang,

As you know raising the compression ratio will help performance, but you have to do it without causing detonation. Improving quench will help prevent detonation.

Have you already got the felpro gasket? If not get the Victor it's thinner and will improve quench some. You can safely mill your head .060 so mill it to achieve 39 CC chambers and you will improve your performance over a standard head.

You may not be able to run as much advance. Time it up until you get detonation at WOT then back off until you don't. Tune your carb to just on the rich side and have fun. I would run 93 octane at 9.5.

GT200Z

 

[addo]

One thing that surprises me is the deck height. It seems more the amount associated with aftermarket slugs for a rebore. Does the bore actually measure stock?

 

[Anonymous]

Some info that I've run across.

"Quench" refers to the area between the top, flat part of the piston and the bottom of the cylinder head. The quench is created by "deck height"- the distance from the top of the piston and the top of the block, PLUS the compressed thickness of the head gasket. Both of these dimensions create "deck clearance". Deck clearance is the same as the quench dimension.

The quench effect is only present on wedge type combustion chamber heads. An effective quench squeezes fresh mixture from the recesses not in the wedge chamber or the piston dish into the chamber/dish.

The tighter the quench distance the greater the squeeze, causing turbulence and better combustion efficiency as a result.

Too little quench distance and the top of the piston begins to hit the deck of the cylinder head at higher rpm/stress. Wear of parts, causing clearances to grow, adds to this concern.

Milling the head does not reduce quench. It does raise CR

Changing to a thicker head gasket adds to the quench distance. It also lowers CR.

Decking the top of the block, on a rebuild, is the only way to effectively reduce quench distance and improve the quench effect. Decking is typically done to level, or create a flat mating surface, on a rebuild. Replacement pistons are frequently built with are reduces pin height to compensate.

Since the deck height on a typical 200 block is approximately .025" and the stock OEM steel shim head gasket is typically .025" a typical 200 has a deck clearance or quench distance of about .050". This is on the verge of too large to have a positive quench effect. A quench height of .035" to .040" is about ideal with stock rods and cast pistons.

I see what addo is saying your deck height stock should be .025, 3/64 is .047.

So with your .050 gasket and .047 deck clearance .097 total quench is way to large for a positive quench effect. If you're going cheap waiting for an aluminum head put it together with what you have and wait until the head is available then rebuild the motor and zero deck the block to achieve a better more effective quench.

GT200Z

 

[Asa]

One thing that surprises me is the deck height. It seems more the amount associated with aftermarket slugs for a rebore. Does the bore actually measure stock?

3&11/16th
or
3.6875

 

[Fred]

You could also try to find pistons that have the piston pin lower in the piston, thus the top of the piston is higher in the bore, thus the distance from the top of the piston to the top of block is reduced. Before you buy pistons for a rebuild, check this. I am personally not in favor of removing any more than necessary from the deck of the block. Thicker decks are more diminsionally stable and reduce flex / wear in the motor. Just my 2¢.

 

[Soldmy66]

I would guess $30 to $50.