Bugeye58

[jlaird]

Check your book please what head is this 12G206 ?

Is this a standard or a good one?

 

[Biff]

Jack,

The 12G206 head is the same as the 12G295 head. Used on a 948 block you will have better breathing and exhaust than any other head. The only 'problem' is that it has a larger head volume than the other 948 heads, i.e., standard 948 head (2A628, 12A1456, 2A629) have a head volume of 25.5 cc, the 12G202 head had 26.1 cc volume, and the 12G206/12G295 heads have 28.3 cc volume. A little off the block and a little off the head will reduce the 'extra' head volume.
If the 12G206 head is good use it. The head alone will produce a little more horsepower (5 - 6).

 

[GB1]

/ubbthreads/images/graemlins/iagree.gif

 

[Bugeye58]

Biff pretty well nailed it, Jack. The 206 has 1 7/32" intakes, a better combustion chamber shape, and vastly better ports than the stock 948 head. I can't remember how much you have to take off to get back to the original volume, but I can figure it out this week. It's not all that much to lose the extra 3cc volume.
Jeff

 

[jlaird]

Although my newly rebuilt head is not on, the block is assembled. I am afraid that it is too late to mill the block.

 

[DrEntropy]

Jeff, does it require the block to be "relieved" to make clearance for the intake valve? My "intuitive part" says "no" but I've not hacked on the Spridget motors... just curious. :drivel:

 

[GB1]

Jack:

I have milled just the head before to cover the difference w/o a problem.

Patrick

 

[jlaird]

Lovely, just the answer I needed. I will pick up the head in Panama City next time George comes to see his cousin.

I'm suer one of you math guys can tell me how much the head needs to be milled?

Thanks all

 

[ThomP]

The amount varies, depending on whether the head has been serviced before. .050 off a standard head will get you close. Tell the machine shop what you want the final chamber cc's to be and they will take care of the rest. I would plan for a 9.5:1 CR for a street engine.

 

[Billm]

I was just reading one of Vizard's books last night and he said to be careful of milling too much off. He says that the limit depends on the depth of the oil galley hole for the rocker towers. With the towers off push a pin down to oil hole and measure how far down it went. You wand a MIN of .060" remaining from the bottom of the hole to the head gasket surface.
Bill

 

[GB1]

Bill you are correct, I believe Jack had his block milled already, so he shouldn't need to take that much off.

Another factor is also the cam lift and rocker offsets, but once again I do not believe that this will be a factor for Jack since he is running pretty much a stock 948, sorry 998.

 

[jlaird]

Oh, I like this thread. This is one I will print out when it is complete.

So, how do you determine your compression ratio?

 

[DrEntropy]

Volume of head CC, volume of "swept area" in the bores, and domed/flat/concave piston considerations.

It's called "maths" Jack. /ubbthreads/images/graemlins/jester.gif

Have you got one of David Vizard's books? "Vizard th' Wizard." They're great to have on hand for this kinda stuff.


EDIT: And 9.5:1 is about as far as you'd want to go on a street engine... Octane bein' what it is (or ~ain't~) these days.

 

[jlaird]

Yea understand that, our gas is awful compaired to what use to be available.

Guess I best get a book.

 

[ThomP]

Here's a quick little web application that you can play with. Compression ratio calculation Second with explaination

 

[GB1]

Wow!

Great Tom, this is a keeper!

Thanks