Comp Ratio??

[664doorfalcon]

I have my 250 at the machine shop right now and i have been doing some cals trying to get to 9.5:1 comp ratio. this is what i came up with does this sound right? any other suggestions?

bore= 3.71 (stock bored .030 over)
stroke=3.91
head cc size=52 cc
compressed head gasket thickness= .053 (Corteco from CI)
have deck milled to .075 from top of piston to top of deck
dished pistons with 7.3 cc dish from CI
does this sound right?

thanks

 

[rocklord]

If the bore of your head gasket is 3.71, I calculated a 9.45 CR.
If the head gasket bore is 3.81, the CR is 9.40.

You're close.

 

[664doorfalcon]

do you think its fine to take the deck down that much?

 

[fordconvert]

Did you CC the heads? Even from the factory they seemed to vary quite a bit and at this age its hard to tell whats been done so CCing them is the only way to know for sure. I had some great plans for one of my builds but the heads turned out to be 16 cc different than what they were supposed to be so I had to go back to the drawing board.

 

[rocklord]

I agree with fordconvert, CC the head before you mill the block. It's hard to put metal back on.

As for the 0.075 deck, that would be OK. Some guys are zero decking the 200 without interference.

 

[664doorfalcon]

i am going to use classic Inlines alum. head with 52 cc. so it should come 52. i heard of people zero decking the 200 but this is a 250 and i thought you cant go down that far on the 250?

 

[kukm66]

compression calculator

http://falconperformance.sundog.net/compcalculator.asp

 

[CZLN6]

Howdy Back 664door:

I decked my 250 block .070" on the rebuild in '01. Many miles and many smiles ago and no problems yet. Unfortunately, the deck clearance with .128" (.075" deck height and .053" of gasket thickness) is a long ways from an ideal quench effect. The CI head will help, along with a performance cam, in managing cylinder pressure and temperature.

Sounds like a great build. Be sure to let us know how it comes out.

Adios, David

 

[664doorfalcon]

Im not sure i understand the quench affect your talking about? can you explain?

 

[cruzingratiot]

have you red this
Static & Dynamic Compression Ratios
http://www.classicinlines.com/CompressionRatio.asp

 

[CZLN6]

Howdy Back 4door:

Sure. Quench effect is produced when the piston in an engine with a wedge shaped combustion chamber head rises to the top of the stroke. The intake charge is being compressed somewhat uniformly until the piston comes closest to the deck portion (not combustion chamber) of the head. At that point the intake charge is squeezed out of that area and into the chamber/piston dish area causing turbulence and improving combustion efficiency. To achieve a quench effect the deck clearance should be between .030" and .040", depending on component materials. Deck clearance is defined as the deck height of the piston, which is how far the piston is below the deck of the block, and the compressed thickness of the head gasket. In your case, Deck height .075", plus head gasket thickness.053"= 128" total deck clearance.

The large deck height of the 250 engines is a hurdle to overcome when it comes to engine efficiency. Commonly available head gaskets ranging from .045" to .055" in compressed thickness add to the problem. We need a head gasket with a compressed thickness of between .030" and .038".

Once deck height is addressed the next issue is the quench to bore ratio. With a flat top piston all but the chamber in the head becomes quench area. With our dished pistons only the non-dish part that faces the non-chamber deck surface of the head generate quench. So, using a flat-top pistons has some advantage in maximizing quench ratio. But, a flat top piston will generate too much CR.

I wish we had talked earlier. I would have suggested that you use pistons from a 255 V8 engine, which stand .085" taller in pin height then regular 200/250 dished pistons. They are also a flat-topped piston with 4 eyebrows, typically of most V8 piston. These along with decking the block to zero solves the problem of too much deck height. The only necessary step is that the basically flat-top must have a "D" shaped dish milled into the top to mirror the shape of the chamber in the cylinder head. Doing this will allow you to lower CR to a manageable level as well as maximizing the quench to bore ratio for maximun combustion efficiency. The modern design of the combustion chambers of Mike's CI head will already be a plus in combustion efficiency.

Hope that helps you. Keep us posted on your progress.

Adios, David

 

[AzCoupe]

Since he lives only a few miles from me, I've agreed to help him along. I'm getting him a set of 255ci pistons, plus we'll cc the chambers before machining the block. He's going to be using a converted log head at first, until he can save up a little more money, but will eventually swap to an aluminum head. This is also a good oportunity for me, as I'd like to dyno the motor with the converted log head, the log head I'm working on, and finally the aluminum head when it gets installed.

 

[CZLN6]

Howdy All:

Wow!!! Good comparisons Mike. I did run some numbers and came up with some rather high readings with 52 cc chamber head. The alloy and optimal head shape of the new head may tolerate higher CR, but I'd be reluctant to go past 10:1 with a factory head. Maybe with the right cam to bleed cylinder pressures? I'd be inclined to start with a 62 cc chamber head and then judiciously unshroud and reshape before I locked in the shape of the "D" shaped dish in the 255 pistons. I seem to recall Jack helping someone to reshape chambers for more of a "Heart" shaped chamber. That was a few years ago though.

What cam are you planning for this combo?

Adios, David