Please check my math for cr

[LameHoof65]

cc'd my head last night after porting and cleaning up the chambers.

chambers were 53, 53, 51, 52, 53, 51, front to back

average was 52.5 and I rounded to 53

if I mill off 10cc's (I am guessing about .050) would bring the average chamber size down to 43cc's. I took this and added up all the figures per the fordsix manual. At this point I am using some of the figures of the 78' head that
was used in the fordsix manual. My head is a '77 200/250 head.

a) piston cylinder volume 533.33cc
b) deck height volume 6cc
c) using the corteco hi-po .038 head gasket estimate about 7cc
d) chamber size (ave.) 53cc (using 43cc) in the formula
e) piston dish 7cc

this puts total volume at 596.33 and then dividing per the manual's formula by 63= 9.46cr

If this is right then let me know---I just want to know if I am doing this correctly. The only two figures I do not have are the a) piston cylinder volume, & the b) deck height volume. Here I am using the fordsix manuals figures. My deck may have been milled previously as the edge of my pistons seem level with my deck at TDC. I will try to get a depth gauge to get a better reading. Don't feel shy about telling me I am screwing this all up. I did excellent in upper level stat classes but am terrible at general math, go figure!

 

[Hot 6t Falcon]

Darwin,

I don't know why, but when I went to http://falconperformance.sundog.net/compcalculator.asp
I got 9.6 to 1

I changed your combustion chamber to 43. the head gasket to .038. and the deck clearaance to .038

Try it.

 

[LameHoof65]

Your right, I didn't adjust for the gasket on deck height clearance. Comes out 9.6:1. A little hot for me. I also used your calculator using the .052 compressed stock corteco gasket, which I also have and it puts me a little closer to what I want 8.9:1. I would like 9.2 or 9.3. I always forget something, it's that attention deficit my dad always said I had.

Okay, using your calculator if I use the .052 gasket instead of the .038 and bring my cc volume down to 40cc then my compression ratio should be 9.3:1. If i am using the calculator right. : right??

So, how will I know if I will have enough clearance for the valve to piston. Using the 264/274 cam may be a little too much at .450 lift.

 

[Hot 6t Falcon]

43 - Combustion Chamber Volume [cubic centimeters]
.052 - Head Gasket Compressed Thickness [inches]

.038 - Piston To Deck Clearance [inches] ???? DOUBLE CHECK THIS

7 - Dish/Valve Relief/Dome Volume [positive cubic centimeters]

9.2:1 = Static compression ratio

Have you checked the piston to deck clearance. The .025 is Ford's ADVERTISED measurement - not what really is. It probably is more like .040.

Have the pistons ever been replaced? Replacements usually are .010 lower in the bore to allow the machinist to mill the top of the block .010 to make certain it is level. Put a bar across the top of the block and get a piston to the top of its stroke. You can use a feeler gauge to check the gap.

 

[LameHoof65]

Okay, I just went out and checked and the clearance I had on the two center pistons at the top of the deck was around .014 using the method with the feeler gauge. I couldn't get to my crank pulley and I didn't turn the crank I just measured the two center pistons in the full up position. I couldn't tell you if they were on the exhaust or power stroke, but it doesn't make a difference does it? so one was .011 and the other was at .014 + the gasket (say the .038) (I took an average of the 2 and got a .013 rounding off and adding to the .038 gives an overall of .051 of deck height clearance right so I plugged this in:

at 53cc no milling to head it is 8.9:1 (using the .038 gasket)
at 43cc milling at approximately .050 it is 10.2:1
at 50cc milling at (I am guessing .015-.020) it comes to 9.2:1

Can you use the average of the two in the up position?

 

[Hot 6t Falcon]

I ran the numbers again with only .013 deck clearance, 43 cc, and .038 gasket, and I got 10.2 to 1. TOO HIGH!

My best suggestion is to NOT mill the head as much as you planned and use the thinner gasket. The smaller the quench (area of gasket and deck clearance) the better for both power and for quelling "ping" or "ignition knock"

With the .038 head gasket.
the .013 deck clearance, and
a 50 cc head, you get 9.2 (48cc gets you 9.5)

good luck with iit
[/u]

 

[LameHoof65]

Yea, I agree, I think a .010 on the head just for a cleanup finish would probably put me in the 9.0 to 9.3 region. This block must have been decked at one time. Thanks Dennis, I will check and recheck this several times I am sure. Once I get the cam and adjustable rockers, I will do a mock up with the old head gasket (after mic'ing it) and use some play doh to see how close those valves come to hitting. 8)

 

[thinman56]

how do you guess or determine the 'volumetric efficiancy' in the calculator? is 80% an average for these motors, or is it related to other factors, like cam grind, quench volume, etc.? everything else looks like you can measure it.

thanks

 

[XFlow_Fairlane]

man I had a deck height of .055" on my motor (78 block so guessing "taller deck for emissions) with my head I came up with 7.75:1...kinda low and it fouls plugs a bit.....but the turbo should be on in a month or so

 

[LameHoof65]

I don't know how you determine the volumetric efficiency but I do know that control of your fuel and oxygen is most important. Turbo jobs will get you closer to that 90% and above mark and I believe fuel injection will even get you closer but when properly tuned a carb can do a pretty good job as well. When adding boost a mass air sensor and fuel sensor with appropriate gauges at the intake manifold that can measure vaccum would be necessary in modulating that control. As well atomization and cooling of the fuel will add to creating a more dense vapor molecularly thereby increasing power and saving on your fuel..Of course I have read a bunch of stuff on this forum and elsewhere....to say I understand it would be a serious misnomer. Sounds like I know what I am talking about doesn't it......well of course I am only speculating wildly. Gee, I love this forum----because I can spill out my lack of intelligence in the area of mechanics and science and know these folks will accept my foolishness and help me correct my mistakes. So have at it, am I any where close to right???