OK, I know this has been done to death but I still can't figure it out. I've tried to calculate it based on the fordmuscle.com calculator but I don't have enough info. I've got a 72 stock short block w/ late 90s iron GT40s heads that have been milled .05. From what I found the stock combustion chamber size on these was 61cc. What I don't know how to figure out is how much this changed by milling the heads. I used standard felpro head gaskets which I think are around .04 thick. One thing I don't know is the piston volume of the 72 stock pistons. The example they gave was -6 for dished pistons but I really have no idea how to find this out. Thanks for any help.
You would have to CC the chamber with the heads off. I wish I could remember how much it changed mine. Too long ago. Is that number right at 50 thou. milling?? Or did you mean 5 thou.?? I think you may be out a decimal place.
No I'm almost positive he said 50 thousandths. Does that seem way off? It sounds like I would have to pull the heads to be exact. What about ballpark, when I used the numbers I do have and their generic numbers I got 9.2:1. I don't it matters that much but I'd like to know, ya know
If he milled the heads .050, then your chambers are in the neighborhood of 52cc. .015 equals about 2.7cc. These are approximate based on calculations of an engine I'm building. Rule of thumb for compression ratio is that removing .060 of material from the block or the heads raises the compression ration by 1. I.e., your engine has 8.5 to 1 CR, you shave .060 off the heads, your new CR is 9.5 to 1. By the way, shaving a head is cheap way to raise compression, but not efficient. You're better off to deck the block or change pistons. A 9.5 to 1 CR with 60cc heads will make more power than 9.5 to 1 with 48cc heads - more volume for fuel and air. My opinion.
I am glad this question came up, since I wanted to know if you can calculate CR based on the psi of the piston doing a compression check. Couldn't you just do a compression check and divide by 14.7 and get a pretty close approximation of Compression Ratio? At least for those of us near sea level? I know, probably not that simple, but it seems that there should be a formula for converting compression readings to compression ratio. Open for correction, as always...
It is way not that simple! here are just a few of the things that affect compression readings: The spring pressure in the valve of the gauge and it's effective seal. The seal made by the rings and valves. The opening of the throttle while measurements are being taken. The overlap of the camshaft. The barometric pressure. On top of all that you can't divide the pressure by atmospheric and come even close to the compression ratio because it is not a linear progression.
The easiest way to get an accurate compression ratio is to tip the engine so a spark plug hole is the highest point and place a fitting with a graduated cylinder on it. You then fill the cylinder completely with oil with the piston at bottom dead center. Take note of the number of CCs it takes to fill it, then stroke the piston to top dead center abd read the amount displaced. Your compression ratio is the total amount divided by the difference between total and displaced amount. Example: 685 CCs total to fill the cylinder and chamber ..............625 CCs displaced (difference = 60 CCs) .............. compression ratio = 11.4:1 It does take some special equipment but it is the most accurate way to find your exact compression.
I knew it wasn't simple nor linear, which is why I asked if there was a "formula". I like the liquid-fill idea. It cancels out all the other variables. But, what is more important...static compression or dynamic compression ratio?
Simple Wiki search found this...And what I was describing was more of a pressure ratio, not compression ratio.
Unless your GT40 heads came from a 93, Cobra, they will now cc at 61. Only the 93 Cobra GT40 heads had a 61 cc chamber, the rest are 64. The Cobra's heads were milled extra an extra pass or two before assembly. The true chamber volume can only be determined by actually cc'ing the heads though. On the E7 heads a .040 mill removes 6 cc's volume. That's going to be true with GT40's too as they shared the same chamber
At .050 removed It just popped into my head about the intake match. Cant remember the numbers for cutting the intake vs heads. You need to remove intake material to keep the gasket contact correct. I'm sure someone else has the numbers. Gotta love this site. Just look at the replies to this thread.
The max you can mill before running into intake fitment problems (the bolt holes fail to line up before you have gasket sealing problems) is .040. If he's got 93 Cobra heads, then that amount will be less. As for needing to mill the intake, first you must do a trial fit to see if anything needs to be milled. Sometimes it doesn't. I milled my Canfields .060. I had to mill a Vic Jr to fit on one motor, but on another block with my 3x2, nothing needed to be milled, but the intake bolt holes needed to be wallowed to compensate for he head holes moving toward the center.
I didn't have any issues w/ bolt holes lining up, used felpro gaskets on the intake w/ rtv at each end. I'm honestly not sure if they're 61 or 64 cc chambers. In any case these are so much better than stock and I got them pretty cheap so I'm happy w/ them, I was just curious about the compression ratio. I'm trying to get some dyno time lined up so I can dial in my carb and timing, and getting some actual numbers for the engine. I got most of my set up from the same guy, he stripped his short block to rebuild the top end and sold me everything off it. These GT40s, weiand stealth, 1.6 roller rockers, edelbrock 650, unilite distributor, and a hyfire 4. Only thing I bought was a cam and pushrods, his was a roller block. Comp Xtreme Energy 218/224 .493/.500 hopefully help the exhaust flow. I put a T5 behind it and I'm hoping for 250-280 at the wheels. Thoughts?
