Guessing Compression Ratio with GT40 Heads

[RPB]

I'm in the process of going through my tired 302 and plan to freshen it up a little. I pulled the motor apart and cleaned up the piston - part number E4ZE6110SB/TRW Marking on the under side. I plan using some GT40 lighty ported Heads off a cobra with the 64/65cc . My concern is what will the compression ratio be ?? I read some place 86 gt's had 8;41 compression and believe the e6 heads to be 62cc. I want the car to have 9;1 compression ratio or a little better. I could always mill the heads a little but if it has 9;1 im not going to mess with it ...

[TopGear85]

Here's the formula from Wikipedia. Bust out your calculator:

[Zap's 85 GT]

I wouldn't sweat it. Just bolt them on. They have been installed onto countless engines without issues.

[grabbergreen84]

Shouldn't you check for and verify piston-to-valve clearance on an '86 engine?

[bwguardian]

I am confused, what are you working on? You first state the piston to have a E4 part number but then state it is in an 86 GT. Do the pistons have four valve reliefs on the top?

[mustangxtreme]

He didn't say what year his car was. The reference to an 86 was him mentioning he had read an 86's CR was 8.4 not that his 86 had 8.4

[RPB]

First let me say thanks for all the replys. I bought the car from someone who converted it to a 8 cyl, so I don't know the history on the motor just that it had e 7 stock heads. After cleaning up one of the pistons and it came back as one from an 86 I got currious about what compression ratio that motor had from the factory. I did read someplace that the 86's had 8'4 compression and just didn't want it that low.. The pistons do have four valve reliefs on them.... I thought the flat tops didnt have four valve reliefs ? Im having machine shop check everything over while cutting crank and he is going to let me know what the compression ratio will be.. The GT40 heads might have been milled a little already so just waiting to hear back as of now.. My brother and I will be doing the assembly and yes we will check the piston to valve clearance so there are no surprises..

[bwguardian]

If the pistons have four valve reliefs in them and are stamped with the E4 part number you more than likely have an 84 short block (cast pistons) with E7 heads. The compression ratio in the 84 engines was 8.4 to 1 and the combustion chambers in those heads were huge. Each engine is a little different and will yield variances. I would pay attention to the combustion chambers cc's and try to get it down to around 58cc-60cc volume. This should put you a little above where you want to be. The original heads on that engine were 67cc-70cc and depending on which version of GT40 heads you have (Cobra heads were 60cc-63cc and Explorer heads were 63cc-66cc) will depend on how much you may need to mill them.

[FM2NOTCH]

86 5.0 HO pistons dont have valve reliefs and they were rated at 9.2 compression.
87-93 5.0 HO had 4 valve reliefs and were rated at 9.0 compression
79-85 5.0 mustang engines were rated 8.3-8.4 compression

the increase or decrease usually comes from the head cc's, not always from the pistons

http://www.fiveohinfo.com/fox/engine.html