flat tops and stock head compression....

Stang_1973 · Nov 5, 2004

Well the first time I had my engine done, the mechanic was able to obtain flat top pistons for me. They were forged but he didn't give me a brand name or other info. So My question is What kind of compression do Forged flat top pistons usually render? I have stock heads but i dont know what size combustion chambers.

66P51GT · Nov 5, 2004

Need more specifics such as compression height, deck height, rod length, stroke, bore, gasket thickness, valve relief size in pistons, head combustion chamber size. Without these, you are just guessing.

Using the following specs:
Bore 4.040
Stroke 3.00
Deck 8.206
Rod Length 5.090
Pin Height 1.600
Head Gasket Size 4.100
Compressed Head Gasket Height 0.041
Piston Valve Relief Volume 5.000

Head Volume: 56.000
Compression: 9.6

Head Volume: 58.000
Compression: 9.4

Head Volume: 60.000
Compression: 9.2

Head Volume: 62.000
Compression: 9.0

Head Volume: 64.000
Compression: 8.8

brianj5600 · Nov 5, 2004

How much would 0 decking it change compression?

66P51GT · Nov 5, 2004

~4 tenths
so if it was originally 9.0:1 it would be 9.4:1

This is changing the deck height from 8.206 to 8.190. Again, this is using stock components which he is not. The .040 oversized pistions could have a different compression height. Best thing to do is mock it up and measure.

HistoricMustang · Nov 5, 2004

I am guessing if they were forged then they were 10.5 to 1 but that is a guess based on the limited information. Believe the HiPo was forged 10.5 to 1 and that is what I run in my ride and they were made by TRW.

HistoricMustang
www.historicmustang.com

Stang_1973 · Nov 5, 2004

Thanks guys.. reason I was asking cause as you see in my sig. I want to get the energizer cam from Crane cams. Their 272 series cam recommends 8.7 Comp. I want to know if there is a chance that i would be under that with my flat tops and stock heads. I eventually plan on getting some GT-40 heads but it will be a while before I can.

jeffnoel · Nov 6, 2004

I'm going to assume that the original engine was the 302 2V. If so your heads have a compression chamber volume of 58.2 and with stock dished pistons had a compression ratio of 8.0. So going to the flattops probably bumped it into the 8.5-9.0 range.

D.Hearne · Nov 6, 2004

HistoricMustang said:
Believe the HiPo was forged 10.5 to 1

HistoricMustang
www.historicmustang.com

Aw, come on now Historic, I know you know your history better than that The Hi-po's had cast pistons.

D.Hearne · Nov 6, 2004

66P51GT said:
Need more specifics such as compression height, deck height, rod length, stroke, bore, gasket thickness, valve relief size in pistons, head combustion chamber size. Without these, you are just guessing.

Using the following specs:
Bore 4.040
Stroke 3.00
Deck 8.206
Rod Length 5.090
Pin Height 1.600
Head Gasket Size 4.100
Compressed Head Gasket Height 0.041
Piston Valve Relief Volume 5.000

Head Volume: 56.000
Compression: 9.6

Head Volume: 58.000
Compression: 9.4

Head Volume: 60.000
Compression: 9.2

Head Volume: 62.000
Compression: 9.0

Head Volume: 64.000
Compression: 8.8

You've hit the nail on the head Most guys just assume they've got a certain comp ratio, when chances are it could be a full point less, depending on the comp height of the pistons they bought. Compression height on 302 flat tops varies up to .030, making piston selection more involved than just grabbing the first thing off the shelf.

flat tops and stock head compression....

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flat tops and stock head compression....

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