Compression ratio confusion

[DarkMesa8]

Alright, I've run through a few of those online compression ratio calculators, and for a 351W stock stroke flat top piston minus 6cc for valve reliefs, with 58cc heads and a 4.030 bore I'm looking at around 11.1:1 CR.

For the 13cc dish its giving me close to 10.2:1 CR, more of my liking.

Are the flat tops really too much for a 351W motor? Is that the reason why most 351W aftermarket pistons are dished?

It doesn't make sense because summit quoted their pistons for like 9.19:1 with a 60.4cc chamber, while these calculators give close to 10:1 CR for the specs summit lists. Why are summit's numbers so low, or why are these calculators so high?

I KNOW its not perfect without actually measuring things, but a full point in CR is a big difference.

I'm trying to figure out if I need to return these dished pistons summit sent me, because I'm going N/A I want a higher compression ratio than what summit tells me with their 13cc dish. If the truth is that 13cc dishes are around 10:1, and flat tops will be too high, than I'll just keep these.

 

[TT91]

Compression is determined by more factors then the above stated. The common mistake is estimating "around" 11.1:1 compression or whatever you're tricking yourself into believing.

The calculation of compression ratios is based on the following formulae:

Compression Ratio = (Swept Volume + Top Dead Centre (TDC) Volume) divided by TDC Volume

Swept Volume = (pi x Bore Diameter x Bore Diameter x Stroke) divided by 4

TDC Volume = Head Volume + Gasket Volume + Deck Volume + Dish/Dome Volume

Head Volume is published or measured in cubic centimeters (cc).

Gasket Volume = (pi x Gasket Bore x Gasket Bore x Compressed Gasket Thickness) divided by 4

Deck Volume = (pi x Bore Diameter x Bore Diameter x Deck Clearance) divided by 4

Dish/Dome Volume is published or measured in cc.


SO, now that you have all that info, taken from Google himself, you need some more info before you go stabbing away in the dark at your final CR.

Bore size?
Stroke size?
Head Gasket thickness and bore?
Combustion Chamber in cc?
Piston Dish?
Piston Deck clearance?

 

[DarkMesa8]

Compression is determined by more factors then the above stated. The common mistake is estimating "around" 11.1:1 compression or whatever you're tricking yourself into believing.

The calculation of compression ratios is based on the following formulae:

Compression Ratio = (Swept Volume + Top Dead Centre (TDC) Volume) divided by TDC Volume

Swept Volume = (pi x Bore Diameter x Bore Diameter x Stroke) divided by 4

TDC Volume = Head Volume + Gasket Volume + Deck Volume + Dish/Dome Volume

Head Volume is published or measured in cubic centimeters (cc).

Gasket Volume = (pi x Gasket Bore x Gasket Bore x Compressed Gasket Thickness) divided by 4

Deck Volume = (pi x Bore Diameter x Bore Diameter x Deck Clearance) divided by 4

Dish/Dome Volume is published or measured in cc.


SO, now that you have all that info, taken from Google himself, you need some more info before you go stabbing away in the dark at your final CR.

Bore size?
Stroke size?
Head Gasket thickness and bore?
Combustion Chamber in cc?
Piston Dish?
Piston Deck clearance?

Bores size is 4.030
Stroke is 3.50
Head gasket thickness is standard Felpro gasket
Combustion chamber cc is 58
Piston dish is -13 for my dished pistons, and -6 for the flycut flattops.
Deck is decked .010.

Can anybody please tell me if I need to go dished or flattop with my 351W?