I have a stupid question on pistons for you guys: Flat top pistons: are they completely flat on top, or are they still notched for valve clearance but sit higher than stock? I'm just not sure what got put in my engine....i bought flat-tops for a .030 over 302, but the machine shop had to go .040 on the engine and swapped in a set of their own pistons. Also, how do you figure compression ratio from the numbers? I'm getting compression at about 140. Thanks!
the 140 you describe is only cranking pressure. It is affected by many, many variables....most influencial is cam timing. Some 14:1 engines can have as low as 150 psi, but my 8.1:1 turbo motor has 195--go figure! Most flat tops for a 302 have 4 valve reliefs in the top of them, though some mid-80's 5.0's have true flat tops--but that limits those particular engines to using the stock cam and stock-sized valves. Hope this helps
Thanks for the response! So chances are that they did put in the flat top pistons like i originally had. That's what i was hoping! So i guess the only way to determine what compression ratio i'm getting would be to do the math.... With flat tops and a chamber size of about 58 or 59 (D0OE heads, 60.4cc chambers, milled an unknown amount), i should be getting around 9.5:1? (Block was not milled and it is not a tall deck).
Compression Ratio COMPRESSION RATIO = (swept volume + total chamber volume) total chamber volume It is important that we understand two terms and their relationship to compression ratio: Swept Volume and Total Chamber Volume. Swept Volume is the area the piston travels through from bottom dead center to top dead center. Total Chamber Volume is all the area above the piston at top dead center. This would include the area above the piston in the cylinder block, the area of the compressed head gasket, the combustion chamber, the valve pocket, and the dome of the piston. The compression ratio is the relationship of the swept volume to the total chamber volume. To start, we need to know the Swept Volume of one cylinder. The size of one cylinder is figured in cubic centimeters. Swept Volume (cc) = cylinder bore diameter (inches) x cylinder bore diameter (inches) x stroke (inches) x 12.8704 EXAMPLE: Cylinder Bore Diameter = 4.000" Stroke Length = 3.480" Swept Volume = bore x bore x stroke x 12.8704 Swept Volume = 4.000 x 4.000 x 3.480 x 12.8704 Swept Volume = 716.62 cc To get the Total Chamber Volume, several things will have to be taken into account: Cylinder Head Combustion Chamber Volume: Find the number of cc in the cylinder head. Piston Valve Relief Pocket Volume(s): Valve relief pockets add combustion chamber volume. Head Gasket Volume: Head gasket thickness adds combustion chamber volume. Deck Clearance Volume: If the piston is above or below the deck of the block, this must be taken into account. Total Chamber Volume = chamber volume + valve pocket volume(s) + head gasket volume +/- deck clearance volume. EXAMPLE: Cylinder Head cc = 72.18cc Piston = flat top with two valve pockets that measure a total 4 cc. Head gaske = 4.000" round and .038" thick when compressed. Deck clearance = The piston at top dead center is 0.010" below the surface of the deck. Gasket cc = bore x bore x compressed thickness x 12.8704 Gasket cc = 4.000 x 4.000 x .038 x 12.8704 Gasket cc = 7.83 cc. Deck clearance volume = bore x bore x deck clearance x 12.8704 Deck clearance volume = 4.000 x 4.000 x 0.010 x 12.8704 Deck clearance volume = 2.059 cc Total chamber volume = 72.18 + 7.83 + 4 + 2.059 Total chamber volume = 86.07 cc Now we are finally ready to calculate the compression ratio! EXAMPLES: Swept Volume = 716.62cc Total Chamber Volume = 86.07cc Compression Ratio = (swept volume + total chamber volume) /total chamber volume Compression Ratio = (716.16 + 86.07)/86.07 Compression Ratio = 9.33:1 The cranking pressure changes with the cam lobe overlap. As the lobe separation angle increases so does the cylinder pressure. ie-less overlap between the intake and exhaust valve
I went to that site and pluged in the numbers and I get 10.03. head chamber 59.0 cc piston chamber 2.0cc (the valve releifs) head gasket thickness .041 head gasket bore 4.100 deck clearence 0 (flat tops) stroke 3.0
I think Tom is correct, my only question is do the pistons actually go all the way to the top? ie-zero deck clearance. The only way to calculate that is to add 1/2 the stroke (1.5) + the rod length (5.090) + the compression heigth of the piston. Subtract that value from the deck height (can't remember what a 302 is, and it is different for 73-77) and you will know if the piston is in the hole or flush with the deck.
That would explain why i have relatively low cranking pressure, my cam has a fairly large overlap, 110deg. Thanks for all the info guys!
