I have a 300hp/350 I'm currently rebuilding. It is bored 0.30 over & has the stock 69 300 hp heads & Silvolite flat tops. I am going to run a comp cams 275 deh camshaft.

What do you think my compression ratio will be with this?

You need more information. Head gasket thickness, deck height/piston piston position, head CCs, do the flattops have reliefs or eyebrows and if so how many CCs?

Formula for CR

CR = COMPRESSION RATIO = CYL. VOLUME @ BDC ÷ CYLINDER VOLUME @ TDC

= 1 + (SWEPT VOLUME ÷ VOL @ TDC)
= 1+ (0.7854 x BORE x BORE x STROKE) ÷ (CCV + HGV + PDV)
CCV = Combustion Chamber Volume, in cubic inches

Note: if volume is given in cc’s then ÷ 16.4 to get cubic inches.

HGV = Head Gasket Volume, in cubic inches,
= Head gasket compressed thickness x 0.7854 x bore x bore
PDV = (Piston Deck Volume) + (Piston Dome Effective Volume)
= (0.7854 x bore x bore x deck to piston distance) + (volume of piston depressions - volume of piston bumps)

Example: What is CR of the engine in #9 if heads have 72 cc chamber, head gasket is compressed to 0.040 inch and flat top pistons give 0.025 deck clearance at TDC?


CCV = 72 ÷ 16.4 = 4.39 cubic inches
HGV = 0.040 x 0.7854 x 4.030 x 4.030 = 0.51 c.i.
PDV = 0.025 x 0.7854 x 4.030 x 4.030+ 0- 0 = 0.32 c.i.
CR = 1+ (0.7854 x 4.030 x 4.030 x 3.48 ÷ (4.39 + 0.51+ 0.32))
= 1+ (44.39 ÷ 5.22) = 9.5 CR

I haven't bought my head gaskets yet. The pistons do have valve reliefs, but I don't know the cc's. This engine had a pretty fresh rebuild on it when I tore it down. I tore it down because I have my car apart and figured I might as well go through the engine while it's out of the car.

The pistons are like new and the cylinders checked out really good, so I'm going to re-use the pistons that were in it.

I went to silvolite's website and they list a 350 flat top piston that says it's 9.7 to 1.

I would like my engine to run good on pump gas. I just wondered what this combo would be.

http://www.csgnetwork.com/compcalc.html try this.