ok im ordering a nice federal mogul/speedpro rebuild kit for my 305 through summitracing. yes they do make kits for them. the part number for the kit is fem-mhp124-000. the only ques i have is it says right clearly in the info about the kit that the piston comp ratio is 8.39:1 but that was calculated with a 64cc head. can anyone tell me what the comp ratio would be with a set of ported 416's that are slightly milled. i know it will be more because the 416's are a 58cc head. will it be a little over 9.00:1 can anyone help me with this. it would be greatly apreciated

 

Just running your raw numbers, the CR they advertised would imply that the pistons have a dish volume of 8.5cc, presuming that they are using a 64cc chamber, standard ring depth (0.025"), standard deck height (0.025"), standard head gasket bore (3.80"), and standard compressed gasket thickness (0.039"). That 8.5cc is consistent with a piston having four flycut valve reliefs on a flat top. Chances are that they are not dished pistons, but you might want to contact Federal to verify that.

Simply changing the heads on that calculation to a chamber volume of 58cc would imply a final static CR of 8.98:1.

You should easily and safely be able to tolerate 9.5:1 on 91 octane if the ESC is funtioning. You can achieve that with those pistons by using a head gasket with a 0.021" thickness, but those can be prone to failure.

A lot of the calculation will be affected by the amount that was/will be milled from the head surface to true it. 0.010" milled off will bump the CR to 9.18:1. Using an 0.031" gasket will further increase that to a net of 9.34:1.

Using a piston with only two valve reliefs (approximately 5-6cc volume) will bump static CR to 9.36:1 with no other changes, or 9.57:1 with 0.010" milled off the heads.

Of course, that is all with a stock bore 305 piston. If you are boring/honing the cylinders to an oversize, the CR will change accordingly. All the details would be necessary to project an accurate final CR.

 

well i looked farther for more info on the pistons and i found that they have a 5.00 dish volume cc. im not sure about the ring depth. my block will have standard deck highth. i was thinking of having it zero decked at the machine shop. would that add more comp? as for head gaskets im using the regular felpro stock replacements, no custom thickness. it is a flat top piston with 4 valve reliefs. for the heads im having 8 to 10 thousands taken off them. it will be stock bore 305 too. just honed. is that enough ifo to help anyone else more. thankyou for your help so far its apreciated