I will soon be putting forged pistons and rods in my engine and i am not sure which bore to go with. 4.030 or 4.040. What are the advantages and disadvantages of the 4.040 set up? Also what is the best company to go with for pistons in a nitrous set up (200-300hp)? I have already chosen eagle forged H-beam rods i am just lost on which pistons to get. I was looking at a set of ross racing pistons but i think that the compression would be way to much for pump gas. They were 12.5:1 on a 64cc head.. what would that be on a 58cc head milled 30thousandts? Thanks for the help.

 

My current motor is 0.040" over, but that is not my preference. When I did the rebuild I couldn't locate a fresh block conveniently and the existing block was already 0.030" over. Stock blocks have thin cylinder walls. In a high-po application high cylinder pressure will cause excessive bore distortion which results in poor ring sealing and lost power. This year I am starting with a "virgin" block and will go 0.010" over.

Rich Krause

 

Save the cylinder walls.... the extra ci is not worth anything. I went 0.020" over on my "381" - based on cylinder wall thickness measurements, desire to keep the walls thick for a heavy shot of nitrous, and the ability to rebuild in the future.

 

Thanks for the help.. looks like i am going to keep the walls as thick as possible. Now what brand piston should i be using? and what compression ratio should i try for.

 

I have had good luck with the JE "Extreme Duty" line, though I wouldn't recommend them for a high rev setup (they are heavy). There are lots of other good pistons out there.

CR is a big topic. Suffice it to say the 8.5-9:1 is a good range for boost and pump gas.

Rich Krause