What does it mean to bore over an engine Does boring over provide for any extra performanc?

Answer 1

Boring an engine is the process of cutting the cylinder bores larger.

The reasons to bore an engine are to rebuild an engine that has worn or tapered cylinders, and/or to enlarge them for more performance.

There is a performance advantage to over boring an engine, but there are other factors involved as well. Simply going from a 4.000 inch bore to a 4.030 bore size and leaving everything else equal (compression, camshaft, rod length, stroke, cylinder heads, etc.) is not likely to show up as a huge gain on a dyno in and of itself.

On the other hand, if you're using a block with wear and/or taper in the cylinders then boring it will give you nice round bores and ring seal will be much better, which of course means more power. Another advantage is that when you have the bores finished you should definitely have the machine shop use a torque plate -- this simulates the stress put in the block from the head bolts and means that the bores at the top of the cylinder are perfectly round when the heads are torqued into place, which means better ring seal. They didn't do this procedure at the factory as it's too time consuming and not really necessary for an assembly line engine that makes 195 horsepower and will never see 6000 RPM's (or more).

Another nice benefit to larger bores is less valve shrouding which helps the engine breathe, and this equates into more power. This doesn't mean one should automatically bore a stock engine block to the maximum oversize it can be bored to right off the bat (in the case of a stock 350 small block Chevy casting the maximum bore size is 4.060).

Guys that are after the very last ounce of horsepower are better off with an aftermarket engine block or a GM "Bowtie" type block as they are cast with much more metal thickness around the cylinder bores and can handle overbore sizes that would ruin a stock 350 GM block.

Another option for guys who want to run a big bore small block is the 400 block which starts out with 4.125 bore sizes and can be bored to at least 4.155.

A 400 block is not as strong as the Bowtie or aftermarket block, but it's definitely an option.