Bore to Stroke Ratio
How is this calculated? Do sbm's have a good bore to stroke ratio? I know that they have a awesome rod to stroke ratio. Machine shop suggested having the main bearing bore alignment checked and corrected if needed and then measure the block deck height and have the block decked and squared. Also he suggested having the longest rods fitted to the shortest ch pistons. Is this procedure worth it or a big waste of time and money?
The easiest way to look at bore to stroke relationships is over square, under square and square.
A square engine is when the bore and stroke are the same number, or close to it. An example would be a 4.030 bore and a 4 inch stroke would be a square bore to stroke ratio.
An example of an under square engine is a 3.940 bore (.030 over 318) and a 4 inch stroke. That’s over square because the stroke is longer than the bore is in diameter.
An example of an over square engine is a 4.04 bore (standard 340) and a 3.313 stroke. The bore is bigger in diameter than the stroke is in length.
As a general rule, an over square engine will make more power. An under square engine will be intake valve diameter limited and therefore induction limited and they produce diesel like torque curves but they will always be down on power and RPM compared to an engine with an over square bore to stroke ratio of equal displacement.
A square engine is in between.
All this means is that as the stroke increases in relationship to the bore diameter the engine will be more induction limited. You can’t fit a bigger intake valve in a smaller bore. And a smaller valve means a smaller port. The smaller port won’t keep up with the air demands you might require. That’s why they get diesel like torque curves but they don’t make power and we all know what that means don’t we???
There isn’t a block out there that doesn’t need to be decked. They should be square decked. It’s not hard to do. Look at it this way. If you bought a set of pistons and half of them had the wrist pin .010-.012 lower than the other 4 (or more…I’ve seen blocks that were .020 off from end to end and that much side to side) wouldn’t you be pissed? You’d call that junk right? So would I because that’s what it is. Junk. So is not squaring the decks.
You are limited on rod length to 6.123 unless you want to turn the rod journals down to 2.100 (SBC large journal size). If you do that you can find longer rods than 6.123 plus you have a much better selection of rod bearing available.
Just like rod length, you are pretty much married to what ever bore diameter Chrysler gave you unless you get an aftermarket block. And any more, you don’t have much of a choice in stroke lengths either. You get what Chrysler gave you or you get a 4 inch stroke. You might still find a 3.79 stroke crank around, but it’s a beeotch getting pistons for them IF a you want to buy your pistons off the shelf. If you don’t mind ordering a custom piston then the 3.79 stroke crank is a good option.
As a general rule, I’m not a fan of over square engines. You can make a bunch of torque but what does that get you? You can smoke the tires easier?
But this is all just mental masturbation because for a 12.5 second car you can use a correctly spec’d and built 318 and do it all day long. Or a 340 or a 360. You don’t need a stroker crank. You can run in the 11’s and not square deck the block (BTDT). You should at least deck the block (you wouldn’t rebuild a set of heads and not deck those would you??? I know I won’t do it) so that should be done. Then to get the decks equal isn’t much more work. Either way the block should at least be decked.
