Mod Motor Tech Forums - Big Bore vs Stroker

First let's look at the stock modular bore and stroke. The stock bore is 3.552'' and the stroke is 3.543''. That stroke is HUGE already. A 351W has a 3.5'' stroke from the factory. They have an extra 73+ CID on the modular motors. This puts it in perspective on how extreme the stock stroke already is.

Now, combine that with a stroker kit and your piston starts leaving the bore. This is VERY bad at high RPMs as the piston becomes extremely unstable. Stroker motors just don't last. The HP/TQ gains are so minimal as well because the valves are still shrouded. The only time you want to use a stroker is when you have a turbo setup as turbos don't really like to rev high either and can benefit heavily from the slightest torque band increase. The reason these are so popular though is because you can use your stock block and you don't have to resort to using the more expensive aluminum block counterpart like you have to in a big bore.

Big bores are what make the REAL power. We're not just talking about HP either. They make great torque and LOVE to rev (which is just what a modular motor needs). Big bores increase the bore from 3.552'' to 3.7''. This is done by taking an aluminum block, boring out the sleeves that are already in it, inserting new, stronger sleeves and boring them out. There are 2 versions of big bores. One is the dry sleeve which doens't cost that much and can handle roughly 20 PSI on a blower. This is the one you want for a mild blower application or NA application. The other one is a set sleeve where they actually bore out the sleeve of the cylinder and insert a new heavy duty sleeve in. It is called a wet sleeve because the coolant is touching the sleeve; this allows for cooler temps. The wet sleeve can withstand basically anything you can throw at it. Aluminum blocks can be either wet sleeved or dry sleeved. Iron blocks can only be wet sleeved. As you probably guessed, wet sleeving is very expensive (about $1k more than normal dry sleeving).

Now, the reason the big bore benefits is this. On the stock motor, the valves are shrouded. This means that the cylinder isn't big enough to support the heads. Basically, the combustion chamber is bigger in diameter than the cylinder. Big bores fix this problem by unshrouding the valves. THIS, my friends, is the secret to why big bores are making the power and lasting as opposed to strokers. Remember this when you want to increase displacement. Oh, and don't even both with the 324'' big bore/stroker blocks as the rod ratio is all fudge up and they are just a problem waiting to happen.

To add, turbo's love exhaust mass. But certain companies will try to tell you, that strokers are ABSOLUTELY needed. They're not. If you really want to push high HP, go with a 5.4, it is THE BEST solution to that problem. A .020 4.6 can support 1000hp in a tight powerband with a turbo, as long as the turbo is sized according to the engine.

Also, you can big bore an iron block, but as Code mentioned, it must be wet sleeved. This process is somewhat cheaper than starting with an aluminum block, but the Darton M.I.D. sleeves needed are in scarce supply and cost ~1,500 dollars alone. The only time i'd recommend using an iron block in this situation is with a NVH 5.4 block used only in an N/A application. No other instance would such a block hold up to boost pressures. The one good side to this scenario, is that iron will cool much better and evenly across the cylinders than the aluminum counterpart. It will never be as tough and strong though.