Maximum safe overbore for 4.6L Iron block

[crimsonangel]

Hey everyone, i'm sort of a little new to all this so forgive me if I made any mistakes. my question is what would be the maximum safe overbore for the 4.6L cast iron block. This summer I plan to completely rebuild the block and I am going to need to overbore anyway since one of the cylinders has a tiny chip in it. I was hoping to reach around a 5.0L displacement from stroking and overboreing. Now I have been getting mixed answers in my search for a max overbore size some people are saying .02-.03-.04 however, I have also seen people saying .06 to .07 being as high as they went, that they just needed better cooling at that bore. What would you guys say as to a max safe overbore?

As to attaining a 5.0L displacement
If a .06 overbore is possible that would make the bore diameter 3.612 radius of 1.806
5.0L = 305 CID
305/(pi(1.806^2)8) = 3.721 would be the stroke I would require

 

[bhuff30]

Build for airflow, not displacement. The difference in bore size will be negligible. Even stepping up to a full 5.0 liters, you will just make basically the same power earlier in the power band. Hence, build for airflow. It is a whole lot cheaper and more productive.

 

[trinity_gt]

I think 0.060" or higher is dangerous simply because the walls become thin. If you are going for that sort of bore you might as well sleeve the thing for a 3.7" bore. Typically on these engines the higher displacement is done with stroke. A 3.750" stroke would put you around 298 cu-in. With a stock 3.552" bore you could get:

Stock you get: 8*(PI*3.552^2/4)*3.540 or 280.6 cu-in.

With a 3.75" stroke you get: 8*(PI*3.552^2/4)*3.750 or 297.3 cu-in.

With a 0.020" overbore you get: 8*(PI*3.572^2/4)*3.750 or 300.6 cu-in.

This is a good way to go because there are cranks on the market already with that stroke and pistons and rods available for that dimension of stroker crank in "normal" bore dimensions (stock and overbores.)

But bhuff is right: the cylinder heads and small overall bore size (necessitating small valve sizes) is really what hurts these things normally aspirated and even after adding stroke you're still trying to pull air through a very thin straw... You might consider going to forged internals on the stock size and just bolting a blower up. Even with just 281 cubes to work with a blown 2V can be a lot of fun on the street.

If you've got cash then a blown stroker would be that much better

 

[Midnight2V]

Adding a power added is a much more effective way to go as the others have said. However, if a significant increase in displacement is what you are after, you might as well go for a Boss 5.0 modular block. It's not much more expensive in the end than punching out your stock block. It will be a lot stronger than a bored over stock block as well, and can support well over 320 CID.