At what point (rpm or hp) does it become necessary (or recommended) to go to 4-bolt main caps?
How much strength difference is there between splayed caps and straight factory style 4-bolt caps?

 

Main caps seldom crack- usually the crank gives way before.

4 bolt main is to hold the cam better, not to make the actual cap stronger.

at least this my understanding. hopefully someone will correct me if i am wrong.

 

There's no magical number. RPM doesn't tell you everything and it's not the barometer of impending disaster. If I were to use lightweight pistons, wristpins, rods then it stands to reason that the rpm can increase and still provide a safe home for the crankshaft whereas heavy components might put the crank out of it's misery.
One very successful dirt track race engine shop here local uses a 500 hp rule. Anything under 500 hp stays with 2 bolt caps (w/studs of course) where as anything over that (their 550 hp motors and up) get nodular 4 bolt caps.... steel being optional $.
Dirt motors are tough on parts so using the same rule of thumb for a street engine is pretty safe. I personally have used 2 bolt caps in n/a bracket race engines with ~600 hp and never noticed abnormal bearing wear. Of course good machine work and precision parts plays a big part in longevity.

So I'd say that for a 500 hp street motor... 2 bolts are fine. Anything above that and I'd be looking for a 4 bolt cap. And the late model (thinner) blocks IMO should stay with the straight bolt caps. Save the splayed for aftermarket blocks.

-Mindgame

 

I agree with Mindgame. Those are good rules of thumb.

 

Thanks for the input.
Do main studs give a significant increase in strength over bolts?

 

Yes, always use studs over bolts when you have the option. Main studs are a great idea with a 2 bolt setup. While they don't necessarily offer more clamping load, they distort the block less.

You can easily mount a good windage tray with the correct studs also.

 

Studs are much more likely to give you the proper fastener strain for a given torque, as they have significantly higher thread quality than a tapped cast-iron hole.

The 505 HP rule-of-thumb seems rather logical. Mindgame's point about good machine work should not be overlooked; good machine work that yields a straight main bore and proper bearing crush will go much further than simply throwing on a set of 4-bolt caps.