How much N/A horsepower will a stock BBC cast iron/aluminum 4" exhaust support?

Id say about 350-400 range.

merc says 750 hp, but i would not

I would say at least 550 N/A power and closer to 600 in a blown application. All this would be if it exits the transom thru tips. If you are thinking about thru the prop then maybe 450 and then you might have reversion/back pressure issues.

Thru transom.

You can't put enough cam into the engine with stock exhaust. It will revert water. I would say 400-425hp. Also, you're loosing power with the stock exhaust. It might make 420hp with the cast exhaust, when in reality it will make 460hp with good exhaust. Good exhaust is also more important in N/A engines.

Griff summed it up perfectly.

What about GLM?

I will be interested to see how much difference headers make on my procharged Pachanga. She was definately making somewhere in the 600 HP range with the stock cast exhaust. I will know come spring!

Here is the deal... I have this 3" Stainless marine system now, made in 1989, old shool, I do not have 2-5000.00 in the budget this year for a Gil, Imco, or stainless marine system, I need to wait one more year at least, I can get good 4" cast stuff for free, main reason is with this system I have now it is almot impossible to do plugs and work on the front of the engines, if I can change to the 4" stuff for free and not lose power I will do it so I can work on this thing easier.

I mean, ther performance of the systyem I have now cannot be good anyway in my opinion, but back in the day it must have been OK if it was made aftermarket?

Like I said, if the cast stuff will perform the same as what I have I will change it.