OK so I hope this is my problem, hopefully one of you can confirm. I have an 89 Formula 242ls. I just had a new big block built for it making 540hp. Anyway I was told the stock exhaust wouldn't flow good, so I found a good old set of merc racing 420 cast manifolds. Now my question is there is only water inlets at the bottom of the manifolds, none in the risor. I was told all I had to do it block off the risor coolant pipes. I did that and there was not much but there was water flowing out the exhaust so I figured it must be ok. Anyway I ran it and the big hose coming out of the water pump was very hard and poped. Is that because I restricted flow and it's only coming out when the thermostat opens??or is something else wrong?? WHAT THERMOSTAT DO I NEED?? HELP​!! I'll try to post pics.
I can't figure out how to post pictures
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Whoever told you that doesn't know what they are talking about.

What risers do have??? Merc 420 manifolds flow water from the manifold to the riser through a hose, so how you got any water out them is amazing.
It also depends on what type of thermostat housing you are running. Look on Genuine Mercury & Mercruiser parts for a plumbing diagram for a 420.

Griff. They are the old wet joint style. I'll have to get the part number off them. I can't figure out how to post pics of them.

you have to be a paying member to post pics. these are the 6 bolt risers? they have a rep for leaking. these guys? Genuine Mercury & Mercruiser parts. Exhaust Manifold / Elbow(Mercruiser System) now whatchu mean block off riser coolant pipes? sealing the holes so they are dry joint? or closing off where water goes out to the exhaust tips. cuz you cant do either! I love that link griff. saving in my favorites.

That's them the 6 bolt ones. Since there is no Riser water inlet I blocked those off on the thermostat housing I had on my stock 330hp. So yeah definitely thinking that's my issue. Too much pressure building up because there was no where for the water to go when the thermostat wasn't open.

Oh. 4 outlets on thermo housing and only 2 inlets on exhaust. Get pipe tees and run the 2 outlets together on each side. Probably bypass the t-stat a little. Then start thinking about new exhausts. Those made good power, but leaked when they weren't rusty and old. Merc stops using their own in-house stuff and buys from somebody else ONLY when they have to.

Exactly. That's what I was thinking of doing, just wasn't sure if that was gonna create another flow issue. But I'll get a T or Y type of fitting so the 2 go into 1 on each side. Yes definitely want to get a better set of exhaust, just maxed out my budget. I did put them together and pressure tested them.

Here is what I did wrong

This is what I think I need

That is the preferred thermo housing. As long as you are gonna run those exhausts, you can hook the 2 exits together from each side for now. Another thing I would consider. Put ball valves on the 2 water hoses going to exhausts. You shut off the engine, flip valves open and drain the exhausts.