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The 520 and 525 are similar. The 520 manifolds are the same as the 496 manifolds, and similar in size and shape to a stainless marine gen 2. If a 520 can gain 48hp in the upper rpm a 525 can lose at least 20-30

That is close to what I seen, the long tubes start breathing better as rpm goes up. Its funny everyone is always talking about the best intakes but then when it comes to getting all that out slap whatever on. What goes in has to come out.

Many years with small motorcycle motors and exhaust was everything.

Thanks for the info. :thumbs

I would love to see how it was set up on the dyno.

Me too, I was ready to buy all new stainless marine or Hardin exhaust at half the price. When I found this sheet I sucked it up and bought new cmi headers. My boat runs 91-92 most days, not willing to drop onto the 80’s over 5-10k. Most on here would spend that to gain 3-5 mph. Why save it to give it up.

Sorry this took a while, the Skater is all covered up for Winter, but here's a pic of the drain system we installed on the 500EFI that's going in my Party Cat right now. The line off the bottom of the T attaches to a single thru-hull fitting mounted in the transom, below the height of the header drains.
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Yes, they're constantly dumping water from both headers out of a single -4 line. I didn't notice any drop in water pressure on the 700s in my Skater when we installed them. It's such a small volume of water relative to the large volume being supplied by the pump that it wasn't noticeable on the gauges. As others that have posted mentioned, it's easy to put a ball-valve in the line going to the thru-hull fitting if you do see a drop in pressure.

Can someone explain me how do you pressure test them when they are on the Engines please?

I’ve done it with compressed air, regulator with air line fitting, then a header to tail pipe jumper cut in half on other end of regulator. Attach that to either the water in at the rail or out at the collector. Cap which ever one you don’t use. A rubber barstool foot with a hose clamp works fine as a cap. Apply air and watch the regulator and listen.

That’ll get you lost of the way there but it’s also recommended that you test with a heat source. You’ll have to pull the headers to do that, but then you’ll know for sure.

I have a large tank that I dump my etops into. They’re connected to air and a gauge. I use the rubber distribution tube and a rubber cap on the jumper. Pressurized to 35-40 lbs and in they go. I watch for bubbles and a drop in pressure. Once that test is good, I pull them and then apply heat via a propane torch to the flange welds where the pipes come into the collector.

The headers are twenty-two years old and no leaks so far. When the engines went back in this time, I added ball valves to drain them into the bilge. I sleep much easier now.

35-40 PSI? what pressure do you have when running. I like the heat idea. I tested mine off but with water, we have 35 lbs here so I used that but didn't go to 35, I think I stopped at 20-25.