why would a small block not produce more hp with aftermarket exhaust, but a 7.4L get up to 30 hp increase over stock exhaust?

There is definetly more power. In my experience anytime you make a motor breath better it makes more power!!

Didnt make it to the lake today, it stormed, gonna try tommorrow. Anyone out there have a small block with exhaust and aluminum intake running a holley carb?
Just curious of the jet sizes being used.

500 RPM, 4 MPH..... I don't think so, Tim.

YEP
Stainless Marine Exhaust, RPM Air Gap Manifold, BG Carb but sent to Nickerson Performance to straighten out. Runs like I stole it.

Found the problem today, pulled all the plugs after a run, found one with cracked ceramic and another that I dont think was firing. Put new plugs in and hit the lake and it ran great.
Thanks for all the input guys!

I'm confused by this too.
Bad tach, speedo ? Using GPS ?

Or did we install lower pitch prop ? Or a ton of slip ?

Using regular gauges to monitor.
Before the changes I could get about 4300rpm and 45-48 on my speedo, this is just a regular speedo, not gps. I have the original review from Power Boat magazine, and this boat outfitted like mine with a 21p ss prop should see 55 @ 4800. After the upgrades with the same prop I can get 4800-5000rpm and between 50-52 on the speedo. I have no idea how accurate the speedo is. Based my observations on the readings of these.

BTW I am using a 21p ss prop