BBC Thru hull exhaust gains?
 

After copying the “330 on Roids” build I still have the thru hub exhaust. I boat between 3800 & 6000’. At this elevation & the mild motor should I expect any gains w/ thru hull exhaust? Would I gain any RPM or speed or just the sound?

Exhaust upgrades with a stock engine will gain you a little. But if you have a performance engine it is required to keep it alive.

I did not read your question correctly last night. I thought you were talking about a exhaust manafold upgrade. Thru Hull exhaust would be a good upgrade. I know 2 people that have done it and you may see a 1 or 2 MPH increase. Make sure you have the right tools a 4" hole saw can be pretty tricky. I am not sure about the elevation factor.

I had a 1989 Sunsation 24' Rocket that had the stock 330hp 454 Bravo 1 package with silent choice exhaust. If I remember the difference on top end between thru the drive or thru the transom was between 1-2 mph also.

Don't fo'get the BIGGEST benny: You'll sound a lot cooler too!

I put mufflers on my through hull exhaust and lost 4-6mph. Took em off and the speed came back. Bet you'll see at least 2mph!

Finally getting around to this. Will I need to rejet w/ the less restrictive thru hull?

If you are running at the same elevation and the only change is the through transom exhaust I would think your jetting would be fine. After checking for any leaks and certain all is in good order and oil temp is up - run it WOT for no more than a minute. imeadately shut down and pull
# 8 plug and check it. Be sure and put in a new set of plugs before the test.
john

make sure where you are boating doesn't have noise restrictions, or your quest for gain can end up in a loss.

Help
Didn't notice the down tube on my diverters are only 3.5". I now need some 3.5" hose but the local supply store is out. I once read a post where a guy used dryer vent hose? I'm leary of trying this if only for tomorrow. Has any one else tried the dryer vent option or have any other options I may be missing.

Worst case, I could block off the tips & use the stock through prop parts. Going to the lake in the morning so I'm running out of time.