JAdamz

Hey folks. I'm interested in gaining a couple extra MPH on a Formula 330SS with 7.4L 454's spinning Bravo 3's. The boat came with through-prop exhaust only.

I'm considering installing switchable exhaust on the boat using both IMCO's Powerflow manifolds and their Sound Advantage switching system, and I was just curious if I could realistically see a decent horsepower/speed increase with the upgraded headers and move to through-hull exhaust.

Thanks!

Pismo10

I got about 3mph which in my single engine Donzi 22 Classic is maybe 30hp.

deboatmon

Gaining 1 mph per 10 hp is most likely the very rare exception rather than the standard. The DONZI may be a nice exception where others might not be so responsive. Gaining 30 hp changing from prop to transom is highly exceptional as well. I will certainly not question anyones gains but the typical gains are probably a lot less. Most people with stock, mild engines gain little if anything changing from prop to transom exhaust other than a more high performance sound. I say this based on 3 years of OSO forum discussions on this very same subject. The good thing is that it's a nice place to start high performance mods.

JAdamz

Yea, I wasn't really expecting a major difference in performance. The 330SS is a pretty heavy boat so it probably would take some decent HP gains to get it moving faster.

The upgrade to through-hull is in part for the "cool" factor of open exhaust, but I figured maybe I could net a little extra speed in the process too...

cuda

My buddy went from thru prop to thru hulls on a 21 Scarab with a 350. He said he picked up maybe 100 rpm.

Pismo10

My experience is by accidentally not opening the silent choice at full throttle and not being able to get over 71-72mph on GPS, then flip the switch and hit 74-75mph gps immediately. Kind of an accidental experiment. 502 Mag in a 3500lb Donzi 22 Classic stock Bravo drive.

Outdrive1

If you are running Bravo 3's you are probably wasting your time. Those drives seem to hit a wall around 55 mph or so. They are real efficient out of the hole and through the mid range but reach a point where they can't really move any more water. I would consider changing to Bravo 1's before I spent any money on any other parts. Start a thread about the performance of a Bravo 3 and get some more opinions.

Secret Formula

I don't know what the HP increase is but I just gained 4 mph on the top going from baffled through hull tips to open through hull tips. This on a Formula 223SS with a Mercruiser 502.

apollard

I measured the exhaust backpressure on my 7.4 BIII - 2.5 psi @ 4700 rpm. Dropping that even to zero isn't likely to help much. Especially since you are out of cam & heads there anyway. Moral of the story: stock exhaust is adequate for stock engines.

Remember that on later model BIII, the exhaust doesn't really go through the prop - take a look at the exhaust tube when trimmed for normal running, the extension from the drive doesn't even go in the tube. Lots of open area there. (early models used the bellow IIRC)

Cool factor is another story, but you won't gain anything measurable on a stock engine.

mysweetone

I too am switching from prop exhaust to a Rex Maring captains call exhaust system this winter in my 216 Wellcraft Eclipse for the "cool" factor. Not expecting any real performance gains but if it happens... hell I'll take it!