Expected Horsepower Increase From Through Prop to Through Hull?
 

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!

Re: Expected Horsepower Increase From Through Prop to Through Hull?
 

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

Re: Expected Horsepower Increase From Through Prop to Through Hull?
 

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.

Re: Expected Horsepower Increase From Through Prop to Through Hull?
 

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...

Re: Expected Horsepower Increase From Through Prop to Through Hull?
 

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

Re: Expected Horsepower Increase From Through Prop to Through Hull?
 

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.

Re: Expected Horsepower Increase From Through Prop to Through Hull?
 

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.

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.

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.

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!

I thought I remembered when I had a 1989 24' Sunsation Rocket in the early 90's when I switched from thru hull with no mufflers to thru the prop the difference was like 1mph. This was with a stock 454 330hp bravo 1 and the boat was 7ft beam and about 3200lbs. More than likely on a larger heavier boat the change will be not measurable.

Bravo 3 is good to a solid 65 mph before they reach their limit of effectiveness.

Leave it alone. Boat cheaply. More power to you. Besides, soon enough we'll "have" to have mufflers anyways. Switchable exhaust doesn't count. Or you could spend that cash on a K*** A** stereo and throw in a recording of a Top Fueler.

Exhaust = $$$$ + stereo to keep up = $$$ + noise ticket = $$$ + new mufflers = $$$$$$$$$$$$

Isn't boating fun?

The stock "Y" pipe exhaust on big block Mercruisers is good to about 350hp. If your motor is less hp than that, you will likely get zero performance increase by going thru transom.

I don't really like seeing a slow boat making loud nascar noises.

If a boat will haul tail, then sure let's make it loud. But our lake has a lot of 45 mph boats with loud exhausts. About the only good thing I can say about that is that the swimmers will hear them coming far enough away that they can swim out of their path.

MC

Loud pipes save lives.

Loud pipes risk rights..

In my experience, at that hp level and the weight of the boat you will not see any increase in speed at all. The stock thru prop exhaust flows just fine for the mild power of a 7.4 Merc.

Years back, I had a new Baja Sport 280 w/7.4L's It was ordered with thru prop exhaust. I had installed the Corsa silent choice system and saw zero gains in speed.

At the time, Gil Marine was advertising speed gains of up to 7 mph with their "Magnum" system. I spent close to 6 grand and still saw no increase in speed.

Unless you are adding more hp, there is no need to spend the money.

...personally, I dislike boats that make lots of noise when there is no need to. Loud, slow boats are not cool.:drink:

My wife and I purchased a 2000 330SS and had Bravo I installed. After rebuilding the 454's to 489's we added some CMI tips and diverters after the stock exhaust manifold. Switching the diverter from below water exhaust to above water exhaust gave us about 0.2 on the gps. If you look at the tack very closely you can almost see the needle move.

It'll sound quicker, so you'll convince yourself there's a gain.

that is what I have read as well,,makes sense since the only people saying they picked up mph have 502s
which make more hp than that.

You will Think you are going faster but with that hull and power it is a waste of $ My 370ss 496 HO's runs 54-55 open or closed Fuel load and wind will have more of an impact