bobl

One thing to keep in mind. The Merc 600/700 have electronic boost control with the intent to maintain hp at the advertised rating under various atmospheric conditions and altitudes. A naturally aspirated engine of equal HP will perform closely under ideal conditions, but in real world conditions you'll see 5-10% power loss with a NA engine due to weather and altitude, more at high altitude. Plus the flat torque curve of t he S/C engine gives a lot more room for propping and they'll not be affected as much with a heavy load in the boat. The NA engine must be closer to it's peak RPM for maximum performance.

Bob Lloyd
Full Throttle Marine

articfriends

iTrader: (1)

When I did my last motor build I went way out of my way to make a good amount of hp normally aspirated as a foundation for a real strong relaible blower motor. I spent the extra time on the dyno developing a n/a tune plus I was curious what kinda hp my motor would make with the blower belt off. We ended up making 657 hp N/A with just about as much tq, this was on a less than ideal set-up due to compression only being 8.2-1 and 540 cubic inches ,blown we made 1057 hp on 92 octane and 1115 hp on a 80/20 mix of 92/110. So I have a motor that could be ran N/A on 87 octane that makes 650+ hp. I have ran the boat both ways, the biggest difference that I can see (aside from the 30 mph it loses running N/A) is running N/A the boat is real sensitive to load and ambient temp. Fully loaded, full tank of gas, 6 passengers and 90 degrees out the boat goes 7 mph slower from its best speed N/A to its worst. Even with my biggest dia/lowest boost pulley pulley the boat loses only 2 mph from best conditions to worst conditions (aside from running with the current or a 40 mph tail wind or something rediculous). The boat really seems to crawl as it picks up its last 10 mph when running N/A. Years ago when I built my first blown engine that made about as much hp as I can make N/A now I don't remember the boat taking so long to pick up that last ten mph, you can feel the difference in tq no way around it and i firmly believe that when a blown application has a large innercooler you don't get that big drop when the ambient temp goes up/air density goes down, I might be wrong, fwiw, Smitty

HaxbySpeed

That's really cool and very interesting. We usually check to see what hp we make without the SC too, but I've never tried to run it in the boat. I think you're right on about the intercooler efficiency too when sized appropriately it can't tell the difference between 5 more lbs boost or 20 extra degrees ambient temp.

pqjack

long,long time ago,when we were just young punks watching the races,a blower was a mystery to us,although the name said it all:it was an add-on monster trying its damnest to blow up the engine...but i got hooked on the whine/whistle of the THING...how times have changed!!
today,they are much more ''civilized'',easily installed,and user-friendly (well,almost),available in different sizes/configuration,and a lot more forgiving-to a point...im running a small one on my 496,and i feel confident i'll be able to come back from the sandbar in one piece....still love the whine....

bobl

Great testimonial Smitty! That's the exact scenario I was trying to describe, but not many people have actually tested both ways as you have.

When I used to run a Whippled engine in my boat I ran many data logs. The inlet air temp changed very little as the air temp went up. However I would see a big increase as the water temp got hotter. Our lakes hit 90+ degrees in August and you could see it reflected in the inlet air temps.

Bob

articfriends

iTrader: (1)

Great testimonial Smitty! That's the exact scenario I was trying to describe, but not many people have actually tested both ways as you have.

When I used to run a Whippled engine in my boat I ran many data logs. The inlet air temp changed very little as the air temp went up. However I would see a big increase as the water temp got hotter. Our lakes hit 90+ degrees in August and you could see it reflected in the inlet air temps.

Bob
Bob/Alex-I have actually ran the N/A set-up about 30 hours of the 130 hours that are on my motor in the boat, had labbed props done for that tune etc. When I go to Fla I bypass my aluminum innercooler and run the boat N/A so I don't destroy the innercooler with salt plus less chance of breaking a drive when that far away from home. I also keep the N/A tuned ecu on the boat in case something breaks on the blower I can take the belt off and inlet connecter to throttle body and be off and running in a couple minutes. My boat runs between 77-70 N/A though so it really feels incredibly slow, Smitty

HaxbySpeed

Yours and Smitty's posts do bring up excellent arguments for the SC. A well developed NA engine is designed to perform in a very specific RPM range. A SC'd motor has torque everywhere and will usually pull well beyond it's designed range. I think you're both very correct that it is much more forgiving of environmental conditions as well. With the marathon guys up here in ultra light tunnel hulls with limited power, you have to dial in the carb the morning of the leg. If you do it the night before you can be down 5 miles an hour the next day and hating life. For Kurt's deal where the power is fairly low I think it would be a tough call between the two though with the SC engine requiring more fuel which adds to the weight + the extra weight of the SC and intercooler. How sensitive are the boats Kurt?

hogie roll

Up to 650hp or so, I'd stay NA. Beyond that, the motor would get quite exotic to make over 700hp NA.

What are you starting with? If you have a 502, taking it up to a 540 or 572 shouldn't be too hard. If you have a 454, you can go up to 496 easily.

Kurt Hamilton

Glad you asked HaxbySpeed. This is where things differ slightly from what the majority on here are running.
The hull is a 21' ski race boat (revolution), it is yet to be built so is going to be brand new. The boat will be used for predominately open water ski racing and island hopping with the kids and wife etc. the dry hull weight will only be roughly 800-1000lbs so fairly light compared to the larger boats most you guys run. However it's only single engined so probably similar power to weight ratios required. Ski racing is huge over here however almost all sterndrive boats racing are dedicated river/lake racing boats and anything over 8 liter capacity is mostly turbo'd or blown (mostly turbo'd). I want something for the open waters where max speeds are down (but still used occasionally) so a forced induction motor isn't critical however we do add 200 liters of ballast + tabs to bury the bow and give a better wash for the skiers, so load is definitely there (I currently have the exact same hull as I'm going to have built but in outboard configuration with a 300xs mercury and it tops out at 83mph but add ballast and tabs to the equation and it's back to 40mph to give you an idea).
The rules for our racing limit cubic capacity to 572ci so I was thinking I would build na to that limit to give me as much torque as possible but keep things as simple and reliable and cheap as possible.
As I said in the beginning I have always assumed my best bet was to build na but then I look at the benchmark in supposedly reliable factory built marine engines and they are using sc. I am pretty sure after hearing all your thoughts I'll stick to the na path. It just keeps things simple and should live longer. I'll happily sacrifice 50-100hp in the long run to know it will cost less to run and take less maintenance to keep running. Thanks for all the advise.