nufced

Greetings from Down Under.

I have a 21ft Skicraft Excel S Sterndrive with a 1995 350Mercruiser 285hp Magnum (carby pre Vortech). It is fitted with an Alpha 1 spinning a Laser II 21inch prop. Weight is approx 1200kg, 2700 - 2900 pounds. It is used for skiing, boarding & footing. The plastic cover on the carby states a WOT of 4400 to 4800.

My WOT is 4200RPM and 54MPH using the speedo no GPS. I did have GPS readings but have since lost them, from memory it was similar to speedo reading.

Anyway, I got the bug for some more HP and after countless hours of reading on this forum, I even got a copy of Dennis Moores SBC book, I went ahead and got the following mods done:
40 thou over flat top pistions (previous were dished)
New 64cc EQ heads with 180cc runners & springs (previous non Vortech small valves)
New roller cam 213/223 500thou lift 110 lobe centre
1.5 roller rockers
Block squared & decked
double row timing chain
measured 10 to 1 comp ratio
Balanced
New BG Demon carby
Thunderbolt IV ignition module (previous Thunderbolt V)

The engine was estimated to be producing 400+hp, giving it at least 115hp more than previously. The engine builder strongly recommended aftermarket headers and it is my intention to replace the standard exhaust manifolds but at $2240 (remember I'm in Australia) they will have to wait a while.

Well as the heading states, after carefully running it in with all these mods WOT is still 4200RPM at 54 MPH. The engine just hits a wall, no surging, no rev limiter cutting ignition. It seems to pull strong to 4200RPM and just stays at that amount The engine builder is a top bloke and wants to come out to observe/test the engine but he is the first to admit he knows nothing about boats, he is an engine man.

The engine still has the same small 1" high x 9" (approx) factory arrestor and standard Mercruiser exhaust manifolds. The exhaust connects to a Y pipe at the rear of the engine and exists through the sterndrive.

Does anyone have any ideas on why I can't get any more revs/speed? Is the exhaust system suffocating the engine? All information I have received indicate I should have been able to pull 5000RPM with a standard engine & exhaust. I wanted the extra hp thinking it would give me more revs spinning the same prop. Now I wonder just how many more revs the original engine could have pulled if I can fix this 'problem' thats stopping it from reving any harder.

I plan on experimenting by removing the flame arrestor and making up some temporary pipe work to exit the exhaust from the manifolds straight over the transom. Problem is it's winter over here at the moment and it's very cold so have to wait for a nice day.

Everyone I speak to back home tell me to to go a smaller prop - 19inch but I think thats only masking the real problem? Doesn't seem like there are many people that really know sterndrives over here.

Anyone have any ideas or thoughts?? Could the 21" Quicksilver prop be incorrectly stamped??

And apologies if I have offended anyone, seeing as it's an Offshore Only forum and my boat is only used for skiing etc. It's just that this site has been a wealth of knowledge for me and there is nothing like this over here.

BenPerfected

For a quick test you could make a run without your flame arrestor. Next, your temporary exhaust/tail pipe mods may not add much if you are planning to still use the stock exhaust manifolds. I see the upgraded exhaust as mandatory to get the expected 400 HP.

Edward R. Cozzi

If you are still at the same RPM and speed AFTER the engine modifications, you do not have an additional 115 hp. It's that simple. Now, you may have changed the horsepower curve, but at 4,200 you still have about the same power if your speed hasn't changed.
Get rid of the cast-iron exhaust and the mini flame arrestor and watch your other mods come to life.
Good Luck. Keep us posted.

ROB FREEMAN

ive found out too , that its not just easy to add power and get speed . thers a deffinate way to make boats go faster , and it costs bucks my wifew has named my boat "the money Pit " and thats exactly what it is !! good luck rob

stevesxm

im not sure i agree w/ the above.... based on what you say (especially the compression ratio increase) i am willing to believe that, all things being equal you probably have at a minimum 85 to 100 more hp.

now... the exhaust that you describe and running it thru the drive is a MONSTER restriction to rpm and will shut that motor right off as you describe.

i believe that f you got rid of the y and got it thru the transom as duals you would see a lot of the revs you were missing... in the alternative i am also willing to bet that w/ the extra compression ratio you bought yourself some serious torque which would lead me to believe that you could put a lot more prop in it and STILL turn the revs you are seeing now.. and as such more speed...

in effect, i am saying that i believe you are being rev LIMITED by the exhaust ... NOT that you don't have the power to make more revs...

in the great scheme of things, 100 hp at 50 mph on a boat like yours w/ a much smaller drag profile then mine should buy you 10 mph i figure as a guess... and from what you describe, i think the exhaust is whats killing you.

zgoo

Just a quick thought, since a new carb and motor is installed double check to see if the carb butterflys are opening fully.

stevesxm

thats a GREAT thought... had forgotten that completely and have seen it occur a dozen times....

Griff

Too make the hp, you need to be able to get air in and air out of the engine.

The small arrestor won't let air in and thru prop exhaust won't let air out.

Have you done any spark plug readings????????
What size carb?????????

nufced

I haven't checked the plugs since the mods, but prior to the mods the plugs were very dark grey to almost black, not oily, more of a carbon build up.

The carby is a 650cfm Demon.

Thanks to everyone for all the feedback. Looks like I need to replace the exhaust manifolds. There seems to be only one option in Australia being the Hi-tek manifold. Are they popular in the States? Any other recommendations, options and suppliers in the States that can freight to me? Need throttle/gear assembly mounts on Starboard manifold and oil filter mount on port manifold.

I will also replace the baby flame arrestor with a K&N . This raises another question, due to clearance issues under the hatch, am I better off using the 59-3264 (339cfm) with 3-3/4" height and keeping my 1" spacer between the manifold & carby OR go the 59-3364 (466cfm) with 4-1/2" height and no spacer between manifold & carby. As above any recommendations on suppliers/forum sponsors that can freight to me.

wwwTOPDJcom

I took a 285hp 1992 350mag out of my boat and put a 450hp BB with stainless marine exhaust in with a bravo. went from
52 gps to 71 gps using labbed props in both cases. I had bought the same book from Dennis but decided to just sell the small block and buy a BB