BAD-HABIT

Well I have my new 358 SBC roller in and would like to know what the steps are to dial in a boat?
My motor will turn 6000 if need be. My cam's recomended RPM range is 12-5200. My intake is from idle to 5500. I am also running the 170cc Vortec heads. I am running Stainless Marine 4" exhaust straight thru hull. I am running a 600cfm carb. SOOOOOO.....Since the cam and manifold make there max power around 52-5500 should I get maximum HP around that? I was figuring around 5300. So do I prop the boat to run at that range WOT? If so after that point whats next? I figured it should be fine tuning the carb/timing and then making any prop changes to keep it in the Max Rpm range 52-5400....Is this the correct way to dial in the set-up? And again how do I determine where I should have my RPM limit at?

Oh well probally sounds like basic questions but Im still learning and I really haven't found anything that describes how all this is determined and accomplished with a NEW higher HP motor.

Thanks in advance!

"Bad-Habit"
86 F3LS

blue thunder

What is the recommended wide open throttle (WOT) recommendation for you stock motor and drive? I assume your dirve is stock. If it is say, 4200-4600, then something on the top of that would be ok... say 4600-4800... not much more depending on drive.
What you do next is run the boat at WOT and see what rpm you get. For every 200 rpm over 4800rpm... you add one inch of pitch to the prop. You change props in this manner until you run at your desired WOT rpm... whatever that may be. There are many factors to consider when deciding of the best WOT rpm. That is how I do it and it alway works.

Hope this helps you out....

BT

BAD-HABIT

Well this motor is not the "stock" motor rebuilt. It is a 89 roller block custom built. I am running a stock alpha one drive. I have talked to Many companies and they all say the same. "400hp and under AND with a small block (less torque) and your o.k. just watch the holeshots and ESPECIALLY getting any air (shockloading)" The stock motor was recommended at 42-4600 rpm's I was running 4900 with a 17 pitch dinged up ballistic prop (On the old motor). I purchased a 21 Mirage and am running only 4400 at WOT. I still have the Ballistic prop but I had it cleaned up and re-pitched to a 19. I haven't tried it on yet though. I have done alot of research and the Mirage or the Mirage Plus seems the best prop for my boat. Should I re-pitch my Mirage down to a 19? That still would only give me 4800 though? If I get it labbed that should get me about 5000 right?
Oh well thanks for the info keep it coming!

"Bad-Habit"

blue thunder

Well, it sounds then like you would need to go down two inches in pitch. That would make a 19 perfect for the 4800 you are after (1 inch per 200 rpm) . I would be wary of going any higher than 4800 with your alpha. At some point the heat will get it, even without shock loading.

Try the ballistic since you have it, then if it isn't what you want repitch the mirage, or buy a used prop. I think 2 inches is about the max on repitching.

My .02

BT

BAD-HABIT

Blue Thunder: My Alpha is on a VERY conservitive x-dimension. So most of it runs under the water. Also I am using Mobil 1 gear lube with a Simtek drive shower. BUT I still didn't get my answer to my question about finding out what my RPM range and WOT should be? Are you saying it is 4800? How is this determined?
Thanks
"Bad-Habit"

crmax

its hard to determine where your peek horse power is ( every motor is differnt) the best way is to dyno the motor the other is just keep trying diffent props till you get the highest mph

blue thunder

Good point CRMax on the max rpm thing. That is definately a good way to find out the optimum combo for top end. Other than that, I would consider the best rpm to be the rpm in which the weakest link in the chain (your drive) can be allowed to have a reasonable life expectancy. My point is... the drive is unchanged from stock. The recommended rpm range from stock is 42-4600rpm. I would think they have some safety factor in there so I guessed maybe 4800 would be a good extreme upper limit if frequently visited. Gear boxes are designed considering bearing pressures and rpm range. When you change either, you change the life expectancy. You are changing both. More hp in motor... more torque (bearing pressure) and higher rpm.

That is my .02 from years of mechanical engineering experience in industry and hard knocks in boating. What you doooo is up to yoooo.

BTW... I like you boat a lot!! It is a similar vintage to mine.

BT

checkmate981

F3 ls ,
The carb seams to be i little small for what you have .I would try something a little bigger .alpha drive , They handle rpm better than touque as long as you keep it cool and leave the prop on the water.
Dave

crmax

yikes! I missed the alpha drive, dont slam that throttle down

Rambunctious

F3LS

Fundamentally, the top speed is when the motor is just at it's peak power output. i don't know for fact, but the 4200-4600 range talked about i think is relative to a small block engine power output. in my liberator I had, the peak power was 4800 stock. and yes I eventually blew out my alpha with stock power!!!. I had a 21 pitch. and would go 53 mph at 4800rpm

i think your cam (5200 tuned cam) is lugging right now at 4200. Waterfoul had this problem for a while till we dialed in. the engine was peaky, once he got over 4700 it came to life.

It seems that most boat engines built don't last long when spun beyound 5100 (500 hp redlines are about 5000-5200) so I wouldn't consider 6000. fine for the circle track, but not steady state WOT.

Remember that power = torque x rpm . at your peak power, you are not at peak torque, but that's ok, because it is power that is important. Power is energy/time (ie. ft-lbs/sec) and that is where you get the most energy to thrust the boat, so peak power is where you want to be for top speed. your drive may actually last longer running a lower pitch at 5000 than it would lugging a larger pitch all day long at 4400 or so. I don't know this for fact..... just my engineering judgement. so I would spin that motor up a little, that's why you built it.

Tomcat on this board has modified a desktop dyno program to overlay the power output of an engine with the theoretical power vs speed curve of your boat. (obtained from your previous top speed and power of your stock engine.) you just need to give him the component specification for your new rebuild and the desktop dyno estimates the power from it's library of components that match your engine. his proram then looks at the two curves and suggests a prop pitch to meet this criteria at the top speed

P.S I own a formula now too


actaully, reading your post again, i think you are on the right track. if you repitch the 21 back to a 19 you say you get only 4800, BUT at 4800 this "Peaky " motor make much more power now than at 4400.. so I think it will push you beyong that 4800 and get you closer to your 4900-5000. then, if you are at 5000, have it labbed and spin her to 5200 and fly!!!

one last thing, check your timing. about 32deg max full advance is what i see on the board.


Tomcat?