Which rpm range should I shoot for

[888]

Just rebuilt my 1987 Mercruiser 260 Chaparral 278xl and im a little confused to what rpm range I should be shooting for. My rebuild included upgrading to Comp cams pro magnum roller rockers, Comp cam roller lifters, Bolted on 1997 Vortec 062 with springs from comp cam, Flat top dished pistons that are .30 over , Comp cam extreme marine cam with a range of 1100 - 5500 rpms, Edelbrock performer intake and a 750cfm Q-jet carb. Before the rebuild I turned a 14x19 pitch prop at 4400 rpms now I turn that same prop at 5500 rpm so Im wondering if i should jump to a 21 or 23 pitch to bring my rpm down or should I leave it be as my cam is rated for 5500rpm. The boat comes up on plane fairly well and tops out at 36mph. Id like to be able to cruise at 3k rpm and stay on plane,right now it takes 3700 rpm to stay on plane. Btw would anyone be able to guess what my estimated HP would be and will my alpha one be able to take it?
Thanks in advance

 

[888]

I forgot to add I also replaced the thunderbolt IV with a Davis United Incorporated marine distributor came with a clear dizzy cap.....Looks awesome in the dark and makes for easy work making sure u have spark or not

 

[NHGuy]

That's darn good pulling for that big a boat. Congratulations on your results.
Do you have any idea what your boat weighs? My Formula is shorter than your boat but it's heavy for it's size which may explain my similar WOT RPMs with a 19 pitch propeller and a Vortec 350. I haven't really run it all the way to top speed, but without trimming it way out it has reached 4400.
Is your 19 pitch a 3 blade? You might go get a 4 blade 19 since your boat is so heavy. Extra blade props are supposedly best suited to heavier boats.
RPMs are where your max HP is. But peak torque is in the upper middle of the rev range.
21 pitch will put you around 5200 or 5300 RPM if I am right. If you go 21 you will lose some holeshot, but will raise your cruise RPM. I think a 21 and a 4 blade 19 are both worth a try. Monitor your fuel consumption and factor that in too.
You have built a big powerplant, don't run it too hard out of the hole or you may break your drive. Just put down your tabs and roll on the gas.

 

[Bondo]

Ayuh,.... I'd try a bigger prop to get the rpms down to 5 grand or there abouts,....

 

[888]

Thxs for replies NHguy and Bondo,My boat is 5500lbs dry but the wife has it plum full of junk so with water,gas,food,her junk and of course cooler full of beer,Im around 7500lbs. Im running a 3 blade SS 14x19 right now and Im eyeing up a 4 blade 14x21. I rarely ever go over 4k rpm and would love to cruise at 22-24mph and somewhere around 3k rpm's. I was told I could be pushing close to 350hp,Sound about right to yinz?

 

[Bondo]

I was told I could be pushing close to 350hp,Sound about right to yinz?

Ayuh,..... I'm pretty sure there's a mathamagical equation that'll tell ya, Exactly the horsepower output,....

But I never was a mathamagician either,....

 

[NHGuy]

Yea go on one of those horsepower calculators. 350 or 360 HP is pretty likely. I bet you are around 400 foot pounds too.
Are your heads 64 cc chamber?
I'd like to do a 383 or 377 some time but I'd probably do a dish piston cause I fear high compression in a boat. Have you figured your static and dynamic compression? If you measured your deck height and know your head gasket compressed thickness you can get those #s. They will give you an idea how much timing to do.
With those flat tops don't put too much advance in it unless you run mid grade or premium fuel.
At least boats have good cooling to help fight detonation.

 

[888]

Yes they are 64cc......I have no clue about deck height, I didnt build the short block. My machine guy did the crank,connecting rods,pistons,cam, and timing chain. Pretty much from oil pan to the pistons is what he did for me. I put heads on and used felpro marine head gaskets. Im running at 32 degrees total and 87 octane. Amazing how much quieter the roller engine is compared to the old engine. Its half the noise. I borrowed a 4 blade 14x23 from a buddy and tried that today,was a dog coming out of hole and barely got on plane..Maybe 30-40 seconds before bow dropped down. I was able to turn it at 4100rpms @ wot but I was into the 4bbl while crusing at 3200rpms. Im thinking a 4 blade 14x19 will do me perfect like you said.

 

[Bondo]

Ayuh,.... Sounds like a good place for a 21" prop,....

 

[888]

Ayuh,.... Sounds like a good place for a 21" prop,....

That a 3 or 4 blade 21 pitch?

 

[NHGuy]

Either. Probably a 3 though. Too bad you aren't here. I have one here that I saved off my old boat. I'd let you give it a try.
Suppose you could buy an aluminum til you are sure of the pitch. Then go find an SS if that's your style.

 

[888]

I prefer SS but wutever works best gets my nod,I just picked up a aluminum quicksilver 14x21 (4) Blade for 120 bux shipped to my door. Gonna be a week or two till i get another chance to test it out....Ill check back with results

 

[888]

I prefer SS but wutever works best gets my nod,I just picked up a aluminum quicksilver 14x21 (4) Blade for 120 bux shipped to my door. Gonna be a week or two till i get another chance to test it out....Ill check back with results

The 21 was just a tad to much so I got a 19 pitch 4 blade and its perfect. Wot is 5200 at 41.7mph and pops up on plane with ease

 

[NHGuy]

Good, plus the extra blade might allow a little more trim, which unwets more of the bottom, which reduces friction, and increases speed/efficiency. Try the trim button!

 

[biggjimm]

Good to hear you've got her ironed out. Sounds like a good setup you've got. You're probably pushing close to 400 HP with that setup. Do you know what the lift & duration of that cam is? I've got a 383" roller that dynoed a shade over 600hp that I built for a buddies jet boat then he got divorced & lost most of his toys & most of his money.... I'm thinking about trying to find a boat to put it in to have a little playtoy for the lake but haven't really got serious about it yet plus I'm not sure of what setup I'd have to have for that kind of power. You just don't find bravo 3's laying around in my neck of the woods. Have fun with yours. Sounds like you've got it right where you want it. Jim.

Ps. If you can find & download a copy of desktop dyno online- that's a pretty accurate program.... If a guy puts the info into the program honestly that is. We've compared it to several live runs & it's usually within about 3-4% of the print out from the dyno.

 

[888]

Good to hear you've got her ironed out. Sounds like a good setup you've got. You're probably pushing close to 400 HP with that setup. Do you know what the lift & duration of that cam is? I've got a 383" roller that dynoed a shade over 600hp that I built for a buddies jet boat then he got divorced & lost most of his toys & most of his money.... I'm thinking about trying to find a boat to put it in to have a little playtoy for the lake but haven't really got serious about it yet plus I'm not sure of what setup I'd have to have for that kind of power. You just don't find bravo 3's laying around in my neck of the woods. Have fun with yours. Sounds like you've got it right where you want it. Jim.

Ps. If you can find & download a copy of desktop dyno online- that's a pretty accurate program.... If a guy puts the info into the program honestly that is. We've compared it to several live runs & it's usually within about 3-4% of the print out from the dyno.

here is link to my cam specs

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=201&sb=2

 

[888]

Good, plus the extra blade might allow a little more trim, which unwets more of the bottom, which reduces friction, and increases speed/efficiency. Try the trim button!

Its hard to find smooth water where i boat but when i do ,She trims up alot further then she used too without blowing out