Prop for custom 350

[Bholub36]

hey guys,

hopefully i get all the information correct to get an accurate idea of what i should be looking for...here goes

custom built marine 350 engine.
flat top pistons 10:1 compression i believe, 78 block
chevy vortec heads
marine comp cam: adv duration 260 int and exh. .440 lift
edelbrock rpm performer manifold
600cfm edelbrock marine carb
brand new sierra manifolds.

i dont know HP of this build which is why i listed it out for anyone who might have a guess. similar builds online are around 320hp?

anyway, alpha one Gen one outdrive: serial number 6192582.

1982 celebrity 188vbr

max speed GPS 45mph at 4000RPM

issue i have...slow hole shot and getting on plane. and seems like low RPM at WOT and slow speeds.

no clue what the prop is on the boat. it must be a cheap ss prop because there is no information on it at all as far as diameter and pitch. i have a spare prop sitting here that is a 13 3/4 by 21p that is brand new. its a mercury marine aluminum prop.

i would like to replace the prop for something thats gets on plane quicker and maybe helps get the RPM up. any help would be greatly appreciated!!

 

[Scott Danforth]

are you sure on the compression? if so you may need to run premium fuel to prevent detonation.
that motor should spin much higher than 4000 RPM. was the rotating assembly balanced?
what drive ratio?

 

[Bholub36]

im not 100% on compression ratio. everything has been balanced. and im not sure what the drive ratio is...the boat had a merc 260hp 350sbc in it from the factory and the drive is the same that came on it. 4000RPM is my max at WOT and trimmed up. just seems so low. could i be spinning the hub on the prop? or could the prop cause this issue not being the correct prop?

 

[porscheguy]

You should be hitting 4600-4800rpm. Maybe more, but 4600-4800 is safe. If you can’t figure what your current prop is, put on the other one and see what it does. That will give you a baseline and you can go from there.

 

[jimmbo]

I think you have the wrong cam. It is actually performing poorer than the stock grind. Of course that might change if the rpms are up, but to do so you would need a prop with less pitch and the end result may still be the same boat speed
What are all the specs of it
Durations @0.050 lift
Lobe separation angle
Lobe centerlines

Who 'Custom built' the engine? Auto engine builders more often than not, deliver an engine not suited for marine use

 

[Bholub36]

I built the engine. machine shop did the short block and i installed the rest. including the cam. i researched and purchased all components and installed. here are the cam specs: comp cam high energy marine cam.

Cam Style:Hydraulic flat tappet

Basic Operating RPM Range:1,200-5,200

Intake Duration at 050 inch Lift:212

Exhaust Duration at 050 inch Lift:212

Duration at 050 inch Lift:212 int./212 exh.

Advertised Intake Duration:260

Advertised Exhaust Duration:260

Advertised Duration:260 int./260 exh.

Intake Valve Lift with Factory Rocker Arm Ratio:0.440 in.

Exhaust Valve Lift with Factory Rocker Arm Ratio:0.440 in.

Valve Lift with Factory Rocker Arm Ratio:0.440 int./0.440 exh.

Lobe Separation (degrees):110

Grind Number:CS 260H-10

Computer-Controlled Compatible:No

Lifters Included:Yes

Lifter Style:Hydraulic flat tappet

 

[Scott Danforth]

I am leaning to wrong cam for marine use. the lobe separation is too close, and my guess without running a dyno sim is the torque starts dropping too soon.

here is your 260H cam (for 4 wheel drive trucks) http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=72&sb=2

Here is the marine cam that I would recommend http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=99&sb=2

 

[Bholub36]

i definitely would have gone more aggressive with the cam but i couldnt with the stock valve springs. the cam you suggested would not work with the stock vortec valve springs unfortunately....i may just throw a 19p prop on and see what happens...any suggestions on diameter for one?

 

[Sea Rider]

Some suggestions :

As a starter to dial best prop need to know precise min-max wot rpm range factory stated for that engine. No wild guessing on this one along a healthy engine, tach included.

Meanwhile remove current prop, take it to a prop shop to have it measured, so to know the exact diam and pitch, take advantage to dignose if hub is in good working order. Inststall second known diam/pitch prop and rmake a wot test run on flat calm water cond with deck weight evenly distibuted, move trim back to neutral once combo is running parallel to water level.

Once knowing current prop diam and pitch of first prop test as indicated the one with less pitch as a starter prop and then go from there maximizing a less prop pitch to pull wot rpm up to improve current hole shot if still happens to be low. Ideal is to dial a prop that revs engine middle to max wot rpm range as usually loaded.

Medium wot rpm for slight more top end speed with slight less hole shot or full wot rpm range for faster hole shot and sligh less top end speed. You decide...

Pitch is all, diam is what prop manufacturer wants to play with....

Happy Boating

 

[jimmbo]

Should have went with a roller cam. More aggressive ramps make the engine think the cam is longer duration. Roller cams also survive today’s oils

 

[Bholub36]

had some nice 1.6 roller rockers i wanted to use, but ended up going flat tappet. thank you everyone for your input! i am going to try out the other prop, see what happens, have my current prop measured, and go from there. i most likely will do nothing with the cam. if the props do not make much of a difference im just going to have to be content on what i have (which as we all know is really hard to do). im not sure if the hub on this prop is rubber or a hydroflow, but if its slipping due to a bad hub i would be very happy. (easy fix).

thanks again!