Ford 302...21 foot fiberglass with cuddy

[Scott06]

Shall I get a 4 blade 15 or a 3 is my only other question on this topic

Personally i prefer 4 blades. My primary use is loaded down water sports. Typically you will see better holeshot, lower planing speed at the expense of a little top speed with a 4 vs a 3 blade.

 

[Jakem]

Mine is loaded down as well..I will go with 4 blade

 

[QBhoy]

I went to a 4 blade 14.5x 17p. It raised my rpm to 4100 ish rpm wot...shall I drop again

Are you getting that with trimming the boat up etc ? What gps speed you at with those rpms

 

[Jakem]

Yes that's trimmed up...speed was about 34 ish mph

 

[QBhoy]

Yes that's trimmed up...speed was about 34 ish mph

I’d try a 3 blade 17”…just in case.

 

[Jakem]

It's a bit slow to get on plane so that's the reason to drop prop size...

 

[Jakem]

Alright...to get a 15p from turning point wich I have the hub for I can only get 15x15...diameter difference a big deal?

 

[Scott06]

Alright...to get a 15p from turning point wich I have the hub for I can only get 15x15...diameter difference a big deal?

No wouldn't worry about the 14.5 to 15" diameter. typically within a prop family as pitch goes down you will see diameter increase. I think the TP 19"Pitch is 14" dia, 17"P is 14.5" dia , 15"P is 15" dia...

 

[Jakem]

So with 10 degrees initial plus the 24 centrifugal you should see 34 degrees total advance by 3500 rpm or so. You can check this on the muffs just briefly rev it. especially with an older mechanical advance distributor that can get hung up or corroded, you just want to verify you Are getting advance otherwise you wont make full power.

if yiu are confident in the tune and tach accuracy, then yes i would drop to a 15 to get revs up to 4500 or so. Not that you have to run it wot but the fact that it is properly pitched so it can reach wot is the important part.

I'm seeing about 28..29 full advance at about 3500..3600 rpm...

 

[Jakem]

Total advance is about 28 degrees at roughly 3600 rpm...initially is 10btdc..

 

[Scott06]

Total advance is about 28 degrees at roughly 3600 rpm...initially is 10btdc..

Would think that should be ok, definitely safe for keeping detonation at bay

Not sure what the stock merc was for that engine. What ever advance limiters were put in there must be limiting it to 14 degrees centrifugal advance.

 

[Jakem]

I will check the limiters...there is spares...12 deg..16 deg and 20 deg

 

[Scott06]

I will check the limiters...there is spares...12 deg..16 deg and 20 deg

I would try and find out what the total timing advance was in the merc manuals

maybe @achris knows what total advance for the ford 302 merc should be

 

[Scott06]

Just found this thread shows total to be about 28 degrees max https://forums.iboats.com/threads/spark-advance-1977-ford-marine-175-hp-302-engine.733308/

not sure i agree with last two posts in thread if graph is total or distributor advance where you would add ten initial to the 24 +/- advance …as to me it reads initial plus the graph for total …

 

[Jakem]

This is in my.book. that came with my boat...I'm guessing the 26 mark is estimate

 

[Scott06]

This is in my.book. that came with my boat...I'm guessing the 26 mark is estimate

Ok same curve that to me looks like 10 intial plus 24+/-2 centrifugal advance. As long as you have the fuel it was rated for in terms of octane and carb is jetted properly should be fine at 34 total