WOT for 5.7 merc

[coopin87]

Ok guys trying to figure out what my WOT should be for my Merc 5.7. I understand the prop acts as a governor so im thinking I may need to go up in size but am unsure what the WOT is. The motor was just redone has world heads on it a little bit of an oversized cam (2 sizes up from stock (RV cam)) High rise intake and a 4 barrel. Not sure if any of that affects my WOT range or not. Any imput would be greatly appreciated.

 

[coopin87]

Re: WOT for 5.7 merc

I found this information ... Is this correct?

Mercruiser 5.7L Engine Specifications​

(V-8 Models) ​

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Models​	5.7L​	5.7L EFI ​	350 Mag MPI​	MX 6.2L ​ MPI​
Propshaft Horsepower (SAV1 Rating)	250[SUP]1[/SUP]​	260[SUP]1[/SUP]​ (233[SUP]2[/SUP])​	300[SUP]1[/SUP]​ (259[SUP]2[/SUP])​	320[SUP]1[/SUP]​
Propshaft Kilowatts (SAV1 Rating)	186[SUP]1[/SUP]​	194[SUP]1[/SUP]​ (174[SUP]2[/SUP])​	224[SUP]1[/SUP]​ (193[SUP]2[/SUP])​	239[SUP]1[/SUP]​
Displacement	350 cid (5.7 l)​	377 cid (6.2l)​
Maximum rpm @ WOT[SUP]3 [/SUP]	4400-4800​	4600-5000​	4800-5200​
Idle rpm in NEUTRAL[SUP]3[/SUP]	650[SUP]5[/SUP]​	600[SUP]6[/SUP]​
Oil Pressure @2000 rpm	30-60 psi (207-414 kPa)​	min. 30psi (207 kPa)​
Min. Oil Pressure @ Idle	4 psi (28 kPa)​
Thermostat	160[SUP]o[/SUP] F (71[SUP]o [/SUP]C)​
Timing @ Idle rpm[SUP]4 [/SUP]	10[SUP]o[/SUP] BTDC​	8[SUP]o[/SUP] BTDC​
Firing Order	1-8-4-3-6-5-7-2​
Electrical System	12-Volt Negative (-) Ground​
Alternator Rating – Hot Operating Amps	65 Amps​
Alternator Rating – Cold Operating Amps	72 Amps​
Recommended Battery Rating (Min.)	375 cca/475​ mca/90 Ah​	550 cca/​ 700 mca/120 Ah​

 

[Don S]

Re: WOT for 5.7 merc

4400 to 4800 is typical on carbed engines of your vintage.

 

[tpenfield]

Re: WOT for 5.7 merc

I am wondering if the bigger cam will reduce your recommended WOT RPM ??? I guess it would depend on if the valve springs were replaced/upgraded as well. There may not be a particular 'spec' to go by, so you may have to guesstimate a bit.

You will like the 4 barrel carb. . .

 

[coopin87]

Re: WOT for 5.7 merc

Well im way over 4800 at WOT more like 5500 rpms with a 21 pitch aluminum prop. I take it I need to go up in size in order to drop the rpm's?

 

[picklenjim]

Re: WOT for 5.7 merc

Rule of thumb is for every inch of pitch you increase it will lower the rpm's by 200. If you change to a stainless prop it will bring it down some also. What is the boat that this is in? Chances are your not going to be able to go over a 23P. An aluminum prop isn't very good to be using with hopping up the motor.

 

[Don S]

Re: WOT for 5.7 merc

Well im way over 4800 at WOT more like 5500 rpms with a 21 pitch aluminum prop. I take it I need to go up in size in order to drop the rpm's?

Get an accurate shop tach and make sure your helm tach is accurate before you start changing props. Make sure you have the proper gear ratio in the drive too.

 

[coopin87]

Re: WOT for 5.7 merc

Its a 95 marada mx3. 21ft without swim platform. the only reason the aluminum is on there is because after everything was done the mechanic recomended I put the 21 on instead of leaving my 19 ss. I dont mind buying another prop just trying to figure out where I need to be as far as pitch before I buy another one. My brother has a three blad 23 ss that I might be able to try tomorrow.

 

[coopin87]

Re: WOT for 5.7 merc

Get an accurate shop tach and make sure your helm tach is accurate before you start changing props. Make sure you have the proper gear ratio in the drive too.

What do you mean proper gear ratio in the drive? I honestly have no idea what this is. how would I check? Tach is accurate I double checked that with a known good one today. Granted I only checked it to about 2k rpms because it was on the hose (very briefly) but if its the same at 2k i would imagine it would be accurate throughout the rpm range. Please correct me if im wrong and I will take the guage with me to the water tomorrow.

 

[Don S]

Re: WOT for 5.7 merc

Look on your outdrive on the port side of the upper in the decal area, you should see the serial number and the gear ratio. What are they? With an Alpha drive, you should have a 1.50 ratio.

 

[coopin87]

Re: WOT for 5.7 merc

Look on your outdrive on the port side of the upper in the decal area, you should see the serial number and the gear ratio. What are they? With an Alpha drive, you should have a 1.50 ratio.

Side sticker is OF694537 1.62R

 

[Don S]

Re: WOT for 5.7 merc

There is your problem, you have a V6 outdrive gear ratio. Not a built up 5.7.

 

[coopin87]

Re: WOT for 5.7 merc

Great So I guess its time to start looking for outdrives then?

 

[coopin87]

Re: WOT for 5.7 merc

What would my best bet be?

 

[coopin87]

Re: WOT for 5.7 merc

Ok so the outdrive is origional to the boat. Motor is not. It origionally had a 305 in it an now has a 350. So my question is simple do I replace the drive (nothing wrong with it works fine) go up or down in pitch or just unload the shotgun into the hull lol. :facepalm:

 

[Bondo]

Re: WOT for 5.7 merc

Ok so the outdrive is origional to the boat. Motor is not. It origionally had a 305 in it an now has a 350. So my question is simple do I replace the drive (nothing wrong with it works fine) go up or down in pitch or just unload the shotgun into the hull lol. :facepalm:

Well im way over 4800 at WOT more like 5500 rpms with a 21 pitch aluminum prop. I take it I need to go up in size in order to drop the rpm's?

Ayuh,.... Quite simply, Try a bigger prop,... If ya get a handle on the rpms, 'n the driveability,... Great....

If Not,.... Trade yer drive for a 1.50:1 ratio, 'n reprop it again....

 

[Don S]

Re: WOT for 5.7 merc

The best thing to do is get a new SEI upper unit for your drive, or get the proper gears in the upper. If your tach is accurate and you are actually getting 5500 rpm, the you are over the max rpm by 700 rpm. Since an inch of prop pitch changes about 100 to 150 rpm you will need a 25 to 28" pitch prop. Which is going to kill your hole shot and getting on plane.

http://www.sterndrive.cc/mm5/mercha...gory_Code=MercruiserAlphaGenIIUpperSterndrive

 

[coopin87]

Re: WOT for 5.7 merc

The best thing to do is get a new SEI upper unit for your drive, or get the proper gears in the upper. If your tach is accurate and you are actually getting 5500 rpm, the you are over the max rpm by 700 rpm. Since an inch of prop pitch changes about 100 to 150 rpm you will need a 25 to 28" pitch prop. Which is going to kill your hole shot and getting on plane.

http://www.sterndrive.cc/mm5/mercha...gory_Code=MercruiserAlphaGenIIUpperSterndrive

So what do you guys think about going with a 23 or 24 and putting smart tabs on the boat ... I can always keep an extra 19 or 21 on the boat for watersports and such.