need more power

[jbh1024]

does anyone have any suggestions on how to get a little more power from a merc 350? i have a 2 bbl carb now. how much will a 4 bbl give me? only looking for about 20 - 40 more hp

 

[HT32BSX115]

Re: need more power

Howdy,

I'm assuming by 350 you mean something like a 350 cu-inch V-8.....maybe 250hp or so?

Even that much increase in HP (40hp) is not going to make much difference in performance. You need more torque.

It usually takes 10hp to give you a 1mph increase in speed. You're only talking about a 4 mph increase in top speed. It might do a little something for hole shot.

That engine is probably not going to see much increase at all going from a 2bbl to a 4bbl.

You can only suck so much volume of air/fuel thru the engine with current valving/cam etc. A bigger carb may make little or no difference.

If you want more HP you'll probably need to go to a fuel injected "350" or "383" Or....a 454!


Cheers,

Rick

 

[bomar76]

Re: need more power

does anyone have any suggestions on how to get a little more power from a merc 350? i have a 2 bbl carb now. how much will a 4 bbl give me? only looking for about 20 - 40 more hp

You made no mention of the year of the block (which I can only guess is a 350 SBC), so there are a number of things that may or may not work for you...

1. 4bbl will give you an increase...MARINE ONLY, and buy one that fits your new intake ...either square bore (weber AFB type) or spread bore (Ughh- Quadrajunk). That stupid Merc squarebore/spreadbore adapter is a huge power thief.
2. Vortec heads...make sure they are off a 350 and not a 305....you will need a vortec style intake also.
3. Roller cam and roller lifters, if your block is drilled for the GM spider it's an easy swap...otherwise, forget it...the retrofit lifters are $$$ compared to the stock GM, and you have to screw w/ a cam button, etc. You will also need the spider, a new roller cam style timing set, a hardened fuel pump drive rod, and to swap the distributor drive gear.

 

[John_S]

Re: need more power

What Bomar76 said, but wouldn't bother with #3 if you are just looking for hp gain + it's a pain (oil pan/timing cover gasket) to do without removing engine. If you are going through that much work, might as well get a slightly better cam.

 

[joseph L]

Re: need more power

Had a 1988 350 magnum. A friend of mine changed the rocker arms to a
different angle. It made the valves open a little more and close faster. You have to make sure you have enough clearance between piston and valve with the new rocker arm. It wasn't that expensive. Also noticed a big difference on throttle response.

 

[bomar76]

Re: need more power

Had a 1988 350 magnum. A friend of mine changed the rocker arms to a
different angle.

The correct term is different ratio.....and going to 1.6 from the stock 1.5 (or so) in a stock 350 you would not have valve clearance or spring issues, but not really much of an improvement for the $.

 

[Coors]

Re: need more power

Ughh- Quadrajunk).

I find them easy to tune.

 

[John_S]

Re: need more power

With stock vortec heads, stock roller cam, 1.6 rockers may be too much lift. Do the math, but if memory serves the exhaust valve is out of the safety zone. The heads would need modding or different oil seals, etc. The pushrod clearance holes in the head will need to be checked and possibly slightly ground for clearance, too. You are also making the roller cams inversion issues slightly worse.

On a stock hyd cam, you can get about 10hp w/1.6 ratio, but need to degree cam. Per Dennis Moore, anyway. Once you are doing that much work, new cam and lifters might be a better option.

 

[jbh1024]

Re: need more power

its a 93 350 mag in a 21 ft lazer. im currently turning about 5000 rpms with a 23 p prop. my problem is i need to come down in rpms but im dragging out the hole as it is. was hoping if i could get a little more power i could go up in pitch.

 

[TilliamWe]

Re: need more power

Well, you don't have Vortec Heads or a roller cam. So what bomar posted is the way to go, but it costs a lot of money. Especially since you block is not set up to accept the roller cam, as he explained. Basically you have a 210 hp rated engine. If your boat was a 96 or newer with Vortec heads, roller cam, and 2bbl, Mercruiser rated it at 250. There's your forty horses. Spend accordingly.
Maybe a different prop is what you need? 4 blade perhaps? Maybe you are slipping way too much at the low end or the top end. Cause at a 5000rpm WOT, that thing should come out of the hole nicely. You're underpropped, since that motor's max WOT RPM is probably only 4800 (maybe only 4600). So you may have a prop issue, not so much of a power issue. The boat doesn't have a bunch of water stuck in it somewhere, does it?

 

[John_S]

Re: need more power

If you have a 2brl carb on a '93 350 mag, someone has modified or changed engine. Assuming you have a standard drive ratio (1.47), the rpms with that prop pitch and size of boat, I'd say you have a 4brl or possibly a TB injected engine.

Going up in pitch is going to make your hole-shot worse. Any hp mods would be most likely adding more at high rpms, with possible sacrifice of the low rpm (hole-shot).

You need to go down in pitch to improve hole-shot, but you will loose some wot mph.

You need to investigate and determine exactly what engine you have and what your wot mph (gps) was. You might be better off looking at tabs or different prop. Vortec heads might still be a good option, too.

 

[JCF350]

Re: need more power

Put a spread bore intake and carb on it. The smaller primaries will boost your bottom end torque and secondaries will give you some more top end power.

 

[John_S]

Re: need more power

A '93 stock 350 Mag "may" have a roller cam. The block would be machined for it, even if it didn't. It would not have stock vortec heads.

The bigger question is what exactly does he have. Hopefully, not a 2brl 210 hp with a 350 mag flame arrester cover.

 

[jbh1024]

Re: need more power

the motor is a 350 mag 250 hp, alpha one outdrive 1.5:1 gear ratio,
4400-4800 rpms WOT. my mistake on carb, its a weber 4-bbl. someone mentioned the prop...i have a reworked quicksilver 23p 3 blade, the guy who reworked it said it is proboably a late 80's early 90's model prop. would a new prop with improved technology possibly fix my problem? my top speed is at a solid 60-62mph on gps depending on water. need to drop about 200 rpms though

 

[QC]

Re: need more power

You sure your tach is right? Calcs out at 15% slip at 62 . . . Could be, but it seems like you could get some more bottom with a vented prop and get the slip lower at high speed too. I doubt you can or need to go any higher in pitch. A vented 4 blade might get you a little better of both . . . 4800 with 10% slip and a 23 is 62.7 MPH . . .

 

[faller]

Re: need more power

Im not to familiar with that motor but an offenhauser 360 dual port manifold might do the trick (if they make one for it). They sure worked good on land.

 

[John_S]

Re: need more power

"need to drop about 200 rpms though"

Running at 5K will not destroy motor. If you are worried, back off a hair on the throttle.

As far as engine upgrades, the vortec heads will give you another 30hp at the prop. They would increase hp over a wide rpm range. You will need a new intake and possibly new jetting to the carb, and misc stuff.

 

[krisnowicki]

Re: need more power

Ughh- Quadrajunk).

I find them easy to tune.

What do you know that we dont?

 

[Coors]

Re: need more power

After years spent on drag holleys, and tuning, which involved taking bowls off, dumping gas on a hot intake, and breathing that for 10 minutes..
all the quad needs is taking off the filter, and swapping stepped rods; tweaking springs- all from the top of the carb.
and the normal problem is usually, the secondary covers are opening too soon- a spring adjustment. After that, the engine will only suck in as much gas as it needs. And the Quad can supply more than it needs.

 

[Maclin]

Re: need more power

jbh1024,

What exactly do you mean by a little more.....60+mph in a 21 footer with 350 cubes is pretty good already! Spinning to those rpm's at 62 mph is almost ideal in my estimation. Any other mods could affect lowend adversely and lean towards water inversion.


There are forumlas to figure it out, but to get more than 1-2mph top speed increase the HP would have to go up more than 40 hp I bet.