Merc 302 vs 351

[pmddouglas]

I have a 1976 IMP 20' -1" Aztec Cuddy Caddy. It has the Merc 888 setup. Going to pull the motor this week. Block might be broke, if it is, should I consider putting a 351 in there instead? If not, how can I determine the specifications required to build the 302 to the 233 hp merc specs? Or should I consider other options?<br /><br />Thanks for any input.<br /><br />Paul

 

[JasonB]

Re: Merc 302 vs 351

The 351 will bolt in its place and use a number of teh same accessories. I fought this same battle 2 years ago when I bought my boat with a locked 302. The 233 was a 2bbl 351. The most merc did with the 302 was a 225 inboard that had a 4bbl. It's not hard to build your 302 to 230hp and many of teh 888's have the better flowing 351 heads I believe. <br /><br />I ended up boring my 302 30 over, installing flat top hypertectic pistons, molly rings, clevite bearings, crane cam/linters/springs, had my 302 heads reworked, and topped it back off with the original 2bbl carb. Not sure what HP I'm making, but my 19' 2800lb wet boat has no problems coming out of the hole like a rocket and tops out just shy of 50mph. Stock max speed was supposedly 40 or so. A 4bbl would be nice, but with the gas prices, I'll keep my 2bbl for a while.<br /><br />I essentially built my 302 to my specs with some help from a number of people on this board. I wanted the 351, but since my 302 block was good, I went the less expensive route and have no complaints. F_inscreename knows these engines fairly well and will likely reply to this post too.

 

[pmddouglas]

Re: Merc 302 vs 351

Thanks for the Reply JasonB. Sorry for mixing up the horsepowers of the 302 and 351. I am hoping that I find that the 302 block is not cracked and can be reused. It is locked up, and I don't know if it is just rusted up, and I also don't know if it was winterized. All I know is that my father in law said it sucked in water and then he removed the water. Probably a bad exhaust manifold. It is the 2 barrel version and I think that is the 200 hp version? I have forgotten the research I did last year.<br /> So a few more questions: There appear to be lifting tabs on the exhaust manifolds. Should I hoist the engine out with those? Or should I remove the exhaust and bolt the chain to the block? If the block is cracked, should I consider using the 78 Ford 351 Modified engine (Non Catalyst Motor)that is in my old pickup as a block to rebuild? I guess I am not Mr. Money Bags and know that I need about 1000 bucks alone to get new exhaust risers and manifolds. I hear they only last 4 to 8 years. I have no idea how old they are other than they are at least 4 years old, because that is how long the boat has not run. So, if I go the 351 route, what accessories from 302 won't work? Intake Manifold? Also, it appears that the 351 and the 302 use the same exhaust manifolds and risers. Is that true? And lastly for now(I am sure there will be more questions) The 351 should also bolt up to the Drive?<br /><br />Thanks again, <br /><br />Paul

 

[mkast]

Re: Merc 302 vs 351

If you go to one of the online parts suppliers, http://www.mercruiserparts.com/, you can crossref. the parts from the 302 to the 351, as in the cams (if you have a factory 302 intake manifold, you'll notice the firing order has been ground off, an a 351 firing order was stamped in)are the same as is the flywheel. I found new, center riser,exhaust manifolds for $150 ea. risers for $90, both engines use the same exhaust parts. The distributor and the intake manifold are the only parts not interchangeable. Go with the 351, lots more torque.

 

[snapperbait]

Re: Merc 302 vs 351

Pandjdouglas.. The 351M (modified) you propose from your truck is a totally different engine.. None of the parts will be usable from the 302 and finding exhaust manifolds to fit a 351M would be next to impossible...<br /><br />You would need a 351W (windsor) if you were to use any of the 302 parts..<br /><br />The 302 can be a very stout engine if done right... The shorter stroke and smaller displacement are actually beneficial.. <br /><br />Shorter stroke means a slightly higher useable RPM range..<br />Smaller displacement means less fuel/air will be needed to fill the cylinders, meaning less fuel consumption...

 

[pmddouglas]

Re: Merc 302 vs 351

Thanks for the additional input mkast and snapperbait. At this point, now I know that the 351M is not an option. If my block is cracked, then I will need to make a decision on the 302 or 351. Do the intake manifolds need replaced as often as the exhaust manifolds? I will probably have a problem finding a good used block. Leaning toward the 302 because of Gas mileage and the higher rpms that it should handle. Wish I knew some recommendations such as compression ratio, cam spec etc., that would work well with stock 2 barrel and possibly a 4 barrel later(Would definitely need new manifold). It won't be a racer, but a cruiser with some get up and go. Hopefully it would be worth what I put in it. Thinking it will be about 3 grand with new exhaust. Based on having someone do the machine work and assemble the long block. I will attempt the rest. Hardest part will be removing rusted bolts most likely. Interior pretty good and lower unit was checked out as being OK per the boat shop. No large knicks underneath.

 

[mojokim]

Re: Merc 302 vs 351

Why not just buy a good assembled shortblock and then bolt on your components. I'm sure you could get well under 3 grand that way. For manifolds check ebay. Dennis Moore has been selling his better than stock cast manifolds and risers on there for a very fair price. I would take your stock 302 heads and have a good valve job and maybe a light port and polish put on them.

 

[OMARiverRat]

Re: Merc 302 vs 351

Hey Paul, are you a Might Mo' river rat as well? Good luck & Happy boating...

 

[f_inscreenname]

Re: Merc 302 vs 351

All right here we go. First of all get yourself a 351W no question about it. A hoped up 302 in a boat is going to be great for about 3 months then look out. You ever notice that “Mercruiser motors” are so week. They keep the power down so they don't blow apart. A 188hp out of a 5.0 (302ci) would never work in a Mustang but people think that is O.K. for a boat. Not me. I want that H.P. any way I can get it as long as long as it is a dependable motor and that is a 351W. You have to love that longer stroke. Much easier on the motor when running at higher RPM’s. As stated above the only differences you will have is the intake and the dist and the ability to get a but load of HP. What ever you do get a master rebuild kit from northern auto parts. Master rebuild kit -cam (your spec), lifters, timing chain, pistons (very cheep upgrades to what ever you want), oil pump, “all” bearings (to your specs), All the gaskets (upgrade marine head gaskets, couple bucks) and freeze plugs (upgrade to brass, 2 or3 bucks) for a 302 or 351W should come to about $275.00 give or take $25.00.<br /> Bang for the buck- the first thing I would do is a cam, four barrel intake (Edlbrock air gap)and a good (edelbrock) 650 carb. If I was building a motor I would use flat pistons, double roller timing chain (advanced or retarded as needed), high volume oil pump..and.................... When you do all that and it comes apart (like a 302 will) it can really p**** you off and I should know I smoked 4 302’s in 2 seasons. Built a 351W for under a grand ($250.00 junk yard motor, $125.00 local machine shop work, $400.00 at Northern (serious upgrades) and less the $200.00 for a bad a** Edelbrock intake. I all ready had the 650 Edelbrock carb. I was pulling at least 300HP out of it (being conservative) . Had it for two seasons then sold it and it went on for two more seasons that I know of (lost touch with the new owner) with no problems. It also seemed to get better gas mileage, I think it’s because you didn't need to push the throttle as far forward as I did with 302. <br />Let us know what you are doing and I will see if I can help.<br />P.S. Familiarize yourself with e-bay real well. There is so many boat and motor parts that go for nothing. I have bought a lot of parts there and if you are not comfortable with buying something like a carb on there because it may be bad you can always get the stuff (like a intake) that has little chance of being defective. I am a little more daring, I bought a Edelbrock 750 marine carb (like new) for $96.00. Never really had a problem with e-bay.

 

[snapperbait]

Re: Merc 302 vs 351

I'm just the opposite of F_inscreenname... <br /><br />Had excellent service out of two 302's that were pulled straight out of wrecked mustangs and set up for marine use... Paid less than $400 for each running long block engine... Both are still in service today... <br /><br />Left the internals bone stock including the hydraulic roller cams... 225-250 Hp? Something too that effect... Put my Commander and PCM 351's rated at 240HP and 260HP (both healthy) to absolute shame...<br /><br />Agreed with f_inscreenname on the air gap intake and 650 carb.. Pop that air gap or performer rpm intake and maybe a smaller 600cfm carb on a stock 5.0 and it will scream, and it will stay together...

 

[snapperbait]

Re: Merc 302 vs 351

Also, before you go making some crazy horsepower, make sure the outdrive can handle it..

 

[Boomyal]

Re: Merc 302 vs 351

pandjdouglas, keep in mind that the 351w is at least 50 lbs heavier, 2.5 inches wider and 2 inches taller. I contemplated going that way for my 20' Gulfstream runabout. It is a medium weight boat with a medium height gunwales and I determined that it would not like the extra 50lbs in the back when I had a boat load of people on board. Plus it would not fit under my one piece fiberglass motor cover.<br /><br />If you want the extra reliable poop and don't want the extra weight and size consider what I'm doing. It cost a few bucks more than stock 302 or 351 but give easy awesome low to midrange torque just where you want it.<br /><br />I'm building a 331 stoker with slightly worked iron heads, an edlebrock performer manifold with a 600 cfm marine holley. If you're interested in the details let me know.

 

[pmddouglas]

Re: Merc 302 vs 351

Wow, I have gotten a lot of excellent replies of useful information. Thanks to all. And if I ever get this project done, I plan on being a Mighty Mo river rat on the weekends. How do I determine what the outdrive can handle in regards to horsepower? I guess I would be OK with 225 horse or better. Of course I would definitely like more, but don't want to ruin the outdrive. It is one of the earlier model of mercruiser outdrives and can't remember the specific model.<br /><br />Dennis Moore manifolds on Ebay? I thought it was highly recommended to stay with Mercruiser for replacement exhaust.<br /><br />Edelbrock Air Gap? Is that the terminology for Intake manifold? Or is that another component?<br /><br />Edelbrock makes a 650 carb for marine use? Very important to stay with a marine carb, correct?<br /><br />I better check my hoist clearances and my engine cover if I consider the 351W. <br /><br />Sounds like I need brass freeze plugs, Marine Head gaskets, double roller timing chain, flat top pistons etc. <br /><br />Once I get this old motor out, hopefully the weekend after next, and tear down, I will make a decision on 302 or 351. Then I will definitely need some more advice for the final specifications.

 

[mojokim]

Re: Merc 302 vs 351

I need to retract the info on Dennis Moore manifolds. I forgot he currently only has them for gm motors. www.mooreperformance.org <br /><br />A 331 stroker is a great idea if you have the funds. These are better than the 347 strokers that you will also see. This is due to better ring placement which improves durability.<br /><br />Marinizing a 5.0 mustang motor is an interesing idea as well. You get the roller cam and the E7 heads which are better flowing espesially with a bit of porting. I was hesitant to mention it because I wasn't sure if the cam profile would be ok for marine use.

 

[JasonB]

Re: Merc 302 vs 351

I used most of F_in's advice. I rebuilt my 302 as follows:<br /><br />Bored 30 over, new valves, cleaned the ports in the heads<br />Hastings Molly Rings<br />Speed Pro Flat Top Hypertectic Pistons<br />Crane Energizer Cam kit including lifters and springs, Grind# 266 H10, .469 lift and 210deg duration, changed to 302 firing order<br />Clevite 77 bearings<br />Melling High Volume oil pump (mistake, use standard volume)<br />Cloyes Tru-roller double steel timing set<br />Pertronix Ignitor 2 ignition upgrade, matching Flamethrower coil, new 8mm Magstar Marine wires<br />Topped off with the original 2bbl Rochester carb (rebuilt).<br /><br />Original HP: 188, New HP: ???<br />Original top speed: 40-42 speedo New top speed: 47.8 GPS before carb rebuild and more tuning.<br /><br />Well pleased with my 302. Although som may comment on the auto cam, I have more than enough grunt for some brutal holeshots and wot is right where it should be. I can't just nail it to pull a skier up, I have to ease into the throttle a bit or I pull them up too fast. I thought about the 351, but the economics weren't there and the extra height/width probably wouldn't fit well in my engine bay.<br /><br />To answer a previous question, the rings on the manifolds are lifting rings for the engine. There is a 3rd one behind the carb on the intake manifold.<br /><br />If you are buying new manifolds and risers, consider going with the center risers if you can get a conversion kit. I wanted them, but the conversion scared me at the time. The stck logs work fine and make handling the engine a bit easier with the rings, but I'm told the centers flow better. There is a company that makes stainless exhaust for the 302 and 351 that is about $1400 shipped from Australia if you are interested. They weigh a fraction of what the cast ones do. I can dig up the info if you want it. If I had the cash....<br /><br />For my 19'er, we get about 3.5mpg at a 3200rpm cruise at about 30-34 mph. with 2 people. 4 people doesn't affect milege much.<br /><br />Northerautoparts.com and ebasicpower.com got lots of my money when I went through this......<br /><br />BTW, be sure to go ahead and replace the exhaust flaps in the bases of the Y pipes while the engine is out. They are probably gone if it inhaled water. Cheap PM.

 

[Boomyal]

Re: Merc 302 vs 351

JasonB, why the following statement?<br /><br />"Melling High Volume oil pump (mistake, use standard volume)"

 

[f_inscreenname]

Re: Merc 302 vs 351

To answer some questions if I can;<br />I had a 888 on my boat when I built my 351W (and the four 302‘s), handled it with no problem. They built things better back then.<br /> As for the mustang motors beating on the 351W, you will never find that kind of stuff (roller cam ect) in a old 302ci. No wonder your 302’s were beating up that stock tired 351W. With equal parts the 351 would have blown those 302’s away. <br /> As for the 351’s extra weight. A good cooler filed weighs 50 pounds. For a stronger motor with more HP I would trade any day. Trust me you will never notice the difference except for the power. <br />Stroker kits. Again Northern auto parts. $699.99 for a 302 to 347 kit. You have to check this place out. <br /> Most exhaust man’s are made by Osco. I wouldn't buy any thing Mercruiser unless I had to!<br />Edelbrock Air Gap. Is the name of the manifold you want to use. Check Summit on line to see what it looks like. Very cool (fuel flow, looks) in many ways.<br />Edelbrock makes any marine carb you want. 650,750,850 ect. They are easy to rebuild and I think the last rebuild kit I bought was $35.00 so if you want to save a couple bucks search e-bay for them. <br />JASONB I to am interested why you don't like the high volume pump. I have been running one for years with no problems. What gives?

 

[JasonB]

Re: Merc 302 vs 351

I don't have any real gripes with the pump itself, but unless they accidentally sent me a high pressure pump instead of high volume, I have way to much oil pressure. I push 40-45lbs at idle with straight 30wt oil. At 2500-3000rpm, the guage is pegged at 80. According to melling, the pressure relief kicks in at 92lbs so I'm not too worried about is as I have good bearings and use high qualith filters, so less danger of problems there. This year I switched the oil to 10w30 full synthetic and now read 50lbs of pressure at idle so it looks like I may drain the synthetic at some point and pour is something thicker like 15w40 or similar. I guess I can say that the engine is well oiled, I just wonder whether I am pitting the bearings due to the pressure or causing other wear somewhere. The pump has 20-30 hours on it so it should be broken in by now. I have also tried a regular mechanical gauge and it pegged it at 80 as well. I have another that goes to 100psi I may try just to see what I am getting.

 

[mojokim]

Re: Merc 302 vs 351

I agree an equally built 351w will outperform a 302. From a budget standpoint a used late model 302 roller motor is much easier to find. Another thing ro consider is how much of the engine work you will do yourself. If you have to pay a machine shop to not only machine but assemble your motor I think you would be hard pressed to beat an already assembled short block. For example Coast high performance sells a 306 shortblock with forged flat top pistons for $1499. You can get a mildly worked set of 302 heads for under $300 and using the Intake and carb noted above have around 230-250hp. Just my .02

 

[f_inscreenname]

Re: Merc 302 vs 351

Hay J <br />After a couple of seasons it will drop off. 20 hours on a motor is still new. That is pretty high though. Like my new van. If it fell lower then 45psi at any time I would be nervous.A couple years from now you will wish you had that kind of psi.