Building a 350: newbie finer points?

A couple questions on a powerplant for my Baja 190 supersport. It currently has a wasted 305 so I'm replacing it with a 350. I know very little about I/Os, but I have a good bit of experience assembling engines so I thought I'd save some cash and build it myself instead of paying someone else $1600 for a shortblock... jeez that stuff is expensive.

Anyway, I'm shooting for 300 hp with a top RPM of 5000. I need to know some things before forging ahead. The dyno simulation below was made using desktop dyno with an off-the-shelf Comp 212/218 marine roller and as-cast vortec iron heads.

- Given the power curve, should I tune for 5000 rpm with out any load in the boat so I still have plenty of power at 4500 and up for the extra weight?

- Is my torque peak at an OK place? Will I be putting too much torque to the Alpha? Will I be on plane by 3000 ya think? That dyno simulation is with a roller cam... should I consider switching to a flat cam to bleed off some of that torque?

- Does a stock marine exhaust manifold flow better or worse than a typical stock automotive manifold? A little better? A lot better? A lot worse?

- I may buy a marine shortblock. If I do, what do I look for in a quality build? I know about brass freeze plugs. Do the crank, rods, pistons need to be forged? Are 4-bolt mains necessary, or just a benefit? What should I look for in the oiling system?

- I have a friend at a plating shop here in town and I already have an aluminum intake... can I just have my intake plated with brass or something? Do I need a marine intake?

- 90% of my boating is in fresh water. Can I get by with aluminum components during the 10% saltwater time? Is a cathoded piece of aluminum as good as an unprotected iron piece?

I know I ask many dumb newbie questions, but I gotta learn somewhere. I have some books coming that should reduce my number of questions.

 

Yo, Curt! How's it hanging? Imagine running into another impala guy in a boat forum...

On the engine questions, Mercury has an alpha drive option for its 350/300 hp mercruiser, so you should be fine. It only has one gear ratio available for that ap, so that might be something to check. But since the rpm are so low, you don't really need to worry much about the bottom end. Mercury doesn't like to let on that they use the generic GM marine engine, so they don't list specs; but you can get them here. One item of interest is that the marine intake is iron on the bottom where the water flows, and aluminum on top. Seems like it would have been easier to just make the whole thing out of iron, but who knows.

On the general engine knowledge, there's not so much of it on this forum. Engines are viewed on this forum as one of several ways to power a boat, not so much as a fun and entertaining hobby in and of itself, so knowledge is limited. Try the hotboat forum instead.

 

Wow! Ironically I wanted to use an LT1 in the boat but I can't find marinization parts

Thanks for the help and the links! I'll take a picture of the boat behind the SS for you

 

LT1 manifolds

If you are still interested in using an LT1 to repower, we can supply HP stainless steel one piece manifolds. Standard units we supply are good for 400+ HP. 5 year warrany and weigh 12kg for pair.

 

The torque curves you show have a dramatic drop at high and low RPM's. Unless you have a very light boat and always run it at higher speeds, the driveability will suck. Also, with such a low RPM torque, getting up on a plane may take quite a while.

 

How would you correct it? Compression is mostly maxed, I've already added 50 lb-ft down low by using a roller cam, and the heads are spot on as far as flow characteristics.

The boat is 2175# dry, so 2600# or so with a person, some gas, and gear. The drive is an Alpha so I don't want to go nuts with torque, either.

 

Here is a screen cap of what Mercruiser puts out as their base vortec 300 mag EFI, and it has dramatically less torque than my build above. Maybe that will help for comparison.

 

Gonzo,

you don't know what you're talking about. Better than 350 ft/lbs of torque from 2000 to 4500 rpm is a huge, fat torque curve. The LT1 is KNOWN for its flat torque curve, because of its intake design, and it's torque peak is huge because of the efficiency of the heads.

[edit] Oh, I see Curtis beat me to it.

 

Here goes Stonebreaker the liar trying to get back at me again. I caught him in a few and now looks for my posts to backbite. Get a life dude.
Curtis 73: the curves you show are only above 200RPM. A boat needs a rather flat curve at the low end to get you up on a plane. Maximum torque and HP alone is deceiving. You need to have torque at the target RPM. Also, the LT1 is a 5.7 or 350. There is no mystery. The bore and stroke hasn't changed in many years. The combination of combustion chamber size, camshaft, timing, etc. give you different characteristics. Marine engines run at full torque or close to it for much of the operating range. A detuned engine may, at times, perform better. A better indication would be the torque at 1000RPM's. If it is equal or better than the Mercruiser then you are in business.

 

Torque is what gets you on the plane. Your going to start from 800 and pull throught to 2000. So from 1000 to 2000 is where you need plenty of it.

There used to be a modification to drill 1/4 holes on the prop exhaust hub to allow the suction on the back of the blade to break. This would allow the prop to turn easier and get the engines revs up to some where where you have some torque with out breaking into cavitation.

Theres not much point in having gobs of it at above 2000 when you cant get there.

 

Well, I'll let my 9-year friendship with Stonebreaker decide for itself. I've found him to be very trustworthy and accurate, but I'm curious about your assessment. Please elaborate.

I think the more important thing here is that I'll be using a cam that is a Melling copy of the Mercruiser cam, bettering it with Vortec heads (which add almost no port volume but adds considerable mid-lift flow) and in general copying or bettering what Mercruiser originally put out.

Personal vendettas aside: Do you have any suggestions for increasing low end torque using a 5000-rpm max? I've already maxed compression and reducing duration would be pretty pointless at this point, so I'm waiting for some suggestions...

 

Just because your software does't show torque below 2000 RPM, that doesn't mean it's not there, how ridiculous. I am running a similar set-up except with a Ford powered (302) with 2010 lb empty weight and am on plane at 2000 RPM, although My boat has less deadrise. Using the same software my torque is 300lb-ft at 2000rpm and builds to 380 at 4500.

Center rise marine manifolds flow much better than stock auto exhaust. Use Small tube headers in you software calculations. Also the software provides HP and torque at the Flywheel. You should reduce the numbers by about 15% to obtain HP and Torque at the Prop.

Pitch your Prop to obtain max desired RPM with Min load - driver and half tank of fuel.

Forgot to mention:
In order to get low End Torque, a cam with higher lobe seperation - 116 deg. and better flowing heads help alot. Although your numbers are great and it should come out like gangbusters.

I have been using an Aluminum intake for three years now (fresh water only) and the water passages are almost like new.

 

Curt,

Try this cam. I had to zip it up to upload it.

 

Curtis' first graph shows an engine combo that clearly provides more low end torque than the stock Mercury setup. In fact the Mercury has no more high RPM power potential yet still sacrifices some low end growl. Even though both torque curves look very similar, note that Curtis's combo's entire torque curve is shifted upward about 50 Lb-Ft compared with the stock mercury. This indicates that his cylinder heads and intake flow better so that he was able to use a cam with a reatively short duration and yet lose very little top end power. The very linear drop-off of torque past 4500 RPM indicates that the engine is 'running out of cam' (lacks sufficient cam duration) for efficient continuous operation above this speed. If he was running into a cylinder head/intake manifold limitation, the drop-off would be steeper and more ragged (not smooth). This is a nice de-tuned high performance build-up, just like you want on an every day marine engine. His engine combo will operate at peak cylinder filling and efficiency at more RPM points than the stock Mercury setup. I was not able to open the second graph, so I can't comment on it.

The OEMS tend to use intakes and especially cam grinds long after they are obsolete. GM and Ford were still using grinds that originated in the '60's into the early '80's when those big boys could easily have afforded modeling software for their mainframe computers and get much better grinds. Both company's high performance divisions were offering such grinds at the time; they were just not showing up in their street engies just yet. I would not be suprised if Mercury has similar habits.

Jimbo

 

Looks like 275 hp at 4500 and 375 tq at 3000. That's simulated with HP manifolds and mufflers. Add 25 to both of those numbers if you simulate small tube headers/mufflers.