5.7L power tips

hwy39 · Dec 6, 2004

Re: 5.7L power tips

1900sx- let me know your results as you progress. I have a 1996 searay 21 with the 5.7L 210hp two barrel myself w/out vortex heads and am also looking into increasing the hp. From my review, just switching from two barrel to a four barrel with out also swapping heads and cam wouldn't do much for hp increase.

John_S · Dec 7, 2004

Re: 5.7L power tips

hwy39, There is a listing in the merc 350 service manual, for a 4brl equiped L model w/alpha. Not sure what years it might have been sold, but is rated for 235HP. I suspect that this is a flat tappet L model with just an intake/4brl change. 25Hp will probably get you 2 mph. The '96-'97 4brl LX and mags probably all have the roller cam, and rated at 250HP. As far as I can tell, same heads as '96-'97 L's. 3, maybe 4mph. Going to the gen+ models, which have the vortec heads, gives another 30hp (280 - using 4brl throttle body injection specs) about 5-6 mph. From here on, it the $ scale starts to really ramp up. blowers, 383 stroker or big block w/bravo

qystan · Dec 7, 2004

Re: 5.7L power tips

Had a look in the Merc parts list. Cylinder head p/n, all serial numbers in year range 1988-95 938-9718 both type of lifters 1996 938-9718 1997 938-9718 1998-01 938-803860T only roller lifters Cam p/n, same for all the years Flat lifter engines 431-5943 Roller lifter engines 431-811658

MGallik · Dec 9, 2004

Re: 5.7L power tips

Vortec heads would be nice and certainly simplify things as they make very good power in the mid range if matched to quench pistons. If you don't use them, there's not enough gain in the heads alone to warrant the cost and work involved. A 5.7 that makes 280+ hp at 4800 to 5000 rpm is not hard to build. All the parts do need to be balanced though. I suspect the 235 hp 5.7L uses the low rise intake and is probably also limited by the 4700 rpm rev limiter in the Thunderbolt V ignition used on the 210 and 235. The cast iron, high rise intake with a Quadrajet or Weber 4 barrel off any early 90's 350 Mag would easily make 260 hp, but not with your ignition. I suggest those carbs because the jetting will already be done. Install a Thunderbolt IV ignition module from an LX or Mag from about the same year as the carb or a Thunderbolt V from a 96 or newer LX or Mag. You'll also need the knock sensor and knock module as our engines aren't equipped with them. Why not? Again a guess on my part but if they aren't used on the 210 or 235 the advance curve and/or total advance must be pretty mild. It really appears that the most limiting factor on performance with stock marine engines is the ignition. FI is hard to modify and another area where the manufacturers can limit your engines power output, 260, 270, 280, nonsense. One is not really any stronger than the other. Torque peak in the right rpm range is the answer. Ignition to match a 4600 rpm dual pattern cam with a high rise dual plenum intake and a 4 barrel is the solution. The Vortec 350 Mag 300 hp is rev limited at 5100, we can beat that with a 4800-5000 rpm cam profile. We can build 430 hp 5.7's. The question is, how much do you want to put through your Alpha? Actually, the advance curves on all the Thunderbolt modules I've looked at are rather conservative. I may try to match a cam to a modules curve rather than buy a high dollar aftermarket ignition. I just want my engine to pull hard to 4600-4800, ultimate power and top speed are not what I'm after. If I can run half loaded at 55 mph or so I'm happy. Had my boat come with a 250 hp 5.7LX, I probably wouldn't touch it. As long as I have the intake off, I'll add a roller cam, looks like a 268 will be just about right, the stock roller is a 258, peaks at about 4500 but I want a bit more to make up for the Vortecs. By the way, I found and bought a brand new 14097494 intake manifold on Ebay.

I guess I now have an Edelbrock 2104 for sale.

John_S · Dec 11, 2004

Re: 5.7L power tips

1900sx, Some non-engine performance mods, you might want to research, are torque-shift prop, and trim tabs (either smart tabs or manual). Lots of info here, on both.

MGallik · Dec 11, 2004

Re: 5.7L power tips

Thanks John. There's a lot to take in. I appreciate all the info and suggestions in this thread so far. It's all very good info. I'd kind of hoped to bring it all together in this thread as an easy place to get a lot of solid information. Looks like the idea is working.

MGallik · Dec 14, 2004

Re: 5.7L power tips

Well, I guess I'm guilty of "kinda remembering" what I paid for stuff. Here's the skinny on the real cost of the parts I've purchased off Ebay so far. Here's the Iron Manifold The Merc Weber 4 barrel Carb The Flame Arrestor The Ebelbrock Intake The Roller Cam Kit I couldn't believe it when the 494 came up and grabbed it FAST, the Edelbrock is for sale.... I copied this post from another thread as I thought it needs to be included as part of the path I've chosen in my "Quest For Fire".

MGallik · Dec 14, 2004

Re: 5.7L power tips

Some discussion on an ignition module was done in THIS THREAD . It looks like I'll use the Thunderbolt IV V8-24s ignition module. It's a lot cheaper and doesn't have a rev limiter like the Thunderbolt V's. This module is used on the 1994 350 Magnum and should come out of the hole like a bear and pull smartly to 5000 rpm, even with a step or 2 up in cam duration. I'll use the stock roller cam as I'm not building a race boat, a 350 Mag that won't abuse my Alpha outdrive will be enough. The next thing will be the exhaust risers. I think the Mag uses a 4" or stainless riser that flows better than the lower performance 3"ers. The standard through prop exhaust will be fine. Through hulls won't help until you're making 350 hp.

Richard Petersen · Dec 18, 2004

Re: 5.7L power tips

I am a experinced CHICKEN. Leave in some kind of REV LIMITER. We are driving over 45 mph? Pedal to the floor? OOPS I just jumped a little wake. Unloaded prop? What could have happened? strange rough idle, tappet noise, goose and clear it out, worse, what a racket.

Bondo · Dec 19, 2004

Re: 5.7L power tips

Originally posted by Richard Petersen: I am a experinced CHICKEN. Leave in some kind of REV LIMITER. We are driving over 45 mph? Pedal to the floor? OOPS I just jumped a little wake. Unloaded prop? What could have happened? strange rough idle, tappet noise, goose and clear it out, worse, what a racket.

It's Not the Over-rev on launch that'll Smoke your motor........ It's the Reload, upon Re-entry that does the Damage.............

MGallik · Dec 19, 2004

Re: 5.7L power tips

But it's also a problem that I've never had in 25+years of boating in boats WITHOUT limiters. Pre 95 you'll find them rarer than hens teeth. Richard, lets build engines here and start another thread for the investigation of the nature, causes, or principles of reality, knowledge, or values, based on logical reasoning rather than empirical methods. Bondo, you hit it right on the head. If you want to get air you have to keep that throttle hand active. The average engine won't hurt itself. I like the idea of a rev limiter, but I want mine to protect the engine and not be a tool of the manufacturer to limit my engines power for the sole purpose of forcing the buyer to pay big dollars for an "upgrade" engine. As I continue through the quagmire that is Mercruiser's most tightly held secrets, I find the ignition to be the greatest limiting factor to what your engines output actually is.

MGallik · Jan 11, 2005

Re: 5.7L power tips

Is there any difference between the elbow mount and distributor mount Thunderbolt IV modules? I'm referring to the plastic and not the old style aluminum modules. What I really need to know is if I can just drill the rivets out of the elbow mount and mount the module on the distributor. Another question, do you think the mid range advance of the V8-24s is a little too aggressive for the stock flat tappet cam? I have my own idea but want a second opinion. Thanks for any info.

5.7L power tips

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