Per GM's Vortec Website, GM only makes one 5.7 marine engine.

Merc sells only one 5.7 motor: the 300 HP 350 Mag.

VP, however, sells two 5.7 motors:

• VP 5.7 Gi - 280 HP
• VP 5.7 GXi - 320HP

I am assuming the internals of both the VP engines and the Merc 350 Mag are all the same. If so, then what are the differences between the two VP motors? 40HP is quite a spread!

Also, to add more confusion, VP seems to now refer to the 5.7 Gi as the 5.7 Gi-300, and is now rating it at 300 HP. On their Website, VP lists the the 5.7 Gi-300 is listed as 280 HP, with the following footnote:

* Due to changing emissions regulations, this engine will be certified 300 hp in California after January 1, 2008.

Can anyone shed some light on all of this?

Different programming, different exhaust. The GXi vents a significant amount of exhaust above the water line when under way. It's often said that the GXi has a different cam but I don't believe that's the case.

To further enhance your confusion, also note that industry standard is to allow for +/- 10% on published HP ratings. But that applies to all engines, not just the ones you mentioned.

Just to be a jerk, I'll point out that Mercruiser does sell the 5.7TKS, a carbureted 5.7 rated at 250hp!

I have seen written that the 5.7GXi gets 1.6 to 1 ratio roller rocker arms and that the exhaust manifolds are different. The manifolds I have seen in photos look the same to me, and I've never removed a valve cover to check the rockers. But almost everyone notes that the 5.7GXi's power increase over the 5.7Gi comes at the absolute top of the RPM range.

For years the 5.7Gi has tested more than favorably against the 350MAG MPI. It's been considered a "strong" 280hp. I think VP just finally decided to call it a 300hp cause they could.

All things being equal and based only on the price increases, I would never get the 350MAG MPI or 5.7GXi over the 5.7Gi.

Some related threads

http://www.boatingabc.com/forums/ubbthreads.php/ubb/showflat/Number/124141/page/0/fpart/1

http://www.boatingabc.com/forums/ubbthreads.php/topics/337823/1.html


I'm going to order one of these covers for my 5.7 Gi and get an instant extra 20 HP

I've heard my 5.7Gi really has 300hp, not the 280 its rated for. It's a strong motor....

More related threads.

http://www.boatingabc.com/forums/ubbthre...html#Post385296

http://www.boatingabc.com/forums/ubbthre...html#Post428637

Well, after reading approx 2,367 posts in those various links, I think I have it. The GXi has the same internals, but different intake and exhuast, which help upper RPM breathing. The GXi also has a slightly higher redline (5,200 vs. 5,000). Also, there may be a difference in the ECM programming, and there may also be a difference in the valve train (roller rockers) - not sure that everyone was in agreemen there.

Bottom Line: The 5.7 Gi is the better choice for most recreational boats. However, those folks looking for max WOT performance might opt for the GXi. Scott L put it best by stating that the $ difference between the Gi and GXi would be better spent on a DP drive.

Thanks!

I left out the rollers on the GXi - it does have them.

You got it, the 5.7Gi & DP is the smart choice. I don't beleive you're talking about a big gain in WOT numbers to go GXi.

Heck, just order a few stickers and you got it. The cover is identical to the older 5.7 Gi.

Did anyone mention ported and polished heads and manifolds for better air flow? You could do that yourself to your existing machine and get a 30 to 50 HP increase out of it? Any engine is just a big air pump and the more efficiently it pumps air the better for you! That's what they did at the factory in addition to the programming modifications that you can probably get from somewhere by looking for them.

I did that to my built 327 4 bolt main that I put into a 1977 Chevy Monza with the stock Saginaw 4 speed and tall gears for the stock Vega 4-banger that it came with. That thing flew right up to about 60mph and then it screamed at some 5,000+ RPM. Of course it was carburated with a finely tuned Rochester Q-Jet as there wasn't any computer controlled injection back then. When those secondaries opened up, you could here it suck gas out of the tank!

Given the plus/minus 10% on the ratings, a "300" horse motor can safely be labeled anything from a 270 to a 330 horse engine. Considering the differences between gasoline from one day to the next, air temps, humidity, etc, engine ratings are just a 'best guess' anyway.

Those roller rockers are the sole reason for the higher redline (they lessen the likelihood of valve float).

Check the rocker arm ratio, too.

Unfortunately deep, the water cooled exhaust manifolds of a marine engine don't allow for the full gains of porting heads. And as you realize, that type of work usually raises the engine's upper rpm power and potential. Well, for whatever reason, Mercruiser and VP would rather keep their engines right around 5000rpm max. I guess since, in theory, you might run your boat there for several minutes. Whereas in your Monza, how long was it actually turning 5000+ rpms? 15, 20 seconds, if that?

Yep, the GXi isn't worth the price increase over the Gi, in my very humble opinion.

Want Power? :

Billy, with the gearing that I had it was turning 5000 RPM every time I got on the freeway. I could get to the bottom of the ramp right quick but topped out at 60mph at some 5000 RPM.

I cannot tell you what the real horspower is of my 5.7Gi -- but I will tell you it gets on plane very easily with 12 people on board

There was a comparison done by Boating Mag some years ago of the two engines. My memory is that they found little difference between the two and that their conclusion wasn't disputed by VP. Save some money and get the Gi.

MIKE

Just the sticker might be even cheaper.

Had mine ported, picked up 100 HP at 5,000RPM, far more torque judging from the throttle response, but my heads were a major choke point as the induction and exhaust had already been improved.

I would not advise anyone to blindly port their own, without some good guidance. Easy to actually make them worse (as well as grind into a water jacket and make them useless) by not having an idea what you are doing.

Biggest thing is smooth rounding of the short side of the turn into the valves, to get the air to follow the turn instead of breaking off and litertally, killing flow at half the valve. That and removing any other odd bumps, lumps, and casting flash.
Too big is bad for your RPM range too, unless the engine is up-sized too.

BTW, polishing has gone away, as it has been found that excessive smoothness actually hurts air flow. The surface boundary layer is actually thicker draging against a polished surface, effectively making the port smaller. The best is the light surface roughness left by the sanding drums while still fairly coarse.

Lowrider, it's been said that Vortec heads for small blocks flow so well, and are already so thin casted that porting isn't necessary, and you can grind to much away, very quickly.

That is definitely a hazzard. Can't claim to know a thing personally about Vortec heads, as, running a big block, have never dealt with them.
I personally sought advice, and consultation (as well as work) from a preofessional head grinder, and, beyond my guidance for what I wanted to end up with, and my aplication and engine build info, I left him to his business, and was VERY pleased.

I would advise anyone who has not ground on heads, or at LEAST been told a great deal by someone grinding on your "other" heads, to give them to a pro before trying to port your own.

The Vortec heads are IMO, from all I have heard, a definite improvement, and possibly the best mid-RPM (like boats run) small block Chevy head available, and as a stock head, reasonably easy to find, at a good price, if undamaged.
A VERY GOOD upgrade part. Even so, prob. only worth 30 or 40 HP alone, unless other things have been upgraded already, or are part of the plan.
Harder to find performance manifolds for them I think, but then again, I have not looked arround. I may be mistaken here.

You can find ANYTHING you want for a big block Chevy, especially the old Mk-IV, The Gen VI not too bad, The Gen V, I'm not a fan of, for several reasons, including aftermarket parts availability.

Now, unlike most folks here, I have run mine well beyond that 5-grand limit, and for lengthy periods of time.

Think we howled her 5,400 for 4 minuites minimum chasing down a friend's Taylor 923 I/O (yea, I know, but it's still a Taylor, and Damien's a good guy), over at least 4 miles, then down into the glass of a deep cove too. Great time. Beautiful lake (Ouachita Arkansas).

So far I limit my 5,900 RPM nitrous runs to 30 secconds or so. LOL

My power peaks at about 5,500RPM, and I have the right impeller cut (prop guys consider this as "prop pitch") to allow me to use all that power. I don't have a gear-change in a drive to worry about tho, direct drive here, 1:1. Extra RPM not a problem to a jet, till you get close to 7-grand.

Thus, as noted in here earlier, considering power upgrades, keep in mind your intended operating RPM. MOST (but not all by any means) power upgrades will increase power all over (if done properly), but will do the most at higher RPM (the big blocks are somewhat (but only partially) an exception to this). If the higher RPM is a problem for the drive, re-consider how to increase power. Most of your engines (in stock form) are safe to 6,000RPM or more, BUT, the limiter is valve float from weak stock springs (that and the rev-limiters some of you all have). :-]