Looking for performance intake parts ssuggestions

[nhuyck]

I have a 1998 Mercruiser 5.7 L 350 carburated small block that has 500 hrs on it. I have to send my cylinder heads off to a machine shop this off season due to two leaking exhaust valves and new seals, and while I had everything apart I was thinking about an intake upgrade. My main focus is fuel economy vs performance as this is just in a cruiser boat and rarely runs over 4,000 RPM....Mainly cruise around 3,500 RP. It gets decent economy right now, but if I could get some decent intake parts (manifold and Carb) for under a grand, I would be interested....any suggestions? I would even entertain an Edlebrock E-Street conversion, but I want to know of anyone who has performed these upgrades...Thanks.

 

[guyjg]

You should be running a 4 BBL carb for fuel economy...drinks les than a 2 BBL. It will run all day on the front barrels but will gulp gas when you hit full throttle and the secondaries kick into play.

 

[kghost]

I would say leave it the way it is... I doubt you would see any gains with any change. Not enough tunability with a carbed engine.

A 4 bbl intake of similar design is just that "similar".

Not having EFI there is no real way to tune it for 3000- 4000 rpms fuel economy beyond the way it is now.........

 

[bondo]

I have a 1998 Mercruiser 5.7 L 350 carburated small block that has 500 hrs on it. I have to send my cylinder heads off to a machine shop this off season due to two leaking exhaust valves and new seals, and while I had everything apart I was thinking about an intake upgrade. My main focus is fuel economy vs performance as this is just in a cruiser boat and rarely runs over 4,000 RPM....Mainly cruise around 3,500 RP. It gets decent economy right now, but if I could get some decent intake parts (manifold and Carb) for under a grand, I would be interested....any suggestions? I would even entertain an Edlebrock E-Street conversion, but I want to know of anyone who has performed these upgrades...Thanks.

Ayuh,.... Do ya boat in Saltwater, or Sweetwater,..??

Raw water cooled, or closed coolin',..??

 

[04fxdwgi]

Unless engine is closed cooling or boat is freshwater ONLY, stay away from aluminum manifolds and stick w/ the factory unit. Raw water cooled with salt / brackish water could make the manifold fail pretty quickly from corrosion.

 

[RicardoMarine]

This is what comes up for the Edelbrock E-Street Power Package Top End Kit's For SBC.
Aluminum cylinder heads and aluminum intake manifold. This intake is dual plane, of which works well for the Marine version SBC.

However, neither will fair well unless your engine is equipped with a closed cooling system due to the PH unbalance of sea water.... be it salt water, or river/lake water........ it just won't work for any length of time.


Your 1998 Mercruiser 5.7 L will be OEM with the full dished pistons. This typically dictates 64cc cylinder head combustion chambers.
If you were to switch cylinder head combustion chamber volume, this must be factored in.... unless you also change pistons.

**********************
As much as we wish this was NOT true, there's not a whole that you can do to increase performance with a change to intake and carburetion alone.
The true performance values come from within deeper into the engine.

IMO, if you want a performance increase... which may include some economy improvement......., stay with the GM Vortec 64cc chamber cylinder heads. Tear the block down, and remove the full dished pistons.
Purchase and install a quench style piston that closely mirrors the Vortec combustion chamber "wedge" area.
http://www.kb-silvolite.com/images/feature/KB735_700pix.gif
This style piston creates an effect that the OEM full dished cannot offer.

A good piston manufacurer sales person can offer you the correct p/n pistons for your application.

With the correct piston underneath the Vortec chamber, and with the correct compressed head gasket thickness......... you'll gain a quench area, aka "squish zone". The Quench Effect allows for changes in Ignition tuning that will benefit low end torque characteristics.
Nothing to the C/R or other engine components requires a change.
See this thread for more info.

Green arrow points to the quench area or squish zone.
Red arrow points to the dish volume that controls C/R.