Max rpm on a 5.7 Mercruiser......

[CliffB]

I have a 5.7L Mercruiser in my old 1989 Skier. It's a great engine, runs really smooth. In the last few months I have added a Petronix II and a matched Pertronix Flamethrower coil. I have also done some maintenance things like put on a new water pump. The cap, rotor, wires and fuel filter have not been touched yet (will rectify that soon). The timing has not been checked.

The engine's max rpm is about 4,400-4,500rpm. That's with a light load (1/2 tank gas, two fit adults), minimal stuff in the boat. At this rpm the thing is hitting about 46-48 mph GPS verified. The prior owner said something about the engine having had some head work done but it was pretty vague exactly what, and the heads look dead stock to me.

I'd like to be able to get this engine to wind up to 4,800 or even 5,000 rpm. If I do all the above stuff (plugs, cap, rotor, set timing, change fuel filter), should this thing rev to 4,800-5,000 rpm with a light load? The prop is a 13X12 cast prop which has been balanced and is in good shape.

Is there a rev limiter on this thing somewhere?

Any help/advice appreciated. Thanks

ps. what are the recommended spark plugs for this engine?

Edited January 6, 2013 by CliffB

[nuttyskier2002]

The correct Mercruiser part # for your plugs is 33-898264001. This is the same plug as the MR43T. However...you can run the R43TS plugs and be just fine. The "S" plug has a slightly longer firing tip that reaches into the combustion chamber just a bit more. It is also a more common plug as the MR43T can be a bit hard to come by. The MR43T does have a stainless steel base as it is a true marine plug. Gap is .035". You won't hurt your engine running it at 4800 to 5000 for breif periods. There is no rev limiter on that engine as before '90 marine engines were still farely primitive as compared to their automotive counterparts. If there has been anything done to the heads that increased your compression...you need to keep an eye on those plugs as you may need to go down on the heat range (42 vs, 43). After you get your new plugs installed, new cap wires and rotor too.........take it to the lake, warm the engine to operation temp. Then run it wide open for about 1 minute. Quickly back off the throttle and shut it down immediately. Pull all the plugs and look at them. They should be clean with a slightly tanish color starting at the tip of the ceramic insulator and getting progressively darker toward the core. It they are very white you are running too hot (or lean). There should be NO silver looking specs on the insulator. If so this indicates aluminum melting off the pistons.

As far as your question about will the engine wind to 5 grand with new igition parts......this is very subjective. Wide open engine RPM will depend largely upon your prop (diameter, pitch, cup and wear. And of course it also will depend on how your boat is loaded.....but I'm sure you already know that.

[Woodski]

A SBC is mechanically capable of running up to 6,000 PRM but in a typical ski boat application the hull drag will be the limiter at about 5,000 RPM and the airflow characteristics of a pre '96 cylinder head peaks around the 4,500 RPM level. No electronic limiter, just physics. In addition to the excellent previous post, other items that will affect your WOT limit will include ignition timing and specific carburetor calibration and perhaps your spark arrestor (some are pretty restrictive). All of the items you comment on other than a change in the base timing won't really change the WOT performance unless there was something wrong or damaged with the ones you are replacing.

The 4400-4500 RPM is consistent WOT limit with that era of engine. If you really are in need of jumping to the 5,000 mark, it will take an improvement in airflow to get there. The key is the cylinder heads, the Vortec style head introduced in 1996 improves airflow and allows the SBC to go from the 265 hp output to 300hp+. A set of cylinder heads and then some exhaust manifold work will get you to where you want to be, and more if you want to continue the upgrade.

Some other options include: base ignition timing (more advance, but you will need to watch for detonation), 1.6 ratio rocker arms, camshaft change. You could massage the existing cylinder heads, but Vortec ones are pretty cheap and a significant improvement.

An additional note: all this assumes you have a 650cfm or larger carburetor, if not that is a restriction and a cork in the system.

[nuttyskier2002]

I installed a pare of Vortec heads on my '96 Malibu Echelon. As Wood says.....you will notice an immediate gain from just this mod alone. You will need to purchase an aftermarket Vortec type intake manifold as the bolt pattern and angle are different. Watch the ignition timing though because the Vortec heads do not require as much advance to get the fuel mixture burnt on time. Also these heads use a totally different spark plug too (R43LTS). Some say these are the best flowing cylinder heads Chevrolet has ever mass produced. I've seen chart numbers that indicate these heads are pretty equal to or better then any head sold or modified for moderate performance use. I didn't perform any exhaust mods on this boat before selling but I'm sure there are more gains to be had here. Have fun with that boat....you will love what you can do with it.