Vacuum limit on Mercruiser/ small block Chevy

[FishStretcher]

I have been listening to Bushwacker's tutelage on cruise vacuum setting for the Mercruiser/ small block Chevy.

So I was wondering two things:

How long can you run Wide Open Throttle before you have to observe the 7-9" vacuum limit?

What is the failure mode, and can we do something about it? I hear exhaust valves. But can this be addressed with better valves alone, and then is WOT all the time possible? Or is it valves and seats and manifolds and pistons...

Finally, I want to see if there is a knock sensor on my engine. If so, then I have a nice trick for a power adder.

[Bushwacker]

Fish,

That's a good question but not an easy one to answer! I guess it boils down to "How often do you want to do valve jobs?!" The exhaust valve is definitely the weak link followed by the exhaust valve seats and heads. Failure mode would be loss of compression and power; worst case scenario might be the loss of head off a valve if you ran it long enough with a leaking seat. Sodium filled valves and Stellite valve seat inserts are the preferred improvements used the Continental/Lycoming aircraft engines.

The Lycoming O-360 used in the Cessna 172 has a TBO of about 2000 hours. A typical duty cycle probably consists of about 2 minutes of WOT operation at takeoff; climb out to cruise altitude would probably be done at 75% power or 5" vacuum. If you figure a typical flight lasts about 2 hours that would give you 1000 take offs/2000 minutes or 33.3 hours of WOT operation prior to overhaul. So a limit of about 2 minutes @ WOT might be a good starting pointAnd remember this is a premium engine WITH sodium filled valves and stellite seats! These engines have the ability to adjust mixture from full rich for take off and climb, to a leaner setting for cruise. They may end up cruising at 5"Hg absolute manifold pressure depending on cruise altitude.

A more practical data point comes from one friend of mine that I made several Bahamas trips with who had a 20 or 21' Wellcraft Nova with a 302 Ford/Mercruiser in it. He worked at Pratt and knew about the 5" vacuum limit, but he was not an engineer, so he didn't really understand the physics behind it. He installed a vacuum gage, and just set the throttle to 5" vacuum, and whatever speed that gave, that's what he ran! He basically ran 5" all the time! After about 300 total engine hours, not only were the exhaust valves burned, the seats were also badly burned and both heads were cracked! And this was on a Ford, which is generally considered to be more durable than the Chevy, since it does not have the siamesed ports and associated hot spots of the Chevy! He ended up pulling the I/O and replaced it with a 225 Merc hanging on the transom.

My other friend that owned the Moesly 21 "Unohu" was quite conservative and cruised and at about 9" vacuum. The boat came new with a 150 hp I-6 Mercruiser, and he did at least a couple of valve jobs on that engine before he learned about the importance of a vacuum gage. He subsequently upgraded to a 165 hp I-6 and then a 260 hp 350 V-8. He cruised at about 9" with the 350 Chevy and got at least 2000 hours out of it. A leaking exhaust manifold that put seawater in a cylinder finally killed it. Pistons shouldn't be a problem unless you lean out a cylinder and over heat the piston.

Don't know if this answers all your questions, but it should be a start. Denny

[FishStretcher]

I was looking at Dart heads, Inconel and stainless valves last night, from 0.281-0.441" stem diameter, with and without sodium filling. I don't need the RPM of titanium valves, I found some heads with beryllium copper seats, I think. It seems to me if I have the power in a small block, I want to be able to use it. All the time. And a lighter small block with good valves might give similar continuous power to a run of the mill big block. But lighter. Especially if one went to aluminum heads, but even barring that, it is interesting to think about.

I am wondering if I burned a valve on the Yamaha F100 on the 20 footer. I ran it HARD this summer.

[Bushwacker]

You probably know this, but if you don't already have FWC, you'll need to add it if you go with aluminum heads, so the weight of the Hex and extra plumbing would offset some of the weight savings of the aluminum heads. One thing I'd check on is the height of the risers on the exhaust manifold because that hull is so deep that the engine is pretty far below the water line. I think Connor said he went to 12" risers on his!

Just curious, how much extra were the sodium filled valves? On the valve seats, I think Stellite, which is around 55-60% cobalt, would be harder and tolerate higher temps than beryllium copper, although the latter probably has better thermal conductivity. Guess I'd research what the go-fast guys use on their engines, as they probably like to run WOT a lot!

[FishStretcher]

I have been looking, I think inconel valves from Manley are $38 each versus 214N stainless. Stellite is hard to find, as is sodium filled (don't recall a price on those). Normal valves are $9 each, I think. Some say the edelbrock e-tec 170cc aluminum head is the ticket and valve seats live longer because of the conductivity of the aluminum head. The bronze valve guides help too.
That might be the route to take. Or an improved valve seat insert. Some of these stock iron Chevy (vortec) heads came with inconel exhaust valve seats in 1 ton trucks. So mercruisers might have them?
I have the heat exchanger FWC system. Holy crap, are riser extensions expensive! And yes, the engine sits LOW in the boat.

Nothing is broken yet so I will wait and maybe build heads next winter. I will just have to set cruise at 5" Hg to make sure I NEED new heads.

[Bushwacker]

Like I said, those aircraft engines run some pretty fancy parts! You only need the nickle valves on the exhaust side. If you can't find Stellite seats, Inconel might be a good second choice, although my experience in gas turbines is that having two hot parts of the SAME material rubbing against each other will create LOTS of wear, so I'd try to find out the exact chemistry of the valve and seat alloys. Some of the nickle alloys also contain lots of cobalt, which is what you want in the valve or seat because cobalt oxide acts like a dry film lube when it gets hot. Looking at my alloy reference book, good old Waspaloy is 13.5% cobalt, Udimet 700 is 15.3%, Udimet 500 is 18%, and Inco 939 is 19%; all have 15-20% chromium too, so they would be very oxidation resistant at high temperature, but none of these are cheap alloys! If you can't get stellite seats, maybe inconel valves with cast iron seats would work ok, as the valve seats won't run nearly as hot as the valves, especially when mounted in an aluminum head. The bronze valve guides are a good idea too, since the wobbly ball-stud Chevy rockers put a lot of side load on the valves. Positive rotators on the exhaust valves would also help. They may be standard on some marine engines.

I'd be inclined to run what you have till it breaks while you're getting another set of heads ready. Connor seems to be getting pretty decent life out of his by observing a 5-7" limit, and he runs with a pretty serious load on his Bahama trips. I'd follow up with him on the riser issue though, as too short of a riser can kill an engine in a hurry!

[Blue_Heron]

Originally Posted by Bushwacker

I'd follow up with him on the riser issue though, as too short of a riser can kill an engine in a hurry!

Riser height for a Volvo Penta may not be the same as for a Mercruiser. Exhaust elbow internal geometry has an effect on how high they need to be. I have 3" extensions on my 350 Mag MPI and the top of the elbow is right where it's supposed to be.
Dave

[FishStretcher]

I looked a while back on riser extensions. I *think* need 4" or more. I think it's $1500 to add 6". Assuming all other parts are copacetic. That said, it has 200+ hours on this install. The local mercury place says make sure the anti-reversion flapper in the bullhorn is in good shape. That is the first/best line of defense, according to them- and the cheapest.

Oh yeah, GM performance Products has sodium filled vortec valves. $50 a pop at Summit racing. I think I would go Inconel with aluminum heads first. After I stumble across some unused cash.