manifold guage on 5.7 volvo

[cdavisdb]

I am the recent and proud owner of a 25 seafari with a 1998 fuel injected 5.7/duoprop. I am adding some additional gauges, raw water pressure, oil temp, and fuel pressure as a way to warn me of impending problems. My main use of the boat is long trips far away from help, so I need all the warning I can get.

Would it do me any good to add a manifold pressure gauge?, fuel economy? trouble shooting? How would I use it? Any other suggestions for additional guages?

Thanks for your input.

Connor

[Bushwacker]

Excellent question! Manifold vacuum is directly proportional to cylinder pressure and is one of the most valuable gauges you can have on any 4-stroke engine! I'm amazed that more guys running these engines don't monitor it! The exhaust valve is the weak link in these engines and to avoid burning exhaust valves you should not run continuously at a vacuum lower than about 5"Hg. I run one in my truck and also observe the 5" limit when towing, dropping it out of overdrive whenever it won't hold speed at 5". The higher the vacuum, the longer the exhaust valve will live. This a very sensitive measurement which can also detect sticking valves, misfires, retarded timing, etc. Good to have as an indicator of engine health/performance in addition for setting cruise power.

The rationale for this limit comes from the aircraft engine industry. The piston aircraft engines like Lycoming and Continental were designed with a "max continuous" power rating that was about 75% of full power. These are expensive heavy duty engines with sodium filled exhaust valves, stellite valve seats, and positive valve rotators that very few marine engines have. If you run through the thermodynamic calculations, 75% of max power works out to be about 5"Hg manifold pressure. I was an engineer with Pratt & Whitney for about 35 years and all the guys I worked with that ran inboards or I/O's all ran them to manifold vacuum, and always cruised at 5" or higher. Frank Brown, who is an outstanding local mechanic that used to crew for Carl Moesly when he was racing the SeaCraft 21's in the offshore powerboat races in the early 60's, said Mercruiser also used to recommend 5" as a minimum limit for a continuous (cruise) power setting.

[BigLew]

Excellent information!!!!

My question is would we ALL benefit from a discussion of instrumentation options and the value of each and how we "mere mortals" will benefit from the education? Any thoughts; Bushwacker, BigShrimping, and CERTAINLY NOT LEAST - Fr. Frannk?

My 2 cents- I vote YES!

[Blackfin26]

Just great stuff...This is what sets CSC apart from the rest...Keep it coming!

[bobbert]

Just went down memory lane to diagnosis problem on 3.0 Mercruiser I/O. Ran great for 20 minutes and then the brown stuff hit the fan. Would not run under 2500, white smoke (steam) out the back, etc.
Compression test cold 150 in all holes.
Vacumm gauge showed a drop from 15" to 5" when things went wrong. Compression check indicated one hole at 120.
Took head off and took it to the head shop. Explained reading to the Head Head guy who said either head gasket or exhaust valve.
Turned out to be both a hot head gasket break and a exhaust valve with a crack AT THE SEAT.
On the 3.0 just in front of the riser is a BRASS plug this is the vacumm tap.
Answer Vacumm gauge is big indicator of engine condition and a problem solver.

[gofastsandman]

Bushwacker, where was your brain when they were making Young Frankenstein?

[Bushwacker]

Just realized I sort of got off on a rant and didn't answer your question regarding other gauges! Monitoring key parameters is a good idea and something we always did on experimental engines to catch problems before they caused a failure. Sometimes it even worked! Did performance work on jet engines for the first 10 years of career at Pratt, and became a bit of a fanatic on instrumentation. It used to drive me nuts when guys would use a 500 psi transducer to measure 30 psi, when they should have used a 50 psi 'ducer! Accuracy is always a percentage of full scale reading, so smaller ducer was 10X more accurate. This is why I prefer mechanical gauges with their 270 sweep where you can use a large scale, instead of the small, compressed 90 degree scale on electrical gauges. Electrical gauges are less reliable, especially on a boat. Digital gauges can provide tons of info, but take some getting used to. Our brains don't process digital info as quickly as position of a pointer on a dial! That's why you don't see many digital gauges in race cars!

The vacuum, water pressure, oil temp and fuel pressure gauges will give you a good idea of what's normal for your engine. Helps to stick a small piece of green tape on gage at "normal" pointer location; then a quick glance will pick up a potential problem.

We used audible alarms for the really critical stuff on the Pratt engines; they can be a pain, as they go off when key is turned on, but many industrial engines use them, so could consider for oil pressure and water temp. Only other thing I can think of is some way to monitor oil temperature in lower unit, but don't know if Volvo makes anything for that; might check the aftermarket.

Be careful with the plumbing on that fuel pressure gauge! I believe Autometer uses a diaphragm with theirs to minimize amount of plumbing with gasoline in it. The diaphragm goes between the fuel pressure source and the tubing running to the gauge, which I think is just a very sensitive air pressure gauge, but calibrated to read fuel pressure.

That sounds like a very efficient power package in your Seafari . . I'm sure the Forum members would be interested in some performance numbers, especially those considering a conversion to outboards! Denny

[cdavisdb]

Thanks very much for the imput. It confirmed my suspicion and I will be adding a manifold pr guage. On the fuel pressure, volvo has a nice access point in the high pressure line, very simple, for once. I like manual guages, too.

I get my baby back from the mechanic this week and will publish some specs as soon as I can get them. My last boat, 24 Seabird, 5.7/duoprop, (its for sale, cheap, hint hint) with stainless props) is very fuel efficient. The weight is the same, so I'm hoping for good things.

Connor

[nestorpr]

Connor, is that the 570SP engine? If it is I have the same one but mated to a 280 drive, I'd love to change it for a duoprop. How does it work for you?

[cdavisdb]

(I think) The 570sp is a pre-vortec GM block. Mine is a 570gsi, the first fuel injected model and I think the block is a vortec, the water pump looks the same. My old setup started as a 570dp, same engine as you with a 290 duoprop Basicly same engine in all of these. The vortec is a lot quieter, but I don't know why.

You would LOVE a duoprop. For me it was kind of like power trim. The idea makes sense, but you don't really appreciate it until you have one. I would no longer even consider an i/0 without a duoprop. Warning, the difference in handling, particularly backing up, with make you crazy for until you get used to it. For me the greatest advantage is low end torque. My Seabird would stay on plane down to 11 knots(oversize tabs and drive all the way down). The ability to run slow, but still solidly on a plane, when it gets snotty is a huge boon. I can't say it runs any faster, but it sure is efficient.