Install "high dome" pistons on factory "low dome" mercury outboard?

[ETX_Engineer_24]

Install "high dome" pistons on factory "low dome" mercury outboard?

I have an '81 Mercury 140 which I am in the process of rebuilding with new starter, trigger, 40-Amp stator w/ regulators, new CDI boxes, wiring, prop, impeller kit, and lastly but most importantly new pistons and piston rings. I purchased the high dome ported pistons not knowing that the motor came with factory low dome ported pistons. After reading multiple posts I've come to the conclusion that there is no problem with this application -if and only if - I use premium gasoline. Logically i was under the impression that the additional 0.1-inch eye brow height may interfere with the allowable clearance at the bottom of the stoke at TDC. So.... I placed the low dome and high dome inside piston hole No. 1 - the lateral distance from the bottom of the piston skirt to a pre-determined location on the block was exactly the same for both pistons; which in turn tells me that clearance is not a problem in this case and there >>should<< be sufficient space at TDC for either pistons to run the specified trajectory without the additional 0.1-inch brow to come into contact with the low dome block. Are there any other concerns of substance that I may be overlooking - prior to installing these "high dome" pistons? Has anyone ever put together such an application? and does it really create about +10 HP?

Thanks

 

[Faztbullet]

You will need to replace all 6 to get the extra hp and its about 10hp.The hp comes from the compression increase which you will loose as you will need to reduce timing to run the chitty fuel offered today.It will make no difference if regular or premium. Even when rebuilt it will only put out about 120hp at prop

 

[ETX_Engineer_24]

I agree. From my understanding - these motors have horrible fuel economy. Do you know of any aftermarket applications or modifications for these units that may that may yield better mpg?

Thanks

 

[ETX_Engineer_24]

From experience - can anyone chime in on whether high domes are applicable on a low dome block for an '81 Merc. 140 - I plan to use a slightly larger prop and the +10 HP may be just what I need (Also - I'd rather learn from another's experience, good or bad, rather than rely on my empirical measurements)?

Thanks again

 

[Chris1956]

Gee, I had a Merc 1500 and now have a Merc 135HPV6, on the same boat. I thought the inline fuel economy was very superior to the V6 engine. Of course the power curve of the V6 is much higher at lower speed. The top end is about the same.

Make sure your motor is mounted high-enough to minimize the drag of the gearcase. Also, as you lift the engine, the trim has less effect on bow-rise. Normally there is a sweet-spot.

 

[ETX_Engineer_24]

Chris1956 - Thanks for the suggestion.

 

[wired247]

Low domes you'll get 130 ish PSI cranking compression. High domes and it will be in the 140 PSI range. If use non ethanol ( or 10% for that matter ) 91-92 octane gas anyway it would be better to use the high domes . If you want to use cheap gas use the low domes.

As far as fuel consumption Ive always found the 10:1 rule of thumb npretty accurate with everything except HPDI motors. Same applies to V6's and inlines . 150 HP at wide open throttle is going to use 15 gallons per hour. 250 HP and you'll use 25 gallons an hour WOT. I plan my trips on it and it works out fine.

You hear all kinds of old wives tales about inlines. Most HP per pound, they use way more fuel than everything else. Stuff that might have been true in 1972 when there was nothing more powerful and they did use more fuel than a 80 HP. mercury HP numbers should always be taken with a grain of salt. Only difference between a 150 and a 140 was a little less compression. A little less compression isnt going to lose 10 HP. What really happened is they released the V6 at about the same time and didnt want to have to explain the difference. In any case that 140 HP inline turned into the 115 inline with no other real changes other than the rating system a few years later.

 

[gm280]

If your NEW high dome pistons were literally designed to work on that particular engine, then you should have zero clearance issues with them. However, if they were built for some other application and could also be used on that engine, then you seriously need to check the actual height from the wrist pin to the very top of the deck height and dome height and check that against the original piston from those exact same spots. And take into account how far the spark plugs screw into the head for that clearance as well. An old hot rod idea was to take some modeling clay and stick it on top of the dome piston and secure the head back on and run the crank up to TDC. Then remove the head and see what clearance you have. Remember as everything gets into operating temperatures there will be some expansion as well. So make sure those clearances will allow everything to still clear when run hard... JMHO!

 

[Faztbullet]

There are no clearance issues between low/high domes. it will be hard to see and feel 10hp increase as it may turn a few more rpm. What kills a inline is the reed block as it so restrictive. When I raced these years ago we cut blocks,made different transfer covers,etc all in order to increase crankcase volume. Then we would crank up the timing till aluminum melted on dome and then backed off a couple degrees, most I remember dialing was about 34 degrees@7000rpm but this was 1/4 mile runs.How are you going to run a 40amp stator???? that will kill some hp!!!!

 

[wired247]

The difference between a 9 amp stator and a 40 amp stator is ...31 amps. 31 amps at 14.5 volts is 450 watts of power. One HP is equal to 746 watts. 450/746= 6/10th of a HP. There are some efficiency losses in the conversion from mechanical power to electrical wattage but 1.2 is a generally accepted power conversion factor so 1.2 X.6 = .72HP. Almost 3/4 of One Horsepower.

The problem with heavy flywheels isnt so much the power it takes to run the 40 amp charging circuit , its the stress it places on the top bearing at high speed. There have been plenty of tests showing almost squat for difference in top speed between styles/amperage/weight of flywheels. The motor MAY rev faster but terminal HP won't be affected much..I used to have a drag car that ran 10.10 second quarter miles all day long. disconnect the 60 amp alternator and it would still run 10.10 second quarter miles all day long with no change in trap speed.

Having blathered on so much about that I don't know that you can convert an inline to 40 amp.

 

[ETX_Engineer_24]

The reason I decided to run a 40-amp stator is because I plan to add multiple accessories to my hydra sports boat. The current draw from a mid range amplifier would fry the 9-amp stator clean out of the water. Per some calculations and a few empirical formulas I interpolated - this addition may yield up to 15-hp reduction @ 4500 rpm. I like the play-doe idea!

 

[ETX_Engineer_24]

The rectifier on an inline-6 can not support the additional 31-amp load. The inline-6 can be converted to a 40-amp setup as long as the aftermarket regulators are able to independently regulate the output. I've seen multiple late 90s - merc V6 models use the same CDI technology with the high power 40-amp stators and what appears to be a very similar wiring schematic.

 

[wired247]

The reason I decided to run a 40-amp stator is because I plan to add multiple accessories to my hydra sports boat. The current draw from a mid range amplifier would fry the 9-amp stator clean out of the water. Per some calculations and a few empirical formulas I interpolated - this addition may yield up to 15-hp reduction @ 4500 rpm. I like the play-doe idea!

It absolutely does not take 15 HP to produce 450 watts of power. Put theory aside and look at actual specs of actual dedicated generator sets. A standard 10 KW generator produces 10,000 watts of power with a 15 HP gasoline engine. That is nearly 700 amps at 14.5 volts. Same deal though. 10,000/746 X 1.2 = 16 HP. I don't know what empirical formulas you are interpolating but your numbers are way off.

Use a 40 amp V6 flywheel and stator with dual Mercury regulators. The timing marks will be off and it wont fit under the cowl but it could be done.

http://www.homedepot.com/p/All-Powe...ility-Cart-Electric-Start-APGG10000/202757614

 

[Faztbullet]

It wont fit under cowling and the end cap height is different between a 40amp and 16amp...

 

[Dukedog]

I like the play-doe idea!

Its called "plastigauge". Been used as long as I can remember. Think it showed up in tha '50's. He must be talkin' 'bout Fred n Barney!



40 stator won't fit 16 (top cap) of any kind except tha very, very rare '91 or later V6, (probly Australian) 16 amp without usin' a "cut off" wheel on it which weakens it considerably ...............jmo

 

[wired247]

No Title

You wouldnt have much room in that inline cover but you MIGHT be able to shoehorn a 93 mm denso alternator in there if you build a bracket set,pulley and fab a bulge in the cover. That would give you an additional 50-60 amps at the cost of about 1.5 HP off your gross . No one makes a kit . Yuu'd have to do it on your own.

Or just ditch the inline and stop interpolating.

 

[ETX_Engineer_24]

Wired247 - I actually meant 1.5-hp which is still quite conservative. I have not yet purchased the 40-amp stater ... and designing a mounting system around the factory design s not really what I had in mind so I may discard the idea altogether. My goal for this project is to keep my Mako/250 yamaha out of the lake and instead use the hydra sports with the 140 inline-6 for that. Thanks for all the suggestions guys!