980 hours on engines - compression test

[tpenfield]

I have 980 hours on my twin Mercruiser 7.4 LX MPI engines. They are going to lap 1,000 next season, as I average 40 hours per year, since buying the boat in 2012. . . 2013 being our first season with it on the water.

I have not done a compression test in a couple of years, so I wanted to do one at this point just to see where things stand. 1,000 hours is when you start thinking about rebuilding engines, etc.

I got a new compression tester (Innova model 3612), and one thing I noticed is that the check valve is right at the fitting for the spark plug hole, which my other tester did not have. The check valve was after a short adapter hose, which would lower the compression #'s somewhat. I seemed to get pretty good #'s with this gauge . . . not sure how much is the actual differences in the gauges (old vs. new) vs. the improved design of the new gauge.

Another thing that I did was to run a decarb a La SeaFoam before doing the tests. I used a method that I learned from Mischief Managed , which I think works particularly well for MPI engines.

I mixed up a 50/50 mix of Seafoam and gasoline in an aux. tank. Brought the engines up to temperature, then switched the fuel to the 50/50 mix. After the mix had a chance to get into the fuel rail, I unplugged the ignition wire on one of the cylinders and let the engine run on 7 cylinders for 2 minutes. Then I re-plugged that ignition wire and went to the next cylinder's wire unplugging and re-plugging until all cylinders had been run for 2 mins getting the seafoam mix without firing.

I then switched back to the regular fuel and let the engine purge the remaining seafoam mix. I'm not sure how much this helped, but it sure seemed like a clever seafoam solution for an MPI engine.

After I ran the engines for about 10 additional minutes on regular fuel, I did the compression tests. Here is what I got . . .

PORT ENGINE

1 - 149
3 - 140
5 - 131
7 - 149

2 - 160
4 - 138
6 - 147
8 - 153

STARBOARD ENGINE

1 - 147
3 - 146
5 - 134
7 - 150

2 - 156
4 - 132
6 - 145
8 - 148

A couple of low cylinder on each engine, but overall i'm pleasantly surprised with the results. Looks like I can hold off on a rebuild for a while.

Thoughts ??? :noidea:

 

[Scott Danforth]

how do the numbers compare with prior years?

 

[tpenfield]

how do the numbers compare with prior years?

Scott,

Here is the history of compression tests, including the one done during the pre-purchase survey. . .



The second one done in 2014 ( '2014C' ) was done same year after a decarb process. IIRC, I used water for the decarb in 2014.

 

[Scott Danforth]

Careful Ted, your inner geek is showing

 

[Lou C]

Man love your charts! My results are scribbled on scraps of paper shoved in my shop manual. The only thing that jumps out at me is the increase in variability (larger standard deviation).

 

[tpenfield]

Thanks, Lou

I think I should run a comparison of the 'old' gauge (not that it was all that old) and the new one, just to see if there is a significant difference in the #'s from each gauge.

I think if I compare the #'s from 2012 to now, that may be the most meaningful. In doing so, the engine has lost a little bit of compression, and one or two cylinders have dropped quite a bit. Maybe I will perform a leak test on those cylinders.

 

[Scott Danforth]

Ted, the cylinders that are dropping seam to be 4 and 5 on both motors. while I dont think you need a rebuild, I would pull the elbows and take a look at the valves and valve seats. you may have a bit of rusting in those cylinders

 

[tpenfield]

Ted, the cylinders that are dropping seam to be 4 and 5 on both motors. while I dont think you need a rebuild, I would pull the elbows and take a look at the valves and valve seats. you may have a bit of rusting in those cylinders

Agreed. Probably would need a valve job at some point. I did replace the exhaust manifold/elbow joint gaskets a couple of years ago. The internals of the exhaust manifolds looked good.

i was also wondering about doing a poor man’s rebuild. . .

Valve job
Piston Rings
Cylinder Honing
Connecting rod bearings
Crankshaft bearings

I am wondering if cylinder boring and crankshaft grinding would be necessary vs. just replacing the bearings/rings :noidea:

Iam am getting decent WOT performance- 4200-4300 RPM and 50 mph GPS.

I did find that the throttle on the port engine is only advancing to 87% throttle (per the Diacom software), whereas the starboard engine is going to 99% throttle. So, if I get the port engine throttle linkage sorted out, I might get a few more RPM’s and an mpg or 2 out of it.

During the sea trail in 2012 I think we got 52 mph. (740 engine hours back then)

 

[Scott Danforth]

doubt you need more than a simple valve job. while at it, clean up the ports a bit, and contour the valve guides.

at 980 hours, you probably still see the cross-hatches in the cylinder bores.

 

[alldodge]

i was also wondering about doing a poor man’s rebuild. . .

You could but I wouldn't recommend it

 

[tpenfield]

You could but I wouldn't recommend it

OK AD, just wondering though . . . you would not recommend it as in not needed, or not the right approach ? . . . In my searching I think my "poor man's rebuild" is referred to a 'refresh' (honing, rings and bearings) vs. a 'rebuild' (machining over-boring, etc.)

Just thinking ahead when I go to sell this boat, it would probably have 1100 ish hours and not having some sort of 'rebuild', even if merely a valve job and lower end 'refresh'. I'm thinking it would be a hard sell. Just not worth going full bore and spending $3-4K per engine. May as well just give the boat away.

Thoughts?

 

[alldodge]

I've done some low end rebuilds (only 2) in my younger days when not having the funds to do a good rebuild. Cylinders do not wear perfectly round. So over time the rings get a set and kind of stay wear there at. Also cylinders flare out toward the end more then the top due to the same wear areas.

Bearings also wear at different angles and most even when polished up have something slight off from perpendicular and 100% flat. Just to many possibilities for a spun bearing and pumping some oil

So its the cost of boring, pistons, crank grind and lifters so long as cam is within spec.

 

[Scott06]

Agree on not doing the poor mans route, done that as stated to save a few buck but was never right. When you're 18 time doesn't matter, we're probably too old for that stuff now.

For all the work to pull them your machinist will tell u what you need. You maintain your boat better than 98% of the people out there so I don't see an issue when you step up. u can find a kid who wants to do a quickie rebuild...enjoy them in the meantime.

 

[tpenfield]

Thanks for the input, everyone.

Just looking at the typical cost of materials and machining, it looks like cylinder boring and crankshaft grinding are not all that expensive. Much of the cost will be in the refresh 'kit' and the cylinder head work. So, I think that it would be worth to go the extra distance, depending on what the lower end looks like in terms of wear and tolerances.

Just looking at a budget, I figure about $1600 per engine in total cost and maybe about $600 in rigging equipment, stands, etc. So, about $3800 for both engines.

I think if I had the cylinders bored, I'd want to do the minimal amount ( 0.001" ???) so as not to add much displacement to the engine, which are MPI. Otherwise I'd need to consider a re-flash of the MEFI units. I know that AD had to have his MEFI unit re-programmed when he did his rebuild. Any thoughts on the cylinder boring vs. re-flashing of the MEFI ?

 

[alldodge]

I would bore standard .030 over and with that you shouldn't need to reprogram. Most of the original tunes are a little fat anyway and have not heard anyone having an issue with standard bore. Could even mill the head a few thousandths if needed to true things up without issue

I had mine remapped because of building for more HP using high lift cam and increased compression ratio. Went from 415 to 530 HP and now have a real cool sounding idle During build I changed to MEFI 3 because 1's were getting harder to find and the 3 has a smoother fuel tables for better overall air fuel ratio's.

Find a machine shop that does torque plate honing. It cost no more then one that doesn't do it, but the bore ensure a better more true bore. The plate is torqued to spec on the block and then bored. The plate mimics the torque that is applied when the head is installed

 

[tpenfield]

Thanks, AD :thumb:

After my previous post, I did the math and realized that a 0.030 over bore is not going change things much . . . I guess you need to stroke the engines to really increase Cubic Inches . . .

I found a web site that does the calculations for the over bore . . .

Standard Bore displacement calc . . .



0.030" Over Bore calc:



So, the displacement is increased by only 1.3% with the over bore. MEFI mapping should not really need to be adjusted for such a small change.

It seems that the typical over bore is 0.030" and the typical crankshaft grind is 0.010" under. (correct? :noidea: )

My engines are pretty tame (310 HP each), and I have no plans on changing that. My main goal is to facilitate the selling of the boat, given its age.

I will have to figure out how to get the engines out . . . :noidea: I've been thinking about building a gantry of some sorts on heavy duty casters, since the boat lives on blocks and stands in the off season. I'd have to lift/remove the rear seat/engine hatch assembly, then pull the engines. Probably would need a clearance of 10' 6" wide and about 8-9' tall to get things out and back in when the time comes. So, I'd have to roll the gantry back away from the stern in order to drop the engines down to ground level, etc.

. . . or maybe rent something that would do the same trick . . . not sure what that would be. :noidea: The key thing would be able to lift 1,000 lbs up to about 9' above ground.

 

[Scott Danforth]

Ted, while increasing the engine displacement helps (0.060 is 468 cubes). a stroker crank helps with adding a larger torque arm

the magic is in the heads and cam and how the air flows

the motor is simply an air pump. your motors should have either 088 or 188 heads. there is a lot of better flowing heads on the market, or you can do some mild clean-up on your heads to help the motor breath. knocking off the casting flash in the runners, radiusing around the valve stem boss, and some mild polishing of the exhaust ports will help

I agree with AD on the torque plate, I also suggest having your crank bore align honed and the rotating assembly balanced.

I went with an internal balance (like older small blocks and the 427). it cost a bit more because i started with an external balanced forged crank, however the motor is extremely silky smooth and will spin up to where the induction runs out of poop

 

[alldodge]

Your boat being on blocks will create some additional issues. Maybe rent a larger wheeled gantry or get a tow truck to come and help

https://www.offshoreonly.com/forums/general-boating-discussion/357086-absence-gantry.html

 

[tpenfield]

Thanks guys.

The heads are 10114156 (Gen6), I believe.

Doing some research. . . There is a ‘Blue Bird Tip Tow 10’ towable engine hoist that can reach to 10 feet high.

 

[Scott Danforth]

Ted,

here is your head 10114156...91-up...oval...OPEN...Gen.V & VI 454, 118cc chamber

If I was rebuilding your motors, I would look for a set of Merlin 269 ovals

https://www.billmitchellproducts.co...269cc-119cc-26degree-2-300-x-1-880-bare-each/

https://www.offshoreonly.com/forums/general-q/90450-how-much-hp-reliably-oval-port-merlin-heads.html