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Chrysler 318 respectible compression ratios?

late start

New member
HI...I'm just about to buy a boat w/318s, 1975, 800 hours, no rebuilds.
We're testing compression tomorrow. As long as they are 'similar', how
LOW can they be before I pass up the purchase?

Thanks,

Late Start
 
If the motor has been sitting for a long time (years) it will have low compression. A good run will bring it back up (free up the rings).

Jeff
 
Your thread title implies compression ratio, but I think what you're after is cylinder pressures.
C/R will determine cylinder pressures.

The OEM suggests that the 318 C/R is 9:1...... and the 360 C/R is 8.5:1.
( PDF page #6 section 2 here)
http://www.chrysler-marine.com/inboards/QMAR318-360.pdf

Chrysler suggests 100 psi, of which sounds rather low to me.
If you use a C/R -vs- C/P table, a C/R of 9:1 suggests a cylinder pressure of 139 psi @ cranking speed.
http://www.healey6.com/Technical/Nominal compression pressure.pdf


Some of the better Tech Articles suggest that a new or healthy engine should give you readings whereby the lowest cylinder reading should not deviate from the highest cylinder reading any more than 10%. Chrysler Marine says that 25% would be acceptable.
That sounds rather generous to me as I've not seen a factor of 25% used before.

Remember that the pressure will be accumulative only during the first two cycles (4 revolutions), after that you'll see real readings.
IMO, I would cycle each cylinder at least 4 x's ( 1 cycle = 2 revolutions).


.
 
Many Thanks everyone. Yes, I did mean cylinder pressures, not CR.

Here's why I sought your advice. The boat (a 1975 32' Trojan Sedan) has been laid up dry for approximately 3+ years, and shows signs of attention
to detail before being put to sleep. (The freshwater tanks were still filled to the brim with anti-freeze, and the like). I instructed the evaluating mechanic to not try to crank the engines, but rather to pull & inspect all plugs and 'squirt' the chambers with Marvel. My idea was to help any 'stuck' rings free themselves during the first cycles of cranking. We did that exercise on Thursday and plan to 'crank' on this coming Monday.
That's the background.

Now, during first cranks on Monday for the cylinder pressure tests, will the readings be skewed by virtue of the Marvel oil we squirted in the cylinders on Thursday? If so, in what direction will the reading tend to go. If, they are 'false', what could then be done to get truer readings?
The boat is on hard stand and we do not intend to try to start the motors...in any case, the carbs are probably plugged w/varnish.

The engines have 800+ hours on the meters. I'm just trying to predict how much life remains before a major is needed...therefore my question:
'What would be a reasonable cylinder pressure to expect for these motors?' BTW, all plugs showed a little bit of rich running...with one showing a hint of moisture!

Many Thanks to all.

Late Start
954.850.8458
 
Here's why I sought your advice. The boat (a 1975 32' Trojan Sedan) has been laid up dry for approximately 3+ years, and shows signs of attention to detail before being put to sleep. (The freshwater tanks were still filled to the brim with anti-freeze, and the like). I instructed the evaluating mechanic to not try to crank the engines, but rather to pull & inspect all plugs and 'squirt' the chambers with Marvel. My idea was to help any 'stuck' rings free themselves during the first cycles of cranking. We did that exercise on Thursday and plan to 'crank' on this coming Monday.
That's the background.

Now, during first cranks on Monday for the cylinder pressure tests, will the readings be skewed by virtue of the Marvel oil we squirted in the cylinders on Thursday? If so, in what direction will the reading tend to go. If, they are 'false', what could then be done to get truer readings?
The boat is on hard stand and we do not intend to try to start the motors...in any case, the carbs are probably plugged w/varnish.


Late Start
954.850.8458
If the engines had been put to sleep correctly, the 3 years should not be an issue.

Here's the deal with oiling the cylinders.... and IMOO:

V engines do not lend themselves to oiling cylinders with expectations that the oil will fill in around the ring landings. Gravity will take it's tole, and the oil will pool at the low side of the ring landings.
Cylinder oiling actually came about many years ago with inline engines and engines that were fitted with flat top pistons whereby gravity did not matter.

As for the oil affecting the pressure readings, technically, yes..... it will affect the readings via the additional volume that the oil provides to the chamber.
Essentially, this closes down cylinder volume, and would/could/may raise pressure.

Basically, and in my opinion only....., we've got several things going against us when doing this.


  • the oil will pool at the low side of the piston ring landings.
  • it will not completely lube or seal the rings, if that is the intent.
  • while the oil remains in the cylinder, it may affect the readings depending on how much volume remained.

I've used the online static compression ratio calculators, and you'd be surprised at the differences when you change a piston dish volume value by only 2 cc or 3 cc . The oil could easily equal 2cc.


I'd get them up and running, and then do your cylinder pressure readings.


.
 
You're better off using a fogging oil. Starbrite offers a good product in a spray can. It adheres to the cylinder walls and won't "pool" like other liquid lubes.

-JJ
 
Many Thanks Rick and ALL,

I must admit that the practice of 'squirting oil' would end up in pooling on the lowest sides of the chambers...I expected this and IGNORED it
hoping to avert 'stuck' piston rings after so long a sleep time. Anyway...tooo late now.

We can't (prefer not to) run the engines for this compression test because of the predicted cost of labor, gummed up carbs, getting water to the
location...etc. Therefore, if we crank...as your suggested 4x cycles, will the readings be meaningful? It seems that of I get at least 100 lbs all around in even proportions I will have acceptable engines once we do start them up? OR OR will the aforementioned 'oil' artificially elevate the
readings?

Your posts have been greatly appreciated!

Regards,

Late Start
 
A couple of suggestions and questions:

Do you have time to pull the carburetors and check them out?
Do you suspect that these will fire up if you have fuel delivery?
Do you have good ignition spark?
Are you able to do a quick dry start? (we can fire an engine up minus cooling water for very short durations without any damage)

I'm reluctant to suggest an alternate fuel supply, but we can do this with extreme caution.
Not Starting Fluid..... actual fuel.

It may be that the carburetors are not gummed up beyond at least allowing them to fire up.
The fuel bowl vent tubes are a direct path to the fuel bowls. With a safe container and squirt nozzle, we can add fuel to these.

All you need is a rather short run to create a bit of cylinder heat, and to allow for some pressure behind the piston rings to free themselves up a bit.
Shut her down, disable the power lead to the ignition coils, pull all spark plugs, and perform your pressure tests.

I think that if you could do this, your pressure readings will be more meaningful.

.
 
To confirm what Rick was saying.....adding engine oil to a cylinder is actually an old trick to test whether a low pressure reading is due to a failing ring. If you get a low reading, then add a spoonful or so of oil to that hole, and it subsequently returns a higher reading.....it's probably a ring. If it remains low, it's probably valve-related.

That said, I wouldn't put much stock on the readings until the Marvel has been cleared from the cylinders. To combine previous points from both Jeff and Rick, it's likely that the motor will return low readings after being laid up so I'd focus more on the numbers relative to one another (looking for no more than 20%'ish difference).

Internally, 318's are pretty tough little motors. If you were to run into issues with an old engine (that's also been sitting), it may be more likely to come from externals (electrical/ignition, fuel system, risers, etc.). I'm not saying that you shouldn't pay attention to the compression/pressure testing.....but I wouldn't put all of my eggs in that basket.
 
Since the objective is to 'confirm' compression, allowing the engines to 'fire for a while' is a great suggestion.
I have done this many times with stubborn gasoline generators in preparation for Florida's hurricane seasons.

So, with DUE caution and safety preparation, we'll give it a try.

These posting have been very helpful...appreciated. I'll report in on Monday with the results. Have a nice weekend.

LateStart
 
The "due caution" is important on a motor that's been sitting.....because you don't want some oddball thing to mess up, what could be, a perfectly good engine.

It's only been three years and I'm sure the boat is up-and-away from the ground but.....a quick story. A couple of years ago, I bought a 1959 inboard runabout that had been sitting in a Michigan field for many years. It was a full restoration job so I had no intention of attempting to start the motor (Ford Interceptor). When I eventually pulled the exhaust and heads, to check the condition, I found it to be well-pickled and in very good shape. It was probably a runner aside from the mouse and/or chipmuck nests, acorns, etc. in the risers and several of the exhaust ports. Again, I'm not suggesting this as a possibility for that boat.....but it's those kind of things that can throw a wrench in starting up a sitter.

Steel wool in the exhaust tips is a good idea for storage!
 
A couple of suggestions and questions:

Do you have time to pull the carburetors and check them out?
Do you suspect that these will fire up if you have fuel delivery?
Do you have good ignition spark?
Are you able to do a quick dry start? (we can fire an engine up minus cooling water for very short durations without any damage)

I'm reluctant to suggest an alternate fuel supply, but we can do this with extreme caution.
Not Starting Fluid..... actual fuel.

It may be that the carburetors are not gummed up beyond at least allowing them to fire up.
The fuel bowl vent tubes are a direct path to the fuel bowls. With a safe container and squirt nozzle, we can add fuel to these.

All you need is a rather short run to create a bit of cylinder heat, and to allow for some pressure behind the piston rings to free themselves up a bit.
Shut her down, disable the power lead to the ignition coils, pull all spark plugs, and perform your pressure tests.

I think that if you could do this, your pressure readings will be more meaningful.

.


I always dry run mine on the hard,before it's dropped in. But remove your raw water pump belt from the pully or you'll burn out your impeller, even though you're gonna have to put new ones in anyway from sitting so long. Just a thought.
:p
 
I always dry run mine on the hard, before it's dropped in. But remove your raw water pump belt from the pully or you'll burn out your impeller, even though you're gonna have to put new ones in anyway from sitting so long. Just a thought.
:p
Yes, I was figuring that was a "given", but I should have clarified that......... Thanks!

I have the Johnson crakshaft pumps, and upon layup, I remove my impellers.
The last thing that I do, is my quick 8-10 second dry start and fogging procedure simultaneously.
This blows the exhaust dry as well.

Two birds/one stone.


.
 
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