Looking at a 1985 315 Commander with 454 340 HP engines. Original engines with 500 hours. Had compression checks done on engines and they came back 110-125. Is this concerning or in an acceptable range?

Thanks for any feedback!!

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New engine compression should be around 130psi per cylinder. Normally when a compression test is done on a used motor what is looked for is even pressure on all cylinders. A 10% difference between the cylinders on the same motor can signal a problem and further testing should be done. A cylinder leak down test can help isolate if the piston rings or valves are leaking causing the low compression reading. Two cylinders side by side can signal a leaking head gasket. 

I’m sorry to have to chime in and disagree on this one. But a 340 horse 454 engine with 500 hours on it is only about half used up on average. It takes 115 psi minimum to burn fuel/air compressed. These numbers are extremely low and should be in the 150 to 160 PSI range. However if all of the cylinders are reading this low I would be more inclined to lean towards using a different gauge to make sure the readings are accurate. then if the readings are accurate I would do another compression test that is called a wet test. This is done by adding a teaspoon or so of oil to each cylinder and retest. If the compression comes up then you have worn rings in the engine. Finally if the oil in the crankcase is not new I would pull a 3 ounce sample from each engine and send it to Blackstone laboratories for an analysis on wear metals and contaminants. It will be the best $35 per engine you ever spent. Hope this helps!

Lee &Charlie,

My cranking compression specs come from Crusader's operation and maintenance manual REV 11/84 chart. Crusader specs for a 350 hp 454 are 130 psi. The fuel octane rating for these motors is listed as 89. The compression ratio is listed as 8.12:1. Low compression 454's are designed for torque and long life, and were originally intended in this form for heavy duty trucks. By the way the oil pressure spec is 30-50 at 2000 RPM.

Dan Stokely 

My concern given the age of the boat vs hours on the engines would be are those hours accurate. 500 hours on a 35 year old engine is 14+ hours per year on average. That screams concern to me. My engines idle more than that just tied to the dock. I agree with Lee that another more comprehensive compression check is in order as well as that oil test. 

From what I am told the boat sat for 6-7 years. I was considering doing oil test, they are sending kits for it.The current owner did do pumps and other deferred maintainace when he got boat five years ago. Don't think I mentioned but they are Mercruisers. Would prefer Crusaders but that's not what it has. Your comment on the hours was certainly a concern I had but the story seems to lack it up, then when the compression numbers came back it concerned me again. I don't think the engines can be scanned for hours? 

Dan's comment regarding lower compression 454's is interesting. If 130 is the optimum number then 110-125 does not seem way out of line?

The boat appears to be a good platform to invest in and I was going to spend money getting it right but a lot of that revolves around the health of the engines. I figured if I kept it, down the road, I'd need to do major engine work but preferred not to do it in the first couple of years. Going to sea trial next week and see how that goes.

Not trying to convince you of anything, My 1975 41’ flush deck has unknown hours. The gauges read 1800 hrs but were not working when I bought and brought her from Ft Lauderdale to Boston in 2003. Those engines are still running with some maintenance through the years. My concern when I see hours like that are if I’m being given the truth about the entire boat. Curious how the oil tests come back. Please keep us updated on your progress. Good luck. 

For comparison, My 85' Crusaders are approaching 1800 hours in New England's short boating season.

I hear you guys. I certainly approach this stuff with what I hope is healthy skepticism. I have a 2000 Donzi 22 Classic with just over 200 hours and a 1995 BW 17 outrage with 300ish hours so they are out there just the exception.

Good comments here.

The engine compression tests do indicate a suppressed value.

What this might indicate is cylinder wall and piston ring corrosion. I would suggest putting a bore scope through the spark plug holes with the piston at bottom dead center. Check for cylinder wall pitting and corrosion marks from the rings setting for extended periods of time.

The suppressed compression numbers also might be caused by the cylinder walls and rings being dry if the engine had not been run before doing the compression checks. Running at full load for a few hours would likely bring back the compression back up if there is no significant cylinder wall corrosion.

I would not put a lot of faith in any pulled oil samples at this point, assuming that the oil was changed just before lay up and that they have not been run for much time before the samples are pulled. The samples will likely state an engine status that is better than what reality is.

If you are serious about purchasing the boat and the bore scope checks are good, then I would suggest to pull the valve covers and inspect the cylinder head surfaces and inner valve cover surfaces for surface rust and corrosion. This is a good indicator of engine interior corrosion status in the areas that you can't see without an engine tear down. I would also inspect the exhaust valve springs for signs of corrosion pitting on them. If there is much spring corrosion, then it would be very prudent to replace them.

Final issue you should also consider is that statistically the big block GM engines have a slightly elevated problem of dropped valves. Not saying that this is a problem under most conditions but in a low hour long term stored engine, if the problem is going to occur, this is the perfect scenario.

Engine valves have a forged head that is friction welded to the valve stem. Valve manufactures are very good at making reliable valves however the heat affected zone is metallurgically different from the base metals on both sides. This area is slightly more susceptible to corrosion leading to weld failures. In normally used engines, this is not a problem since operating the valve at design temperatures tends to inhibit this issue however in engines with low hours and extended idle periods between run times the corrosion issue is exacerbated.

Assuming you decide to go through with the purchase, you should factor in an increased risk factor for potential dropped valves occurring in the engines in this boat. Depending on the storage environment and exactly what was done when the engines were layed up greatly affects the probability of the problem. If you are adverse to risk,you might want to consider replacing all of the exhaust valves in the very near future. If you want to play the odds then fine, however if you do drop a valve at some point in the future, replacing all of the valves would then be indicated.

This is one of those situations that I would have much higher confidence in the engines if they actually had significantly more hours on them.

You will not get a proper test on a stone cold engine that has been sitting all winter - It will read lower than it should - test should be done on a warm engine/throttle open/ignition disconnected/normal cranking speed- the readings you got are what I would expect for a good engine that was stone cold . 


This topic has generated a lot of discussion, The best way to find out if the motors are ok is to do a sea trial. Testing them to see if they work together, or if one is weak. At full throttle the motors should be able to get to 4000 min 4400max rpm and should be close to the same at max rpm. If the motors pass these tests they are ok, if not - no sale. End of story

Had boat surveyed and we sea trialed Monday. Survey was good and engines ran well. Was rough but engines had no problem at 4200 RPMs. So we bought it. Thanks for everyones input, it is appreciated.

Now we move to getting all of the little stuff right. I'm sure I'll have a few questions there.

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