First leakdown test, what does it mean, exactly?
 

I'm looking at buying this Formula 303 with 502 mpi's, One motor was "rebuilt" last year and the other could be original with 573 hours on boat. I've never had a leakdown test done before so I am not familiar with the readings and what they mean exactly. It seems to be preferred over a compression test. I asked the shop to do both but we all know how that goes....
Here are the results I got from the shop:
LEAK DOWN TEST
STBD PORT
1 20% 27%
8 12% 20%
4 15% 20%
3 25% 25%
6 15% 20%
5 15% 25%
7 12% 20%
2 25% 30%

What do you guys think of these numbers? Apparently, they didn't do a compression test too, like I asked. Do I need one of those too?

Thanks!

Im no pro but know enough to answer a bit! When i do leak down on small blocks, brand new out of the box, they have like 4-5% leak. What it means is they really hold well the pressure put inside the cylinder
a good used engine may leak up to 15-20% and still be very good. Now maybe big blocks for boat might be built a little looser allowing higher base leak down number not sure. But all the numbers you gave seems to show a rebuilt engine and an older one, but both pretty good. Of course the older one will leak more in the futur depending how it is run, but i would not be effraid to buy with those leak down numbers! Will see what other engine guys might bring on that subject,

Generally a new big block built for marine use will see around 10-12% leak. 15-30% is considered normal for a used motor. A leak down test is far superior to a compression test. There is no reason to do a compression test as well. It will not tell you anything that a leak down test won't tell you but a compression test will not always tell you everything that a leak down test will. Sometimes a motor will actually "pass" a compression test but "fail" on a leak down test but never vice versa

the reason one does a cylinder leakage test is to find out where it leaks and how much or the per cent of the leak.my bet is that most of the leakage the op is seeing is from poor valve seal,it is time for a valve job,new springs&head gaskets.

Thanks for the post, but the reason I posted this is because I am trying to BUY this boat and the owner allowed me to have the boat brought to a Formula dealer to have the boat "checked" out. Mostly for the motors, since I read that the 454 Mag EFI's and 502 MPI's of that era typically had compression issues. So, rather than being caught off guard, I was able to have the motors checked. From what i can tell, the Port motor is the motor that has not been rebuilt. How much longer can this run at these leakdown numbers? Are they in the healthy range?
I don't want to buy the boat and then have to rebuild a motor right after (all things considered). I am not a WFO driver, mostly a cruising, occasional almost WFO and canal idler, especially this time of year with the holidays and associated canal cruises.

the numbers you posted imo indicte that the engines will need work in the near future,you should use those numbers as a barganing tool because sooner or later you will be getting the work done.

What would "perfect" compression be with no leakage? I think I read somewhere that 150ish is about the best one could expect on a 502 BBC, but even with that someone posted there would still be 10-15% loss even on a brand new motor. I believe the STBD motor was rebuilt and has only like 10 hours on it.

the engine could show good numbers on a standard compression test,that will not change the facts that the leak down test show.edit in, a new endine should show under 10% cylinder leakage on a cold engine and less on a warmed up engine.

Compression test numbers are really not that useful. Too many variables. On a compression test, you are looking for all cylinders to be aprox the same. If all of the cylinders are worn about the same, you would never know unless you had the numbers when that particular engine was new. A leakdown test is what tells you what is going on. On that engine with 25-30% leakdown it is tired. But the object of the leakdown is to see WHERE it is leaking. Either through the exhaust valves, intake valves or rings. This tells you whether it just needs the heads redone or the whole engine gone through. Hope this helps.

good explination joe,that is what i was saying but sometimes i am not so good with typing the words.with leakdown numbers of 25 to 30% i would expect a lot of valve leakage,the person who did the test should have noted where the leakage was going.

i will also add that most engines that have poor valve seal normally have tired valve springs.the hp500 is famous for this because they use a valve spring that wears out usually between 200 to 250 hours.i rebuilt a couple hp 500 efi engines recently ,i don,t know what they had for hours but both engines had several broken valve springs.the ones that were not broken were ready to break.

If the price is reasonable I wouldn't pass on the boat based on the leakdown check. 303 is a solid safe hull, and will take a beating. I know this from experience. Head job isn't too expensive when time comes for that. I'm thinking you could probably run it for a while based on the driving habits you posted.

Hopefully this youtube video can be opened up here for viewing. I have noticed that in the past couple of weeks they do not even show up in the threads for linking up youtube videos.

youtube video here if one can see it and open it up. ----- Anyways Dart and Reher Morrison will disagree about a leak down test is the test to perform only. How I know about this video and class --- because I was in it personally and the topic went into more depth than shown on the youtube video.

IMO both tests should be performed. Compression and leak down testing. Neither test is 100 percent that the motor is great to go. If one had a broken valve spring and or a very weak valve spring(s), leak down testing can show good. If one had a broken rocker arm - a leak down test can show good. I have had this before and were compression tests shown bad numbers but leak down tests shown good numbers.

I just was told that they did a compression test "sampling" of cylinders with different leakdown percentages. The compression numbers seemed ok to me.
The Port motor, #8 = 130 (20%), #5 = 128 (25%) and #2 = 125 (30%).
The Starboard motor, #8 = 140 (12%), #3 = 135 (25%), #5 = 138 (15%), #2 = 135 (25%).
FWIW.

Compression means very little as lots of people have already said.it basically just tells you if it has enough to fire.
Leak down is a true indicator of the engines health

If you like the boat go ahead and buy it just keep in mind it's either going to need springs,A valve job or new rings.

I would agree with this. The springs get worn out and no longer control the valves very well. This typically causes wear on the seats and valve faces. Most of your leak down is going to come from the heads. Very little is typically from the cylinders themselves unless you have rings that are broken or bores that have rusted and become very out of round.

you say this like you have one or are a powerplant guru:daz:

You say that like you already know the answer...........smartazz. Not even close to being a powerplant guru but I have spent some time beating on a Formula. Right?

or all of the above.

More than likely top-end related on the one that’s lower; very typical of the 502 MPI. The exhaust seats start to go away. Use it as a bargaining chip. 527 hours on the short block is nothing unless it hasn’t been maintained or had the bejesus beat out of it.

I am certainly no expert as some of these guys who have responded are, but my take on the leakdown situation is:

If you have the percentages, they should have told you if the leakage is through the top end (intake/exhaust) or the bottom end (rings). Last year, I was looking at a particular boat which had a 502 with around 380 hrs. I wanted a compression and leakdown test done before purchase. It was checked at a very reputable dealer in AZ., and I was told that the leakage was about the same as yours, all through the intake and exhaust valves. Mechanic said that it needed a valve job. Again, I am not an engine guru like some of the guys here and I was a little skeptical at first. The boat had been sitting up for a while and I really didn't know how long or hard it had been run before the leakdown test was done, but all the numbers indicated a problem with the top end of the engine. Anyway, long story short, they did the top end, I bought the boat and it runs like a new one now.

As stated, once you find out if it's the top end, bottom end or both, you can use that info to make an informed decision in deciding if you want to purchase the boat or not. If I really liked the boat and it needed a little TLC, it would not bother me to have to do a little maintanence. I would just use that as a bargaining chip to negotiate price. Good luck.

Did they warm the engines up before doing the leak down?