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.