Hey guys,

So my built short block is going on it's fifth year/ 15k miles and since I planned on turning the boost up this year I thought it would be a good idea to check the health of the engine. I started with a leak down test which showed about 13-14% in each cylinder. The compression test showed really close to 160psi across the board. I was actually surprised at how little variation there was between cylinders. My friend who is a BMW mechanic said the leak down results are a little high. I did some research and my results seem to be rather normal for these engines? Is this something I should be worried about? Since the engine is getting up there in miles and the goal was 800hp this season I figured changing out the rod bearings wouldn't be a bad idea. Other than that the setup will be staying the same. Thanks in advance for your replies/opinions!

 

Did you check for leaking air anywhere? At 13-14% that seems pretty high but your compression test seems decent. Almost like a valve is slightly open for the leakdown tests. For reference I have 3 yrs and 20 k ish miles on my built engine ( built by a craphole shop so I don't trust it for a second) and it has leakdown results between 2&4% across the board. But the beauty of the leakdown test is it can help you pinpoint the leaks as opposed to the compression test that just tells you when you're screwed or not.

 

I could here bubbles in the catch can which I'm assuming is crankcase/rings.

 

It's a little high. fresh, fully broken in engines are typically <5%, as they wear you'll see it creep up to 10%. Anything less than 10% is great. 10-15% is OK, 15-20 I would definitely call meh, anything over 20 is suspect and should be further inspected for an issue.

With your compression at 160, I wouldn't worry about it. My stock shortblock is at 165-170 compression with 7-9% leak down.

 

I was a little worried about the leakdown but felt a little better after doing a compression test. The engine is coming out for rod bearing replacement and i've been throwing the idea around of a possible rebuild. On a side not... At the end of last season my catch can was filling up with oil rather quickly which never happened to me before. Around that same time is when I started doing some pretty heavy fifth gear pulls. I was thinking I was starting to get some blowby but after some further inspection I think the vented dipstick dube + long highway pulls are the cause.

 

Im leaning towards a rebuild while the engine is out. Can anyone else comment on those leakdown numbers?

For the record, leakdown and compression were done on cold engine.

 

That's probably why you're getting slightly high leakdown numbers. With 160 compression I think a complete rebuild is a little overboard. If you had bad compression numbers then that would be one thing.

I'd redo the tests with a warmed up engine.

 

I will redo the test tomorrow with the engine warmed up. Are leakdown tests typically performed on a warm engine?

 

So I repeated the test with a warm engine. The results were:

1. 8%
2. 8%
3. 10%
4. 12%

Now the engine probably cooled signifacantly by the time I got to the last cylinder since its very cold out. What do you guys think?

 

are you pulling the head? if the motor is out won't hurt to measure the cylinders for roundness and taper, and if they are within spec do a quick hone and put fresh rings in.

 

I didn't plan on pulling the head.

 

You should be able to spin new bearings in with the motor in the car..

 

^^^^^^this

 

Tranny is getting rebuilt so I figured it would be easier to pull the engine/tranny together since i'm doing it in my garage. The bearings wont help my leakdown situation though! Do you guys think those results are acceptable?

 

If it was my car.... The motor is out. Tear it down, measure it out. 25k in 5 years, its clearly your racecar and probably gets driven as such. The leak down is approaching the "meh" zone, although not terrible, so I doubt there is major wear/damage. Re-ring and new bearings is cheap. You can hone the cylinder yourself with a ball hone and electric drill.

As for your blow by, I agree it was probably because of the dipstick vent. I was running STM's vent fitting in the front balance shaft inspection hole, and it filled the catch can in 600 miles. I removed that line and it doesn't even get oil in the catch can now, just condensed crankcase vapor (smelly as *** yellow **** lol)