Kboi12

I recently turbo charged my track car while keeping the internals stock. I was considering doing just rods, but, thought I should get as much seat time possible with the original motor and then fully build one when this motor gets tired or explodes.
To get an idea of the engine life I thought id give it a leak-down & compression test. This engine has 25-30hrs naturally aspirated and 3 hours turbocharged@10-13psi. Car made a solid 235whp@10psi & 255whp@13psi on a mustang dyno when I installed the turbo. Here are leakage and compression results.

Leakdown:
Cylinder 1: 4%
Cylinder 2: 15% ---> Leakage is heard through the oil cap so I believe its the rings.
Cylinder 3: 6%
Cylinder 4: 2%

Compression(psi):
Cylinder 1: 140
Cylinder 2: 127
Cylinder 3: 140
Cylinder 4: 140

Obviously cylinder 2 has seen better days but 1,2 & 4 are healthy. What is your guys take? Motor feels healthy but never had a car where 1 chamber is so off. Should I be planning a rebuild sometime soon?

Mr Plow

I take it this is the first time you have tested compression and leak down on this engine?

I'd probably have a look in the bore with piston 2 at bottom to see if there was any obvious doom. If nothing obvious I'd do another track day on it and retest - maybe even test halfway through the day. It'll soon tell you how urgent the issue is. If there is little measurable difference - I'd rinse and repeat if you want seat time so long as no crazy oil consumption etc.

Your approach may depend on how important the parts on the original engine are to you to be used in any rebuild.

Kboi12

Yes this is the first. Last event I money shifted 3rd-->2nd and thought it be a good idea to see if I slapped a valve or bent something. Car ran hard for another 20 minutes after the over rev. Part of me suspects I could have bent rod 2. Might consider measuring TDC and comparing to cylinder 3. This a ridiculous assumptions?

themonkeyman

Hmm, valves yes, but I’ve never heard of anyone bending a rod from an overrev. Bent rods are a cylinder pressure issue, either from combustion or hydraulic pressure. I’d also recommend as above, looking at the walls, but if it runs ok, send it.

Gee Emm

A bent rod is easy to diagnose. My tool here piston depth measurement tool

With the plugs out, measure the depth of the pairs 1&4, 2&3. Never mind the crank position, the pairs should be at the same point (if you don't move the crank) and the depth measurement should be the same (same for each pair, not same for all four unless you have it at exactly the midpoint of the stroke). My 1 and 4 have a 6mm difference, that is pretty conclusive, didn't bother measuring 2&3 as that was enough for a teardown. Symptom was a bad knock (presume rod touching bottom of cylinder). No over-rev involved, no detonation heard. Logs yet to be downloaded to see what story they tell.

Looks like you are on top of this but others may be interested in what is involved.

Watterson02

I bent a rod on my first motor from over-revving it. Moneyshifted at a drag strip from 4->3 instead of 5th. Datalog showed 8350rpms. Me being the dumbass I was at the time, I didn’t check it. A month later it blew the bottom end out. I was running 10-12psi taper. No more than 220wtq for sure.

Kboi12

better get that patent. Already made my own! So piston depth checks out. I redid the leak down and I get the same numbers cold on cylinder 2. I put my ear on the exhaust and it’s definitely the exhaust. Looks like I wasn’t careful enough. So 15% leak down on a exhaust valve. Are valves something that deteriorate quickly when damaged? Or should I just send it?