Nate99

I'm trying to get my motor refreshed a bit to make it competitive in Supermiata S2 (140whp max per the rules). The car made 128 whp on a Mustang dyno in Feb with the following mods:
-RB header
-Catless midpipe
-Borla muffler
-K&N intake
-Stock mani (no square top)
-MS3 and a dyno tune
-Pump gas

Before diving in and throwing $ at it, 949 suggested doing a compression/leak down test on the motor to figure out what's going on. Note this is my first time doing either of these tests, but here are the results I got:

Cylinder 1: 145 190 psi compression, 50% leak
Cylinder 2: 144 190 psi, 43% leak
Cylinder 3: 147 185 psi, 35% leak
Cylinder 4: 135 187 psi, 60% leak

I don't trust the leakdown numbers. I did the test with the motor warm, made sure I was at TDC on the compression stroke, etc., but I used a buddy's Harbor Freight gauge and my result seemed to vary +/-10 percentage points depending on how well I could hook up the quick connect each time...

The thing I can say for sure is that I could hear air coming out of the valve cover (oil fill hole) and the dipstick tube on every cylinder. I heard nothing through the TB, and don't think I heard anything through the exhaust. No air bubbles in the radiator during the test on any of the cylinders.

I was hoping I could do a head refresh and square top to get most of the power I need, but based on this test it seems like the problem is more in the bottom end. What do you all think? Can I draw any conclusions here or do I need to redo the test?

EDIT: tested compression again with the engine warm

emilio700

iTrader: (3)

TLR

confess I did not read your post. Just read the compression numbers. That motor is toast. We retire motors when one cylinder reaches 10%. Unfortunately you can no longer find healthy used NB Motors as they have been out of production for about 15 years now. You will need to rebuild it. Don't just do the head or the bottom end. You need a full rebuild.

The good news is you don't need a race engine. An oem rebuild with cheap synthetic oil and an oil cooler will run 200 hours provided you don't over rev it.

TNTUBA

iTrader: (1)

Exactly what E said.

Anything north of 10% is costing you wins. Anything north of 20% you can feel in the "butt dyno" and north of 30% would be tired for a street motor.

tomrev

Before you write off the engine, are you confident you checked the comp. correctly? You note the engine was warmed up; was throttle fully open, and 5 full comp. strokes per cyl. reading? All plugs out, yes? I agree with you about not trusting the HF leak down; with that much leakage, she'd be running like ****, so also possible your comp. test set is wrong too. Have a friend with a second test gauge you can use to confirm #'s before you go engine shopping. Worth a try.

turbofan

iTrader: (1)

Definitely want to confirm the test was done correctly. Those leakdown numbers are severely bad. Compression is low too, though, and that test is simpler. I'm betting that engine is a goner. Like E said, it's become almost impossible to find ready-to-run used engines from a junkyard, probably best to do a full rebuild and know what you have.

18psi

iTrader: (76)

Some HF comp testers read low. real low. I had my perfectly healthy nb2 bp at 150 on one, and then 200 on a different brand. Just sayin

Midtenn

iTrader: (15)

In to comment on HF testers. Had a HF compression tester read 120-130psi on all 4 cylinders for a 11:1 cr motor that made 160whp a few months later. Quality tools should be used to determine health of motor before rebuilding. Many part stores rent/loan them out.

sixshooter

iTrader: (12)

Just like chassis dynos they are only good for measuring against themselves. In this case one cylinder to another.

emilio700

iTrader: (3)

FWIW, the deadest engine I have ever tuned to squeak up to the S2 power cap was down to 10% average in all cylinders, or thereabouts. That motor had all the fixins: BP4W, square Top, RB header, mandrel bent 2.5" exhaust, good CAI. Barely made 137whp peak but didn't quite make the power area. So it was slow.

18psi

iTrader: (76)

correcto-mundo

ryansmoneypit

iTrader: (6)

My 2 cents on leakdown.... even a cheap one should work ok. But how do you know? plug the test end and make sure the two gauges align at the same pressure readings. If they are all the same or close, and similar to your compressors output pressure, then you are in pretty good shape, test equipment anyways.


The fact that all the pressure readings are similar along with the leakdown numbers... I bet that engine has been HOT a few times. No ring tension left allows all this air to escape through the rings into the crankcase and out the breather.

Nate99

Guys, thanks for all the opinions so far, much appreciated.

You actually made me realize that I tested compression cold and totally shouldn't have. I did the leak down test when the engine was warm, and I meant to go back and re-do compression when warm and totally spaced it.

I just re-ran the compression test with the engine warmed up, all plugs out, throttle wide open, 5 compression strokes per cylinder, as you suggested, and got some pretty different numbers (I edited the OP to update these):

Cyl 1: 190 psi
Cyl 2: 190 psi
Cyl 3: 185 psi
Cyl 4: 187 psi

Also note that my compression tester is not from HF, it's from Innova (although that may or may not be any better).

So does that change everyone's recommendations? Seems like at the very least I should try the leak down test again with a better gauge, maybe?

turbofan

iTrader: (1)

That's more like it.

Re-test leakdown I'd say.

curly

iTrader: (25)

That sounds ok, and you're most likely testing the incorrect cylinder for your leak down. Remember that 1 and 4 are up at the same time, and while they're at TDC, 2 and 3 are at BDC. So bring cyl 1 or 4 to TDC with a long 1/4" extension down the spark plug hole of one of them, and test cylinder cyl 1. If it seems super bad like 40-60% leakage, swap to cyl 4 and try again. If that seems better, rotate 360 degrees and recheck cyl 1, then rotate 180* and check cyl 3, then 360* and check cyl 2.

Nate99

Cool, I'll try that. The other thing I noticed that seems to be causing a lot of variability in my results is the gauge. I am using the HF gauge basically the same way as in this video:
I get the extension hose screwed into the plug hole, then I set the needle to the "set" range on the gauge. Then I hook the gauge up to the extension hose and take the reading. I noticed that how quickly I can attach the quick disconnect between the gauge and the hose (around 3:45 in the video) can change my reading by about +/-10 percentage points, which seems stupid. Am I doing this totally wrong or something?

codrus

Leakdown testers are kinda finicky beasts, IME. Often times the quick disconnect fittings will leak a bit, which obviously throws off the readings. You also need to make sure to inspect the O-ring on the hose that goes into the spark plug socket.

Also, I find that sometimes it just gives me an oddly low reading, and I can make it get significantly better by rotating the engine a little ways one direction and then back to TDC. Someone suggested to me that this was caused by bits of carbon getting stuck between the valve and the seat and holding it open a crack -- supposedly this happens when you turn it over by hand because there's little to no pressure, but when the engine is actually running there's enough gas moving around to prevent it. I dunno, but I figure nothing I do with the crankshaft is going to make the gauge read erroneously high, so I try it a few times and take the highest value that I see as the correct one.

--Ian

ryansmoneypit

iTrader: (6)

You shouldn't be disconnecting anything while testing. You let the air flow, set the gauge test pressure (100psi keeps math easy) and read the loss gauge. If you try to rotate during the test, use a big 1/2 in. Breaker bar. Not a 3/8 ratchet. You will need fingers later in life.

I need to start making YT videos. A leakdown instructions should be about a 20 second vid.

tomrev

I'm glad you did a re-test; I couldn't see your engine making the power it does with such crappy #'s. Plenty of folks omit one or more needed steps when running a comp. check, and then think they have an issue that isn't there. Cool news!

codrus

Yeah, don't try to spin it with pressure in the cylinder, sorry if I wasn't clear.

--Ian

Nate99

Yeah, the HF gauge doesn't work like that. It's stupid. I bought a new gauge that works the way you describe and can see how it's much better. I re-ran the leak down test with the new gauge and all four cylinders were under 7% leak (e.x. I set the left gauge at 90 psi and each cylinder read >=84 psi on the loss gauge). So, I think the motor is... fine? Those numbers seem almost too good for a motor with as many race seasons on it as this one has.

So now the question is, if the motor/head is fine, how do I get from 128 to 140 whp to be more competitive in S2? Would the square top and maybe an E85 tune get me there?