Originally Posted by Will Race 4 Food

To test for leakdown, you need:
Air compressor
Leak down tester.
B&B or others make them, either a big single gauge or dual smaller gauges, they show from 0 to 100 %. (Leak)

You hook it up to the spark plug hole, connect to your compressor, set at the air pressure, get your piston on TDC first, and back off your rockers on that cylinder so both are closed. Off you go, it shows you leakdown. Once you've done that a bit, you can do other things too...

It shows you how well your cylinder is sealing, or leaking, as the case may be. Do it when the engine is fresh (new), then run a few passes, note the difference!! and then way down the track. Diagnoses problems. Unlike a compression tester, which is affected by compression ratio, cam timing, etc, this gives you a real indication of how the cylinders are sealing. Do it hot and cold, play with it and learn. Professional engine builders use them all the time. VEERY useful tool IMHO.

Originally Posted by faster67

I start by pulling all the plugs and coil pack/valve covers. I spin the engine until the desired piston is at TDC (or very close) and then I pull the intake/exaust rockers. If you watch the rockers it's pretty easy to see when the exhaust valve is JUST about to seat and the intake valve starts to open (overlap). The exhaust valve will come down just before the intake starts to come up.

You should be close enough to TDC around there. If you're way past TDC or way too far before TDC the compressed air will force the piston down in it's bore. I use a half inch drive ratchet to spin the engine. Just for safety's sake, pull the ratchet off when you pressure the cylinder up. Who me? No, I've never spun the engine around and had the ratchet smack something. Since the spark plugs will be out by then, it should be pretty easy to spin. If it's a 6-speed car be sure to set the parking brake and pull it out of gear.

If an intake valve isn't sealing you will hear a hiss coming from the intake. If an exaust valve isn't sealing you'll hear a hiss coming from the exaust. If a ring isn't sealing you'll hear air escaping into the crankcase when you pull the oil fill cap.

At least that's the way I do it...

Originally Posted by Mike K.

Pretty easy. Get a leakdown guage via summit and remove all the sparkplugs from the holes. Hook the guage up to your air compressor and inject 100 psi of air into each cylinder at its TDC. You can find TDC by injecting say 20psi and listening for TDC, or also it's right after the crank goes from super hard to turn to super easy.

Once at TDC, inject 100 psi, and look at the 2nd leakdown guage on the tester. If it says 90, then then that cylinder has 10% leakdown. Also you need to listen to where it is leaking from. If it is going into the oil pan, you have blowbuy, i.e. ring/piston issues. If it is out the intake, then your intake valve is fuxored. Likewise if you here air escaping out the exhaust, it's your exhaust valve. If you see bubbles in you coolant than you have a head gasket issue.

It's a really cool test to do, especially being able to hear where the problem lies. Also anything above 10% is real bad. 7-10% is still bad but ok. 4-7% is pretty damn good. Under 4% and your leakdown percent is rivaling race cars. I could be wrong slightly on the above percentages but basically anything up to 10% is ok, with 9-10% being iffy.

Originally Posted by LowETz

Compression test:

Compression guage needed. Line from compression guage to any given cylinder via spark plug hole. *DISABLE THE FUEL PUMP / INJECTOR / IGNITION SYSTEMS, by pulling those fuses before cranking over the motor. Remove the spark plugs. Block the throttle to WOT position. Crank over engine at least 7 compression strokes, and watch the guage.

Low compression on the first stroke, followed by gradually increasing pressure on successive strokes, indicates worn rings. Low compression on the 1st stroke, which doesn't build up during successive strokes, indicates leaking valves or a blown head gasket. Record the highest guage reading obtained for that cylinder.

Repeat this procedure for all other cylinders.

Cylinder compression pressure should not be below 100 psi on a stock piston/head setup. Max variation should not be more than 30% between cylinders.

If readings are are below normal, add about 3 squirts of oil to the cylider and repeat procedure. If compression rises after oil is added to cylinder, rings are bad. If the compression doesn't go up, the problem is at the valves or head gasket.

If 2 adjacent cylinders are showing an equally low reading, odds are it's a blown head gasket. If you see the compression is way down or varies greatly between cylinders... do the leak down.

Leak down test:

This test will show condition of upper end components (ie): guides, seals, seats(bent valves), rings, or head gasket, and pinpoint where the problem lies. Motor "should" be as close to operating temps. as possible.

Leak down guage/compressed air needed. Line from compressor to guage, line from guage to any given cylinder via spark plug hole. With piston at TDC(compression stroke), apply air (I usually do 100 psi). Guage will show % of leak down for that cylinder.

Originally Posted by TXCAMSS

I just did a compression test on my H/C setup and got on average 225 psi. The highest was 228 and lowest was 215. the rest stayed close to 225 psi. I know several cars with non-shaved heads and fly cut pistons that are around 165-190 psi.

Originally Posted by ONETA2TRY

How to properly do a leakdown test

first and foremost use a quality Leakdown Tester

step one: remove all spark plugs

step two: remove radiator cap

step three: prop throttle blades wide open

step four: turn engine over until both valves are fully closed on the cylinder being checked

step five: screw in leakdown tester hose

step six: set regulator pressure to 90psi and note the difference between the two guages per the chart depending on which tester being used - MAC in this case

step seven: depending on ring package used, leakdown will possibly be slightly better on a warm engine than cold, but still show a noticeable difference if there is a problem.


Things to note: Anything over 7% leak on a street engine is bad; 10%+ is really bad.

Also, note cylinder to cylinder differences; if there is a variance, it's possibly worn or washed out rings, either from just age or improper tuning.

Also, note while doing a leakdown to look in the radiator for any bubbling (which is possibly the head gasket) and or any hissing through the intake. That could possibly be a bent valve or both valves are not fully closed.

Also, if two cylinders side by side show both lower than tolerable percentages, then it most likely head gasket blown between those two cylinders.

Originally Posted by mikey

Leak down should also be done at TDC with the engine warm if possible. Top of the cylinder is where the cylinder sees the most stress and tends to warp up there first. You can set your air pressure to 10 PSI and move the crank by hand to get TDC, then set the pressure back to 90. I do leak down at TDC and BDC to check for concentricity. For an LS1, we consider anything over 10% to be bad. LS1's are not known for their well sealing rings... lol. We've had SCCA cars leak 8% on all and win races.