Author: Michael McCoy

A leak down Test can be performed on a warm engine. The appropriate leak down testing apparatus will be required, along with a good air compressor. You should not have a cylinder leakage of more than 10% in any cylinder for a fresh/healthy motor. You should also not have more than 5% variance between any two cylinders. If you do, then you will need to further investigate the cause of the leak. More than 20% leak down is an unhealthy motor and you should investigate further and find the appropriate course of action for repairing/replacing.

You will need to make sure you are exactly at TDC for the tested cylinder, and on the end of the compression stroke/start of the power stroke. If you are 360 degrees out, you can have a leak from the intake or exhaust valves which will make your results useless.

If you have a leak down of more than 10%, you should be able to track down the source of the leak, which will help determine the best course of action for repairs.

One of the benefits of a leak down test is that it can help isolate the problem areas. You should be able to hear the leak, or use a plastic bag and some tape to help locate leaks. An audible leak out of the intake indicates a problem at the intake valves (either valve seat issues, bent valves, etc). An audible leak from the exhaust (and you will still be able to hear the leak at the exhaust tip) will indicate an issue with an exhaust valve. If you can hear the leak in the expansion tank (with the cap removed) then you have a blown head gasket, cracked head, or an issue with the block. You will generally hear some leakage in the crankcase. A little is normal, but a lot is bad. This is why the leak down tester has a gauge.

Be sure to record your readings for each cylinder, and we recommend testing each cylinder more than once as a sanity check.

Compression Tests should be performed on a warm engine, with the fuel pump fuse/relay unplugged (and the fuel rail drained/depressurized), and the throttle should be held fully open while testing. These pressure values will be less at higher elevations, and can be lower on engines with more mileage. Aggressive cams or cam timing will also reduce the measured cylinder pressure during a compression test, but should still give consistent readings. You should not have more than 10% variance between cylinders.
Typical compression test results should be approximately 150-170 psi per cylinder for ~8.5:1 CR, 210-250 psi per cylinder for ~10.5:1 CR.

Before you have us tune your car, there are a number of important things to verify. If there is a “small problem” that is unknown before the dyno session, it can nullify

The pre-dyno check list is similar to the pre-track check list:

• – Test the intake system for leaks
• – Check, clean, or replace air filter
• – Check engine oil level
• – Check coolant level
• – Check brake fluid level
• – Check wheel bearings for excessive play
• – Check that all suspension components are tight
• – Check or replace fuel filter
• – Bring plenty of fuel

It is very important to ensure that your vehicle is in good mechanical health before tuning it on the dyno. If you are using a custom tune (NA or turbo), we first send a base tune. This tune should be very close to the final tune in idle and part throttle drivability, so will be sufficient for startup and leak checking.

Before the tuning session begins, it is best to make sure you are tuning a healthy engine. We suggest you complete a compression test and a leak down test prior to your tuning appointment.

Please know that even if either of these test results look good, you can still have an engine that has some mechanical issue and the tuner/calibrator should be able to recognize any negative symptoms and consult with you about the consequences of further tuning your engine.

Make sure your air filter is clean and in good working order. Cleaning or replacing your air filter at regular intervals (according to the manufacturers instructions) is always a good idea, especially before a dyno tuning session.

Vacuum, pressure, or smoke check all intake, charge pipe, intercooler, turbo, and vacuum lines. If it is a turbocharged car, pressure test the system to between 1.5 and 2 times the target boost pressure. Couplers can appear sealed and hold vacuum or low pressure smoke just fine, but leak when re-shaped by high boost pressure. Smoke test, vacuum test, or light pressure test naturally aspirated cars.

Materials can deform both elastically (returning to its original shape and structure) and plastically (permanently changed in shape, structure, or both). Every material under any sort of load (lets consider a fastener in tension) is deformed elastically, even if it is such a small amount that it’s impossible to notice. The strain (deformation, elongation in this case) in the sample is proportional to the stress on the sample (force over area) times Young’s Modulus which is a property of the particular material.

Stress vs strain: E, the slope of the curve in the elastic range, is Young’s Modulus. This is an indicator of the stiffness of the material.

When the stress on a test sample reaches the elastic limit (aka the yield point) of the material in question, the deformation becomes permanent. For example, a head bolt or stud will elongate slightly (and temporarily) as it’s tightened up to its yield point, then it will begin to elongate more per unit of stress applied to it, and the deformation will be permanent. The deformation at this point is generally uniform, that is to say it occurs evenly along the length of the bolt, until the ultimate tensile strength of the fastener is exceeded and it fails (no longer provides the correct tension). At this point, a noticeable “necked down” section appears If more stress is applied, and the bolt will eventually fracture (break). Some fasteners, such as BMW head, main bearing, and rod bolts, are designed to be torqued past their yield point and deformed plastically when installed. This leads to a joint that is very resistant to fatigue in the fastener, because the tension in the bolt cannot be released and reapplied. The bolts also strain harden as they are tightened, increasing their ability to hold further loads applied after they are installed.

What does this mean to me?

Never reuse a bolt that requires angle torque, or that BMW requires be replaced. BMW specifies torque angles for fasteners that are torqued to yield because they are a more consistent way to specify the installed strain in a fastener than a torque value that can vary with the specifics of the application, such as lubrication on the threads.

Also, whenever a torque value is specified with a particular lubricant on the threads, ensure that it is always used. Failing to do so can lead to different amounts of stress in the fastener (and thus different clamping loads) at the same applied torque.