LD85

Here are the leak down specs, look OK?

Plug/Input PSI/ Leakage

#1 70-67
#3 70-64
#5 70-64
#7 70-64
#2 70-64
#4 cant test
#6 70-62
#8 cant test


[Modified by ld85, 1:54 PM 10/11/2003]

SWCDuke

70-67 good
70-64 okay
70-62 so so

If it's a driver, drive it. If it's a racing engine, it needs an overhaul.

Duke

norvalwilhelm

They are really low to me. You want to be high in the 80's if not the low 90's. I guess it depends on the gage but I am in the 93-94%. I have a new gage that measures the flow in liters required to keep the cylinder at 100 psi. My old gage gave me the 93% readings.
To me the motor needs overhauling.

CORKVETTE1

is 70 all the cranking comp you can get?

LD85

I did a leak down test where I put 70PSI into the cylinder at TDC.

Then looked at the second gage which would read the amount of pressure it in the cylinder which was the number in the second column.

So, 70 psi in,,, but the cylinder would only hold say,,,, 64PSI,,,,,

All of the numbers were withing 10%-12% which some say is OK, but is it?

I will call the short block builder tomorrow for their input.

norvalwilhelm

Id85 I was wrong . I am not familiar with your gage. With mine you set the regulator at 100 psi and it read out the persent leak. My news very accurate gage reads amount of leak in liters starting with 100 psi.

SWCDuke

Commercial compression gages are dumbed down so dummies can use them. They're designed for 100 psi supply pressure and if the cylinder holds 95 psi you have "five percent leakdown".

There is a principle of fluid mechanics called the "critical pressure ratio", and it is equal to 0.528. If the downstream absolute pressure is 0.528, or less, of the upstream absolute pressure (remember were talking RATIO here not difference) the flow is choked and is purely a function of leak area. Regardless of how much upstream pressure you apply the flow rate past the leak area is sonic and neither the velocity nor mass flow rate will increase no matter how much upstream pressure you apply.

With atmospheric pressure of 14.7 the upstream absolute pressure to achieve critical flow is about 29 psi, which is about 14 psi gage pressure. Since the pressure drop between the two gages due to the metering orifice is less than critical, the flow rate is a function of pressure DIFFERENCE and will remain constant as long as the downstream pressure (the pressure in the cylinder) is above the critical level.

So if you have a cylinder that measures 64psi with 70psi supply pressure it will be 74 with 80 psi, 84 with 90 psi and 94 with 100 psi.

Since I'm not a dummy I prefer to to quote leakdown as the actual pressure differential such as "six psi" rather than six percent.

Duke


[Modified by SWCDuke, 7:03 PM 10/12/2003]

LD85

Correct,,,, it was 64psi with 70psi supply pressure then 74 with 80 psi, and 84 with 90 psi , but I did not go as high as 100 PSI.

gkull

Id85 - It's best to use 100 psi which is the standard test. The last time I did mine it was every cylinder under 3 %. But it's also was also just born in May with only a few thousand miles on it.

Since I have an auto tranny I wedge my 1/2 inch breaker bar with a socket on it to the crank. You don't want the motor to rotate when you apply air. Manual tranny guys are lucky.

They say it a truer test at mid stroke where the bore is most worn. But I don't bother because that requires backing of all the valve lash.

SWCDuke

Bore wear is always at the top where the rings slow to a stop and reverse as the cylinder pressure forces them against the wall on the compression/power stroke. With proper igntion timing, peak cylinder pressure occurs just past TDC. This wear is usually called "taper" because it only occurs at the top of the bore. That's why you do leakdown at TDC. At mid bore the rings usually see hydrodynamic lubrication, so there is no contact and little wear. The rings slow and reverse again at BDC, but since there is no cylinder pressure at BDC, there is usually little wear.

Duke

CORKVETTE1

i believe the higher pressure you put to it the more it will leak

if i was you do it again but do it at 100 psi so we have a standard :cheers:

LD85

Well its neither here nor there now, I will pull the engine and put in a set of new rings, and heands and intake this winter, I think the bottom line is that for it being a new engine, it leaks more than I wuold like.

My problem was that my minram intake did not seat on the heads and before I realized it I sucked alot of oil into the cylinders , essentially, polishing the cylinder walls with oil.

gkull

SWCDuke - It has been my experience that the thrust face of the bore tend to ware out in stroker motors.

I understand what your saying about old motors that actually have a TDC ring ridge. But even those have the bore worn out of round because of piston thrust. I had a 393 stroker with 5.7 rods that was junk and severly scuffed on the outboard bores without any ring ridge. Who I bought it from was doing high rpm all the time racing. Also it was externally balanced with lots of heavy metal added to the crank.

SWCDuke

Okay, I hear ya. A stroker with a small skirt piston can certainly wear or scuff the major thrust surface of the bore, but such wear is not normal on OE engines used for normal road service.

Running an engine to the limits before it is fully warmed up is a common cause of piston scuffing.

Duke

arnold

Apparently, I am, (a dummy) cuz I recognize percent of leakage. Your example has the percentages changing with difference input pressures. I agree using 100 psi (or somethin' close) as a std. That or read whatever input pressure used, on a percentage scale. IOW, with 70 psi input and 64 psi output, the result is about 9% leakage, from my POV. Maintaining the 6 psi drop changes the percentage to 6 with 100 psi, in your example. OR, am I missing somethin'? :)

SWCDuke

"Percent leakage" is part of the dumb-down process. Percent of what? If you have 100 psi supply pressure and the downstream gage past the calibrated orifice reads 94 psi, it's called "six percent", but it is really SIX PSI, which is proportional to the square of volume flow.

A leakdown tester is really a flow meter that uses the pressure drop as a surrogate for actual leak flow rate in CFM.

Alternatively, you can say that flow rate is proportional to the square root of the pressure drop. If one cylinder shows 3 psi leak down and another is 6 psi, the latter has 41.4 percent more actual flow volume leakage. (The pressure doubles and the square root of 2 is 1.414)

If you understand the concept of critical pressure ratio that I explained previously, then you understand that once the downstream pressure pressure (which is the pressure in the cylinder as measured on the downstream gage) is above critical then the flow rate WILL NOT increase regardless of how high the supply pressure and cylinder pressure is. Understanding the concept of critical pressure ratio is necessary to understanding what leak down data actually means.

The pressure difference between the supply and downstream (cylinder) pressure is a representation of the actual air flow rate, so whether you use 100 psi or 50 psi supply pressure or any pressure that keeps the downstream gage above the critical, the pressure difference will be 6 psi, because the actual leak rate in CFM is unchanged no matter how much you increase the supply pressure.

This is why I prefer to quote leakdown numbers as "psi" not "percent".

Duke

69 N.O.X. RATT

So where do you want the piston in the bore to get the most "important" reading ?

The instructions say BDC, but this would seem to give an optimistic reading due to the decrease in bore wear at BDC

SWCDuke

It is normal to test leakdown at TDC of the compression stroke because on most engines the greatest wear occurs at the to of the bore. Maximum cylinder pressure occurs just past TDC, which places maximum radial force on the rings, and the low piston velocity precludes hydrodyamic lubrication.

It's impossible to measure at BDC with the valves operational as either the inlet or exhaust vavle is open. It can be measured at about mid bore on the power stroke, since the exhaust vavle usually doesn't open until the piston is more than halfway down.

Wear at midbore is generally not an issue (absent of scoring/scuffing type wear, which is considered abnormal) because it is generally believed that the rings experience full hydrodynamic lubrication where velocity is greatest.

Duke

69 N.O.X. RATT

Thanks Duke

Lionsden

In addition to Duke's infomation

Having done leak-down and compression checks for many years there are many variables that also need to be considered.

One of the biggest is to have the engine at operating temp before either the leak-down or compression check. Different expansion rates of the parts, e.i. heads, block, piston, rings etc, can have a big effect on readings. The engine at operating temp is really what maters.

I don't know if there is a standard as far as orfice diameter for lead-down testers. The guage accuracy or lack of can also effect the readings. Comparing your readings to someone elses can be misleading.

Cranking rpm on compression tests can have a big effect on outcome.

What is as important or maybe more important than actual numbers is the comparison of the readings between the cylinders. Whether you use 60, 70 or the standard 100psi how do the reading compare to each another.