leak down and compression test.

[big-n-italian]

wondering what a leak down and compression test is.

is this a necessary step before adding a supercharger? how is this done? what happens if a cylinder(s) fail(s), do the rings need replaced? in additional info is appreciated on this subject. thanks.

 

[Joseph Dell]

compression test checks to see how well the cylinders are holding, well, compression. This is a function of two things: valves sealing properly and rings sealing properly.

as long as your percentages between cylinders are within 5-8%, you are fine. if 9 out of 10 cylinders are 150psi but one is 90psi (or anything 100 and under) then that cylinder is weak or dead for some reason. what is the reason? could be piston...could be rings... could be valves.

a leakdown test is a little more accurate (but i still have never done one) and uses air to see a) whether it is intake or exhaust that is leaking, and b) whether you are maintaining compression or not.

tool for a comrpession test is a "loaner" at auto-zone or $30 to buy. remove all spark plugs and screw this thing into the spark plug holes one at a time. with the fuel pump relay removed (so that you don't dump fuel in the motor), crank the car 2-3 "revs" and then check the gauge on the compression test tool. Do this for each cyl and write down the #'s.

good luck! if they are all the same or close, then you are fine. if one or two is off, those cyl's are suspect.

JD

 

[1TONY1]

Read this:
http://www.vclassics.com/archive/leakdown.htm

I wouldn't say it's necessary if your car is running as normal, but it sure won't hurt to check it. If you do check it you will have a reference (save the readings) to check against in the future. If a cylinder just "fails" because of a s/c you will probably need more then rings With proper tuning there should be no failure within a reasonable power level for the type of parts in an engine.

 

[Joseph Dell]

Good link Tony!

One more details:

The times i've done compression tests on vipers, i've learned the following:

- in one case, a ring land and part of a piston were gone (on an 01).
- in one case, i bent a valve so it wouldn't seal and so compression in that cyl = 0.
- in one case, a rod was bent. showed a lower compression than in other cyls.

In many cases, it all looked fine!

JD

 

[big-n-italian]

<< tool for a comrpession test is a "loaner" at auto-zone or $30 to buy. remove all spark plugs and screw this thing into the spark plug holes one at a time. with the fuel pump relay removed (so that you don't dump fuel in the motor), crank the car 2-3 "revs" and then check the gauge on the compression test tool. Do this for each cyl and write down the #'s. >>

would you say that it takes 5 to 6 hours to do this test? sure doesnt sound like it...

 

[Joseph Dell]

um... 20-30 min if you are slow.

all you do is

a - unscrew all spark plugs
b - screw tool into plug #1
c - crank motor for 3 seconds
d - repeat on all other cyls

with a "bump starter" it takes even less time b/c you don't have to go inside the car to crank it.

5-6 hrs? maybe if you had no arms, no legs, and were floating in the ocean...

 

[1TONY1]

What do you call a man with no arms and no legs floating in the ocean?


Bob.

 

[big-n-italian]

ok, here is what is going on. i have contacted a well-known tuner about fine tuning and dynoing my supercharger setup.

they have recommended a leak down and compression test and are wanting to charge me $75 per hour for this test, which comes to $375-450.

in all honesty, i had a good idea what this test was, but i posted the question to this forum because i was wondering if i was "missing something" when i was told it would take 5 to 6 hours to complete. i didnt think it would.

next, i have read alot about the VEC2, and think it would suit my intentions fine. it appears to be a fine setup. i already know it isnt as sophisticated as the AEM setup, the price difference makes it obvious. but when asked about installing the VEC2 setup, i got this response:

<< "We can surely install and tune with the Vec 2 but I don’t know how well the VEC2 will handle varying weather conditions, altitudes etc. We will utilize whatever it offers as a tuning solution for the power we are making. We will be regulated by its level of how sophisticated it communicates and handles engine control." >>

if i am wanting my car used for street use only, do i really need to worry about the altitude in IL?

i just want to make sure i am being steered in the right direction and not being given the "scenic" route, and i think the 5-6 hour LD&C test quote weirded me out a bit.

 

[big-n-italian]

also, i am running 6.5 lbs of boost if that matters. also, the car runs fine with no smoke, doesnt burn any oil, oil changes look fine, everything looks normal, etc.

 

[J DAWG]

I assume you have a Roe? I thought the cards Sean had were pretty close on the s/c???


Do the compression test yourself and forget about the leakdown unless your car feels or makes weak numbers on the dyno. Before I paid someone $400 to do it, I would buy a leakdown tester(which I have, I think it was $70 bucks-Proform) and get my girlfriend out in the garage and have her hold the brake in while I went thru the cylinders.

Not rocket science and doesnt take 5-6 hours in the least nit, especially for a "tuner"

John

 

[big-n-italian]

i am sorry, i have installed a vortech.

 

[J DAWG]

VEC2 will work fine at that boost level. I know alot of Paxtons with much more boost running a simple split second box.

 

[HOdbleFman]

Make sure you crank the same number of times for each cylinder. The VEC2 will compensate for altitude since it has its own MAP sensor. The VEC2 still uses the stock PCM for IAT readings and compensates for a rise in temperature based on the stock enrichment table. I'd go with the AEM for sure. My car drives better than it did when it was stock. I had a VEC2 and I didn't really think the driveability was all that great.

 

[Jack B]

The real truth:

The compression test gives you an approximation of the ability of the engine to hold cylinder pressure.

A leak down test will give you the same measurement, but, it is far more accurate and will isolate any individual cylinder issues. When you go nitrous or SC, a problem with a bad ring can result in a lean out of that cylinder and culminate with a lifted ring or burnt piston/cylinder. A leak-down test can find such.

As was stated, the leak-down tester is under $100, and takes about 2 hours to perform, the trick is finding TDC. Go to Eastwood's (internet),they have a neat device (cylinder stop) to find TDC. A leak-down test is cheap insurance.

 

[1TONY1]

The real truth:

The compression test gives you an approximation of the ability of the engine to hold cylinder pressure.

A leak down test will give you the same measurement, but, it is far more accurate and will isolate any individual cylinder issues. When you go nitrous or SC, a problem with a bad ring can result in a lean out of that cylinder and culminate with a lifted ring or burnt piston/cylinder. A leak-down test can find such.

As was stated, the leak-down tester is under $100, and takes about 2 hours to perform, the trick is finding TDC. Go to Eastwood's (internet),they have a neat device (cylinder stop) to find TDC. A leak-down test is cheap insurance.

If that's a mechanical cyl stop I would pass or turn the motor very carefully by hand. I agree with Jack on the 2 hours. A bit longer for the lea,down vs compression test. Why even do the tests ?? They are wanting to cover their butt in case there is already a problem in the engine. I do not blame them for that.

 

[big-n-italian]

<< They are wanting to cover their butt in case there is already a problem in the engine. I do not blame them for that. >>

thanks guys. i have no problem with a little CYA - but it is the 5-6 hours at $75 per hour i have a big problem with especially when i know and hear that it should not take more than 2 hours.

it immediately makes me wonder about the rest of the service and corresponding charges.

 

[Joseph Dell]

For the record, a leakdown could take 2 hrs. A compression test, no way. 1 hr max, but more like 30 min.

JD

 

[Jack B]

Tony:

It is not really a stop, it actually is a slide that has graduations. It will move up and down with the piston, and you are right, you rotate it by hand. As soon as you see it stop moving you know that is tdc, you then reference that vertical postion as tdc, all of the cylinders that follow will be the same.

 

[1TONY1]

Tony:

It is not really a stop, it actually is a slide that has graduations. It will move up and down with the piston, and you are right, you rotate it by hand. As soon as you see it stop moving you know that is tdc, you then reference that vertical postion as tdc, all of the cylinders that follow will be the same.

Ah....that's pretty cool. My method is to use a 1/4" drive extention 10" long. Still need to be careful though. The Eastwood tool looks safer especially for beginners. I might have to try one.
Eastwood tool link ..click HERE

 

[Joseph Dell]

you mean that getting a 3/8 extension stuck between the piston and the cyl bore is bad???

oops...

no wonder i always lose compression after i set valves...



JD