[turbobaby]

Anybody know what the % leak for fully built motors should be? Are they typically higher than stock motors?

[jays05]

I dunno about leakdown but compression will be the same or a bit less. Mine gave 120psi across the board, while the stock engine with the same gauge read 135psi.

You should probably see a bit higher leakdown % due to larger ring end gap, p/w clearence, etc.

[Neanderthal Racing]

5% is acceptable

[caballero_sti]

CP100mm I got 120psi all around is that normal? still never have a change to get a leakdown done.

[AR Fab]

The compression test is not an acturate way to measure the sealing of your engine rings and valves. The amount of pressure read by these gauges will change due to compression ratios and camshaft grinds. If they are within 10% of eachother your O.K. The leakdown is a measurement of how well the rings and valves are sealing....A decent condition engine will be no more then 20% / Good condition should be between 10-15% / very good should be below 10% / or you can get one of ours at 2-3% after our 40min recommended breakin period.... which is considered excellent.

Howard

[Flycaster]

Boy, I dunno about those #'s Howard - for an N/A car, yeah, but for a FI car, a 15% leakdown can be pretty ugly. With big boost, you're going to blow cam seals and completely block the oil drainage on a BB CHRA unless you have one helluva CCV system in place - and that won't be the OEM case breather system, either.

[Brandi@ARFab]

Originally Posted by Flycaster

Boy, I dunno about those #'s Howard - for an N/A car, yeah, but for a FI car, a 15% leakdown can be pretty ugly. With big boost, you're going to blow cam seals and completely block the oil drainage on a BB CHRA unless you have one helluva CCV system in place - and that won't be the OEM case breather system, either.

Well in F1 with multi-million dollar budgets anything more then perfect is unacceptable. When we have a cylinder thats more then 10% we go home from racing. But I have seen alot worse without having oil seal problems. What if all the leakdown is in the exhaust valve? Your saying that the oil seals will be leaking and the OEM case breather will be affected? These numbers are a tool and if you do not know how to use it or properly use its information leave it in the toolbox or have someone else use it.

I have a customer that ran his engine last year with over 35% in 3 cylinders and still ran low 10's. He has inspected the block and found that the factory cylinders are out of round by .004-.005". It didn't smoke and still made very good power up to 35psi. The valve job was less than fair though(hence the 35%).

Remember the leakdown test is a static test not a dynamic test. Some cylinders will leakdown really high when not at operating EGT temps with 5-15psi in the cooling system.

I was not referring to F1 cars or CUP cars...I was referring to 90% of the cars posted on this site. Typically, the numbers I gave are acceptable to the industry standards for production cars. But the values do represent an issue in the system.

Howard

[Flycaster]

Originally Posted by Brandi@ARFab

Well in F1 with multi-million dollar budgets anything more then perfect is unacceptable.

Howard, that was an "I", not a "1" - I was talking about "forced induction" motors, not the multigazillion dollar F-1 race motors. Sorry for the confusion, I should've spelled it out.

Again, my point is with a 15% leakdown loss, you're gonna have a helluva of blowby and crankcase pressure with a motor running big boost pressures. I think that's way too much loss to still be called "good."

[AR Fab]

Originally Posted by Flycaster

Howard, that was an "I", not a "1" - I was talking about "forced induction" motors, not the multigazillion dollar F-1 race motors. Sorry for the confusion, I should've spelled it out.

Again, my point is with a 15% leakdown loss, you're gonna have a helluva of blowby and crankcase pressure with a motor running big boost pressures. I think that's way too much loss to still be called "good."

No problem...

I will stick with my statements from above about the %'s and conditions of the engines. It is an industry standard....could the leakdown at 15% be better? YES ... Can the engine last and make power? YES ... Will it have an excessive amount of blow-by? Depending on where the losses are. YES & NO


Howard

[Flycaster]

Originally Posted by AR Fab

...Depending on where the losses are...

I'm not with you here - compression ring blowby is compression ring blowby, no?

Anyway, I agree those are the standards for typical N/A engines, heck most of the guages even indicate green up to 20% LD loss. And if that is still "good", it certainly makes me feel a whole lot better since that gives me a looong way to go...not that I'm feeling antsy since at my last check at 25K miles my worst jug pegged only a 6% loss. I'm a happy camper.

[AR Fab]

The leakdown test shows the percentage of leakdown in several areas not just ring leakdown....it will show if your valves are not sealing and if the headgasket is leaking also...

You are in good standing at 6% only if its past the rings....but if its in the intake or exhaust valve or to the water jacket you could have other problems...

The leakdown test, when properly executed can tell you alot about combustion chamber sealing.


Its not all about ring blow-by..

Howard

[Flycaster]

Ah, ok, it was just your wording that confused me. I've always understood blowby to refer only to the ring loss, and the other 3 areas as "leaks". Oh well, we're one for one here.

[gabedude]

When I did my leakdown and got good numbers, I was very unhappy. Damn heads. Hopefully it is just an intake valvestem seal.

[Brandi@ARFab]

Originally Posted by gabedude

When I did my leakdown and got good numbers, I was very unhappy. Damn heads. Hopefully it is just an intake valvestem seal.

What is the engine doing that makes you unhappy and believe you have an intake valve stem seal problem?

[mxpop]

Originally Posted by Flycaster

Boy, I dunno about those #'s Howard - for an N/A car, yeah, but for a FI car, a 15% leakdown can be pretty ugly. .

Funny, I thought you were talking about a Fuel Injected car, not F1 or forced induction.....

When possible, I bench check the motors I do for both compression and leakdown. I rarely notice any meaningfull change from freshly assembled cold to broken in and warm etc.

I wouldnt be happy to see over 5% on a proper built new motor. However, 10% on a used motor doesnt seem to make measurable losses and I have even seen 35% motors seemingly run fine but should be fixed. A dry sump motor can probably handle higher leakdown as it will help seal the rings better while running (assuming the losses are mainly ring leakage).

FWIW, I build engines from air cooled utility twins, 14500 rpm motocross engines, 17k rpm N/A motors that make over 200hp per liter to turbo motors with 800hp+ and the principles of leakdown are the same throughout. As mentioned, compression is a factor of piston, head, cam, cam timing, decompression mechanism, etc design and cant be used to determine condition of a motor unless it's simply to compare it to when it was new.