Leakdown %

[SSTOM]

What is the ideal % of leakdown on a 260? How will the leak down hurt wot performance? Thanks Tom

[Jay Smith]

Our 260 engines after a fresh rebuild will leak down at 2% or even less. I suggest rebuilding race engines @ 8% but only if 1 max 2 holes are leaking that bad and all the rest are OK. In all out competion if one would race a competitor ( all things being the same )and he's sealing @ 2% and the other guy is leaking across the board @ 8-10% the "leaker " has NO CHANCE in taking the win.. ( If more than 1 to 2 bores are leaking more than 8% most of the time that motor has lost its competitivenes against motors that are sealing ) I suggest running a lake motor no longer than a 10 % leak down that has been found. A motor can have decent cranking compression and still have a unacceptable leak down and tons of potiential power be left on the table. A motor with an unacceptable leakdown will be noticalbly down on power but this condition happens sometimes slowly and is hard to feel the transition from good to not so good until one day it becomes VERY apparent something is getting VERY lazy thats why a leak down test periodoticly is key to staying on top of your peak performance . When a 10-20 % leakdown is found that means that that bore is only containing 80 % of it potiential power. Some guys "run" engines at > 20% leakdown and call it good to me thats goes against the grain of what one would own any high performance vehiecle for...

My opinion, good luck

[bill mason]

Hey Jay (or anyone) can you give directions on math?? Lets say for intance the top number before turning engine is say 100, when leakdown is done it reads 95. Do you subtrack the 95 from 100 then devide the the sum by 100 to get 5% OR do you just divide the bottom number by the top number (100 into 95) to get 9.5% leakdown??? Thanks for the math class (in advance)

[RichS]

Divide the leak pressure from the test pressure and subtract from 1.00.
1.00 - Leak/Test = Leakdown%
1.00-95/100=5% leakdown

[BigRedAlli]

Should you do leakdown on warm or cold engine, or does it mater? I would think the results woulb be a little higher with warm engine, unless the cold oil in the cyl's sealed beter.

Also,do you use just the TDC resluts? Usely when you start going past TDC, seems that the leakdown increasses futher down in the cyl. Is that a sigh of cyl. ware, or just the pistion cocking slightly due to the leverage from the rod?

How much more leakdown do you get from top pin pistons, due to the ring gaps lineing up?

Sorry for all the questions.Just trying to learn.:D

Thanks...Marty

[BigRedAlli]

Pm"s Welcome... :D

[bill mason]

also interested in if warm or cold engine makes any difference?

[TheRickster]

I always do a leakdown with the motor warm. THis came from doing automotive racing engines where the head bolts would distort the tops of the cylinders. We would use a torqueplate to simulate the head being on the block and I would bolt everything I could on the block, water pump, motor mounts, bell housing, etc... and also fill the block with hot water to get the block up to temp for honing. Temperature does weird things to metal, especially dissimilar ones that have different expansion rates. Anytime I test compression or leakdown I always try and get the motor up to operating temps.. After all you are trying to see what kind of shape the ring seal is in.. The motor runs at temperature so that is where you need to be concerned about how well it is sealed....

My opinion,
Rick

[bigbore]

obviously when the piston strokes down past the cylinder port's it's gonna loose pressure (intake and exhaust)as bolth ends are open further down the line.:cool:
in turnnthis means the only thing a leakdown test is good for is combustion
chamber,with each piston at t.d.c.,which means that compression is gonna be the way to go with any variences in-between cylinder to cylinder (it'll let you know if there's something wrong in one of the holes)the other thing to do is keep track of your running hours n re-ring every (600) or less,or TAKE THE HEADS OFF N CHECK THE CYLINDERS OUT FOR ANY BURN MARKS OR SCARING.

[sosmerc]

I like to do the leakdown test after getting the engine up to temp. And it is important to use at least 100 PSI incoming pressure for the test as some ring designs seal better at higher pressure.
Some pressure leaks past the rings at the end gap and this might explain why some engines (after a re-build) start out with a higher than desired leakdown percent...because the end gap was excessive due to: cylinder being bored improperly, or the ring sized improperly. Or perhaps a new ring being used in a cylinder that was just "cleaned up" and still out-of-round. Cylinders can show good compression and still have very high leakdown numbers above 15%...but usually this indicates there is damage to the piston and cylinder wall. I see alot of general use motors out there running with over 15% leakdown and the customers are not aware of any problems. It's kind of fun to pull the exhaust covers and actually show them the damage and then discuss the "future" of that engine.

[Hottrucks]

How much more leakdown do you get from top pin pistons, due to the ring gaps lineing up?

Sorry for all the questions.Just trying to learn.:D

Thanks...Marty

I'm not sure about top pined pistons, DO the gaps line up?????? I don't think so but have never seen them either?

[sosmerc]

So I guess one could wonder whether leakdown would be higher if the end gaps line up, vs. end gaps that are offset. You would think that leakdown would be slightly higher if the gaps lined up, but who knows.

[chad202]

Has anyone ever had a motor that was perfect with a compression test but leakdown was bad. The reason for asking is: if you do compression test on a regular basis and record the numbers, when you numbers starting falling or straying off a little, then it's time to maybe check the leakdown or just rering?

[sosmerc]

Yes, I have checked engines that have had good, even compression, yet the leakdown numbers were suspicious. Upon inspection I found scored pistons and cylinder walls. The rings were still free enough to somehow provide some sealing, but obviously these engines would not provide normal power. The L3 and L4 Mercs seem to exhibit these characteristics often. Merc eventually made a thermostat change (from 143 to 120 degrees) to eleviate the scoring problem.

[GTO GEORGE]

Compression testing numbers ( lbs/sq. in) relate more to port timing relative to compression chamber size, ie, 145 lbs is a lower compression ratio than the same motor with "tighter" heads that may show 200 lbs. Raise the exhaust port with the 145# heads and the pressure will drop, maybe to 110#, depending on the amount of the porting. These measurements illustrate "peak" pressures with a certain porting/ combustion chamber volume combination.

Leakdown testing shows how well a ring/ bore/head gasket/ valve(4stroke) combination is sealing. If you put 100# in a cylinder and it only holds 80#, then you have 20% leakdown; it's leaking somewhere and therefore when combustion occurs, it's not pushing 100% on the piston and the total force of the combustion event is wasted. This test illustrates pressure holding ability @ tdc with any porting/ chamber volume combination.

[Hottrucks]

I would think that you would want to do a leak down from tdc to the top of your exhaust port it would be the only way to insure that the hole is sealing when it needs to be ( sort of the combustion stroke )

[props4u2]

I would think that you would want to do a leak down from tdc to the top of your exhaust port it would be the only way to insure that the hole is sealing when it needs to be ( sort of the combustion stroke )

I agree, that's how I do it.


Lee

[GTO GEORGE]

You can't do a leakdown at any other place BUT tdc on the compression stroke; the chamber has to be sealed, ie, the piston is covering the exhaust port or in a 4 stroke, the valves are on the seat.

[BigRedAlli]

Thanks Guy's...

Marty

[Balzy]

You can't do a leakdown at any other place BUT tdc on the compression stroke; the chamber has to be sealed, ie, the piston is covering the exhaust port or in a 4 stroke, the valves are on the seat.
That's not true. You can work your way down until you open a port. That's how you check a lot more of the bore than just the very top.


Has anyone ever had a motor that was perfect with a compression test but leakdown was bad. The reason for asking is: if you do compression test on a regular basis and record the numbers, when you numbers starting falling or straying off a little, then it's time to maybe check the leakdown or just rering?
Oh yea !!!!!!!!! Big Time !!!!!!!!! 145 to 147 across all on the compression gauge. 1 1/2% to 2 1/2% on 5 holes and 45% leakdown on one hole !!!!!!!!!!!!! Look at the pic !!!!!!!! :eek: :eek: :eek:



Guys, go here and read my instructions if you want to know how to do a leakdown test properly. http://www.freewebs.com/bttwracing/leakdowntesterinstructions.htm

[Jay Smith]

OUTSTANDING EXPLINATION GEORGE !:D :D :D And right on target !
You da man !

Thanks Mike a GREAT testimant that a compression test ain't SQUAT verses a leak down test :

NOTE: the now FAMOUS Balzy Leak Down Tester attached giving an accurate reading of the health of this puppy's rings and bores....

[Fast Fred]

ok, jumpin in hear,
Jay Smith
Supporting Vendor
Our 260 engines after a fresh rebuild will leak down at 2% or even less.

them is Top Pins?, so less than 1% can happen? and at 1% or less, thats tellin me she is Solid, No leaks on the head gasket, rings, and no cracks?:cool:
thanks
FF


yes, top pins Do line up the gaps, if your runnin two rings.:cool:

[Fast Fred]

:eek: :cool:

[Jay Smith]

Believe what you choose.......

[Fast Fred]

thinkin i'm askin if i got it right. cuzz on some Modas it's hard to tell ware the F%$# the water leak is comin from. so i'm Askin:cool:

[Riverman]

I remember some time ago there was a piston ring called Total Seal that could achieve 100% sealing. It consisted of 2 rings in each land that fit inside each other, so there was in effect no ring gap. A friend had them in his big block gasser, and they worked!

Jay, I forgot to thank you for the shirt. It's a beauty!

[Jay Smith]

That you Jeff , because of you I have ICE .......;)

[Fast Fred]

thinkin i'm askin if i got it right. cuzz on some Modas it's hard to tell ware the F%$# the water leak is comin from. so i'm Askin:cool:


:cool:

[Jay Smith]

Guess I'm confused Fred ,:confused:

What does bore/ring seal ( leakdown ) have to do with finding a water leak ??????
Leakdown numbers are a test to observe the health of a bore to ring mating combo and or the lack there of with the ability for the ring/bore seal to contain and hold a known amount of pressure introduced above the exhaust port.... One would come to the conclusion that this test is only accuarce if there is no head gasket or sleeve leak :cool: ........

Oh and BTW a Drag Motor only has 1 ring Fred and it will leak down a 2% or less when sealing and healthy ( round hole - round piston with fresh ring ;) go to the bank with it !!! )

Good luck,

[d&smach]

I found a problem with compression testing. I had a motor that had 150 lbs. after rebuild. When cranked over the compression would jump to 150 quickly. I had an overheat, almost stuck, situation. Afterward the compression was still 150, but it would take alot more cranking to get there. I suppose if you are good you could watch the compression gauge speed taken to get to max. pressure.

[BrentShaw]

Do single ring pistons tend to have a higher leak down that 2 ring pistons. might be a dumb question but I was curios how they would compare with all other things being equal?

thanks Brent

[rpm racing]

Do single ring pistons tend to have a higher leak down that 2 ring pistons. might be a dumb question but I was curios how they would compare with all other things being equal?

thanks Brent

YES, single ring piston have a much higher leakdown number with only the one ring.

IMO, a single ring motor after break in will have around 5%, when it gets over 10% you better be concerned. The service life of the ring in these motors at HIGH rpm is well under 10 hours in my opinion.

[mr_velocity]

Guess I'm confused Fred ,:confused:

What does bore/ring seal ( leakdown ) have to do with finding a water leak ??????
Leakdown numbers are a test to observe the health of a bore to ring mating combo and or the lack there of with the ability for the ring/bore seal to contain and hold a known amount of pressure introduced above the exhaust port.... One would come to the conclusion that this test is only accuarce if there is no head gasket or sleeve leak :cool: ........

Oh and BTW a Drag Motor only has 1 ring Fred and it will leak down a 2% or less when sealing and healthy ( round hole - round piston with fresh ring ;) go to the bank with it !!! )

Good luck,

Wouldn't the leakdown also tell us if there was a leak in the head gasket (water getting into the cylinder while its runnng)?

I got a FF motor, just got the new MegaBaltz the other day so I figure I would put it to the test. Both at TDC and then dropping the piston so it was just above the ports I was getting less than 1% across the board.

[Balzy]

mr_velocity,

Yes, a leakdown test will show a higher leakdown if you have a water leak somewhere. It will pick up any leak of any kind. If you do a test on a motor with a sealed cooling system (car motor with a radiator) sooned or later you will see bubbles in the radiator. The air has to go somewhere........

One would come to the conclusion that this test is only accuarce if there is no head gasket or sleeve leak :cool: ........
I think what Jay is saying is that if you have a water leak, you won't get an accurate reading on your ring seal.

[bigshrimpin]

This might be a stupid question . . . but how do you clamp a merc flywheel to prevent it from spinning during the leakdown?

[rpm racing]

This might be a stupid question . . . but how do you clamp a merc flywheel to prevent it from spinning during the leakdown?

Put a socket on the crank nut in the middle of the flywheel and rotate and hold it with a BIG breaker bar. It is much easier with 2 people.

[Balzy]

On my instruction sheet I suggest you use a bigger ratchet rather than a breaker bar. If you end up going over TDC it will just spin over and not take you for a ride like a breaker bar can.......:eek: :eek: :eek:

[Riverman]

I used a 1/2" ratchet with a LONG pipe on the end to make it easier.

[Jay Smith]

PERFECT or near PERFECT leakdown has EVERYTHING to do with 3 things , a perfectly round straight bore with a proper hone/ plateau brushed ( no egg ) a perfectly round piston and the correct end gap that is flat to each other mating surfaces.If any of these 3 conditions aren't met your seal and or leakdown results are high....

BTW: I HAVE build some single ringed Drags that fit these criteria and will leak down at 2% or less at time of its 1st running for an hour at the shop.. I will agree that this condition is very short lived ......

Have a good one,

[Fast Fred]

:cool: i was thinkin it was sayin (the test) just a bit more about the cyl as a unit, and thats what i was lookin for.:cool: ;) thinkin i be needin one, never tryed it.

and just once again, top pins runnin two rings can test out to 1% and or less?
BTW: I HAVE build some single ringed Drags that fit these criteria and will leak down at 2% or less at time of its 1st running for an hour at the shop.. I will agree that this condition is very short lived
ok missed that one the first time:cool:


thanks
FF

[Fast Fred]

hears some top pins, US1 top pins.
http://img117.imageshack.us/img117/4268/p5260001cs8.th.jpg (http://img117.imageshack.us/my.php?image=p5260001cs8.jpg)