jhelms

iTrader: (2)

A friend of mine had a leakdown test in his LS1 the other day, all the cylinders were around 200 (is this psi?) except one, and it was 175.

He wants to know how bad this is and if he can keep driving it around and getting on it. Eventually he wants to build the motor but now just isn't the time.

They also said from the leakdown test that they could tell that the leaking was on the exhaust side, so they predicted a burnt valve - that sound right?

If this is the case can he still keep driving the car?

The Derek H

The are supposed to be within a certain percentage of the highest one. Mine were all between 185-195 except for a cylinder that I had blown a ring on in my stock short block. I don't think its anything super serious but check the specs. Unless he's running into issues with the car he might be ok. Have him contact someone like Nick Agostino or Geoff at Thunder Racing.

The Derek H

Oh and just to mention it most dead cylinder show very very low compression. Not even close to 175

jhelms

iTrader: (2)

[quote]Originally posted by DerekHLS1Freak:
<strong>The are supposed to be within a certain percentage of the highest one. Mine were all between 185-195 except for a cylinder that I had blown a ring on in my stock short block. </strong><hr></blockquote>

I think they said the other cylinders were around 200.... so is that ok?

just me

that is NOT a leakdown test. That is a compression test.
A leakdown test pressuerizes the cylinder with air and measures the amount of air leakage and is read in a percentage.
A leakdown test can pinpoint where the cylinder sealing problem is, rings, valves headgasket etc.7-9% leakdown would be considered good for a stock block.
do a real leakdown test on the low compression cylinder.
15% or more and it's tear down time IMO.

Dean98TA

When you say 7%-9% for a leakdown test, what are you taking the percentage of?

I understand the compression test and wanting all cylinder pressures within a certain percentage, but how exactly does a leakdown test get performed?

just me

This link will explain leakdown testing-
it's in PDF format.
http://www.goodson.com/instr/LDT-50.pdf

Steve
www.tech-ls1.com

LS1derfull

To add to justme's info:Be carefull leakdown readings as hi as 15% can occur from compression rings lining up temporarily. This condition usually corrects its self in time. I have personally corrected hi leakdowns of 20% and more on gen.1 small blocks and gained nothing in performance(mph in 1/4). As far as exhaust valve leakage I would run the engine hard on the hiway and retest hot. You may be able to clear carbon off the seat.Sometimes you have to remove rocker arm and tap end of valve lightly. If retest indicates problem still there before you pull the head you could try this:remove rocker and spring, chuck up valve in hand drill and spinlap the valve into its seat.[racers trick] Good luck.

just me

I'll use my own 98 ls1 motor as an example.
This took place about 2 months ago.
I pull plugs regularly as I am a poweradder guy.
I noticed one cylinder had a weak firing plug.
Ran compression test first.
All cylinders at 195psi except for the cylinder showing a weak plug reading was down to 170psi.
Now a cylinder still building 170psi is not really a problem.But suddenly dropping 20-25psi from the last compression test raised an eyebrow.
Then I did leakdown testing.
Cold motor, the suspect cylinder showed 20% leakdown.A couple more cylinders showed ~15% and the rest around 7-9%.
Then did the test on hot motor.
All cylinders except the suspect cylinder now at 7-9% so that shows the difference in hot and cold tolerances.
Suspect cylinder still at 20% though.Not good.
All leakages past the rings.
The motor used no oil, and ran very strong,it was running 100mph+ in the 1/8th with 2.73 gears a couple weeks prior and felt even better even with the poor sealing cylinder as I had added some more tweaks.
But the plug reading and then the leakdown test do not lie so no sense kidding myself there is no problem.
So I pulled the motor.

Tear it down and find 2nd ringland cracked about 2" long on the suspected cylinder.
Remove rings on piston and piston ring land chunk falls out in hand.
Same old story.
Steve

LS1derfull

Steve I see that I went a little off coarse on this one,but your readings on the bad cylinder were good for the amount of damage you found.Now most important is that you keep it from happening again. I believe LS1 has hyperutectic pistons from GM that use large ring end gaps(compared to forged) because of good comb. heat transfer. I have also seen that GM has tightened ring gap standards on these newer engines because they can(less bore distortion with ls1 design). This makes the ls1 more powerfull stock, but touchy with power adders. I would use forged pistons with ring gaps set up for blower/nos. There is also the possibility this happened from detonation so make sure you are not having any oil consumption problems, this destroys octane of fuel causing detonation. LS1.com has threads on oil consumption fixes. Let me know what you decide.

just me

LS1derfull ,The piston cracked the 2nd ring land from heat.
That particular cylinder went slightly lean on a dry 200 shot once about 1.5 years ago.It was my own fault as I knew I was pushing the fueling.
When you over heat up these hyper pistons they give.
It wasn't detonation. Never hurt a plug , never lost a gasket but I did get the egt up to high.
I actually ran with this cracked piston for an entire year <img src="gr_eek2.gif" border="0">
I know this as when I pulled the heads last year I saw the white exhaust valve and scored cylinder.
I figured it might have had a cracked land at the time but I took a chance and put it back together anyway.It had held compression for quite a while but finally it started to go.Well I ran several more times on the bottle and then 10psi boost for a few months and it finally had had enough.I actually ran even faster when I converted to the blower.
The scoring I saw last year was caused by tiny bits of piston from behind the cracked 2nd ringland that got caught between the rings.There were 2 of these and they were nicely polished flat riding up and down the bore for the last year.
There were also several that got caught behind the ring.
I am not all that amazed at this. I ran with 4 cracked 2nd ringlands on a stock block 5.0 and never even knew it until the motor was torn down when I built a real motor to take it's place.
Pulled that one apart and 4 pistons fell apart when removed from the cylinders.
Again that motor had seen a lot of bottle and then converted to blower.
So I have seen this before.
I know I am pushing my luck with the cast pistons and the blower.But I am pretty good at anti detonation measures.
The blower is no where near as hard on them as the bottle.
Nitrous just kills hyper pistons no matter how good you are with the stuff.
It's funny ,everybody worries about the top ring lands. But I have now broken 5 2nd ring lands on cast pistons.
Melted off a big chunk of top ringland on a good forged piston though.
I don't want to put a lot of money in this thin sleeved early 98 block.

[ December 11, 2001: Message edited by: just me ]</p>

jhelms

iTrader: (2)

[quote]Originally posted by just me:
<strong>that is NOT a leakdown test. That is a compression test..</strong><hr></blockquote>

They were supposed to do a leakdown and a compression test for him, I'll ask what the leakdown numbers were.