mchicia1

iTrader: (5)

I have a 99 C5. Was bone stock...I wanted to keep a stockish feeling car but maximize my power so I went with AI ported 799's, kooks headers, and an ls6 manifold. I got everything bolted together, started it, and it was running extremely poor...I tested fuel/spark and everything checked out. I then checked compression and got ZERO on 5 cylinders and very low on 3 cylinders. So I popped the VC cover off and on one of the dead cyls I took off the exhaust rocker and pushrod. I then set my PR length checker to 7.250 and stuck that under the intake rocker...turned over the motor and instantly got good compression...Obviously these are probably too short, but I wanted to set it to something extreme just to see if I got any compression.

I was running 7.375" pushrods but obviously these were too long and causing my valves to stay open during the compression stroke. I ran the car like this for maybe 5 minutes and didn't go above 1500 rpms or go in any gear. Just wanted to see if it would get better as it warmed up because at the time I didnt even think of PR length causing the issue.

What are the chances I banged my valves off my pistons? The cam is still the stock 99 cam, so less than .500 lift.

msydow

i doubt it since you got your compression back when you swapped the pushrods. bent valves=no compression every time (85%-90% of the time).

mchicia1

iTrader: (5)

60% of the time, it works every time!

Maybe I should do a leak down test just to be sure? I mean should be easy right? just remove all rockers to hold the valves up? Or is it not that simple and I actually need to find TDC?

1313of1600

iTrader: (7)

I just today took the head off my LS1 and did have all 4 of the intake valves on one side tap the pistons. The pistons had small marks on them(the deepest mark was .025"). Took the head to the shop and all the valves were still good and strait. I'm running .601 intake lift and .610 exaust lift and comp 926 springs on LS6 heads that were milled .002 to flatten them out. The tap was from floating them and bouncing off the rev limiter at 6800rpm's for a split second. Engine still has great compression and no noise.

180ls1

iTrader: (4)

is your car cammed?

cleonard

if the valve was bent it would be zero compression or really low. yes you can pull the rockers and do a leak down.

1313of1600

iTrader: (7)

Also on the stock cam that your running with the pushrods that your running i really doubt that you tapped anything. I think that most likely that engine was having a hard time due to the valves staying open/closed too long on both intake and exaust and you were just not getting good compression and a good burn. May i ask if your running a stock cam why are you not running stock length push rods, 7.4's? Why are you thinking that 7.375 are too long? If your not tuning the idle then its gonna run like **** when you start it up and wont get better as it gets warmer. If you really want to see if the valves are tapping put a piece of clay on the piston dish put the head on with about 4 bolts and turn the motor with a breaker bar and the crank bolt one complete rotation. Then pull the head and slowly take off the clay and measure the depth of the indention and you will know if its gonna tap and or what your clearance is(or if you cant get the motor to make a complete turn then you for sure are tapping LOL)

vettenuts

iTrader: (4)

Posted in your other thread as well but I think you are fine. Hitting a piston is usually a result of over rev and loss of control or bad cam timing (timing chain on wrong). Since you were holding the valves open and some cylinders had a little compression, they were likely barely off of the seat and well pretected within the combustion chamber. As previously stated, the compression came right back which is an indication that everything is OK.

farmington

iTrader: (14)

If you do a leakdown test, the piston has to be right at TDC or else the air in the cylinder will actually push the piston down. You'll need marks on your balancer every 90 degrees. When you get #1 cyl at TDC and both valves are closed, just follow your firing order, turning the crank 90 degrees, and checcking the next one in order.

mchicia1

iTrader: (5)

Yes stock cam...

Well, yes the pushrods were way too long even at 7.375. The 799 heads must already be shorter than 853's or something...combine that with a .030 mill and ls7 lifters and you get a shorter pushrod. I am going to measure better this time LOL.

mchicia1

iTrader: (5)

That makes me feel a lot better! I am sure I am perfectly fine then. The lift on the ls1 cam is only .498 and it has very low duration.

mchicia1

iTrader: (5)

Ok so no leak down test then .

1313of1600

iTrader: (7)

Wow they were milled .030 that's a lot to take off!! LS7 lifters are not really any shorter then the LS1's it is actually an visual illusion. I guess the heads must be shorter is right! If it turned and you did nothing but idle and you didn't hear any pinging or tapping then you didnt tap. Again it is gonna idle like **** till it's tuned but check your clearances for sure once you get correct push rod length!!

mchicia1

iTrader: (5)

Yeah, 60CCs

I actually expect it to idle perfectly...stock cam man!

Too hard to tell if I had any tapping since they were new lifters, the motor just sounded like **** on first startup. A lot of bad noises occur when you run barely on 3 cylinders lol. I will find out soon enough though.

1313of1600

iTrader: (7)

Stock cam or not you still need a tune, more air with same fuel and better flow of exaust and changing the displacement by milling the heads= needing a tune!!!

mchicia1

iTrader: (5)

Of course I am getting a tune! LOL.

I just meant I dont think it would require a tune just to idle or drive around town for a couple of days before I get it tuned (no wot obviously).