Hey guys I wanna pick your brains on some theory... since im far past tearing my hair out trying to fix the car I just wanna figure it out... For a while now ive been having a issue with my 72 nova 355 sbc with gt45 turbo its been lacking power drastically and has had issue with oil leaks and oil spray from the valve cover filler holes. After trying every different thing I decided to go back to the basics so today I did a vacuum test,a compression test and a leak down test on the engine to try and trace the root of the issue. First I ran the car with a vacuum gauge on it and noticed the idle vacuum was a little low even when I advanced the timing the vacuum didn't change much it sat around 10-12, not a crazy cam so I believe it should have been a touch higher. Anyway I then did a compression test on the engine and found pretty much good and even readings through out about 150psi give or take 5-10psi from highest to lowest. Then I did a leak down test I only tested cylinder number one but from the result I got I assume the rest are about the same, nevertheless when I did the leak down I found a pretty decent amount of air coming from the dipstick tube and when I pulled the breather it blew out a lot about the same as if you were whistling... Help me out how can I have good compression and bad leak down results am I right in thinking this is my issue, am I doomed to rebuilt the engine again? thanks in advance
the percent leakage gauge showed a very small percentage maybe 8-12% I disregarded the gauge reading because there was a lot of air coming from the crankcase... engine was originally rebuilt like 2 years ago since then I have had many driveability issues with it but it is a sbc 355 entirely forged rotating assembly file fit rings edelbrock roller cam scorpion full roller rockers afr aluminum heads 75cc mls gaskets high rise single plane intake manifold professional products throttle body EFI gt 45 turbo with 6btm MSD box
The leak down gauge should work it's not been used very much but with the amount of air coming out of the dipstick and oil cap I figure something's gotta be wrong regardless of what the gauge says
The compression test measures how much air is pulled in and compressed in a fairly short time period. Reasonable pressure numbers and consistency basically means that you don't have major restrictions on the air in or out and no gapping holes in the pistons, or hung up valves.
A leak down test is a lot different, it basically measures the sealing of the rings and the valves (no moving parts, just how much do they leak. Without testing all of the cylinders you don't have way of using the information.
Depending on what your crankcase ventilation system is you may to much blow by for the system to handle and you are building pressure that is pushing oil out wherever air can get out.
What is your boost limited to? What is your static compression ratio? What about dynamic compression ratio at maximum boost?
I have a pdv system on but hve also tried the crankcase evac system in the exhaust, tried all different type of baffles lighter oil
static compression ratio is 9:1
boost is about 5lbs currently not sure what the dynamic CR equates to with 5lbs of boost though
You said the rings are a file fit.First are the pistons make for blower or turbo applications??. Those normally have the top ring lowered on the pistons to help avoid the heat generated by a blower or turbo. It's that plus the blower or turbo settings of the ring gap that keep the ring's ends from butting against each other.When that happens either the rings break or twist out of the ring lands. You might be able to scope the bores to see if that has happened.You want to look at all the cylinders when you do that. And borrow or rent a good scope to do it with. Not HF junk.
ive done leak down tests before... the too,instructions say to use 100-150psi shop regulated air i used 100psi as my test psi if i lowered the regulated air at the tool the leakage percent would climb in other words at 100psi the gauge would read 8-10% leakage at 80psi the gauge would read slightly high leakage percentage at 60psi the leakage would be even more however that just be the functionality of the tool until you get to the right pressure for the test
So that being true the leak test does not prove that this is my issue because there is not enough time for the air to leak out when the engine is running
A leak down test tells you what percentage is leaking from what cylinders and where the leaks occur. Race cars leak less than street cars.How much leak down you will tolerate is up to you. as the engine leaks more it indicates wear or damaged parts. A fresh engine leaks less and makes more power and smokes less,,,,
well 8-12% , if they are all doing that.. and you have a turbo.. if it's 8-12 % from a 100psi air flow
what type of air you thinks getting past the rings when the turbo is spooled up..
go test them all.. was the file fit rings gap set up for the extra heat of the turbo?
gear head that was one of my original thought when pressure is being introduced to the cylinder by way of turbo the amount of leakage has got be greater... the percentage gauge reads 10% leakage but it sure feels like a whole lot of ait to me
Ok guys here's what I found after leak testing all cylinders
1-150psi comp 15% leak 2-160psi comp 8% leak
3-150psi comp 40% leak. 4-155psi comp 21% leak
5-140psi comp 28% leak. 6-150psi comp 14% leak
7-150psi comp 14% leak. 8-150psi comp 12% leak
All of which I feel at the breather hole in the valve cover and the dipstick tube. I suppose the next step is to start tearing the engine apart to probably due a set of rings right? What do you guys think?
I have seen that on newly built engine and the compression test was fine but leak down where up to 40% and the problem the owner was having is the oil would be dirty in less then 100 miles. Even after 4000 miles it would not seal up.
This was a Corvette 327 that was .040 over went .060 and torque plate honed the cylinders problem went away.
your case I think the rings ends butted or you got into detonation and broke the ring lands.. and/or rings.
the wrong cross hatch for rings..
iron/moly/chrome all want different set up's..
boring the block with torque plates but they used bolts, you use studs..
oval cyl..
I'm sure I don't have anything like that going on but now that I think about it I did have an issue with the car running really rich may have washed a few of the cylinders down... If I got the engine re honed with a torque plate could I then do a re ring? I would really hate to have to have it bored out even more then buy pistons then I have to re balance the rotating assembly
Rehone will not do it as it takes at least .005 to hone with a plate to round up the cylinders. A good hone needs to be used for the best result.
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