Compression/leakdown test results

[Joe C.]

Re: Compression/leakdown test results

Pardon my ignorance please; how do I tell the difference between the two?

Positive stop valve seals are made of either Teflon or Viton, and have a spring steel retainer wrapped around it to hold it fast onto the valve stem boss. These almost always require machining of the cast valve boss, although I understand that there are some available from GM for SBC that do not require machining. They are permanently fixed in place on the boss, and provide constant and "positive" sealing to the stems. "Positive" means that pressure differential between the crankcase and the induction system will allow a small amount of oil to pass, which is all that is needed for a SBC.

http://www.summitracing.com/parts/EDL-9725/

Umbrella type seals are used in conjunction with the standard o ring seals. They are fixed elastically to the valve stems and ride up and down with them. They do NOT form a positive seal, which is more important for a BBC, with its poor top end oiling. They function by preventing much (but not all) of the oil splash which is shed from the springs from clinging to and dripping down the valve stems and through the guides.

http://www.summitracing.com/parts/CCA-502-16/?rtype=10

[Larry T.]

Re: Compression/leakdown test results

I pulled a valve cover and did my best to sneak a peek through the springs. It was tough, but what I believe I see is a white material (teflon?) wrapped with an aluminum colored metal. So it seem I have the positive stop type.

[Gary R.]

Re: Compression/leakdown test results

I know this is a year old now but I'm experiencing the same thing with my 350. What did you find out? The engine is using about the same amount of oil as you did. I had some time today to start to get back to this car. It usually just sits in my garage along with the others.

Some facts
350 .020 overbore, Speedpro flat tops, speed pro file fit rings,moly top.
SR Torquer heads, ported & polished. Performer intake, 1256 intake gaskets matched.
Compression test, 180-205 psi. Partial leak down 40% on a couple of cylinders.
Oil use had increased, initial break in and first 800-1000 miles little smoking. Now, about 2500-3000 miles smoking all the time idle, driving,and WOT.
I may pull the intake and check the gasket and runners.

[Clem Z.]

Re: Compression/leakdown test results

i owned 5 different BBC engines in cars and trucks and none got more that 600 miles per quart of oil. the original ring package was not very good when it came to oil consumption. one good thing the exhaust system never rusted out.

[Domenic T.]

Re: Compression/leakdown test results

I am leaving the forum now but I had that problem after I rebuilt one of my engines and it was the break in.

I was told to get on it hard to get the pressure behind the compression rings and it worked.

I to had about 3K and was babying the thing.

Also to test the oil thru the guides. Just run the car on the street in 2nd gear up to about 5k and let off the gas till you get to about 2k, then give it the gas again. If you see smoke when you do so it is getting oil past the guides.

This creates high vacume that will suck oil thru the intake guides.

If its rings that are not yet broken in you will see smoke if you get on it in 3rd gear and reach high RPM's.

This was common (rings) in the 60's when GM used chrome rings in their cars. They never broke in and smoked all the time till you beat it a little.

GM stopped using chrome rings for that reason.

I was told by service writers to go out and run cars hard. They belonged to elderly people that were normal drivers. We also had to de-carbonize some that were knocking (carbon knock), yes the carbon built up on top of the pistons and hit the heads.

At first we pulled the heads and scraped them then GM came out with a pint can that you poured in your carb at about 2k till the engine stalled. You let it stand for about 1/2 hour and started it and it cleaned it up.

Prior to that we poured water in the carbs with wide open throttle and put the water in at the rate as to just keep the engine running. This was to steam clean the carbon off the pistons at the dealership but the buba's got carried away and warped a few valves and bent a rod or so.

Don't take any thing apart till you check ALL the SIMPLE things.

DOM.

[Rich W.]

Re: Compression/leakdown test results

Gary if you have 40% leakdown in a few cylinders then you need to know where its leaking to..exhaust pipe..up the carb..did you do a wet or dry test..anyhow..I know you are a smart guy..if it turns out to be rings put some carbon eater in the plug holes and let them loosen up ..works good if you let it sit for a day or so..just change the oil once your done..all the crud will end up in the oil pan..should loosen up sticky rings without a problem.

[Gary R.]

Re: Compression/leakdown test results

Hey Rich,
I don't have much faith in those leak tests from today, I found the hose was leaking and didn't really get to do the test correctly.Plus this car has headers and those are real joy to work around!!

The compression test is more accurate from today. I use a locking gauge so I know the highest reading is locked in.

[Dick W.]

Re: Compression/leakdown test results

Compression and leak down are both useful tests. Do your leak down test first and then do a dry compression test and then a wet test. You will have a very good set of numbers to work with.

My guess is, as Dave said, your engine is not broken in. Many moons ago, when those cars were new, Perfect Circle suggested that you get your engine up to operating temperature, find you a road with very little traffic, accelerate from 35 to 55, let off the throttle and coast down to 35, repeat the accelerate/decelerate cycle 10-15 times.

[Clem Z.]

Re: Compression/leakdown test results

Valve seals may be a cover up for wear on valve guides. Big blocks are more prone to guide wear more than small blocks in stock street engines. When rebuilt were the guides replaced or knurled? Also the cast iron guides are better that the bronze for street use.

Big block replaced guides need to be sealed from the water jackets. Also there is not a lot of material around them for press fit, so use a good sealer.

BBC heads had the valve guides bored after they were installed and sometimes the bore is not centered in the guide so replacing the guides is not a good idea. the best way is to just sleeve the original guides with cast iron sleeves.

[Jim D.]

Re: Compression/leakdown test results

I've had/built several BB cars, including a brand new 1970 LS-6 Chevelle, and have never had one use more than a qt. of oil in 1500 miles. I know everyone says BB's use oil but if they're built right, they don't. As far as ring seating goes, unless you're using old style chrome rings (why would you?), the new rings seat within 50 miles if broken in correctly.

[Gary R.]

Re: Compression/leakdown test results

Hi Dick,
Yes I have to get some more time to really get the leak down test done. The numbers I was getting weren't adding up compared to the compression test numbers. The leak tester line was leaking at the crimped end! New hose too.

I have done plenty of hard loading during the 3000 miles but it didn't help. The problem is getting worse instead of better. The first 800 or so miles I had no oil burning issues.

[Joe C.]

Re: Compression/leakdown test results

Hey folks, some of you should learn to pay attention. Gary stated that his engine is a 350, not a ratmotor.

Gary,
This speculation is meaningless unless you provide some useful data, namely: dry/wet compression tests (of course with all plugs removed and the throttles blocked wide open, but you knew that already), and an accurate leak-down test, most commonly conducted with 100 psi flowing pressure at the supply side. 5 per-cent or less leak-down is target for specially prepped race engines, 10-20 per-cent is about average for street engines, and anything more than about 25 per-cent is cause for concern. On all cylinders, note where the air is leaking. Oil filler/breather indicates rings, tailpipe indicates exhaust valve, and carburetor indicates intake valve.

[Clem Z.]

Re: Compression/leakdown test results

Hey folks, some of you should learn to pay attention. Gary stated that his engine is a 350, not a ratmotor.

Gary,
This speculation is meaningless unless you provide some useful data, namely: dry/wet compression tests (of course with all plugs removed and the throttles blocked wide open, but you knew that already), and an accurate leak-down test, most commonly conducted with 100 psi flowing pressure at the supply side. 5 per-cent or less leak-down is target for specially prepped race engines, 10-20 per-cent is about average for street engines, and anything more than about 25 per-cent is cause for concern. On all cylinders, note where the air is leaking. Oil filler/breather indicates rings, tailpipe indicates exhaust valve, and carburetor indicates intake valve.

the first post says L-72 and i am not up on all these "L" numbered engines but that is a 425 HP 427

[Joe C.]

Re: Compression/leakdown test results

the first post says L-72 and i am not up on all these "L" numbered engines but that is a 425 HP 427

You mean that mousemotors use "L" prefixes as well as fatmotors? I'll be damned!

Gary is not the first poster. He didn't exactly "hijack" the thread because it has been dead for more than a year, now. Gary just did the smart thing..........he picked up on a "dead" related post, which follows with immediate notification of all of the "players" within the "dead" thread. Genius.....................but that's why he's an NCRS nerd just like the rest of us.

[Timothy B.]

Re: Compression/leakdown test results

Gary,

What type of pcv breathing does the motor have. Is the pcv baffled to prevent oil from being sucked up into it, this is more important than people think.

I once built a 350 that had the same problem and then re-ringed and still the same. I now believe the problem was a combined high volume oil pump and poor pcv arrangment. So much oil up top and the baffles could not keep it away from the suction of the pcv.

The cranking compression pressure numbers sound very good to me..