I've done quite a few leak downs since my first in '71. Up until about the mid 90's I thought leak testing was valuable and a lower number meant more power. Then I started hearing about not putting too much stock in leak downs and how they really aren't a realistic picture of what's going on when an engine is running. I decided to leak my current engine while it's out of the car just for some data. The car ran it's quickest 1/8 and 1/4 mile ET's in October in decent but not great air and a 10 mph head wind. My leak numbers are between 17 and 26 with one at 32. These are way high for anything I've built in the last 40 years. So I guess the guys that say leak downs aren't realistic are correct.
I leaked it a second time in 60 degree garage and #4 went from 24 to 60% and mostly all out the exhaust. I tapped it with a hammer and it went to about 55%. I thought about Tom Worthington's situation a few months ago. I pulled the valve spring off and spun the valve by hand while bumping it against the seat using a lapping motion even though I couldn't see anything on the valve. Re-leaked it and it read 20%. If I had the 60% number first, I probably would have pulled the heads off. I can see some little grains of oxidation in the exhaust port so I assume a little bit was holding the valve open. Hard to believe the 230 lbs of seat pressure wouldn't have overcome a little trash like that. Any ideas?
Posts: 466 | Location: Maryland | Registered: January 23, 2007
Happens more often than you think to a lot. Normal occurrence here if i let the car sit. Warm/cold and don't fire it for some time in the shop when running Q16. I was surprised at the amount of minor rust on the seats and faces. Bore scope. Q16 wants to pull moisture. Say for instance. I pull the car into a cool shop with a warm engine, guaranteed you will see it. That why i do the LD hot and cold.
Posts: 1422 | Location: Under a Truck | Registered: August 23, 2013
Originally posted by Ron Gusack: I've done quite a few leak downs since my first in '71. Up until about the mid 90's I thought leak testing was valuable and a lower number meant more power. Then I started hearing about not putting too much stock in leak downs and how they really aren't a realistic picture of what's going on when an engine is running. I decided to leak my current engine while it's out of the car just for some data. The car ran it's quickest 1/8 and 1/4 mile ET's in October in decent but not great air and a 10 mph head wind. My leak numbers are between 17 and 26 with one at 32. These are way high for anything I've built in the last 40 years. So I guess the guys that say leak downs aren't realistic are correct.
I leaked it a second time in 60 degree garage and #4 went from 24 to 60% and mostly all out the exhaust. I tapped it with a hammer and it went to about 55%. I thought about Tom Worthington's situation a few months ago. I pulled the valve spring off and spun the valve by hand while bumping it against the seat using a lapping motion even though I couldn't see anything on the valve. Re-leaked it and it read 20%. If I had the 60% number first, I probably would have pulled the heads off. I can see some little grains of oxidation in the exhaust port so I assume a little bit was holding the valve open. Hard to believe the 230 lbs of seat pressure wouldn't have overcome a little trash like that. Any ideas?
Yea. don't leak a cold motor, especially one that sat a while that has steel seats! The ex usually rust a bit and cause false leak values. This can happen on the intakes too but more common on the exhausts. Leak down values are just to see that all is OK relatively speaking, so your looking for a similar number in all cylinders. It's just a static test and the real ring seal works in a dynamic world.
Leak Down testers are just another tool. And they don't always tell the truth. Back in the late 1970's when I was class racing in Sand Drags, I had a highly Turbocharged (30 psi for the day) engine. At a particular raceevent I did a leak down and it was horrible. That day I set the National Record in the Class. So take it for what it is,.....maybe just pointing you in a direction to look at.
Fellow racer and servant of the Lord of Lords John 14:6 "Creekside Racing Ministry" MFI system, ProCharger Non-intercooled [6.02 @ 229 or 3.91/660' soft tune and killing power above 6K rpm] Ron Clevenger
The engine in my wife’s dragster had about 300 runs at the end of last year so we pulled it out to freshen. With a couple races left I was looking at the plugs and #8 was black, oily. I replaced it and checked it after she made a run and it was black again. After that she ran the quickest ET she’s ever gone with it.
We pull the engine apart and find that #8 cylinder has the moly from the rings gone, so that’s likely where the oil on the plug came from. I’m sure if we leaked it down that cylinder would have leaked terrible. But yet the car went the quickest to date and was the same amount slower than my car as it was all year. Since we are measuring performance to the thousandth of a second I would think we’d see a difference but like Al said this stuff happens in a dynamic world and apparently it was happy enough to go fast!This message has been edited. Last edited by: 183N,
Posts: 743 | Location: Upstate NY | Registered: July 02, 2013
Yeah I agree with all, but I didn't always think a leakdown was just to provide some direction. I was at E-Town for the Summer's in the mid 70's and I was stalking Jenkins like I did every year. I go to his pit and Lombardo and I think Thacker are at the front of the car with the hood off. I'm standing right behind them, as close as I can get without going inside the little rope. They are doing a leakdown and they don't seem happy. They are bumping the balancer bolt with the breaker bar. I can't see the gauge. Lombardo says "Go get Bill". When Thacker leaves, I can see the gauge and it's on 4, which was as good as I could get our engines. Bill comes over and does the same jerking deal with the balancer and the gauge never moves. Bill turns to walk away and says "Yank it." I'm thinking I gotta find a way to get mine better and the next engine I did I got it down to two on most cylinders and that's the best ever for me. Many years later I started hearing good builders poo poo the leakdown test. Interesting stuff, thanks for the replies.
Posts: 466 | Location: Maryland | Registered: January 23, 2007
I use a Cranking compression check at the track and always log the reads. Also watch the crank case vent for blow by, changes or increases usually tell you a story. Especially on blown engines.
Fellow racer and servant of the Lord of Lords John 14:6 "Creekside Racing Ministry" MFI system, ProCharger Non-intercooled [6.02 @ 229 or 3.91/660' soft tune and killing power above 6K rpm] Ron Clevenger
Remember, the majority of valve seats are iron. There is always a valve or 2 or 3 open, and the seats will rust. Rust will skew a leakdown. Storing your racer in a trailer will build moisture inside. Parking on the ground outside your house will allow the moisture to penetrate the plywood floor of the trailer and with valves open the seats will rust. Much like a boat engine, most will leak 50% on the exhaust and some more, but they run like a top. Leak will get better on most after running and warmed up so the seat returns after the rust gets pounded away. Most guys just leave their cars outside the trailer at a multi day race, and the dew,humidity, moisture take a toll on the insides, as well as the ever forgotten master disconnect switch. Storing your car in an enclosed trailer, mount a de humidifier in it and when the car is home plug that thing in. Dont believe it? all the steel bolts stop rusting, the aluminium stops powdering, and the trailer stops sweating. I get 5 gallons every 2 or so weeks here in Florida. And it is clean water so use it in the cooling system.
Posts: 868 | Location: ft laud | Registered: September 02, 2004
Agree 100% with Bad News on the dehumidifier. You will see a huge difference in general corrosion on everything in the trailer, tools included.
I really don't see the need for doing leakdown on a typical bracket engine. Will probably get more information from frequent oil changes and inspecting a magnetic drain plug. Simply connecting air to the cylinder and listening for leaks is probably adequate. By this point there is likely a problem and this is a diagnostic check, not preventative maintenance.
I too experienced rust on exhaust seats when I was checking valve springs over the winter. Could hear the leakage in the exhaust and I was concerned. It sealed right up after running.
Matt Ward
Posts: 1395 | Location: South Carolina | Registered: March 20, 2004
Originally posted by Big Steve: I think they are a good tool to find out if something drastic is broke, bent valve, burnt piston etc
I tagged an exhaust valve in the burnout at the bracket finals, round 2. I heard the "ting" noise and it sounded bad as I idled up to the line. Thought sure I was screwed, car ran dead on all day. Went red at 20 cars.
Also, the amount of air applied to a leakdown is critical to the reading you'll get. My fancy Snap On leak tester only applies about 60 p.s.i. and will show horrible leak numbers on a big bore engine. My old school dual regulator leak tester applied 100 p.s.i. and gets much better results, but you have to hold the crank while applying the air.
"Despite the high cost of living, it remains popular." Dave Cook N375
Posts: 1880 | Location: Indy | Registered: November 21, 2008
