Attended a tech session today at a local garage where I got to watch a
pretty neat demo of a leak down tester. The host (Craig Vaughan, Foreign
Car Enterprise) was trying to show us the importance of good diagnostics
and good compression.
He was using a dual face gauge attached to an air supply. With dual
faces you can see how much air is going in and how much air is going
out. When you get 50 pounds going in and 40 pounds going out you have a
leakage of 20% -- that is a number you can work with to improve. The
'average' car can be expected to have leakdown in the 10 to 12% range. A
good valve job and high-tech rings can bring leakdown to the 2 to 3%
range. That is almost a 10% compression improvement all by itself. (He
is a distributor for Total Seal piston rings by the way.)
Back to the tech session: He had a '67 Jag roadster in the shop and
suspected some compression problems. On one hole, he put 50 pounds of
pressure at TDC through the spark plug hole and was only holding 40.
There was a noise. Taking the radiator cap off we could see and hear air
bubbling up. No brainer! Head gasket. On the adjacent cylinder, he
repeated the test. There was more noise. Coming though carbs. Cracked
valve.
This was done after a compression check that identified those two holes
as suspect. I thought that was a pretty neat diagnostic method. Two
gauges and he was pretty sure where the problem was (compression test)
and what the problem was (lead down test). Cool.
I just may need to get one of those leak-down gauges to complement my
compression teseter. Summit has one in their catalog that is a Tavia,
has the part number TAV-08015 and is priced at $63.69. Their compression
tester is part# SUM-900009 for $21.99 (older catalog not listing their
toll free number 330-630-3030. of try their web site at
http://www.summitracing.com).
Bob Allen, Kansas City, '69CGT, '75TR6
|