valve adjustment with "racing" cam

[Jeff Berk]

I double checked the "mouse trap" springs and all were good. I replaced the valve cover and tried to run a compression test using a gauge I borrowed from a parts store. Cyl 1 read 145 pounds. The gauge then stopped working at that point and would not hold a pressure on any subsequent tests including on cylinder 1 so I returned it and got another gauge from a different store. It read 90 psi on all cylinders including cylinder 1.  I'm assuming it was a bad gauge so I'm holding off on a wet test until I can find another gauge and work out a way of testing a gauge against a tire pressure gauge.

 

[240260280]

It is good they are all the same.  I think the test gauge is the problem.

If you have the spindle in the normal spot, try starting it up!

[grannyknot]

Jeff, when you did the compression test were the SU's being held fully open? You won't get proper reading other wise.

[Jeff Berk]

Grannyknot: I did not have the SU's held open. Is that needed to minimize restriction to the intake valves? 

I'll keep that in mind for when I get a new compression tester. The engine likely has good compression because it is relatively low milage.

[grannyknot]

7 hours ago, Jeff Berk said:

I did not have the SU's held open.

That's the reason for low compression numbers, it's like trying to take a full breath through a straw.  An engine is essentially an air pump, a restriction anywhere in the air path will reduce the flow.  That last compression testing gauge you used is probably just fine.

All your spark plugs are removed for the test right?

[Jeff Berk]

We had nice weather so I warmed up the Z and retested the compression.

1) 125

2) 125

3) 125

4)130

5)123

6) 128

I had warmed up the engine but by the time I got to doing the test it cooled down a bit.

[grannyknot]

That's pretty consistent so that is good,  the numbers themelves  are a bit low but that maybe because the engine cooled off or probably the engine just needs a good run at highway speeds to loosen everything up. The rings may be gummed up from sitting so long.

Edited December 3, 2018 by grannyknot

[siteunseen]

You should also do some downhill gear down braking.  That will suck the rings against the cylinder walls.

[Zed Head]

The numbers are so consistent that your test method seems fine.  I think that "racing" cams can give lower cylinder pressures because the valves close later, to get more charge in to the cylinder.  The gauge hardware can give lower numbers also.  The tubular extension that makes it easier to reach the plug hole will give lower numbers for sure.

Is it an L28 head or modified L24?  You could have dished pistons under a head designed for flat-tops.

[240260280]

The made in China compression testers look like testers but not all of them are.

 

If the hose was not low expansion rubber then it effectively acts like a balloon/air cushion and the compression will read lower.

The calibration of the dial gauge could also be off.  If you have an air compressor with a good gauge or some pressure gauges then you can cobble a compressor to your compression tester and see if the gauges align as you pump up the pressure.

Edited December 3, 2018 by 240260280

[Jeff Berk]

I'll have to try some of that down-hill engine braking. With that low-restriction exhaust it'll sound great!

From some paperwork I have dated 1986 (1 year after "restoration") the 260Z has a '78 280Z block with a 280Z fuel injected head that was installed at that time. From some other paperwork, the 280 engine was new. The paperwork says It was ported, milled (?) , "trick" valve job (?),  the block was bored 20 over. There was something about the pistons in the paperwork but I cannot find that now.

As you can tell by the question marks, I don't know what some of  these upgrades are but I suspect they could impact compression. The car had 61,000 miles at that time and now has 75,000 so there shouldn't be much wear on the engine since then.

I need to check the calibration of the gauge but that was a loaner gauge. I ordered one and will look into a way of checking it's calibration.

Jeff

 

Edited December 3, 2018 by Jeff Berk

[Zed Head]

The key finding is that they are even across all cylinders.  Unless you plan to take the engine apart there's not much to gain by measuring again.  It's just a number.