Need Compression Advice

[Marty Rogan]

Hi All,

I am looking into buying another Z on Saturday. This one is an early 71

with a stock L24.

The owner did a compression test which showed these results:

From back to front: 135, 150, 140,145, 150, 150.

Of course, I plan to do my own test to confirm.

I looked all through my Haynes and FSM and could not seem to find the stats

on minimum acceptable compression. I know you want compression in each of

the holes to be within 10% of each other. Five out of six look good. Is

the 135 at or below the minimum, and do you think that is something to be

concerned with?

Looking forward to your opinions.

TIA,

Marty

[72zcar]

could be the #1 cylinder and rings are worn...or could be the valves need adjusting. I am thinking if the valves are too tight in # 1 then it could cause low compression in that cylinder, maybe someone else could confirm this.

[NH-75-Z]

Have been posting on a number of Z-car tech forums

a similar question.

Most discussion on compressions readings your seeing is

that they are on the moderate low side, 170 being a new

engine.

My high is 150, my low is 125/130. Most responders have

said its valve stem seals, just to live with it till time for a real

overhaul.

Engine still runs fine at these pressures, so I don't think its

a clunker, just not real fresh.

[240 in OZ]

Factory minimum spec is either 115 or 125 from manual. Quoted from memory as I dont have the manual here at work.

[shyfty]

what do you do if they're over the top? I've a '77 with the L28E. it's been sitting for 7 years. I did a test today and my numbers are (front to back) 215, 215, 215, 190, 220, 215. as an aside question, this will be my first time ever working on a motor (the '77 also being my first car ...a real beauty, though a little rough around the edges) and, well, where should I start? that 190 I know at least tells me that a new head gasket is needed. so, while I'm at that, what else should I do? btw, the previous owner dropped the FI system and mounted a triple Mikuni setup (which, I have no idea about either )

[240 in OZ]

Hello

I suggest that your engine or head has been modified to produce larger cylinder compression. Without knowing more of the history of your car and the engine it will be difficult to tell what mods have been made.

Regards

Joseph

[shyfty]

will I have to just get down to it and start taking apart the motor to see what's been done? well, for that I first need to know what to look for....... after examining it, would I be able to tell the normal compression? I mean as "such-and-such" mod adds "this much" compression..and then just add and adjust?

sorry...newbie to the extreme

[Mr Camouflage]

Originally posted by NH-75-Z

Most responders have said its valve stem seals, just to live with it till time for a real overhaul.

Valve stem seals wouldnt affect compression readings at all, unless they were really worn and letting a lot of oil into the combustion chamber, in which case they would actually increase compression readings, unless your rings are really worn.

If your car is blowing alot of white smoke and you have good compression on all cylinders, then it's probably worn valve stem seals letting oil into the combustion chamber causing the smoke.

If you have poor compression and smoke then its probably worn rings (and mayby valve stem seals also).

If you have poor compression and no smoke then its worn valve seats (which are differerant to the valve stem seals, which are oil seals to stop oil dripping down the valve stem into the combustion chamber). A Valve seats is where the valve sits s against the head to trap in the compression.

If you squire oil into a cylinder then re-compression test it you should get a better reading unless you have worn valve seats, or I think broken rings.

I'm no expert so someone tell me if i've got it wrong, I wont be offended. I need to rebuild my engine or head beacuse its smoking heaps just on idle.

[NH-75-Z]

Hey Mr. Camo... this is what happened ...

I got this 75-280 off E-Bay, had it shipped 2000 miles to NH.

It started and ran "OK" but sluggish. Checked timing and it was

15 deg retarted!!!

Fixed timing, and the thing roared!! Took it out and rev'ed it

up to 4 grand no problem, BUT then I noticed that it would

smoke bad on deceleration, like coasting down a hill. It did not

do that before I adjusted the timing and had it running good.

Most responders have said that on deceleration , manifold

vacuum is highest, thus its probably sucking oil down into the

cylinders causing said smoke.

However... I have squirted oil into the cylinders and the

compression increased in all...one even by 20psi.

Odo says 91,000 but maybe its 191,000 ???

Am expecting a new compression test kit this wk and will be

doing a bleed down test with compressed air. Will that indicate

stem seals, rings,etc.???

What did I do when it rev'ed to 4K for the first time in many

years ... break a oil ring??

Z-On ... Gene:ermm:

[Mr Camouflage]

No Your rings sound fine.

Smoking on deceletation is a sign of worn out valve stem seals. You are sucking oil alright, but down the valve stems into the combustion chamber. I wouldnt worry too much about it, unless it a problem for the authorities in your area.

You'd have to take the head off to put new seals on the stems i believe, so if you do that, make sure you reseat the valves with some vavle grinding paste, and one of those valve lapping tools that looks like a mini plunger.

Valve stem seals have nothing to do with compression, they just stop oil from being sucked down the valve stems, the same way the rear main crank seal stops oil dripping out the back of your engine.

Maybe it was smoking a bit before and you didnt notice it. I've heard that most cars will smoke a bit if they've been driven like your grandma would drive it for years, they you hop in and give it a bit, even newer cars. But thats only what i've heard.

I have no idea what a bleed down test with compressed air is.

Also over rich mixture may cause engine smoking.

I should compression test my engine.

[carguyinok]

You can replace them with the head on. There is a fitting that you can screw into the sparkplug hole that will hook to your air compressor. This will keep enough air pressure in the cylinder that the valves will be pushed up on the head and stay there.

Then you can remove the rocker arm and pop the lash pad collet and retainer. Then lift off the springs. You will be looking at your seal after that. Just make sure you dont push the valve stem down hard enough to brake the air pressure seal between the valve and head or the valve will drop.

Looks and sounds MUCH harder then it is.

[NH-75-Z]

Have heard 2 ways of doing in head seal replacements.

Your method of using the compressed air to hold valves

in place, and I just got the new compression test kit that has

that feature.

But also heard that if you take a long length of 1/4" nylon

rope and stuff it into the cylinder via the spark plug hole, then

it will prevent the valve from dropping down too far!!

In either case, I've ordered the stem seals, valve spring

tool and will be changing them soon.

The National Z-Car convention is coming to New England

this October, I just don't want to smoke out the poor folks

driving behind me during the events!!!:tapemouth

Z-On ...