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Engine compression quesstion


Zak's Z

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This might be a silly question, and I did search thru the archives for an answer first.

When I pulled the engine out of it last year, I did a compression test but Im not sure if I did it right. My numbers were all between 150-155 across the board. I remember the car was warmed up, and I tried it with the choke open and with it closed..I think these numbers are with it closed. I left all the plugs in except the one I was testing.

Its too late to recheck now, as the motor is on a stand...so my question is what are the numbers supposed to be? and am I o.k. with my numbers? I read somewhere as long as they are all close to each other, thats good.

I dont know the history of the motor, I know the previous owner told me he never messed with it, i assume its stock. He said it didnt burn any oil, and every time I started the car before I took it out, it started right up. It did run rough at about 4000-5000 RPM if you floored the gas, but I assumed that was due to the carbs...it did idle beautifully.

Any feedback would be appreciated,

Zak

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Hey Zak,

I'm sure you will get different answers on how people check their compression. But, I warm up the engine until it reaches operating temp. Pull all the plugs. Disconnect the coil. Start with #1. Open the throttle and crank the engine until the needle tops out. Usually 3 or 4 compression strokes.

Your numbers look real good. Basically what your looking for is that all cylinders read about the same. Which yours are. Each engine is different and you never know if the engine has been worked on. For example, two identical engine's. Both in new condition. One is stock and the other had had the head shaved, or has had piston work. The one with higher compression will give you a higher reading.

Hope that helps.

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Yup, Ed's procedure is the way I was taught to do it decades ago. Done that way, I would like to see numbers between 170-180 psi or so, and a spread of 5-10 psi from high to low. Differences of less than 5 psi are within the limits of measurement unless you have a very high-dollar compression gauge, so you can easily round to the nearest 5. (My L28 was 180-175-180-180-180-175.)

Done your way (other plugs in, and throttle closed) your numbers sound decent, nothing to be concerned with at this point, I'd say.

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Thanks guys,

As I said...This was the first thing I did before I pulled the engine, which was a week after I bought the car, which is the first time I really started learning about this kind of stuff. I am determined to learn as much as I can, and I appreciate all the feedback over the last year.

I guess I'll roll the dice and leave the motor as is when it goes back in. All my $$ has gone into new floors and restoring the frame.

The shop that is doing the metal work let me come in and work on other parts of it, and I love every minute of being bent under a wheel well with a heat gun 2" from my face while Im scraping off old undercoating, with degreaser running down my sleeves!

Zak

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What difference could it possibly make having the throttles open? I would think that at such a low RPM count as the starter is capable of producing that there woud be no appreciable difference.
More than you might think. If the throttles are closed, the engine can't suck as much air. Creates a vacuum in the manifolds, so the cylinders aren't starting the compression stroke with air at atmospheric pressure. So the numbers are artificially low. Not a huge issue, as you are often looking more for delta (difference between cylinders), which should still be fairly valid.
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  • 2 weeks later...

I am guessing that you may need to freshen the head while you have the engine out. Valve seals are your number one concern. I would definetely put a new crank seal in the engine. I will cost you a few bucks and all you need to do is take the flywheel off and on. Think about the clutch disc, T/O bearing??? You have done all of the work. It would be sad to have to do it all over again to fix a chattering clutch or leaky seal or ????

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If you are putting in a new clutch kit, you simple need to remove the flywheel, take the seal cover off the back of the engine, press/put a new seal in the cover and reinstall. It will take you 15 minutes to get the new seal on.

If you have not seen any oil burning or not using any oil, you can skip the valve seal replacement. It is a major job. I have a '71 240Z with 87,000 miles on it. If I was going to use that engine again, I would definetely replace the valve seals. If you cannot DIY, just pull the head and take it into a machine shop. They will resurface the valve and seats and put new seals in. This refreshed head will really smartin' up an old engine.

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ZSAINT - Thanks for the info...I'll start getting that ready over the winter. I'm pretty much shutting the big stuff down over the winter, its too cold in the garage!

MONTOYA - I read that article. I clearly didnt do that compression test the right way. The only useable data I have is that they are all close.

Thanks guys,

Zak

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