How much compression should my engine have? i did a test and with the engine warm it was between 150-159 in every cylinder. just wondering what everyone else is at.

 

147.5 on two cylinders and 150 on two cylinders.

 

Seems a bit low for a 9.2:1 engine, i was getting around 155psi on a 7.8:1 engine.

 

you cant base compression test results off of an engines static compression ratio...

as long as the highest and lowest cylinders are within 10% of each other, and theyre over 100psi, then theyre fine.

 

Why not? Seems like the readings should be linear, a 7.8:1 engine gets around 155psi and i've seen a 8.5:1 get 175psi.

 

If the static CR held true than a 9.2:1 CR motor would test at 9.2 X 14.5 = 133 psi

 

Valve seal & ring fit affect the measurement, as does expansion of the air as it's compressed and heated.

Valve timing/overlap will affect it , The speed at which the engine cranks affects the readings some also due to valve & ring seepage.

I would expect it to be bit higher in a warm engine due to the better ring fit.

Did you do it with all 4 plugs out when measuring one? It cranks faster that way.

 

When I did it, I went by the SI GM tech thing. It has you pull the fuel pump fuse and start and let the car run till it dies, then pull all the coil packs and do one plug at a time. I also did it on a cool motor to avoid any possible damage to the threads in the head for the plugs.

 

They don't want you pumping fuel into the cylinders and it's always best to pull the plugs cold to prevent damage to the aluminum head threads.

 

I just looked in the official service manual, and it says basically what hunterkiller89 said. If < 100psi you have a problem, and if the balance is > 70% between cylinders you have a problem. 70% balance differential is a shitload IMO

They give no other specific guidence of compression in the service manual... which is pretty odd in my experience.

It did say to make sure you have the fuel system disabled and to have the gas peddle at wide open when performing the test. I can tell you from doing a lot of compression checks on cars, having the peddle floored does alter the readings a bit for me. It also says to remove all coil packs & spark plugs.

 

seems like my compression is in check then. i remember my old 1990 integra had 190psi in each cylinder with almost 170,000 miles on it, so i wasn't sure.

 

SCR doesnt mean **** in a compression test. DCR definitely would though, but even still, there are other factors.

 

Old dead thread revived but I did mine today at 42k miles and have 160, 160, 152 & 155 on cyls 1-4.

Looks like I'm still ok from what others posted.

 

I've seen some very inconsistant numbers tossed around the site as far as what guys are coming uo with for test results. I did mine a while back the exact way you did and ended up with almost exactly with the same result as you. Mine were 145 147 145 148.

I'm going to take a wild guess and say that the guys coming up in the 170-180psi range are most likely introducing fuel into the combustion chamber (by not disarming the fuel system) which is skewing the test results. Again it's just a wild guess, but judging from the results I have seen from propper compression testing, somewhere in the 140-155psi range seems pretty dead on for a healthy LNF

 

Like the OP said earlier. there are way too many variables in play to state any kind of an accurate specification for combustion chamber pressure. EXCEPT MAYBE IN A LABORATORY SETTING. Compression RATIO has absolutely nothing to do with combustion chamber PRESSURE. There is NO direct relationship between VOLUMN and PRESSURE. Understand the RATIO is just that. The RATIO of combustion chamber VOLUMN with the piston at TOP dead center versus BOTTOM dead center. Generally speaking, if the engine makes enough pressure to start the engine easily and the cylinder PRESSURES don't vary by more than 10-15 PSI across the board you're good to go. When you have a problem that causes the pressure to be less in ONE (or more) cylinder, like worn or broken rings, badly worn cylinder walls or even bad head gasket, that's when you will hear/feel a MISS. And THEN it's time to do a cylinder leak down test where you put compressed air into each cylinder, one at a time, with a guage and monitor how fast the pressure leaks out. And it WILL leak down at a rate determined by the condition of everything seperating the combustion chamber from the outside world. If you CAN'T detect a MISS, the engine starts easily and fuel mileage isn't off the charts engine compression shouldn't be one of your worries.

 

With the ring land failures posted in the past and knowing how I beat on the LNF, I feel better knowing mine is ok. I have 20" vac at idle and it runs fine, I just needed something to do this morning I guess.

Thanks, Buellfooll. Nice to see someone on here older than I. Not many of us here.

 

What compression tester setup are you guys using? Where did you buy it?

 

autozone? any car place should have one

 

I would stick with the 10-15 percent rule..

if you think you have a problem do a cylinder leakdown test..

 

I guess you have to be an "old" guy to know about manifold vacuum. Especially in this day of turbo/super charged engines. In the right hands the vacuum gauge is probably the best piece of test equipment in the box.

 

Very true. I look at vacuum right away when I get in someones car that has a problem, and they never understand why.

Actual numbers on the compression tester can vary from one gauge to another as well, that's why you look for an even reading across the board, not a certain number. My LJS and my boy's sentra both read around 185-190 on healthy motors. When I blew a piston, #4 dropped down to 140ish psi.

 

First car in 1974 was a used and abused Red '65 Malibu SS 327 4bbl 4spd, Zoom clutch, headers, glass packs & dual side dumps. Got it from my brother when he went into the Navy. Took all I had to drive that car.

GoodYear Polyglas F70-14's on chrome reverse wheels with ChevyII center hub caps.


Yeah, I'm officially an old guy.

From what I've seen in this thread 150 psi is about the norm for the LNF.

 

Hi, I did a compression test with a compression tester from autozone and I was getting 80-85 throughout all cylinders. That was with me turning on the car and then turning it off after a second or two. Was I doing this wrong? I mean reading now what you all had to say I'm getting concerned. Do you think the tool was just cheap?

 

If it has a long hose, 12" or more it's going to take several strokes to reach it's peak.

IIRC, the service manual says to pull the fuel pump fuse, idle the engine until it dies.

Pull all 4 coil packs & plugs.

When you take the comp reading, hold WOT and roll through 4-5 compression strokes. I cranked it until I saw the peak, took 6 or so with a 16" hose.

Mine was 160 to 165# after replacing the injectors & seals at 50k miles.

 

What do you mean by turning it on and off? You arent' trying to start the car are you?

Maybe see if you can get someone to help you that knows a little more about what they are doing.

Why are you doing a compression test, what problems are you having?