Rallas

iTrader: (9)

I did a compression check last week on my freshly rebuilt (<3k miles) 96 engine after noticing that the bottom end is pretty weak compared to my wife's factory 97. Above 4500rpm it scoots and sounds/feels very healthy. I did some extensive porting and polishing while rebuilding the engine as well as some valve deshrouding in the combustion chamber. I knew I was dropping the compression ratio with every little bit I removed from the chambers so I tried to geustimate the material removed and shaved the head accordingly to make up.
My lazy *** should have CC'd the combustion chambers to make sure I had it right. Suspecting the lowered compression I measured 160psi on most cylinders, Cyl 2 had 157 psi, so no large variances. No improvement with oil so I do not think it is a ring problem. I might do a leakdown check, but I doubt it is a mechanical leak problem, just low compression. From my hanes manual it looks like a 9.5:1 should read 192 psi, 9.0:1 should read 182 psi which makes me wonder if I am below 8.5:1 then? I have looked for some formulas to calculate CR from compression readings but these are rather inaccurate when you are dealing with the compression of gasses and valve overlap and other variables.
Luckily I am planning on installing a MSM turbo setup this fall so it is not all a loss. Too bad the motor was not built for a big turbo and lots of boost so I could take advantage of the lower compression. I am just worried that even with the turbo I will end up with a gutless engine below 4000rpm.
I would take the head off and shave it some more, but with all that effort I would go ahead and get a 99-05 head while I am at it.
Does the lower compression lead to lower gas miliage as well? I cannot seem to get above 27 mpg.

Savington

iTrader: (31)

When was the last time your compression gauge was calibrated?

JasonC SBB

Do a compression test on the wife's 97.

If it's the same, the gauge is off. Where is the check valve located in the gauge? In the gauge, or in the end of the hose that goes in the plug hole?

If it reads higher, check your cam timing. A retarded intake will cause low compression readings and a weak midrange.

Some simple porting should improve power m ore than that lost by reduced c/r.

1slowna

iTrader: (1)

^ I dont get over 27mpg with my 1.8 either, usually 25-27, i have trouble keepin my foot off the floor though. Its stock and slow so i gotta floor it everywhere.

Sparetire

Lower compression can make for less mileage, but a half point should not make a huge difference.

And I doubt the car will be gutless with a tiny turbo setup, regardless of what your CR is.

falcon

iTrader: (13)

My 8.7:1 1.6L with 3000kms on it is 180 across the board.

Rallas

iTrader: (9)

The porting really pepped up the top end. It almost feels like VICS kicking in when I hit 5000rpm. Its the valve deshrouding that removed combustion chamber material and could have changed CR.
I used 2 different gauges that read within 2 psi of each other. Also checking on a warm engine for best seal. The wifes 97 read 175psi on all cyls but cyl 3 that read 165. I guess that one is ready for a head refresher. Will have to deal with that later (110k on that engine).
They are both HF gauges, no idea where the check valve is. I want to say it is down on the spark plug end of the hose.
I checked the timing belt and cams several times before dropping the engine in. I might just pull the cover to check cam timing too.
I am running MS with no maf and cone filter on factory 1.8 intake tube. I know some cold air intakes will shift the power band up, but not this much. I don't know if that could be a factor too. I can't break a tire loose unless I dump the clutch, but it still zooms up top, even more than my dad's 99.

Rallas

iTrader: (9)

Last week before I drove it on track I pulled the valve cover, front covers and belts off. Everything is aligned perfectly. So it is not the cam timing.
After running the track day the car seemed to run much better, almost like I really broke it in on track. It only has about 2500 miles on the new engine. So yesterday I went and did another compression check about 10 minutes after I got home (45 min drive from work). This time I could only get 150 psi across all cylinders with the same gauge. WTH!!!
I did not have any mechanical issues on track, it ran like a champ with coolant temps below 210 even with 96 deg outside and it didn't use a drop of oil. I am really beginning to suspect the two cheaper gauges I have. I am going to see if I can borrow a gauge from a mechanic friend of mine to see if his Snap-On gauge reads the same pressures. I picked up a leakdown tester as well to see if I can figuire out what is going on.

It just seems strange that all the cylinders would wear evenly that fast.

chicksdigmiatas

iTrader: (11)

If you are happy with it, and it isn't losing oil, I would say you are good.

flounder

My 99 got new rings, hone and a 3 angle done prior to boost and had 205 psi afterward. Now after 6k miles, it's still 205 all 4.

Rennkafer

iTrader: (3)

What procedure are you using to do the compression test?

sixshooter

iTrader: (12)

Are you compression testing with the throttle wide open? If not, you should.

nitrodann

Six beat me to it.

Dann

Faeflora

iTrader: (2)

I see 150psi on all four. Cold. doesn't change with throttle open or closed. No oil squirt in cylinder. 9:1 FM wisecos. FM said that's fine

compuatic

iTrader: (1)

Best way to measure compression is to do the following:
1. loosen or remove all plugs
2. engine should be warm
3. bridge the battery with another battery on a running car to ensure it's not sagging
4. crank with the throttle wide open until the gauge no longer climbs any higher. usually 5-10 cranks to be safe.

mazpr

iTrader: (1)

Make sure fuel pump is off.

On the 84mm 9@1, straight from the machine shop, has not even left the garage yet mine reads consistent 180 on all fours with minor slight variations of 1 to 2 psi.

Have you done vacuum readings?

Before the honing, new Supertech pistons 9@1, on my first setup with 1.8 dohc, 10@1 miata pistons, (only two layers on the head gasket/instead of 4) would read 210 psi.

Rallas

iTrader: (9)

So I did it all over again after getting home last night with a nice hot motor just after stopping. With throttle closed I got 155psi, throttle open 160psi, addeded some seafoam directly to the cylinders and it barely went up to 163psi. I don't thnk this is enough to show a sign of ring leakage. Then I did the whole seafoam treatment that I know works on all my higher miliage engines and saw no difference. I don't think I have anything wrong at the moment, just lower compression. Hopefully this lowered compression will help me build the dependible mildly boosted track car I am shooting for.
If I get the time I will do a leakdown test in the next week or so.

k24madness

iTrader: (6)

I bet the lower compression is from shaving the head. That retards intake cam timing which in turn reduces compression. Install an adjustable intake gear and advance the cam 3-6 degrees and the compression will increase.

spoolin2bars

iTrader: (1)

that makes sense only if he shaved alot off the head. another possibilty is that the rings may not be fully seated despite the mileage. could be why the car felt better after that trackday. there was a story in GRM about a rebuild he did on an old volvo (iirc) where the compression was just not what it should be. after checking everything, nothing was found and the car ran fine, so he just dropped it. thousands of miles later, the car suddenly felt faster, more powerful. they checked compression again and it had gone up something crazy, like 20+ points!
so in regards to that, i wonder if the op used or is using synthetic motor oil. if so, he needs to be running regular dino oil for the first 10k or so to ensure the rings seat properly.

shlammed

iTrader: (8)

with cheap guages, dont care about psi being low, more about being consistant.


i have a guage that is horribly inaccurate in terms of real PSI, but can show variance between cylinders.

i have tested known good honda b series engines with ~11:1 compression that have shown 190psi where other (more expensive and accurate guages) would show upwards of 230psi.

Mine is from princess auto (Same as us harbour freight)