New rebuild compression results
 

This is how I went about rebuilding my engine. I completely tore down, added forged rods with new bearings and piston rings. I had the head decked, new valve seals and a 3 angle valve job.

Haven't ran the new engine rebuild but 100 miles or so. Decided to do a compression test and all my cylinders are at 150psi. Is it just me or does that seem low?

When I test I take 1 spark plug out at a time and turn the motor over. I don't open the TB or take all the plugs out, that's just how I test. Any thoughts?

Warm or cold engine? Did you try putting a little bit of oil in each cylinder and then test?

You need to prop the throttle open, and have all the plugs out to do a compression test. First dry, and a tablespoon or so of oil to wet test.


Oh, and 100 miles may not be enought to fully seat the rings.

Warm engine, had noticed my water pump leaking so figured time to test everything else.

I figure I will retest tomorrow with TB open and the plugs out. But this engine has several hours of run time on it.

When I rebuild my engines I do several heat cycles to try and seat the rings. I will run the car for 45mins to an hour around 2k rpms then shut it down and let it cool off. Then repeat. Some days I just let it idle for an hour or so periodically throughout the day.

My biggest concern is that I am getting white smoke out of the exhaust. I know with a new rebuild this is possible, it only smokes a little after it gets warm and you rev it up. It will go away the more it heats up and today I noticed I had no smoke at all.

That may be from blowby (rings) or it may be valve seals. Mine smokes like the Marlboro man at start, and then nothing after it warms up.

What are good compression numbers on a rebuilt engine? I have heard up to 210 per cylinder

209 is "new". Anything over 190 I would consider very good.

Try it again with no plugs, and a stong battery. If it's still blowing smoke I'd say rings weren't fully seated yet. 150 seems pretty low.

I will try a dry and wet test tomorrow, then tearing it down to fix this damn water leak. Will post results tomorrow! Thanks rleete

This is pretty much the exact opposite of how you're supposed to break an engine in. If you're lucky, the regular driving you (hopefully) did was enough to seat them despite your best efforts to glaze the cylinder walls.

Take all your plugs out and try again. If the numbers are still now, leak the motor down. If it comes back with leaky rings, I wouldn't be even slightly surprised.

Lol.

How are you suppose to break an engine in? I do this on my v8s, whats the difference? hell I have built an engine and raced it that same night running a 8600rpm chip. Enlighten me?

I do put a load on the engine of course, (if that's what you are saying I am doing wrong) I drove it short periods starting from a slow roll in 3rd or 4th and let my rpms rise.

This is a good read http://www.mototuneusa.com/break_in_secrets.htm

Pretty much this. The way you are supposed to break an engine in is by alternating between full load and no load, over and over again, slowly increasing max RPM until you're wringing the nuts off the motor.

Letting the engine idle for an hour is a bad idea for any gasoline engine. It's a HORRIBLE idea for a brand new gasoline engine. Letting the engine camp at 2000rpm for 45 minutes is just as bad.

i just did a total engine rebuild in my dirty garage. 9.5:1 84mm supertechs, bore&hone, etc..

Wait engine to warm up. Then drive it like i stole it. Did compression test and it was 202-203-201-203PSI :party:

My results at 500 miles on a fresh rebuild were 185psi across all 4. I've been meaning to test it again.

Who built it?

Just my .02... But like Sav and Baz said/posted....

Do it along the lines of the motorman break in.

You have to load the rings into the cylinder wall.

Fire it up, check for leaks, idle no more than 10 mins to get some heat in the block... Go drive it.

I drove between my heat cycles, like I stated I put a load on it going from 3rd or 4th and let my rpms rise. (my first startup it ran for 20-30 minutes and my father took it out to do the break in) That is the only was to simulate load as for our engine dyno you can simulate load just by a dial.


This is what I read up last year that some spec miata guys are averaging.

As for my results, I let the motor turn over 5 times and each cylinder is about 180psi, turn it over 7-8 times and I am pushing 200 per cylinder. That is on a dry test with no oil added to the cylinders.

Then engine is fine, drive the ---- out of it.

I plan on it, just waiting for Abe to build me an awesome setup!