engine Break-in: compression question
 

I have a recently rebuilt 12aSP, and I just clicked over 1,000 miles yesterday. I've driven it VERY lightly during break-in, no serious loads, varying engine speeds, etc. I did a compression test today and was shocked to discover only 60lbs on each rotor!
Is this normal for this stage of break-in, or is it time to visit the engine builder with some molotov cocktails?
What's compression normally like during break-in, and when can I expect it to be as much as it will be?

Most of the engines that I build have about 80-100 right after the rebuild, and up to 120 when the breakin is Done.

It really depends on what was reused, and how bad they were

did you test it properly? warm engine?

also how many old parts were used?

In this case, all seals and springs were replaced with new, irons were spec'd and one (center, I think) was replaced, and rotor housings were replaced with like new (no chrome flaking). Rotating assembly was spec'd and one rotor had to be replaced. A few donor motors were used for their good parts, and the housings still had to be purchased from a dealer. In some sense, this almost qualifies better as a new engine, because of the parts used.

Maybe try another compression tester to make sure?

ideal compression as per mazda is given at 250 rpms and at sealevel. odds are your starter isnt at 250rpms and your not at sea level

lol! I'm at just about 8,000 ft...

Altitude can definitely make a difference. Explain how you performed the comp test and with what type of tester.

I used a hand-held, analogue guage, screw-in type compression tester. I screwed the gauge tightly into the trailing spark plug hole (easier to reach), with the leading spark plugs still installed, the coil wires grounded out, and the fuel pump disconnected. The engine was at normal temp after driving around town a bit. I cranked the engine over with the starter, no throttle, and watched for pulses on the guage, and got them. This guage has a valve that allows it to hold pressure (a little button by your thumb-pretty common IME), so after I confirmed that I was getting 3 rythmic pulses, I left the valve shut, figuring that this would give me a max reading on each rotor. After several rotations, the pressure was still only reading approx. 60psi.

Always use the leading plug hole to check compression. They are about 10X bigger than the trailing, I dont know if it matters for sure but my guess is that it does.

I like to take the lower valve out of the compression tester, usually its just a screw in valve like a tire has. Once the valve is out, you wont get the pressure hold feature which is what you dont want. Then you just watch for the needle to bounce. You should see even bounces and where they peak would be your compression. Remember to check compression on a warm engine.

Is the car sputtering? Loss of power? If its running fine like a rebuilt engine should, than who cares about a compression tester. You can tell if your motor is losing compression just by the way it performs. So go out and floor that bad boy and see what its got!

i guess another question to ask is what were your readings just after the rebuild?

keep the throttle wide open when compression testing that should help aswell

60 is way to low for a rebuild

Throttle plates must be fully opened to draw in the max amount of air to be compressed. Step by step details for performing an accurate test is listed in the FSM, but it may not be comprehensive enough until the 87+ edition. It makes no difference betwen the trailig or leadig plug hole, however, I trust the leading/lower better when performing the bounce test to get an idea of compression for each face.

To get the gross/highest reading of each rotor, don't use the release button on the tester. There is a table chart on the FSM that has adjustment figures to compensate for cranking rpms and altitude. Chart is in the 87 FSM, Sec. 1, p. 11.

Also keep in mind that piston style testers may not yield the same results as a rotary specific tester and trying to read the bounce is subjective at best.

Thanx, I'll try it again and see what happens. For what it's worth, the car does feel like it's got gargantuan balls for an N/A 12A, the compression thing was mostly a curiosity. Mostly;-)

are you using a rotary compression tester?

Dont think he is , How many people on here have one.

I had to make my own digital one, but still.

I would say 95% of people on here use the Piston Version without the oneway valve.

many things forgotten have you youngling....
 

Three major issues:

not sea level changes a bit...

closed throttle plates..... changes a lot

and .......TaDa.... ported engine. this changes it the most. depending upon how it was ported you could have changed your intake exhaust overlap quite a bit which makes for very screwy readings at best.

try a measure the compression on a freshly built bridge, or perry port they are even worse due to the overlap.

kenn

According to the official procedure in the FSM the *trailing* plug is supposed to be used, on a warm engine.

I wouldn't necessarily trust the PSI readings given by a piston-engine comp tester, only the bounces.

But if I were you, I'd take it to a dealership or rotary mechanic and get the test done right, on the proper equipment. I mean you just spent over a thousand bucks on a rebuild, you might as well spend the extra fifty on getting a proper test done to know where you stand.

One thing that helps is to hook booster cables to your battery and to another running car. It sounds weird, but you get better cranking that way if your battery and cables aren't new. I replaced my battery, battery cables and starter, and when we did the test I was cranking 300rpm!

Jon