Here's my problem, I recently purchased an 85 Camaro 305 TPI motor for my 89 camaro. I cleaned it all up and painted it, when I went to take a compression test all of the cylinders had over 200lbs of compression. I had the motor semi-apart to put a new timing chain on it, still had the plastic one. So I'm guessing that nobody messed with it. Anyone got any ideas?

 

maybe you didnt line the timing marks up correctly and the valve timing is off

 

Doesn't sound like anything's wrong to me.

 

I'm not saying there is anything wrong with it, I'm just curious to why this motor has that kind of compression when nothing was done to it. As for the timing being off, the motor fires and runs fine so I don't think that is a problem. Plus if the timing was off I don't think the compression would be so even across all the cylinders.

 

The actual numbers on the gauge don't mean a whole lot.

 

yeah, just that it's consistant. my 9.5:1 c/r 305 put out 190lbs. my 10.2:1 305 puts out like 230+

 

run the cast numbers on the cylinder heads on mortec

 

That is so true. The actual reading is a function of your static compression ratio, the condition of your rings & valves, and the timing event of the camshaft.

A fairly mild camshaft will produce a fairly high reading at low rpms but if you installed a longer duration camshaft; it will produce a lower reading just because of the "valve overlap". Even though they have the same static compression ratio.

So, as Apeiron/Chris stated, the actual reading does not have much meaning as there is a wide range of factors that affect the actual reading. It's the consistency between the cylinders that matter more.

 

The "valve overlap" is more a secondary effect of valve timing in this case. I think what you meant to say was the closing point of the intake valve has a huge effect on cranking compression. The intake valve closing point is at the opposite end of the intake lobe ramp as the overlap period. So a lot of intake valve cam timing will delay the intake closing, and therfore probably raise the overlap period (depending on the cam's LSA).

Scot

 

HI I HAVE THE SAME SITUATION. I BOUGHT AN 87 305 TPI. AND ALL BUT TWO OF THE CYLINDERS MEASURED OVER 200 PSI WHEN I RAN A COMPRESSION TEST. I DID A LITTLE MINOR VALVE ADJUSTING. AND THEY ARE NOW ALL BETWEEN 205 AND 210 PSI. IT WAS RUNNING GREAT WHEN I GOT IT. SO I HAVE NO PLANS TO PULL THE HEADS RIGHT NOW. I READ THAT CARBON BUILD UP CAN CAUSE HIGH READINGS. BUT FROM WHAT I CAN SEE IT LOOKS PRETTY NORMAL. I PLAN ON INSTALLING IT IN MY 89 TRANS AM NEXT WEEKEND. GOOD LUCK WITH YOUR PROJECT. ---------------------------------------- TIM ---

 

190-215 psi is NORMAL for a 305. Most 305s of that era had 9.3-9.6:1 compression.

 

How’s the Trans Am today Tim?