scotty

what should be the compression in each cylinder of my motor?????what is a good compression reading? its a 318 5.2L 77 aspen,thanks

TVLynn

Probably around 130 psi stock. Ideally for street preformance 165 psi

scotty

anyone got any input? is 130-165 correct?

PK1

I have yet to disagree with any of TVLynn posts. He knows his stuff. You prob have 130 ish numbers if all is healthy. As long as they are all close you are ok. If you are get the numbers post them and we can show you how to trouble shoot with the tool to diagnosis the leak down.

As you "cam up" the motor it needs cylinder pressure to work. I run 165psi + or - 2 psi in each cylinder for the cam.

bremereric

I looked in my 1977 Service manual...318 was a dead 8.6:1

Minimum Compression Pressure 100 psi and a maximum variation of 40 psi between cylinders.

scotty

no idea of the cam in it ,,it was re-built befor i got it supposidly, ive only had it a short while,but mr aspen who works on her with me says theres no way the motor is stock ,he guesses close to 275 hp so who knows whats in there and from 145 stock thats a pretty big jump ide say ,thats why i wanted to dyno it,ill post the numbers tomorrow when i do it,, update havent done yet

Archer

scotty -

I don't recall what the compression number are supposed to be, but I was always taught that what you are looking for are differences. IE, one cylinder a lot lower than the rest, two adjacent cylinder, etc.

Archer

78doubleR

130-165 is a good range. the closer all 8 of them are to each other the better


Not to thread jack but i will be getting some pics to you in the afternoon tomorrow of the trim.

scotty

thanks

bremereric

One other thing my service manual said to do was make sure it was warm...with all the spark plugs removed and at wide open throttle.

scotty

that dosent make sence to me,,what does a wide open throttle have to do with it ,,you just check the pressure when you crank the engin over throttle has nothing to do with it???am i missing something ,,and wouldent you check 1 plug at a time then remove the next one?????

Coronet 500

All plugs out= easier on the starter motor.

Open throttle= easier on the starter motor and won't suck gas in.

scotty

and dis connect the starter coil so no spark ,,right

Garys69RR

The reason you open the throttle when checking compression, is to allow the piston to draw in a full cylinder of air on the down stroke. Then you have a more accurate check. Throttle closed equals less air drawn in on the intake cycle, therefore lower psi on the gauge.

PK1

yes, disconnect coil power prior to power getting to the coil. letting it fire the wire can cause problems.

The throttle blades need to be open so the cylinders can fill easily.

I also recommend stopping the fuel or the intake and cylinders will be wet. Not mandatory of course but if you are going to crank alot to get use to the tools and readings it wont hurt. You dont need that fuel washing past the rings and getting in the oil.

scotty

hows the fuel disconnected easily?,,im a newbie

scotts74birds

I prefer cylinder leak-down tests. They give more info where the losses may be.

Mr.Duster

you could do a static pressure test of the cylinders and then a running one and the running one i believe should be within 80% of the static one to be okey. What Tyl said sounds pretty much right.

PK1

If fuel is electric, kill power to it and let the motor run dry. If mechanical disconnect before pump.

You DONT need to kill the fuel but it sounds like that you may be new to this procedure and might take your time and do the cylinders a couple time etc.... It saves cylinders from being washed in fuel.

I usually warm up a fuel injected car and pull the fuse to pump and let the car die. On a mechanical pump i let it warm up and pinch the line with flat paddle vise grips if there is a rubber section ( not recommended on week old hose but it works in a pinch ) and let it die.

The only other thing Id recommend is make sure the battery is up to snuff or park your car so it can be jumped if you kill the battery..

Good Luck. Let us know what you get.

scotty

ya its mechanical so a set of vice grips befor the carb will work ill just totally disconnect,,iv egot extra hose length so ill cut a fresh piece befor i re-connect,,so theres no probs with the hose after