LA 318 compression numbers

[pjc360]

Friend of mine has a 1981 dodge power ram 150 with what i believe is a 318, but it could be a 360.. I have not looked for any numbers on it.
It's got a tiny little 1 barrell carburetor on it. It's got a ton of starnge smog and emissions stuff on it. Anyways it was running really rough and he brought it to me to look at.
First thing i did was a compression test on it, Every cylinder on the drivers side was at 100 psi, this is cranking for 5 seconds with the gas pedal to the floor and with the engine warm. I found 2 fouled plugs on the drivers side, plug number 3 and plug number 7. Passenger side cylinder number 2 read 95 psi of compression and cylinder 6 read 95 psi compression and the rest were at 100 psi. Cylinder 2 and cylinder 4's plugs were fouled. And cylinder 8's plug was oil fouled.
So it was only running on 4 cylinders. And his plug wires are junk and all taped up. But anyways we are at 4000 feet above sea level, are these compression numbers good or bad? What kind of shape is this engine in compression wise?
My 360 magnum has 150 psi on every cylinder at 4000 feet above sea level, so his numbers are down compared to mine, but i dont know if they are horrible numbers or not?

 

[sireland67]

The numbers are a little low, but they good thing is they are all about even.
I would give it a good tune up and see how it runs from there.

 

[RustyRatRod]

I am sure it is a 2 barrel. They do look quite small. Compression numbers do sound low. If you can post all of the numbers, it would be better. If they are all low but within 10% of one another, it is probably just a worn engine.

 

[pjc360]

Cylinder number 1 is 100 psi, cylinder 3 is 100 psi, cylinder 5 100 psi, cylinder 7 is 100 psi.
Passenger side. cylinder 2 95 psi, cylinder 4 100 psi, cylinder 6 is 95 psi, cylinder 8 is 100 psi.
Thats what the compression test revealed with the engine warm. It's got a burnt out fan clutch, a leaky radiator. And it puffed out some blue smoke when he fired it up.
He is wanting to make this truck a solid runner. And doesnt know wether to start putting money into the engine thats already in it or go a different route.
I told him if it were me, i would save up 2 to 2500 bucks and get a 360 magnum out of a wrecked dodge ram from the junk yard and get an intake and carb and distributer for it and new plugs and plug wires and get the timing and carb dialed in on it and i told him i think he would be more then happy with the results.
But he doesnt quite have the money for a project like that.
I told him the compression numbers of what he has now and he asked me to find out if the engine is in good enough shape too even be worth buying a new 4 barrell intake and carburetor for.
He wants to get rid of the 2 barrell and smog crap and run a 4 barrell intake and carb and get a new distributer that has adjustable mechanical advance so he can get higher initial timing in the engine and new plugs and new wires and a new radiator and fan clutch.

 

[DaveBonds]

Do you have access to a leakdown test kit? That could tell you how well the rings and valves seal and if it will respond well to a tune up.

I'll bet you that part of your low compression problem is coming from a worn timing chain.

When timing chains stretch, it retards the valve timing against the crank/ rod/ piston.

Ultimately, it's negative affect on cylinder pressure is that the intake valve closes late, after the piston is already on it's upstroke, so you lose compression for some of the total stroke. This also diminishes flow, coming from the intake, as the piston counteracts it's draw from the intake runners. Even a good double roller on a stock cam at 0° can be improved with a timing chain/ gear set that allows a little more advance on the valve timing, so the intake valve closes even sooner.

To check timing chain stretch, put the engine at TDC going clockwise and have your friend eyeball the distributor rotor with the cap off. Move the crank counterclockwise until you see the distributor budge and stop. Give about 1° or so margin of error and that is the amount of retardation the valvetrain has. 3° advanced from 0° stock makes a big improvement, so if you are seeing more than that in retardation, it will respond well to a good double roller timing chain set with multiple keyways to set more advance. It also helps dramatically with consistency in ignition timing.

Static compression ratio numbers advertised were down around 8.5:1, which were optimistic. I'd imagine it's somewhere in a 7.8:1 range, stock.

Hope this helps.

 

[67Dart273]

Also, don't discount that the gauge may be inaccurate.

 

[krazykuda]

did you try squirting a few squirts of oil in each cylinder and retesting the compression? If the numbers come up, then you have some wear in the rings/bores. If they stay the same, you may need to look at the valves.

I usually see 100 to 120 on well used 318's. These numbers are really close from one cylinder to the other. How many miles are on it?

If he wants to keep run ning it, throw some fresh plugs and wires on it.

If it has 100k or more, throw a nice double roller timing chain on it (Mancini Racing sells a nice Edelbrock true roller double roller for like $65). The stock chains are crap and stretch/chip teeth real easy. I've gained up to 2 MPG by replacing an old worn out timing chain with a good double roller (at least equal to a stock 340 chain). At today's gas prices, it would pay back pretty quickly.