What do I do now??

[85MudMonster]

Okay, I'm at the point of rebuilding the engine in my truck. However, I wanted to make sure the compression was good on it, so I tested it (I did not however test the valves or rings by use of motor oil in the chamber.) I know that one of my next steps would be to go back to the ones that tested a bit low and squirt some engine oil in the, but it's late and i'm tired now (and I'ts getting new heads put on it anyway). Here's what I've got:

#1: 140psi
#2: 148psi
#3: 130psi
#4: 148psi
#5: 145psi
#6: 135psi
#7: 137psi
#8: 130psi

The engine is a 1985 350 4blt main.
My question it this:

Are these numbers bad enough that the engine could need new rings, and how much difference between cylinders is acceptable?

[VileZambonie]

Well if you are rebuilding it you'll be putting new rings in anyway won't you?

When you did the compression test did you have a battery charger on it? choke open wide open throttle? 4 compression strokes?

[HAULIN IT]

Mudder, I question the "rebuilding" also. Are you just changing some parts (cam, intake, exhaust) ?
 Those numbers are low for what would be ideal, however the difference among them is nothing to indicate that there is a problem & would make me think that there is no real sealing problem, just bum because of the smog engine.
If you are planning to install new heads...put some on with 64-68cc chambers, this will raise the compression/cylinder pressure. Lorne 

[85MudMonster]

sorry should have been more specific. I did the compression test so that if the rings are good I won't have to pull the motor and do a complete rebuild on the engine. I plan on putting newer heads on it, same factory 929 truck cam, 1 3/4" tube headers, weiand action + intake manifold, and a new carb (I'm thinking about a holley truck avenger 670 cfm, with annular discharge)

Also going to put in new main and rod bearings.

I looked in the Chilton's and it says that the maximum variation between cylinders should be 20psi or less.

Vile, Why would I need a battery charger on it? The choke is electric and the engine was cold so I'm gonna say that the choke was closed. And as far a compression strokes I didn't count, I just cranked it for a while until the gauge stopped increasing in pressure.

[VileZambonie]

When you do a dry compression test it is ideal to have consistant cranking RPM, max air flow (choke open and Wide Open Throttle). Disable Fuel and ignition (disabling fuel is difficult with a carburetted engine). I would just rebuild the whole thing. Do it right the first time (don't make fun of me blowing up my 454 lol)and know everything is up to par. I agree with Lorne too on bumping up the CR

[85MudMonster]

Well, I was thinking about it and when I do the rod and main bearings I could just pull the pistons out then and put new rings in and shove them back up in there, it's not like the rings are outragously priced and if it'll help stop any excessive blow-by that I might have I guess it'd be worth it.

The heads that are going on are from off a 1987 L89. They are 64cc heads, i'm not sure that the heads are that are on the truck now.

[VileZambonie]

Are you not planning on just pulling out the engine? You should just yank it out. We'll walk you through the rebuild if you need help.

[85MudMonster]

^ That was exactly what I was trying to avoid.

[frogman68]

Really you cant know what will need to be done till you tear into in at the least I would think the cylinders will need to be honed for 200 bucks (Courtesy of Ville in another thread) you can get this

http://store.summitracing.com/partdetail.asp?part=SUM-SBCKIT1-000&autoview=sku

Not sure machine shop prices in your area but in MD its 5 bucks a piston to press off and press on

So pull the engine tear into to check the Main and Rod clearance make sure it is with in tolerance send the rods out to be done and hone the cylinders

Ps Crank for a 350 is like a 100 bucks at autozone with the bearings machine shop here wanted 150 to check the crank and no bearings

[85MudMonster]

If I wanted to do that I'd build the 2 bolt 350 I've got sitting on my engine stand already, but I found out this one in the truck is a 4 bolt so I was going to use it. But didn't want to have to pull it out cause I've got nowhere to put it.

Also on a seperate note about compression....

Why is the compression pressure between 130psi and 148psi when the compression ratio is only 8.3:1

if you take Atmospheric pressure 14.7psi x 8.3 = 122psi shouldn't that be the compression pressure?

[frogman68]

If I wanted to do that I'd build the 2 bolt 350 I've got sitting on my engine stand already, but I found out this one in the truck is a 4 bolt so I was going to use it. But didn't want to have to pull it out cause I've got nowhere to put it.

Also on a seperate note about compression....

Why is the compression pressure between 130psi and 148psi when the compression ratio is only 8.3:1

if you take Atmospheric pressure 14.7psi x 8.3 = 122psi shouldn't that be the compression pressure?

I think it would be alot harder to do this with the engine still in you can get a engine stand for like 50 bucks

[choptop]

Back up a little here. Are you having problems with it like oil consumption, smoking, fouling plugs, misfiring, etc? I dont see anything about the compression numbers that justify a rebuild, unless there are other problems which is why you are checking it out. I wouldnt throw a hot cam in it, and hi compression heads, with nitrous without rebuilding it, but if tis running good, I wouldnt touch it. If your just changing the heads like Lorne suggested, it'll probably be fine. But if you hop it up and are going to take it out and thrash on it, you will find the weak point in the bottom end soon enough. If your just gonna drive and enjoy, then go for it.

[VileZambonie]

static compression ratio is never the same as dynamic. So your calculation will not work. Just like CID is theoretical.

When you started this thread you said you are at the point of rebuilding your engine. So what are you trying to do because there is confusion now. Are you planning on rebuilding or trying to avoid rebuilding? Clarify your goals so we can offer the best advice.

[85MudMonster]

static compression ratio is never the same as dynamic. So your calculation will not work. Just like CID is theoretical.

When you started this thread you said you are at the point of rebuilding your engine. So what are you trying to do because there is confusion now. Are you planning on rebuilding or trying to avoid rebuilding? Clarify your goals so we can offer the best advice.

Right. Duh  I wasn't thinking my Bad.

I guess I'll just get a stand for it then. I mean I was going to replace the bearings anyway, the only thing I was trying to avoid was having to pull the pistons out and put new rings in it, but I'm wanting to make about 400 ft/lbs of torque so I guess I better pull the pistons and hone the block and put new rings in it. So much for having it ready to run by spring...

[frogman68]

static compression ratio is never the same as dynamic. So your calculation will not work. Just like CID is theoretical.

When you started this thread you said you are at the point of rebuilding your engine. So what are you trying to do because there is confusion now. Are you planning on rebuilding or trying to avoid rebuilding? Clarify your goals so we can offer the best advice.

Right. Duh  I wasn't thinking my Bad.

I guess I'll just get a stand for it then. I mean I was going to replace the bearings anyway, the only thing I was trying to avoid was having to pull the pistons out and put new rings in it, but I'm wanting to make about 400 ft/lbs of torque so I guess I better pull the pistons and hone the block and put new rings in it. So much for having it ready to run by spring...

Look at this http://www.compcams.com/camquest/default.asp it will tell you (not sure how accurate) what you will get with whats in the engine