RTH: 283SBC compression test numbers - too good to be true? (1 Viewer)

[RWBeringer4x4]

Hey all. Decided to check the compression of the 283 in my FJ40 this morning.

For reference, it's a 1965-67 era 283, from a pickup according to the casting numbers. It has power pak heads and I THINK a RV Cam but that is just based on a receipt from the PO. Rochester quadrajet carb and HEI distributor.

I think it's running a bit rich, but it doesn't smoke at all aside from the usual condensation and soot at startup.

Anyway - I ran the truck up to operating temperature, then cut off the fuel supply to drain the carb until it died. I pulled all the plugs, opened the throttle plates on the quadrajet, disconnected the HEI. All of the plugs appear to have significant oil fouling. They were legitimately "wet." Granted, I have no idea how long these plugs have been in the truck. They were in it when I bought it 3 years ago. I've driven 500 miles on them in that time.

Assuming the worst due to the fouling, I checked the compression. The results? 139-150psi across all cylinders.

Divide by 14.7 atmospheric pressure and that's like 10:1 compression. I could have sworn even brand new that 283's were like..9:1 or even 8.5:1.

Here are some shots of the plugs:

And last, the compression checker hose, because it was pretty fouled by the end of it too.

What do you all think? Is this too good to be true or do I basically have a brand new, incredibly leaky 283?

 

[bikersmurf]

I'm not sure about your math, but here at sea level I'm used to seeing 155-160 for a 9:1 ratio.

Last time I checked I had +/- 195 PSI across the board. 10.25:1 was an option for my 69 SBC 350.

Your compression numbers are even, and I'd suspect you've got an 8.5/9:1 ratio.

Btw, those plugs are toast... I'd put them back in, use some orange engine degreaser to clean the mess, put in some new plugs, and go from there.

 

[RWBeringer4x4]

Yeah I have new plugs that are going in. Is that much fouling normal?? I guess it's impossible to say given that I have no idea how long they've been in there...

 

[bikersmurf]

They look dark, which is usually a result of running rich. Given you've only put on 500 miles in the last 3, it's impossible to say much, could be just from running cold and not getting hot enough to burn clean.

 

[shawnfj40]

I would say that you have a great 283 that needs valve seals.

 

[shawnfj40]

And yes plugs as well.

 

[terrx]

The compression numbers are fine. Change plugs, tune it and drive it. If it only sees 500 mi in 3 years another 20-30 years should be no problem...

 

[RWBeringer4x4]

They look dark, which is usually a result of running rich. Given you've only put on 500 miles in the last 3, it's impossible to say much, could be just from running cold and not getting hot enough to burn clean.

Yeah, especially given that I didn't change them when I bought it. Could be 3 years old, could be 30.

I would say that you have a great 283 that needs valve seals.

It sat for a long time before I bought it so this wouldn't shock me. It leaks from every other seal, why not the valve seals too? Haha

And yes plugs as well.

It got new plugs today. Let it get back up to operating temperature, took it for a spin around the block to hopefully blow out some cobwebs.

The compression numbers are fine. Change plugs, tune it and drive it. If it only sees 500 mi in 3 years another 20-30 years should be no problem...

Well, it has only seen 500 miles because I've been busy fixing it. Hopefully it will see a lot more than that down the road.

Thank you all for the responses. This engine needs to be pulled and relocated in the bay. This was an effort to determine whether or not I should replace it at the same time.

I think this old block gets a clean bill of health, so it's going to get yanked, resealed, and then dropped back in with a Downey bellohousing and a new clutch behind it.

 

[handcannon]

The most important thing is that all the cylinders are real close in PSI numbers. And, I agree that the valve seals are probably needing changed, which can be done without having to take the heads off.

Don

 

[RWBeringer4x4]

Yeah. The plan is to replace virtually everything cork or rubber on it when it's out of the truck. If it needed machining, I was going to pawn it and buy a 350. Sounds like that won't be the case though, and in THRILLED because new seals are (relatively) cheap.

 

[DPV7070]

I'll be curious to see what your compression is when your valve seals aren't leaking oil in the cylinders. Isn't that like doing a "wet" compression check? Either way, they're all pretty even, so I'd say run and gun buddy!

 

[RWBeringer4x4]

I'll be curious to see what your compression is when your valve seals aren't leaking oil in the cylinders. Isn't that like doing a "wet" compression check? Either way, they're all pretty even, so I'd say run and gun buddy!

Yep, I'm almost certain you were right. I didn't bother with a wet test because of this, and because frankly the compression was acceptable "dry."

It might come down a little but it wouldn't surprise me if it truly was close to that good. It doesn't smoke, really at all, there's no coolant in the oil or vice versa. It has always seemed like it was in pretty decent shape but the fact that it pukes oil from every oriface and the exhaust manifolds have completely rusted through in places had me worried.

Looking more and more like the seals just went bad from sitting.

 

[bikersmurf]

As for seals, sometimes a 'rebuild' gasket kit is more economical than buying a whole set of gaskets individually.