Logo
FULL SITE MENU

Put a deposit on a pair of 8.1’s yesterday....compression test #’s

B

bobct

Advanced Contributor
My wife and I took a long drive down to look at a pair of ‘02 8.1’s HO’s with Hurth 2:1 straight drive, 8 degree down-angle transmissions. The boat is being parted out and sat for a couple of years. I was able to hear a cold start from when it was last winterized. The tank was removed so it was running off a gas can. They definitely have some deferred maintenance but started and ran well. I let them idle for about ten minutes and then did a compression test. I was going to do a leakdown but wasn’t sure that I would have power to run my compressor which turned out to be the case.

Results were:

Port - 175,170,175,165,165,175,150
Stb - 160,170,170,175,166,172,170,180

I think the #’s would be a little higher if it was fully warmed up and run under load. Obviously the 150 stands out on the port engine. I did that one twice, back to back with the same results. Maybe a little carbon build up on the valves on that cylinder? Once I get them home I can do a compression and leak down test under better conditions.

What do you guys think of those numbers overall and especially the 150? Also, both flex-plates were rattling but there were no shafts connected so not even a little bit of a load. Both engines seemed to idle great so it’s not clear yet if the rattling is actually the flexplates or rough/low idle.

My plan is to get them home in the next two weeks and get them setup and running in my garage and go through then top to bottom, replacing all of the consumable parts. So a bit of a project but these 8.1’s are impossible to find in really any condition.

One immediate question, I’m not quite clear on the cooling setup. One engine had both exhaust manifolds replaced. The setup on this boat was elbows bolted directly to exhaust manifolds, no riser in between (which I see as an option in the parts manual). Unlike the 454’s, are the 8.1’s raw water which is why they were replaced? I thought Crusader exhaust manifolds on all models were sealed glycol cooled. I know that some other 8.1’s, non Crusader do have RWC exhaust manifolds.

thanks
Bob
 
Last edited:
Ignore the FWC question, I found the cooling flow diagram. So it looks like one part of the elbow is RWC so only one component to replace. I’m not sure why the exhaust manifolds were replaced on one engine with Osco’s. I’ll clean them up/and repaint and just replace the elbows on both engines since I have no history.
 
The compression numbers aren't ideal but well within the service limits GM has published. as far as the root cause(s) the leakdown test would help there.

Can't recall if you posted the hours accumulated....if they are over 750, it may be worth reconditioning the heads before you do the swap....

They made them with both FWC and RWC. Because of the was the coolant circulates, additional parts change with the desired configuration. Are both engines RWC?
 
bobct said:
Ignore the FWC question, I found the cooling flow diagram. So it looks like one part of the elbow is RWC so only one component to replace. I’m not sure why the exhaust manifolds were replaced on one engine with Osco’s. I’ll clean them up/and repaint and just replace the elbows on both engines since I have no history.
Click to expand...

Whether fitted with a Closed Cooling System (either full or half), or raw water cooled, the exhaust elbows are always Raw Water cooled.
 
So the hours are reported to be in the 450 range, take that for what's it worth. Agreed, I can get them home and then decide on head work after doing a fully warm comp and leak down test.

On the cooling system, I didn't think RWC was even an option on the 8.1's. From the Crusader docs I read, it states "Full freshwater cooling system including exhaust manifold". So I never even gave fully RWC a thought until now...ugh. The water flow diagram which I have in front of me says ("Typical") and only shows RWC flowing through the elbow.

I assume "fully" RWC does not mean raw water cooling the block correct? (my assumption was those days are long gone). If fully RWC, does that just mean exhaust manifolds in addition to the elbows? Not ideal but maybe not a deal breaker. How would I verify that short of removing a drain plug from the manifold?

Bob
 
Last edited:
just did a little digging and compared the diagrams for RWC vs FWC. There is an extra hose coming out of the exhaust manifold on the FWC setup which I can briefly see in a video I took before I left (lucked out doing that). So, it looks like the only consumable exhaust parts will be elbows, it currently does not have the optional "sandwich" riser which I see in diagrams between the exhaust manifold and elbow. This looks like a much easier maintenance wise compared to my current riser/elbow arrangement on the 454's.
 
The diagrams I have access to show a full RWC configuration exists.....the easiest way to tell if the block is FWC/RWC is the presence of a heat exchanger...HX = FWC. A full FWC system includes the exhaust manifolds, a half system excludes it. I think the crusader diagrams also have a 'non-heated riser' option which can't exist on the engines you looked at (no riser, only elbow).

agree that the center riser style is much easier to work on than the older log styled stuff..
 
Based on my knowledge, there is no evidence to support the existence of a full RWC configuration for these particular engines. The presence of a heat exchanger (HX) does not necessarily indicate a full FWC system. In some cases, HX can refer to a separate closed-loop cooling system for the exhaust manifolds, which is not considered a full RWC system.

And yup, I think the term "non-heated riser" is typically used in reference to older engines with risers that do not have an integrated heat exchanger. It is not applicable to these engines, which do not have risers at all. Instead, they have elbows that connect the exhaust manifolds to the exhaust risers.

Finally, while I agree that the center riser style is generally easier to work on than the older log styled stuff, the specific design of the risers and manifolds can vary depending on the engine model and year, but I have no doubt that the engines in question do not have a full RWC configuration.
 
bobct said:
My wife and I took a long drive down to look at a pair of ‘02 8.1’s HO’s with Hurth 2:1 straight drive, 8 degree down-angle transmissions. The boat is being parted out and sat for a couple of years. I was able to hear a cold start from when it was last winterized. The tank was removed so it was running off a gas can. They definitely have some deferred maintenance but started and ran well. I let them idle for about ten minutes and then did a compression test. I was going to do a leakdown but wasn’t sure that I would have power to run my compressor which turned out to be the case.

Results were:

Port - 175,170,175,165,165,175,150
Stb - 160,170,170,175,166,172,170,180

I think the #’s would be a little higher if it was fully warmed up and run under load. Obviously the 150 stands out on the port engine. I did that one twice, back to back with the same results. Maybe a little carbon build up on the valves on that cylinder? Once I get them home I can do a compression and leak down test under better conditions.

What do you guys think of those numbers overall and especially the 150? Also, both flex-plates were rattling but there were no shafts connected so not even a little bit of a load. Both engines seemed to idle great so it’s not clear yet if the rattling is actually the flexplates or rough/low idle.

My plan is to get them home in the next two weeks and get them setup and running in my garage and go through then top to bottom, replacing all of the consumable parts. So a bit of a project but these 8.1’s are impossible to find in really any condition.

One immediate question, I’m not quite clear on the cooling setup. One engine had both exhaust manifolds replaced. The setup on this boat was elbows bolted directly to exhaust manifolds, no riser in between (which I see as an option in the parts manual) sites like funzpoints offer reviews about all sites like Funzpoints 2023 Unlike the 454’s, are the 8.1’s raw water which is why they were replaced? I thought Crusader exhaust manifolds on all models were sealed glycol cooled. I know that some other 8.1’s, non Crusader do have RWC exhaust manifolds.

thanks
Bob
Click to expand...

I have recently met a similar situation and concluded that the low compression numbers for the port engine, especially the 150 PSI reading, are a cause for concern. This suggests a potential problem with that cylinder, such as worn piston rings or valve seals. I have done a leakdown test to measure the amount of air that escapes from each cylinder.

And I concluded we have to pay much attention to address carbon buildup on valves that can impede compression and engine performance. I consider using a fuel injector cleaner or professional valve cleaning services.

And I also replace flex-plates because loose flex-plates contribute to rattling and vibration.
 
Hi, just seeing this now. Yes, I did get both engines installed and running well BUT wound up doing a full rebuild. While the compression numbers were ok, the fatal mistake I made was not scoping the cylinders. Once I got them home, I could see the cylinders walls were pretty scored from sitting and prior coolant and/or raw water leaking. Low hours = bad.

I had them bored 0.030 over. If I had my borescope, would I have walked away from buying this pair? That’s a tough call. There are so few 8.1’s that come up for sale and obviously location is a key factor. These were pretty far away (4 hours) but still a drive.

I still have one nagging issue on my Stb engine which is a 200 RPM surge at a precise RPM range. No codes and virtually all sensors replaced (no codes). I work around it by going faster/slower but I need to fix it. This is pushing me to get the Foxconn gateway and then I can see the data realtime.
 
Back
Top