Compression Readings All over..... What did I do :-(

[HVAC Cruiser]

Engine specs.
Chevy 383
Pistons flat top +5cc
heads 76cc
cam Comp cams xtream marine 256
intake Edlelbrock Performer EPS
Carb Edlebrock marine 750 cfm

I am a real upset camper today. I started having engine problems a few weeks ago the carb was acting up, wouldn't take adjustment ( sucking air through the throttle shaft. I ordered a new edelbrock 750 and performer eps manifold. When I took her out she really was not acting right, the sea's were bad so I came back home.

I checked for dead cylinders found #1, 5 & 8 dead as a door nail. Tried swapping out plugs, wires cap to no avail

Yesterday & today I decided to get to the bottom of my problems. I ran a compression test and was shocked at the results

Cyl 1-80#, 3-120#, 5-120#, 7-175#
Cyl 2-150#, 4-110#, 6-130#, 8-100#

I did 2 squirts of oil and redid the test, the pressures really didn't change maybe 2 lbs

There is no oil in the antifreeze (closed cooling) there is no water in the oil.

I am thinking the heads, but every cylinder????? these are the the chevy 76cc castings that are prone to cracking.

If I busted rings I'm thinking I would have metal in the oil , making a racket etc...

Anyone have any thoughts??????


EDIT- Thought about it S@rew it Guess my season is over, I am going to pull it and go through it AGAIN. I might have washed the rings with the bad carb I am getting some blow-by

 

[HT32BSX115]

Re: Compression Readings All over..... What did I do :-(

I am thinking the heads, but every cylinder????? these are the the chevy 76cc castings that are prone to cracking.

If I busted rings I'm thinking I would have metal in the oil , making a racket etc...

Anyone have any thoughts??????

Howdy,


Before you assume you have broken rings (that wouldn't necessarily produce metal in the oil), You might try a leak down test to help determine where the loss of compression is occurring.

I wonder if you may have had a brief detonation event.....

 

[Don S]

Re: Compression Readings All over..... What did I do :-(

With that cam, I am wondering if you are getting water in the cylinders at low rpm, from reversion.

 

[coheej]

Re: Compression Readings All over..... What did I do :-(

Was this a recent rebuild. You might pull the valve covers and make sure the valve adjustment is still where it should be. If not you may be losing cam lobes.

Also even with low compression, you should not have cylinders that are completely dead.

 

[HVAC Cruiser]

Re: Compression Readings All over..... What did I do :-(

HI Guys,

I am trying to wrap my mind around what happened.
HT don't know about detonation BUT, I got a single backfire at 4k-4500rpm before I replaced the carb. The engine is fresh with only something like 15 hrs total on her. When I built her I really was not happy with my #'s My piston to wall was out about .0001-.00015 on the loose side. I had talked to my machinist that honed the block and shot-peened the pistons ( he was supposed to check it all for me and see if it needed to be bored) and I got NO WORRIES..... loose makes more power. So I put it together against my better judgment.

Never in my life would I have gone along with some of the things I did with this engine if money wasn't so tight when I was building it

Hey Don,
I was wrong with the model # , its a comp cams extreme marine here's the link to the cam card XM256h 212 intake & 218 deg exhaust duration at ..050 lift. It is a very, very mild marine cam with that duration I shouldn't have suffered from reversion. BUT if you think that might have caused it, will raising my idle TO 800RPM after I fix what messed up prevent it from happening again?

 

[Bifflefan]

Re: Compression Readings All over..... What did I do :-(

I highly doubt the rings are broken, just because most of the compression numbers are low. And breaking all the rings at once, thats high odds.
Im thinking more to the cam and or lifters.
When did you put it in?
Also the valve guides, maybe they are hanging the valve open.
A leak down should give you a direction to head in.

 

[HVAC Cruiser]

Re: Compression Readings All over..... What did I do :-(

Was this a recent rebuild. You might pull the valve covers and make sure the valve adjustment is still where it should be. If not you may be losing cam lobes.

Also even with low compression, you should not have cylinders that are completely dead.

Yes it is there is maybe 10-15hrs on her at most

I ran the vales down yesterday, everything was still perfect. Up till they clicked, down till they just stopped, then 1/2 turn. I counted backwards as I was adjusting them and they were still on the money.

That's exactly what I was thinking the cylinders shouldn't be completely dead..... Unless we put together bad exhaust valves, that would give poor compression and Reversion as Don suggested hammer the piston and rings.

I think, but I am rusty a bad exhaust valve would kill the cyl. completely if it is bad enough. This would account for the poor compression numbers. With the setup I have I am running a theoretical 10.4-1 CR that's a +5 piston (flat top) , 76cc heads and a .041 compressed 4.06 head gasket with a .016 deck height.

 

[coheej]

Re: Compression Readings All over..... What did I do :-(

HI Guys,

My piston to wall was out about .0001-.00015 on the loose side. I had talked to my machinist that honed the block and shot-peened the pistons ( he was supposed to check it all for me and see if it needed to be bored) and I got NO WORRIES..... loose makes more power. So I put it together against my better judgment.

QUOTE]

.0001 - .00015" is negligible. If it was .001 - .0015", this still would not have caused any problem.
I don't understand shot peening the pistons, maybe rods but not pistons.

When you did your breakin, did you put a zinc additive in your oil to protect the cam. Also did you keep the rpm up to about 2000 rpm for the first 20 minutes.
This assures that the cam lobes are getting oiled.

 

[coheej]

Re: Compression Readings All over..... What did I do :-(

Yes it is there is maybe 10-15hrs on her at most

With the setup I have I am running a theoretical 10.4-1 CR that's a +5 piston (flat top) , 76cc heads and a .041 compressed 4.06 head gasket with a .016 deck height.

With a flat top piston with valve reliefs (I assume) would be a -5cc, not +5cc. Your compression would be closer to 9.5 - 1.

 

[HVAC Cruiser]

Re: Compression Readings All over..... What did I do :-(

I highly doubt the rings are broken, just because most of the compression numbers are low. And breaking all the rings at once, thats high odds.
Im thinking more to the cam and or lifters.
When did you put it in?
Also the valve guides, maybe they are hanging the valve open.
A leak down should give you a direction to head in.

I agree, I am going to do a leak down test to her tomorrow. My 6th sense keeps saying, heads,valves etc... When I built the engine I wound up real unhappy with the machinist. I found bad valves, stem tips chewed after I got the heads back from him. Like a jerk I didn't check to see if any were bent.

I have to say I used to go to Jack Merkel (spelling) for all my machine and preliminary checking work and the guy was always on point, phenomenal work. That was until he charged me for an angle mill job on a set of dirt track heads but didn't do it. Like a jerk I didn't talk to him about it (maybe he forgot, benefit of the doubt) Instead I looked for a new machinist. I Have Never Found one Better :redface:

 

[Don S]

Re: Compression Readings All over..... What did I do :-(

Reversion isn't caused by bad exhaust valves, it's caused by valve overlap. On race engines, they are normally at WOT or near WOT. On most family boats, you see a lot of no wake zones, and low rpm while docking etc. That is when reversion really shows it's ugly head. At those low rpm, moisture is gathering in the cylinders. If you shut if off, things are wet and they rust.
Over propping makes it even worse.

 

[Bifflefan]

Re: Compression Readings All over..... What did I do :-(

shot-peened the pistons

Either he knows something I have never heard of, and while that is possible, I have built many, many engines in my day, or you need a new machinest.

You shot-peen the rods and maybe the crank not the pistons. You will totally kill them.

 

[HVAC Cruiser]

Re: Compression Readings All over..... What did I do :-(

With a flat top piston with valve reliefs (I assume) would be a -5cc, not +5cc. Your compression would be closer to 9.5 - 1.

LOL, I was waiting for that....... You do your math LOL

It takes -5cc from the chamber, the dome would be a +5

Its a -5 on the combustion chamber its a +5 on the piston. When you calculate Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-') When you look at a pistons specification you will see the manufacturer gives the spec - for Dome + Dish

Here is a good on-line calc that I use to make it easy for me Calculator

 

[HVAC Cruiser]

Re: Compression Readings All over..... What did I do :-(

Either he knows something I have never heard of, and while that is possible, I have built many, many engines in my day, or you need a new machinest.

You shot-peen the rods and maybe the crank not the pistons. You will totally kill them.

Iv'e been drinking a little sorry LOL the pistons were glass beaded the rods were shot peened lol

 

[Aloysius]

Re: Compression Readings All over..... What did I do :-(

Maybe KNURLEd the pistons. Running clearance isn't real important, but the condition and clearance of the rings in the RING LANDS is critical. If the rings lands were shot peened, the pistons are toast..the rings will NEVER seal in the lands.

 

[Bifflefan]

Re: Compression Readings All over..... What did I do :-(

Comes out to 9.605862127298298:1
That 9.6 all day long. Not 10.5 as you thought.
You cant use a +5 for the piston with flat tops. They have dents and hold space, not take it up.
Even with a 5 its 10.1522, which is 10.2.
5cc is quite a bit.

Running clearance isn't real important

Umm, yes it is...

 

[HVAC Cruiser]

Re: Compression Readings All over..... What did I do :-(

Reversion isn't caused by bad exhaust valves, it's caused by valve overlap. On race engines, they are normally at WOT or near WOT. On most family boats, you see a lot of no wake zones, and low rpm while docking etc. That is when reversion really shows it's ugly head. At those low rpm, moisture is gathering in the cylinders. If you shut if off, things are wet and they rust.
Over propping makes it even worse.

Hi Don,
Well, I didn't build a race engine. The duration on the cam is real low and was designed for pleasure boats good fuel economy.

I could be wrong but I am thinking a bad/burnt exhaust valve cold potentially suck water it on the intake stroke. This would also account for my dead cylinders and low compression


Hi bifflefan,

My brain is starting to hurt here's a link to a comparable piston although they show a + 6.1 if you look at a dish it shows a negative number a flat top or dome as positive. That's how I figured things, that the dish being a negative is showing the cc;s that it subtracts from the piston volume and the + is adding to the piston volume

EDIT- I am not doubting you, me being lazy I use an online calculator that actually figured it to 10.445286110101534
And yes if I change it to a - I get 9.415910262230176

EDIT #2 Guys this is a 383 3.75 stroke not a 3.48 a 3.48 with a +5 comes up to 9.35 with a .041 compressed head gasket

 

[coheej]

Re: Compression Readings All over..... What did I do :-(

The piston cc size is telling you what the effective size of the dome is. If the piston is a flat top with no valve reliefs, then the dome volume would be 0 cc. If the piston has valve reliefs or a dish, then it will be a negative cc. If the piston has a very small dome and valve reliefs, then it still could have a negative cc if the valve reliefs have more volume than the dome. If it's a bigger dome, then it will have a positive cc.

After reading all of the posts, I'm still not sure, do you have a flat top with valve reliefs or do you have a dome with valve reliefs?

I agree that if you have the flat top with valve reliefs that your cr is about 9.44 - 1.

I hope you're running 92 or 93 octane in that.

 

[HVAC Cruiser]

Re: Compression Readings All over..... What did I do :-(

I Have Completely lost my Mind!!!!!

I knew I was rusty but I didn't think I had completely lost ALL Concepts :redface:

I thought about the calculator I used and started to look for pistons in case I have to rebuild her. I see the manufacturer is showing - for dome and + for dish and flat which makes sense. You take away vol for dome you add vol for dish.

The online calc I use says "Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-')"
So in essence I needed to use a - for a manufacturers + and a + for the manufacturers - DUH :redface:

 

[HVAC Cruiser]

Re: Compression Readings All over..... What did I do :-(

The piston cc size is telling you what the effective size of the dome is. If the piston is a flat top with no valve reliefs, then the dome volume would be 0 cc. If the piston has valve reliefs or a dish, then it will be a negative cc. If the piston has a very small dome and valve reliefs, then it still could have a negative cc if the valve reliefs have more volume than the dome. If it's a bigger dome, then it will have a positive cc.

After reading all of the posts, I'm still not sure, do you have a flat top with valve reliefs or do you have a dome with valve reliefs?

I agree that if you have the flat top with valve reliefs that your cr is about 9.44 - 1.

I hope you're running 92 or 93 octane in that.

Sorry, just realized our posts are crossing each other. I am cooking dinner in between sentences LOL

Yep I always use 92 or 93 octane.

Boy Its really amazing what I have forgotten in the past 20 years