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Crusader 270 rebuild LOW compression

I

itselliott

New member
I had my Crusader 270 Hp 's rebuilt my a Marine specialty shop here in Michigan.

The shop ran the engines in on a wet test stand prior to my taking them.....
After my reinstall, I ran them for 20 minutes at 2000rpm prior to launch......Then,I ran the boat 120 miles at various speeds on the very first run.....never attempting WOT.
After 30 hours of use this summer, I changed the oil and went for a full out run.............the boat fell flat on its rudders..............3700-3900 rpm max.....26 MPH, a huge loss from before the rebuild. 4400-4500 at 32 MPH

Sent the Q jets out for a re do........Cliffs Performance ,....Mount Vernon Oh. The carbs are (IMO) perfect..............but still no WOT.
THEN!!! I did a compression check............130 to 135 on all 16 cylinders!! I think that the shop missed decking the block after a + .020 bore. Or maybe a too fat head gasket?
I no longer have the crane or needed equipment to pull the motors myself and naturally the shop is on the East side of the state and I am on the West Side. Oh, and only a yard style trailer.
I saved the oil filters and a cup of oil from each engine on the first oil change...........visually, I see Nothing in the oil.

How screwed am I?? where do I go from here?? how does a new build come up with such low compression?
 
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An incorrect piston profile and cylinder head combustion chamber combination may lead to low cylinder pressures.

First, find out which piston profile was used during the over-haul. Not the brand name, not the piston material.... we need the actual piston profile!

Secondly..... find out which cylinder head casting numbers were used.

Post this information here.


We DO NOT combine these parts without knowing exactly which ones are compatable with one another.



Next question.... how long were these run prior to the actual installation and under-load use?


.
 
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Are you positive the motors are timed correctly, with the spark advance hitting its proper maximum? I suggest putting a timing light on them and seeing exactly how much advance you're getting.

Then there's plug readings. Run her at WOT for a mile or so, then shut them right down--no idling allowed! Pull a few convenient plugs and read the colors.

Unless there's a ton of blow by past the rings, those motors should still make lots of power with those compression readings.

Jeff
 
Elliot, the reason for my specific questions are:

1... for examples:

full dished pistons with the 76cc chamber cylinder heads will not offer the proper static compression ratio.
(this may explain the low readings and low performance)

full dished pistons with 65cc heads, will offer the correct S C/R.
(pressure readings would be much higher)

flat top pistons with 65cc heads would offer an excessive S C/R.
(pressure readings would be excessive, and may lead to detonation issues)

BTW, when checking cylinder pressures, the nature of the pressure gauge is somewhat accumulative, so it's best if we pass through 2 or 3 compression strokes ..... or at least 5 or 7 crankshaft revolutions per cylinder.



2... (not mentioned earlier) incorrect camshaft profile may cause low cylinder readings while testing at cranking RPM.


3... if the engines were run for too long a period with no load on them (I.E., the shop ran the engines in on a wet test stand prior to my taking them), you may have missed a very important phase of piston ring seating.


As for Jeff's comment re; ignition advance, you'll want to use a timing light to see not only what BASE advance is, but what the progressive advance is doing at above idle speed up to approx 3,200 rpm. (should be in the 26* to 28* range @ 3.2K rpm)
If not correct, and if on the low side, no gasoline engine will perform to it's optimum.


In my opinion, your rather even cylinder pressure readings (130 to 135 on all 16 cylinders!!) may indicate an incorrect piston profile/combustion chamber volume combination.


The shop will need to provide the piston profile information, and you can get the cylinder head casting numbers by removing valve covers.


.
 
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itselliott said:
I think that the shop missed decking the block after a + .020 bore. Or maybe a too fat head gasket?
Click to expand...

The above would be more important if the shop used a Quench Effect style piston, and was after a proper quench dimension.

In part, this is yet another reason for asking my questions.


.
 
Re :timing...Set @11 initial with 28 full verified at the flywheel with an electronic light.

The heads are 1986 OEM...but I do not currently KNOW the numbers. I have thought of the piston issue, and DO have a call in to the builder..doubting if I will get good answers this far down the road.
What issues are related to a "no load" run in?
Coil is firing a STRONG spark.....when I pull the coil wire to check compression, plugs are slightly sooty however,
AC MR43T.....but they havent been changed out when the Carb was re built......previous idle was Rich the base of the electrode is tan.
IF I pull the head......am I able to determine what piston is in there? Can yuo post a foto of the various piston tops you're refering to?
 
Sounds like the motor MIGHT be running a tad rich. The plugs in my Mopar 360 (inboards) tend to run a very light tan thanks to ethanol gas.

Jeff
 
Ayuh,.... While I wouldn't be Happy at 135 psi compression,....

This is a dock boat,... Just how much sealife is hangin' on the hull,..??

It don't take much to overload the motors,....
 
just getting the part number and brand of the pistons shouldn't be that hard. You may want to ask what cam was used as well....

when does the timing come full in (rpm-wise)?

Like Jeff said, they aren't the best compression numbers but they are decent and the engine should be making decent torque..
 
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itselliott said:
Re :timing...Set @11 initial with 28 full verified at the flywheel with an electronic light.

BASE Advance and Total Advance numbers are meaningless without an associated RPM.
My question would be: At what RPM is the full in 28* occuring????



The heads are 1986 OEM...but I do not currently KNOW the numbers. I have thought of the piston issue, and DO have a call in to the builder..doubting if I will get good answers this far down the road.
What I meant earlier... was that if the GM style full dished pistons were used underneath the 76cc chamber cylinder heads, your S C/R will be too low for good performance.

What issues are related to a "no load" run in?
We need pressure behind the piston rings in order to push them out and against the cylinder walls for proper run-in or seating!
In order to generate this pressure, we must put the engine under a varying degree of loads.


NO LOAD operation cannot create the pressure that is necessary.
If we miss this opportunity.... there's no going back!


Coil is firing a STRONG spark.....when I pull the coil wire to check compression, plugs are slightly sooty however,
AC MR43T.....but they havent been changed out when the Carb was re built......previous idle was Rich the base of the electrode is tan.

IF I pull the head......am I able to determine what piston is in there?
Yes... absolutely!


Can yuo post a foto of the various piston tops you're refering to?
Start reading here at post #7.
This thread was not intended to show us that the full dished piston cannot be used ...... but rather to demonstrate what I'd call a more correct method for building a SBC Marine Engine.
Click to expand...
 
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