sixtysixtom

The shop doing work on my engine did a compression test and the results were:
#1 125#
#2 & #3 120#
#4 110#
#5 125#
#6 105#
#7 125#
#8 110 #
This is an original 327/300 bored .060 over with a slightly livelier cam. with aluminum intake and holley 650 carburetor.
They say the compression should be 150# per cylinder, and that a rebuild is in order, or replace the engine. I think the 150# number is a little high, what do you guys think? I know #6 has an issue. This engine was rebuilt in '08 and has less than 5k on it.
Thanks
Tom

MikeM

Those are pretty lame numbers. A recently rebuilt engine like yours shouldn't have compression consistent with what the shop said.

But, unless you are having trouble of some kind, it should still run reasonably well. If it runs well, you can drive on for awhile or you can spend a bucnch of money fixing it.

On the other hand, since the compression is that low, this short of time from a rebuild, you compression may deteriorate pretty quickly.

63split63

Before you spend a lot of money , spend a little and have the compression checked again somewhere else .
Get a second opinion from someone who dosen't re-build or sell re-built engines .

Bill

sixtysixtom

Bill:
This shop is a reputable corvette restoration shop. They do not build or sell engines.

MikeM:
While the car is still in the shop I was thinking that in the long term it would be wise to pull the original engine and put in a brand new engine. Over and done, no more problems. Save the original for when the car is sold.

MikeM

You didn't say why you had the compression checked. Just wondering.

AZDoug

I would say a leak down test is in order to determine if the problem is rings, or valves.

That is pretty low #s. My 350/327 was about 180 PSIG +/5 on all cylinders.

Doug

sixtysixtom

MikeM
I was going to have the shop dyno-tune the engine and they were doing some preliminary tuning and noticed a lot of smoke emanating from the block breather tube. Said it looked like an old locomotive chugging along at the higher rpm's. That's when they decided to do a compression test.
Tom

63split63

I would still get a second opinion with a second gauge, then the leakdown test .
JMO Bill

MikeM

That's a pretty sure sign of blowby but not always. But the compression readings kinda' confirms blowby.

Looks like you'll have to select your priorities. If you want it to run top notch, you'll need some work. Maybe a lot of work. If the way it runs now is "okay", you should be able to drive on for some period time without problems.

427Hotrod

I would try to figure out what was done during the rebuild. Quite often replacement rebuilder pistons are about .025" or more shorter that originals...add in a set of composition gaskets and a valve job that sunk the valves somewhat...and you magically have an 8.5 compression motor (at best) and then throw in a rowdier cam and you instantly have low cranking compression.

Now all the blowby can all be tied to it. maybe hypereutectic or cast pistons that got detontated a few times and rings lands are destroyed? A leakdown ck will tell you a lot.

JIM

GeoffM

Just been through a similar scenario, IMHO your at 60 over now , there may not be anywhere to go with that block, sonic test will be the decider. as they guys have said drive it as it is or go the whole route.To keep my block because it is a number matching car we sleeved it, lost cubic inches but keeps the numbers. Not a cheap exercise.
Geoff

MiguelsC2

Do you have a working relationship with this dyno shop? They may not sell or build motors. But they sure make money off of the labor.

Sounds like you have talked yourself into a new motor.

Sure wouldn't hurt.



Can you go back to the shop that built the motor?

66since71

I'm in the "get it checked again" camp. The values are too similar, cylinder to cylinder. I'd expect significant problems to produce a few really bad cylinders with the others nearer normal. The livelier cam will lower compression readings too.

And i'd check the PCV system before accepting smoke from the breather as a problem, unless it's setting off fire alarms!

Harry

65tripleblack

My engine gives me readings between 225 and 240 with a solid roller cam whose 0.050" durations are in the 250-255 range.

The OP never states how the compression test was done. Never states if oil added caused the readings to rise.

Compression readings, per se, are not always an accurate indicator of engine condition. The most useful thing about them is reading the VARIATION in pressure between cylinders.

sixtysixtom

Hi guys:
Thanks for all the responses, and Happy Easter!
To answer a few questions and address some comments:
The restoration shop doesn't actually do the dyno work, they shop it it locally. The dyno shop has not even seen the car yet.
The last rebuild was done in '08. That rebuild was due to cam failure. we replaced the cam with comp cam 280H. Which I had been running before. The pistons are forged pistons which were reused, just replaced the rings with a quick seating set. Could be they were too soft. Also honed the cylinders and did a 010/010 grind on the crank.
I thought the engine was running pretty good since the last rebuild, hadn't been any real issues, just wanted it dyno'd to see what hp I had and to give it a good tune-up.
Maybe I was too quick to jump on the new engine, but it would preserve the number matching engine I have and give me no worries. Hopefully.
I will suggest they do a leak down test before I make a final decision to see what's up. If its just bad rings should I resleeve all the cylinders back to original bore and start over? Or is it less expensive to just buy a long block and add my peripherals. As know machine work on engines is not cheap, and I don't want to tie up the car for a longer period of time, as the driving season is approaching here in the northeast.
Thanks again for all your feedback
Tom

sixtysixtom

Hi Guys:
Thanks again for all the responses. I had a leak down test done on the engine Tuesday. The results are:
#1-15%, #2-30%, #3-17%, #4-14%, 35-19%, #6-60%, #7-16%, #8-53%.
As you can see, the right side of the engine suffers the worst blow-by.
Anyway, I have decided to pull the original engine and will build a new engine. I already located a newly machined .030 over block with forged crank and pistons for a very good price from a co-worker. Hopefully within a few weeks I'll be on the road again.
Tom

67*427

Did the leakdown test show where the air was going on the bad cylinders? With less than 5000 miles on the engine, I'd pull the right head and have a look. I wouldn't be surprised to find a bad head gasket and that would be a lot cheaper than a new motor.

65tripleblack

Sonic test the prospective block before you buy it.

MiguelsC2

That would make sense given the low ratings are all on one side between #6 and #8. #2 isn't good either. Maybe the builder got a phone call when he was torquing the head? Or it didn't get re-torqued after break in.

Westlotorn

Originally Posted by 67*427
I wouldn't be surprised to find a bad head gasket and that would be a lot cheaper than a new motor.

Sure has all the indication of a blow out between the two cylinders. If you get a little extra timing involved the cylinder pressure can lift the head allowing the gasses to burn right thru the gasket into the adjoining cylinder.
Possiblity # 2, if you ever got this engine hot the back cylinders are most vulnerable to heat damage. If the heads were hot they expand and crush the gasket. When you tear down you find a couple loose head bolts a result of the crushed gasket, the gasket does not rebound after a crush unless is is the newer MLS design. A crushed gasket can always be measured after the fact and show you the area that got hot. If it is supposed to be .039 compressed it might measure .032 to .036 in the area that overheated. One lean cylinder from an intake leak can do this and all other 7 look perfect.