High Compression values

I,ve been retired from Cat dealer 16 yrs. now but while I was working yet Cat never listed compression figures that I ever recall. They did maybe 25 yrs. ago recommended using the leak down test for cyl. & valve conditions by pumping air on top the pistons at TDC firing stroke & check the leak down of air after the supply was shut off and go from there and I preformed that on lots of engines & worked but that,s all I can tell you unless in these later years they came out with compression specs.It also would give you an idea too if a chamber seat was leaking or head gasket and would sort out which chamber was bad or would show if a head was cracked too if compression was getting into the coolant by leaving the cap off and observe the coolant at the top if bubbles was showing in it.As for your problem you think you have it seems if it was too high it would have caused problems long before this.Maybe if you know where another tractor is or engine with the D333 in maybe see if they would let you check their engine compression and see what you come up with.Also it seems if it is too high it would be hard turning over on start up too but just my opinion.

Don't know of any compression specs for Cat but it would seem that 22-24 bar (319-348psi) would be way to low. Got some old notes here indicating 450 psi as being good and 400 psi still ok on a D315 engine. I don't think you have a problem.

Thanks for your prompt replies!

We would like the thought that everything is fine, as we have no idea why the pressure should have increased.

Of course you are right, we don't have compression specs from Cat. We will check our specs for the leak down test.

But, and I did not make that clear in my last post, the pressure seems to have increased.

The compression has been tested once about 7 and once about 8 years ago and it showed both times values from 19 to 24 bar (275 to 350 psi). (According to german books about engines this is a reasonable value for diesel engines).

The cylinder head had been removed about that time too, but it is not possible to reconstruct whether the second compression test was before or after that work. (Probably after.)

So it looks as if the compression has increased during operation.

(If, of course, all measurements were correct....)

Anyone an idea how this could happen?

Thanks for your replies
merry Christmas

Ole

It,s pretty hard to believe that compression will increase over the years and maybe the first time it was read wrong maybe but just guessing or even the second time but all the years I rebuilt engines I never seen or heard of one increasing but usually the other way. I don,t think I,d worry about it for now as long as it,s running and starting ok.

I've never taken a compression reading on a diesel, but I have been told that the normal way to do it is to thread a gauge into the injector opening and then start the engine.😮

Perhaps the readings you are getting are only from using the starter to turn the engine over.

If the starter is used to turn the engine over for compression check, the condition of the batteries, cables, connections and starter etc will have a huge effect on the result.

I don't know if this helps, but there are two ways to test compression on the International/Navistar diesels I work on in the automotive industry. There are compression specifications for just one compression stroke of the piston, or 3 to 4 compression strokes, allowing the engine to "pump up" the gauge until the reading stops increasing. The single stroke reading is always lower than that of several strokes. If the compression tests were done differently it could explain the different readings. I have never compression tested my old Cat diesels, but 500+ psi sounds good to me. The modern diesels I work on have tested at 650 to 700 psi in new condition with everything at proper spec., and they have only 25% to 30% bearing surface area of the old Cat engines I have been into. Of course modern clearances and bearings are a lot different that they used to be. My opinion is that your engine is probably good.

Hi Toby,
Sounds good to me😄 😄

CC,
Never heard of a compression test where you just plug er in and let er rip.

You can get two different readings on an engine that has removeable precombustion chambers depending on whether it is done with chambers are in or out,it will be higher if the pc's are out,testing compression on a diesel is done with the fuel off,all the injectors out unless there is a glow plug hole to use,the engine should be at normal temperature or as close as and is cranked over on the starter,I too never seen any specs for what the compression should be for any diesel but there is a formula to make it up using the bore,stroke and ratio,usually what most people go by is take a reading and the cyl that is a lot lower than the others is the culprit,so cracking the pipes is just as effective and a lot cheaper than the gauge and all the adaptors for the hundreds of different makes that I come across in the repair business
AJ
Happy Xmas all

Hi Ole,
After putting pencil to paper and looking at some rough calculations I would tend to agree with your lower numbers.......however I can't think of anything else that would increase the compression pressure. There is a chance that parts were used during the rebuild that could effect the compression ratio such as pistons, changes to crankshaft stroke, rods or maybe cylinder head but more than just a head gasket thickness change. Could the changes in CR coincide with the engine change???? Records seem a little vague.

If in fact you are increasing that much I think you are correct in having concern as both the D330 and D333 are somewhat famous for spinning rod and/or main
bearings. (along with cracking heads)