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Old 23-02-2013, 23:50   #16
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The compression ratio will not give the compression obtained with a gauge. Due to various inefficiencies, slow cranking, the gauge itself, etc. the reading will be significantly less than that computed from the ratio. Your only guide would be experience or the shop manual.

If it starts easily when cold the compression is probably good enough to suspect your smoke problem lies elsewhere.
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Old 24-02-2013, 01:00   #17
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Re: Perkins 4.108 compression

It's just new ! give it a chance! just don't rev it above 1800 for a while ! I change the filter at 2 hours of steady running, and add the needed oil. I tear the filter apart and see if I have to much metal. Then I change the oil and filter at 50 to 75 hrs. and by then its well broken. If ya don't run her much it'e gonna take some time for her to settle in !! Just sayin don't get to worried Ive seen 20 or more of these old girls do the same thing at fresh start up
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Old 24-02-2013, 01:56   #18
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Re: Perkins 4.108 compression

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Originally Posted by sww914 View Post
If the pistons were too small it would be burning a LOT of oil. Is it using any oil?
Hi SWW914,
I don't think the pistons are too small, I think they could be too short.
If they are too short the compression would be down.
That's why I need the correct compression range (min / max) for this engine.
It appears to be a very well guarded secret, as I have not been able to get a useful answer spending a couple of days on the internet & calling local engineers.
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Old 24-02-2013, 09:32   #19
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Re: Perkins 4.108 compression

Not a word at least that I can find in the manual

http://www.richardandjudybell.com/wp...hop-Manual.pdf
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Old 24-02-2013, 09:40   #20
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Re: Perkins 4.108 compression

As others have said.... This engine needs to be broken in, and for sure the issue will resolve itself...

As for your curiosity because you want a number...

Compression should be WAY WAY higher than the low 300's PSI. The simple answer is far less complicated than the thermodynamics PVT (pressure/volume/temperature) calculations....

From Wiki:
where is the cylinder pressure at bottom dead center which is usually at 1 atm, is the compression ratio, and is the specific heat ratio for the working fluid, which is about 1.4 for air, and 1.3 for methane-air mixture.


Your 22:1 calculates out to 1098 for a "perfect ideal".... I would suspect that 80% should be expected on a newer "broken in" rebuild...


My quickly depreciating 2 cents...
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Old 24-02-2013, 09:52   #21
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Re: Perkins 4.108 compression

I would run the engine for 50 hours or more UNDER LOAD before worrying. Run it at various speeds. You could do some of this at the dock in gear. The rings need to seat in the cylinders. Also be aware that you must a have a bolted on compression gauge to test on the diesel. A hand held one wont work. The compression on my Perkins 4-236 was 335-350 If I remember right... dont take that to the bank.
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Old 24-02-2013, 10:32   #22
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Re: Perkins 4.108 compression

Rather than do a compression test you can do a "leak down test" This is how you test aircraft engine compression. In this way it does not matter what the compression ratio is. I suspect they might also do this to racing car engines. Briefly, you put a reference pressure into the cylinder and measure how much leaks out.
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Old 24-02-2013, 10:55   #23
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Re: Perkins 4.108 compression

seman, refering back to post 4, where you say the injection guy is blaming the low compression???? That makes it seem like there has already been a compression test done. Is that the case, and if so what did the numbers show? Getting in contact with an engineering rep from Perkins (not the local sales people) should get you a near enough number for the freshly overhauled engine. Are you doing the run-in according to the manual? How many hours does the manual say before it is completely broken in? Good Luck with it.____Grant.
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Old 24-02-2013, 11:19   #24
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Re: Perkins 4.108 compression

If ya had the right stuff to check the compression, It would have to be OVER 300lbs, but theres more to it then that ! ya have to have PLENTY battery power to spin it fast enough to get a proper compression reading ! Thats one of the reasons theres none listed in the manuals! because in most instalations it's almost impossible to ck it anyway ! Just give it a chance to break in and give her a little tune up after some hours of running and chances are she will be just fine !!!
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Old 24-02-2013, 11:44   #25
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Re: Perkins 4.108 compression

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If ya had the right stuff to check the compression, It would have to be OVER 300lbs, but theres more to it then that ! ya have to have PLENTY battery power to spin it fast enough to get a proper compression reading ! Thats one of the reasons theres none listed in the manuals! because in most instalations it's almost impossible to ck it anyway ! Just give it a chance to break in and give her a little tune up after some hours of running and chances are she will be just fine !!!
Nah, with the right machined gauge adaptor, my 4-236 spun over nice and fast.
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Old 24-02-2013, 15:17   #26
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Re: Perkins 4.108 compression

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seman, refering back to post 4, where you say the injection guy is blaming the low compression???? That makes it seem like there has already been a compression test done. Is that the case, and if so what did the numbers show? Getting in contact with an engineering rep from Perkins (not the local sales people) should get you a near enough number for the freshly overhauled engine. Are you doing the run-in according to the manual? How many hours does the manual say before it is completely broken in? Good Luck with it.____Grant.
Hi Grant,
Thanks for your post.
You are right, I have had 2 compression tests done. all 4 cylinders are within 5%, very good.
But that doesn't help, I need to know what the compression should be.
I am in a hurry, trying to sell the boat.
With the amount of smoke I get on start up nobody will believe that the engine is ok.
With the heat exchanger / exhaust disconnected the smoke appears to be grey.
Thanks again.
Walter
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Old 24-02-2013, 15:48   #27
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Re: Perkins 4.108 compression

The Perkins 4-135 manual indicates a compression ratio of 21:1 with a pressure at 200 rpm of 427 psi.

That may not be exactly the same as a 4-108, but it should be close.

Chuck
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Old 24-02-2013, 16:13   #28
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Re: Perkins 4.108 compression

Make sure the engine is getting to temperature of 85 C or so. (180F)
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Old 24-02-2013, 17:56   #29
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Re: Perkins 4.108 compression

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Originally Posted by Seman View Post
Hi Grant,
Thanks for your post.
You are right, I have had 2 compression tests done. all 4 cylinders are within 5%, very good.
But that doesn't help, I need to know what the compression should be.
I am in a hurry, trying to sell the boat.
With the amount of smoke I get on start up nobody will believe that the engine is ok.
With the heat exchanger / exhaust disconnected the smoke appears to be grey.
Thanks again.
Walter
Wlater,

There are no quick answers when it come to breaking in a new/rebuilt engine. It takes time for the rings to wear in properly.

On these old technology engines, required a proper break-in oil. You can still get break-in oil from some of the JD tractor stores.

You need a good oil with a healthy amount of ZDDP none of the modern oils have the proper TBN number these engines need.

If you don't run in the new/rebuilt engine properly she will smoke and slober oil and exhaust gasses past the rings for ever.

What oil and viscosity are you using, and what is the ambient temperature?

Now after you have the right oil you need to run the engine at about 25% load for a couple hours, then increase it to 50% load for another 4-6 hrs, then 75-80% load for a good ten hours these are total run times, and don't have to be consecutive.

Then change the oil, to the proper viscosity for temp, you need low ash, and high ZDDP oil.

The only way to get that oil now is with an additive. It hasn't been manufactured since 2007.

Now if she still smokes after break-in, and the right oil, then you need to look at the engine t-stat, your engine should run at a temp of 170 to 190F.

Also the injector pump timing is critical, and can only be set properly by bringing the #1 cylinder up to top dead center on the compression stroke. Then drop the valve spring on the #1 intake vale, let it sit on the top of the piston, Then set up a dial indicator and move the engine by hand to get the reading to top dead cent by measuring the top of the valve stem. Then counter rotate the engine back, about 1/4 turn, look up in the work shop manual degrees BTDC set the pump timing, rotate the engine forward until the dial indicator moves to the required point, the adjust the injector pump to the timing marks.

That's it.

lloyd
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Old 24-02-2013, 18:31   #30
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Re: Perkins 4.108 compression

There should be no problem with getting the engine turning fast enough for a compression test, since all of the other injectors will be out, and you will only have compression on the single cylinder that you are testing. If the engine turns over slowly in that configuration, then you have more problems than just compression. Another note that I am sure the OP doesnt want to hear is that you can have good compression with bad oil burn if the oil scraper ring is installed wrong. 4 hours wont tell you much. I agree with the group here saying run it, even if you have to hire someone to go out and roar around the harbor all day to put the hours on. Run the engine under the load conditions that the overhaul manual tells you to, so that the overhauler cant blame it on improper run in. I can understand your stress since this involves the sale of your boat. Good Luck with it.____Grant.
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