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Old 23-02-2013, 00:16   #1
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Perkins 4.108 compression

Hi,
Would anybody know what the compression on a fully rebuild Perkins 4.108 should be?
The engine has only done approx. 4hrs.
A lot of light blue smoke, smells like diesel, when first started.
I suspect low compression due to incorrect pistons fitted.
I don't want to remove the head to check the pistons at this stage.
I have the engine specs but compression is not listed.

Thank you very much.
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Old 23-02-2013, 01:00   #2
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Re: Perkins 4.108 compression

Quote:
Originally Posted by Seman View Post
Hi,
Would anybody know what the compression on a fully rebuild Perkins 4.108 should be?
The engine has only done approx. 4hrs.
A lot of light blue smoke, smells like diesel, when first started.
I suspect low compression due to incorrect pistons fitted.
I don't want to remove the head to check the pistons at this stage.
I have the engine specs but compression is not listed.

Thank you very much.
One Mans rebuild...does not mean a REBUILD.

It could mean anything from 2 spray bombs and new gaskets...to well??

A true rebuild of a Perkins engine to me would be new Liners, new pistons, a new crank and cam, A new/rebuilt Injector pump, and injectors(by a proper Diesel Sytems builder), an new fresh water pump, and a a new oil pump, as well as a new sea water pump, and all the other little parts.

but not to digress.

What ever makes you think the piston's are the reason?

Blue smoke on a diesel is also very subjective.

Especially with only 4 hrs. Any new/rebuilt engine is likely to smoke a little blue until it is run in properly to seat the rings.

Perkins has to my knowledge never published a compression table, for any engine they built. Instead they caution that each cylinder should be with in 15% of each other.

Next the only way to make a compression test on a Perkins is to have a Donner Injector refitted to a compression tester, that is in the range of 300 psi.

Lloyd
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Old 23-02-2013, 01:04   #3
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Re: Perkins 4.108 compression

Quote:
Originally Posted by Seman View Post
Hi,
Would anybody know what the compression on a fully rebuild Perkins 4.108 should be?
The engine has only done approx. 4hrs.
A lot of light blue smoke, smells like diesel, when first started.
I suspect low compression due to incorrect pistons fitted.
I don't want to remove the head to check the pistons at this stage.
I have the engine specs but compression is not listed.

Thank you very much.
Run it in and then asses its performance. All rebuilt engines (to whatever degree of rebuild) need to be run in to allow correct ring bedding etc.
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Old 23-02-2013, 04:50   #4
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Re: Perkins 4.108 compression

Thank you for trying to help.

The engine is running.
The injection pump & injectors were reconditioned as well as the complete motor, i.e new sleeves, pistons, bearings etc . the head was done as well.
The company recond. the motor is blaming the fuel injection for the smoke, the injection specialist is blaming the low compression for the smoke!
If I could get concrete figures on the compression, at least I could pin point who is talking bull & follow up!
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Old 23-02-2013, 05:32   #5
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Re: Perkins 4.108 compression

I stole this from one of Chief Engineers posts so credit goes to him.


Compression Ratio is 22:1

22 x 14.7 = 323 psi




A 1:1 ratio is equal to 0 PSI. 14.7 PSI is equal to a 2:1 ratio. Just multiply your ratio by 14.7 to get PSI, or divide PSI by 14.7 to get ratio.




Read more: How do you convert compression ratio and psi
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Old 23-02-2013, 05:41   #6
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If they rebuilt your injection pump your timing might be a bit off. Try rotating the pump.
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Old 23-02-2013, 07:07   #7
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Re: Perkins 4.108 compression

If the pistons were too small it would be burning a LOT of oil. Is it using any oil?
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Old 23-02-2013, 16:28   #8
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Re: Perkins 4.108 compression

Quote:
Originally Posted by Tellie View Post
I stole this from one of Chief Engineers posts so credit goes to him.


Compression Ratio is 22:1

22 x 14.7 = 323 psi




A 1:1 ratio is equal to 0 PSI. 14.7 PSI is equal to a 2:1 ratio. Just multiply your ratio by 14.7 to get PSI, or divide PSI by 14.7 to get ratio.




Read more: How do you convert compression ratio and psi
Thank you Tellie, but unfortunately it's not as simple & 323 psi is not correct, as there are a lot of other factors to be considered to calculate the compression.
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Old 23-02-2013, 17:03   #9
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Re: Perkins 4.108 compression

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Thank you Tellie, but unfortunately it's not as simple & 323 psi is not correct, as there are a lot of other factors to be considered to calculate the compression.

Hmm. Then how can a 4.108 be considered to have a ratio of 22:1 for compression testing? I would think it would have to be mighty close. Even then it would be a reference to test each cylinder for differentially acceptable pressures. I'd be interested in your findings once you nail down the right pressure.
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Old 23-02-2013, 18:49   #10
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Re: Perkins 4.108 compression

I always thought the compression ratio was the difference in volume at LDC and TDC. And wikipedia agrees with me.

Quote:
The compression ratio of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity. It is a fundamental specification for many common combustion engines.
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Old 23-02-2013, 18:59   #11
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Re: Perkins 4.108 compression

How could you have pistons that were to small? Not likely. Rings improperly installed etc. but the pistons are going to be the right size because you don't really have a choice with piston size. Maybe old pistons with worn ring lands but with new rings they are going to work fine for a while. Look eleswhere.
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Old 23-02-2013, 19:00   #12
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Re: Perkins 4.108 compression

Plus the volume of the combustion chamber.
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Old 23-02-2013, 19:34   #13
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Re: Perkins 4.108 compression

If the engine has been rebuilt and only run for 4 hours it will make light blue smoke, it's caused by oil burning as the rings and liners 'bed in'.i.e mating surfaces smooth together to create a gas/oil tight seal.
Here's a few tips for a long life from your Perkins, don't run the engine on idle for more than 5 minutes, if tied up securely to the dock put it in gear with a light load, or, if you wish to go cruising don't thrash the engine, treat it to light loads, don't rev it out full until it's done around 200 hours, always always when you come in from a cruise let the engine idle for about 5 mins to cool and relieve the thermal stresses around the head. Check/monitor for leaks in the the coolant and oil level everyday. Change the oil and filter after 250 hrs. Always keep an eye on the temperature gauge and don't let it overheat and it will last you a lifetime. All the older engines smoke when starting until they reach operating temperature, it's normal don't worry about it.
Good luck and safe cruising
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Old 23-02-2013, 21:08   #14
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Re: Perkins 4.108 compression

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Originally Posted by DeepFrz View Post
I always thought the compression ratio was the difference in volume at LDC and TDC. And wikipedia agrees with me.
I always thought the same. Maybe I'm missing something but if Perkins claims the 4.108 is a 22:1 I just figured they knew best.
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Old 23-02-2013, 23:36   #15
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Re: Perkins 4.108 compression

22:1 is the compression ratio. It means the engine compresses the air mixture down to 1/22 of the volume of the cylinder at the bottom of the intake stroke when it reaches TDC. That's what causes the air to heat up to the point where a spray of diesel fuel burns.
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