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Whats the difference between a small cam cummins and the big cam cummins engine?
Small Cam/Big cam Cummins
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Done both. I use the old CPL book to know if it's built to spec or not. Just did two
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We should look more at the camshaft itself to differ the 'small' verse 'Big'. This has nothing to the cam being big in lift or small in lift. It pertains to the physical SIZE of the camshaft as a whole. If you see an older 855 series Small cam you will notice the camshaft itself has a smaller diameter journal which entails smaller lobes, opposite is true for a big cam, large journals equals larger usable lobe size which do tune differently however the large journal cam can accept higher rates of acceleration at the lifter. The cam follower plates are also different in size where you adjust your valve timing.
I had a slight argument with some older co workers on this when I told them there was no such thing as a "big" cam or "small" cam at which they were thinking on a whole different perspective as I was.
I had a slight argument with some older co workers on this when I told them there was no such thing as a "big" cam or "small" cam at which they were thinking on a whole different perspective as I was.
Lobe size and injector plunger drive rates were a byproduct in afterthought of the original fix which was updating and repairing the ever-so-prevalent cam bearing spin in small cams. When big cams started having the same issues, tho later in engine hour life, they dropped the running oil pressure from 54/60PSI to 38/42PSI thru II and III series noting it was not tube load but wash out load that spun the bearing. Easy argument fix in cam size is
1) parts order ship weight
2) the Cummins stamp and small cams and the BIG CAM stamp on the latter
They do time and "tune" at same specs as early big cams tho fast cam timing was in truck and Ag use for SC, BCI, II, III and early IV... "Back 2" 7inLB click stop T. Either 14 and 27 or 11 and 23 could be used depending on valve set at OBC or 2 back plus hot and cold or dial indicator methods. .070"(+/-) was normal injection event in all early Cummins which was supposed equal 19* static in the "over 100 different" pistons and 40+ different cams and injector sets, tho the 176BK's turned up purdy often for injectors to match OEM rail... Good post tho
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Interesting, I haven't heard of wash out and tube load. What exactly are these occurances?
tube load " pressure excerted on cam follower by lift of injector lobe thru injector tube" parasitic shock. injector plunger bottoming out. bearing washout " high oil pressure washing out bearing's, small cam including the 743 series ran 55 to [email protected] rated speed but held together, bc cummins 35 to 45lbs @ rated speed ran forever
Thanx Fest. :thumbsup bike didn't go so well... :bang
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I wonder the point of the higher pressure?
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I was told by an old Cummins engineer that they went to lower oil pressure struggling to make horsepower....
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I had two certificates on the Big cam 4 and here is what we was told. first low oil pressure gives the engine less drag due to less friction on moving parts so less HP is robbed. Then higher oil temps will help keep oil cleaner. And the main thing was the low oil pressure lub system was disigned for the Big cam 2 for a better filitraion.The oil pump pressure is raised at the pump and this will let oil be pumped thru the filter whether it wants to or not instead of by/passing like the previous system.It will by/pass but has a spring on that vavle which needs 55 psi to open the by/pass valve.The older systems only need like 15 psi difffernts between in let and out let of the filter to by/pass so that means when the engine is started and cold alot of the oil is by/passed around the filter to the brgs.I think the oil pump relief valve is around 120 psi. That is too much pressure to brgs so down stream on the oil galley there is a steel line going to a regulator valve on the oil pump that the down stream pressure is regulated to 35 to 45 psi and that is after the filter so this means most of the useable oil is filter more than older systems.
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This brings me back to the rule of thumb of oil pressure in a racing engine, 10psi per 1000 rpms. Could a performance diesel be used under the same thumb? Granted heavier parts but still....
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Are the bearings in a race engine smaller in surface area compared to a diesel engine? I guess my reasoning is psi is force based on area so if a bearing surface is larger you get more force at the same oil pressure.
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A high performance diesel is almost always forced air, so there is no vacuum stroke other than at idle, also there is no throttle plate to create large scale vacuum so the loaded part of the bearings is more uniform compared to an air throttled engine, which suck the load onto the opposite part of the bearing surface during deceleration when most failure occurs. Another factor is that the diesel is designed to run at full load for long periods from the factory. If the internal clearances are too great between bearing and shaft, the leakage exceeds the capacity of the oil passages and the oil film is not maintained, but the pressure at the passage may still be high.
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I'm not sure pump oil pressure plays a very large part in keeping journals and bearing surfaces apart anyway. Do the math, the load is thousands of PSI across the bearing surface, and the thing that keeps them separate and oil in between is viscosity and the "wedge effect" of a plain bearing filled with oil....
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Quisiera saber si en una cummins small cam ntc855 puedo poner todas las partes de un cam4 grande, como turboinyectores multiples, pistones top stop, etc., o si voy a tener algun impedimento
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Very concise answer.
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