Need pistons....

[Diesel Power]

all the while you can make it balanced by weight you can never balance the pressure difference in the hole that is larger. no matter how much you know, you cant change or balace that part of it, that is what i was getting at, merry christmas to you too

There are way too many other differences in these engines to worrie about 1. 2 cubic inches.....



For example the side intake plenum----chokes the #5 and 6 cylinders very bad, what oxygen fill rate differences do you think happens to those 2 cylinders verses the rest? Big difference!



Now lets look at the exhaust manifold, the CR manifold is again very restrictive in the #4,5,6 outlets, what do you think backpressure does?



Believe me it's WAY more of a issue than 1 cubic inch... ...



Merry Christmas.

 

[HerbW]

Did you know oversize pistons weigh the exact same as the standard ones. They balance them that way on purpose to keep things in balance. I've used Wiseco pistons in one cylinder before and the engine purred. It was a gasser though.

 

[chipster]

all the while you can make it balanced by weight you can never balance the pressure difference in the hole that is larger. no matter how much you know, you cant change or balace that part of it, that is what i was getting at, merry christmas to you too

On engines that I`ve run one oversize piston in, there was NO difference in the compression tester reading. If you run 1 . 020" oversize piston in your cummins that comes out to . 628 cubic inch more than a cylinder with a standard piston. and IF you hop up the engine to make 2 horse power per cubic inch(720 hp), the oversize cyl will make a whopping 1. 256 more horse power!!! you can`t measure that small a difference on an engine dyno, much less a chassis dyno. Let`s worry about things that will really hurt the durability of the cummins engine !!! :-laf:-laf Merry Christmas again, your friend and mine, Chipster

 

[JWadford]

On engines that I`ve run one oversize piston in, there was NO difference in the compression tester reading. If you run 1 . 020" oversize piston in your cummins that comes out to . 628 cubic inch more than a cylinder with a standard piston. and IF you hop up the engine to make 2 horse power per cubic inch(720 hp), the oversize cyl will make a whopping 1. 256 more horse power!!! you can`t measure that small a difference on an engine dyno, much less a chassis dyno. Let`s worry about things that will really hurt the durability of the cummins engine !!! :-laf:-laf Merry Christmas again, your friend and mine, Chipster

i cant argue with you , the pressure thing, you just dont understand. psi is a square inch thing, not a cubic inch measurement, the kind of pressure i am talking about is not measured with a compression tester, 100 psi over 10 square inches is 1000 pounds, 100 psi over 11 square inches is 1100 pounds, so with compression being equal across all six with a dead even balance, the pressure put onto the rod journal will be greater on the overbore hole, there is no way around it. your right, you wont see it on a dyno, they measure power output not cylinder force and crank deflection, while you worry about 1 h. p. , i worry about crank walk/breakage and vibration,the things that hurt the durability of a modified cummins. so i will leave it at i agree to disagree.

 

[brods]

Updated calcs

Interesting issue. The 05 manual also says that standard and oversized pistons can be mixed on an engine. To put some simplified numbers to it:



standard piston = 4. 020" diameter = 12. 6923 square inches of surface area

. 040 piston = 4. 060" diameter = 12. 9462 square inches of surface area



A difference of 0. 2538 of an inch of surface area. The difference in cylinder volume works out to about 1. 2 cubic inches.



3,000 psi is supposed to be a more accurate peak combustion pressure for our engines.



0. 2538 square inches x 3,000 psi = 762 pounds of extra push on the larger piston.

Sounds like a lot... but to put it in perspective, at the moment of peak pressure the downward force applied to the piston is:



stock piston (12. 6923 x 3,000) = 38,077 pounds

. 040 piston (12. 9462 x 3,000) = 38,839 pounds



It works out to about a 2% difference. Still not too bad and if Cummins says its ok, that goes a long way. Even so, once the motor is out might as well spend a little extra to fix it all the way... .

 

[KcoleKennyc]

Been building engines for 50 years and machining them for almost as long. I have been asked to bore one cylinder and been asked to turn one throw on a crank. Have not and will not do either. It may or may not affect the balance of the engine but don't think that a good quality machine shop would do this. Imagine what would happen years later when the average mechanic attempted to rebuild the engine. Unless he has micrometers and measures each part, he will probably install the wrong parts. Had a Kubota block in my shop a few months ago with a scratched cylinder. The customer wanted to do just the one cylinder and I refused. I guess the owner didn't believe what I told him so he called the number in his owners manual. They told him not to bore one cylinder oversize, bore all three. I have bored several of these blocks and have sleeved many of them for Cummins dealers but they have never asked me to bore a cylinder and not all.

 

[Diesel Power]

Interesting issue. The 05 manual also says that standard and oversized pistons can be mixed on an engine. To put some simplified numbers to it:



standard piston = 4. 020" diameter = 12. 6292 square inches of surface area

. 040 piston = 4. 060" diameter = 12. 7549 square inches of surface area



A difference of 0. 1257 of an inch of surface area. The difference in cylinder volume works out to about 0. 6 of a cubic inch.



I think I've read the peak combustion pressures can be as high as 3,500 psi, so lets go with that as a worst case.



0. 1257 square inches x 3,500 psi = 440 pounds of extra push on the larger piston.

Sounds like a lot... but to put it in perspective, at the moment of peak pressure the downward force applied to the piston is:



stock piston (12. 6292 x 3,500) = 44,202 pounds

. 040 piston (12. 7549 x 3,500) = 44,642 pounds



It works out to about a 1% difference. Can a production engine operating in the real world produce power from each cylinder that consistently falls within1% of all the other cylinders?



The idea of using mixed size pistons in an engine rubs me the wrong way too. Looking at some numbers it sounds a bit more reasonable.

You are forgeting about compression ration (static) and dynamic CR (running CR)



It's dynamic pressure that matters, static psi will never really matter, camshaft changes all that. First a stock engine never see's 3500 psi... . but those figures are good but not accurate.

the bigger piston surface will nessecitate a bigger bowl to allow the same CR as the stock piston, so volume will change psi, more volum accually makes for less psi

If the over bore piston is the same CR it will have no measureable difference in cylinder psi during opperation.



Few things to also remember..... a bigger piston needs more PTW clearance, and a bigger end gap, again these will lower dynamic cylinder psi, and increase some blowby... .



Trust me, it been done for 50 years and makes no difference, also if it were not true cummins definatly would not reccomend it, nore would i or any other builder

 

[HerbW]

Probably if you're going to take the block to a machine shop for bore job you could do them all while it's out of the truck. Here's another two cents of info. All my oversize pistons I've ever bought were stamped on the top the oversize like . 030. I just put four new ones in my 4 cylinder boat engine in a rebuild. Then if you ever pull the head, the oversize is right there to warn mechanics that it's been bored out. You should always return everything(pistons, lifters, push rods, etc) to their original spots anyway. The oversize would go into the oversize bore. I replaced all four pistons on my boat cause all the replacement pistons I bought had rings included and I wanted new rings throughout. Then the whole engine has equal wear life and durability.