Need help with some issues after no. 6 cylinder replacement...

[vineyardmh]

I have a suspicion that no matter what the intake air temp.... the final 'compressed' temp will be the same, if not pretty dang close. (as long as everything else is constant). cold AiT = more molecules to 'excite' during compression...more heat gain. Hot AIT = less molecules to compress, less heat gain. where: where

P is pressureV is volumen is the number of molesR is the universal gas constantT is temperature (K) So, lets say we compress this cylinder (with a perfect seal, no leakage) at the speed cummins cranks their engines to start. Then suddenly stop the piston at TDC, and we have a 'hot' cylinder gas of 650 degrees, with a psi reading of 750 psi. ( in a perfect scenario). Then we allow the gas temp to return to ambient. the law above says 'double the temp, double the pressure'. But that assumes instant compression, not a slow compression (our engines during cranking) which is already dissipating heat to the head and cylinder walls! The OP statement was correct on his calculations....WITHOUT temp factored. My question was his original value of 14.7.... because our compression gauge reads 'zero' at atmospheric idle, not 14.7 Is this becoming a chicken or egg first... need heat to make psi, or psi to make heat? Maybe the dude just needs to rebuild his engine:lmao:

As I said ...I am a simple engineer who likes to do ''back of envelope" calcs and simplified modeling that perhaps closely models what we CAN do in the real world without expensive lab tools. Your thought experiment is too complex to do in the real world. But we do make assumptions - during piston compression cycle, we assume minimal heat of compression lost to the piston and cylinder walls, we assume minimal leakage past rings....and the compression is nearly instantaneous enough to 'approximate' Boyle's Law. My P1xV1/T1 = P2xV2/T2 gives results of 388 psi ...and if we subtract 15 to go from PSIA to PSIG ....we are in the ball park. But the error of calculation is that maybe the gas is much hotter than assuming T2 is only at the ignition temperature of diesel (483 deg K) ...maybe it is hotter. But it is hard to reliably measure without a nice science laboratory.

But - if we work reverse and assume that a 388 psi is reasonable at a minimum to give good auto-ignition of diesel ....then compression test values of under 250 psig would be problematic....and your statement that "Maybe the dude just needs to rebuild his engine" is likely very correct!

(I have read of some college students performing the advanced measurements on a diesel engine - heavy instrumentation so that they could show real measurements and compare to theoretical modeling. Piston speed (speed of compression) and chamber pressure (before and during combustion) were fairly straight-forward to measure, but getting pre-ignition gas temperatures were more complex and required several methods of getting reliable/accurate temperatures (and they need to be both high speed and high resolution)! Of course - as backyard mechanics - we don't have the time/tools or money to run these science experiments - nor do we want to!)

[freedomisntfree]

Rancherman...... I will gladly rebuild the engine if I knew for sure that is what it needs.... The whole point of my thread was to try narrow down where my problem is. If ccompression is too low then I rebuild the motor that simple. Since my compression is consistent somewhat low.... Or not low ... I'm still confused whether 250 is low normal or just plain shitty .... I will not rebuild until I make sense of this... I rebuilt the bad cylinder and every other cylinder looked great with hone marks still present. Already have $3k into this doing the #6. sent from my cell...

[ISX]

I've been looking, and looking.......I have yet to see the spec for a VP truck. The service manual in the downloads section here explicitly omits the spec. I can go on cummins quickserve and my factory service manual is public for my 12 valve and it says 350psi for a new engine and 300psi for a used one with none being more than 100psi variance. But 12 valves run 17.5-18 compression (never can get a straight answer which one). So where is the spec for the 24v's? Then here we have this article http://articles.mopar1973man.com/general-cummins/34-engine-system/144-compression-testing and he says the stuff isn't listed. But he says 375 across the board for a good running engine which all 24 valve engines have less compression than a 12V which I already found has a new engine spec of 350. Sooo left with throwing your hands in the air again. Theoretically you could calculate the pressure and then compare it to the pressure you measure and if its within a certain percentage then you would be able to call it good. I think.

[rancherman]

Rancherman...... I will gladly rebuild the engine if I knew for sure that is what it needs.... The whole point of my thread was to try narrow down where my problem is. If ccompression is too low then I rebuild the motor that simple. Since my compression is consistent somewhat low.... Or not low ... I'm still confused whether 250 is low normal or just plain shitty .... I will not rebuild until I make sense of this... I rebuilt the bad cylinder and every other cylinder looked great with hone marks still present. Already have $3k into this doing the #6. sent from my cell...

Sorry about that! I got a little sideways, off track, so to speak! Sometimes the who-what-where-why-when gets in the way of a simple problem. calculating 'ultimate' pressure doesn't get your truck out of the shop any faster! my bad. Do a leak down test to see where it's all going.. and go from there. your bottom end might not be as bad as you think. (you said blow by is minimal) so I'd suspect valves, cracks in head next.

[Ilikeoldfords]

You might check the valve guides. I was missing half of my valve seals because some nimby didn't adjust his spring compressor before putting it all back together. Just with 11 seals missing, I was going through a quart every 300-400 miles. Smoked a lot too. Got those replaced and now don't go through hardly any oil.

[freedomisntfree]

Sorry about that! I got a little sideways, off track, so to speak! Sometimes the who-what-where-why-when gets in the way of a simple problem. calculating 'ultimate' pressure doesn't get your truck out of the shop any faster! my bad. Do a leak down test to see where it's all going.. and go from there. your bottom end might not be as bad as you think. (you said blow by is minimal) so I'd suspect valves, cracks in head next.

No worries man. Yeah.... Now the leak down test I haven't done , I need to put that on the list. Bad part is the shop I use is extremely busy so anything need to be done it will be in shop atleast few weeks. But I will def get that done asap.. sent from my cell...

[freedomisntfree]

You might check the valve guides. I was missing half of my valve seals because some nimby didn't adjust his spring compressor before putting it all back together. Just with 11 seals missing, I was going through a quart every 300-400 miles. Smoked a lot too. Got those replaced and now don't go through hardly any oil.

Valve guides...... Hmmmm that is interesting. I really don't know that deep into the motor so maybe I will get more input on that... But I will do some research. sent from my cell...

[freedomisntfree]

OK... Its been a while but I'm back with some info, I haven't done anything to motor such as valve guides or anything major. I have noticed that I have been using a hell of a lot less oil. After the motor was refurbished with new no. 6, I used royal purple synthetic. Which I was told I wasn't supposed to for that break in. Well that said and done just all of a sudden stopped burning so much. I figure the synthetic oil wasn't allowing proper break in?? Next thing, after getting on the throttle under load, I noticed oil in valley area below injectors. It would burn away then show up again after getting on the throttle again. It wasn't coming from the valve cover but somewhere close. I grabbed a 10mm socket and found the intake plate ( below grid heater) bolts were a little loose.. Truck sounds a little better and doesn't seem to smoke as much. Still has a whomp whomp sound thru intake but doesn't seem as bad. Any thoughts

Alright gentlmen.. I got my results for compression test. Injectors were pop tested and all checked out to be working normal. Cylinder 1 - 245 psi 2 - 250 3 - 250 4 - 250 5 - 250 6 - 255. I still have that whomp whomp sound thru intake. and as far as I know still burning oil.. I just figured out the edge has settings within each setting. Going to mess with that to see if it gets rid of some idle haze. I'm going to top off the oil and run it some more to ensure it still burning oil. Thnx for any more suggestions u have... And appreciate you guys that following up. sent from my cell...

[Ilikeoldfords]

Another good test you can do to check the health of the motor is a blow by test. You hook a blowby meter to your crank case vent tube and it measures precisely how much blowby you have. I will tell you if the rings or cylinders are worn versus the head. Most diesel engine shops have one of these tools. I was able to borrow it from the local cummins shop for the day.

[freedomisntfree]

I never thought of that.... I actually have a radiator pressure test pump kit I bought at a yard sale once. Had no use for it just thought I would someday. I can rig something up for that and also boost leak test.. good idea man Thnx. I extended the blow by hose so it should be piece of can tapping in to itsent from my cell...

[fire176969]

You don't want to block off the blow by hose completely to test, it will be very inaccurate. Sent from my DROID BIONIC using Tapatalk