Melting pistons.

[McRat]

This topic had come up on CompD since the CR guys are having problems with it. Here's my opinion on it:

It's not widely posted, but yes, Duramaxes with big power have been melting pistons for at least 5 years now, back before 30+ deg timing. First one I know of was Micheal "SuperDiesel". Very few trucks that have had this happen have ever posted about it, since they tend to be high-profile trucks. I can't say who, other than Micheal, since I don't discuss other folk's failures unless they make them public first, which probably is not going to happen.

Casper's OEM engine was run at up to 754HP on fuel, and when it finally shrunk the factory rods, there was zero piston erosion on factory pistons. It ran a lot of timing and 120+ trap speeds, but rather tame rpm (under 4000).

So we built the engine to resolve the rod problem. And we added a .070" jet of nitrous occassionally. Soon there after, we saw our first piston melting. So we blamed it on nitrous, since I knew of other trucks at high HP running nitrous that had melted pistons. This engine had run at higher RPM's, up to 4500.

But recently, I ran an engine with no nitrous exposure at all, but ran it at 5200+ RPM. Erosion again.

My current thinking is that high angular pulse duration (a necessary evil at high RPM) is the culprit. Angular duration is the % of time the injector is open per rev. If your injector is open for 36 deg of crank revolution, you are open 10% of a revolution. If you drop that to 30 deg, it's about 8%.

High timing has been blamed for piston erosion, but I'm not convinced. This problem was occurring back when you couldn't go past 22 deg timing, except it was happening at lower HP.

To reduce angular duration, I'm now running really big (100% over) SoCal Diesel injectors and lower RPM. I will run more HP though, and longer WOT run time than is seen by most other trucks. When we tear down the engine, we will see if that is the answer.

 

[RENODMAX]

Interesting Pat. We talked about this when I was down there. Time will tell if big injectors keep the pistons from catching on fire

 

[super diesel]

Were these the better pistons you were running Pat, or were they the stockers that you saw the most recent erosion with? Hi EGTs, heat soak and extreme piston pressures scare the bejouls out of me any more. I'm thinking there maybe a few different factors involved here that any one of them can account for this happening?

 

[TIM Z]

Great info again Pat, thanks for posting.

There should be no shame in an engine faliure, its how you learn to correct the problem.

 

[S Phinney]

I'm sure the rpm"s have something to do with it. You are get more heat soak that would also be hurting the pistons. Interesting theory Pat.

 

[McRat]

Were these the better pistons you were running Pat, or were they the stockers that you saw the most recent erosion with? Hi EGTs, heat soak and extreme piston pressures scare the bejouls out of me any more. I'm thinking there maybe a few different factors involved here that any one of them can account for this happening?

So far, I've just run cut stock pistons. The new build is using SoCal/Mahle forged pistons.

 

[McRat]

Great info again Pat, thanks for posting.

There should be no shame in an engine faliure, its how you learn to correct the problem.

Here's the problem. Engine failures are used for advertising purposes against competing shops, so failures are hidden as much as possible. This is why I won't discuss other truck's failures unless it has already been made public.

The shops involved think this is to their advantage, and they are probably right. There is no advantage in having a public image that your shop is the only one with problems.

But as Duramax hotrodders, this works against us. Not only do we not know what we are up against, it can make us buy mods we don't need, and not buy mods we do need. Simply because we do not know what fails and what doesn't until it breaks on our own truck.

I've made a lot of enemies calling a spade a spade, and as time goes on, I get more and more pressured to hide info as well, and yes, I do hide some stuff. Many shops won't sell to me, because if the parts they sell break or don't work as is advertised, there is a high chance I'll post about it. They claim it's because they are afraid I'll copy their parts, but there is no basis for that historically. It's funny that the same shops who think I'll copy them call me and ask my opinion on projects.

Seriously, I don't manufacture anything, and since selling stuff that I've tested and found to be good mods is interfering with my racing and hotrodding, we are probably closing down the McRat Racing business after our financial year ends.

 

[TIM Z]

Yep, Exactly,

It why the editors of a magazine wont tell you what air intake, exhaust or tuner is best for a paticular vehicle.

Plus a hundred other examples.

But it sucks. Poeple want to buy the best product available for their vehicle, but it seems we have to learn the hard way, thru time and money and hassel.

This is why small time hobbyists appreciate the truth from racers like you.

We might be 3 years down the road to building an engine, but we can see thru HONEST posts what works and what dont.

I talked to you on the phone bout a year an a half ago about the PPE Hot +2

You gave me your honest opinion on it, and i bought from you, Glad i did, it was a bad ass tuner i ran for over a year till i found out more on EFI live.

So any way Its too bad People hide failures . It hurts the whole community like said above.

wish we had a Section for , Best Pistons, Best Exhaust, Best tuning , ETC with proven results, Not opinions.

Tired of all these products needing protection cuz one out performs them.

 

[Accelerator]

Great topic pat,
While i cant say i have ever melted a d-max piston, or even a dodge piston, i can say that i truely feel that i have prevented these problems thru a better understanding of how the CR system works, IMO the high rail pressure is the absolute cause at high RPM and some timing situations.

now i know high timing has been used as a cop out many times but let me explain first,

Rail pressure is delivered in PSI (pounds per square inch)

Timing is delivered in degrees Advanced, sometimes retarded, but useally advanced. advanceing the timing of any injection event cause's a rise in cylinder PSI, and it cause'es the burn to accelerate before TDC.

Nitrous is used to complete a fuel burn that would other wise be ineffiencent, in the process of completing a fuel burn, nitrous creates a higher cylinder PSI and temperature "can" result from this more complete burn.

fuel will burn @ temp "A" without nitrous, with nitrous the fuel will start to burn at the same or lower temp, because the nitrous makes the fuel burn more aggressivly or should i say easyer, than with out because there is more and colder oxygen present. if this nitrous assisted burn is not accounted for on the exhaust side of the engine you WILL have a higher EGT and will heat soak the piston's more redily, which in part can cause a melted piston.

how does timing play a part?
well IMO it add's more cylinder PSI than a lower setting would, and adding a catalist, such as nitrous will add even more cylinder PSI, and not to mention if you have a exhaust resrtiction, it adds up to a high pressure and dangerous engine combo. This is why backing down timing is so important, you lowering heat from PSI, and your delaying the fuel burn untill right at TDC or shortly after, which is where it should be.

Now add all this up with high RP and you have a atomic bomb going off inside of your engine, atomization is good @ 23k or so and you can make plenty of power if the RP stays there, but as you go higher with pressure, guess what, the timing also increases, and so does the cylinder TEMPS and cylinder PSI. this combo is whats eroding or washing the piston tops away.

nitrous is easy to blame but it's not the core problem, incorrect tuning is. i can say this with full confidence, because i have yet to melt any engine down, and these are rules i have followed that have contributed to my engine building success.

Now, i want to help in this tech thread, but i dont want the typical bashing session that you perodically allow, other wise i will not add one bit more of knowledgable info here.

 

[LBZ]

Great topic pat,
While i cant say i have ever melted a d-max piston, or even a dodge piston, i can say that i truely feel that i have prevented these problems thru a better understanding of how the CR system works, IMO the high rail pressure is the absolute cause at high RPM and some timing situations.

now i know high timing has been used as a cop out many times but let me explain first,

Rail pressure is delivered in PSI (pounds per square inch)

Timing is delivered in degrees Advanced, sometimes retarded, but useally advanced. advanceing the timing of any injection event cause's a rise in cylinder PSI, and it cause'es the burn to accelerate before TDC.

Nitrous is used to complete a fuel burn that would other wise be ineffiencent, in the process of completing a fuel burn, nitrous creates a higher cylinder PSI and temperature "can" result from this more complete burn.

fuel will burn @ temp "A" without nitrous, with nitrous the fuel will start to burn at the same or lower temp, because the nitrous makes the fuel burn more aggressivly or should i say easyer, than with out because there is more and colder oxygen present. if this nitrous assisted burn is not accounted for on the exhaust side of the engine you WILL have a higher EGT and will heat soak the piston's more redily, which in part can cause a melted piston.

how does timing play a part?
well IMO it add's more cylinder PSI than a lower setting would, and adding a catalist, such as nitrous will add even more cylinder PSI, and not to mention if you have a exhaust resrtiction, it adds up to a high pressure and dangerous engine combo. This is why backing down timing is so important, you lowering heat from PSI, and your delaying the fuel burn untill right at TDC or shortly after, which is where it should be.

Now add all this up with high RP and you have a atomic bomb going off inside of your engine, atomization is good @ 23k or so and you can make plenty of power if the RP stays there, but as you go higher with pressure, guess what, the timing also increases, and so does the cylinder TEMPS and cylinder PSI. this combo is whats eroding or washing the piston tops away.

nitrous is easy to blame but it's not the core problem, incorrect tuning is. i can say this with full confidence, because i have yet to melt any engine down, and these are rules i have followed that have contributed to my engine building success.

Now, i want to help in this tech thread, but i dont want the typical bashing session that you perodically allow, other wise i will not add one bit more of knowledgable info here.

It would be interesting to see some charted logging of cyl press in relation to timing and temps just to see where things are and how changes affect the pressure/temp and by how much. I agree-restriction in the exhaust plays a bigger part than many think.

 

[super diesel]

I have seen in the past that my EGTs increase rapidly as I back off the timing (testing some years back). Have you seen this as well Pat? Wasn't this talked about in the past? Just trying to get a solid grip on what's happening too.

 

[ChevyDieselLLY]

:nothingtoadd: subsrcibing

 

[TrentNell]

I have seen in the past that my EGTs increase rapidly as I back off the timing (testing some years back). Have you seen this as well Pat? Wasn't this talked about in the past? Just trying to get a solid grip on what's happening too.

I have seen this also, mainly from too much of the spray happening too late in the power stroke , i think the only true way to reduce timing is with bigger injectors and less pulse width .

Getting the fuel in at the right time is crutial . with big pulse width too much fuel is being sprayed both on the compression stroke and power stroke affecting egt's, and double affecting cylinder pressure from trying to compress so much fuel on the compression stroke , and from the expansion of the overly hot air caused by the high egt's .

 

[Mike]

If infact erosion is caused by rail pressure, is this erosion pattern the " star " shape?

How many folks here have tested pressures and heat with cylinder pressure monitors of some sort?

 

[Accelerator]

I have seen in the past that my EGTs increase rapidly as I back off the timing (testing some years back). Have you seen this as well Pat? Wasn't this talked about in the past? Just trying to get a solid grip on what's happening too.

IMO, by delaying too much timing, you are also delaying fuel ignition, which means that the burn is now focused later in the stroke or even far after TDC, not allowing the heat to rid itself as fast as a more advanced timing event would......make sence.

Quicker ignition = quiker heat expolsion = lower EGT's

Delayed timing = delayed heat expolsion.= higher EGT's

 

[Accelerator]

If infact erosion is caused by rail pressure, is this erosion pattern the " star " shape?

From what i have seen yes, in most cases it is, or there are holes where the fuel hits the pistons...

 

[TrentNell]

From what i have seen yes, in most cases it is, or there are holes where the fuel hits the pistons...

How is the star shape caused from too high if rail pressure ? Not saying it isnt just trying too see your point of veiw . I tend to think the star is from spraying too much fuel on the compression stroke ( before ignition happens ) but i am no expert . that star shape has been very present on a motor with 1 or more very poor injectors , basicly over spraying 1 cylinder or multiple .

 

[McRat]

First, yup, small timing raises EGT's in my experience, but I'm not sure EGT's are responsible for piston melting. My EGT's have been going down ever since I first ran a 12 second ET. Back then, burying the pyro (1600+) was normal, and it occured at 1/2 track. Melting pistons will occur as low as 1450 deg in my experience.

Fuel pressure is might be a contributing factor, but I doubt it. Simply because when I had lips on the pistons of the OEM engine, it didn't melt the pistons with dual fuelers programmed for max pressure. Only after I started running more RPM did it happen.

 

[Mike]

First, yup, small timing raises EGT's in my experience, but I'm not sure EGT's are responsible for piston melting. My EGT's have been going down ever since I first ran a 12 second ET. Back then, burying the pyro (1600+) was normal, and it occured at 1/2 track. Melting pistons will occur as low as 1450 deg in my experience.

Fuel pressure is might be a contributing factor, but I doubt it. Simply because when I had lips on the pistons of the OEM engine, it didn't melt the pistons with dual fuelers programmed for max pressure. Only after I started running more RPM did it happen.

This was discussed a year or two ago on this board wasn't it?

 

[McRat]

Yup. At the time I blamed nitrous. Imagine my surprise when we stopped spraying and it still happened.

My opinions are just that, opinions. And they often change as time goes on and I learn more.