Educated feet

[Abull78]

So I am reading a bunch of threads pertaining to turbo charging, I may or may not be responsible for some of them.

I keep coming across EGT scares and lifting head scares. I am sure that it has happened for some and I am sure it will happen for others in the future but, I wanted to point out something that I see keeps getting over looked.

.........and that is engine speed.


High EGT's and high boost at a low RPM are the time tested recipe to lift a head or window the block at a cylinder. Jamming a bunch of heat and boost into your motor when it is spinning slowly means each cylinder is under much more stress than it would be if it was spinning at a higher RPM.

There is a reason NOS systems have minimum RPM settings that you should fire it at, there is a reason that race motors of ALL persuasion run high rpm, it's to spread the forces that combustion is generating across lots of rpm. I have yet to see a low rpm race motor, something that is a torque monter that pulls a tall gear through the quarter mile.

That being said, I see guys that have lifted heads at 12psi and then i see guys that are running 25psi and no problems and I think that boils down to having an educated foot. Mashing the throttle and blowing plumes of black smoke which will light a turbo faster than easing into means that you are gonna come up on boost at a really low RPM, and you could lift a head. Picking a tall gear up a hill towing or not means you are boosting and gererating heat at a lower rpm and you can lift a head.

I have always set my trucks up to deliver enough fuel to destroy the motor BUT, i dont drive around with my foot to the floor all day long, I don't lug up hills in tall gears. My usual scenario is winding first up to redline, when i hit 2nd, 3rd and so on, the rpms arent gonna drop below 2500 which means i am spreading all the fuel and boost over more combustion cycles. RPM's are healthy for motors in moderation. I think you are much more likely to hurt something trying to torque your way through than using your God given rpms to keep the motor from blowing itself apart.

Thats my .02, do you agree disagree?

 

[dyoung14]

I will put my personal experience in here, as you may or may not know up until recently i would turn these engines to 4500 rpm all the time, my mods was 4500 rpm spring, pump almost maxxed would loose a lane of traffic if i wanted too, ram-air intake, no soup bowl, better floeing exhuast, timing maxxed as far as i could twist the pump, i could lay down 0-60 times in around 11 seconds, and pull the 7% grade mountian at over 70mph and out run our 94 psd with ease, i had alot of power to be N/A it was alot of fun, but after i tore the clutch out it kinda cured my lead foot for a while, till i can afford it again, but in all the time i did this to my engines i never had an EGT gauge so EGT were unknown and i never melted a piston or blew a gasket or anything, i pulled the heads on my motor im using right now and the gaskets were perfect and every piston was perfect minus the 2 that ate glow plugs,

Something i recomend if you decide to go crazy with the fuel timing and rpm like i did is cut the tips off the glowplugs and cranks it with a wiff of either, if you dont its going to eat the glow plugs plus it will lower the compression a little bit,

I hope to one day be able to afford to build a mean engine that will spin around 5000 rpm and smoke newer diesels with ease, but if i une my engine just right now i can blow any almost stock 94-97 psd off the road, so im pretty happy with it

 

[Diesel JD]

I had never considered the possibility that boost down low could increase the heat and dynamic pressure in the cylinder. So what you're saying is that 10 psi at 1200 rpm would be a lot harder on the engine than 10 psi at 3300 rpm or even 2500? I always assumed more rpm would mean more pressure because of the pressure from the compression of the engine which should remain about the same, boost which increases with rpm because more fuel is being burned and more fuel being burned means more exhaust gases which spin the turbo faster=more boost. Add to that the kinetic energy created with more rpms and some of that energy is wasted as heat. That was my thinking but I'm no genius when it comes to physics so maybe you could explain why that is wrong. If you're correct this is a very good argument for a non wastegated turbo and/or a boost reference. These things will not let you overboost the engine until you have some engine speed and boost respectively, yet you can keep on boosting till you run out of fuel or rpm, or both or blow up the engine.

 

[rjjp]

He is correct, high boost is harder on the engine at low rpms, than it is at redline.

 

[Abull78]

I had never considered the possibility that boost down low could increase the heat and dynamic pressure in the cylinder. So what you're saying is that 10 psi at 1200 rpm would be a lot harder on the engine than 10 psi at 3300 rpm or even 2500? I always assumed more rpm would mean more pressure because of the pressure from the compression of the engine which should remain about the same, boost which increases with rpm because more fuel is being burned and more fuel being burned means more exhaust gases which spin the turbo faster=more boost. Add to that the kinetic energy created with more rpms and some of that energy is wasted as heat. That was my thinking but I'm no genius when it comes to physics so maybe you could explain why that is wrong. If you're correct this is a very good argument for a non wastegated turbo and/or a boost reference. These things will not let you overboost the engine until you have some engine speed and boost respectively, yet you can keep on boosting till you run out of fuel or rpm, or both or blow up the engine.

let me answer it 2 parts. First, low boost fueling. From a non competition standpoint, low boost fueling is bad. You are jamming fuel in that should be accompanied with a specific amount of boost, so you get high egt's until the boost comes in too cool it down and provide better burn.

Second, low rpm boost. In a turbo application we are jamming boost into a cylinder on the intake stroke, after combustion we have that boost PLUS the exhaust generated by combustion PLUS everything has expanded due to the heat. so we have a high volume lower density charge exiting the cylinder. At a low rpm that cylinder see's prolonged pressure from the boost entering the cylinder and while it is exiting. Pushing the exhaust out the cylinder is powered by another cylinder during its combustion stroke AND again at a lower rpm these cylinders are seeing these pressures for a longer period of time.

now lets add RPM, we are doing the same thing but we are minimizing the amount of time that stresses are being applied to the cylinder, we also have the kinetic energy that is the rotating assembly aiding in exhausting the cylinders.

Rpms are stored energy, the more rpms you are spinning= more energy stored in the rotating assembly= less energy required to accelerate that rotating assembly.

 

[Abull78]

so if you can agree with me on that point, what I am getting at is that I am sure its possible to lift a head with 12psi and most likely that is gonna occur at a low rpm and that 25psi of boost at high rpm may not lift a head. Obviously there is a lot more that goes with that but the basics being that spinning motor higher prior to commanding full boost is much safer than pouring the coal to a motor at idle or low rpm.

 

[Diesel JD]

I can see how that makes a lot of sense. Also I don't care to see any boost till I'm past the torque peak and closer to max HP, before that the torque of the diesel gives me all I need anyway.

 

[kas83]

I can see how that makes a lot of sense. Also I don't care to see any boost till I'm past the torque peak and closer to max HP, before that the torque of the diesel gives me all I need anyway.

But you want a little bit of boost, that builds steadily, to complement the torque peak and broaden the torque curve. This will make for a more linear torque curve that will pull like a freight train.

 

[Abull78]

But you want a little bit of boost, that builds steadily, to complement the torque peak and broaden the torque curve. This will make for a more linear torque curve that will pull like a freight train.

Again, i think thats personal preference. I dont care about off idle boost. Ill take a little lag in exchange for more boost up high.

 

[Agnem]

So can I assume that you want to take your motor to 4500 RPM, and see how much boost it will take? It would be nice to turn your theory into fact.

 

[dsltech83]

Did I mention that this forum is awesome-you guys have alot of great experience that this 26 yr old newb can use!!

 

[towcat]

Here's a couple of my personal notes on driving these trucks for a living.
In order to make power, you have to spin these motors in order for them to make power. My non-turbo'd F450s with 4.63 rears and 5.13 rears needed to be running between 2k-3k rpm to make power. with my '92, it likes running from 2500 to 3500rpm in order to keep the EGT's and water temps down.
My governor is set at 3800 on all my trucks. with the F450's they will run all day pinned to the governor. Have I tried going higher? Nope. More than one IH tech over the years gave me a warning about taking it over 400rpm. I figured tachs are not an exact science, a 200rpm cushion is good. Believe me, your truck will tell you where it is happiest. Sounds crazy but all you have to do is listen.

 

[Abull78]

So can I assume that you want to take your motor to 4500 RPM, and see how much boost it will take? It would be nice to turn your theory into fact.

you are correct sir!

 

[sle2115]

you are correct sir!

Sounds like the only thing stopping you is your educated feet!

 

[Abull78]

we will see tomorrow when i start recording vids.