Safe boost?

Katavic918 · Jan 20, 2016

I turned up the fuel on my d turbo ldt a while ago. I have a boost gauge installed but not egt monitor. I don't know what my max boost is since I don't let it go above 15 psi max and I don't let it go past 10 psi when pulling a long hill. So my question is what are my egts doing under these conditions. I try to take it easy and operate under the principle that if I don't think a 6 ton truck should be able to do this, I let off. Thanks all.

SCSG-G4 · Jan 20, 2016

Boost means nothing, EGT's must be known! If you go over 1200 F, you will not be running for long! Pistons will melt!!!

patracy · Jan 20, 2016

Sorry Mike, but I have to disagree with that general statement. EGT and Boost BOTH matter. At least to head gaskets.

As to the OP's question. That's kinda impossible to answer without the gauge. Any response you get is speculation at best.

swbradley1 · Jan 20, 2016

Oh, I love guessing games. My guess is that on a long hill your EGT is going up. Do I win?

;-)

porkysplace · Jan 20, 2016

patracy said:
Sorry Mike, but I have to disagree with that general statement. EGT and Boost BOTH matter. At least to head gaskets.

As to the OP's question. That's kinda impossible to answer without the gauge. Any response you get is speculation at best.

Either way the proper gauges are a whole lot less money and work than a new motor.

doghead · Jan 20, 2016

There is a field method to set fuel rate in the lds manual, using only boost measurement.

jimk · Jan 20, 2016

EGT rise as rpm drop (boost drops when rpm drops too). The relationship is very linear and predictable. My LD got a D and was turned DOWN to ~15psi . Says a lot about how much fuel was being wasted. At 15psi my 1971 M35A2 (LD/ White/ turbo) is almost incapable of hurting itself. This applies no matter what I do, nor how high the ambient temp is, or how heavy the towed load. On a few occasions I have seen the temp rise very slowly to around 950, post probe. This has always been at very bottom of operating range. Downshifting drops temp 100*F and pronto . Gauges are important (and recommended) but after running around for years I could confidently run without them.

I also saw this, daily, during my 25y of driving big rigs The early U models had egt gauges. The last 15y all my CH models did not. These all ran ~36pis (intercooled) and were impossible to damage. The only thing that I found that could hurt a Mack engine was the French, they got involved with the company...

Katavic918 · Jan 20, 2016

doghead said:
There is a field method to set fuel rate in the lds manual, using only boost measurement.

I believe that's what the military did. I think stock is 7 psi. At that boost you can pretty much run them as hard as you can without damage.

Katavic918 · Jan 20, 2016

swbradley1 said:
Oh, I love guessing games. My guess is that on a long hill your EGT is going up. Do I win?

You can pick either the stuffed bear or the giant inflatable hammer.

Katavic918 · Jan 20, 2016

porkysplace said:
Either way the proper gauges are a whole lot less money and work than a new motor.

No argument there.

Flyingvan911 · Jan 20, 2016

My deuce with LDS will get up to 12 psi or so and occasionally peak at 13. EGT's are just below 1200. Here's a little report my brother and I made.

Katavic918 · Jan 20, 2016

Flyingvan911 said:
My deuce with LDS will get up to 12 psi or so and occasionally peak at 13. EGT's are just below 1200. Here's a little report my brother and I made.

Good info. Thanks for the report.

rustystud · Jan 21, 2016

jimk; The only thing that I found that could hurt a Mack engine was the French said:
You got that right !

JasonS · Jan 22, 2016

Katavic918 said:
I believe that's what the military did. I think stock is 7 psi. At that boost you can pretty much run them as hard as you can without damage.

The field method is described in the LDS troubleshooting manual: boost pressure is 20" Hg (9.8 psi) at 2400 rpm and 10" Hg (4.9 psi) at 1600 rpm. Is the LDT boost specified in any of the manuals?

74M35A2 · Jan 22, 2016

You guys should drag Wildchild467 into this. He has a turned up pump, a mix of LDT/S parts, and a 15psi Issy boost gauge. When he steps on it, the needle swings around and hits the 0psi rest pin. His EGT can also go past 1250F with no load in the bed or trailer on the truck. Additionally, he also has a ton of blow-by, just rebuilt the engine, and has tons of blow-by again. Leak down rates of 35%, cranking compression of 340#, leaking rear main, and a slipping clutch. Cause so far is TBD..... He is fueling with nearly 100% WMO. He may be interested in some actual experiences here.

breaktime · Jan 22, 2016

Combustion temp is a lot higher than egt. It varies in the chamber depending on where and when. Peak is about 3500*F. It is cooler right next to the cyl wall. Temp drops after the fuel is spent and the cyl vol increases. It drops again as soon as the ex valve opens. A little cooling by port wall, that's EGT. Back to combustion. These hot high pressure gases are not supposed be below the compression rings. If they are they will heat the piston skirt. Hot pistons expand. Too far and they wipe the cyl wall lube and start scuffing. That makes big heat from friction. Friction exceeds power. The piston seizes. Feels like a brake application. In extreme cases a rod will fail or the piston will weld to the wall. If caught it in time, left to cool... they shrink, makes clearance, the motor may turn fine. In most cases aluminum is smeared into the rings grooves forever locking them. What started as bad blow by is now really bad blow by.

Anyways, blow by can overheated pistons. Hot pistons have hot crowns. Hot crowns raise egt.

TsgtB · Mar 24, 2017

My replacement engine is right out of the can (92 refurb Toole).
I even have a print out of the dyno (somewhere)
the FDC came bypassed already (in the can), and feels like they turned the fuel WAY down.
Now with new pyro/boost, I am seeing MAX 6lbs and 900* @ 2200rmp (post turbo).
I plan several test runs before any adjustments, loaded and unloaded on hills.
I wanted to see if there was a known average for # flats to make initial adjustment.?

It now needs a little foot pedal to get running. and slow to spin up. (with fresh batteries).
I'm going to take the cover off my pump and compare it to a pump from the original engine, that
always fired with a bump of the button... ( but had bad head issues). (check fdc rod for lengths)
I'd like it to start strong again, and get her to a safe boost, I dont haul anything now, so it's recreational.
(sorry the meds have taken effect)
Please advise.

patracy · Mar 24, 2017

Since you have gauges, tune by them. I'd turn it up 2 or 3 flats to see how it responds and go from there.

rchalmers3 · Mar 24, 2017

I'll second the opinion that tuning by the gauges is the way to go.

How you use your truck and the terrain you operate in are additional factors in how long you can expect to hold acceleration.

I set mine up to be able to pull the longest hill in my area, with the EGT's maxing at 1300*. All the other pulls I do in my hilly area are shorter and do not create the same increase in temps.

Rick

The King Machine · Mar 24, 2017

I suppose there are two ways to skin this cat.

Set it without the possibility of damage by setting fuel cautiously. If you are planning to let other people drive you truck this might be the best way to go.

I went the other way, the safety margin is managed by the operator.

-FDC bypassed
-Digital EGT and boost gauge.
-Fresh injectors.
-Fresh filters.

Since I did the heads last year and upgrading all of these parts I have put about 2400 miles on it in all types of conditions. Summer, winter, high altitude, sea level, mixed fuels, straight diesel, loaded/unloaded, etc.

In the right conditions I've seen my boost peak at 17-18 psi, I am also able to reach 1300 deg. without trying on hills, so I'd say I'm heavy on the fuel. She sure does purr like a kitten though. Very little blow by and solid compression. Pump Diesel she smokes very little after initial start up. It will burb a small cloud out when I leave the light.

I have noticed variances when I run mixed fuels. The numbers change, EGT's seem to climb faster. I might be mistaken however.

This old girl sucks wind at high Altitude that's for sure!

Safe boost?

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