I removed the Cat in the exhause a while ago and experienced higher boost levels but I didn't think about any disadvantages.

I got back from a 1200 mile trip Biston- Toronto yesterday wit my SDL.
I experienced a problem with fuel economy (20 mpg now versus 25 mpg before) and I wonder if this comes together with the seriously higher boost pressure during all rpms now compared to how boost was before Cat removal . (I have to say that I also have a new fan clutch in and it was mentioned some economy drop related to this but..).
As this was the first long trip with highway driving after cat removal I wonder if you can help me out here.
The data I can experience today is as follows.
Boost pressure now during highway driving (horizontal highway, constant speed-no acceleration)
today:
6 PSI @ 60 mph
10 PSI @ 70 mph
12 PSI @ 75 mph


data before cat removal (from some older records)
before:
4 PSI at 60 mph
6 PSI at 72 mph

As I was driving mostly 70 -75 mph boost pressure has almost doubled about to what it was before:

Questions
1) Can you explain why boost pressure went up that much after cat removal?
2) It is a fact that I experience these higher boost levels. What does it do to the engine?
It was quite cold during my trip, around zero, so I couldn't figure out if the turbo boost also creates a temperature problem. engine had about 90 degrees constantly.
I know that a diesel runs with a overdose of oxigen or air, but to what extend is this helpful or contradicts economy?
Assuming that the former boost pressure was sufficient for a clean combustion, what are the benefits or drawbacks of a higher pressure?
3) Could it be that a higher pressure creates more inner resistance resulting in bad fuel econonmy
Is there a higher resistance within the engine?
4) Is there any method to reduce boost pressure during constant driving if this is a problem for economy. Is the boost controller I installed a problem?
5) What are 'normal' boost pressure numbers you experience at 60/70/75 mph?
Martin

1: Because the exhaust is getting to the turbo much more efficiently without the TrapOx restricting it.
2: No operational drawbacks from higher boost. Your right foot controls the fuel, not the turbo.
3: Most of the energy used in compression is returned on the rebound stroke. Higher boost should make little difference.
4: You didn't say anything about a boost controller before. THAT's why you have much higher boost pressures.
5: Before my VNT, 8psi/10psi/13psi. With my VNT, 11psi almost all the time.

If you want better economy, slow down. It takes 2x the power to drive at 75 than it does at 55.

Boost signals the ALDA to back-off fuel de-enrichment but that's only to give the engine fuel it can use.

Sixto
93 300SD 3.0

The engine still only uses the fuel it needs to maintain speed.

Sounds about right, your cat was plugged worse than I thought!

I think the removal of mine is one reason my SDL scoots so well. To run the 75mph, you'll need about that much boost you're seeing now.

How much are you seeing at full throttle?

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

I'm not following this logic.

The turbo must spin at a given speed to produce a specific boost level. This boost level is required to push the vehicle through the air at 60 mph. Let's just say that it's 5 psi.......for example.

If the boost is increased to 7 psi, the ALDA will deliver more fuel and the vehicle will accelerate to a higher speed.

Now, if the cat is then removed, the backpressure is reduced, thereby allowing the engine to increase in speed because the load on the engine is reduced. The vehicle would then accelerate.

To control the speed of the vehicle, the pedal would be lifted somewhat and the fuel would be reduced slightly........this should have the effect of reducing the boost level.

The entire concept of higher boost levels with reduced exhaust restriction is counter-intuitive to me.

Yeah something doesn't make sense. Did you change anything else? What else is not stock?

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Because a plugged TrapOx will significantly slow down, expand, and cool the exhaust before it gets to the turbo.

The turbo will make as much boost as it can all the time in a diesel. By removing a significant restriction, it's much more efficient at converting the exhaust energy into mechanical energy to spin the turbine..

It was not that TrapOx that was removed, it was the cat under the car...

A pal of mine had a Turbo Buick about eight or ten years back, the cat was SO plugged up the car would boost about 3psi only and barely run. After we removed it, jumped to 8psi just like it was supposed to... and ran fine.

Didn't the kid from IN with the SDL have a REALLY plugged up cat too?

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

I accept that.

However, with the restriction in place, the engine must consume more fuel and produce more exhaust volume to achieve the required horsepower to maintain a specific vehicle speed. This would produce a specific boost level.

Without the restriction, the engine can produce less exhaust volume (at higher temperature) because, as you noted, it's more efficient at converting the exhaust energy into mechanical energy.

With less exhaust volume, the turbo speed will be lower and the boost level should be lower.

Even still. Slightly different concept, same effect.

If the under car cat was plugged, it would cause high pressure at the turbo outlet. The turbo works on pressure differential. The higher the pressure difference between the turbo inlet and outlet, the more efficient it can be.

Makes sense to me.

Having any restriction anywhere on the exhaust system will lessen the boost. However, I do not understand why your mileage is so low. I regularly cruise 70-75 in my SDL and get around 28MPG

__________________
87 300SDL - 215K Miles !!
99 F-350CC Dually PSD - 190K
86 300SDL - 189K
All on B-100

Head wind, tires, fuels, etc.

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

I removed the Cat underneath the car not the big ball.
Before I cut that thing out I installed a boost controller (the thing with the spring) and adjusted to about 14 PSI max. After I removed the cat I had to turn the controller back a lot because boost went up to 20. Also I removed the ALDA a while ago which makes the car much better off idle but should not influence boost.
I removed EGR and ARV a while ago. New fan clutch. Anything else is according specs.

High pressure: If you tell me you experience the same boost pressure at your SDLs then this calms me down.
I understand that these boost levels are higher than combustion needs.

I understand that a diesel always runs on too much air (compared to a gasser) but if there is enough air at let's say 5 PSI why should the turbo produce 10 PSI, unless it really has no side effect like power loss or increased temperatures.

Someone said the higher boost pressure does not change anything significantly? Economy, Temperature etc. Can you comment and explain what happens exactly in the engine if there is enough air in the chamber for combustion and then you increase the air pressure? I imagine there is more resistance at compression stoke (piston has to work harder to compress the amount of air) but is there a return of this energy?

Does engine temperature change with higher air intake pressures?

Someone also mentioned that he experiences like 28 Mpg at 75 mph and similar boost pressures. I don't know what the difference on my car is and why my car has such bad economy.
I have good injectors (Bosch however- not Bossio), clean inlets, clean air filter, good fule filters. engine should be in perfect shape.

Before removing the Cat I had like 25 mpg at 75mph. Now I experienced 20 mpg.

According also Brian's logic there is less resistance for the engine with a removed cat so the engine should operate more economical and result in better fuel economy. But the opposite is the case.

It was quite cold during the trip and winter diesel, winter tires might have some influence but there was nothing really special.

I put in a 4 inch exhaust on the truck compared to stock 2.5" less boost pressure after installation
On the boat with a new engine installation mechanic will measure boost pressure in exhaust making sure there is no restriction Too much restriction engine doesnt last............

__________________
commercial fisherman diesel in my blood in my boats trucks and Dear old Dad has had me drivin them since i got my first license in 1968
1986 300 SDL 427654
1999 Chevy Crew Cab Dually 225423
1986 300 SDL 287000 Dad's
1987 190 Turbo 158000 Mom's (my inheritance)