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Old 08-03-2004, 12:51 PM
Join Date: May 2002
Location: Blue Point, NY
Posts: 25,396

Old300D is correct. If there was an "ALDA" on the wastegate that would sense the decrease in absolute pressure, and thereby allow the wastegate to remain closed until higher boost pressures were achieved, it certainly would be possible to maintain 26.7 psi, absolute, all the way up to 10,000 feet or so.

It's still possible now. Just dial up the boost on the turbo to about 15 or 16 psi or so. Better put a boost gauge in the cockpit, however, and keep the boost below 12 at sea level. You would need a table of the maximum boost allowed at different altitudes to not melt a piston.

Alternately, and probably preferable, you could put an EGT gauge in the cockpit and monitor that.
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Old 08-03-2004, 02:32 PM
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Join Date: Jul 2004
Location: W. Mass / Burlington Vt.
Posts: 132
Originally posted by gnolo
...when I was going down the hill using engine brake, time to time the engine was running jerky and shaking the whole car. I was obliged to accelerate a little bit to stop that. Once I left the moutain, it didn't appear anymore.
I noticed this too, and it was accompanied with white/blue smoke out the exhaust. Any ideas as to what it could have been?
1983 240D automatic, Orient Red, 174K
1983 240D manual, Biscayne Blue, sold at 341K
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Old 08-03-2004, 10:16 PM
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Join Date: Jun 2001
Location: Northern Calif. (Fairfield Area)
Posts: 2,225
If your car has not been altered, the pressure switch on the intake manifold will limit you to 15 lb boost and dump the boost pressure to the pump.


Gearing down doesn't really slow the engine. The lower gearing increases engine speed, but compression still keeps the engine turning over, which is why the Jake Brake was invented. The Jake Brake opens the exhaust valves which relieves compression and slows the vehicle.

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Old 08-03-2004, 10:26 PM
Join Date: May 2002
Location: Blue Point, NY
Posts: 25,396
Well, if 15 psi is the limit, then you can still get an additional 3 psi over the factory limit of 12 psi. This would allow you to achieve sea level power at an altitude of 6200 feet.

You could, of course, bypass the overboost protection valve and get whatever boost the wastegate setting will allow.
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Old 08-04-2004, 12:20 AM
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Join Date: Jan 2000
Location: PA
Posts: 5,440

You don't just "dial up" a turbo. The turbo will pump so much air depending on the design and size of the impellers and how fast it is going. The boost pressure is controlled by the waste gate opening and closing. You can change the waste gate opening for higher pressure, but you can damage the engine that way.

The turbo will keep full boost prsssure at higher altitudes until it reaches its design limits or doesn't have enough exhaust flow thru it to keep it turning fast enough.

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Old 08-04-2004, 08:14 AM
Join Date: May 2002
Location: Blue Point, NY
Posts: 25,396

Maybe I was not clear. To increase the output of the turbo from 12 psi to 15 psi, by "dialing it up" as you say, you would simply adjust the wastegate.

I think we all understand that the wastegate is the only control that the turbo has.

If you are at an altitude of 6200 feet and you were to adjust the factory wastegate setting of 12 psi and raise it to 15 psi, the engine would then put out exactly the same power as it would at sea level. You will not damage your engine. Now, if you return to sea level, and you do not readjust the wastegate back to 12 psi, then you can do some damage.

I cautioned the need for a boost gauge and/or an EGT gauge in a previous post.
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Old 08-04-2004, 08:35 AM
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Location: Milford, DE
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The genisis of your argument seems to be that the turbo system will only deliver atmosphere + 12 PSI (or 15, or whatever the upper limit is). So If you are running you car at sea level and have 12 lbs of boost you would have ATM (14.7) + 12 for a total ot 26.7 absolute. Under the same conditions at 10,000 feet it would be ATM (11-12) plus 12 for a total of 23-24 PSI absolute.

I'm not sure if you are right or wrong but why couldn't the engineers make the operation of the ALDA compensation based upon absolute pressure? Doing it this way would ensure that full boost (26.7 absolute ) would be reached without regard for base atmosphere pressure - why would they have to use atmospheric pressure as the baseline for boost control? It seems designing the system to respond to absolute pressure would be a better way to go and it would improve high altitude response.

I've done quite a bit of high-altitude driving and from my seat-of-the-pants perspective my turbo diesels feel every bit as strong at 9,000 feet (once boost is up) as they do at sea level. I think I could detect a 5-10% power loss.

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Old 08-04-2004, 09:15 AM
Join Date: May 2002
Location: Blue Point, NY
Posts: 25,396

I think you have two things confused.

1) The engineers did provide "aneroids" in the ALDA that certainly do respond to absolute pressure. When at sea level, the ALDA will deliver a certain amount of fuel, with respect to the boost level that it sees. At altitude, the ALDA knows that the absolute pressure is reduced and it reduces the fuel accordingly.

2) The turbo, however, does not know that the vehicle has been driven up to 6200 feet. It puts out its normal 12 psi just like it would at sea level. The 12 psi is added to the absolute pressure at altitude and that is what the engine gets. Now, could the engineers have provided an aneroid compensator for the wastgate? Of course they could. But, the turbocharged engine loses only a bit of power at most reasonable altitudes (see aforementioned calculations) and it certainly would not be cost effective to try and recover that remaining 10% or so.

Now, with regard to your seat of the pants feel that you have the same power at altitude that you do at sea level:

At sea level, you have 14.7 psi absolute pressure, prior to the boost. When you get the boost, you have 26.7 total manifold pressure. You certainly notice this difference in power. It is nearly an 81% power increase. You are quite pleased.

At 6200 feet, you have 11.7 psi absolute pressure, prior to the boost. When you get the boost, you hve 23.7 total manifold pressure. The difference is a rather astounding 102% power increase. You now think that you have the same power as you do at sea level, becasue the kick that you feel is even greater than the sea level kick.

In reality, the vehicle has 12% less power. This would add about 1.5 seconds to your 0-60 times. Think you would notice that? Might not unless you pulled out the stopwatch.
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