I've read a number of times that on turbos the ALDA is only active in the pre-boost mode.

Are people certain this is correct?

I can operate my car such that the engine is at ~2200 RPM and I have 2" of boost or I can operate my car such that I have ~2200 RPM and 10" of boost. If the ALDA is not involved in fuel metering once the boost kicks in how does the pump know to deliver a greater quantity of fuel at the 10" of boost over the 2" of boost?

I'm just trying to understand. I don't claim to have any great wisdom on the matter.

Thanks.

If it was only active pre-boost, then why don't all the NA cars have one?

AFAIK, the ALDA measures boost, and limits fuel until boost catches up to accelerator input.

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The ALDA "Allows" the Fueling Rate to increase BASED upon the Boost PSI it "Senses".
[To put it the reverse:If the ALDA Doesn't "Sense" the Boost...Fueling is Limited
to Normal rates. You can have 35 P.S.I. of Boost and get NO Increase in "Power
Made" without the ALDA's allowing Fueling rate to Increase.]

Which is why: If the ALDA system's Vacuum/Pressure Plumbing is Kaput/Clogged
NO Acceleration Increase/Multiplier response to increased Boost.

Gotta Have: Increased Boost AND INCREASED FUELING to "Scoot".

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'84 300SD sold
124.128

If I follow you then what you say makes sense. Which means the ALDA is involved in the operation of the boost.

They do.

If you take the ALDA off you still get boost. I suppose it still is involved.

whether you have boost or not is independent of the ALDA. The ALDA responds to boost pressure and based on boost pressure increases fuel rate.

NA engines can have ALDA, but the signal for the operation does not come from boost pressure, it has to come from elsewhere. These are often called partial load distributed advance mechanisms and can be controlled by an altitude sensor or some such device.

this is my story and I'm stickin to it.

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Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others

I think stating that the alda adds fuel with boost is going a little to far, all the alda is capable of doing is pulling fuel away by pulling back on the rack (you can do the same thing with your foot) it cannot push the rack further forward.

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1996 s10 zr2 with a 6.5 turbo diesel
1996 c2500 suburban with a 6.5 turbo diesel
1981 300sd with a turbo 617
Monte carlo figure 8 car

NA's don't have ALDA's, they have ADA's. The ADA senses altitude or ambient air pressure.

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1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
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1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

"If I follow you then what you say makes sense. Which means the ALDA is involved in the operation of the boost."

1.ONLY in the sense that the ALDA allows more Fueling (Or Retards it) in response to the Boost
,Which creates More power and RPMS (Or Not).
[More RPMS Usually means More Boost,BUT you can "get put into a corner"
trying to explain all the VOODOO involved with Turbine/Compressor Flow Rates. ]

__________________
'84 300SD sold
124.128

Here's some reading! http://www.peachparts.com/shopforum/showthread.php?t=203907&highlight=alda+function

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92 300D 2.5L OM602 OBK #59

BBDC,

I appreciate it.

__________________
'84 300SD sold
124.128

I understood the ALDA to be involved both in altitude compensation and boost response. When I read numerous postings that say, "The ALDA is NOT involved once the boost kicks in ..." I become confused.

Reading the link that BBDC provided confirms my thoughts that the ALDA is involved in boost operation.

Thanks.

ALDA is also the means for administering overboost protection to the IP. It will reduce fuel when the pressure switch exceeds 1.2 bar, down to the level of a normally aspirated car.
But this would be a very rare event and may never happen in your lifetime with the car... it's there to protect broken wastegate issues.

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Cheers!
Scott McPhee

1987 300D

I gather it'd be more accurate to say it restricts the fuel until the boost comes on. But the effect is the same -- more boost = more fuel.

fuel rate rises with boost to a point. Im not sure where that relationship ends (im sure it is dependent on adjustment) sorry i cant help, time for someone smarter than me.

__________________
1996 s10 zr2 with a 6.5 turbo diesel
1996 c2500 suburban with a 6.5 turbo diesel
1981 300sd with a turbo 617
Monte carlo figure 8 car