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Q: How does the turbocharger work?

Subject:      Re: [RAM] Turbo Operation info .....
From:         Drdonnelly <>

The exhaust gases are split by the manifold into 1,2,3 and 4,5,6 which means that the exhaust pulses are balanced by the firing order of 1,5,3,6,2,4--sequential pulses to opposite halves of the manifold.  The two passages lead into two ports in the exhaust or turbine housing of the turbocharger.  These ports turn in a circle around the shaft of the turbo and also get smaller as they go around.  There are slots all the way around that lead into the center point, to the turbine fins, so all the gases at some point in their rotation around the shaft centerline go through the fins and out the back end of the turbine housing to the exhaust pipe. The temperature differential combined with gas flow spins the fins, and the greater the temperature differential, the faster the turbo shaft will spin.  The rate of spinning is not just airflow related, because boost, related to turbine speed, increases with fueling much more than with rpm.

The turbo shaft is spinning because the exhaust fins (vanes or blades) are turning from the exhaust going through them.  On the other end of the turbo shaft is another set of fins that move intake air, compressing it and pushing it out through a compressor housing.  This housing works in the reverse of the exhaust housing.  This compressed air is the "boost" that goes through the aftercooler and into the intake manifold.

Why use a turbocharger instead of a supercharger driven by the engine or an electric motor?

First, lets look at how much power the turbo compressor is using. These figures came from a Cummins/Holset presentation at a TDR rally:

Generally, 1/3 of the heat energy obtained from burning the fuel in the cylinder is transfered to the crankshaft in a diesel engine. Another 1/3 of heat energy is dumped into the cooling system, and the last 1/3 escapes through the engine exhaust. This means that an engine producing 100 hp at the flywheel also dumps the equivalent of 100 hp into the cooling system and another 100 hp into the exhaust system.

Now, a well designed turbocharger converts 1/3 of the heat and pressure in the exhaust stream into compressor power. This means that an engine producing 100 hp to the drivetrain is using 33hp from the exhaust to power the turbocharger. At 200hp (near full throttle), the turbocharger in a Cummins B can extract 66 hp from the exhaust to power the turbocharger.

So, where does that leave us?
Up to 80 hp (for a 245 hp ETH engine) of relatively "free" power is being used to power the turbocharger compressor. It would take a very large electric motor to produce 80 hp, and power for this motor would have to be produced by the engine alternator, which would consume that much power from the engine to generate the electric power. Likewise, an engine driven supercharger would remove up to 80 hp from the crankshaft to power the compressor.

Bottom line: A turbocharger uses the waste heat to produce boost, without consuming fuel to power the compressor.

Which turbocharger Is On My Truck?
Turbocharger Identification
HX 35 Turbo Housings H1C/WH1C HX35W Turbo HY35W Turbo
PN cast on turbo housing: Turbo housing size (cm2)

& Clamped



3519297 21
3519410 18
3522778 18
3521927 16
3580651 14W
Ram Diesel Turbocharger Specifications    (stock engine)
Year Engine Rating Max Boost Range Wastegate Holset
Turbine Housing
(sq cm)
89-90  160 HP 22-25 psi 22 psi H1C 18
91-92.5  160 HP 15-19 psi 17 psi H1C 21
92.5-93  160 HP (CPL 1579) 18-21 psi 19 psi H1C 18
94  160, 175 HP 15-18 psi  17 psi WH1C 12
94  160, 175 HP 15-18 psi  17 psi HX35W 12
95-98  160, 175, 180 HP 15-18 psi  17 psi HX35W 12
96-98  215 HP 21-23 psi 23 psi HX35W 12
98-00 ETC 215, 235 HP 18-20 psi 20.8 psi HX35W 12
01 ETC 235 HP Auto
and Manual
20.5 psi 23 psi HY35W 9 single port
01 ETC 235 HP Manual 20.5 psi 23 psi HX35W 12
01 ETH 245 HP 20.5 psi 26 psi HX35W 12
Why is the HY-35 turbocharger used on 2001-2002 trucks with an automatic transmission?

A truck with an automatic transmission has to meet soot emissions standards when torque converter slip allows the engine accelerate quickly. This does not usually happen with a manual transmission because for the engine speed to increase, the truck must accelerate - and that takes time. The HY-35 builds boost a little more quickly than the HX-35 does when the engine load is light, so from an emissions standpoint the HY-35 is better suited to stop-and-go driving with an automatic transmission.

FWIW: The HY-35 and HX-35 are functionally equivalent under most engine loads, and the wastegates on both turbos are set for the same boost level. The HY-35 does not breathe as well as the HX-35 at higher speeds under a heavy load. The choke point of the HX35 is around 35psi; the HY35 chokes at about 29psi.

What is the "intercooler" and what does it do?
What is a "silencer ring"?
With Ring
with silencer ring without silencer ring Turbo
Without ring

What is "boost"?
What does the "Wastegate" do, and does it work along with "Boost"?

From Joe D.

The factory uses a small exhaust housing so that boost can be built very quickly, like small tube headers induce exhaust flow faster.  However, the turbo could overspeed and/or the exhaust restriction from small exhaust housing ports could choke the flow and horsepower at higher rpm.  Therefore, a wastegate is used.  A hole, about 1" diameter, is drilled into the side of the rear exhaust housing port.  A flap valve connects this hole to a bypass port leading directly to the exhaust pipe.  Therefore, when the wastegate opens, some of the exhaust flow is bypassed from the turbine fins.  This wastegate helps to limit total boost, because it opens at a preset amount of boost. Enough gas is bled off so that boost effectively is limited to that value (18-23 lb for most B engines).  The higher rpm hp is still choked by the small housing ports, but for emission purposes, it is more important to build some boost very quickly so that fuel can be allowed without smoke.

The wastegate operation is simple.  A pressure line goes from either the compressor housing (e.g. the 180 hp engine) or from the intake manifold (via a T fitting on the AFC housing on top of the injection pump governor) over to a pressure motor attached to the turbo exhaust housing.  The wastegate flap is held closed by the heavy spring in the pressure motor, through a linkage. When boost pressure to the pressure motor overcomes the heavy spring, the linkage opens the wastegate flap.

Can I turn up the wastegate for more boost to increase the power output?

     Turning up the wastegate will increase the air volume that is stuffed into the cylinder. This *may* reduce the EGT, but will not increase power. How do you get more power? One way: more fuel = more power. To increase the power, the fuel rate must be increased. The boost should only be increased to match the fuel curve for smoke and EGT control. Several tests have been done where a truck was run on a dynamometer, the boost was increased by blocking the wastegate signal, and the truck was re-tested on the Dyno. In every case reported, no power increase was measured. 

    The 12 Valve engines do not have a problem with excessive boost.     For improved performance on pre 94 engines, a 16 sq cm turbine housing is usually the best compromise between low speed boost and high speed exhaust restriction.   The stock 12 cm housing on 94+ engines is OK up to about 240 HP, beyond which the small housing restricts the exhaust and raises EGT. A 16 cm housing provides enough low speed boost for an enhanced engine, and the exhaust runs cooler at high speed.

    The engine computer on ISB engines is designed to prevent excessive torque from causing damage to the drivetrain, and monitors the boost for "excessive" power. The computer will reduce the fuel rate (cut back the throttle - it is throttle by wire anyway!) when the boost exceeds 21 psi on 215hp and 235hp engines.

From Cummins:  

Cummins builds engines with the proper combination of turbocharger, pistons, timing, camshaft and injectors to meet EPA regulations. Our Engineers determine what Holset turbocharger will produce all the right air flows and boost pressures to make the engine perform at a certain horsepower and torque and also meet EPA requirements. The EPA fine for modifying an engine that does not meet their regulations is $25,000 per occurrence.

Cummins does not publish turbo boost pressures, however, our data sheet and performance curves show maximum intake manifold pressure at full load and maximum rpm:

Engine Model (Dodge) Intake Manifold Pressure (max)
6BT5.9-160 (12 valve) mechanical 34.0 in.Hg, 13.6 psi
6BT5.9-175 (12 valve) mechanical 38.0 in.Hg, 15.2 psi
6BT5.9-180 (12 valve) mechanical 39.0 in.Hg, 15.6 psi
6BT5.9-215 (12 valve) mechanical 51.0 in.Hg, 20.4 psi
ISB-215 (24 valve) electronic 39.1 in.Hg, 15.6 psi
ISB-235 (24 valve) electronic 40.0 in.Hg, 16.0 psi
ISB-245 (24 valve) electronic 51.0 in.Hg, 20.4 psi

NOTE: Remember that this is with the engine on the test stand, pulling maximum horsepower with dynamometer load making the engine work at 'full load'. It would be very difficult to duplicate maximum load on the engine in the vehicle.

We would recommend monitoring intake manifold pressure at the intake manifold, since that is where you would measure the amount of pressure going into the cylinders. You could do it at the cylinder head but the manifold is the preferred location. Due to charge air and after-cooling components between the turbocharger and engine cylinders, monitoring turbocharger output is not a true indication of air intake manifold pressure.

From Ted Jannetty:

Air fuel ratio has all to do with all internal combustion engines, in the case of Diesels

Everything is a balancing act. Simple test if you have a boost and egt gauge:
Lower boost a little at a time, you will see EGT drop then rise again as you go down, find the point were you have balance for you combo.

How fast does the turbocharger spin?

From the Holset Turbocharger Fundamentals page:

As you put your foot on the accelerator the wheel starts spinning faster. Small Turbos (e.g. Holset H1C's or HX35's) can spin at up to 140,000 rpm, that's 2333 revs per second or over 2000 km per hour.

What is the max. "allowable" boost pressure for 12 valve engines?

The HX-35 and HY-35 turbochargers are rated for a maximum boost of 44 PSI. To safely reach 44 PSI, the turbine housing MUST be sized to keep the choke point above 40 PSI. The 12 cm2 housing chokes at about 32 PSI and the 9 cm2 single port housing chokes at somewhere below 30 PSI, so be careful about pushing the stock turbocharger beyond 30 PSI!

Well, Cummins used to say 30 psi was max, but no one running 35-40 has experienced failures related to the amount of boost, so now they seem to say 35 psi or so is OK!  Of course, the turbo is spinning at or over 200,000 rpm. At any rate, the Rams don't see the upper boost levels and turbo rpm for much of a duty cycle.

From Dave F:   
1 pound of boost for every 10hp is the maximum engine requirement. Overboost does not increase power. Excessive overboost increases the manifold EGT and reduces fuel economy due to the increased exhaust pumping losses. The "choke point" of a turbo charger is defined as the maximum exhaust volume the turbocharge can pass before exhaust back pressure rises drastically. Operating a turbo beyond the choke point is inefficient, drastically increases EGT, and can cause serious damage to the engine and turbocharger.

Good article from Bruce Malinson: "The compressed air or intake manifold pressure enters the combustion chamber and forces out the burned hydrocarbons or exhaust. This process is called scavenging the combustion chamber. Now, what happens when all of the exhaust doesnít get evacuated out past the exhaust valves and through the turbocharger? Simply put if the exhaust backpressure is greater than the intake manifold pressure, the exhaust will enter the intake manifold once the intake valve opens during valve overlap. This is not a good situation and you certainly donít want this taking place in your engine. The results of excessive backpressure are high exhaust temperatures, poor fuel mileage and a tight running engine. What do I mean by a tight running engine? If your engine feels like you always have to push it, your foot is always into the throttle, the engine feels as though its being choked at highway speeds it just doesnít want to glide along the highway, this is what I call a tight running engine. Too small of an air cleaner can also cause this problem, however in this article we are going to concentrate on the turbocharger and exhaust system. In todayís society everybody wants more boost or intake manifold pressure and to obtain more boost you decrease the size of the turbine housing (exhaust housing) of the turbocharger. Now that the exhaust has to pass through a smaller orifice or turbine housing, the velocity of the exhaust increases and the turbine wheel (exhaust wheel) spins faster which in turn spins the compressor wheel forcing more air into the intake manifold. Now all of this may sound good so far however once the size of the exhaust housing is decreased the piston on its upstroke must now work harder to force the exhaust out of the combustion chamber through the turbocharger."

If I have turned up the injection pump, can I simply disable the wastegate instead of adjusting it?

From Ted Jannetty:

You Do need a wastegate for many reasons;

  1. to control turbine shaft speed and keep it at a safe limit
  2. to control back pressure in the exhaust manifold.
  3. to prevent excessive heat in charge air.
  4. to control boost so proper Air/Fuel ratios are maintained.
  5. to prevent HIGH EGT from backpressure.

21 Psi will support 350 hp flywheel with clean tail pipe and EGT well within spec. When you raise boost you raise charge air temp which in turn raises EGT for all reasons listed above.

Every turbocharger is sized to run at a specific boost level, where the turbo can make COOL Boost, when you go above that it makes HOT boost, NO GOOD!!!

So many people are concerned with backpressure when it comes to turbine housing size, BUT NO ONE takes that in to consideration when disabling wastegates.

Don't do it You Don't Need it, it won't make any more power, Probably less.

When my throttle snaps closed, there is a brief whoosh or whistle sound from the air filter area. What is causing this?

When the engine is loaded, very high turbine speeds are required for the turbo charger to provide intake manifold boost.  When the fuel is suddenly cut off, the exhaust volume collapses and the turbocharger rapidly spins down. As the turbine looses speed, it looses it's ability to sustain the manifold pressure and compressed air rushes backward from the manifold and through the turbocharger.  The momentary airflow reversal is not harmful and produces the short pSSHTT sound.

How long should the engine idle to cool the turbocharger bearings?

 This is from the Ram Service Manual

  My cool-down comments -

     -- Dave --

   From the Holset opertaing procedures page:

What is the max Allowable Exhaust Gas Temp?
Engine Load
Exh Manifold
Turbo elbow - Sustained*
Turbo elbow - Peak
Torque Peak 1250° F 810° F 900° F
Rated Power 1250° F 950° F 1000° F
* Aproximately = 1250° - (10° x boost psi)

Exhaust Temp discussion form From the Cummins Mail list

Subject:   Re: 1997 Dodge Cummins Max boost PSI?
From:      Josh Berman <>
Date:       Tue, 22 Oct 1996 10:14:33 -0500
To:          cummins

At 09:27 10/22/96 -0500, you wrote:

> I just installed an exhaust temp, boost pressure and engine oil temperature
> gauges on a 1997 Dodge 215 hp Cummins pickup truck. What are the "normal"
> and Max values, especially for the turbo boost?

Lucky you! Is it your truck, or were you helping out a pal? Here are some #'s for the 215 HP Dodge rating:

Oil Temp: should not exceed 250-260 deg. F at any time (oil tends to break down, drastically reducing its life, if it runs at temperatures higher than 250-260 deg. F)

Joshua Berman             MidRange Service          Cummins Engine Company Cummins    Homepage:

What is likely to be damaged by high EGT?

From Joe Donnelly:

Which cylinder runs the hottest?

Cylinder #1 has an obstruction of the intake port caused by a casting boss for accessory mounting bolt placement plus a forward facing valve port which causes the intake air to run a maze to reach the cylinder. These reduce the airflow to the #1 cylinder, causing it to run richer than the other cylinders. Often, the exhaust manifold gasket has been cooked on cyl #1, while the others are fine.

Subject:     Re: boost and exhaust temperature
Date:         Tue, 4 Feb 1997 10:04:01 -0800
From:        Blaine Hufnagle <>
To:           cummins

At 07:24 2/4/97 -0800, you wrote:
> 1. Max boost is about 25 psi, and exhaust gas temperature (egt) is about
> 1200-1250 degrees (probe in the exhaust manifold, about 0.7" before the turbo
> flange). Are these correct? I have heard about the 25 psi before, but the
> egt is a lot higher than Josh Berman showed for cpl 1550 at 900 in the
> manifold.

This sounds AWFULLY high. Your turbo housing *should* be glowing red hot at these temperatures, as should your exhaust manifold. (Look at them after dark with no lights.)

Mack recommends that exhaust temperatures never exceed about 1000 degrees or so (below the turbo). The ones I've seen, when loaded 12,000 lbs over gross weight, will run "only" about 800 or so, under full boost of 28-30 psi. Mack also recommends that you never exceed about 1050-1100. They say to "reduce engine power or downshift" to bring down engine temperature.

> 2. What egt is safe max for instantaneous, brief, and prolonged periods for
> my cpl?

I'd say Mack's recommendations are pretty close to right on.

> What "safety factor" is built in here?

Absolutely none. Remember that most oil will burn at about 450 degrees (that's with a FLAME.) The turbo is oil-cooled. Even though it has a high flow rate, you can't take that much heat out that fast.

Indeed, the cast-iron turbo housing and exhaust manifold start glowing around 1150 or so. It is worth noting that the crystal structure of iron-base metals changes at 1200 degrees, and as such 1250 degrees (give or take a few) is the post-heat temperature of choice for tempering in the "quench and temper" process. (I can go into the metallurgy if necessary by request.)

This is perhaps the most important reason to have a cool-down period of three to five minutes at idle when you've come in and parked, especially after a loaded run. No-load exhaust temperatures are well under 300 degrees (the lower limit on Mack pyrometers). The cool-down time allows the moving exhaust gas to cool the turbine and housing, and the high oil flow will cool the rest of the turbo down to something a little less resembling Hell itself.

NOT doing so is THE fastest way to coke a set of turbo bearings.

> 3. In December, Josh Berman wrote that the wastegate allows use of a smaller
> turbo housing for faster boost, without raising cylinder pressures too high.
> I have read that the early trucks (1989-90) used an 18 housing, and
> gave up to 25 pounds of boost also. I read the 215 hp uses a "different"
> turbo from the other 96-97 trucks. Is that just the wastegate opening
> setting, or is the housing bigger/smaller?

dunno, but you've got to flow a little more gas (and of course, fuel) to get the torque required.

> As a comparison with the early
> truck's turbo, what would be the max boost from my turbo if the wastegate
> didn't open/wasn't there?

A wastegate simply allows the exhaust gas to bypass the turbine and hit the exhaust pipe directly. I'd assume that the boost available wouldn't change at maximum, as the wastegate is closed at that time, and doesn't effect max pressure, except maybe to limit it at times when it's not really needed (such as coming down from speed). But I'm not sure as to the method of operation on how it actually does this.

I was always under the impression that wastegates allowed the use of a smaller turbo (to increase spin-up rates) while keeping maximum pressures manageable at high-speed. Can someone correct me?

> I gather the housing on my turbo is smaller than
> 18 to bring in boost earlier. If the wastegate sticks open, I suppose
> I would get no boost, and egt would be too high (?) and power low.

EGT won't go up unless boost goes up. There's a direct correlation between EGT and engine load. There's also a correlation between engine load and manifold pressure. Since you won't be getting manifold pressure high enough to fully load the engine, you won't ever get the EGT up high enough to worry. (or so I thought.)

> wastegate stuck closed, and boost climbed (to what pressure?), at what point
> would a problem develop, if any?

I'd imagine you'd be safe to 30-35 PSI or so... Although I wonder if you'd get that high. The Mack I drive at work is equipped with a boost gauge, and the highest pressure I've ever seen peaks at just under 30 PSI. Note that this is loaded to over 90,000 lbs and going up a grade. Normal cruise boost runs a "sedate" 12 psi or so. (note: this is on a Mack E7-350 V-MAC.)


Subject:   Re: boost and exhaust temperature
Date:       Tue, 4 Feb 1997 13:57:31 -0800
From:      Blaine Hufnagle <>
To:          cummins

At 14:06 2/4/97 -0500, you wrote:

> my service manual ('90) says max exhaust gas temp of 1290F... and max
> manifold pressure of 26psi. it, however, does not state where the
> *STANDARD* measurement location should be...

Mack's MAP sensors are a hole drilled in a seemingly random location in the manifold, although I've seen the sensors usually placed alongside the #4 cylinder.

Mack places the pyrometer thermocouple about six inches DOWNSTREAM of the turbo.

> a conservative operator would do well to heed Blaine's comments above...

You might be surprised to find out that only a half a dozen or so fellow drivers (out of about 50) follow these suggestions at work; most of them charge full throttle into the yard, then immediately shut down. God only knows what kind of damage is being done.

Even though said practice is actually mandated by stickers in the cab...

(I learned said tactics when driving a turbocharged John Deere as a youth.)

>> (the lower limit on Mack pyrometers). The cool-down time allows the moving
>> exhaust gas to cool the turbine and housing, and the high oil flow will cool
>> the rest of the turbo down to something a little less resembling Hell itself.
>> NOT doing so is THE fastest way to coke a set of turbo bearings.
>> again, Blaine has echoed my practice and is well to be heeded...

Also probably why the failure rate of automotive (i.e. sports car) turbos is so high... Scream into the garage and immediate shutdown.

For those of you who do not desire to babysit your vehicle for three to five minutes after running, there are idle timers available to do the job for you.

>> EGT won't go up unless boost goes up. There's a direct correlation between
>> EGT and engine load. There's also a correlation between engine load and
>> manifold pressure. Since you won't be getting manifold pressure high enough
>> to fully load the engine, you won't ever get the EGT up high enough to
>> worry. (or so I thought.)

> Blaine, are you sure on this one?

Actually, I seem to have stuck my foot in my mouth. :-)

Boost won't go up until EGT goes up, by definition: The turbo extracts the energy to provide boost from the heat in the exhaust. If there's no heat, there's no boost.

> it *seems* that with no appreciable manifold pressure, the governor will be trying
> to give full-fuel and egt would skyrocket...

All the extra fuel will do is make soot. As manifold pressure increases, soot will decrease. Under the conditions you specify, the fuel isn't being burned completely, thus maximum heat isn't being generated. The exhaust won't start to get hot until the boost starts to come up; it's a process that feeds itself. The low-MAP EGT will come up slowly, thus giving the turbo the energy it needs to function, but it won't really get started until the engine can produce enough gas flow for the thing to behave itself (around 1100 or so RPM in most industrial motors.)

> really-California-ish)... and you'll soon overload the engine, if manifold pressure
> never builds

Well, I'd assume anyone smart enough to drive one wouldn't leave an engine in this condition long enough to overload the engine; they'd change gears (or the tranny would do it for them, in the case of a slushbox) and get the engine into an RPM range where boost and power can be better utilized.

In older engines where smoke limiters aren't used, this condition often winds up hard-lugging the motor, great clouds of sulfurous smoke, but bringing it up to speed pretty fast.

> (it'll behave like an unaspirated engine with an intake and exhaust restriction,
> won't it?)

Probably. But remember that at low RPM's, those restrictions are actually your friends, as they keep flow velocity high and thus increase your combustion potential.

> my feeling here is that the hoses leading into/out-of the aftercooler
> (in the Dodge application) would blow-off the fittings before
> engine-damage would occur

I dunno... I'd imagine they could safely handle at least that much... Also worth noting is that you could blow intake/exhaust manifold gasketry....

> (you *sure* do not want to have manifold pressure great enough to unseat the
> intake valves... think about exhaust valves and exhaust brakes, too... :) that's why
> you need heavy-duty exhaust valve springs with an exhaust brake...)

I'm not sure here... There's twice as much valve motion when an exhaust brake is in action than when not in action; I think the heavier valve springs are to prevent valve floating, not hold exhaust side pressure.

Or are you talking about an Extarder(tm) versus a Jake Brake?


Subject:   EGT & Boost
Date:       Thu, 27 Mar 1997 11:43:48 -0500
From:      Josh Berman <>
To:          cummins

Hi everyone,

This is getting back to a thread that was raised at the beginning of February... what are the "limits" on EGT and Boost? Well, we typically quote EGT specs measured a couple of inches downstream of the turbo (after the turbine wheel), because this allows the thermocouple to be in a cooler exhaust stream (=longer life), and it is also safer for the turbo (in case the probe breaks off, the turbo doesn't eat a chunk of metal at 90,000 rpm).

That being said, the limit for CPL 1550 ('95 manual, 175HP @ 2500RPM, Chrysler rating) is 900 degrees F, and the limit for CPL 2023/2175 ('96/'97 manual, 215HP @ 2600RPM, Chrysler rating) is 950 degrees F. It's not that we're that concerned about the temperature of the exhaust after the turbo; we're interested instead in what we call TIT (Turbine Inlet Temperature). If this gets too high, then you start to think about damage to manifolds and turbos. Since turbos typically drop 200-250 degrees F across the turbine, and the limit for CPL 2023/2175 is 950 F, the max TIT you should see is right around 1200 F.

Now, that's not to say that your EGT CANNOT exceed 950F, but if it does, it shouldn't be for long. Short excursions to high temperature are unlikely to damage the turbo, but if you run at high temps for a long time (tens of minutes), you're probably asking for trouble in the long run.

Joe asked about what would happen if his wastegate stuck closed/open. If the wastegate stuck closed, you'd get a LOT of boost at rated. I don't know exactly how much, but it's a lot. You would also overspeed the turbo, which might lead to turbo failure. However, if the wastegate stuck open, you would be in much better shape (so to speak). If you look carefully at your fuel pump, there is a small line running from the manifold to the fuel pump. This is the AFC (Air Fuel Control) line, which allows the fuel pump to compensate for boost. We have to have this because if you floor the pedal @ idle (no boost), and the fuel pump delivered all the fuel it would when you floored the pedal @ full boost, you'd get a lot of black smoke. So, when the boost is low, you get what we call "No-Air Fuel". As the boost comes up, you get more fuel. If the wastegate stuck open, you'd never build boost, so you'd never get off of no-air fuel, and you'd have low power.

You can have a similar problem if the AFC line gets a hole in it. You will still build most of the boost (though some of it will leak out of the hole in the AFC line), but you won't get more than "No-Air" fuel, so you will still have low power.

As far as the old/new turbos go, there is difference in boost: for example, the '93 160HP engines have a max boost of 35 inHg (17 psi), while the newest '97 215HP have a max boost of 51 inHg (25psi). The new trucks do use a new design turbo, but I don't know exactly why it was changed (some serviceability, some performance, some ???).

I hope this is interesting. Feel free to post any questions; I'll try to answer them in less than the 1.75 months that this series of questions took :-)

-Josh B.

Joshua Berman        MidRange Service           Cummins Engine Company      Cummins Homepage:



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