[Vwdiesel] Turbo vs. Non-turbo [was My TURBO test (more scientific)]

Tyler "Casioqv" Backman casioqv at usermail.com
Tue Apr 8 05:28:30 EDT 2003

I said correct me if I am wrong, but I still don't think that I was, and
I explained my reasoning below. Feel free to elaborate if I
misunderstood what you were saying, etc. I also disagree that one
shouldn't compare gasoline and diesel engines. I think a lot can be
learned about both by comparing their differences, and that it is
required to determine if you want to use a gasoline or diesel engine.
Such comparisons have caused me to use diesel engines for both towing,
and long distance driving, but gasoline engines for both racing and
short distance driving. This setup has saved me a lot of money, time,
and effort.

 > Not really.  It is dependant on a number of factors.  TIming, fuel
 > boost enrichment, camshaft design which dictates at what rpm your
torque and
 > hp will peak, etc. Mine makes full boost any time you press on the pedal.
 > It's all in the setup.

An engine can't make full boost anytime you press on the pedal. I am
talking about low rpm operation (1500-2300rpm or so). At 1500rpm a VW TD
motor should produce enough power to drive the car, but not enough
exhaust gas to spin the Garrett T3 enough for full boost, especially
without full throttle (which would likely just lug the engine at these

 > Depends on the definition of perform. Quarter mile times or how far
you go
 > on a gallon of fuel. The diesel will win the latter hands down.

Generally when one talks about "performance" with cars they are talking
about acceleration (or possibly handling, braking, etc.). I wouldn't
expect to open a catalogue that says "Performance Parts" and see
products intended to improve fuel economy. The definition of a word is
whatever common meaning the sender and receiver have in mind. Many words
are expected to have different meaning than their dictionary definitions
when talking about cars on a internet e-mail list ;)

 > But that is not the reason why the turbo is there...
 > It all boils down to VE, or volumetric efficiency.  The higher the
VE, the
 > more you can get your engine to do in terms of work and efficiency. If a
 > greater percentage of the cylinder swept volume is filled in a given
 > stroke you have potential to burn more fuel, to make more power, or
to burn
 > the same fuel more efficiently and completely.

You don't think companies put turbos on almost all modern diesel engines
at least partially because it makes them respond more like gasoline
motors? Automotive manufacturers know that how the vehicle drives sells
it a lot more than some power or fuel efficiency statistics on a sheet
of paper. If that's all the turbo were put there for, then engines would
have much bigger and less responsive turbochargers that are more
effective at improving power and efficiency.

 > Other factors at work here.  Cam design, timing, injection timing,
 > etc.  Peak torque in a tdi design cam at 1800 rpm.  Not so with your
 > You have to turn faster to get peak, around 2800 or so IIRC.   The
butt dyno
 > is pretty poor rating system actually. If you do the trailer pulling
 > scenario, if you could get each runnig at the peak torque rpm range, you
 > would be in the sweet spot for efficiency and power.

Those variables are not sufficient to account for the huge difference in
feel between a non-turbo Volvo D24, and a TDI. In fact, I am certain of
it because the turbocharged D24 which has basically the same "Cam
design, timing, injection timing, advance, etc." feels more responsive
than a TDI.

 > Low VE.  Low power.  Horsepower is a factor of RPM, and at higher
rpm, the
 > NA engines don't have a very high VE, so lose out on the power.
 >  You can dyno anything. You get a result in foot pounds of torque.
you can't
 > describe the volks diesel as huge rotating mass either. My 425 cummins,
 > yeah, sure, but not the little volks.
 > Remember horsepower is a function of RPM, so anything that revs
slower, may
 > have lots of torque, but has lower horsepower due to the lower RPM's.
 > Horsepower is a dynamic thing, torque is more like a snapshot in time
at a
 > given rpm, a function of cylinder pressure actually. Think of an itsy
 > F1 race engine delivering 1500hp at some unearthly rpm figure like
 > It has such a high HP figure BECAUSE of the rpm.  Think in terms of there
 > being a series of torque producing events per revolution, and if you can
 > cram more of those into a minute, and measure them you get horsepower.
 > Measure ONE event, you are measuring torque. Design a cam profile and
 > system that can deliver constant flow through 15000rpm, you get a lot of
 > horsepower...

I was aware of all this, but I was just suggesting that a small
dynometer that a car manufacturer such as Volvo might use to test it's
gasoline engine cars, might not put sufficient load on a diesel engine
to measure the full torque that the engine is capable of. A dynometer
usually works by having the cars wheels (or engine directly) accelerate
a large heavy drum, and derives torque from its radial acceleration,
because the mass of the drum is known. If the drum were of sufficiently
small mass, the diesel engines slow rev characteristics would prevent a
proper reading. Take for example, no load on either engine. If both
engines (gasoline and diesel) with the same torque curve are given full
throttle at the same, time, clearly the gasoline engine will reach
maximum rpm quicker; therefore there must be a margin of error that must
be minimized with acceleration drum of very great mass. Many
acceleration drums that I have seen allow the car to accelerate quicker
than it would in real life (especially with a trailer) and could easily
be the cause of this sort of error. I even wonder if the Dynometer at
the DEQ was able to put as much load on my non-turbo diesel as it gets
climbing a steep grade, if it would still pass the exhaust opacity test.

 > No.  Exhaust temp is what you set it at with fueling and timing. It
can be
 > really hot (1300F) or cold (600F) You have less chance of an exhaust
 > overtemp condition in an NA actually. Ther rest is very true.

My understanding is that Gasoline engines tend to have much higher, and
more sustained high exhaust temperatures, but it is less of a problem.
In fact, almost all modern turbochargers on gasoline cars are
water-cooled, while diesel turbochargers are not, but still last longer
because they run so much cooler. I know that if I open the hood on my
Gasoline Turbocharged Volvo after a quick full throttle run the
turbocharger is glowing bright red, but my turbodiesel doesn't even
after towing a trailer at 10psi of boost all day. Why do turbochargers
last longer on diesels, despite the fact that they run in constant boost
(compared to only occasional boost on gasoline engines)? It can't be
because of the corrosive effects of diesel exhaust, because corrosion
isn't what wears turbos out, usually the seals fail, or the bearings
wear out and cause the compressor or turbine to hit their respective

 > No.  They use more fuel.  Nothing is free- it takes fuel to make power,
 > higher VE allows more fuel to be injected, making more power.  Turbos
 > make for a more efficient engine to allow getting more power from the
 > package. Fuel economy is best in small NA engines, and now tdi's due to
 > electronics and direct injection. The TDi is a different cat... How
to put
 > this clearly... The TDI engine has the ability to make more power, due to
 > factors effecting overall dynamic efficiency.  Therefore, it needs
less fuel
 > to handle an identical load when compared to other engines. It has
less to
 > do with the turbo and more to do with other tricks... swirl technology,
 > injection management, camshaft parameters etc...  But if you compare the
 > same engine, turbo, and non turbo, say, a 1.6 d vs td, the first
 > applies.  The turb engine will use more fuel.

With gasoline engines of the same size, the turbocharged one will almost
always get worse fuel economy, but with diesel engines the turbocharged
one will almost always get better fuel economy despite increased power
output. With a turbocharged diesel, the additional air allows for a much
leaner condition, which promotes more complete burning of the fuel, and
therefore more power and fuel efficiency. The turbocharger itself
requires very little additional power from the engine, (from
backpressure) because most of its energy is extracted from the exhaust
heat, which would have otherwise been lost (this increases the
efficiency of the engine). One also saves fuel by accelerating quicker
on energy that would have otherwise been lost, and therefore spending
less time accelerating to speed (despite increased fuel delivery). Many
studies have been done, some of which by Gale Banks Engineering proving
that a turbocharger can increase both power and fuel efficiency on a
diesel engine. My Volvo turbo diesel gets 6.5mpg better than the
non-turbo diesel, and the turbocharged suburban gets 2mpg better than
the non turbocharged one, and even more when pulling a trailer. I'd be
willing to bet that most other list members experience the same thing.
The ECOdiesel has a turbocharger just for increasing fuel efficiency,
and only adds a few horsepower because it has no fuel enrichment. If it
were more efficient to run a diesel without a turbocharger, than why do
virtually all semi-trucks have them, when any savings in fuel that they
could make would go strait to improving profit margins? I also suggest
that most of the improvements that one sees in a TDI over, say a 1.6 NA
motor comes from the very advanced turbocharger that it has. The
turbocharger reduces noise, smoke, increases fuel efficiency, power,
etc. I think a lot of the fuel efficiency also comes from the direct
injection, but it has been shown that a TDI can produce the same power
and fuel efficiency when equipped with mechanical direct injection as
with common rail EFI, and there are a few people on the TDIclub site
that are doing this.


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