Today's Message Index:
----------------------
1. 06:09 AM - Crossover exhaust vs straight pipes (Gene Smith)
2. 06:52 AM - Re: Corvair (Gary K)
3. 07:08 AM - Turbo VS.. Supercharger again (Alex Balic)
4. 07:13 AM - Re: ram air (Gary K)
5. 07:16 AM - turbo vs. supercharging (Gary Casey)
6. 07:16 AM - Re: Exhaust plume drag (Gary Casey)
7. 08:01 AM - Re: Crossover exhaust vs straight pipes (James R. Cunningham)
8. 08:30 AM - Re: Re: Exhaust plume drag (James R. Cunningham)
9. 08:32 AM - Re: ram air (James R. Cunningham)
10. 08:43 AM - Re: Turbo VS.. Supercharger again (Randy)
11. 11:26 AM - Re: ram air (klehman@albedo.net)
12. 04:07 PM - Re: turbo vs. super (Doug Dodson)
13. 07:38 PM - Re: Crossover exhaust vs straight pipes (Archie)
Message 1
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| Subject: | Crossover exhaust vs straight pipes |
--> Engines-List message posted by: "Gene Smith" <esmith6@satx.rr.com>
Since everyone is into the exhaust subject, I need some advice/info/opinions on
straight pipes vs a crossover system...I have an RV-4 (160 Lyc) with Alan Tolle
crossovers and I'm thinking about going to Larry Vetterman's straight pipes...I've
heard that the straight pipes give as much or more back pressure than
the crossover, which doesn't seem logical to me, but may be correct?..Any info
would be appreciated..........................CHEERS!!!!.......................Gene
Smith.
Message 2
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--> Engines-List message posted by: "Gary K" <flyink@efortress.com>
isn't there a Corvair list already somewhere? Just making sure you knew
that, there are plenty of people building and using Corvairs there (wherever
that is - Yahoo?).
Gary
----- Original Message -----
From: "k.jones" <kevin-jones@snet.net>
Subject: Engines-List: Corvair
> --> Engines-List message posted by: "k.jones" <kevin-jones@snet.net>
>
> I'd appreciate anyone who is preparing (or has prepared) a Corvair engine
for aviation use getting in touch with me off the list at
kevin-jones@snet.net.
> Do not archive.
>
>
Message 3
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| Subject: | Turbo VS.. Supercharger again |
--> Engines-List message posted by: Alex Balic <alex157@direcway.com>
I looked into both supercharging, and turbo charging the SVX motor that I
have installed in my Velocity. Both cost about the same to install, both
would be able to be designed for the same boost at takeoff, but the problem
that I came across with the supercharger installation is that it is
(obviously) connected directly to the crank shaft of the engine, and has a
fixed drive ratio. This fixed drive ratio means that at maximum engine RPM
you will get maximum boost from the Supercharger. This is not a problem
with boats and cars, since they operate at sea level generally, but for an
aircraft, it creates a problem with system design, since in order to obtain
full boost at altitude, you will need to dump boost at sea level, or use
some type of transmission/slip clutch arrangement to de-rate the
supercharger at higher ambient pressure of sea level. I investigated as
many options to do this as I could find, but finally decided that all of
them would be a lot less reliable than a turbocharger/waste gate system. My
current Garret T3 hybrid system will produce 4 pounds of boost up to about
12,000', and drop off as a normally aspirated engine would at higher
altitude, and at sea level, the turbo is simply throttled back by bypassing
it with the waste gate. I have an intercooler to control the inlet charge
temperature, but I would have used one either way, so that creates no
difference. One interesting point I need to investigate is the suggestion
that I received recently, that turbo over speed might be possible, because
at some point, the exhaust will be driving the compressor against reduced
resistance due to low ambient pressure at high altitude, I have been trying
to research this effect with no avail, - anyone out there have any
information/experience with this?
Message 4
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--> Engines-List message posted by: "Gary K" <flyink@efortress.com>
Looks like a good list here, there seems to be a lot of experience. I'm
planning on trying a ram air intake for my subaru with Bing carbs - any
tips? I'm not sure whether to try a scoop or a NACA duct on top of the
cowling. I've seen a small scoop used on the MCR01's with Rotax 912 and
they claimed at least a few % power increase. I'm not doing it just for
the power increase, my water-cooling system could use more air thru the
radiator so I'm thinking of modifying the cowling again and this is one of
the options. By providing a separate air source for the intake, I could use
more (all) of the existing cowl inlets for the radiator. Just curious if
anyone has any experience with this.
Thanks,
Gary
Message 5
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| Subject: | turbo vs. supercharging |
--> Engines-List message posted by: "Gary Casey" <glcasey@adelphia.net>
This is a great discussion with lots of inputs, so it's easy to lose track
of who said what when. Here's some more comments:
I said:
<<In the pressure ratio ranges we are talking about
>a well-matched turbocharger compressor section could operate at 80 to 85%
>efficiency, while I would bet that a belt-drive centrifugal compressor
would
>be about 70 to 80%. A non-compressing blower has an efficiency of about 60
>to 70%.
I forgot who said:
<<I completely disagree. A turbocharger and a supercharger using the same
type of compressor will have exactly the same efficiency, there's no magic
attributed to the source use to turn the pump.>>
I'll have to disagree with your disagreement. Just because two devices are
of the same "type" doesn't mean they have the same efficiency. The
turbocharger compressor section has had the benefit of being under
large-scale development for many, many years - more money spent doesn't
guarantee results but it helps. It also benefits from being able to be
driven at higher rpm's. Even though it wouldn't have to be more efficient I
think you'll find that the units available are somewhat more efficient than
belt-drive units. That being said I'll admit I don't have compressor maps
of belt-drive superchargers. Perhaps someone on the list does and could
share one.
On the subject of inlet heating due to conduction from the exhaust side,
yes, that is a problem but not a big one. The oil-cooled center section is
the primarily isolation and a good design may include a radiation shield
around the turbine housing.
Someone said, talking about exhaust pipe breakage on a turbo compared to
belt breakage:
<<These two failures are radically different. One can easily kill you and
everybody aboard, the other is an annoyance.
If the belt breaks on a super charger, you loose boost, you fly slower to
your destination....>>
On this I tend to agree - the biggest problem with exhaust pipe breakage on
turbo engines is the fire hazard, not the loss of boost. On the other hand,
a well-designed turbo exhaust system will be very, very unlikely to fail
catastrophically. It will much more likely crack first and be detected.
Since the exhaust system is pressurized a crack will produce more leakage
than on an NA engine and therefore pose a CO intrusion issue, if not a fire
hazard from the jet of exhaust gas. A counterpoint - a belt failure on a
belt-drive supercharger is probably much, much more likely to occur simply
because there hasn't been as much development and the belt is highly loaded.
And it may take out the alternator at the same time, creating two failure
modes at once. Is that a big or little deal? Don't know.
Another comment degraded the "complexity" of the drive system on a turbo.
It is basically very simple and the whole system has only one moving part.
the complexity is in the plumbing, not the drive system. The supercharger
may look like it has a simple drive, but all the gears and oiling system
components are hidden from view. Even the bearings that support the
compressor wheel are more complex. The turbo usually uses a simple sleeve
bearing that has quite a bit of clearance, while the supercharger, because
of the accuracy required to hold the gears in alignment, requires a ball or
roller bearing support.
What I would like to see is a belt-drive supercharger with a scroll design
that allows a straight-out departure of the air - I think that would
measurably improve the efficiency by allowing velocity recovery. Also, how
do you mount the compressor up against the front cylinder of the engine
without squashing the inlet to the compressor?
There was also a post about the desirability of forced induction in general,
implying that any designer would be nuts not to include it. There are lots
of tradeoffs in design, mostly relating to cost and performance.
Performance means efficiency, power output, weight, reliability, etc., not
just power output per displacement. It might be a very rational choice to
just increase the displacement of the engine, leaving it NA rather than
adding the complexity of forced induction. Even in aircraft applications
that is a tempting thing. Since the passengers are (usually) not
supercharged most flights are done below 10,000 feet. Because of the
tradeoff between power and reliability most engines are cruised at 75%,
which can be obtained without forced induction up to about 8,000 feet. The
NA engine and passengers are fairly well matched and coexist fairly well.
Push the operating envelop higher, though and forced induction becomes
almost mandatory as even if the aircraft could go higher the owner (the one
that makes the $$ tradeoffs) is not likely to be happy with the reduces
speed. That's why I want to include forced induction in the design.
Gary Casey
Message 6
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| Subject: | RE: Exhaust plume drag |
--> Engines-List message posted by: "Gary Casey" <glcasey@adelphia.net>
<<As you know, the only way that the plume once exited can transfer drag
loads back into the airframe is through the shear strength of the plume
at the tailpipe exit. What is the shear strength of that plume?>>
Not so. Even though the plume (we're talking about the exhaust gas stream
after it exits the pipe) is disconnected from the airframe it still has an
effect on airframe drag. It obstructs the flow of air just as though it
were a solid pipe, until the airstream bends it into conformance. This
"obstruction" increases the pressure of the air ahead of it, increasing the
upstream pressure for the airframe as well. That is just the same as when
two cars are driving side-by-side: The drag of each is more than if it were
driving alone on the road. If you turn the pipe to the rear how do you
separate the effect of reduced drag and jet thrust? No way to do that as
far as I know, except by engineering calculations. This would run counter
to the arguments of some that have said that empirical data is the only
proof possible. My position is almost the opposite - if the result can't be
explained or predicted by engineering principles it is suspect. I did a
thrust prediction (can't find the results now) for 300 hp and found two
things - the contribution is very small, and the optimum exhaust pipe
diameter is not far from the typical one used. Conclusion: Point the
exhaust pipe rearward and use normal practice - it isn't worth the effort to
get much more involved in the subject.
Gary Casey
Message 7
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| Subject: | Re: Crossover exhaust vs straight pipes |
--> Engines-List message posted by: "James R. Cunningham" <jrccea@bellsouth.net>
Other things being equal, I would expect straight pipes to result in
more backpressure than a good crossover system, mostly because of the
lack of scavenging in the straight pipe system. But be warned, the
above is a gross oversimplification. If Archie chimes in on this, pay
more attention to him than to me.
JimC
Gene Smith wrote:
>
>...I've heard that the straight pipes give as much or more back pressure than
the crossover, which doesn't seem logical to me, but may be correct?.
Message 8
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| Subject: | Re: RE: Exhaust plume drag |
--> Engines-List message posted by: "James R. Cunningham" <jrccea@bellsouth.net>
Gary Casey wrote:
> It obstructs the flow of air just as though it
> were a solid pipe, until the airstream bends it into conformance.
Of course it does, but the energy required to decelerate the flow has no
way to impact the airframe unless the wake directly impinges on the
airframe at some point downstream. I don't buy your following assumption
of the magnitude of extensive changes in the upstream pressure gradient
(more follows).
> This "obstruction" increases the pressure of the air ahead of it, increasing
the
> upstream pressure for the airframe as well.
Total pressure, static pressure, or dynamic pressure? How much lateral
spread in the increased pressure and how do the components tranfer?
Have you mapped it with pressure probes to confirm the extent? What
effect does pressure recovery have on it? At the range of Reynold's
numbers involved, how substantial is the contribution of aft pressure
recovery in reducing that drag?
> If you turn the pipe to the rear how do you
> separate the effect of reduced drag and jet thrust? No way to do that as
> far as I know, except by engineering calculations.
Engineering calculations are far and away the easiest way to do it, and
I highly recommend that anyone interested in the subject do them.
Combine a differential drag count and the jet thrust calculations to
obtain a total. It will be reasonably accurate. It's pretty basic
stuff, but for those who aren't familiar with it, Barnes McCormick's
text gives a good description of how to go about doing a basic drag
count, using a Cherokee 180 as an example (note that there are typos in
a couple of the listed drag coefficients).
> This would run counter to the arguments of some that have said that empirical
data is the only proof possible.
I agree. These may be the same folks who believe that a 747 or Airbus
should be designed and built by trial and error.
> My position is almost the opposite - if the result can't be
> explained or predicted by engineering principles it is suspect.
I agree.
> I did a thrust prediction (can't find the results now) for 300 hp and found two
> things - the contribution is very small, and the optimum exhaust pipe
> diameter is not far from the typical one used. Conclusion: Point the
> exhaust pipe rearward and use normal practice - it isn't worth the effort to
> get much more involved in the subject.
It usually amounts to about 2% to 5% of the total thrust at typical
cruise speeds and for typical engines and props. Enough usually for
about 2 to 3 knots if you ignore the tailpipe outlet. Optimising the
outlet for desired crossover altitude can add another 1 to 2 knots. I
suggest making three seperate slip-on outlets with different opening
reductions, which will give you 4 easily available combinations that
take about 5 minutes to implement on the aircraft as a ground
adjustment. Since the input for the calculations takes only a minute or
so, and the computer spits out the results in a fraction of a second,
what effort is required to run the numbers?
Jim
Message 9
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--> Engines-List message posted by: "James R. Cunningham" <jrccea@bellsouth.net>
If you use a NACA duct, be sure it is located in a high pressure area.
Gary K wrote:
> I'm not sure whether to try a scoop or a NACA duct on top of the
> cowling.
Message 10
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| Subject: | Re: Turbo VS.. Supercharger again |
--> Engines-List message posted by: "Randy" <rnvcrothers@comcast.net>
Alex,
Have you looked into the following approach? A 1 1/2" butterfly valve
is installed between the SC and engine to act as a waste gate. The valve is
actuated by a servo like you see in electric trim and autopilot systems.
The servo motor is controlled by a programmable controller with feedback of
the pressure in the charged section of the intake. Since there are still
situations that could result in overboosting, (failure of the servo, very
rapid throttle closure, etc), a pressure relief valve is also included in
the charged section. The result is a fully automatic supercharger
operation. The valve is adjusted as needed to maintain the set pressure up
to the critical altitude.
The drive belt is my biggest concern with the superchargers. There has
been one instance that I know of in this design where the SC belt took out
the alternator belt and the timing belt resulting of course in sudden quiet
engine syndrome. From what I have seen, it takes a large crankshaft pulley
and a small SC pulley to get the needed ratio. This results is a fairly
small contact area for the SC pulley and perhaps an even more critical
pulley alignment situation. A wider belt and fences to trap the belt in
place were added to the design mentioned to solve the problem. Belt
tensioners are being considered but they require another bearing and other
complications that for now are being avoided.
The absolute worst flying nightmare I can imagine is an in-flight fire
at high altitude, for this reason I prefer supercharging. If I were to
install a turbo charger I would also want a GOOD fire suppression system to
go with it.
Randy
----- Original Message -----
From: "Alex Balic" <alex157@direcway.com>
Subject: Engines-List: Turbo VS.. Supercharger again
> --> Engines-List message posted by: Alex Balic <alex157@direcway.com>
>
> I looked into both supercharging, and turbo charging the SVX motor that I
> have installed in my Velocity. Both cost about the same to install, both
> would be able to be designed for the same boost at takeoff, but the
problem
> that I came across with the supercharger installation is that it is
> (obviously) connected directly to the crank shaft of the engine, and has a
> fixed drive ratio. This fixed drive ratio means that at maximum engine RPM
> you will get maximum boost from the Supercharger. This is not a problem
> with boats and cars, since they operate at sea level generally, but for an
> aircraft, it creates a problem with system design, since in order to
obtain
> full boost at altitude, you will need to dump boost at sea level, or use
> some type of transmission/slip clutch arrangement to de-rate the
> supercharger at higher ambient pressure of sea level. I investigated as
> many options to do this as I could find, but finally decided that all of
> them would be a lot less reliable than a turbocharger/waste gate system.
My
> current Garret T3 hybrid system will produce 4 pounds of boost up to about
> 12,000', and drop off as a normally aspirated engine would at higher
> altitude, and at sea level, the turbo is simply throttled back by
bypassing
> it with the waste gate. I have an intercooler to control the inlet charge
> temperature, but I would have used one either way, so that creates no
> difference. One interesting point I need to investigate is the suggestion
> that I received recently, that turbo over speed might be possible, because
> at some point, the exhaust will be driving the compressor against reduced
> resistance due to low ambient pressure at high altitude, I have been
trying
> to research this effect with no avail, - anyone out there have any
> information/experience with this?
>
>
Message 11
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--> Engines-List message posted by: klehman@albedo.net
Try to pay attention to the pressure distribution aroung the cowl. For a
conventional single engine Cessna type airframe, the top of the cowl
from the prop to about half way to the windshield is usually a low
pressure area more suited to an air exit than an intake. Closer to the
windshield tends to be a high pressure area same as for a car.
I would guess that inside the cowl on the pressure side of any baffles
would be as good an air source as any in most cases, as long as it is
before any radiators or exhaust heating.
Ken
Gary K wrote:
> --> Engines-List message posted by: "Gary K" <flyink@efortress.com>
>
> Looks like a good list here, there seems to be a lot of experience. I'm
> planning on trying a ram air intake for my subaru with Bing carbs - any
> tips? I'm not sure whether to try a scoop or a NACA duct on top of the
> cowling. I've seen a small scoop used on the MCR01's with Rotax 912 and
> they claimed at least a few % power increase. I'm not doing it just for
> the power increase, my water-cooling system could use more air thru the
> radiator so I'm thinking of modifying the cowling again and this is one of
> the options. By providing a separate air source for the intake, I could use
> more (all) of the existing cowl inlets for the radiator. Just curious if
> anyone has any experience with this.
>
> Thanks,
> Gary
Message 12
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--> Engines-List message posted by: "Doug Dodson" <dodsond@qnet.com>
Actually, there is much talk in this tread about the power increase at sea
level. There, the difference between the turbo-supercharger and crank
driven supercharger have been well covered.
At altitude, on the other hand, not much has been said. Fact is, at
altitude, the turbo-supercharger can actually create the desired amount of
boost without additional power extraction because the backpressure at the
exhaust outlet drops as the static pressue at altitude drops. The wastegate
is opened at high alitiude because the turbocharger would create too much
boost if not controlled.
The altitude performance of the turbo-charger is far superior to the
altitude performance of the super-charger in every respect. The systematic
issues of heat management in the intake system are identical for both
systems. The heat management issue in the exhaust system is the only realy
disadvantage to the turbo system at altitude.
The fuel consumed to drive the turbo-charger resulting from loss of
efficiency due to exhaust back pressure is quite insignificant when compared
to the fuel required to recover power extracted to drive the super-charger
directly from the crank. Total cost of ownership will depend largely on how
the turbine components in the exhaust stream are designed and operated. In
current typical applications, this is very mush a pilot attentiveness and
workload issue, not so much in cruise flight as in descent, landing and
shutdown.
Douglas L. Dodson, Jr.
Glasair II-S FT
Flight Test Engineer, CFI-A,G
-----Original Message-----
From: owner-engines-list-server@matronics.com
[mailto:owner-engines-list-server@matronics.com]On Behalf Of Scott
Subject: Re: Engines-List: turbo vs. super
--> Engines-List message posted by: Scott <scott@tnstaafl.net>
Another shameless theft from teh same email list
The intention with this thread is to shed some light on the theories behind
supercharging the modern engine, and the wonderful advantages it gives us
in both speed and basic engine design issues. This post is also to try and
give people a better understanding of supercharger systems and how they
work to make such a simple thing as an engine even simpler (speaking in
terms of tuning). I personally am tired of seeing people bicker over the
differences between the two basic designs, and would like everyone to note
the vast similarities among them instead. So without further delay... I
will start with this simple statement:
"It makes no sense to build a naturally aspirated engine if what you are
looking for is good power output."
To understand why this is so obviously true, we need to break the concept
of an engine down into its simplest parts. There are 4 main parts to an
engine that concern airflow:
-the induction system
-the cylinder head(s)
-the exhaust system
-the camshaft(s)
Now if you think of the engine in terms of airflow and forget about fuel
for a minute, it becomes a very simple matter really. What we want to do is
best flow air through the cylinder head, from the induction system to the
exhaust system and then out into the world again. This is best & most
naturally accomplished by pressure variation, because as almost anyone with
a high school education knows, air naturally flows from areas of higher
concentration (pressure) to lower concentration (pressure). Now let's
assume for a minute that we are talking about an engine at or near sea
level, well we can just forgo the exact physics of things and say that at
both the induction system's inlet & the exhaust's outlet we have equal
pressure (just under 15psi absolute pressure). So in order to flow air into
this system we must always be working a balancing act between the three
fundamental sections of the engine, which are exposed to each other only
through the camshaft's orchestration of the valves. So forget everything
else you know about engines and start thinking of what's under your hood in
this way for the rest of this post :).
NA ENGINES (naturally aspirated)
These must work within a maximum pressure variation of 0psi (which is
really hard to create without massive pumping losses) and 14.7psi (maximum
atmospheric pressure @ sea level). To add to the basic problem of how to
flow air into and out-of this system, both ends of the system start out at
the same pressure, meaning air doesn't naturally want to go IN or OUT. This
can be accurately termed as a "pain in the ass". Now engineers and
enthusiasts alike have long been fascinated with how to make power from
this setup, but I am talking specifically about supercharged engines here,
and as I already stated "It makes no sense to build a naturally aspirated
engine if what you are looking for is good power output." So forget about
how you can best accomplish this through piston movement and it's effects
on cylinder pressures, and understand that it's just a whole lot easier to
get an engine to work if it's supercharged.
FI ENGINES (forced induction)
From a pure engine design standpoint, it makes MUCH more sense to
pressurize the intake system than to run NA. When only the intake system is
running under pressure well above atmospheric, it becomes perfectly obvious
that air is going to want to flow through the engine exactly the way we
want it to, and both cam timing & exhaust sizing becomes much less
important to getting the system to work right (as it was before in NA
setup). The air will naturally want to flow into the cylinder head, and
then after the very strong power stroke (thanks to all that air) it will
naturally want to flow out into the lower pressure exhaust system
afterwards. Everything in the engine will be working at pressure above
atmospheric and the pressure differences will be greatest in the induction
system, so all air will want to exit out the tail pipe quickly and
efficiently. One other thing should be said here: turbos technically ARE
superchargers. A supercharger is ANY device that pressurizes the intake to
above atmospheric pressure, and turbos do this exactly like superchargers
do. The only difference is in how a turbo gets the energy necessary to
perform it's job, and also that the turbo contributes to supercharging the
exhaust system (or more accurately a portion of it, the exhaust manifold).
THE CASE FOR SUPERCHARGING
Since a crank driven s/c (s/c = supercharger) is what people are normally
talking about when they use the term supercharger, I will no longer say
"crank driven" to make the distinction between it and a turbo. Now using a
supercharger makes a ton of sense simply because it only has a direct
effect in pressurizing the engine on the side we want it to, the induction
side. Since pressures will always be higher here than in any other part of
the system (except of course during the engine's power stroke, but that's
always sealed off from the rest of the system so we can forget about that
complexity), it's very easy to make this combination a powerful one. NA
engines often use large amounts of valve overlap to get the whole system to
work properly at higher RPM, which has obvious drawbacks in that it's
possible for the intake system and exhaust systems to interact in a
negative way (since they operate at similar pressures). It's sometimes just
as easy to get air flowing backwards through the system as it is to go
forwards in an NA setup, which is one reason camshaft choice is so
important to where in the RPM band best power will be produced. And here is
where the beauty of supercharging is; neither valve overlap amounts nor
perfect exhaust system designs are completely essential to keep everything
flowing in the right direction. No matter how long the exhaust is exposed
to the intake system through valve overlap, air should NEVER pass backwards
through the system unless the supercharger stops working.
THE EVIL OF SUPERCHARGING
The evil of supercharging is that some of the power we finally get from
combusting the air/fuel mixture must go back into powering the
supercharger. So here we have designed this whole system that works so
well, yet we have to power it with some of our hard earned torque. This is
not a good thing, but then again nothing so simple is ever going to come
for free. Do superchargers work? Of course they do, which is why many
racing engine uses the technology unless the rules prohibit it. The net
result is more total power from the system, but a portion of this power
must be sapped from our output to make it all work.
THE CASE FOR TURBOCHARGING
This section is easy to write, because it's exactly the same thing as the
supercharger portion. We have all of the same advantages, except for one
major benefit. That benefit is that turbocharging runs off what is largely
wasted energy, so that damn drawback of needing to power the system with
some of our hard earned torque is removed. In this way, a turbocharger
addresses the one main drawback to using a supercharger, but as you will
see in a second the supercharger addresses the one main drawback of
turbocharging.
THE EVIL OF TURBOCHARGING
Hopefully you now understand why it makes so much sense to forgo designing
engines for NA use and just supercharge the sucker instead, at least when
we are talking about how to best make power. And if you have been following
what I have said, you will also understand the bad effect turbos have on
our little perfect world of pressure variation.
A turbo is an ingenious little design that harnesses the wasted kinetic
energy we dump out through the exhaust system to actually force more air
into the engine. This is good for the same reasons that supercharging is
good, but it has one major drawback: it of course increases the pressure
within a portion of the exhaust system. While turbocharging a motor
increases the amount of air that can be flowed into it, it has a negative
effect on how easily we can flow it back out again. This weakens our
positive pressure difference between these two fundamental sides of the
engine, and causes both cam timing & exhaust system design to again become
extremely important to making good power. This is most certainly not a good
thing, but can a turbo overcome this drawback with the other inherent good
it possesses? It certainly seems so, because in most current forms of
racing where the rules don't probihit the use of tubos or slap restrictions
on their use, the turbo reigns supreme in terms of engine power output.
Now I didn't post this to make a statement about which system will work
better for your intended use, because the answer is (as usual) "it
depends". Sorry, but if there was such a clear cut answer do you really
think people would still be debating this topic? A long time ago someone
would have proven everyone else wrong, and either turbos or superchargers
would no longer exist. Remember, these systems were designed and in use on
production vehicles long before most of us were born, so it's not like this
is a new debate. The purpose here is to educate people on exactly why we
would want to supercharge or turbocharge an engine in the first place. Also
I wanted people to see, from a basic and theoretical perspective, how each
system is different in its function and it's relative pros & cons.
Hopefully this discussion of basic theory helped some of you come to a
better understanding of FI engines, and that my leaving out any real world
examples actually made it easier to understand. I have written an article
concerning the technical differences of the common systems, giving examples
of different supercharger designs and their advantages/disadvantages. You
can read it HERE
(http://af.datablocks.net/vbulletin/showthread.php3?threadid=425). And
please, any questions or comments, post 'em here.
Thanks for reading, peace!
Message 13
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| Subject: | Re: Crossover exhaust vs straight pipes |
--> Engines-List message posted by: "Archie" <archie97@earthlink.net>
> --> Engines-List message posted by: "James R. Cunningham"
<jrccea@bellsouth.net>
>
> Other things being equal, I would expect straight pipes to result in
> more backpressure than a good crossover system, mostly because of the
> lack of scavenging in the straight pipe system. But be warned, the
> above is a gross oversimplification. If Archie chimes in on this, pay
> more attention to him than to me.
> JimC
Ditto here, provided these are "really" tuned.
I have seen ac designs that appear impressive, but have very little effect.
If the opportunity arises to check a set, check the firing order pulses, and
how they affect flow. If that checks ok, then measure the amount of volume
that each tube contains. This can be done with water measure, also.
All primary tubes should hold the same volume.
I personally prefer a four into one system, incorporating a collector.
Unfortunately for aircraft, torque enhancement is via a longer collector.
Archie
> Gene Smith wrote:
> >
> >...I've heard that the straight pipes give as much or more back pressure
than the crossover, which doesn't seem logical to me, but may be correct?.
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