So, why do designers use small turbos, or multiple turbos so that full
boost is attained at low rpm (1500 rpm in the new BMW engine, for instance)?
Also, why do turbos have a pop-off valve to limit boost at higher rpms?
Methinks your statement doesn't accommodate those facts.
On Sat, 16 Sep 2006 06:44:11 -0700, "Floyd Rogers"
Small turbos produce less boost than larger ones, but they also reduce
lag, no matter what the engine RPMs, for the same displacement engine.
It's that lag they are working to reduce, with the side effect of
producing more boost at lower RPMs.
But, turbos do provide more power at higher engine RPMs.
No. Turbos do not produce power. The *ENGINE* produces
power, and all engines produce more power at higher RPM.
It doesn't matter whether the engine has a turbocharger, supercharger,
or has none.
Ed actually got it right in his first sentence, then blew it in the 2nd.
Well, gosh, if I knew I'd be dealing with some kind of pendatic
asshole, I'd have used the words "allow for" instead of "provide." Or
substituted "airflow" for "power".
But see if you can follow along. More airflow means more oxygen. More
oxygen means, if one meters the air properly, one can burn more fuel
with that increased oxygen. Burning more fuel per rev means more
power. Thus, for those who can't seem to follow along at home, turbos
provide more power.
It was clear to everyone except you.
Actually, I was right the whole time. I just didn't want to type so as
to preclude every pedant from getting his panties in a bunch.
Well, I lost track of the attribution, but I think the argument that
"turbos work better (see, I avoided "power" altogether) at higher engine
RPM" is a bit off-track. When you get right down to it, the
turbocharger's function is fairly independent of engine RPM. If you put
a turbo engine in high gear and lug the snot out of it at 2500 RPM with
the throttle WFO, an appropriately-sized turbo for general street use
will *still* spin up and deliver the same amount of wastegate-limited
boost that it will produce with the engine spinning at 5000 RPM. That is
because there's sufficient volume flow through the system. Now the
*engine* will probably still produce a bit more power at 5k, for exactly
the same reason that a normally-aspirated engine will produce more power
at 5 grand. But OVERALL, turbochargers tend to BROADEN, FLATTEN, and
FILL IN THE LOW END of the torque curve of an engine when compared to
the same size/type engine configured for normal aspiration. So I would
still contend that turbocharged engines really shine at the LOW end of
the RPM range, because thats where the boost provides such a big
advantage over normal aspiration, where the induction system is
typically tuned to work best at near-redline RPM.
That's really true of all forms of forced induction, not just
turbocharging. The Reno air racers (on my mind because the races just
concluded yesterday), particularly the Rolls-Royce Merlin powered ones,
run absolutely enormous boost pressures (over 110 inches of mercury,
compared to 60" HG max for the stock engine). But they only increase the
RPM by a relatively small percentage (3300-3400 RPM instead of 2900 to
3000 stock). The horsepower doubles compared to stock, though.
It depends on the compressor map. But at low RPM, the engine doesn't
move as much air, and the turbo won't spin as fast. Obviously, there's
a physical limit to how much air a turbo can push, but you'll approach
those limits on a street car pretty high in the RPM range.
No it isn't. The amount of air moved by the engine determines the rpm
of the turbine wheel.
Have you ever owned a turbo car? This last paragraph is patently
untrue. DAGS and educate yourself on the subject.
Yes it is.
The turbo is driven by exhaust gas, and that's not as dependent of
RPMs as it is on throttle position.
IOW, half throttle at 1500RPM will produce more exhaust than cruising
at 2000RPM, and thus more pressure from the TC.
Yes, but that's determined by more than engine RPM.
OK, then tell me, at 2000 rpm, how many liters of gas per minute goes
through the turbo of a 2.0L turbomotor?
Now, tell me how many goes through at 4000rpm? Remember that the motor
is an air pump, and that volume of gas going out is directly related to
how many times per minute each cylinder gets filled. Unless you're
claiming that the air is somehow not making it to the engine
That makes no logical sense. The volume within the cylinders is fixed.
More revs means more gas.
Show me the math.
How? Show me with MATH. I want to know where this air is going. In
my car, once the air moves past the MAF, it's going to the exhaust.
There ain't nowhere else for it to go.
I have no idea. I do know that the turbo runs off the exhaust, and the
exhaust is determined by both throttle and engine RPM.
No, I'm claimin that the exhaust amount diuffers mased on more factors
than just engine RPMs.
No, not on a gas engine; you are more right for a diesel, but even
then, as more power is produced (throttle opened) the exhaust flow
I'm not a math wizard, but I know how turbos work.
Like I said, I'm not a math wizard.
However, it's not hard to demonstrate. Have someone open up the
throttle of a car while you hold your hand near the exhaust. You will
notice an immediate increase in pressure that does not correlate to
the increase in RPMs.
Also remember that, as you open the throttle in the car, you let in
more air/fuel, before the RPMS can rise at all. And as the throttle is
held open, more air/fuel is still allowed into the cylinders; this is
how gas engines accellerate. That increased air/fuel exits as exhaust,
and makes the turbo spin faster.
Isn't your MAF before the intake manifold? At the least, it's before
the valves, if it's in the manifold. But the MAF will be downstream of
the turbo's output, thus the air charge from the turbo goes through
the MAF. And, of course, once it leaves the MAF, it eventually goes
thru the exhaust; is there's a turbo there, it drives the turbo. Which
then delivers air to the MAF.
No. Exhaust volume is determined SOLELY by how much air gets into the
cylinders. The cylinders' volume is fixed, and the more times you fill
them, the more air comes out the other side. Period.
The calculation is thusly: One exhaust cycle every four revolutions
multiplied by total engine volume is 500 x 2. 1000 Liters.
Prove it. BTW, 2000L exhaust at 4000 RPM. If you made a graph, you'd
see it was linear.
You keep saying this like it's true. OK, prove it.
So far, I haven't seen anything that supports your opinion on the
subject. But feel free to cite some reference.
And yet your making claims that require more than simple math to prove.
This is not physically possible. The volume in the cylinders is fixed.
14:1 air:fuel is the "perfect" ratio, and modern turbomotors use an MAF
to keep it that way. But only so much volume will go into each
The lag doesn't even take a half second at 2000 rpm.
The more exhaust there is, the faster the turbo spools. You don't get
more VOLUME unless you fill the cylinders more often (higher revs.)
It is. It's right after the air filter, in fact. Before the turbo.
Which, like the rest of your stuff, comes from where? I don't know of
any turbomotor where the MAF is after the turbo. But I'd be willing to
learn - show me.
Now, before some pedantic Floyd comes in and grouses about the above, I
have made a more simple picture of it than reality (the boost map is a
complicated surface, not a curve or a line) but the basic stuff is
Bill, feel free to look this stuff up. It's out on the Web if you want
to actually know, instead of imagining that you do. And if you find
something that directly contradicts what I said, go ahead and post it.
I'm man enough to admit when I'm wrong.
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