> When stopped at a light, the pressure sometimes
> falls to almost zero. Yet, the car keeps ticking.
> But, at the other end of the spectrum, it rarely
> sees 4k on the tach. This is not a race car. It
> is my daily commute.
Oh, I'd better backtrack a little on my prognistications
of doom. I've personally lost 1.25 B motors to bearing
failure brought on by low oil levels, but they were
because I was combining 1+ G loading from modern race
tires with 6500+ RPM levels on the track. John Ross and
Andy Banta have lost motors too, again on the track. BUT:
With careful maintenance, the B motor *as a street engine*
can last many, many tens of thousands of miles, and when it
finally does join the choir invisible, as must we all, there
are few engines cheaper to rebuild (the Chevy V8 has cheaper
parts but more of them to buy, and other than that, the
Spridget and the VW Bug are some of the only cars that cost
less to do a full rebuild.)
> So, why are you guys running such high rpms?
If you ever use your net.points, I'll take you for a ride
in The Green Car and demonstrate. Then you can tell me how
how 5800 RPM at the exit of Turn 10 at Sears Point in an
MGB compares with Watkins Glen in a TR4-A. :-)
> Scott #1, time to jump into another expose'.
> Where do the torque curve and the horsepower
> curve peak, relevent to each other, for a stock
> 1800 engine?
About 5200 RPM is the drop-off point on a pre-smog
motor; it drops to 4500-4800 or so on the later ones
with worse breathing. Fix the ports and you can get
7000 RPM with continued (if marginal) power; to get
any real improvement, change the cam.
> Does raising the horespower output
> necessitate raising the rpm?
They're related. Horsepower is work over time (33,000
lb/ft/min, specifically). So if you get more power strokes
in the same time, you can get more power, right? Well,
maybe. There's a sweet spot in any engine (or in any
sausage machine -- no, I PROMISE not to talk about the
sausage machine any more) at which the power produced by
each combustion cycle is relatively constant regardless
of RPM. The bottom of this sweet spot -- which may be a
couple of thousand RPM long -- is generally (but not
always, and not necessarily) the torque peak. On stock
MGBs, I think it's about 2700 RPM. The top of this sweet
spot is the horsepower peak (again, on MGBs it's about 5200
RPM). (One goal of the racing/sports gearbox, BTW, is to
keep the engine operating within this range, which is the
powerband; on a well-engineered sports or race car, a shift
at one end of this band will put the tach right at the other
end of it.)
Basically, below this range the motor isn't turning quickly
enough to do all that much work, and the piston velocity isn't
high enough to fill the cylinder completely so each combustion
stroke is less effective than it will be at optimum. Likewise,
above this range, there isn't enough time to fill the cylinder
completely, and the result is that the combustion efficiency
drops and power for each stroke falls off.
Where this gets tricky is when the power per stroke drops but
the strokes per minute stay high enough (relative to the drop
in per-stroke output) that the overall engine output is still
better than it would be if you shifted up into the next gear.
That's where you need a chassis dyno, so you can do a thorough
analysis of the engine's power output throughout the RPM range.
As a general rule, engines with long strokes and narrow bores
(like most British engines, as discussed in the RAC H.P. essay
of a few days ago) tend to generate their maximum power fairly
low in the RPM range. Two reasons: first, the longer stroke
gives more leverage to the rotation of the crankshaft, so there
is more power available even at low RPM; second, the longer
stroke means that at high RPM, the piston has to travel so fast
that it can't fill the long cylinder as easily, and therefore
the brake mean effective pressure (BMEP) for that cylinder
drops because there's less air/fuel in it to combust. And
BMEP (yeah, I know, I just introduced it here) is an important
factor in power output.
> It seems to me that
> if it were possible to raise hp but not rpm, the
> engine would last longer.
More or less. That's the premise behind great humping
thrustmobiles--er, US-built V8s. Basically, the less
stress you put on a motor, the longer it lasts (given
equivalent construction techniques and quality). For an
interesting bit of calculation, consider this:
Specific output is defined as horsepower per liter. There
are a number of ways to raise specific output, but they
usually exact a cost either in complexity (meaning production
expense and maintenance expense) or in longevity, due to
greater stresses on the components. The domestic V8 gets
its power by having larger cylinders in which greater volumes
of air/fuel can be detonated; the advantages from the standpoint
of durability is that you can drop the specific output into a
level where the engine is less stressed and still come up with
a total power output that's very effective.
Let's take a look, though, at a 1967 MGB and a 1993 Mustang GT,
just for fun.
Power Displacement Specific Output
MGB 96 bhp 1.8 liters 53.33
Mustang 210 bhp 5.0 liters 42
This is one reason the Ford V8 is such a nice package for transplants
(right, Roland?) -- it's relatively understressed, its displacement
and basic design give *instant* throttle response, and in the case
of the Ford small-block, it's actually fairly light and compact
for a V8. And the 25% reduction in specific output gives it a
long and fruitful life peeling the tread off tires and filling the
coffers of John Law. But we Britcar owners can snobbishly say, Oh,
but its *specific output* is so LOW... as we drink our Samuel Smith's
Nut Brown Ale and hurl darts.
Doesn't someone on this list have a newish Lotus Esprit Turbo?
I think that was the winner in the specific output wars, with
(if memory serves) 119 bhp/liter (262 bhp, 2.2 liters). At least
for production cars, of course; the BMW F1 engine was the all-time
winner, with something over 1000 bhp (rumored up to 1500 for
qualifying) out of a turbocharged 1.5L four.
> Whence do Miq and I get to collect our team.net
> points, thence?
And now a word about our CONTEST!
Second place, with 7 points, goes to Alan Costich, for
his "Ich bin nicht ein Berliner, but I've been to Munchkin!"
His call arrived at 9:12 AM PST, according to my voicemail.
And the first place award of Ten net.points is Miq Millman,
with an "Ich bin ein Berliner, 'all" at 9:03 AM PST! Congrats!
(Gee, too bad I didn't have this contest last week, I could
have announced that I was sending you a Cashier's Check for
$1500! :-)
Net.points, as I explained to Alan yesterday, can be used to
buy a wide range of valuable merchandise. In the San Francisco
Bay Area, for example, 7 net.points plus $6.50 will buy you a
round-trip ticket on Caltrain between Mt. View and San Francisco!
Or with only a $1.50 copayment, you can buy a caffe latte at many
fine dining establishments!
What we *really* need -- and maybe Mark Bradakis can work this out --
is to install the UUBP program that I've just received the man page
and specs for. Here, let me transfer it from my other computer...{|@
NAME
uubp -- Unix-to-Unix beer protocol
SYNOPSIS
uubp [- acefghlqy] site
DESCRIPTION
Uupb allows the user to transfer beer, ale, or other fermented grain
beverages between network sites. Using TCP/IP (telecommunications
protocol for imbibing pilsners), uubp encodes beer from a local
file system into packets suitable for FTP (fermentation transfer
protocol) delivery at a remote IP site.
Example:
% uubp -c"AMBER" -f0.7 -y0 -q2 198.137.240.100
The preceding example sends two six-packs (-q2) of amber ale (-c"AMBER")
with a fizziness quotient of 70%, brewed using yeast of type 0 (saccharo-
myces cerevisiae) to IP address 198.137.240.100, which is the IP address
for the White House.
RESTRICTIONS
Both source and destination sites must be running uubp-daemon. In
addition, local restrictions exist in many areas for the transportation
of alcohol across state lines. The Electronic Frontier Foundation is
currently involved in litigation to ensure the ability to distribute
beer through the uubp protocol according to the 21st Amendment. To
support the SIG of EFF devoted to this cause, join the Homebrewers of
the Electronic Frontier Engaged in Winning Electronic Independence and
Zeroing Establishment Nonsense (HEFEWEIZEN), or send mail to
hefeweizen@eff.eff.org. Be sure to include the entire text of
this manual page.
NOTES
Relax. Don't worry. Have a homebrew!
--Scott "And I hear the chairman of the committe is Craig Shergold" Fisher
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