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BSFC and Exhaust Gas
Temperature (EGT) Information ;
In the old days of dyno
testing , the dyno instrumentation
back then consisted of measuring
Torque and engine RPM .
The dyno "luxury options" were also the ability
to measure
fuel flow and exhaust gas temperatures , now these
options
are standard equipment on modern dynos.
Fuel Flow
readings are used to measure how efficiently
an engine was at turning
each pound of gasoline or methanol
into HorsePower , in other words ,
how much fuel was being
consumed per HorsePower . The resulting
terminology ,
is named "BSFC" or Brake Specific Fuel Consumption
.
Brake = meaning as measured on a dyno water
brake
Specific = results obtained relating HorsePower to
Fuel Flow
Fuel Consumption = the amount of fuel consumed in
Lbs. per Hour
the BSFC equation is ;
BSFC =
Fuel_Consumed / UnCorrected_Brake_HP
.450 BSFC = 225 Lbs. Fuel per Hour / 500 UnCorrected HP per
Hour
Note= UnCorrected HP means the raw , uncorrected HP
as measured
in the current dyno room weather conditions , and as
yet
this raw HP has not been corrected to Standard
Weather
Conditions (ex=29.92" Baro, 60 deg F, 0 % Rh)
or has any
other corrections such as Friction Torque
or to account for inertia
losses due to acceleration,
have been made to the raw dyno room
HorsePower numbers .
BSFC numbers are a means by which you can
judge an engine's
efficiency. Also, BSFC numbers are often looked
at
as representing "Rich" or "Lean" engine conditions .
That concept
is somewhat true, but not totally true or
exactly how the BSFC
information should be interpreted .
Examples of BSFC and EGT
number misconceptions are ;
1- An engine is being dyno tested ,
and is tuned to its best
and proper Air/Fuel Ratio , and its BSFC is
recorded .
If some person were to pull off a spark plug wire
during
the dyno test, the BSFC would instantly skyrocket !!!
The
resulting BSFC would then be higher , and could easily
be
misinterpreted as going RICH , but in reality, the
Air/Fuel Ratio would
have basically remained "unchanged" !
Both an EGT temperature probe and
a Oxygen or Lambda sensor
on exhaust would indicate the engine went
richer also ,
but in reality an Air/Fuel Ratio meter measuring the
engine's
"Consumed" A/F Ratio would have remained unchanged .
2-
An engine is being dyno tested , and is tuned to its best
and proper
Air/Fuel Ratio , and its BSFC is recorded .
If for some reason , the
distributor came loose, and
ignition timing went retarded, the BSFC
would also "skyrocket"
All it did was start burning the same
correct A/F Ratio later
and was continuing to burn as it went out
exhaust ports
as you were pretty much wasting it and sort
of
blow-torching the EGT probes
This last example would be "doubly
perplexing" to the
novice observer because the BSFC will be higher
falsely indicating the engine went "Rich" , and the EGT temps
will
be very much higher , falsely indicating the engine went
"Lean"
,....both occurring at the same time and instant !!!
In reality
neither the engine went "Rich" or "Lean" ,
instead the "consumed"
Air/Fuel Ratio remained unchanged !
There are many more examples of
EGT , BSFC, and A/F Ratio
misconceptions, but the above 2 examples are
primary reasons
and sources of racer confusion .
Sometimes i've
seen a few racers come to my dyno and dyno test
their engines ...we
make a few pulls, and i tell them from the
computer readouts what the
A/F Ratio is or what the EGT temps are;
Some racers respond by
;
1-if i tell them its rich ...they want to increase ignition
timing
to change the jetting
2-if i tell them its lean, ....they
want to retard ignition timing
to change the jetting
Then i ask
them ;
"How can increasing or decreasing ignition timing "
magically "UNSCREW" and "REPLACE" that jet in your carb
?????
The "Consumed" A/F Ratio remained basically the same,
no
matter what you do with ignition timing .
What they "perceive" as
Rich or Lean moving the ignition timing ,
are the EGT temps growing
hotter or colder depending upon which
way you move timing ...and the
BSFC numbers getting higher
or lower depending upon which way you move
ignition timing .
Moving ignition timing will NEVER unscrew and
magically replace
the jet in the carb with a new jet size !
You
should tune by the "Consumed" Air/Fuel Ratio readings ,
then look at
EGT temps , ...then and only then use these
EGT temps as a tuning
"referrence" and not as the
"Last Word" in proper engine tuning
!!!
If timing is too retarded , the EGTs will be very high
for a
particular A/F Ratio and Compression Ratio/ Boost psi
I go after
ignition timing 1st...we keep moving ignition timing
a step at a time
one way or another, to find best combo
of torque/hp...then we go back
and play with jet sizes
to fine tune ...if you go after jet size 1st
with wrong timing,
sometimes exhaust conditions during overlap can
affect
jetting slightly , if the racer happens to have mostly
header
specs on edge of being wrong , or intake specs or
overlap period on
edge of being wrong for rpm range.
Then after the engine is tuned
for best average torque/hp
curve,....note the EGT temps as a reference
!
but only as a reference , not the final word in jetting .
I
use EGT probes to tell fuel distribution and other things ,
i never use
or heavily rely on EGT as A/F Ratio determination
Using the EGT probes
for fuel distribution studies is
their greatest value !!!!
You
can't ask some other racer what EGT is best for you .
example= no
matter how well you tune a 9:1 normally
aspirated engine, its EGT's
will be higher than a similar
properly tuned engine with 15:1
Compression Ratio .
....no matter what you do , the 9:1 CR engine is
going to have
higher EGTs because it will waste heat out exhaust more
than
a 15:1 engine, or any CR higher than the 9:1 CR reference point
.
If you asked a group of racers that all had their
engines
perfectly tuned and maxed out with the best possible
ignition
timing curve and A/F Ratio ....
one racer might say 1350
deg F ,
another might say 1250 deg F ,
another might say 1150 deg
F,
another might say 1050 deg F
.....they could all be correct and
have the very same A/F Ratios
.....the varying EGTs depending primarily
upon Compression Ratio,
Volumetric Efficiciency percent % , and
Combustion Chamber/Piston
top design .
You can dyno test the
world's most perfectly tuned engine
with the most stable , most perfect
A/F Ratio ,..it might have
1150 to 1250 EGT for an example
.......
then dyno test this same engine at zero deg TDC ignition
timing
advance under full load for a half an hour or so,
...you're
going to completely melt the headers off the engine
in half an hour,
thats is if it lasts that long
and doesn't 1st cycle/snowball into
severe preignition
then detonation ! all the while its A/F Ratio would
be perfect ,
but the EGTS will have gone thru the roof !!!
that is
if you still have a roof in the dyno room
and it wasn't burnt in the
fire caused by the melting headers .
Another thing you have to
watch out for is
where are you placing the EGT probe??
1-at 12
o'clock or so ...
2-How far away from heads exh port flange
???
3-How deep is the EGT probe protruding into header pipe diameter
????
4-EGT probe type and response time
all these factors
greatly infulence EGT temps making
it very much impossible to directly
compare EGTs between racers
if no standards are set to probe
placement
So basically , if you walk around the pits at a National
Event
and ask the SS racers that have CR from 9 to 11:1 mostly
,
they'll tell you 1250 to 1350 EGT and maybe a few at 1450
you
ask the ProStock and Comp guys with 16+:1 CR
they'll tell you 1000 to
1150 deg F
maybe some of the very large Mountain type engines
with
HiCr tell you 850 to 1000 deg F EGTs
Hi
Compression Ratio = Lower EGT temps
Low Compression Ratio = Higher EGT
temps
Correct ignition advance(32+ to 40+ ign deg range)= Lower
EGT temps
Wrong ignition advance ( below -25 or so deg range)= High
EGT temps
..... burn slower and later in stroke , continuing to
burn
while exhausting and if severe...cycling/snowballing into
rising
coolant temperatures, preignition, then
detonation.
Rich A/F Ratios = Lower EGTs (also extremly
lean, misfire)
Lean A/F Ratios = Higher EGTs ..burn slower
and later in stroke ,
continuing to burn while exhausting if
severe...cycling/snowballing
into rising coolant temperatures,
preignition, then detonation.
Some more aspects of EGT tuning
...
Example-> 4 cyl engine
Engine #1 cyl #2cyl #3 cyl #4
cyl
RPM EGT EGT EGT EGT
5000 1350 1250 1300
1325
....
....
8000 1390 1370 1390 1390
i would be worried
about the
Number # 2 Cylinder out of the 4
Look at the "RATE
of CHANGE" in EGT temps
Number #2 cyl starts off low , ends
still lower than
others but its RATE is quicker
during a short
600 RPM/SEC dyno acceleration test
you might see engine's EGTs like
above example
but if you were actually racing this same engine
in a road course or relatively long race
the number #2 cyl would
grow way too hot ,
its EGT would out pace other cylinders and cause
failure ???
Look at "RATE of CHANGE" !!!!
also at the lack of
"RATE of CHANGE" in EGTs |
