Re: OT- fuel mileage & fuel injection questions
- From: no@xxxxxxxxxxxx (jimbo1490)
- Date: Fri, 23 Jun 2006 06:09:14 GMT
I can't believe you guys have missed THE MOST IMPORTANT PERFORMANCE
BENIFIT OF EFI!!! I mean if you have built a few hotrod engines, it
should be fairly obvious! Think back, those of you who can :D to a
time when most cars still had carbeurators. When you went to the
drags (think 1/4 mile NOT Kweer Eye) all the really fast cars could
usually be identified by the hulking tunnel ram intake manifold
protruding though a big hole cut into the hood. Why did they use
those manifolds? Anyone? Yes, you in the back...That's right, the
tunnel ram intake provided for more cylinder filling because of its
long runners and straight flow path, both of which maximize charge
inertia to 'ram charge' the cylinders. Properly tuned , a tunnel ram
equipped engine could easily exceed 100% VE.
"Well golly", you might ask, "why not just put a
tunnel ram on EVERYTHING?" Thinking back to our night at the
drag races, do you remember how that tunnel ram equipped motor ran?
Oh yeah it ran great down the track :nod: But I'm talking about how
it ran before that. Did you see how long it took to start? Did you
notice the mechancs squirt gasoline down the intake with a spray
bottle while the driver hit start? Then when it finally woke up, it
seemed like thay had the idle set way up high, and it STILL ran like
crap...until about 3K revs.
Yes the dreaded tunnel ram intake was the only way to go really fast,
but with considerable cost ITO 'signal' to the carbeurator causing
ridiculously hard starts. Even after then, if RPM's fell below about
2K, the fuel would become seperated from the air charge and
precipitate onto the walls of the manifold or even puddle at the
bottom sometimes followed by a severe lean backfire then fire :mad:
This was definitely NOT a 'streetable' intake....not yet. It seems
there were good ressons street intake manifolds looked the way that
they did, with their short turned runners and tiny plenum volumes.
You see, if you are going to introduce the fuel charge way up at the
top of an intake manifold, then the manifold is not going to carry
just air, but a fuel/air mixture. THIS IS A PROFOUND DIFFERENCE! If
we want that fuel to stay homogenously mixed with the air (and we
do!) we must be strictly constrained when it comes to the design of
our intake manifold. It CANNOT look like a tunnel ram unles you can
put up with piss-poor starting and drivability.
Then along comes EFI systems. At first, they were mostly electronic
replacements for carbeurators. They even used the same intake
manifolds and simply bolted on a TBI unit instead of a carbeurator.
Nice enough, but no big difference. Then the multiport systems showed
up. Now the injector points had multiplied and moved out to the ends
of the intake runners. The manifolds began to change also. The
plenums got much bigger, something you could never do with
carbeurators. Finally, the runners began to take on 'exotic' shapes
like straight up then turned to one side or the now common 'nest of
snakes'.
Simultaneously, engineers began to notice that they could now push the
envelope of what constitues a 'street' cam lobe profile. Electronic
engine management coupled with modern intake manifold design meant
that there would not be any fuel charge in adjacent intake runners
until the precise moment it was called for. This had a profoundly
positive effect on idle quality when using a 'big' cam profile. So
now with the long runners, large plenum and multi port injection
right at the cylinder head, the adjacent cylinders no longer 'saw'
each other. This is not unlike the situation with motorcycles. Have
you ever noticed how freakin radical a cam they can get away with?!
Why is that??!! It's because there IS NO INTAKE MANIFOLD to muck up
adjacent cylinders with out-of-sync pressure pulses and unwanted fuel
charge. Each cylinder on a bike is fed by its own carb down a
straight pipe.
We don't usually get the luxury of that sort of engineering on a
street auto engine, but with EFI, the results have got pretty close
to that.
So, the advent of EFI allowed new freedom in intake tract design. We
now use manifolds on street legal, smog equipped cars that OUTFLOW
race day tunnel rams built in the 1970's. And we run pretty big cams,
too. EFI has allowed all that. THAT'S the difference.
Jimbo
.
- References:
- OT- fuel mileage & fuel injection questions
- From: Eric R Snow
- OT- fuel mileage & fuel injection questions
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