ox sensor on sequential efi,high overlap cam....

[Greg Hermann]

>Jemison Richard wrote:
><snip>
>
>>         [Jemison Richard]  Turbos actually do care I believe.  Most turbo
>> cams are wide center line, mild duration, moderate to high lift cams.  You
>> don't want to lose all that pressure out the exhaust pipe!  Also, cylinder
>> filling is largely taken care of my the pressure developed by the turbo.  So
>> you don't need cylinder scavenging.  You already have a large pressure
>> differential across the cylinder to remove combustion gases and recharge the
>> cylinder.
>>
>>          Also,  grab your favorite cam grinders spec sheets or catalog and
>> compare hot street and turbo cams and check me out on this.
>>
>>         Rick
>
>Two points, Removing the combustion gasses is a problem with Turbo motors
>as the
>exhaust pressure can be higher than the intake. They take a standard, mild cam
>and crutch the exhaust by openning it sooner; more time to blow down. Net
>result
>is wider CL.
>
>You're right though, boost is better than lotts of cam and RPM, but a turbo at
>high rpm will still need a big cam thou not as big as a na motor.
>
>My point was (originally), that we should tune the idle by driveability and
>'dead recon' the idle mixture from there and forget all that learning stuff (at
>idle). Just use the O2 for a reference. That assumes that we can easily change
>the map and have a good trimmer for the map sensor.
>
>My 482 idled like a kitten with a Predator carb, alcohol, and roller
>cam..... it
>just dribbled raw fuel out of the headers at idle, not enough to clean the
>floor, but almost. (another sick try at humor).
>
>later  Tom

Hey Tom--

1. Sick humor is the very best kind--the sicker and less politically
correct, the better the laugh!!!

2. Turbo motors DO care. Yes, you can make up for HP lost to bad intake
timing and bad intake tract flow (all the way from the turbo to the intake
valves) with more boost, BUT this will cost you higher exhaust
back-pressure, higher temperature, worse durability, lower HP/boost ratio,
and lower fuel efficiency in order to drive the turbo that much harder!!

The pressure losses (pressure drops due to flow friction) in the intake
tract are directly proportional to the DENSITY of the air going through
them. THEREFORE, and ALSO CONTRARY to popular belief, a turbo motor cares a
little MORE than an NA motor does!!! The fact that you can cover up this
inefficiency with more boost is what leads to the "doesn't care" idea!!!
You really want somewhat bigger (and as smooth as possible) ports and
intake tract on a boosted motor (rpm range for rpm range) than you want on
an unboosted motor so as to make up for the higher friction losses from
flowing denser air!!! (Also true onna supercharged motor, as better porting
will lead to lower parasitic HP to drive the blower for a given output HP!)

I agree that you should open, and close, the exhaust valves sooner on a
boosted motor (wider lobe centers, and retarded cam) to (1) get more,
longer,  blowdown and (2) avoid blowing so much mixture into the exhaust.
But, know that the earlier exhaust opening point reduces your effective
expansion ratio, and thus reduces power and efficiency! Which also means
that turbo motors would like shorter rods (longer piston residence time at
BDC) if it were not for the fact that the piston side loads go so $#%^ high
with high cylinder pressures and short rods!!!

Other point is that HIGH lift is good on turbo cams, as are roller lifters
and sharp valve accel rates, cuzza the proportionally higher flow losses!
Shorter duration, higher boost, and lower revs will give the same power,
with better efficiency and better durability, EVERY TIME on a given turbo
motor, provided that you have a good enough chamber and strong enough motor
to do it without detonation or auto-disassembly!!! (If you doubt me on this
one, just ask Cummins!!)

This was the great folly of the boost restrictions on turbo Indy motors
years ago---it made the motors MORE expensive, and shorter lived to boot,
instead of the other way around, cuzza they hadda turn faster to make their
power!!!! A rev limit wouldda been a much smarter rule to get them to where
they wanted to go!!!

Regards, Greg