This section exists
to help give back to the community by explaining some of the technical
challenges we've faced along with their solutions.
6-Bolt in a 2g Kit and Converters
6-Bolt in a 2g Kit - this product was developed
to allow a 2g transmission to be mated up to a 6-bolt engine
without bottoming the torque converter into the pump and trashing
the transmission. This allows 2g cars to use a 6-bolt engine,
which has generally been the preferred DSM engine to use in
racing purposes. This also allows adapting a 2g transmission
into a 1g vehicle (with other changes) and using the Precision
Industries 9.5" billet converter. The smaller converter
allows a higher stall speed and the ability to run a larger
turbo, particularly without nitrous.
Torque Converters and Stall Speed - a properly
matched torque converter to the engine's power curve is the
most critical aspect to getting down the track at the best ET
possible. This is especially critical in a high boost application
as the balance between too tight down low to get the turbo spooled
and too loose up top is particularly difficult to achieve. The
goal is to be able to come up to launch boost within 4.0 seconds,
then also pull the engine down a minimum of 1000rpm on a shift,
preferably closer to 2000rpm. It is relatively impossible to
get an 11" converter too loose for the torque and rpm range
the 4g63 runs. The challenge with a restalled stock converter
is getting a turbo bigger than a ~60mm inducer on a 2.0l or
~65mm inducer on a 2.3l spooled up without nitrous. This means
a k-factor at about 230-240 or stalling a bit over 3000rpm at
0psi boost on a 2.3l. The 9.5" billet converter from precision
industries can be set up considerably looser. It can easily
be set up to spool a 74mm turbo on a 2.0l, but slip up top with
this loose of a converter becomes a real issue. This can be
about 4200-4500rpm at 0psi with a 2.3l or a k-factor around
340. The Kiggly Racing drag car runs a converter with a k-factor
in the 270-280 range. Below is a graph from a 5.31 1/8mi pass
with a 1.245 60', showing the timing and boost levels.
K-Factor - this is way more meaningful than
calling out stall speed and the only thing that makes sense
for a turbo car. Stall speed is a function of torque. Torque
is a function of boost. As boost goes up, the engine makes more
torque and this forces the torque converter to stall higher.
Torque is directly proportional to the absolute pressure in
the intake manifold, making about 5.0ft-lb per psia per liter
(this is a reasonable approximation for all engines). The governing
behavior is
k-factor = rpm / square root of torque
or rearranged to be
rpm = k-factor * square root of torque
Tips and Tricks for Spoolup - unfortunately
there is no free lunch, but there are some general guidelines
that work well. More timing is generally a bad thing for spoolup.
Turbos harness heat energy and mass flow; and the more timing
you run, the colder EGT's are and the less energy exits the
exhaust port. Also as you run it richer, the mass flow increases
a little bit. In general, play around with the tune down low
to see what is the minimum timing you can run before it starts
to flash lower and then add in a bit of fuel again until it
starts to flash lower. I've found I can pull out a lot of timing
without losing flash stall (18deg on my car currently), but
too rich and it loses stall speed quickly. The minimum timing
and richest to still make good power will spool the turbo the
quickest, prove it to yourself with datalogs. Advancing the
intake cam will also always make more low end torque and better
spoolup, but at the expense of top end power. This is the reason
the Kiggly Racing drag car has a MIVEC intake cam setup.
©2011 Kiggly Racing, LLC
