Supercharged Dyno tuning advice/ checklist:
Good to know:
-On a stock compression
motor (10.4:1) it's usually best to pull 1.5 - 2 degrees per pound of boost.
Stock piston ringlands are the weakest link; they will destroy themselves
quickly with detonation. It's best to stay at least 2 degrees away from any
knock appearing no matter what.
- Spark Advance also depends
on the cam used, if using a cam with more overlap (smaller LSA) the cylinder
pressure will be less because part of the boost is flowing out the exhaust
value. Reduced cylinder pressure means less density. Less density equals slower
flame front, meaning more spark advance is optimum. So you can see it depends
on the camshaft too.
- Compression ratio is a
huge factor against detonation and anyone running greater than 6 psi should consider lowering their static compression.
Pulled timing gets pretty beyond that point.
- Thanks to Scott Peitzsch for reminding me about intercooling and its
effects on intake temperatures. I meant this as a primer for basic 6psi kits,
but it is crucial to discuss. Because of
the way superchargers compress layers of air the air temperature increases with
compression. Intake air temperature needs to be monitored. Intake air temps
need to be consistent to get consistent results. Higher temps require timing to pulled to prevent detonation. The solution is to lower temps
via an intercooler, instead of just pulling timing.
- You will need to get it
dynoed with a wideband o2 sensor to tell what the fuel mixture really is.
Anyone with a blower that hasn't calibrated it on a dyno with a wideband is
crazy. It's just not worth guessing. You probably won't want to go leaner than
12:1 - 12.5:1, but that's a little lean for a forced induction motor in my opinion.
The additional fuel helps to cool the combustion chamber.
- Any decent dyno should be
able to hook up a boost gauge as well, so you can see how much boost you are
really seeing in relation to exact RPM, so you know when to start retarding the
timing and how much (1.5 - 2 degree * psi)
- The reason we lower the
timing in the 90, 95, 100 Kpa in the spark advance table is because the scan
tool tends to report spark advance higher at WOT. Ignore the scanner data (per
Ed W. it can read 4 deg over actual at WOT).
- The important thing is NO
KNOCK, no matter what. If your knock sensor is giving knock always, then
replace it before you tune, false knock reading sucks.
Let's see if I can mentally
do a checklist here for you blown guys:
___ Get extra O2 bungs
welded in the Y-pipe, without these, you can't monitor actual Air:Fuel
(Your dyno shop me be able
to weld these in, if not a muffler shop can, make sure to use anti-seize on the
plug) If you run only one wideband, run the driver's side- it tends to be
slightly leaner.
___ Verify the dyno shop has
a wideband o2 sensor that can bolt into your y-pipe bung, also see if they can datalog boost referenced to RPM, as well as fuel pressure.
If on a tighter budget you can always bring a fuel pressure gauge with a long
hose to make sure the fuel pressure isn't dropping. If the shop has no
wideband, find someplace else.
Work quickly; remember time
is money at a dyno shop. Just don't make mistakes from being careless.
___ Set
up an appointment for the dyno
___ Have a datalogging tool
to monitor knock, and injector duty cycle. I recommend TTS Datamaster.
Stage 1:
___ Initial WOT timing for a F-body is 33 degrees at 2800 on up, Y-body at 3200 on up. The
timing below those rpm ramps up to these points in a smooth curve. Start with the timing pretty low, 23 degrees
is super safe, 25 might be more realistic. The first run you want to monitor
these things, and get a baseline for how much boost is building and at what
rpm.
___ Make sure no knock
appears at these lower Spark Advance numbers, if it does, then it's often false
(assuming good 93+ gas) check knock wires are away from spark plug wires and
replace knock sensor if necessary.
___ Make sure the ignition
is not breaking apart at higher rpm. If it is and there is no knock retard,
then check/disconnect any MSD/Crane ignition box- they can often go bad,
especially if they got wet (ask Ernest) Closing spark plug gaps to 0.035 and
going one temp range colder spark plug is a common setup as well.
___ Make sure the fuel
pressure is not dropping as the RPMs increase (signs of in-line or in-tank pump
needed)
___ Make sure the injector
pulse width's are not maxing out (extended times at 85% duty cycles are bad for
GM and Ford SVO injectors), 90% is bad for Lucas) TTS Datamaster is a great
tool to use, since it calc's duty cycle for you.
___ Take a look at the
wideband AFR (Air: Fuel ratio), is it higher or lower than 12.5:1? Since you're
blown- it's probably lean, so you'll need to increase the PEvsRPM table to add
more fuel. Generally it takes pretty big steps to move 1 point of AFR. I find
often a PEvsRPM value increase of 12-14 steps is needed to move one AFR (ie. 13.5:1 to 12.5:1 ) on a
Naturally Aspirated setup.
___ Check Intake air temp,
higher temps cause detonation, you can’t get the best
tune with inconsistent temps. Consider an intercooler if one is available for
your supercharger. Intercoolers reduce intake temps greatly, fighting
detonation greatly.
Steady the AFR curve at
12-12.5 depending on your religion, 12.5:1 makes more power, but 12:1 cools the
combustion chamber more. Once the AFR is regular at 12-12.5:1
after a couple more runs, then you can go to stage 2.
Stage 2:
___ Set the Spark Advance
tables back to stock values at 90,95, and 100 kpa for this next part.
___ Take a look at the boost
psi and at what RPM the numbers occur. Match the datapoints up with the Spark Advance table, this how we'll
fit the curve. Look at the boost level for
each rpm checkpoint, and subtract 2 times that value for each RPM point on the
spark table from the original Total timing of 33 degrees.
Example:
Say at 3400 rpm I see 2 psi of boost. The timing value at that point was 33
degrees, so subtract (2*2) and change it to 29 degrees at 90, 95, and 100 Kpa.
At 3800 rpm I see 3 psi of boost. The timing value at that point was 33 degrees, I subtract (2 * 3) and change it to 27
degrees. These numbers are picked out of
thin air for example, nothing more.
We are basically mirroring
the effect of a Boost Timing Retard by doing this. You may also want to try 1.5 times the psi of boost as a subtraction factor if no knock is seen.
At 10.4:1 it isn't smart to go beyond that.
This should help you fit a
nice curve taking into account the boost per Rpm.
Each time you change the
timing curve you will need to make sure everything still passes the Stage 1
criteria, the AFR must still be 12:1, no knock, no injector max, no fuel pressure drop.
Providing everything passes Stage
1 and Stage 2 has a timing curve reducing 1* or 2* the boost psi, then experiment by incrementing spark one degree at a
time, making sure no knock occurs and horsepower is increased. If horsepower stops increasing, stop
advancing!
A Maxed
MAF at 471 gm/s (or 510 gm/s) is very common with blown setups. What's
important to realize is that the airflow is still increasing above the maxed point, but the PCM only sees it at 471 in the air:fuel calculation. For this
reason the PE vs RPM table may need to have a very
large increase at the point after the MAF maxes,
balance the AFR throughout the entire operating RPM and you don't need to worry
about the PCM not seeing more than 471/510 gm/s air mass.
A dyno tune is a bit more gentle than drag racing a car, as the dyno's weight is often 3000lbs where our cars are
3400-3800+ lbs. You may want to run 12:1 or to take this into account, pulling
two degrees of timing once the tune is dialed in is a fairly common practice.
It's your car, so it's up to you. Remember- No Knock.
Hope this makes sense,