Category Archives: Test Drive

More Cadillac XLR baseline Virtual Dyno runs

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XLR Retest Sunday 3 way Comparison

Couple of quick runs today to gain more data to see what consistent results look like.  The red and blue runs are both done today, from two different test locations.  The green run is run 3 from yesterday at the same location as the blue run.

I altered my test method to lock the XLR into 2nd gear for the test, so that we get a longer look at the torque curve and so that we see how the hp falls off or sustains from 6000-6700 rpm.  XLR retest sunday flat blue

For today’s test Engine Coolant Temp stayed at 199F so out of ECT retard, and the advance was a healthy 25.5 degrees at 6558.  Piston protection kicks in at 6000 RPM and the engine adds 0.5 air fuel ratio AFR to command 12.0 as shown.  The LH2 appears to like a bit more rich, so that’s something to look at another day.

XLR sunday retest calc hp

This chart shows the XLR PCM’s calculated HP and Torque values, not corrected for conditions, and not smoothed.  This graph corresponds to the blue line on the Virtual Dyno chart.  I am including it here as another point of comparison for what the XLR calculates is happening during the run.

My conclusion is that today’s runs are consistent within a range of yesterday’s run.  I prefer today’s method of locking the gear so that we get higher RPM info.

Tuning the Cadillac XLR – MBT #Motorama @hptuners

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Torque and horsepower are related, and if we increase torque everywhere across the RPM band, horsepower will go up as well.   What we want to know is what timing or advance will give us the most power?  But also, knowing that more timing can mean more heat and more detonation, what is the minimum timing for the maximum torque from the engine (MBT)?

I recently purchased a 2007 Cadillac XLR with the 4.6L LH2 V8 DOHC VVT engine. I will be tuning this platform and publishing my experiences.

XLR MBT

GM helpfully includes a reference table used by the PCM which shows the Max Torque Timing vs RPM vs Cyl Air based on their experiments with the engine design.  This table is shown above for the XLR from the factory calibration file for my 2007 XLR 4.6L.

I highlighted the two rows 0.56 and 0.64 cylinder airmass because at WOT the engine flows from 0.61 to 0.58 cyl airmass.  So those are the rows of interest.  If we get the timing to the values shown along those rows at various RPM values (without detonation), we will maximize the output of the engine.

XLR compare XLR tune to MBT

For this sample I captured the actual advance from the engine, the actual measured cylinder airmass, and then calculated the ideal advance from the MBT table to compare to the actual advance from the engine result.  What we see is that with the heat and other factors the engine is 3 degrees retarded from MBT.  In this range for this engine I believe that is a 15 hp difference.

Now, since some of the gap is CAUSED by intake air temperature (IAT) retard, and some is caused by engine coolant temperature (ECT) retard, we may not be able to get to MBT in the summer heat here in Texas.  But during tuning that is the spark advance I will be shooting for.

Separately, I will be looking at the fueling the engine prefers, the transmission tuning, and transitions.  If you have other ideas or recommendations please reply below.  There is not a lot published on the RWD LH2 Northstar V8, so if you have experience with what worked for you please share.

 

Cadillac XLR & Virtual Dyno conventions

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Another thought on this topic.  My XLR got new tires along the way, and uses Michelin Pilot MXM4 tires, 235/50R-18.  The calculated diameter for this size would be 27.3″ per Tire Size Conversion, or 27.25 according to Virtual Dyno.  Tirerack lists the actual diameter for this size from this Manufacturer as 27.4″.  One would also assume that is on a new tire, and the diameter will shrink as the tire is driven and wears?  I have decided to use the Virtual Dyno theoretical calc of 27.25″ based on tire size of 235/50-18.

As a convention, I am going to put test equipment into Driver Weight, and remove gas used from car weight.  So Driver weight will include laptop, hptuner interface, etc.  Car weight for the XLR is 3647, which I assume is full of fuel.  So on the sample runs tomorrow I estimated the car was down 3 gallons at 6 lb/gal and deducted 18 lbs.

Virtual Dyno doesn’t adjust for altitude, so I am going to leave that out as well.  Plano is at 600+ feet of altitude, so we will just know the readings are lower than sea level.

Frontal Area & CD — in 2004 preproduction test C&D quoted a frontal area of 22.4 ft2 and a CD of 0.35. Cadillac marketing said the CD was 0.31, which is what I am using.  In 2006 XLR-V testing C&D quoted CD 0.33 x frontal area 44.6 ft2.  That is a large difference.

From a thread on Virtual Dyno on HpTuners, The quick way to calculate frontal area is (((w*h)(0.85))/12)/12 = Frontal Area.

For the 2007 base XLR, W=72.3in,  H=50.4in so for a first approximation (((72.3*50.4)*.85)/12/12)= 21.5 ft2.

xlr car profile

Okay, so that settles a convention for Weight, Driver Weight, Drag Coefficient, Frontal Area, Tire Diameter, and I updated the gearing to match spec for 2007.

XLR close top updated for conventions

This chart only shows runs from yesterday.  The next task is to get more data with the same baseline to look for consistency.

UPDATE:

Studying the HP Tuner results, it appears yesterday’s run actually was lower hp due to higher engine coolant temps.  The ECT negative advance kicks in within that range.

xlr premium comparison 5995 rpm

This is the ‘before’ or premium comparison file showing 192F temps and 25.5 degrees of advance at 5995 rpm.

XLR comparison run2 5995 rpm

This is yesterday’s run showing 5995 RPM has only 23.5 degrees of advance because engine coolant temp has hit 207F.

XLR comparison run2 5995 rpm ECT advance

This is a file from hptuners showing the ECT base advance adder going from 0 at 194F to -3 at 212F, so in this case at 207F adding most of (13/18ths)  -3 or -2 degrees of advance.

So the XLR made less power yesterday due to reduced advance from approx 299.1 hp to 290.6 hp due to those 2 degrees of advance.

This type of intricate timing and fueling in response to a variety of inputs makes back to back testing difficult, but I appreciate the insight that the tools like hptuners provide.