Category Archives: Test Gear

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.

Cadillac XLR Top-up Testing

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This evening I ran some top-up comparison tests to help calibrate and establish a baseline.  Previously I had tested with the XLR top down, enjoying the convertible.  I was focused on monitoring calculated hp/torque, so aero wasn’t a factor.  However, as I am comparing using Virtual Dyno, then aero is a factor and I wanted to know exactly how much of a factor, and to establish a top-up value.

VD XLR Comparison Premium Top Up

[click to zoom in, back to return here] [This chart was based on incorrect tire diameter — see replacement chart below]

The Red line is the previous, Top-down test at 256/239 whp/lb-ft.  Tonight’s comparable run from the exact same location is the Green line at 245/259 whp/lb-ft.  The Blue line is another run tonight at a separate test location at 235/247 whp/lb-ft.  Tonight’s runs were lower HP, higher Torque.

With the top up or down, the variance is in CD, or coefficient of drag.  In my previous setup I wrote that I modeled the top-down impairment as a 0.05 CD adder.  For today’s test I have left that delta alone, but I have revised the base CD to the 0.31 that Cadillac marketing materials said for the XLR, instead of the 0.35 one of the early magazine reports quoted.  So Top-Up CD is modeled at 0.31, and top-down at 0.36.

I am using ‘absolute pressure’ from the dew point calculation for baro pressure for the virtual dyno calc.  This takes altitude and local barometric pressure into account.  Thanks to Richard Shelquist!

xlr premium comparison tuner calc

My conclusions are that tonight’s runs were lower results than the previous baseline.  This may be normal day-to-day test-to-test variance (unknowns not accounted for), or it may suggest that the top-down drag disadvantage for the XLR is not as great as 0.05.

I enjoyed the XLR all day, and it was out in the sun prior to the tests.  The important thing is to capture sufficient baseline data and to establish a baseline, and eliminate test to test variability.  I may have to either only run tests first thing in the morning to avoid heat soak, or only run tests with the car heat soaked (!)

XLR tire diameter

Update:  the previous Virtual Dyno setup used 27.73 as a tire diameter, and I see from the calibration file it should be 27.24.  This changes the results to following table:

Virtual Dyno XLR Tops Down comparison tires

Which is less than expected.  I am including both charts for now to document my confusion, and to assist in analysis.  Yes, tire size has a significant impact on calculated results.  With SAE adjustments for conditions the 320 hp rated LH2 should make approximately 256 whp; I am uncertain why today’s results are 226-236 whp.