Cadillac XLR LH2 Tuning – Tune 6D Better is not always better

Today’s test Tune 6D was to keep the early VVT changes from Tune 6C with some late changes back from Tune 6B.

I have also slightly modified my Virtual Dyno settings.  Virtual Dyno uses barometer and intake temperature inputs to correct to an SAE standard.  Each test run captures the local baro and intake temps as the XLR sees them, but they tend to vary slightly across the run.  The new change is to take a mathematical average of the baro and intake temps for each run and use that average value in the Virtual Dyno for that run.  Previously I had settled on a fixed 29.83 for baro and did a simple 2-point average of the intake temp at the beginning of the run and the intake temp at the end of the run.

I have restated 6B, 6C, and 6D in the graph below with the new methodology:

XLR 6D comparison to 6B and 6C


This graph is only today’s test and the 6B test:

XLR Tune 6D and compare to 6B

Although 6D kept the earlier 4800 RPM VVT changes from 6C and the 5200+ RPM changes from 6B, it also had some lower advance around 5200 based on KR (knock retard) from the 6B run and some clean up and corrections.  I am not sure if the KR on that run needed less timing in the file or was unique to that run.  Another solution would be to go a little richer at that point, which is what I will try on follow-up if the next baselines show the same KR.

Next I plan to return to a duplicated of Tune 6B, (as Tune 6E), reset the baseline, and then modify from there.


I have re-visited all of my tune logs, and I suspect that the adjustments to the MAF in Tune 4 were good outside of WOT, but not in the WOT range.  I have extrapolated from the pre-WOT range to correct the WOT range of frequencies.  This would have effected everything from Tune 4 (right after Volant install) to now.

Cadillac XLR LH2 4.6L V8 Tuning — VVT Tune 6a-6c

One change for the Cadillac Northstar in the LH2 variant was the addition of variable valve timing.  Today’s test is a slight change in the VVT timing on the intake and exhaust to see what impact on our test results.  I also restored the IAT retard at higher temps (no impact today).

XLR base VVT timing

This table shows the base VVT timing.  I am focused on 5200-6800 rpm, around 0.56-0.60 cylinder airmass.

XLR Tune 6 VVT Deltas

These tables show the changes for Tune 6a.  The intake cam is degrees of advance, and the exhaust cam is degrees of retard.  For Tune 6a I retarded the intake cam from 5600-6800 rpm and ALSO retarded the exhaust cam in the same region in order to keep the overlap the same.

XLR Tune 6a VVT test

For this test we are focused on 5600-6800 RPM.   Different days, different conditions, so I see that today’s test (BLUE) is healthier at lower rpm as well — ahead by 5 whp at 5200 rpm.

XLR Tune 6a VVT zoom

For this chart I have zoomed in to just the 5300-6600 range.  Today’s tune (BLUE) appears to do much better at 6000 rpm, (9 whp at 5900-6000 rpm) although less better at 6600 rpm (5 whp)

Looking at the timing tables, I kept the retard on the exhaust at 4 degrees across even though the retard on the intake cam could not be that high (because it was already at 0).

For Tune 6b I will reduce the exhaust retard to match the intake retard in each area — that was my intent for the test.

XLR Tune 6a peak hp

This is the HPTuners VCM Scanner shot at peak calculated hp of 312.5 hp at 6638 RPM.

Timing is a healthy 29 degrees of advance including 2.8 degrees of IAT advance.

Less Timing?

I have been thinking about the overall test results I am seeing — and about how the XLR seems to get better results hot than cold (adjusted for temperature).  I suspect the LH2 would actually like LESS timing instead of more.

XLR Main Spark

In the main spark table (high octane), the OEM calibration lowers the timing as the cylinder airmass increases.  This trend occurs up to the cylinder airmass at WOT — 0.56 to 0.64 — where it has gone flat for the rest of the table.  At high temps the engine gets retard from IAT and ETC which pulls the timing down.  At lower temps it gets advance from IAT which pushes the timing up.  I wonder if it wants less timing.

Next test:  match the exhaust cam retard to the intake cam retard exactly.


I adjusted the VVT timing to a) start intake at 5200 and b) sync up exhaust with intake better.

xlr tune 6b run 1 1254 s3

In this chart the update run is in GREEN, the earlier today run in BLUE and the comparison run Tune 5D in RED.  What we see is close match between blue and green EXCEPT in the 4200-5800 range where starting the retard of the intake cam early picked up 5 whp in that range.

xlr tune 6b zoom s3

This is the same chart zoomed in.  Around 5700 the 6B change is almost 9 whp above the 6A tune. At 5200 we are still at high torque so approx 0.63 airmass.  The OEM intake advance was 12-11-8-3-2-2 from 5200-5600-6000-6400-6800-7200 at 0.64.  My test 6B is 8-7-4-0-0-0 intake advance.   It may actually want to stay at 7 degrees from 5600 to redline?

The second part of the test, trimming the exhaust to ensure overlap remained the same and try to get back to upward slope at the end of the red run into the green run.  This seems to have worked but we’ll see how it looks in future tests.

Update 2:

Started test of tune 6D which expanded the VVT mods down 400 rpm and up through 6800 RPM.

xlr tune 6c vvt 1050

The yellow line is the 6C run.  Comparing the 6C and 6B curves, the 6C appears to start earlier, which is what we wanted.  However, it does not hold up from 6000-6800 RPM.  For the next Tune, 6D, I will return to 6B above 6000 RPM.

This run had another interesting incidence of freezing the VCM scanner, and then tilting the XLR.  I ended up in limp home mode, where I was able to use a code reader to reset the DTCs, after which the HPTuners VCM scanner was able to work as normal.  The test run for 6C may have been effected, but seems to still have been strong.  I am not sure what is causing these interrupts, but they are exciting.

I am glad to see 6C repeat the strong hump from 6B.  I plan to mix 6C below 5500 and 6B above for 6D.



Cadillac XLR tuning LH2 Tune 5d Piston Protection AFR

The default XLR programming includes 0.5 AFR richer tuning at 6000 RPM to cool the pistons.  Ideally there would be a timer associated with it — say after 10 min at 6000 RPM it added — but there is not.  For this tune I removed that variable in order to study the impact from 6000-6400.  Since  there were few other changes in the tune we are focused on the 6000-6400 RPM range.

Specific Changes in Tune 5D:

  • raise piston protection rpm to 8192
  • reduce IAT retard
  • point advance reductions around 5400 rpm to eliminate slight KR

XLR Tune 5D test 1

The blue line is today’s test.  The red line is Tune 5B in hotter weather.  From 6000 to 6500 rpm the Blue line goes up 4 whp from 224 whp to 228 whp instead of staying flat as in prior tests.

The Virtual Dyno test results always seem muted in cooler air, which is counter intuitive.  I wonder if there are other factors such as the air pressure in the tires (reduced in cooler weather) which are impacting the test results.

XLR Tune 5D test 1

The calculated hp in HP VCM Scanner showed a peak of 316 hp.

XLR Tune 5D test 1 peak torque

The calculated peak torque was 308 lb ft.

Overall I was glad to see a positive impact from 6000-6800 rpm and no knock retard.  I am uncertain why cooler weather tests seem to record lower results.  My intuition is there is a variable I am not controlling for.