I see talk about the differences between the XKR and XKR-S in terms of power and that it's a combination of tune and exhaust.

Something I've noticed over the past few months of having a tablet running AutoMeter's DashLink is that no matter what mode I'm in, when I have the pedal to the floor, the throttle plate never goes past 90%. I've confirmed this with my Foxwell NT510 scan tool. I've never seen this mentioned in any of the discussions and while I don't think that extra ten percent can make another 40 HP- especially at the last part of the plate opening- I do wonder how much of a contribution it makes.

I'd especially like to hear what a tuner has to say, but I'm a bit disappointed they don't take a more active role in this board.

 

Mandrake, I applaud your interests, but I do not expect much response from tuners. Basically you are on your own....
I would start my research by comparing throttle body part numbers. If the XKR-S throttle body has a different part number, then consider buying an XKR-S throttle body and doing an A/B dyno test.
Same part numbers? Then try testing your throttle body. There must be an electrical "signal" to the throttle body requiring it to open the flap. Your particular throttle body may itself be "subnormal". All this requires some reverse engineering.
Good luck.

 

Maybe the "90%" is an arbitrary number and does not dictate the physical position of the throttle plate. Maybe its in Degrees? I dunno. I'd think a person would have to observe the throttle plate position while at full throttle, but that may be a bit difficult. Maybe a borescope with a record function, tapped into the runner directly in front of the plate?
It is an interesting theory though........
Does anyone out there with an XKR-S want to check your throttle position reading at WOT???
Now I want to check mine, with both Torque and Dash Commander.
On that subject............... does anyone know if the supposed FORD only "FORScan" work with Jags? Maybe I'll try, for grins. FORScan works great with my F150, I just modified some files to activate my trailer brake and some other minor stuff. Lots of stuff in FORScan.

 

The simple answer is that aftermarket scan tools don't read/display the throttle angle correctly. You will see very different numbers if you read these parameters with an SDD.

 

Usually the PCM keeps track of the min & max throttle values and regards them internally as zero and full. So it doesn't matter if min is 7% or 9% and max 90% or 95% (or whatever).

They'd not be the cause of "missing" HP.

 

Its simple, the ECU regulates the torque (so power) via the throttle, and that one is programmed to only allow a certain torque at certain rpms.

So by just taking away the torque limiters, you get the full power potential.

Then you can tune for more (ie ign, a/f etc) for even more.

 

I got the torque limiters removed or raised but throttle is never above 82.5% in stock or limiters raised in the tune

 

That's because an aftermaket scan tool cannot read the throttle position correctly.

I checked this on my own XKR.

Torque App sees my throttle position as 15.4% at idle and 78.8% at WOT. SDD sees it as 6% and 100% respectively.

 

QUOT I'm using the same torque you app so make sense

 

I'm not a tuner but i'm very close to one...

I've explained the differences between the XKR and XKR-S tunes several times.

The key to the XKR-S making more power is the exhaust, the XKR-S tune has a torque limiter raised which allows the engine to make the most of the improved efficiency from that exhaust.

 

How exactly would a software "torque limiter" be programmed? It would seem to use a Torque Value as a LimFac, that there first must be a Torque Meter, AKA Dynamometer...

 

Torque is estimated by the PCM using a number of values and measurements.

A number of factors are taken into consideration to determine the volumetric efficiency of the engine. Like the number of cylinders, volume of each cylinder, manifold pressure, exhaust back pressure, etc. There are even values in there for the supercharger pulley ratio, diameter of the throttle body, and others too.

This enables a torque estimate to be made at each rpm, throttle position and load.

So the PCM "knows" what the torque "should" be at any given time, and it has an upper limit as well which it is not allowed to exceed, if it reaches that value, it throttles back.

 

So, it's a 'guess'. Doesn't seem too technologically advanced to me. Put a single resistor inline with something-or-other and the whole system is fooled into making 640 HP.

 

Nope, you couldn't be more wrong.

There are tables and tables and tables of set values, and they all have to correspond, otherwise the PCM will ignore the incorrect values.

For example, the "RaceChip" piggyback modules, they fool the MAP sensors, by lowering the reported manifold pressure before and after the supercharger, the idea being that if the manifold pressures are measured "low" then the PCM will allow more boost. This doesn't actually work.

The PCM knows at a certain rpm, load, throttle positon, elevation, and Mass Air Flow rate, what the manifold pressure should be. If the manifold pressure measurements are not plausible, outside of the allowable tolerances, it simply ignores them and runs on the MAFs alone.

If you try to cheat the MAF signals, the PCM recognises they are "wrong" and ignores them, and runs on the MAP sensors instead in a speed-density mode aka MAFless tune. Actually on the later cars with Bosch ECU's you can unplug both the MAF's and it drives just fine!

The PCM's in these cars are very smart, you can't fool them.

This is why any mechanical modifications to the car don't make much power on their own, or will in fact lose you power. Like fitting high-flow cats (or no cats) and an open exhaust with greatly reduced backpressure. You lose power. Because the volumetric efficiency has changed, and the torque estimates don't match up with the pre-programmed values any more.

I went through exactly this with my own XKR in the last few weeks. Changed the cats, put on the straight through exhaust, and it had less grunt than with the choked up cats and stock exhaust. So we changed the exhaust backpressure values in the tune and it was back to normal, actually even ran a bit quicker than expected.

Any mechanical modification to these cars MUST be corrected for in the tune, otherwise you won't get the full benefit of the changes, or even worse, you go backwards.