So on my 08 XKR I have a tune and pulley upgrade. next logical performance mod I thought is the high flow cats, but Im finding conflicting information as to results from this mod.
I there solid proof such as dyno or time slips that high flows have a positive effect on a 4.2 supercharged?
Apricieate any help

 

What a slippery slope.
There are those who have stated that although their Pulley And Tune helped performance, all it did is to make power at lower RPM and not necessarily more of it; Supposedly raising both their 0-60 times and the 1/4 mile times. I do believe one person also stated that although the times where higher in the 1/4, the trap speed was higher mph.
There are also those who absolutely SWEAR that ANY increase in exhaust flow reduces back pressure, thereby LOSING power. (I can not for the life of me figure out how this would be possible, except in maybe the ultra-extreme, say ten-inch quad pipes)
I truly hope there is an answer out there, as I would like to pulley-tune-cats my own 5.0 SC.

 

Theories are great, but actual proof wins.. Take my tune experience with OE tuning, felt massive gains on the lower end but on the drag strip I have actually lost time and trap speed. Working with Eurocharged now hopefully their tune will show actual gains...

 

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CeeJay, its because back pressure is a dynamic. If you increase pipe diameter you lose velocity at a given flow. The higher your initial flow at a set velocity, the more back pressure is created in a given pipe.

A "free flowing" pipe with a several foot diameter and 20' of piping, will mostly just create turbulence if you try to promote flow just with your breath blowing into it. At that low flow rate, and low velocity, a smaller pipe diameter is needed to actually get a flow along the pipe to happen.

You want to size your exhaust diameter based on the velocity and mass that is flowing into it. These change with engine RPM, and throttle position. At low RPM, on an exhaust designed for maximum flow at high RPM, back pressure is increased at the lower RPM, resulting in a loss of power. It is not that going to a smaller exhaust creates more back pressure at low RPM, and thus more torque, or that going to a larger exhaust, creates less back pressure, and thus less torque at lower RPM.

Each time an exhaust valve opens, it creates a pulse of high pressure. The length of this pulse is determined by the exhaust velocity, exhaust mass, and inner diameter of the pipe. At higher RPM, you release more pulses in a given time. If your pulses are too close together, they fight each other for space, and make turbulence, resulting in increased back pressure. If the pulses are too far apart, they lose velocity as the pulse will dissipate its pressure into the void as it travels the pipe. Ideally, you have the length of each pulse such that they travel very closely together, yet neither cramming, nor dissipating.

Catalytic converters, partially break up the pulses, dissipating their pressure and velocity. The less dissipation, and the less velocity loss, the less loss the system will have. Catalytic converters are sized such that the area of the input and output pipes roughly equal that of the area of the space between the obstruction. The less area the obstructions take up, the less disturbance, and the better the flow.

So its a dynamic situation, where you want to tune things to the amount of mass at that mass's velocity you are actually exhausting from using the engine. For maximum horse power, you tune to a very narrow range for the upper RPM that the engine is capable of. This results in extra turbulence, lower velocity, and higher back pressure at the reduced flow rates of lower RPM, or part throttle running.

If you decrease the restriction of the cats by internal design, but the diameters remain the same, you will be reducing a restriction across the entire rev range, you will gain power. If you keep the same internal design, but increase the diameter of the cats, you may get better flow at higher RPM's, and reduced flow at lower RPM's.

 

Very interested in seeing what they come up with and what your before and after numbers are. They are one tuner I have spoken with and considered. Are you doing a dyno tune or just their premade maps?

 

Well so far I have the premade Eurocharged tune and by comparing just the overall feel to the premade OE tune and to stock ,Eurocharged tune feels very much like stock with no difference in every day drive-ability. On top end I cant feel the difference either but hoping that a dyno will show otherwise. The main reason that I decided to ditch OE is because of lack of support so hopefully Eurocharged will be different but OE tune felt a lot more torquey in lower to mid range. I have sent Eurocharged a few emails regarding the lack of low end torque in comparison to OE tune , and will be taking the car to a dyno and to a track later and compare to stock.
Fingers crossed

 

Well i'll tell you my story... my 2005 XJR is running 100 cell cats, and to be honest I did not see any gains on the dyno, the 1/4 mile or the 1000m sprint compared to "good" stock cats.

So here it is.

The XJR has always pretty much run 13.3's @ 106mph, +/- a little.
It was dyno'd here in Australia and made 330rwhp.

Some time ago the car started running like crap in the upper rev range, there's a long-winded thread about that story here https://www.jaguarforums.com/forum/x...solved-137070/ but to summarise things, the cats had choked up, and it had happened so slowly and progressively, I hadn't really noticed.

And when I changed the cats to 100 cells, problems were solved, and the car felt like it had picked up a heap of horsepower. This sensation was purely because of how much power it had lost due to cats choking up.

The car has since been back on the dyno, the 1/4 mile and the 1000m runs. It made 320rwhp, trapped the same 106mph and trapped the same 210kmh on the 1000m, just like it had a couple of years earlier before the original cats choked up.

So on my XJR at least, it seems there was not actually any noticeable gains had from the 100 cells vs. the good factory cats.

If your car has high mileage, or you have a failed/choked cat or cats, then yes changing to a more free-flowing cat will probably help to restore some lost power.

But I really have my doubts now that changing the cats alone "adds" horsepower.

Some other things to mention;

A friend here has a naturally aspirated XJ8, with very low miles, he did a 100 cell cats & exhaust & he is convinced that there was an improvement in performance (unfortunately the car was not dyno'd before changing cats).

Another friend here has a naturally aspirated X150 XK, he changed the cats to 100 cells and had a tune done on the car, it is definitely quicker than a stock car.

Another friend here has an F-Type V8S that's had several different tunes in it, he had 200 cell cats fitted and he felt (**** dyno) that the car lost some bottom end response & torque after fitting them to the car, and it was more or less the same in the upper rev range, just louder.

So my train of though is, perhaps the naturally aspirated cars benefit more from the free-flowing cats than the supercharged ones?

I don't know, but my experience with the XJR is new free flowing cats are better than old stock choked up ones, but probably not any better than good stock ones.

 

im gona side with cambo on this one .
i have 1.5psi power house pulley / 200cell magna flow cats and 2.5'' pipes from the manifold and an x pipe / two silver bullet chambers and two step through glass pack rear boxes all 2.5''.
and i struggle to come close to the factory 0-100kph time .mine is about 5.8sec at the mo .although traction is tricky.
I'm about a quarter or half second off factory .
and consistant 13.6s and trap speed of between 105 to 107mph .
its slightly better than proclaimed stock quarter times .
although this is all a massive gain over when i purchased my car and my cats were blocked with crap .
it shows i have just about replenished all the power my car had lost in 170000km.
but it sounds 100 times better than stock !!! love it . chuck them on you won't regret it .

one thing i did notice though when i shifted the x pipe from just in front of the diff to just behind the gear box , so i could add the center chambers .
it did shift the power curve . and now has a much stronger low end to mid range punch than before .
although that could have also been the addition of the chambers in conjunction .
sorry for the lack of definitive proof . and lack of awesome quarter mile times .
but it all was still worth doing in my opinion .

 

Just wondering... what would be better to spend the money on instead of cats?

 

It makes me wonder with all of these customs exhaust companies claiming dyno proven gains with their exhaust, some fairly significant, if it is a bunch of BS...

 

I swapped the single muffler for two magnoflow style mufflers and the car became a total dog. Had no power, sounded lousy, and was big old waste of money. Would have rather given the money to feed & shelter the homeless... Two mufflers in the trash.

If you do go with an aftermarket solution, make sure it is engineered and tested.

 

In my opinion, the intercooler system, anything you can do to reduce the charge temperature will be hugely beneficial.

And so, I had a chat about this exhaust/cats subject, with my ex-JLR friend who writes tunes.

Here's what he said;

-------------------------------
⁠⁠⁠Exhaust backpressure is very important on the torque based PCM's, i.e. the 2006 onward models like an X150, since this parameter is used to calculate engine VE (volumetric efficiency).

So, put in an x-pipe or remove the cats (or install 200cell, 100cell, etc) and the fuelling would be off, because the VE of the engine no longer matches the stock parameters, resulting in high LTFT/STFT %'s.

This would limit the perfomance somewhat and/or cause some hesitation in certain RPM/load regions.

VE, in turn, is used in the torque calculation (driver demand, calculated indicated torque, which are compared to the estimated torque table in the PCM). In HEAVY cases SOMETIMES it may even trigger limp mode (been there, done that).
-------------------------------

And this makes perfect sense to me now, since we had the experience of flashing the factory XKR-S tune into a standard R and it did not make good power. Yet we have also flashed the stock XKR-S tune into other cars that were fitted with the XKR-S exhaust (with the x-pipe) and it made great power.

There are dozens and dozens of maps/tables and factors in the PCM of the X150, even things like the supercharger pulley ratio are in there and can (must) be modified. So with an X150, any mechanical changes you make to the car, be it exhaust/cats, pulley's, intakes, they have to be acommodated for in a custom tune.

Yes the PCM will adapt somewhat, but it won't be perfect, and in some cases I guess you could even go backwards, if the tune is not matching the mechanical changes.

 

Very interesting. Yes keeping to cool will always help with power or at least maintaining it.

As far as exhaust back pressure, with modifications, how do you calculate the new numbers the PCM needs to see?

 

So, in light of Cambo's post #13, here's the question for all of us 4.2L XKR owners who removed fuse 19 to keep the active exhaust open at low rpms: Was this a shot in the foot? Did this reduce back pressure and also reduce low-end torque and/or HP?

Even if it did, the improved sound is worth it.

Next time I take my XKR out for a spin, I'll put fuse 19 back in and see what my butt dyno thinks. Will have to wait for a nice day.

 

A nice bottle of single malt to help erase the memory of the money we've wasted on other mods.

 

This post does not address the removing of converters but, something similar. When I used to own my '08 NA XK I changed the factory cat-back exhaust to a very popular aftermarket exhaust with claimed extra horsepower.

Prior to making the change I did a set of 70-110 MPH acceleration test.

Following the change to the new free flowing exhaust I also made the same set of 70-110 MPH tests over exactly the same road, under the same temperatures.

I probably still have the numbers somewhere but, the change to free flow did show a significant reduction in the acceleration times, something close to 3 seconds. That was very significant. I was shifting at redline during the test.

However: the negatives: I picked up a very bothersome low end drone. Worse, my low end torque dropped significantly. Most noticeably, acceleration uphill at low RPMs suffered greatly, meaning everyday driveability suffered. I found myself manually downshifting at those situations to avoid the lack of accelerating torque, combined with bad droning.

I drove the car with the free flow for quite a few months. When I finally went back to the stock exhaust it felt like a huge improvement. After all, how many of us are spending appreciable amounts of time driving near redline, where the improved flows really manifested?

 

I don't know myself, i'll have to ask, I think there are some references which could be used to provide a fairly accurate estimate, since the backpressure of the standard system is known.

Since the valved exhaust is a factory thing, it will be accounted for in the stock tune. Perhaps not with them locked open all the time though, there is a detailed method as to when/why the valves are opened. See below;



Basically under 3000rpm is where things might not be "perfect" if you've pulled the fuse...

And this thread I find particularly interesting ==>> https://www.jaguarforums.com/forum/x...esults-172981/ the chap put a valved exhaust on his XFR, and did dyno runs with the valves open (i.e. straight through exhaust) and closed (normal exhaust) and the car made considerably less power with the valves open...

 

Cambo very interesting info
Although by looking at the thread you provided, he is loosing 25hp with open exhaust because of the super rich AFR, maybe if he just tunes for the open exhaust he will gain all the lost hp + some,, Just a thought. Maybe its the same story with the cats removal AFR must be tuned for them being out...


Back in the days I had 92 Lexus SC400 with open exhaust and free flowing intake and gained something like 30whp by tuning correct afr ratio...

 

Yes for sure, any mechanical changes you make need to be adjusted for in the tune.

In saying that, I still think it's possible to go too far when it comes to removing exhaust restrictions / reducing backpressure. Happy to be proven wrong though...