I got a question via Private message and thought it may be a good idea to post it out in public so that others may benefit.

"Anyway back to my questions, when I ran into him @ Browns Lane in the late 90's he was saying the XJR was detuned from 450 hp to give it a broader torque curve. I had also heard this from someone else. Do you know what they did to detune the engine? I have a 2000 XKR that has been highly modified, including one of Andre's Twin Screw supercharges with a true 3.5" fresh air intake & ported cylinder heads as well as water methanol injection & a Killer Chiller. I am wondering if my ECU can be tuned, & if so how. I had a look at the website of Unichip & they do not list Jaguar. Any thoughts would be greatly appreciated as this car has been my hobby for the last 10 years.Thank you in advance."

Ok, I think first of all you've done the right things- tackling the intercooler and porting the cylinder heads. I wouldn't personally go down the twin scroll or lysholm style supercharger route. They may be more efficient and the tank temps will be lower- they're also quieter but because they do internal compression work you'll loose fuel economy at part load. The Eaton Rootes style super chargers don't do internal compression work and simply supply a volume. This is also true of the latest phenomenal TVS 4 lobe series. This is why they can be bypassed at part load and the parasitic losses are minimal. You can see evidence of this on the latest AJ133 and the X351s superbly low CO2 ratings. (CO2 ratings are closely related to fuel economy) Just compare then to comparable BMWs and boosted engines of similar power. Also because the Rootes style blowers are positive displacement pumps they have tremedous transient response.


I do

 

In the past I have rebuilt superchargers and I think that this is misleading. No pump makes pressure. The pressure is the result of some restriction or closure downstream of the pump. That makes the pressure. The Rootes "blowers" were used to scavenge the cylinders on the 2 cycle Detroit Diesels. Drag racers took the 6-71 blowers, spun them fast enough that they supercharged engines. The straight lobed Rootes tend to pluse as they move the air. There are Rootes units that have a twist to the lobes to stop the pulses. This can be seen on autos (Eaton Rootes). The twin screw is popular for air compressors and they can move air. However once you get to a certain point of flow, you need to put oil into them which gets seperated out after the screws. They do make heat.

One thing that the Rootes can do is spin with just vacuum power. The lobes don't touch and you can even drive one without the belt in place.

 

A twin scroll or Lysholm does internal compression work a Rootes type blower or its modern TVS equivalent supplies a volume. Regardless of definition- the bypass on the Rootes type blower gives a massive fuel economy improvement at part load. This also means that when you go to say, a 2.2:1 pulley ratio, at part load it barely effects your fuel consumption. With a twin scroll however, these is a huge fuel consumption penalty- and the best route is to clutch it out (like Mercedes has done)-except this solution is riddled with many other pit falls.

It is true that the Rootes type design has its roots rooted in 2 stroke scavenge pump development, for engnies like the Detroit Diesel, or Rootes Lister (in the UK Commer vans) etc

 

Funny enough I am not experiencing at all the fuel penalty you are describing, and think if there is one at all it will be very low (i.e. hard to notice a difference), and compensated when you use the positive pressure. The reason for the latter is that the roots is less efficient and takes more power to turn, thus you are getting more power with the twin-screw for the same fuel amount. This benefit of better acceleration (for the same amount of fuel) is not part of the European fuel consumption tests, so most likely it isn't an important factor for manufacturers when it comes to shine with fuel consumption.

Even though the twin-screw is internally compressing air (which is what makes them so efficient), the roots also has to move air (which it does less efficient) and push it thru a little hole (the bypass). Of course when running low power (i.e. cruise) the system is running in a vacuum, so very little air is going thru the system, and thus also very little effort is needed for both, and iirc I saw somewhere it would just be 1.5 hp for the twin-screw.

Now in top street trim, the fuel consumption goes up as the twin-screw needs to turn faster, however it is only about 10% more than when I had the original Eaton on it, and this is with about 480 rwhp.

When I down tune it (to about 70-80 rwhp more than the stock Eaton) I even think it is more fuel efficient than the stock Eaton. Problem is I can’t test it long enough as I miss the power, and swap back the right pulley ;-).

For some comparison, when I tested the stock Eaton with a bigger lower pulley (iirc 15-16 psi), my fuel consumption shot up about 10% yet my top end power was still around stock.

 

I have to ask with all of your knowledge of these cars, what are you going to do to your car ? What modifications do you recomend to others? I do realize that most people just talk about upgrading their cars, but there are a few of us who actually do & your recomendations would be greatly appreciated.

 

The fuel economy penalty talked about is documented from the engine dynos: When talking of fuel economy and engine modifications it is best to talk about BSFC or brake specific fuel consumption. If talking about these engine mods while then referencing vehicle mpg figures or seat-of-the pants figures it muddies the waters: transmission variability, differential wear, and other tolerances have a big effect along with your particular vehicle spec. Generally the transmission will have a much bigger effect on fuel economy then small engine changes. In the engine world we're fighting for 2-3%. 10% is HUGE!
There is little doubt that the twin screw system is far superior to the eaton rootes blower in terms of throughout of power and efficiency. The screw device is also quieter and the lower charge temperatures should allow the use of a bit more ignition advance as your knock limit should be improved- although for the man in the street to to try to optimise the ignition to the knock borderline is risky! This is all at full load-WOT. Not part load.
I MAINTAIN that the screw compressor pays a BIG penalty in terms of fuel economy at part load. A rootes blower takes about 70 Bhp to drive at full load. This is from parasitic losses/mechnical and pumping work. That is HUGE. This is not hearsay- this is fact- on an M112. The mechnical efficiency of this device is about 83-92%. So most of this parasitic loss is due to pumping. However as stated before, the Rootes blower is bypassed at partial load so only the mechnical losses are incurred and a very modest pump loss through the recirculation valve.
The Twin screw device will certainly incur less than 70 bhp collective losses at peak power and full load (because its a more efficient device) but at part load it will STILL incur both pumping losses AND mechnical parasitic losses because the nature of the device means it doesnt respond to a bypass valve. The only way to take it out of the loop is to declutch it which is very complicated (Some Mercedes did this and the current VW Golf 1.4 TSi does this). This may not be a big deal if you can live with this. In addition the twin screw device has a non linear charge delivery response so there is an slight adverse impact on throttle response compared to the Rootes device.
As far as I''m concerned Im offering very technical and professional proprietary perspective on the technical development of our vehicles. If this is contradicted in a disrespectful way, with anecdotal evidence or accounts of Big block chevys at the Carlsbad race track etc etc, or seat of the pants vehicle 'evidence'-I will now not respond to it. My time is precious and I can't be bothered. Asking questions is fine- esp in order to understand. Not agreeing is fine, but in a respectful way.

 

I will cover this in a seperate thread-please look out for it. The main issue for me is that this Jaguar project CAN'T interfere with my 1970 Dodge Challenger RB wedge/Hemi project nor my new job/supervisor position in Chicago!

 

English is not my main language and it might be that I have used too short phrases that my thoughts didn’t come across as again it was not my intention to insult you in any way!

I do understand that a twin-screw (or the scroll one) is doing its compression inside of the casting and compression = work, everyone will agree here I think.

Initially when I started this project, I was also looking at how MB used a clutch type unit to decouple the supercharger when under part load, but the mechanisms where just too big to fit so dropped the idea. I actually rather have liked a system that looks more like the gearless transmissions, but of course that would be way to complex and probably use even more power. But now as I see how low (in my terms) the actual consumption is, it is in my thoughts neglectable, and not worth the effort for just trying to be a perfectionist.

Normally I stay shy of when people on boards speak about their MPG figures as it is dependent on so many factors (tires, tire pressure, driving style, driving conditions, weather, fuel etc), you just can’t use these figures as measure to compare.

For myself I use only figures based on at least thousands of miles, with at least hundred drive cycles with similar driving to get me an idea (not a fixed value, just a good idea). And so far my fuel consumption is up by 7% compared to my stock car, and not to be picky I choose 10% in my initial post. For me this is a very low figure (especially when you compare it to other cars which have similar hp outputs). My stock car (and with my driving style) used approximately 13.6 liters per 100km and I am using now about 14.6 (both according to the board computer).

The figure will be lower when I turn down the speed of the SC (=less work), but as said I just can’t keep up with that for long (so at least the same amount of thousands of miles) to get a good idea, though the indications are definitely lower consumption. Besides the inaccuracy of the real world driving, these figures can’t be compared as I am sure the twin-screw I am using now is also different to the ones you have tested with in the past, especially by the fact that I am using a higher compression ratio amongst others (higher ratio = even more power consumption, but also more efficient at boost levels above).

I know that fuel measurements outside of laboratory has a large margin of error, which is also why I said/think that if there is a penalty it will be very low and not to be noticed. I hear you are saying there is a penalty, but when you speak about 2 or 3% as being big, then I can see from where you are coming, and this would indeed not be measurable by me with my driving style (nor been seen as a penalty as it is nothing compared to the other nice benefit of extra hp form the TS ;-).

Out of interest, which twin-screw (type/volume/internal compression) did you do the testings on, and what was actually the difference you have measured?

To the speculation I made (when I said “if there is a penalty”, so not meant as your work was wrong); In normal life the mpg will be different from the manufacturers claims (based on several field tests, and this does not mean I think the manufacturers are doing a bad job, just that in general we do not drive in the real world as is being required to test the vehicles under for whatever reason), I was speculating that there could be a difference in MPG in a positive sense for the TS, as once you go into a positive charge the TS will give you more power for the same amount of fuel, which could compensate a little for the part load situations. I can’t remember what the manufactures had to test their cars under, but from what I can remember the time allowed to accelerate to certain speeds was hideously long (more like minutes than seconds), and that would of course work against the TS. What do you think of this?