My friend with a stock SType R and I dragged a few times last night after work. We raced a little over a 1/4 mile. I'm happy I won the 3 times but barely a full car length. Hell if I didn't have the exhaust mod and filter would I even won at all?!

My car is in top condition, great tires, etc.. I had a 1/4 tank of gas no passenger. He had a passenger and his gas was about the same. I really thought I'll beat him a lil more convincingly. I know... same engine and tranny... I know... but still! I'm also aware that the Silverstone and earlier 4.0 are a couple tenths faster because of the couple hundred pounds added to the 4.2 cars. I've seen times of 12.9 - 13.2 posted for these from the mags and 13.0- 13.4 posted for the 4.2 with the str having 13.3 to 13.6.... if it was the 4.2 XJR I would be surprised with its light weight aluminum body.

I need to get a lower pulley for xmas and go get him again. I may go for some custom magna flow hi-perf cats. What else can I do without breaking the bank? I'm only trying to have a 12.6 2005 XKR.

Juke

 

NOS should do the trick - I've seen it on the F&F films, everything goes blurred apart from lights either side flashing past really fast

 

No thanks... My car has an extended warranty! But it would be fun... maybe Diesel and Tyrese will race the winner.

 

Juke, you might want to read the extended warranty paperwork. Modifying the pulley or exhaust, and/or engaging in street racing, may void it anyway.

 

I have the lower pulley mod and it does make a big difference. However, the biggest difference I've seen is from the addition of an LSD and rear axel ratio change to a 3.27 (vs. stock 3.06). However, I understand that a rear axel ratio change is not possible on a 4.2 liter XK8/R. I can't remember the reason.


Mark

 

On the exhaust it's not an issue and with the lower pulley it's okay. My Indy says on the pulley he'll install it as an "oem replacement" but on the upper pulley it'll be an issue because of the grinding necessary to install.

I don't street race on neighbor roads. We were at an industrial park and the Fed ex guys thought our cars were 2014 models!

 

If you modify the exhaust, you lose boost, your going to have to upgrade the lower pulley for sure.

 

Really? I thought freeing up the exhaust would help not hurt a SC car?I

 

It helps the high end, not the low or mid-range. Jaguar states that the XKR tops out at 175 mph. Unless you're on the track and have very large b*lls, it is unlikely any of us will ever see that speed, especially since the ecm limits speed to 155 mph. What's important to me is the low and mid-range. This is the reason why, in addition to the lower pulley mod, I changed the rear axel ratio but left the exhaust alone. That, and I also use my XKR as a real GT should be used, for very comfortable long range cruising.


Mark

 

It is a misconception that it is backpressure that improves low end torque, it is really exhaust velocity that is important.

An engine is essentially an air pump, and removing restrictions on the outlet side is a good thing for power. Less work goes into moving the waste gas out and more work goes into turning the wheels.

On a supercharged car, since the blower is directly tied to the crank, it moves an amount of air proportional to the rotation. An Eaton isn't a perfect positive displacement device but for simplicity you can think of it that way.

You may in fact decrease the observed boost level because with less restriction to get the exhaust out, more fresh air can get in. And since there is a fixed amount of air being pumped into the manifold with every revolution, the pressure goes down, but not necessarily the volume of air that gets into the cylinders.

Lower boost can actually make more power in the right situations.

The thing that a big exhaust system can do is decrease the velocity of the exiting gases. At a high velocity, they create a sort of vacuum that helps to sucks the exhaust out of the cylinders.

Going with a large pipe, decreases exhaust velocity until you get to higher RPM, where the air pump is making more exhaust gas and the velocity comes back up to get good scavenging.

So, you can lower boost and make more power and you can also have a big free flowing exhaust and lose power in the ranges you spend most of the time.

It's all about balance.

 

Great post

 

You will never know which car/driver is quicker until you take it to the track. Was your traction control on? That can kill performance. I beat many quicker cars because of better reaction time and/or 60 ft time. Also, a full car length is nothing to sneeze at.

 

Rear gears is what our cats need. The DOHC small 4.0 displacement engine, struggles to get in its powerband fast enough.

3:27 gears would be the perfect all around solution, however,

1. I understand this will cause a "incorrect gear ratio" error of some sort, and any tune, even minor, ive been told is $800, even if all you have changed is the gears.
2.Don't know of anyone that has limited slip setups and gears for us.

Maybe someone can correct me if there is a better solution to this issue.

 

I've been told that doing the lower pulley on a 4.2 is a waste of money because it is already bigger than a 4.0 (which is what the lower pulley upgrades were made for). I'm told the difference is negligible. The other thing about a 4.2 is they screwed around with the sensors and you can't put in a lower rear end (they do make them) limited slip without throwing a code. There are a couple folks working on a fix for this but I'm waiting until one of them reports it actually works as that part isn't cheap and I'd hate to have to take it out again. If you do go to a smaller pulley you will have to do something about the intake temp as it will spike. The Eaton's really heat up the air and the more you work them the hotter the air gets removing the power you would otherwise gain. Consider a chiller killer but both isolate the cooling circuit and put in a second pump otherwise you don't get enough heat transfer. I didn't do either and it isn't getting cold enough (I do have the smaller pulley).

 

They have ford cobra units on ebay under $200 that includes two fans. That will really help on the heaton lol.

 

This is true. You can get the 3.27 rear axle ratio out of another Jag, I forget which one. XKRacer can help with that, however it does require reprogramming the ecm. This costs about $2000. Also, as stated above, This will not work on a 4.2 model. I bought mine from another forum member. But for the record, it is now seriously quick off the line. If I forget and step on it when a red light changes, I scare the sh*t out of myself.


Mark

 

It is the Mexico region XK8 that had a 3.27 gear.

 

That's true but there is also another Jag, one of the XJ variants. I just can't remember which one. My original 3.27 rear end broke a tooth and I had to replace it. I bought one from XKRacer.


Mark

 

Wow good stuff!

 

I pmed him and sent a email separately, no response, maybe hes on vacation