So I’m looking to upgrade my supercharger on my XJR. I’ve been looking at upper and lower supercharger, pulleys, I have been looking at forms people recommend running only one due to heat. So I am confused on which one would be better the upper or the lower pulley if I were to go with one, which one gains the most horsepower and torque efficiently. I am planning on doing a bigger intercooler, upgrade the supercharger coolant pump and possibly doing water methanol injection. Once I’ve done all that, would I be able to run both upper and lower pulleys and how much power would I gain if I run both?

 

I think i can answer your question since I have done all of those upgrades on my XJR.

All powerhouse UK products.

You are in the right sequence.. You should do the lower pulley last. It won't be a plug and play situation. You can look up my old posts.

You want to make sure and resolve of all the mechanical and electrical problems first.

Yes.. You can feel the big low end torque gain...

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Is there written anywhere data where is measured the intake air temperature in real world situations ?

 

We have made testings quite a lot. If you have stock engine and increase Eaton speed by chaanging the pullies you mostly change the power and torque crve up or down. If you change bigger lower pulley for 163mm - 179mm one you lower the curve by 500-1000rpm ( please check carefully that the pulley has still a working harmonic balancer ) . Top power is 10-20bhp more. You can fell big lowe and middle rpm torque
which is good for daily driving. The 4.2/4.2 charge coolers have their limitations: Capacity, design and water inlet/outplet sizes. You can get a bit better system with bigger intercooler radiator and a bit better flow IC pump but those are not enough for longer WOT. If you want to have good power up to redline you have to install water/meth system. Generally I would say that Eaton lower + upper pulley change at the same time is an overkill. Even a small SC speed increase way result 30-50F IAT increase. IAT2 is a slow sensor and not so how much it really helps to avoid knock especially by retarding the ignition on AJ26SC where the knock control is very limited.

We have tested 4.2 charge cooler bost pressure drops before and after cores. Our TS made 25PSI before the core ( measured from the " V8 part " ) but 17.5PSI under the cores. 4.2 cores are same size than 4.0 ones. 4.2 core cells have been upgraded to have more cooling capacity but sinze the core cell sizes are so small ones they do not flow as well as 4.0, which I believe are good enough good for Eaton flow. When we saw this we made one speicial pair of charge coolders. The empty ones. Unfortunately the SC makes so much heat and use E85 and big water/meth system, the IAT down to the engine was way too high at cruise speed and WOT. At least we saw 29PSI straight down to the engine just before the diff broke during a hard pull. Based on these experiences we have a route to go. This just needs some extra motivation. Even we have started the process it's not just plug and play.

 

Mind if I jump onto this thread with a related question?

I'm thinking in a few years of getting the primary chains replaced and as the front end of the car will be apart it seems daft not to get a bigger bottom pulley fitted. Is that a sensible thing to do with no other mods? I don't really need more performance, I just like the idea of being able to say it's got 400bhp as it's a nice round number unlike 375bhp they quote for the AJ27.

 

So I have basically been thinking this quite right. I have never fully believed that big ic radiator alone with the bigger pump will give way better cooling.
Basic meth injection system using windscreen washer container as combined windscreen wash and meth injection tank is the way to go. When replacing lower pulley that is.

 

There have been a few XJR engines destruction tested on here over the years, if you search you shall find it's all been very much done before. Killer Chiller kits were all the rage a decade ago.

Crank pulley install tips
Check for backlash on the S/C coupler before overdriving it, slack means refresh of the blower coupler at the least.
Rock the blower nose pulley back and forth, does it feel tight? or like its catching up?

Have the shorter belt, and fresh bearings for the idler/tensioner pulleys.
Make sure the install has been centralized and dynamically balanced or use the later pulley.
New crank bolt, cone and seal at the back of the damper. 375Nm or it will back off.