So I installed a boost gauge in my center vent in the clock position. Pics coming.
my issue as I suspected I am only getting 10psi of boost. Maybe a bit more but not 13 to 14 which seems to be what I should have.

Im thinking possibly a weak fuel pump. The car runs fine but initial start up takes a little longer than when warm which I have read can be a clue.

What else should I look at for low boost on a good running car.

 

Obviously vacuum leaks. Not my forte' with these but the knowledge is here.

 

I presume you are talking about X308 (4.0 lit.) supercharged. The system has a variable boost and the full boost will only come at about full throttle. How did you check for the boost level?

 

Yes x308 4.0, full throttle multiple times through the day.

 

10PSI is same than some others seen for stock 4.0 supercharged. Factory says 11.6PSI for 4.0, but looks like 9-10PSI in real life for these quite old cars.

 

If you want a bit more boost, you can install a slightly smaller (6%) pulley on the supercharger. This is an easy upgrade as it does not require machining of the snout, nor chipping of the ECU, and you can use the same belt. Another thing affecting the boost is the efficiency of the charge cooler. When the intake air gets too hot, the ECU will start reducing the boost. With a smaller supercharger pulley, it is advisable to also install a higher efficiency charger cooler radiator and a higher flow electric pump for it. You can get a lot of info on these upgrades if you search the forum.

 

How accurate is the gauge? Have you checked it ? Not sure how the fuel pump could affect the boost which will be purely related to crankshaft RPM.

 

I found other posts referring to the two fuel pumps being an issue with performance if one pump isn’t working. Not sure how it affects the boost but I assume a lean condition leads to reduced boost.

sounds like my 10psi May be normal based on other replies although the factory spec I read was 13.6 psi.

 

13PSI for 4.2

 

Check that the supercharger by-pass linkage operates freely and the small vacuum hose is connected. It is hard to see between the intercoolers - a small extension mirror helps.

 

Extract from the Jaguar Service Training Course 881, for AJ26/AJ27:

11.6 psi is the maximum boost pressure declared. This could be achievable in certain ideal conditions - new air filter, with denser air (on a cool day at sea level) etc.

 

Not quite. The Jag supercharger system is "power on demand" or "boost on demand" type. The higher boost comes in mainly during acceleration and depends on how hard you want to accelerate. During cruising at part throttle, the boost is much smaller.

 

AKA bypass valve doing its job

the more wheeeee you hear the more boost you are making

 

Love the boost whine

 

I don't know how ECU can adjust the boost just because it does not control the bypass valve? Valve is controlled solely with vacuum by the same vacuum line what goes to the evap etc. Atleast this is how it is made in my car. I did even have massive vacuum leak in the particular line and it did not affect to the supercharger at all.

 

In the case of too high intake air temperature, the ECU will reduce the throttle opening and reduce the engine output. The reduced throttle opening will also create higher intake vacuum which will open the supercharger bypass valve and reduce the boost. Happened to me once when, during a spirited drive, I used almost full throttle repeatedly a number of times in succession. The power/boost reduction was momentary and was back to normal as soon as I eased off a bit. After a few seconds of "rest" I could again use high boost. The SC cooling system appears to have some limitations - it is not designed to sustain "street racing".

With regard to your case of having good boost even though you had vacuum leak in the line related to the operation of the SC bypass valve, if there is a vacuum leak it will result in the bypass valve not opening or not to opening fully. This will cause higher or full boost all the time. How the bypass valve operates is that, at high vacuum the valve will open meaning low boost; at low vacuum due to wider open throttle (or due to leaks in the vacuum line) the valve will close meaning high boost. The valve is normally kept closed (high boost position) by its spring; the vacuum acts against the spring to open the valve and reduce the boost.

 

Yes. That is how it works. Ecu does not directly control boost level. I knew this and that's why I was asking it. But don't know how to ask in English in certain way..