Does anyone here know what thickness decompression plate would be needed to drop a 84 XJS's compression down to 9:1?

 

Or would changind pistons or shortening stroke be better to drop compression?

Just doing some research on a twin turbo setup

 

Pistons would be the way to go. The ones from the 1995-1997 XJR supercharged 6 might fit.

 

ive read that wrx pistons work, what all entails on getting them to fit?

 

find the volume of the combustion chamber in an HE. and then find your overall volume minus combustion chamber volume to get your cylinder volume.

then take Cv/(X * Bore + V )
X = plate thickness
V = combustion chamber volume.
Cv = cylinder volume.

then play with your X values until you achieve a 9:1 ratio.

i think im on the right track here.

 

... but curious about what type of power numbers you're looking to make with this setup? Any thought as to location of the turbos - rear mount?

 

any idea what the measurements are on those variables?

 

Nothing super crazy as for numbers, it will still be a street car. And will have front mounted turbos.

Like i said, im just researching right now, but im mainly thinking about doing this just for the adventure or turboing an xjs, not to spin out 1000hp

 

no guessing required.

knowing bore and stroke allows the calculation of swept volume

knowing swept volume and compression ratio allows calculation of the combustion chamber volume

knowing the above and the new compression ratio allows calculation of the new required volume of the combustion chamber

the thickness of a plate is determined by the enclosed volume divided by the area.

the stock bore and stroke can be found on any search engine worth the name

No ... WRX pistons will not fit unless it was raining horseshoes when the two engines were designed by different design teams at different times in different places.

 

The simple answer is it depends on what engine you use as the CR for the HE is from 11.0 to 12.5:1 and the pre HE 7.0 to 10:1. So depending on what CR you start with the decompression plate will vary.

WRX pistons are feasible but-----

You will need to bore the sleeves as - stock they are 92mm.

The wrist pin is different and I don't know if there is enough meat to machine them to accept the Jag pin. Wrist pin size is 0.906 Jag is 0.937 and Chev is 0.927. maybe a combination of sleeve the rod and machine the piston - just seems a bit too much machining - would probably not cost much more to buy forged pistons.

I do know that Holden 202 6 cylinder pistons have been used with great success here in Aus these are 90.xxmm can't remember exactly.

You DO NOT want to use shorter rods the way to lower CR is to either

Decompression plate - not my favorite as it reduces squish but can be used quite effectively.

Lower piston compression height by moving the pin higher in the piston.

Lastly open out the combustion chamber.

 

So... 500RWHP?

I'm always interested in these developments. In the mid 90s on the Porsche 928 forums, a few intrepid souls started out with supercharging the V8. Now there are three competing (and field tested) bolt in supercharger kits.

Prices on the 928 kits run from $5500 for 400 RWHP to north of $9k for 500+ RWHP. I would love to see somebody develop a bolt in solution for the V12 XJS.

Keep us posted, and good luck!

EDIT:

Oh yeah, there's also a complete 928 twin turbo solution, but this is around $15k installed. Still, the workmanship and aesthetics are incredible. Potential for lots more power than the superchargers, too.

http://www.kuhnperformance.com/gallery.php

 

theres a thread i started about a year ago, with a TON of good info about twin turbo setups on an XJS.

but realistically, your never gonna get there with an HE motor.

theres a guy that trolls Jaglovers who has a flathead twinturbo in a zonda inspired kit car, with a 2mm decomp plate.

 

what does it take to get the 202 pistons to fit?

 

Link for the OP:

https://www.jaguarforums.com/forum/x...project-62150/

 

Any change in pistons will require the sleeves to be honed. Max over bore on the standard sleeve is 93.5mm.

The 202 pistons will require machining of the pin holes and a bore and hone of the sleeve. These pistons are used by the guyas who race HQ Holdens N/A and make about 200hp.

6MKRY9096

These ones are dished and they are what some guys here in Aus use for the 202 turbo's, they will not hold big HP the I6's make about 250HP with these - so figure about 500 for the V12.

Subaru cast pistons seem to hold up much better. I know the stock WRX can make 300hp (I have had 2) on stock pistons so the V12 should easilly hold 600hp.

 

Did some research, for a set of custom forged pistons made for the xjs will run about $1400-1500 for all 12. Seems like the most logical option instead of backyard-engineering some other make pistons to fit in the jag. And honestly it would probably be alot cheaper to compared to buying 12 other pistons and having to machine the hell out of everything

 

And the pistons will be designed to handle boost and machined to match the correct CR needed

 

Quote:

Originally Posted by jake21

Did some research, for a set of custom forged pistons made for the xjs will run about $1400-1500 for all 12. Seems like the most logical option instead of backyard-engineering some other make pistons to fit in the jag. And honestly it would probably be alot cheaper to compared to buying 12 other pistons and having to machine the hell out of everything

I started researching this almost 2 years ago and these are my findings based on doing the job properly and cost effectively.

WRX Hypatec pistons 93mm good in the WRX for an easy 300hp. Thats 75hp/cylinder x 12 = 900hp, so they are up to the task and cost about $750.00 for 12.

Rods Chev 6.0" I beam in either 2.1 or 2.0 journal (I'd go 2.0) good for 600hp in a V8 so 900hp in the V12, up to the task. Bush the small end to accept the smaller WRX wrist pin. This should not cost ore than a couple of hundred dollars.

Crank - offset grind the crank to accept the smaller journal rod, Just make sure the journal is radiused to relieve stress. 2.1 journal will give about 0.185” extra stroke and 2.0 about 0.285”. giving 6.0L and 6.2L respectively. $600

From my understanding and also some assumptions. I have not yet disassembled my engine to check these. Anyone with definitive numbers please feel free to chime in.

Assumptions
HE chamber volume is 33cc
Head gasket is 1.092mm thick with 93mm bore.
Zero deck height, meaning the piston crown is level with the deck at TDC
Deck height 221mm

WRX pistons stock are
Dish volume 13.5cc
Compression height 31.5mm


So based on the following
Stroke = 74mm
Bore = 93mm
Gasket compressed thickness = 2.0mm
WRX pistons
Chev 6.0 rods


Deck height is -0.1mm

CR would be 9.2:1

By using a copper head gasket the thickness can be changed quite easily. $400

Total cost for above not including labour is around $2000

 

Is there any benifit to go with the whole new setup with rods and crank grinding rather than just getting the custom pistons machined to fit the current components?

 

Yes the Chev rods are significantly lighter somewhere around 100g per rod that's over 1kg of rotating mass, plus lighter crank as the journals are slightly smaller. You do not need to do this you could just have new pistons made with the wrist pin in the correct location for 9:1 CR and 3mm over size as you will need to hone the sleeves anyway.

Also if you are going to the trouble of rebuilding the engine the crank regrind is quite cheap I was quoted $600 for the offset grind, more cubes are always good. And while you are at it if your engine is a rope seal have it converted to lip seal, no mods to block needed.