New member. Have been perusing the forums for several days.

Noted a fair amount of discussion on forced induction, and the consensus seems to be that the HE 5.3s are a poor candidate because of the high compression.

Several interesting solutions floating around.

Recently read a thread that the pre-HE pistons were dished while the HE are domed.

Just curious if a possible solution, besides custom pistons, to reduce compression, might be to use pre-HE pistons in an HE, or potentially have the HE pistons machined a bit to lower compression, assuming there's enough meat on top.

Tried searching for thread discussing, but "he" isn't exactly a unique word and doesn't get me anywhere.

 

Actually H.E. Pistons are flat tops.
I too, have been looking at options for lowering compression for a possible forced induction engine build. The problem I see with using the pre-H.E. pistons for FI is that they are cast, rather than forged, so you really don't have the strength you want for that application.

Check out the last 3-4 pages of the "so has anyone actually built a 500hp HE" thread. I think that was the one where someone was recently discussing using the pre-H.E.s, or scooping out the H.E. pistons.

 

Thanks Jagzilla. I've seen that thread, but I don't know if I read it all the way through.

Is that your car in the signature of your post? It's a beauty!

So the HE pistons are forged then? Perhaps there are some pre-HE forged replacements out there that wouldn't require a full custom set?

I'll hit that thread and see what I can learn.

 

all OEM factory V12 pistons are CAST aluminum

 

Sorry that I worded that in such a way that it seemed the H.E. pistons are forged. Like Ron said, they're all cast.

Since you would need to upgrade to custom forged pistons, you may want to do the same with your rods. I've heard conflicting reports as to whether they are cast or forged.

The crank IS forged (except for the last 2 years of production...94-96). I have no clue why they would use an expensive forged crank, and then plug the holes with cast pistons, and possibly cast rods. Perhaps the length of the crank, and the stresses involved with pushing a dozen plungers required that they be forged for longevity sake.

At any rate, the 5.3 crank is relatively easy to stroke to 78mm, with the rod journals milled down to accept Chevy rods with a 2.1" BE (much cheaper than forged Jag rods). And since you already may need to buy forged rods anyways, you might as well take the plunge, and have your crank stroked....which will not only increase your displacement, it will also lower your compression ratio.

 

Been reading up on that very thing.

I have to say, though, that it's hard to get any solid info reading through old threads.

If I decide to go with an XJS, I'll probably end up with an HE engine. Early 90s seem to be the most prevalent near enough to go get.

This probably takes the original thread off topic but, I live in a rural area with lots of twisty roads and drive a lot. I enjoy shifting, so a manual conversion is a definite must and I'm factoring the cost of that in from the start.

Will probably go with Megasquirt to manage everything and ditch the factory fuel and ignition systems, which leaves me open to a lot of possible mods.

So given that, is there any sort of consensus on what works best for my intended situation? Like best gearbox choice, rear end ratio. Should I just stroke the engine and leave it otherwise stock or are there other modifications I should pursue?

I guess I'm trying to figure out if there's a path already worn that I should follow that is something like a known-working combination.

 

That's a Photoshopped version of JagZilla, just to see how it would look if I used the rollup rear quarter windows from my convertible donor, to make a one-of-a-kind "hardtop" XJS. I'm still undecided about doing it. It's a lot of work, the difference is so subtle that most people wouldn't even notice the difference, and it most likely compromises the rigidity to some degree.

 

All Jaguar V12 rods are forged. TWR changed the rods because they broke a couple due to flaws in the forgings. For street use the stock rods and pistons are fine.

IMO the cast pistons will stand up to low boost, heck Subaru cast pistons stand up to 300hp in a 4 pot, so 500hp in a 12 should be a doddle.

The issue I see with pre-HE pistons in a HE is you will end up with a CR of about 6:1.

 

Would be interesting, but definitely a lot of work.

Am I correct that after the mid 90s facelift (or sometime after), the door windows went frameless? Hard to tell from pics.

 

Search for 2 old threads:
1. Quad Turbo XJS Build--in which I make a horse's a$$ out of myself early on.
2. Twin Turbo V12 Project

They're both quite lengthy, and they both have great info on building a forced induction Jag V12. I've just bumped them both up to the top of the forum for you.

Also read this article: Bradley Smith XJS in Detail

Good technical info there on the route one member took to supercharging his. Bradley was a great guy, and put well over $100K in his car. I almost bought it for about a third of that several years ago.

Aside from forced induction, a manual transmission will change your car more than any other modification you can invest in. A lot of people in the US use a Tremec 5 or 6 speed (drivenman.com has a kit), or, they use a Borg Warner T-5 out of an 80s-90s Mustang. Both are pretty plentiful in junkyards and on ebay. If you go with a 6-speed, I recommend the Richmond ROD 6-speed over the Tremec, because the Tremec is just a double overdrive tranny, giving you only 4 fun gears. 5th & 6th are overdriven for hiway cruising, and gas mileage. However, the Richmond box is a single overdrive, with 5 fun gears. It also has straight-cut gears, and is infinitely stronger than the Tremec, but, it's a high performance, aftermarket-only box, that was never offered in a production car, so it's both rarer, and much more expensive.

Standard mods that cost little are: better exhaust, replace belt-driven fan with an electric, cold air intakes, and a shift kit for the Turbo 400 (I used a TransGo kit in mine when I had an auto in it).

If you go with forced induction, you won't have any choice. You'll have to go with Megasquirt, Electromotive, or, some other engine management to handle spark and fuel. The stock setup isn't tunable by the user. You'll also want to upgrade the injectors to supply more fuel.

 

Thanks for the bumps and the other info!

 

as said,

preHE pistons in an HE engine, about 6.1 comp ratio.

He pistons in a Pre-HE engine about 16-1 ratio.

if you use Methanol fuel you just might make it work, around 5-6 MPG tho.

 

Been reading as much as I can digest. Lot of info out there!

The concept of reducing compression with a decompression plate is a new one for me.

At first I thought it kind of "hokey", for lack of a better term, but I'm starting to come around on it.

I originally assumed separate head gaskets would be used above and below the plate, but after some further research, it appears the plate mates to the block metal-to-metal (with some appropriate sealant) and then the gasket is used between the plate and the head.

It would seem this would be the most cost effective way to alter the compression, considering everything else has to be done in multiples of 12 (besides swapping heads). That said, I haven't found anyone online to quote a cost.

Another advantage is it's essentially reversable and transferable. Also, depending on expense, it would seem somewhat easy to "play" with different CRs by varying the plate thickness.

But I wonder about throwing off the valvetrain timing. I imagine you would need to have some sort of adjustable cam gear on one or both heads to compensate and possibly have issues with chain tensioning and length. But at least you don't have 12 chains to worry about...

 

I'm not sure that attempting to seal it to the block is a good idea, because the V12 block is essentially an open coolant jacket, so the seal is crucial. You can see what I mean in this photo. You can also see what happens when that seal is breached.


For that reason, the plate would likely need to go above the head gasket. That's just my speculation though.

The thickness of such a plate could possibly be compensated for by the timing chain tensioner, as I doubt that it would be anywhere near equal the length of a single chain link.

Good luck trying to find anyone who offers such an obscure part off-the-shelf. Its most likely something that would have to be fabricated ($$$).

If I go forced induction (and that's a big IF), I would just have some forged pistons made that will give me about a 7:1 CR, and, work out the plumbing to twincharge it. The superchargers will compensate for the low CR off idle, and the turbos should kick in seamlessly at higher RPMs. One can afford to dial in plenty of boost to make big hp, with such a low CR. Oh damn, now I've given away my evil plan.

 

Be a little positive, not a evil, but a divine

 

Chrisser, check out the recent posts on the 500bhp thread. That may be the best, most cost effective solution for lowering the CR of a 5.3 while also ensuring that the pistons will stand up to the boost.

 

The XJRS pistons?

Sounds interesting, but also sounds like they may be made out of unobtanium. Just learning about Jag sources, but the heritage site shows them as not available.

Any advice where I can find detailed specs on the engine, specifically pistons, rod, crank dimensions? Been googling around without much in the way of exact figures. Found a v12 service guide online, but it's for the 6.0 and I don't know what numbers apply to the 5.3

On a whim, I checked RockAuto Parts Catalog for pistons for an xjs to see if the specs were listed. They had no listing, but did have one for an xj12 of the same era. Was the 12 in the xj mechanically identical to that of the xjs? Curious if it was a catalog error or a difference I didn't know about.

 

Go here, and download "The Book"
Kirby Palm's Jaguar XJ-S Help Book
Its a searchable PDF book, with tons of great info. The engine info & specs you want start on page 38

The XJ12 used the same motor as the XJS, but, the motor in the XJRS was a whole different animal.

 

let me understand this.

you dont even have a car yet, and you have big plans for a much modified supercharged engine!

HMM,, very interesting.

 

We have several reliable unmodified utilitarian vehicles. I'm looking for an interesting project car.

Seems better to know what can and can't be done before buying it.

For example, knowing what can realistically done to the 5.3 to increase displacement, it may make more sense to buy with a blown engine or an sbc transplant and plan for sourcing a 6.0. Or, if an engine were to come available, it might make sense to buy it first, rebuild/modify it, and then find a car to put it in later.

Or I may just say to heck with it and plan on keeping the 5.3 basically stock and just drive it as it came from the factory.