I’m pretty sure this must have been posted before but not the best at sifting through forums. Would the heads from a pre HE engine be a direct swap if I wanted to put them on an HE engine? If not, why or what would have to be changed?

 

No. Pre HE heads are completely flat and have no combustion chamber. The combustion chamber is in the top of the pistons. The HE heads have a combustion chamber in them. You'd have to change the pistons. Fuel injection would also not be correct. Not sure whether the cams changed or not.

 

Ok thanks. Wasn’t sure if it was possible or not. Thanks for the info

 

Yes, this will work with some caveats, but as mentioned, the HE pistons would probably cause an insane compression ratio. You'd either need pre-HE pistons or see how much you could cut off the top of the HE pistons (but they'd be flat w/no bowl combustion chamber. It'd probably be easier just to do the calculations and order up a set of pistons. The rods are exactly the same. Although you *might* want to consider using a better rod/piston package w/ rods that weigh less than 1lb (950grams or so) each). If you use I believe an 89.5+ block you'll have the one piece rear seal instead of the rope seal, but either will work. You need the sizing tool for the rope seal version. I can't remember the diff in the cams. I don't think it's a lot but it is published. You'd have to run the calculations on that to make sure the lift and timing didn't interfere with the pistons. There was also the full flow vs bypass flow oiling system, and the way the ancillaries up front are belted... but that's more about setup in the car than head compatibility. - and of course there'd be the issue of creating correct fueling probably needing an aftermarket ECU and rolling road tuning time.


~Paul K.

 

That’s a lot to change. Reason I was wanting to know was I currently have an 88 HE that runs just fine, but I have a line on a 6.0. From what I’ve read, the hp output on the 5.3 was 242, the HE was about the same maybe a little higher, and the 6.0 was 318 and 333 (depending on the year). Was wondering if the pre he heads would/could provide more hp to an HE v12 and if it would be easier/better just to swap in the 6.0. One plus (to me anyways) is that the 6.0 has coil packs vs the 5.3’s distributor and miles of plug wires

 

Either change has it's issues. The 6.0 swap will require rewiring the car for the 6.0 ECU and ignition system. No XJS had coil packs on the 6.0, only late XJ12s built after XJS production ended had them.
In addition, the 6.0 will not bolt up to your existing tranny as the bolt pattern on the bell housing changed when they upgraded to the 4 speed automatic from the TH400. You could upgrade to the late tranny at the same time.
​​​​​​So either swap will have it's challenges, but the 6.0 would be more satisfying.

 

Being as it appears you are in the US of A, and I think?? 1988 still got the Lo Comp engines in the 5.3.

By that I mean 11.5:1, as apposed to us 12.5:1 Comp , and very little emissions junk.

That gave us about 300+ HP, and simplicity.

Removing the emissions is one way of opening them up, if legislation allows in your market.

The larger (XJ40) throttle discs, wakes that sucker up, as does proper Cold Air Intakes.

Cam timing is SPOT ON, usually the A Bank is one Micro Tooth retarded. NOT a 5 minute task, as that huge$$ chain tensioner needs to be played with, and 20+ hours labour to install, if you are blessed.

ALL the little things are SPOT ON, and that is time consuming, and many give up, "it runs, runs well, suits me" is the usual response.

Remember also, you have a 2.88:1 rear ratio, and that makes the car a Grand Tourer, as designed.

The 6ltr, with the 4speed trans ran a 3.54 (DONT quote me), and compensated with the converter lock up.

All mine were that spot on, and on one huge trip I decided to see just how quick this thing really is, I ran out of "brave pills" at 260KPH. NO idea of the tacho etc, I was GLUED to the road, wife simply said "260, slow the hell down". BUT, that sucker was still pulling like a train, so I dont know the top end at all, just theories, and theories are rubbish to me.

 

The Americoan version had 262 hp both pre HE and HE. The compression on American Pre HE was 7.8-1 while the HE was 11.5-1
The horsepower on the 6.0 was 314 In America and 328 in The rest of the world.
Normally with higher compression comes more power but not in our case. The timing on the Pre HE was 38 degrees while the timing on the HE is 17 degrees.
The real reason for the HE was the up coming change in California Smog regulations. Since That accounted for over 50% of Jaguar’s sales. Loss of that market would doom Jaguar who went deep into debt to design and develop the V12.
While there is a slight improvement in fuel mileage due to the HE combustion chamber other changes made about the same time were the real reason fuel mileage greatly improved.
In the 3/4 of 1977 switching from the old Borg Warner transmission to the GM Turbo 400 was the biggest single improvement. Transmission slip went from over 4% to less than 2%. Other improvements around that time were a change from a final drive from a 3.07 to a 2.88, improvements in the EFI, and detail shift improvements.
The design of the HE head ( Mays head) was patterned after the Buick head of the 1920’s and later Chevy’s used until the late 1950’s called the fireball.

Now with regard using the early Flathead ( pre HE) on later engines such as the 6.0 it is possible in that it will bolt right on. But the stock pistons won’t allow it. Clunk. That’s right, the valves will hit the pistons.
With a little machine work you can use the early pistons 7.8-1 on a 6.0 liter by removing some of the top of the piston and relieving room to allow the valves to clear.
Now your compression will be around 14-1 which will call for use of E85 gasoline. (85% ethanol).
plus bigger injectors and an aftermarket ECM.
The good news is you’ll now have close to 375 hp.
The camshafts are the same from 1971 - 1997
just different clearance ramps and ways to measure them. Lift remains .375 at the valve and duration once adjusted for the different measuring method is the same. Reground camshafts to a higher lift and more duration can add as much as another 100 hp. But all that extra power will be at higher RPM. Nothing below er 4500 will be improved.
Due to the firing order of the V12 the idle will remain smooth.
Cosworth designed pistons also can be used resulting in a more practical compression ratio. Plus they are forgings that actually weigh less than the factory castings. Weight can also be removed from the stock connecting rods balance pads. The result will be a much quicker reving engine.
There are other pistons than can be modified to work for example the GM 3800 is suitable and a set of 6 costs just slightly over $100 from Summit racing. So less than $250 for our V12’s. ( pin size is wrong but a simple modification for a machine shop).
In addition there is a Venolia forging that can be modified by J&E
Why go to all that effort?
The early head is a much better head. It’s what all the racers used or patterned their heads after. Flow is multiples better then the best work out of the HE head.
The HE head is pretty limited to around 450 horsepower after a great deal of work while the Flathead ( pre HE) has made over 800 horsepower.
Now there are a set of jpg patterns to make the heads Group 44 had made floating around someplace. That’s the easy path to 600+ horsepower.

 

As jal1234 noted, going to a Sedan 6.0L is an issue because the Sedan used a Nippondenso injection and EDIS ignition system that was never fitted to the XJS. Also correct is that the car came with a 4L80 that required a trans controller, AND caused the block to be marginally changed to add some meat/mounting points for a different drill pattern. These.. are... solvable issues. 1. You can get a 4L80E controller, OR you can re-drill the undrilled bosses on the bellhousing pattern on the back of the block to fit the Th400. You're almost better off going with a 5 speed TKX kit from The Driven Man there, but that would add another $6-$8K in parts cost (likely). 2. The dead simplest thing to do is to spend 40K Kroner (er...$4K USDish?) for Ole Moebeck's packaged ECU/EDIS ignition system for the V12 (you'd have to check if he has a 6.0L map yet).. but that's probably the easiest way to get the 6.0L slapped in and up and running with the least hassle (less so than trying to transplant and wire the Nippondenso ECU out of the sedan). The 6.0L has a slightly different oil pan, but I don't think that's a problem, and you could swap the sandwich plates on the bottom if you had to. The different full-flow oil system might be an issue (pipe routing). I'm not 100% sure on that one.

Going to flatheads can be worth while IF you're committed to a big expensive project w/ lag time in parts sourcing. But it's far far less bolt together than the 6.0L option even though THAT in itself will present a challenge. Digging into a V12 is a substantial thing too. It's not like digging into a Chevy 350.

~Paul K.

 

There is no horsepower difference between the Pre HE and the HE. Both have 262 hp The 244 hp is for the carbureted engine. So the EFI engine makes 18 horsepower more than the carbureted engine.
Yes 314 net horsepower with the 6.0.
Some of the power ( about 30 hp) can be added back by simply. Removing the mufflers. Using the rear resonators to keep the noise and resonance to a reasonable level the intake is also muffled and opening that up is reportedly good for another 10 horsepower, 20 if you add cold air. ( but your fuel mileage will decrease)

 

I presume it's all the smog equipment on the engines that Jaguar had to put in US-bound cars that always affects the bhp differences. The original pre-HE engine in UK cars made 285bhp, with the P-Digital pre-HE making 300bhp. When the HE was introduced, it was quoted at 299bhp for the UK. By the time the "standard" 6.0 came around, I presume either US legislation had reduced or else the EFI engines were just more efficient? UK "standard" 6.0 XJS were quoted at 308bhp, whilst the earlier XJR-S engine was quoted at 318bhp.

Paul

 

In a sense. US ( and Canadian) cars until the 6.0 had lower compression than ROW cars. Also from about 1975 they had catalytic converters that were not fitted in the UK market. By the time the 6.0 cars came along, the UK cars were equipped with catalytic converters and the compression was standardized at 11:1 for all engines

 

Actually it’s more than just smog and compression. The advance curve on the distributor and in the later ECM program are different as well.
British octane rating allows more advance and compression. Some of that is a red herring in that American ratings are RON and MON divided by 2. While England uses the single method.
It’s been a while but I think our 93 octane is close to England’s 99 octane. Now that England is getting 5% and 10% ethanol I believe they will be very close.

 

Can you put an HE distributor into a 6 litre and and run it, with the amp etc.?

I thought I saw a mechanic do this on an E Type V12 upgrade.

Rob

 

Yes some of the blocks even have the bearings pressed in while all of the blocks are machined.
Realize the block is pretty much the same it’s the 5.3
the only real difference is the stroke on the 6.0 is 1/4 inch longer.
You can still take the bore out to 94mm ( from 90mm ) however the 6.0 has a better rear seal and a bigger oil pump.
The first batch of crankshafts are all forgings. However the last batch 1995 and later are sintered Iron. They are fine on the street. And even OK for racing. However they don’t have the ability to offset grind as strong as the Forgings do. Those you can offset from the stock 2.30 inch down to 1.94 picking up as much as .960 additional stroke. ( using aftermarket rods.)

 

Wow so many ideas! I may take this one guy up on his offer (has 2 mid-late 80’s xjs’s for $800). Both have the full original drive trains so I’d have 2 engines to experiment on. Time really isn’t an issue, money and finding the parts is more of an inconvenience. There are several machine shops around and several more shops that specialize in hot rodding and street rods, so plenty of options. All the while I’ve still got my beauty running to still be able to enjoy the drive. Hard part is figuring out which paths to take.

As a side note, speaking of Group 44. A couple of months ago I was able to get in contact with one of their members about the wheel well flares. Name is John and he’s with (may own) a company called JBL Specialty. He can still produce the flares they used if anyone is interested. Send me a msg and I’ll give you his contact info

 

One option is to turn the car into a group 44 tribute car. Either for the street or take it vintage racing. Nice thing about vintage racing is no contact is allowed. It’s real racing but racing with gentlemen and Ladies who respect cars and drive with control.
Eliminate all the messy options on the V12 and remove the EFI. Then you’ll see a beautiful engine which stock is plenty fast. But it’s easy to double the power.
The remarkable thing is chances are very good the engine itself is in great shape. With the extremely short stroke the rings don’t wear the way they do on longer stroke engines.
swapping to a Chevy manual gearbox is a relatively simple thing to do. You don’t need to buy an expensive kit. If you can drill and cut aluminum you can make the required adapter. Then your choice is 4-5-6 speeds. In Vintage racing the 4 speed is just as good as the 5-6 speed. If they are all in good condition.
I’m preparing mine and my budget could be as little as $2000. I’m going a little more so it will actually be closer to $10,000