V12 mixing & matching Good or Bad?

(A.J. Simpson) #1

Hi guys,
I have gathered a treasure trove of parts through the years and I’d like to put some of this stuff together. Please feel free to shoot down the idea but explain why. If moving forward makes any type of sense feel free to fill in the gaps or point me in the right direction.

Somehow I’ve managed to accumulate my fair share of 5.3 & 6.0 HE engines, Pre HE flat heads, AJ6 Engineering Pre He intake trumpets, new HE Pistons, new cylinders, completely rebuilt & upgraded Getrag R154 transmissions, Getrag dogleg 5 speed trans, Lister 5.3 5 speed 3.54 parts car, and many more parts that are to numerous to mention.

I’m guessing a standalone ECU will have to be sorted but that conversation is outside the scope of this fact finding subject matter.

So here are the two scenarios that I have in mind:

  1. Mount Pre He heads on a 5.3 block
    The reasoning is the 5.3 is over square which (IMHO) makes is a much sweeter engine and much more lively and free revving.
    Is there any additional hp to be gained?
    Is this worth the trouble?

  2. Mount Pre He heads on a 6.0 block
    The reasoning is the 6.0 starts with about 325 hp right out of the gate.
    I’m not trying to answer my own question here but should this head swap be all about hp with nothing else to be considered?
    Is there any additional hp to be gained?

I am fairly certain that utilizing the Pre He heads in either scenario will require custom dished pistons but my guess is I’d have to have at least one Piston as a guide in order to have new ones made some specified spec, which brings me to the question of compression. What would be the ideal compression to be built into the pistons that will allow the engine to run on high octane fuel throughout the country? More specifically I’m thinking of places out west like California. Many places only have 91 octane and these days one really has to sniff out fuel stations that sell Ethanol free fuel.
What head gasket to use?
Because where talking 12 cylinders I guess that means the added expense of two sets if they are normally sold in sets of 8.
What about rods? Would new rods be required?

It would be foolish of me to think that any one so called upgrade would in and of itself make the world a better place. Finding nirvana will encompass the full breath of the build and will include implementing an AJ6 intake runner, two larger throttle bodies, Euro down pipes, full custom 2" exhaust (curtesy of an old contributor Rob Wade), 5 speed transmission, 3.54 or 288 rear?, and 18" wheels.

AFAIK (correct me if I’m wrong) other than refinement there doesn’t seem to be any real differences in any of the camshafts between V12 models. Having said that, I do have a pair of Isky cams although I don’t know the profile.

Should there be any fear of enlarging the cylinders to accept larger pistons? If so, how best should this be accomplished? Would boring be safer if the cylinders were mounted in the block first or would free standing be ok?
Again, correct me if I am wrong, AFAIK boring the cylinders will yield way more torque than it will hp. HP will come from the compression built into the dome of the piston in relation to how well the heads are setup? Which would be the reasoning behind utilizing the flat heads in the first place? Although, I’m sure Chad Bolles would beg to differ.

The purpose of the exercise:
I spend a fair amount of time driving between coasts and anyone that has done it can share their own experiences dealing with altitude and mountain passes. Colorado, Nevada, and California quickly come to mind and IMO driving around just about anywhere else is child’s play in comparison. Out west is where you truly get to see what you brung.

As an aside, I picked up a really nice low mileage 4.0 XJS convertible from guy in Sacramento over the holiday’s. He told me that he had checked the oil and charged a new battery that had gone flat from sitting in the garage (I still have to track down the cause of the parasitic battery drain). He handed me the keys and I climbed into this completely unknown XJS without ever once pulling the dipstick, nor checking the tire pressure. I gassed her up and proceeded to drive the 2k miles home. On two separate occasions the car failed to start after cranking (panic set in) but after waiting a few minutes she fired right up both times (haven’t had a chance to look at it yet but hoping it’s a ground wire). Morel of the story, I love the car but will never look to drive it across country again. That little 4.0 engine makes for a great local grocery getter but a cross county runner, not so much.

I’m looking forward to the prevailing words of wisdom.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #2

The way I’d probably go is with the pre-H.E. heads on the 6.0. You will need to calculate what sort of compression height you’d need to achieve a reasonable compression ratio. And then you’ll probably need to order custom pistons; I doubt if any of the OEM pistons would yield a reasonable compression ratio.

While you’re ordering the custom pistons, you might want to go ahead and bore the liners out by a couple of mm, and then design the piston ring grooves to utilize some commonly-available rings. There are zillions of engines out there with bores in the 92mm range. If you bore more than 2mm, I think you’d want to make custom liners that press-fit into the block instead of slip-fit.

And to answer your questions about performance: Peak torque is pretty much a matter of displacement, so both the longer stroke and the bored liners will add torque. HP, though, is a matter of breathing; if you don’t improve the breathing, enlarging the displacement merely moves the peak torque down the RPM range so the peak HP remains nearly constant. Breathing is the bane of the Jaguar V12, so if it were me I’d focus on it.

You might want to spring for some custom cams, something with a bit more lift. The .375" lift of the stock cam is pathetic by modern standards; an LS engine typically has something in the range of .600" lift, which is how they get so much HP out of a two-valve engine. Of course, they use rockers with a 1.7:1 ratio to get that lift; you’d never get that much with direct-acting cams. But you can get closer to .500", I think.

I live in Florida, but it’s my understanding that engine knock is reduced by driving to higher elevations. The lower compression means less detonation. However, I think the gas companies make use of that fact and sell lower octane fuels at higher elevations. You should think long and hard about whether you want to have to run octane boosters in your car.

(Robin O'Connor) #3

That was the one thing that I took away from the post, like most things in life it comes down to the lowest common denominator, as Kirbert states do you really want to worry about finding octane boosters out in the boonies.
Living in a small country has its benefits we can get what gas we need almost everywhere :slight_smile:

(A.J. Simpson) #4

“You will need to calculate what sort of compression height you’d need to achieve a reasonable compression ratio.”
Ok, so let me get this straight. First order of business is get a Pre He piston and mount it on the crank then measure the cc’s of the dome, the deck height and the combustion chamber of the head. Armed with these numbers then it’s just a matter of adjusting the spec of the piston accordingly. Do I have that right so far?

Question is, what to do with those combustion chamber numbers? What is a reasonable compression ratio that will allow the use of 91 octane gasoline?

“While you’re ordering the custom pistons, you might want to go ahead and bore the liners out by a couple of mm, and then design the piston ring grooves to utilize some commonly-available rings. There are zillions of engines out there with bores in the 92mm range.”
I am not at all excited about enlarging the liners unless it is absolutely necessary and certainly not enough to warrant custom liners.
I guess what I think that I am hearing is that the overall problem is the head has a bad case of COPD if you will.

It sounds like I am mistaken in thinking that using the flat head would have solved the breathing problems.

As far as custom cams are concerned that would certainly take this to another level way beyond my rudimentary grasp of things.

There is always room for improvement but it’s my feeling that torque is not a problem that either of these engines suffer from.

(A.J. Simpson) #5

Yeah, that was my point from the beginning. The engine would need to be able to run on 91 octane fuel. I couldn’t be bothered with carrying around cases of octane booster.

(A.J. Simpson) #6

It dawned on me that maybe I’ve been thinking about this the wrong way. Perhaps the question shouldn’t be about how to use the Pre He heads.
My simplistic way of thinking says measure up the cc, adjust piston compression ration to some ideal number, clean up the head really good and then your done.

Instead, I can’t point to nor have I heard of a single success story of anyone that has done or attempted to merge these two engine components together.

Why is that? It is pretty hard to imagine that I am the only one that has ever thought of such a thing? I think not.

There are any number of knowledgeable minds on this forum alone that could probably whip up a successful recipe of what I’m proposing on the back of a napkin while have a cup of coffee.

(Paul Wigton) #7

All true: being a Colorado native, and having driven a mile or two, over most of the paved passes in Colorado, I never saw the lack in any good, well-sorted 5.3 over the ones Ive been over, including all the highest passes…

All were capable of breaking the posted limits uphill:wink:

(Paul Wigton) #8

That is correct, and it has to do with the reduced partial pressure of oxygen at altitude.

My 9:1 E Type did and my 10:1 Rover does just fine on 91 RON.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #9

You don’t really need to get the pre-H.E. piston. You can make all the needed calculations on paper. Note that there are 3 or 4 distinct versions of pre-H.E. piston with compression ratios from 7.5 up to 10 I think. Reportedly all have exactly the same compression height, the difference is in the size of the bowl in the crown.

The pre-H.E. head doesn’t have a compression chamber, it is flat. The valves aren’t quite flat, so it’d probably be helpful to actually “CC” the “chamber” as it were. BTW, there’s a tiny step in the surface just outside the diameter of the valve seat, and there was a thread here claiming significant performance improvements by polishing that little step smooth. Apparently flow in/out of the valve trips over that little step.

You know the stroke; on a 6.0, it’s something like 78.5 mm, somebody can confirm. You can measure the length of the conrod; I think all conrods are the same on the V12. Somebody here should be able to tell you the deck height and the thickness of the head gasket. And there are web sites that can tell you how to plug all those factors into a formula to determine compression ratio. I would suggest you aim for around 10 to 10.5 or so.

While you’re having custom pistons made, you might as well specify recesses for valve clearance. Especially if you take my advice on getting a high-lift cam.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #10

No doubt. Perhaps worth noting, though, that if you wanna get serious about driving at higher elevations, I think you want a turbo. While a normally-aspirated engine feels anemic in thin air, a turbo will continue to provide full air supply to the intake.

(Paul Wigton) #11

Yep…a thing Dr. Moss tried to show, back in the 20s, I think, up Pike’s Peak.

As it pertains to how to make a Wee-12 run well, I accede to your MUCH deeper knowledge base. I’d love to drive a sedan with a massaged, more modernized version of the 12!

It took technology about 4 more decades to prove him right! Tweety ran strongly up the Peak, but one could easily feel the loss of power at those altitudes. He wasn’t as fast, but I could STILL rage up, fairly well!

Here’s a (not-great) video, going up Mount Evans…

(A.J. Simpson) #12

Pardon the interruption
Rob Beere Racing has the answer & solutions
Big $$$, though

(ronbros) #13

lot of theory going on here , who has actually modified a Jag V12 engine internally??
i have and here is what i did, 26yrs ago, plus many other mods !

so the cam lift on a preHE head lifts .375 thou. off the seat! partialy shrouding air flow, jag engineers left it that way for manufacturing purposes !

they did some further testing and found that by milling .050 thou off the flat surface, put the valves .050 deeper into the cylinder, also reducing shrouding!
something akin to .040.050 to more cam lift at the valve !
Dyno proved it by 10/ 15 more HP at same RPM!

and to add ALL 4cycle poppet valve engines , the most restrictions are always the stupid VALVE, everything else just adds more flow restrictions!
you may have to alter chain tensioner , i didnt have to , but i think engineers designed it that way!

(Mark Eaton) #14

Aren’t the two blocks the same? I thought the difference was in the crank and pistons.

Perhaps you never hear about anyone doing this because they treat all the blocks the same? All the discussion is around swapping to the HE heads instead of using HE??

Only difference I was aware of in the blocks is the drilling for the bell housing. And I think there are 3 different patterns for that, depending upon vintage.


(A.J. Simpson) #15

I’m just not sure if the differences in the rear main seal is a result of the different cranks or a different configuration?

I’m not sure that I understand what treating all the blocks the same means?

Exactly, but as far as I am aware the move to the HE head was because of government regulations in an effort to save fuel or something along those lines. At the end of the day didn’t the fuel savings amount to 2 or 3 miles per gallon between heads?

You could be right about the 3 different patterns, I was only aware of two patterns.

Unless I misread the offerings on the Rob Beere website it seemed apparent that there is a market for using Pre He heads and custom JE pistons. They even offer enlarged press fit liners that Kirby suggested looking into. Apparently there are more than a few V12’s running around with 400+hp engines out there that are either undercover street cars or purely track cars, I for one have never seen or heard of any of them.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #16

Actually, they are quite different. Perhaps the most significant difference is the rear main seal, which is a rope seal on earlier cars but a great big honkin’ lip seal on the later engines. That seal is that large because they had to make the land the same diameter as the flywheel mounting flange. And to make it all work, the lip seal is mounted in its own little cover that bolts onto the back of the block.

That big seal has been known to blow out. Before putting a transmission on, drill and tap for a ring of screws around the edge to positively retain that lip seal.

Another difference is that the later block has a “waffle” appearance to it, all over. Presumably that means Ford made it either lighter or stiffer or both.

There’s a difference in the oil pump. The early oil pump is NLA, anybody needing a replacement oil pump has long needed to buy the newer style pump and an adapter kit to fit it to the earlier engine. Note that the early oil pump didn’t seem to have a problem, I can’t even recall anyone having problems due to inadequate oil supply. I also can’t recall a Jag V12 oil pump measuring within specs when disassembled; I am convinced they were not within spec coming off the assembly floor.

And yeah, there are three different bolt patterns for the tranny. The first was to fit the Borg-Warner. Second was a Jaguar V12-unique pattern to mate to the GM 400 to make sure no GM 400 out of some other car would fit. Third was a Jaguar V12-unique pattern to fit the later electronically-controlled transmission, again to make sure you couldn’t get a replacement transmission out of a Caddy.

All that said, I think the crank bores, liner bores and deck height are the same for all Jaguar V12 blocks.


50% increase over HE was claimed.

(A.J. Simpson) #18

Ah, I vaguely recollect the mileage increase was not the HE head alone but along with the introduction of the 2.88 rear end.

On a percentage basis 50% is a huge number but in reality we’re talking about an increase of around 5 miles per gallon from around 10 miles/gallon on the Pre He up to about 15 miles/gallon. Of that 5 mile increase I don’t know the breakdown between the 2.88 rear end and the May head.

If those numbers have any validity at all then it it makes the point of the 2-3 mile increase, but overall you don’t lose much mileage with a Fireball head but what you gain in breathability should be significant.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #19

Jaguar considers that all part and parcel of the same change. The May head allowed much higher compression, which provided more torque, which enabled the use of the taller gearing.

If you separate the effects, put a 2.88 diff in a pre-H.E. or shorter gears in an H.E., I believe you’ll find most of the MPG improvement – which, according to reports from owners in this group, generally went from around 12 mpg to around 16-18 – came from the heads.

Word also is that the harder you run it, the smaller the difference; the MPG benefit of the May heads is at light throttle, if you’re always driving with your foot in it you won’t see much difference.

(Scott Horner) #20

I have been through the exercise of building a bored and stroked Pre-HE V12. While my engine uses an original rope seal block and crank. If I had the ability to use a lip seal block and crank I’d take it in a heart-beat.

In my engine I had the liners bored out to 93.5 mm. I didn’t have to search to hard to find a machine shop that could do this for me. The liners also needed to be shortened to account for the longer crank stroke. Your 6.0 liter liners should already be shorter.

Semi-custom pistons (2 sets of after-market 6 cylinder mustang and falcon pistons), modified for a compression ratio of 10.2:1.

Total capacity is 6.4 liters

ECU modified by AJ6 Engineering, but the engine is in a 1992 MY car and is running a Marelli ECU complete with front and rear sensors - So it is definitely to mix and match components to suit.