[Lumps!] Engine + trans suggestions for 81 XJS

Lumps
1

I’m considering acquiring a 1981 XJS. The HE motor runs but
badly and appears to be a nightmare of cobbled parts. The
car is otherwise in excellent condition.

I’m thinking of installing an aluminum V6 motor with FI and
its transmission in an effort to improve the weight
distribution and fuel economy.

It has been decades since my last swap.

Suggestions, please.–
gypsy 85 california vanden plas
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2

Although on paper the weight savings and smaller displacement of the V6 seemed to point to better mileage, the reality is the car will likely not be any more fuel efficient than with a larger V8. To move these heavy cars around, a smaller engine will run higher into the power band and as a result not even close to its optimal fuel efficiency point. So the end result would be a car that is slow, gets disappointing mileage, and likely was expensive to build since everything would have to be re-engineered for the install. I get questions like this may 3-4 times per month, and the callers are usually very disappointed to hear the reality. That’s bad news.

The good news?? If you do a fuel injected V8 conversion, you can expect as much as 25 or more mpg on the freeway. That’s pretty darn good for a car that weighs 3800 lbs, has 300 hp (if you pick the right V8, like an LT1) and is also fun to drive. What’s not to like???

I hope that helps

Andrew
www.JaguarSpecialties.com

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3

In reply to a message from A sent Sat 3 Mar 2012:

Andrew,
Is the LT1 a better choice than the LS* engines? Been
wondering this myself.
Bill–
The original message included these comments:

The good news?? If you do a fuel injected V8 conversion, you can expect as much as 25 or more mpg on the freeway. That’s pretty darn good for a car that weighs 3800 lbs, has 300 hp (if you pick the right V8, like an LT1) and is also fun to drive. What’s not to like???

–
'75XJ6C about to be unlumped, '86 XJ6, '67 FHC, '75TR6
moultrie, ga, United States
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4

May I jump in here? IMHO, the LT1 is the best from “bang for the buck” perspective, they’re practically being given away now that LS engines are plentiful and getting cheaper every day. I recently saw a LS1/tranny combo go for $1800 plus shipping on eBay! BUT, the LS is better, more modern, more powerful, more efficient, etc. Either way, you won’t be sorry…go with the LT1 if you’re on a tight budget, LS otherwise.

Chuck Cote’
Atlanta, GA

Andrew,
Is the LT1 a better choice than the LS* engines? Been
wondering this myself.
Bill

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5

In reply to a message from Charles Cote’ sent Sun 4 Mar 2012:

Yup:

After years of struggle, I got my LT1 doing what it is supposed to.
I am very pleased with it.

But, I agree, the LS series engoines are even better.

Either is miles away from a sick V12.

If bucks are really an issue, a careful survey of the V12’s ills
and costs to fix are in order. When right, it is a fine engine.
High maintenance, though. Very intolerant of low maintenance.

I would not mind fussiong with one for the ‘‘fun’’ of it.

Gotta fuss with mine today or tommorrow. Battery discharge issue.
At worst, a gone diode in the alternator, at best just a closed
circuit as in boot, interior light,etc. Or just cuz it was idle for
a spell??

Carl–
The original message included these comments:

May I jump in here? IMHO, the LT1 is the best from ‘‘bang for the buck’’ perspective, they’re practically being given away now that LS engines are plentiful and getting cheaper every day. I recently saw a LS1/tranny combo go for $1800 plus shipping on eBay! BUT, the LS is better, more modern, more powerful, more efficient, etc. Either way, you won’t be sorry…go with the LT1 if you’re on a tight budget, LS otherwise.

Is the LT1 a better choice than the LS* engines? Been
wondering this myself.

–
Carl Hutchins 1983 Jaguar XJ6 with LT1 and 1994 Jeep Grand
Walnut Creek, California, United States
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6

In reply to a message from cadjag sent Sun 4 Mar 2012:

I went to wikipedia to find out about LS engines, but all I
come away with is confusion…

I definitely want aluminum.

Could someone suggest a beginning model year, please?
Perhaps a recommended displacement also?

Are these motors as picky about bore:stroke ratio as the
early small blocks were (think 307 CID, a disaster IMO
whereas a 302 was as good as it gets - again IMO)?

What transmissions are normally used with the LS? (Might
the trans be a big installation headache?)–
gypsy 85 california vanden plas
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7

In reply to a message from gypsy_vp sent Sun 4 Mar 2012:

I have to disagree with your assessment. The 307 is simply
a 283 with a 327 crank. We used to build these pretty
routinely and they are a great combo.

A 301 is the reverse - 327 bore, 283 crank. The Z28 version
is stronger of course, but this was rarely done by any hot
rodders. Who wants to lose 25 cubes? The Z28 302 has a rep
for power, but frankly, it’s a high-winding thing that would
not do well in a heavy Jag.

I would pass on an LT1/4. The LSx is the motor to choose
these days. Much better aftermarket support and fewer
unusual parts to break. Priced an LT1 distributor lately?

While the Vette, Camaro, GTO 5.7 and 6.0 are sought after,
the 5.3 and 4.8 liter versions are common as dirt. Around
here they are usually $500 engines. Another $500 will get
you the transmission, wiring, and ECU. With no more than a
cam swap you have over 300 hp on tap and still with decent
mileage.–
1983 XJS coupe
Orlando, FL, United States
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8

In reply to a message from gypsy_vp sent Sat 3 Mar 2012:

I have an '89 XJS that I am planning on doing a V8 swap. Has
anyone had any experience with a big block chevy into these
cars? I know the kit is available, but not sure if mgp is
achievable…? Any experience?–
FFIGHTERDADDY/ 1989 XJS
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9

In reply to a message from ffighterdadd sent Thu 24 Oct 2013:

Naah, but GPM probably is.

Why not come up to that kind of torque and power in a smaller and
lighter package? The LS series engines.

And, what trans would you back the BBC with?

let alone ancient carb vs FI technology???

Unless, you have a fresh BBC and tras ready to go, I’d look ahead.

Just my thinking, do whaht appeals to you.

I’m very pleased with my LT1, plenty of torque and
driveablity,although short of what a BBC could do. But, as the BBC
pulls in for fuel, I’ll cruise by in ;70 x 2.88 OD!! Engine just
barely ticking over!!

CArl–
The original message included these comments:

I have an '89 XJS that I am planning on doing a V8 swap. Has
anyone had any experience with a big block chevy into these
cars? I know the kit is available, but not sure if mgp is
achievable…? Any experience?

–
Carl Hutchins 1983 Jaguar XJ6 with LT1 and 1994 Jeep Grand
Walnut Creek, California, United States
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10

What is MGP?

The big block will fit, but with it comes a host of issues:

  1. Weight- that engine is no lighter than the original V12, so the car will still be very nose heavy
  2. Cooling- a special custom built radiator will be needed
  3. Exhaust- there are no standard GM manifolds that will fit the engine and chassis, and no off-the-shelf headers either, so custom headers are required.
  4. Hood clearance- this is tight on most carb’d XJS conversions- it is even worse on the big block.

Unless you already have this motor sitting on your garage floor, I’d recommend instead going for a built small block, perhaps a stroker. The weight savings going this way (the car loses 300+ lbs) makes a HUGE difference in the handling and performance, and there are none of the issues above to deal with. Better yet, step up to an injected LS engine and the possibilities are endless, and all in a package that easily fits the car with again none of the issues above.

I get maybe 1 or 2 calls a month from customers thinking about a big block install for an XJS, but once they hear/understand all of the issues that go with it, they all go the route of a built small block or later model injected engine. It just makes more sense to them…

That’s my $.02

Thanks

Andrew
www.JaguarSpecialties.comOn Thursday, October 24, 2013 8:16 PM, ffighterdadd cb350f73@yahoo.com wrote:

In reply to a message from gypsy_vp sent Sat 3 Mar 2012:

I have an '89 XJS that I am planning on doing a V8 swap. Has
anyone had any experience with a big block chevy into these
cars? I know the kit is available, but not sure if mgp is
achievable…? Any experience?

FFIGHTERDADDY/ 1989 XJS
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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11

FWIW, I’ve decided not to go forward with my 86 XJ6/LT1 swap, lost interest in it, too many cars and not enough time. So, I have a freshly rebuilt (0 hours) mildly “improved” LT1/4L60E setup about to be put up for sale. Car is in beautiful condition, the roller is for sale as well, I’m thinking I’ll net more if I sell it in pieces.

Charles Cote’
Atlanta, GA

In reply to a message from gypsy_vp sent Sat 3 Mar 2012:

I have an '89 XJS that I am planning on doing a V8 swap. Has
anyone had any experience with a big block chevy into these
cars? I know the kit is available, but not sure if mgp is
achievable…? Any experience?

FFIGHTERDADDY/ 1989 XJS
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

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//please trim quoted text to context only>On Thursday, October 24, 2013 8:16 PM, ffighterdadd cb350f73@yahoo.com wrote:

12

In reply to a message from A sent Fri 25 Oct 2013:

I’ve done both SBC and BBC swaps with an emphasis on performance.
I agree with Andrew’s assessment for all-round street performance.
I don’t think a BBC makes sense these days unless you are planning
a 540 with aluminium heads. At that stage you have performance
objectives which are getting hard to replicate economically with a
SBC or LSX. I don’t think the driveability or gearing is an issue -
4L80E with 2.88 powerlock will support a stout BBC, aftermarket
EFI is the same price BBC or SBC. But headers are
difficult/expensive, you will be sticking through the hood, and
you’ll need a quality cooling system.

Cheers, Andrew Robertson, New Zealand–
The original message included these comments:

I get maybe 1 or 2 calls a month from customers thinking about a big block install for an XJS, but once they hear/understand all of the issues that go with it, they all go the route of a built small block or later model injected engine. It just makes more sense to them…

–
Andrew Robertson
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