The Jaguar straight 6 was a decent engine but never had the potential of other inline 6 engines. Probably the cheapest thing is a performance cam and exhaust for bolt on. All of the other suggestions are the normally good approach to any performance build of any car engine by doing better internals and having them balance & blue print the internals and get everything to flow better. The drawback of the Jag 6 is that to do a decent performance street engine by a quality engine rebuilding shop familiar with Jag's is about $20K at places like Classic Jaguars and others, Team CJ stage one engine specs and pricing You would get about 300 HP if you go to that level of a build and be a good street engine. Getting more than that would be pushing it and needing more forced induction and other things that may not make it as reliable and a more stock rebuild with mild upgrades your more likely only going to see around 170-180 HP. If you can save up and get a fully built Jag 6 with about 300 HP, that would be to me the best keeping it all Jag and be enough to have reasonable fun; then if you had more money it would be nice to go to and ITB fuel injection setup which should give you a little more power but more importantly more drivability and get rid of using carbs which is very appealing but big money as that will likely be another $10K easily if not more.

Going non stock as I did with the GM LS1 aluminum block V8 I had been running put out about 400 HP and to me that is the power that really makes the 3.8s feel alive and be on par with today's performance sedans. That was achieved with just a mild cam, exhaust upgrades and ECU tuning with no other internals nor fancy induction, just stock normally aspirated power of the LS1. I am almost finishing the rebuild of that LS1 with a fully built, forged internals, ported head, cam, upgraded induction, etc. that is your typical performance build of any engine but the difference is that it will put down 500 HP and be still a reliable street driven sleeper.

There is no cheap way to get power out of a Jag and while my approach provides more serious power, and the engine is half the cost of a built Jag engine, all of the mods to get it to fit, etc. still make it an expensive but that is the reality no matter how you go. One potential swap that might be less work to install and provide about 400-550 HP would be upgrading a Nissan RB or Toyota JZ twin cam 6 as those are similar in size to the Jag 6 but put down way more power without a fully built engine. It would still require a lot of work but a stock RB or JZ with just a mild cam and exhaust with ECU tuning will be like my LS1 V8 in power.

 

A couple of things here Primaz.

First a standard 3.8 XK engine produced 220 hp and up to 265 hp in the XK150SE with the straight port head so where you get your figures of only 170 to 180 hp from I am not sure unless you are saying with mild tuning you can get an extra 170 to 180 hp.

Secondly you keep referring to your S Type as a 3.8s but actually you have the LS1 under the bonnet but have not mentioned what CC it is producing. The LS1 can be anywhere from 4.8 to 8.4 litres so what size is yours. There is an old saying in motor sport that you cannot beat litres and in the 1960s this was proven the case when the big Ford Galaxie with its 5.8 litre V8 was taking on the 3.8 Mk2 Jaguar in saloon car racing and beating them but only just. An American 4.8 litre V8 is possibly not producing that much more power then a 3.8 XK engine where as an 8.4 has the litres advantage.

If we go back to the original question about how to cheaply get more power out of the original Jaguar XK engine I think we have to go back again to the 1960s and take a leaf out of Colin Chapmans philosophy when he said “Simplify, then add lightness”. The LS engine weighs two thirds the weight of the XK engine as it is all Aluminium whereas the XK engine was a cast iron block. If you want an XK engine to go quicker lighten the car. I remember going to Brands Hatch with my father in the 1960s. He had a 3.8 Mk2 he raced. It was a bog standard family car 6 days a week but we would turn up at the pits on a Saturday or Sunday and between my brother, myself and my father we would strip everything out of the car. that could be unbolted. My father would then go off racing and the family would use the seats that had been removed to sit on in the pits watching the racing. End of the day the car was put back together and we would all drive home for supper.

 

An engine produced in larger numbers and more recently designed will generally be more powerful and cheaper to upgrade. Almost all the rational logic will take you in that direction.

 

Going back to the Rob Beere pistons, they are probably designed for a high compression ratio and the crowns are too thin to machine down to a compression suitable for normal road driving.

 

As I say above, The Standard Eagle E Type Capacity is enlarged to 4.7-liters, enough for an output of 380 horsepower at 5,750 rpm and 375 pound-feet of torque at 4,000 rpm. 0 to 62 mph under 5 secs. Top speed over 175mph.

We should bare in mind the age of the XK design. Did OK for the day. It will always be hampered by it's long stroke & high piston speeds. Governed by tax purposes of the time.

 

I like the arrangement of the spark plug leads on that DB4GT, very neat. The pistons look a nice design and 'modern' compared with the more common offerings for old XK engines. Did a quick Google search and found some on ebay at a mere £2700 ... !

 

The 1960's ratings of 220/265 were much more created in the Advertising department than the Engineering department and have long been considered an exaggeration. It's fairly common for a good condition triple carb E Type to make around 180hp on a modern dyno. If you recall, the most powerful version of the XK engine was the late Series III saloons with the big valves and EFI and it was rated at 205hp in Europe with 9:1 compression, 171hp in North America with 7.8 compression.

As a data point, I have measured my 3.8 in an S Type that has been modified with EFI and distributorless ignition and I achieved 163 hp. I know it was going lean during the run, so I know there is a bit more power to be gained, but it was an otherwise standard engine and exhaust.I know tweaking the air-fuel ratio isn't going to get me an extra 60 hp!

 

I remember Ron Beattie saying that production as installed engines with full ancillaries delivered 160 to 180 HP or maybe 10 hp more on a kinder dynamometer. That covered the range from 3.4 to 4.2. In those days, it was usual to quote HP for engines with every possible auxiliary removed, hand set ignition and mixture and anything else to give a bigger number. Or alternatively, take a competitor's number and add 10%. Ferrari were known for quoting cavallini (little horses). The same went for Aston and all the US. However, if I remember correctly, he claimed more than 200 HP for his own twin SU 4.2 engines.

 

I totally agree that HP numbers were exaggerated just as MPG numbers are but if one company lies about the HP by 10 to 15% you can guarantee all the others are the same. I also understand that the quoted HP numbers are based on power at the flywheel not at the road tyres so what you measure on a rolling road is always going to be down on factory figures.

 

Its begs the question....

A full race XK engine from Crossthwaite and Gardiner is about £70k.

What if they built a batch of road engines, brand new, 265hp, what could the price be and how many would sell?

 

A mere bagatelle to a gent like you ~ yes, Aston spares aren't cheap.

 

The HP figures Jaguar marketed were not what was real, if you put them on a real dyno they would put more like 170-180HP instead of the 220HP they list. To get real power as I said the cheap way is a mild cam and exhaust but that will not add a huge amount. As others have said and I agree the real way to get the stock block to gain performance you would basically build the engine with better quality internals, ideally forged crank, etc. port the heads, enlarge the intake/exhaust ports with a 3-5 angle radius, etc. That is NOT cheap for a Jaguar as even a more bone stock or stock with minor cam will cost easily $15K or more. To build what others have said which is the normal approach to get power out of any engine that will set you back $20K + and the more real power to expect for a street high performance engine would be around 300-320 HP.

Team CJ stage one engine specs and pricing
they use forged pistons, ARP studs, ported head, upgraded cam, larger intake/exhaust ports, along with their standard rebuild items. It will cost you another $1K to $2K for forged connecting rods.

I did mention other options as the Nissan and Toyota twin cam straight 6 engines would both do 400-550 HP in relatively stock trim and when you build those up the same way I and others have said, the Nissan & Toyota straight 6 will do a reliable 600-750 HP in a high performance street build and in full race will do 1000 to 1500 HP and still be way more reliable than most other engines at that extreme. Doing any engine swap is not easy and not without additional costs so there is really no cheap way to gain any substantial power. I am running a 5.3 LS1 aluminum block/head V8. I mention what that does as having driven stock and modified I found that with around 400 HP the car has good power and will do 0-60 in 4.0 seconds. My old engine was an aluminum block/head 5.3 truck engine with a mild cam, ECU tuning, headers and the rest bone stock which created 400 HP. Wait until January and I will show you what those same internal mods mentioned will due to the same 5.3 without boring it out, just better internals, etc.

Now the 3.8s Jaguar is not an XK so it weighs a lot more and in stock trim it only does 0-60 in 11.5 to 10.2 seconds depending on the tranny. Thus even with a decently built stock engine it will be much quicker but really to enable that sedan to have fast performance you really need more like 400 HP, but with a quality built stock with 300-320HP it will be fun. It will be nowhere near 0-60 in 5 seconds due to the weight of the 3.8s four door sedan but maybe 0-60 in 7 or 8 seconds if you are lucky. To get a fun more peppy ride you can do that with a 3.8s with a fully built high performance Jag engine but it will be over $20K assuming all of the carbs and tranny, etc. are in like new condition. That would be a fun car but people should know it will be an expensive cost to get there...

 

The cheaper way to get a faster 3.8S/Mark 2 is to put in an AJ16 engine, at least it stays Jaguar.

 

I think thats quite a tricky/tight install though.

 

For the supercharged ( in the photo) yes, Normally aspirated is much easier.

 

There are a few NA AJ6/16 into MK2 conversions. The dimensions of the two engines are quite close. I was planning on it myself until I became concerned about the UK DVLA and possible loss of historic status/identify.

​​​​​​There are even some Mk2s that have received V12 engines. Another 6-cylinder possibility is the V6 from the modern S-type. By old Jaguar standards, it's very compact. The only one I'd consider is AJ6/16 as it maintains the character and is close to being a relative of the old XK. Beyond that, they are as robust as any engine you can find, give you as much power and more torque than a highly tuned XK6 and can be bought for a few hundred pounds.

 

You may think that a bit tight but that does not look that tight as the LS is a tighter fit If one wanted to keep it Jaguar then the AJ16 with supercharger would be a good option as that will give you about the same power 300-320 HP as doing forged internals/porting/cam/increasing valve size/etc. with the the old era Jag straight 6 but lighter and more trouble free as it eliminates the carbs and the typical cold start and other issues with that older technology. I can see why the Project Utah Jaguar restomod decided to use the Toyota JZ as that will give more power/reliability but they opted for ITB fuel injection to give it that more classic look, wider RPM range, and the sound thru their exhaust is very sweet. The V6 to me would not be a good option as that would be a tight fit and while it can be done, at that point you are better off doing the Jag XKR V8 or an LS V8 to give you more power and more torque.

 

The supercharged Jaguar V8 engines are probably an easier fit as the supercharger sits on top. Personally, I prefer to stay in line 6. I like centrifugal superchargers. They occupy little space, are efficient over a wide range and can be matched to nicely compensate the drop in volumetric efficiency of the engine as speed rises. The downside is that they cost a lot compared with any other kind of supercharger.

 

I'm in the process of converting a Daimler DS420 limo to the 6 cylinder supercharged engine. I chose the 6 over the V8 for the electronics, as the 6 has stand alone control modules for the engine and transmission and everything else can be left behind in the donor XJR.. The V8 cars are all networked with Canbus, and if something is removed from the network, nothing works. I suppose there are aftermarket controllers, but the Jaguar V8 isn't a well supported engine for controls, same with the Mercedes transmission. I have gone down the road of tuning an ECU with Megasquirt in my S Type and I don't want to do that again.

 

Yes, I can see that's a problem with the V8. You're more or less forced to use all the donor drive train and the electronics. If you take only the engine and make your own ECU, you have to find a suitable gearbox. You're right, it's easier to deal with the Eaton stuck on the side of the engine. Of course, you have a lot more space in a DS420. It will be an interesting and exciting car.