I've not managed conclusive measurements, but I'm also of the opinion that, with a tall block, the series 3 manifold hits or is very close to the top corner of the footwell part of the bulkhead and the RHD steering column. It might be resolved by tilting or raising the engine a touch. Or by lowering the column and adjusting the bulkhead. My idea is to redesign the manifold by chopping the runners a few inches out from the injectors and welding on new pipe work giving longer, upward curved runners. At present, it's just a general concept in my head.

As to pistonheads, guilty.

 

I think my efi plan is a pair of Jenvey throttle bodies on a twin manifold.

 

I forgot to mention the simplest EFI manifold for a straight port head. That's to turn the series 3 manifold upside down. It makes access to the injectors a bit awkward.

 

I've read about that.

I'm still thinking I want to look more carefully at a triple webber manifold and some spacer adapter jiggery pokkery fron Jenvey that might bring the throttle bodies up out the way a bit. Them take cam sensing off the dizzy drive and go fully sequential.

We can but dream.

 

Interestingly, since the HS6 carbs are off my 240, tonight I was looking under the bonnet, I looked at the inlet manifold and thought them there holes look bigger than 1.5", so I whipped out my handy steel rule, sure enough the manifold is bored for 2" carbs.

All I need is a 4.2 bottom end and a pair of 2" carbs.

 

The HS6's are not bad carbs. The 3.8's were originally HD6, so the same 1.75" bore. The later Daimler limos with the 4.2 used the HIF44, which is a 44mm bore size, which converts to 1.73".

If we look at the area of the throttle plates, a pair of 1.75" carbs is identical to the throttle plate area of the EFI Series III manifold in the 4.2, which supported 205hp. It was known at the time that the flap door type air flow meter caused a reduction in power of 12%, so the throttle should be able to support 230hp. That being said, for a given area of throttle plate, an EFI throttle body will flow more than a carb, as the carb has the restriction of the jets and bridge. However, if you want to save some cash the HS6 carbs you have now will do a fine job on a 4.2 with the right needles.

 

Yes. Totally agree.

But 8's would make more noise!

I like the HS type, I know my way round them very well and I like the way the manual choke lowers the jet.

 

There are not many aftermarket companies with a lot of options for the Jaguar engines so to me what makes more sense is to either:
* Lower your performance expectations and just rebuilt the stock with mild cam, ported heads with larger valves but have it balance & blue-printed so you get as much
as can with a mild stock setup.

* If money is not a concern and you are stubborn about keeping it an inline 6 Jag, then you go for more race level forged internals, and do not waste your time with
carbs but instead go with ITB & EFI with modern coil packs and a custom ECU to really push the power as much as you can but all of that is very expensive due to the
limited vendors and companies that are available. I have ran cars with Webbers, and the better Mikuni triple carb setups and they are nice but not great to go
long distances with different altitudes, etc. so you are better off with modern ITB/EFI for the extra money.

* For big power but keeping inline 6, go for the Toyota or Nissan twin cam turbo modern RB or JZ engines as they will make the big 500+ HP and be reliable and trouble-free; you can put a custom engine cover, etc. and put a Jag logo or leaper to fake out non car guys and it will be a cool ride with way more performance.

* go for the Jag V8 but that will be even more money and hard work with limited aftermarket options; the inline 6 Jag with supercharger already mentioned is probably
the easier Jag swap

* or go with a GM LS V8 as those are so easy and so many options of aftermarket performance and ECU parts and vendors

The problem when you go beyond just a stock Jag 6 with mild cam and mild internals is that you will spend way more money chasing mediocre performance and end up spending more than either a non Jag 6 or a lightweight LS V8. That is ok if you know what you are getting into and the outcome. The Jag 6 with mild cam and internals can be fun with a little pep to it but to get big power you better have a big wallet and and an even bigger one if you want to keep it all Jag.... that is just the facts....

 

From coast to 1800 metres (6000 ft) on an Alfa, Webbers require no fiddling if you are just doing a trip & not optimising. No driveability issues. I used to do it every year for the F1 GP & a few other times. You don't have to stop & re-jet the thing.

 

The person who originally asked about upgrades is in Australia, GM LS engines are not exactly thick on the ground there. Can't recall if it was this forum or another where an LS engine was suggested to a person in the UK and they replied that the starting price for a plain 5.3 there was $10,000. So outside of the USA an LS can often be ruled out in the basis of cost.

If you're not in the USA, getting the aftermarket parts for an LS can be a problem. Many US vendors don't ship outside the USA, and of those that do the shipping is often laughably expensive. As an example I need some copper sealing washers for flare connections on an AC line. I found some from a USA vendor for a dollar each. Shipping to Canada was going to be $93 - yes, almost a hundred US dollars for something that they could have put in a envelope and sent as letter mail.

 

In Australia and the UK, you can get the Nissan RB twin cam which will be an easier swap than an LS and have the same power.

 

Yes and no. As a power upgrade, only the turbo versions are worth the effort and, from a quick look at eBay, they seem to cost two or three thousand and up - still cheaper than modifying the XK. The significant advantage of a non-turbo over an XK6 or AJ6 is in dimensions and weight. At that point, it's a question of how much more power you want and then if you want a turbocharger. For a lot of power and a turbo, the Nissan makes sense. Otherwise, it's stay Jaguar.

 

Not sure if the OP expected all of this but sticking to his original question, and assumed "budget" the cheapest ways to increase power are to go to a full loss electrical system and remove any power driven units like PS. Or add an injection system like NITROUS and wait for the disaster that will happen sooner or later. Most of the comments to the potential of the XK engine are spot on but Jaguar themselves were able to get 350 HP out in experimental versions. Jaguar did have twin plug heads but quickly plugged off one hole as they proved not worth the effort and added a negative factor toward reliability.

 

A further note on power figures. Firstly, useless without knowing the torque curve. Secondly, they used to quote SAE ratings which were higher because ancillaries including water pump could be removed and blueprinted engines with ported manifolds were allowed. So not a true reflection of stock engines. As has been said it was all about advertising and I think most manufacturers used SAE and drew curves with thick pencils on short axis. Later figures were DIN at the wheels rating so more realistic. The 245 or 265 often quoted for a 4.2 is at 5400rpm, the engine has a 5000 red line so the curve is still rising after the red line. Highlights the difficulty of getting flow through an under square engine. I recall a magazine test of a stock E type only making 137 mph and having sluggish acceleration. The factory said it had problems and returned it after a week when it did a best top speed run of 153 at 6125 rpm. An un-fettled motor would not have been safe at those revs. The 4.2 is relaxed in terms of offering decent acceleration in a heavy car without having to change out of top gear and nice if you want to stick to legal limits with only the occasional indulgence in 3 figure speeds.

To me the fun is that with a nicely tuned stock engine, a classic Jag is still a revelation in performance and surprises a lot of modern drivers. The torque curve on a 3.4 upwards makes a very satisfying overtaking response, but of course it is limited compared with a modern Porsche, for example. To me it is the whole characteristic and sound of the XK pulling strongly and very willingly, it is a wholistic pleasure.

I do wonder how you manage for insurance. When I put a hot engine in a mini, I had enormous problems insuring it for daily driver mileage and had to pay for an engineer nominated by the insurance company to sign it off as safe.

The matching between inlet ports in the XK head and the manifold was not very consistent and a useful increase in power can be had from fettling these. The works did that for press cars and referred to them as "fiddled manifolds". I think there is a record of a works memo stating that about the Mk 1 3.4 supplied to Mike Hawthorn.

IMO, the sensible output from a 3.8 is the honest 250 hp offered by the Coombs style conversion of twin HD8s on a fiddled manifold and a flowed B type head (that's not the same as the straight port E type head) and a light weight clutch and damper. That makes for a lively car. If you want match an AMG Merc with a sub 4 second 0-60 time then obviously 250 will not cut it. I have a modern V8 XF for that sort of thing and to be honest keeping a clean license in the UK would be a challenge unless you resort to false license plates, as many do. Road racing is becoming anti-social here. Some resort to track days and find themselves not so fast as they thought. There are so many cameras. I see them braking from 120 down to 30 for the speed camera and they look like twits making up for some sort of inadequacy.

When I was involved in amateur motor racing, old classics with big hot modern motors never managed good lap times on UK circuits such as Silverstone, compared with performance euro boxes or small light racers with high power to weight ratios and nimble handling.

That said, I understand the attraction of squeezing some more power. It is nice to at least be respectable. I risk attack here by saying I am not a fan of the 4.2 XK 6 engine. It was a stretch too far for marketing reasons and good for getting more torque at lower speed. Quite impressive for cruising in an XJ6 or DS420 but not a sporty motor, the engine was past it's sell by date, the tooling old etc. For racing the works concentrated on better head design etc. More capacity meant more weight moving and so less high speed durability. The 4.2 is not long lived at 6000 rpm. I think modern balancing and engineering has gone some way to improve those limits. But all this is very expensive.

The cheap option is to flow the head and fettle the inlet manifold.

 

1. sniper
2. make intake
3. install
   
   
   
   
   

 

I think WBO would like to call it a Jaguar.