Maybe someone can enlighten me, as the answers/information seems to vary considerably.
What model Jag came with the straight port head? and there seems to be some reference to the S3 XJ6 having the same–is this correct?
Hi
I assume that you want to keep a twin carb set up
The straight port head that I think you need is one from a 340 saloon
This is compatible with your car and has the rev counter drive at the rear.
The XJ6 straight port head has two extra water ports at the rear which have to be blanked off and the
rev counter drive is not there however the fuel injection engine has larger inlet valves.
The manifold on the inlet for a 340 has provision for 2 inch carbs same as the 420 but comes normally with 1 3/4 carbs and the top hose on the rad is at right angles as opposed to straight
My car being a 380 has this setup.
If you need a picture I will try to send you one
Cheers
Brian
The term “straight port” refers to the angulation of the inlet port between its inlet and the inlet valve. It’s a bit of a misnomer because it isn’t actually completely straight on the straight port head, just somewhat straighter than on the B head on the Mk2.
They were fitted to a lot of XK engined cars; see Tim Dearing’s summary below.
The reason that people like the S3 XJ6 head for added performance is that as Brian says the inlet valve is 1.8715" rather than 1.75". This gives 15% more valve area and around 30% more flow.
AFAIK pretty much any XK head can be fitted to any XK engine. I am not sure of the exact details for every iteration. As Brian says the XJ6 heads may have extra water ports which need to be threaded and plugged.
Inlet manifolds are also head specific, and for example you may need to put a later inlet manifold on if you fit a straight port head, although again I do not know all the details. Tim’s summary covers some of this but he doesn’t cover the 4.2 heads.
XK head info.pdf (1.2 MB)
Really?? A genuine one, as in one of about 11 or so made? Did they come with the same SPH as 3.8 E Type without the extra coolant holes at the rear? BTW, I believe they were called a 340 3.8, rather than 380. Is this correct?
Hi Kevin
The picture that you show is indeed a 3.8 340. I engine number on from mine I have seen this car.
The text with it is slightly incorrect as there were 13 cars in total as confirmed by the Jaguar Heritage archivist. The term 3.8 340 is rather awkward to explain hence when one says 380 everyone knows what you are talking about… There was never any official listing for either term.
The head is the same as the E type but the manifold is for a twin carb set up
No extra water ports at the rear
I did a full two-page report in Jack White,s mark 2 section in the Jaguar Drivers Magazine
and both he and Paul Skilliter have driven my car
cheers
Brian
Hi Kevin
I have just found my article
It was in the February 2016 JDC magazine under the Jack White’s Mark 2 section
and was very detailed.
Incidentally I was in your area a few years back staying at Rotorua and visited Matamata
to see Hobitland and toured around the North Island .Very beautiful and scenic
Brian
Brian, are you having issues with your account?
Thanks Andrew–this is very much appreciated.
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Thanks Brian
you added some clarity to my murky water of understanding.
Hi Nick
I have been paying to your account but password and details seem to be confused
If you can sort this I would appreciate this
Thanks
Brian
I sent you a private message, Brian.
Hi Brian,
Thanks for the reply. Small world, I live 1\2 hour from Rotorua.WRT your car, does that mean there are 2 extant? What engine and chassis numbers do they carry ? Do you know the horsepower? How does it perform compared to a standard 3.8?
Thanks.
Kevin
Hi Kevin
, As far as the 380 goes there are 4 existing, all in the UK Engine numbers start with SE
With the straight port head, 2-inch carbs, and free flow air filter it can rev up to nearly 6 thousand rpm
and in top, it pulls 4,500 rpm (around120mph) with a 3.07 Etype diff around the long circuit of Le Mans which I did quite a few years ago when we were invited to do 3 1/2 laps on Race day
I am glad you still drive on the right over there! and by the way, I met the Kiwis rugby team over here when they won the cup in 2015 They are enormous guys and very friendly can you spot Sonny Bill
Brian
That’s him Officer, the one in the bow tie. I’d recognize him anywhere! I fink he plays 'alfback! 
3.07 diff? Is your car auto? Surely you can’t pull that high a ratio with overdrive. My 3.4 auto Mk1, converted to o\d ran about 28 mph per 1000rpm with a 3.54 diff and struggled to maintain 3000 rpm in o\d top, or about 90mph. 3.07 with o\d equals about 32mph\1000rpm. Even with the standard 3.77 diff, IIRC 5500 equates to about 145mph in o\d top.
My 3.8 ‘S’ had a 3.07 diff and that pulled it ok, not a rocket ship on take off 
Hi Kevin
Yes the 380 has a bw35 auto box and I pull away in D1 On kick down in second I am around 85mph and the car pulls well in top. On a motorway" somewhere in Europe" we managed 100mph
for one hour and I estimate we used almost 10 gallons of fuel. I put this down to the 2 inch carbs.
I think that is the price you pay for power? I haven’t changed to manual because my Heritage Certificate says this is the last auto made (3rd April 1968) The advantage of the 3.07 diff as Robins 3.8S is that motorway driving is much more relaxed although in UK no one seems to drive at 70mph
I understand the logic behind keeping such a rare car original (ish) Certainly there should be no issue refiiting the original diff head if needed, and also expect the car is a better tourer with the high diff. I just didn’t think that even a 3.8 such as yours would manage a 3.07 plus overdrive. Both my E Types had 3.07 but of course, more power and less weight. At what stage did the big Mk2s get the BW35? I thought it was only for the 2.4 and Daimler 2.5.
Robin, I take it your S Type is\was also auto? I doubt even a 4.2 would handle 3.07 plus o\d ; works out at something like 2.36.
Yes, that’s about twice the rate you have when going 60, air resistance and torque converter losses go up and it’s a large engine going fast. Maybe it’s a bit too rich up there too. Still a lot though, 28l/100km, can’t get that with a same size more modern engine…
Really? I drove my 3.4 Mk2 today and it couldn’t pull the skin off a custard.
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