Looking for more performance. 3.4l Mk2

So I have just bought a 3.4 Mk2. It’s in good condition mechanically with a solid, recently rebuilt engine and 5 speed manual gearbox, but I want a bit more poke when I push the accelerator.

Options I have in my shed include:

Several E Type heads; both 3.8 and 4.2.
A set of D Type grind cams.
Several carby options including manual choke twin HS8 SUs and triple HD8 SUs.

I also have the parts and expertise to build a 4.5l motor with either standard E Type or big valve S3 XJ6 head and if I really wanted to I could do fuel injection on that motor too.

I’d like to start with head and induction.

Given what I have, what would you suggest my first move should be?

What is your existing compression ratio pistons? I believe the 7, 8 and 9 to one were available depending on market and other factors. All had to do with the amount of dome on the piston.

I think I would be inclined to go with the ‘D’ type cams.

See if you can find a copy of " How to powertune Jaguar XK engines" by Des Hammil. Everything you need to know is in there. I don’t know how much you want to spend, but it sounds like you are capable of going big. Personally, I would go for better low speed torque for road use, so gas flow the head, but not radical and keep valve sizes standard. I’ve often wondered how a straight port head would go with 3 x 1 3\4" instead of 2" carbs would go; maybe loss of power at high revs but increased gas flow lower down? Based on 2 E Types, 3.8 with 8:1 and 4.2 with 9:1 , both 3.07 diff, the 3.8 had better overall performance and MUCH better tractabilty, as in 20mph in top gear and accelerate cleanly. Despite much work, 4.2 would never do that, although later fuel could have been a contributing factor. (1970’s vs 2000’s) You might find that SPH with 2 x 2" will be fine, plus no hassles with engine bay clearance. Find that book , a mine of information, both technical and practical.

Good for you Andrew! It’s widely considered that the 3.4l is the smoothest of the bunch and this is my personal experience. Was it previously an auto? If so it may still have a 3.54 diff - can make it a little slower out of the hole than the 3.77 that came with MOD cars. I think Ian’s car (England) has a few performance modifications - in a more modern 3.4l (from memory). Maybe we will hear from him. Paul.

Yes it was an auto Paul and yes I’m keen to see if what they said about the 3.4 is true.

I’ve got a lot of potential options, so many in fact that I’m a bit unsure as of where to go.

AS far as the engine number goes it’s 8:1 but it’s anyone’s guess where things are now. First priority will be a compression test, if and when work lets up for five minutes.

Yes, the clearance issue for the front carb, plus the simplicity of using the same inlet manifold does make the 2" SUs look attractive.

You’ve reminded me I have a copy of Des’s book. I’ll pull it out and have a look.

3.4 also the most robust version, I believe. Back in the day, Coombes modified Mk2’s for racing or road and quote figures for a modded 3.8 with gas flowed head, 9:1 pistons. 2" carbs ( only 2 I believe and B Type Head) and exhaust mods. 0-60mph 6.9sec, 0-100 16.2sec and standing quarter 14.7 sec. They claim the car was quicker to 110mph than a standard E Type. With modern tuning parts and 60 years of new knowledge, you should be able to improve on that, but that’s still a pretty lively performance.

To my mind there is a very short list of where to start:

  1. Fit a modern ignition, for ease a 123, for more configuration, the EDIS/Megajolt system. This lets you ensure that the all-important timing is optimal. If the timing is crummy, which it will be from anything based on the ancient contraption of whirring magnets and springs, nothing you do will help much at all.

  2. Just go straight to EFI and get that side optimal as well. You can mess around with any number of carb and needle combos, but at the end of the day it’s worn out old tech and there are better options. For looks you can even make it look something like original.

The XK is an incredibly forgiving engine and will run passably on crummy timing and flakey mixture, but the difference it makes to actually having it running properly is vast. Plus, there are lots of mods that promise a few hp here and there. In my experience, to really notice a difference you need a pretty large increase. Most of it’s just placebo and lies.

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I have a SPH with 2 x 2" HD8s in my Mk2 and it’s a tight fit, though it’s a 4.2 not sure if the overall size is any different to a 3.4. I want to do EFI on mine as I’ve found the mixing and matching of parts, especially with upgrades, leaves tuning and perfecting to be desired.

Are you using the manual choke setup on the HD8s? Are you using the stock Mk2 inlet manifold or something else?

I spent the weekend tuning the car and trying to adjust the throttle linkages to actually get full throttle and also get it to return reliably to idle. The lack of access to the linkages is making me think seriously about making up a cable throttle and manual cable choke arrangement.

It’s interesting comparing the 3.4 Mk2 with my 3.8 and 4.2 E types. To get the 3.4 to perform you need full throttle. I understand that it has less torque and more weight than I’m used to but it feels like it would really improve with a better head and bigger carbs.

My motor is a 4.2. It has an intake manifold from (I believe) a 420 sedan, which fits the straight port head and holds 2" carbs. The carbs have a AED on them like the original saloon. I didn’t install the linkage but it looks to be original to the factory 3.8.

IIRC, the 1 3\4" carbs and the 2" ones are the same size manifold face to aircleaner face, so there are no additional clearance issues and the Mk 2 aircleaner bolts straight on, although the ports in the manifold and aircleaner need opening out. Don’t forget that the 240 used SPH with 1 3\4 carbs.

After another drive I’ve realised what I’m missing in comparison to my E types.


So my plan is to build a complete new motor. I have a 4.2 short stud engine and straight port head. I have set of new “D type” profile cams.

I expect to be ready to install by the end of the year.

Yes, you can’t beat it. Over the years I’ve owned or driven plenty of cars with plenty of Voom, but the torque of the XK IMO ( limited, admittedly) can’t be beaten. Especially memorable was the 4.2 MoD S1 XJ6, which was awsome in the 50-80mph region. Flick out of O\D and just go. Other cars which may have been faster 0- whatever, often needed the snot revved out of them to go, by which time any passing opportunity was often lost, where the XK engined machine could be gone. Same applies to 3.8 Mk2 and esp E Type. As the saying goes “horsepower sells motor cars, but torque wins races”
Mk2 with 4.2 should go pretty well especially if you don’t tune it too highly. Great project.

I think that is your best bet!

Wouldn’t a stroker crankshaft add some science & fun to the equation? 4.7L should haul the extra weight around nicely.
Might take it out of the realm of low budget/use what you have laying around, however.

I have considered that, and in fact that was my original plan. My thinking is that the 4.2 XK engine is already a stroker by virtue of it being significantly over square and I’m not sure what compromises would be made to gain an extra 400ccs. The additional cost of a new crank and rods, or special pistons, also put me off a bit.

It seems that I can only overbore to 40 thou, and that only gives an extra 100ccs. Given that I’ll replace the liners in the short stud block I have with top hats anyway I’ll probably end up going back to standard.

I’m hopeful of generating 260hp or so and loads of torque with what I build.

Off to raid the piggy bank.