XJ6 ser3 3.4 carb engine compared to earlier 3.4-3.8

I am fitting into my 1968 240 a UK spec 1985 XJ6 series3 3.4 engine with AUD 357 SU carbs from a 1968-71? series1 4.2 XJ6, how does this engine compare to the earlier Mk2 3.4 and 3.8 engines regarding BHP, torque, internal components, cooling, water passages etc. is it a better engine? anything is better than my seized 240 engine though!

engine number is 8A16366-H

A 2.4 has a much shorter stroke compared to all other XK 6 cylinder engines. This does 2 things, First, Because of the longer stroke, a 3.4 or 3.8 will have a good bit more torque compared to the 2.4. What is the rear axle gear ratio in your car? Were 240’s geared lower to compensate for the more modest torque?

Second, do not be surprised to see that the 3.4 and 3.8 are significantly taller than the 2.4.

Rear axle changed for a Power Lock from a 3.8, the 240 had a very low ratio, the 240 was a better engine than the 2.4 as it had SU carbs and a straight port head but it was not worth rebuilding after sitting for 37 years! was wondering how the XJ6 series3 engine compared to an earlier Mk2 3.4 and 3.8, I bought a rusty/damaged XJ6 with five speed box for £1000.00, sold the box for £900.00, will be using the MOD box from the 240.

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240 is around 133bhp , not sure why , but my book changes to Din on the XJ6 , 3.4 XJ6 is 161 Din , 4.2 with fuel injection is 170 Din

I would think the bhp would be about the same as a 340 , 210bhp , and a 420 with s/p head 2 SU HD8’s around 245bhp

3” stroke for the 2.4, 3.3” for the 2.8 (same block per Wikipedia), the late 3.4 has a stroke of 4.2”, so different heights

Actually 205 in the old world!

What was the 240 ratio, 4.55?
I think the late 3.4 has long studs like the 4.2 but as a smaller version - not sure.
Whereas the 2.8 was based on the 2.4!

4.55 rings a bell,

I believe the late 3.4 has the same Siamesed bores as the late 4.2 and a stronger rods / crank,

Actually you best drop Jaguar World a e-mail and let them know they are wrong :mask:

The US cars with cats have 170@4500, the UK cars we’re talking about have 205@5000.
I like the 2.4 engine though is it really not worth rebuilding?

I ended up with the good running 3.4 engine for a nett £100.00, to rebuild the seized 2.4 £1500.00…£2000.00? and I wanted a larger engine so for me a no brainer, I sold the straight port head from the 240 for £250.00 and still have the manual choke SU carbs to sell so from my point of view a good deal, I generally only upgrade things when they need replacing or rebuilding so again a cost effective build although it seems to be getting to the spendy stage now!

Ian, if you look at the red inner wing you can see where I have moved the wiper motor mounts out to get the air filter in, hopefully!

Shame… but more power!
Are the mounts the same (I assume yours are at the very front of the block, so should bolt up there), and can you use the 3.4 water pump without too much modification? Better compare and adapt it now before it’s in the car half way.
Oh and the oil pan, you already asked about whether it fits.

The sump is almost identical, no mods needed but I do still have the 240 one, mounts bolt straight up, oil filter housing from the 240 is being fitted as there is no room on a RHD for the XJ6 spin on, Mk2 water pump also going on due to the length of the XJ6 nose, keeping the electronic ign from the XJ6…

Sounds good!

Sounds like a good result. But if I were in your shoes I would have searched for a 2.8 litre engine and spent the money to have it bored to make 3 + litres. It would go really well in the 240 body. It is a shame Jaguar never offered this engine in 3 litre form, if a 4.2 can be made to rev (and I had an Etype that easily went over 6000), then the short stroke would be a beauty.

The smallest engines were all tax-dodgers for countries (F, I, E, L ?) that had cheaper bands below 3L.

Back in the day the 2.8 was not well thought of, crankshaft bearings from memory? I did consider the 3.0 route but the 3.4 chose me, it was salvage on Copart, and remember this is/was an economy build, I am trying hard not to end up under water! I mentioned in the restomod thread that my first car was a £100.00 MGA, I drove that car to Greece for my honeymoon in 1975, just 50 miles on the clock, had only been on the road for three days! well I still have that car and I will hopefully be restoring it for a second time as the next project, and I have this plus another steel one to restore, maybe,

Piston burn was the problem IIRC. A peculiar geometry thing where the 2.8piston crown could melt under load if coked up? Roger Bywater mentions it. I think the oval journal thing was 4.2s.

Exactly right. Fixed mostly by heavier pistons. They never babied them when test driving so it didn’t happen until they were sold.

Talking about pistons , this was out my 240 :thinking:

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When the 3.4 engine size was resurrected in the mid-seventies it was really just a small bore version of the 4.2 with siamesed bores, liners and straight port head and was some way removed from its illustrious ancestor. This 3.4 always had S.U. carbs like its namesake but was cursed with the appalling AED which the 4.2 was fortunate to escape from for its final years.

From the AJ6 Engineering article.