As the title reads will the 420G engine bolt into the 340? Could I use the intake and carbs from the 3.4? I might have a chance to buy the 420G with a good engine. I'm needing something to power my 340.

 

Given up on the 3.4 litre, the 3.4 litre is the better engine ?
The 4.2's larger bores made the block and the head prone to cracking.

Don't know if it will fit, but if the 420 has triple carbs, it won't.
The intake may fit, but the carbs won't allow it to breath.

 

This has been discussed on here on many an occasion. Yes the 4.2 engine will fit. Is it an easy job, No. Engine mounts and block are different if I remember right. As Jeff has stated triple carbs on the 4.2 but you could swap the whole intake over from the 3.4 but not sure this will work. The 3.4 was actually the better engine. Better refined and smoother. Lots of the ancillaries might not swap over but you can only tell that with the engines side by side.

Try reading this post and answers. https://www.jaguarforums.com/forum/m...change-244539/

Personally I would take what you are going to buy the 4.2 engine with and give it to an engine builder to rebuild your 3.4. In the end you will have an engine which will be basically new not a worn out 4.2 that someone has said is a good engine until you try and get it running.

 

I believe the 340 has a straight port head, so the intake from the 340 should bolt directly the 4.2. An early 4.2 should be fine, it was the ones from the 80's that typically had cracking, The water pump might be different, I'm not sure if the 340 one will directly swap over. There was different impeller sizes, and I don't know what the 340 has. The motor mounts will move directly across, the oil pan might be different, but the 340 one would move across.

 

We have covered this. On top of block cracking you would have to move to 2 X 2" inch SU carbs & manifold like the 420 compact. The 3 carb motor won't fit. There are many other pitfalls that would cost. Anything is possible but costs would be excessive. Find a 340 straight port head engine & rebuild. It can be bored to 60 thou max should you wish. Direct fit. Magic smooth revving engine with great performance. Your head shaving will take to CR (compression ratio) of 9 to 1. Don't exceed 9 to 1 unless you can get (98/100 Octane) gasoline or you will suffer knock (pinging) or have to retard timing to undesirable levels.

https://www.jaguarforums.com/forum/m...4-swap-257876/

https://www.shopbhp.com/products/jag...ns-x-6-2791303

 

I already went through that with my S type. The 4.2 will fit after you solve all the issues, and there are many. I ended up rebuilding my 3.8 and a 4.2 engine mounted to a Stand. Pretty to look at, useless for my S type.

 

Yes ~ the head & block will fit. Then the trouble starts & costs start mounting e.g a pair of 2" SU Thermo Carbs at huge cost. Manifolds accessories etc.. And the list goes on & on. Spot on Jose.

 

Why couldn't the 340 manifold and carbs bolt right on? If the 340 is straight port, it will fit. The earlier B type head has a different bolt pattern, so a Mark 2/ S Type intake will not bolt to a 4.2 head. It's ever so close, but not quite the same

 

Thanks. I was just pondering this swap. Think I'll start over again.

 

Because you are throttling the engine if you fit a 4.2. You require 2 X SU 2" HD8 carburettors on a 4.2 engine minimum & that restricts output by at least 20 bhp from 3 carbs. The 3.4 takes 2 X HD6 X 1-3/4" SU carbs. (I believe that some cars had HS series carbs but still 1-3/4")

One thing I'm not sure about is if they used the same straight port size head from 2.4 to 4.2 ~ one would doubt it. The port size should steadily increase with capacity but this is Jaguar. Maybe they used the 4.2 E Type straight port head on all later models with only the heated manifold & carbs changing.

Then you had the 3.8 litre 340's ~ Very few built 8 to 10 of them. Mainly for police but some went to private people. They were badged 3.8 340"s

 

I'm not fully convinced that 2" SU's are required. On the Daimler DS420 with the 4.2 they used a pair of HIF44's, which is 44mm or 1.73" bore. The compression is slightly lower than a North American spec Series III XJ6 with EFI and the rated power is within 7hp. I think that power difference is more attributable to carb vs EFI, rather than the carbs choking the engine.

The power rating of the triple carb version of the 4.2 is a product of the marketing department, rather then the Engineering department, the 265hp rating is known to be wildly optimistic, and 180-190hp is what is generally achieved on modern dynos. That is in line with the power being less than the same compression ratio EFI application.

I did some diving into the parts books, and it's interesting. The 340 head is part C27495 from engine 7J50001 and subs, the earlier engine is C26201. The Mark 2 3.8 is C14956, which is the same as a 2.4. In the 4.2's, the Mk 10 4.2, 420 and series 2 E Type are the same C26202, E Type Series 1 4.2 is C23241. The later 420G is the same as a 420 ( which makes no sense to me, as a 420G should have had the post-1968 block with the extra two cooling passages at the rear and the longer head). The E type and 420 heads differ because of the tach generator on a Series 1 E Type and not on a 420.

A long winded way of saying that the 3.4's use at least two different heads, and they are different from the 4.2, 3.8 and 2.4.

Intake manifolds also differ, the early 3.4 is C14651, 3.8 is C14809 and 340 is C27916. Looks like thermostat housings and starting carb piping changed with the 340, as well as to the straight port pattern and 2" carbs will also fit this manifold.

 

I'm just comparing 420 power output with 4.2 E Type output & 420G. Bed time.

 

Sorry I was falling asleep last night. Had been up all night doing mod duties on MBWorld. >60% of members are from the US so I keep US hours in Cape Town.

I agree with SAE & Marketing exaggerated power outputs ~ you are spot on, Same with Aston Martin which is going to come into this conversation as their Tadek Marek 3.7 to 4 litre inline 6 engines in standard form in DB4 to DB6MkII are very similar to the XK apart from being all aluminium with dry sleeves/liners. They even look the same.

Heads are somewhat what I expected we don't know respective port sizes & finishes & alignment with manifolds etc. There is your noted confusion with some & expectation post-1968 block & longer head. I don't know any better than you do.

To cut through all this & get to the nuggets: Carb diameters do make a big difference. Is the 1.73" US spec?. Would not make sense on the Daimler as it never sold there. More a low RPM high torque at low revs stately machine. Aston also played with 2 carbs on early DB4s, then upped to 3 X 2" SUs on standard engines & as capacity increased from 3.7 to 4 litres. Vantage engines generally used 3 X Twin side draft Webers. (Vantage was what they called their performance versions of the range) badged on the chrome strakes on the engine compartment exhaust ports in the front fenders just ahead of the doors.

Enough background. Our experience in SA has been that the 3.8 S type benefits considerably from fitting 2 X HD8's of 2 " diameter performance wise. If one then Flow Benches/gas flows the inlet manifold & head you really get a large benefit without an exhaust change. (Manifolds & pipes) which if changed would likely enhance performance even further.
The Tadek Marek engines on the Astons show exactly the same benefits of going up in carb diameters.

I would not try & run a 4.2 XK engine on less than 2 X 2" HD8 carbs for performance a la 420 compact. I believe the same benefits would be seen as we see here on the 3.8 litre engines with 2" carbs & B head. I unfortunately do not have the rolling road before & after dyno traces as I have kept my car dead standard for concours. This has been done by some of our club members & they run away from the pack of standard cars when we do club runs. I drive my car. It's not a trailer queen like some. If I can't drive & enjoy it I don't want it. I just want to put it in the concours once to have our efforts rated. Covid caused cancellation of last year's concours or I would have been there.

 

I agree with what everyone has replied. I'd add the following:

Without getting into performance upgrades, the only significant advantage of the 4.2 over the straight port 3.4 is torque. To take advantage of it, you'll want to change the back axle ratio to 3.31 or 3.07, which entails an expensive rebuild.

Carbs and inlet manifold: the simplest choice would be the 2 inch SUs and manifold from a 420. These should bolt on. However, the carbs would probably require overhaul and tuning. You can keep the triple SUs of the 420G, but the battery has to move to the boot, the wiper motor may have to move and the inner wing will require some re-modelling. Air cleaning will have to be improvised. (As an aside, I understand that 2 inch SUs are good even on a 3.4 and it's a relatively straightforward upgrade, given rebuilding and revised needles etc)

The 340 has the all synchro gearbox with well suited ratios. Is the 420G an automatic? If so, there's the complication of transferring, flywheel starter etc for the 340 box.
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Is the 420G transplant possible? Yes. Will it save money over restoring the present 3.4? I very much doubt it, if it's to be well done. Is it worth the effort? Not for me. Would I do it? No. Honestly, if I were prepared to go as far as to swap my 3.4 for a secondhand 4.2 , I'd probably forget XK engines altogether and buy an AJ6/16.

 

Concerning heads and manifolds, I'm fairly sure that any straight port manifold should bolt to any straight port head. The ports on B-type heads do not align with straight port manifolds. The B type manifold can be opened out to take 2 inch SUs.

Any head can go on any block, though some water passages have to be blocked on later heads.

From what I've read, at least some of the 3.8 litre 340s had 3.8 litre engines as fitted to S-types. Presumably, these had B-type heads. If so, it raises the question of their advantage over a straight port 3.4?

 

I was wondering about your comment about going to bed and thought surely it was morning in South Africa!

Nope, my Daimler is UK spec. For our OP, I wasn't thinking of ultimate performance, it was more of a "will it work", and a pair of 1.75" carbs will work nicely on a 4.2. It can certainty haul my 6,000lb Daimler around and can cruise at 75mph down the highway. If the OP is just looking for a low cost engine to get him going, then a good used 4.2 on his carbs can certainly work. Optimum? No. Serviceable? Yes.

I know the triple SU's on an E Type engine is over carbed. I've seen an engine on a dyno and the carb pistons only lift about half way up. I doubt that much if any performance would be lost with 2 x 2" SU over triples.

 

Changing to an AJ16 is a whole league more work. A 4.2 shortblock swap could probably be done in a weekend, or a few days at most an have a running car. It's a bolt in conversion and all of the acessories and transmission can be reused from a 340. The AJ16 will need to have a new fuel system, with new high pressure lines, new cooling, the electrical will need to be changed to negative ground, ECU's will need to be wired in and a spot to mount them, the engine and transmission mounts will need to be fabricated, new driveshaft (propshaft) made, exhaust fabricated etc. The AJ16 is actually a fairly wide engine with it's accessories mounted, I have serious doubts it could be mounted in an unaltered Mark 2/S type engine bay.

Doing an AJ16 swap is orders of magnitude more work than changing a 3.4 to a 4.2, and not something I'd try on a budget.

 

They were exactly as fitted to the S Type including aircleaner (paper & across the top of the engine.) 240s with straight port head were very similar except they had the crossflow aircleaner with 2 trumpets extending from the end. 340 3.8 had the downpipe connected to the convoluted hose to the front. Yes 340 3.8s had B Type heads.

 

I agree on the effort, but once you go one step out, why not make it three? And you do end up with a lighter, more powerful, reliable engine that should last for years.

 

That AJ6/16 engine is a tough & trouble free bugger.