S-Type Engine

RX8 Project – Part 4, The New Engine

Following part 3 where the original rotary engine proved to be a lost cause I decided to research possible engines  that could be swapped in but there were a few criteria and limitations I had:

Size – The RX8 has a reasonable size engine bay overall but due to the size and position of the standard engine there are some limiting factors to consider. I’m aware others have swapped V8’s (among others) into RX8 shells but this generally involves extensive modification of the engine bay, steering rack and even front cross member due to the length of the engine.

Weight – The RX8 is famous for being very balanced largely due to the compact size and resulting low mounting position of the standard engine. No standard piston engine will quite match up but I wanted to get as close as reasonably possible.

Power – The standard RX8 was available with either 192 or 231 bhp and so I wanted to get to ideally the upper figure (even though mine actually started as the lower 192 model) or even exceed it if possible.

Cost & availibility – I wanted  and engine that was cheap to buy and for which spares were cheap and readily available. This was always intended to be a budget project to swap the engine more cheaply than replacing it.

After considering a huge number of options from things people have done before (VW 1.8t engine) to completely off the wall ideas that would probably upset all the RX8 purists (1.9 turbo diesel?) I eventually came across a couple really  promising candidates – the Mazda KLDE and Ford AJ series engines. These are both very compact aluminium construction V6’s which should be short enough that no modification to the front cross member should be needed. It became apparent pretty quickly that the KLDE was hard to find and attracted a comparatively high price so I ruled this out.

The AJ V6 is related to the older KLDE and is available in a few flavours. It was produced as the AJ25 and AJ30 (2.5 and 3.0 litre respectively) and were used in a a range of cars in slightly different configurations including the Ford Mondeo ST220 (along with US Contour and Taurus), Jaguar S-type and X-Type along with several others. Some (including the S-type version) have VVT.

S-type V6
S-type V6

So after an eBay search and a hard earned £165 (including delivery) later I had this prime example of an AJ25 from an S type Jag sat on my driveway. At this stage I went for the 2.5L because the 3.0L version attracts a more premium price and since I had no real idea if it I would ever get it all together I bought the cheap version. Since the block is identical for both the logic was if I made one fit and decided I just didn’t have enough power I could swap all the custom parts over to a 3.0. Clearly there’s a lot of extraneous parts on here I won’t be using and once much of this is stripped the true compact size of the engine is a bit clearer.

Stripped AJ25
Stripped AJ25

There are a few reasons I picked this version of this engine. One was that the Mondeo version, which is more common, has the water pump driven from and extended camshaft on the rear of the engine because in the Mondeo the engine is in a transverse orientation. To fit the engine to the RX8 the engine will need to be installed longitudinally and the rear will have to be very close to the firewall so this is a non-starter. The Jag version has the water pump front mounted so avoids this problem. The Jag version also includes direct acting mechanical bucket cam followers and VVT. Sadly the 2.5L generally only offers 200bhp in this configuration so it’s a little down on where I really wanted to be but the torque is 240 Nm compared to the 211 Nm peak of the 231 bhp RX8 and a considerably wider torque band so it should still go well.

Around this time I found out that the Noble M12 uses this same 2.5L engine running as a twin turbo at 325 bhp. The later M400 uses the 3.0L version of the engine but they again two turbos to it and get something in the order of 425 bhp out of it with minimal additional modifications. Reports from Noble suggest it is capable of even more but was limited because they were planning on upping the power later selling this as  another model but due due to the change of direction and ford taking the engine out of production this never happened. More info can be found here : Noble M12 History

I also made the decision to keep the RX8 gearbox so I could retain the standard carbon prop shaft in the RX8 so next up is the challenge of making an engine made by Ford, which was salvaged from a Jag, fit the gearbox from a Mazda!

More in part 5…

9 thoughts on “RX8 Project – Part 4, The New Engine”

  1. Hi

    How is your engine swop going? I came across your site when researching changing the head on my Noble for the Jag items. What are your plans with the ECU?

    Incidentally I run well over 700bhp from the engine with a single turbo, I’m after more hence swopping to the VVT heads.
    With the 3.0L If you swop the rods for stronger items and machine oil squirters to the block you can reliably make up to 500bhp without any other bottom end changes (using the standard pistons) and only a set of stiffer valve springs… so you have a lot of potential if you choose.

    Interesting project, good luck with it.


    1. Hi,

      Sadly real life continues to hamper progress but it’s moving forwards. Funny you should mention the oil squirters – I’ve actually been working on installing these recently and will put up a post soon about how I did it. I’ve also got a set of forged rods and heavy duty rod bearings waiting for the rebuild. Most of the guidance came from a website about the history of developing the Noble engines. I originally started with the 2.5 as it was intended to be a budget swap project but as I’ve mentioned elsewhere as soon as I decided to turbo that changed a bit!

      I’m hoping to get a reliable 350 bhp from the final setup, should be a healthy amount for a pretty light car! I figure if it all works I can always buy the 3.0 engine and swap over everything I’ve built/upgraded but I’ll need a whole new drive train by then but it’s good to know everything else is pretty strong!. Interesting you should mention valve springs, any idea at what point this becomes necessary?

      ECU wise I’ve got a Megasquirt 3X with an added cam sensor board which with any luck should let me control all the VVT along with sequential injection and COP ignition..

  2. Valve springs become necessary at approaching 400bhp and its when you start turning the boost up, of cause thats the ford rocker springs rather than the better jag bucket design so you might be good without… 350bhp will be very reliable if you have oil squirters and a decent ECU.

    Very interested in how your doing the squirters yourself, i’ve actually been planning over the last few weeks to mock up my own template to allow me to do exactly that myself rather than paying £400+ to a machine shop per block.

    1. I’ll write a guide on how I did the squirters when I get time, but the basic idea involved a long series 3.2mm drill bit, a lot of care and plenty of lubricant or you’ll seize it and shear the drill bit in the block (trust me, that’s annoying to fix!). On the bore side I drilled the hole out to 5mm to maybe 10mm deep using an extra long bit from the top of the bore then tapped them M5 to accept a screw in brass nozzle.

      The land area on cylinder side is fairly large so there’s a reasonable degree of tolerance the problem comes on the end cylinders because the lands are much smaller making going out the side rather than the top a serious risk. That said the 3.0 block should give you more room due to the increased bore.

      When I get my block back from the guys doing to hone & balancing I’m happy to measure everything up for angles etc if it’d help?

  3. Hi, I’m very interested in what you’re trying to achieve.
    I also have a dead RX8 but want to do the AJ swop.
    This information you have posted will help me out massively when I have the funds in place to start my swop.

  4. All makes sense regarding the oil squirters. My plan was actually to drill through the existing ones in my block into a spare crank and use that as the template… Id weld tabs to the crank so that it can only bolt in one position.

    However your way sounds like a better idea, will take a look at that first.

  5. Not sure how I never seen this before but I’m currently doing a very similar project, (aj30 onto rx8 6 speed box) but putting it into an old opel kadett! ☺️ Looking into making up flanges for inlet and exhaust manifolds and making engine mount to incorporate the oil cooler if your interested in getting them to let me know! I’m sure we can get discounts if we get them made together! Send me a mail at galliwally@hotmail.com I’m sure we can learn from each other regardless! ? I also have gearbox adaptor plate made and flywheel mods done!

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