I have always wondered about the above question. Any benefits in terms of the driving experience? Do the nikasil engines produce anymore power than the steel lined engines btw?

 

Hi,
The differences between the AJ26 and AJ27 are minor. I rememeber being at Bridgeend writing the manual to inform the workers of why the change was being made. Ive forgotten alot now, but lets see if I can give a high level overview.
The biggest change was going from the 2 stage variable cam phasing units (unisea) to the Denso continual cam phasers. This gives a slight torque advantage at the mid range and they claimed a 50% quicker response- but it really is barely perceptable. The biggest advantage is the ease of getting the car through emissions due to the ease of control. The pistons have bigger cut outs to accomodate the 8 deg earlier intake valve opening (obviously not on 3.2 and SC cars)
The AJ27 has 3 angle valve seats and therefore flow marginally better but you'll barely notice the difference on an engine dyno nevermind a vehicle!
I seem to rememeber that the intake cam period shortened from 240 to 230. This would have some benefit in terms of refinement.
The injectors are air shrouded and atomise better with larger number of holes- but again this doesnt benefit performance or fuel economy- just getting the engine through LEV2 emissions of the time. You can see physically under the bonnet of an Aj26 engined car that they are side fed while the later AJ27 engined air shrouded injectors are top fed.
Physically under the bonnet you'll see the AJ26 engined cars have a large throttle vacuum recepticle resevoir that you dont get on the later AJ27s as the cruise control was handled by the differently -all by the ECU.
Higher capacity oil pump is used to feed the VVT units with more oil pressure.
EGR valves elmintated for NA models and retained for S/C models only.

Regarding Nicasil vs iron liner- in theory nicasil should have an advantage in terms of lower coefficient of friction and therefore more power. However all the dyno data and motoring data I saw showed no difference at all. This means that any advantage is probably so small it is lost in the engine to engine tolerance and resolution of the measurement system

 

Wow, thanks for the detailed response! It makes a lot of sense that there was hardly any difference since I have not seen anyone on this forum say to go for the AJ27 over the AJ26 for any reason with the exception of updated water pump and etc.

 

Was there an addition of upstream O2 sensors to the AJ27?

 

The Hego (Heated exhaust gas oxygen sensor) sensor is certainly different on the AJ27 vs the AJ26, it is wideband and and more accurate. Also- the catalyst brick density changed for Aj27 and the size of the front brick relative to the rear brick and the size of the air gap between. This was for purposes of more sophisticated OBD. However the AJ26 cars has HEGO and UHEGO sensors also- they were just configured differently.
I'm attaching pictures to show what I mean- that covers the AJ27 emissions upgrade engines.

 

I know this post was a while ago but you seem very knowegable on the subject. I have put a 2001 xjr engine into my 1998 xjr , and i am not sure what parts i need to replace in oder to make it run ?

 

was it a direct swap? did you get it running?

 

I believe you need a 98 or 99 xjr or xkr engine. With an 2000 or newer sc engine you will need modifications. This post discusses a similar situation.

https://www.jaguarforums.com/forum/x...ht=Aj27s+aj26s

 

I have done a write-up on my experience with a customer that put an AJ27 engine in a 1998 XJ8. He did a good job but forgot the timing segments in the torque converter driveplate. You will get CKPS/CMPS relationship faults!!!!!!!!!!!!!!!!

bob