Citroen XM Forum

In the general case (I don't have specific knowledge of the PRV engines) a correctly tuned inlet manifold can make a significant improvement to the available torque, so the 24V manifold might improve the 12V engine, but probably not at high RPM. BUT... On a MAP injection system, fuel metering is based on a map of the volumetric efficiency of the engine, adding a differently tuned inlet manifold will change the volumetric efficiency, so the fuelling will not be optimal, unless the ECU is remapped.

Manifold length will make a fair difference depending on air speed, a shorter intake path will perform better at high engine speeds and a longer runner will work better at lower engine speeds in terms of cylinder filling, by developing an active intake manifold you end up with better performance across the rev range rather than fitting an intake that works best at just one air speed. The engines are the same capacity and given the maximum operating speed one will not require a larger inlet path, or surely they would have just fitted larger diameter inlets and called it a day?
Of course, the extra valves are part of the increased cylinder fill efficiency but not the whole story, fit the 24v with the same exhaust and manifold setup as the 12v and I would guess you would struggle to tell them apart.
The fact is the 12v is tuned to be quiet and have a smooth cruiser style power delivery and the 24v is tuned to give extra power but at the expense of noise and smoothness of delivery, I would guess when that manifold shortens the path to the valves it becomes noticeably more vocal and gives a surge of power.

This manifold is probably the reason you couldn't buy one with an automatic as there were a lot more powerful engines than the 24v the HP18 was mated to and it coped fine, who knows.

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xantia_v6 wrote:In the general case (I don't have specific knowledge of the PRV engines) a correctly tuned inlet manifold can make a significant improvement to the available torque, so the 24V manifold might improve the 12V engine, but probably not at high RPM. BUT... On a MAP injection system, fuel metering is based on a map of the volumetric efficiency of the engine, adding a differently tuned inlet manifold will change the volumetric efficiency, so the fuelling will not be optimal, unless the ECU is remapped.

Yes it would need an aftermarket engine management system that could be used to actuate the manifold diverter valves as well, it would be interesting to try but alas parts are getting thin on the ground now and if you were going to fork out for a good engine management system the obvious choice would be to supercharge it to 6psi and be done.

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there is no doubt about it the 24v is a messy engine visually

Gulp, I am not sure I agree with you there Dean and if it does that is only because of the increased size of the lump and a smaller plastic cover to show off the really clever bit! Here are some pictures for comparison with the 12v plus a diagram of the 24v inlet.
With regards,
Matthew T.

A matter of opinion, of course, but the Tron engine cover of the 12v hides a lot of birds nest, I think the Citroen/Peugeot designers were very proud of their inlet manifold (or proud they got it past the accountants) and rightly so. It looks like the cover was just there to frame it, I think it would have looked better with it covered as it has prettier rocker covers than the 12v engine.
Thanks for the document anyway Matthew, have you got the full writing with that so I can translate and have a geek out?

Anyway, we digress! The new plugs are in and it is smooth as smooth can be, two plugs had no continuity unless you tried to bend them which is great news although suspected!, so.........the moment Russ has been waiting for.........the first Drive
https://youtu.be/fIUiisyh-z8" onclick="window.open(this.href);return false;


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We want impressions from THE REAL 1st drive.

Not exactly the long distance cruiser I was expecting.

I was in the driver's seat and the car moved I don't know what you two's definition of driving is but Russ wanted the first drive on film and here it is

That damn oil cooler is leaking I think which is a pain, it must be leaking when it gets hot or it could just be some residual coolant being shaken off by the engine running, it was only a couple of drips I noticed after packing up for the evening.
I can't rule out completely making a mess of the piston liner seals as there is a fair bit of mayo in the oil cap each time I have run the engine but all its done is idle to 90c coolant temperature and been turned off so I am praying the mayo is just from sitting and idling so much (please let it be that).
The radiator fans are not cutting in either which is odd, I will have to look into that, I just had the A/C running which turns both fans on low speed and that kept the temps down to a steady 87-92c.

Even if I have messed up the liner seals (I will be damn annoyed) but the conversion and problem solving is done, the engine is good, the wiring is sorted and it all works very well, the test drive will hopefully happen at the end of the week depending on time I get to nail these niggles down.

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I can't see the engine idling even without coolant would damage the liner seals. There isn't much heat being generated adn the oil will be dissipating all that is.

Possibly the cooler is leaking internally.

I don't know.... What was that! We need more... that was just testing the creep of the gearbox!

Can we actually register a speed on the speedo next drive.

Mind you it's sounding great, I'm quite Hope you get the other niggles sorted without too much fuss.