Citroen BX 16v Rotrex compressuer

Post pics of your car in here
Post Reply

Citroen BX 16v Rotrex compressuer

Post by badlyworntoy » July 11th, 2012, 1:41 pm

Right something a little different to what we’re used to reading about here. Having read Guy’s latest book (several times) and him saying to me “Oh Ian I love Superchargers” when visiting his works one day .I think its only right that I publish some of my work here with the Rotrex Superchargers. While the Lada project is on temporary hold as its in Leeds having an new Exhaust system fitted, I’ve found myself with a few days spare to turn my attention back to my previous project. It was pushed outside due to failure of the Flow Diverter valve on the Hydraulic system when I tried to reverse it down the drive for the first time. I’d simply ran out of time/patience and money with it after that and its sat outside for around a year waiting for me to get the interest back again.

This is a car I’ve owned for about 5 years but only set out to supercharge it in 2010 having stumbled across the core components on an internet auction site to do this sort of conversion.
For those who don’t know Rotrex are a Danish Firm which manufactures small compact belt driven centrifugal superchargers. These are very compact units around the size of an alternator therefore are generally easier to retro fit into engine bays and have some advantages over conventional blowers. One of the main benefits is the way it can be applied with relative ease and without major changes to the standard components and layout in the vehicles engine bay. The AFM (Air Flow Meter) is still used in the intake of the Rotrex. The best circuit is to have the throttle positioned upstream of the supercharger,(i.e before the intake) this then will minimise the work the supercharger has to do and therefore should reduce fuel consumption under part load due to the thin air situation around the impeller at throttle angles less than 90 degrees
The simpler approach and the one I used to get started was to keep the throttle housing on the manifold which ket it simple, however to do this I have fitted a re circulation valve (dump),this is so at low engine speeds the impeller in the charger at engine overrun/deceleration doesn't back up air in the compressor housing and cause any racket. This in simple terms means that the dump valve will prevent surge in the discharge pipe work by allowing air to recirculate back to the intake under vacuum condition pretty much identical to a turbo system really.
You can depending on the model, gearing and physical size of the compressor wheels get gains of up to 100% power but the model and application I have will typically tend to see around 50% increase in power. So on a 160bhp Peugeot Mi16 engine I should be seeing around 240bhp.

Now I’ll apologise in advance as I know most people that use this forum are bias towards the Fiat/Lancia products, but for the sake of the supercharger conversions please bear with me.

I’d thought long and hard about the Rotrex conversion and did a lot of research and to my horror discovered that knowbody seemed to have any idea how to workout what the optimum compression ratio needed to be (Guy Please tell us the correct way if you know, I still don’t). I’d decided/guessed that the best way to factor in maximum performance and reliability was to get the Comp ratio down to around 9:5:1. from 10:4:1. Now this may sound silly but I wanted to know why Volkswagen had seen fit to lower the comp ratio on their 1.8 G60 engine from the normally aspirated 1.8 GTI engine when essentially it’s exactly the same engine?. They must have know something I don’t. I then spoke to GMC motorsport in Scotland who are an authorised Rotrex dealer and they said that its fine to run the Sp30/74 on the Mi16 engine and leave the comp ratio as standard (but That they’ve never tried it!). they advised me the comp ratio wants to be no lower than 9:0:1 in any application as you will loose to much drivability at the bottom end, I asked what formula they use to work this out and they didn’t seem to want to tell me.
I think its probably ok to run 0.7-0.8 bar on the comp ratio 10:4:1.....but you will need a powerful live ECU which is capable of retarding the ignition enough under load and a huge intercooler to keep the air charge temp as low as possible and Fuel with octane additives….maybe, any thoughts?

By far the easiest way to reduce the comp ratio is to get a later engine with the DFW code as opposed to the earlier D6C high comp engine (both still 1.9cc) The later engine had a slightly more modern Bosch level 4.1 motronic 3 row ECU which ran a knock sensor and a air charge temparture sensor and a catalitic convertor. This engine ran a reduced comp ratio of 9:7:1 which is ideal really when you do the maths. I struggled to find one of these engines but I did manage to managed get a set of DFW pistons, these have a 0.85mm shorter crown than the D6C ones. I also Commissioned another company In Scotland… Hiflowheads to build me one of their full blown stage 5 head force inducted heads which has 35.05 inlet valves with 6mm stems and 30.7 exhaust valves with 7mm stems. These are cut with 3 angle valve seats which recesses them further into the combustion chamber reducing the compression ratio even further. They had the CC figures from a similar head and had done the calculations which I’d based my engine on, So now 9:5:1 is the optimum ratio ideal on 98 ron pump fuel and standard ECU They is and MF2 piggy back triggering two auxillary injectors from a 2 bar map sensor under boost.

So a stand alone engine management system is where I’ll be going once I’ve got the engine functioning correctly on its original management. I purposely omitted to modify this area for the time being in an attempt not to bog down the BX project to the point of never getting it going again. I’ve learned in the past that its better to do things in stages as you also learn a lot from running a car between the stages of tune rather than going from standard to mental in one go. I’ve also learned that a lot of the time most of the stuff you plan isn't necessary or needed.
I have a spare Omex 600 management system I could use on my BX but for the time being getting it up and running on standard management with a piggyback auxiliary injector driver is the more sensible option. I’d like to see how it performs on the rolling road before I go down this route. I actually think that if I do decide to take it to the next stage of tune I’ll go to the Netherlands with DP-engineering who do lots of turbo/supercharger conversions on Peugeot and Citroens and go for a plug and play Vems system with MAP, Pieter there says that my car with such a high spec could easily achieve 300 bhp but only time and money will tell

So some pictures of this stage 5 head which apparently has been designed to work with forced induction in mind. I’d like to know what Guy thinks of this, I did briefly mention it to him and he thought the concept didn't sound right butwith more info would have a look. Apparently so I was told by Highflow heads one of the only screw ups Peugeot made with this engine was the design of the exhaust ports and the exhaust manifold, On flow test the exhaust ports flow around 120 CFM which is ok but as soon as you add the manifold into the equation it drops down to about 65 CFM which is dire. Peugeot corrected the problem with the later 2.0 XU10J4 adding Siamese ports per cylinder. They copied this design on my 1.9 head to combat this problem as much as possible. Its also had a big big valve conversion to improve flow with Inconnel Inlets at 35.05mm and Inconnel exhausts at 30.70 which is the absolute max you can go to on the Mi16 head
I’ve also fitted a set of Kent Vs34 twin valve springs and titanium retainers This is supposed to prevent any blowback on valves under boost, these are also more suitable for the long duration cams needed for getting the torque in the right area.
I've also fitted the obligatory kent adjustable pulleys and a set of Sierra Cosworth 57X exhaust manifold studs.

From an application point of view it took months of thinking and fiddling on to get it all in. For example I had to move the radiator up 3 inches and forward about 2,Therfore had to raise the header tank aswell I've had to find the room for all the intake and discharge pipe work as well as 2 oil coolers a intercooler a charge cooler, charge cooler radiator and a electric fan plus not compromising anything that was already there before I started, its been difficult I must say. Having a sound background in Classic car restoration/welding as well as equipment like lathes, milling machines tig welders etc always proves invaluable when you're doing this sort of thing.

However there has been a lot more work getting all this in the BX than I first thought they would be. Anybody who knows hydraulic Citroens knows that the Hydraulics and associated components are everywhere on a BX so making a supercharger system thats a professional fit is difficult and expensive.

Just a couple of points to note in the Pictures:- The lap angle on the Supercharger belt should be ok due to the low resistance of being centrifugal of course. If it was a roots or screw it would have to be a toothed belt. The timing belt tensioner attached to the sump is designed to get extra tension on the charger pulley to prevent slipping

Let me have all your thoughts good and bad

siamese ported exhaust ports
003.jpg (110.07 KiB) Viewed 10387 times
close up
004.jpg (107.19 KiB) Viewed 10387 times
005.jpg (135.96 KiB) Viewed 10387 times
005-1.jpg (95.41 KiB) Viewed 10387 times
Inlet ports
007.jpg (63.87 KiB) Viewed 10387 times
Piston height differences
015.jpg (113.7 KiB) Viewed 10387 times
Wet liner assembly
008.jpg (138.29 KiB) Viewed 10387 times
completed engine
008-1.jpg (158.03 KiB) Viewed 10387 times
Body shell pepairs and modifications.This is a box I had to make in the inner wingas they was no room left to put a Air Filter.
020.jpg (124.7 KiB) Viewed 10387 times
Repairs and additions of bracketry for oil coolers and chargecooler and assiociated parts
019.jpg (144.33 KiB) Viewed 10387 times
Mocking up the Pace Chargecooler
021.jpg (109.06 KiB) Viewed 10387 times
Mocking up intake pipe work and air filter
006-2.jpg (120.89 KiB) Viewed 10387 times
bx 006.jpg
Range rover P38 intercooler mock up
bx 006.jpg (528.73 KiB) Viewed 10387 times
Bit of stone chip and paint now the modifications are completed
017-1.jpg (100.16 KiB) Viewed 10387 times
Trying to make pipework fit and look like its supposed to be there is difficult
023.jpg (166.59 KiB) Viewed 10387 times
010-2.jpg (179.54 KiB) Viewed 10387 times
012-3.jpg (180.29 KiB) Viewed 10387 times
006-4.jpg (150.02 KiB) Viewed 10387 times
001 (2000 x 1500).jpg
Pile of parts ready for assembly
001 (2000 x 1500).jpg (665.61 KiB) Viewed 10349 times
005 (2000 x 1500).jpg
005 (2000 x 1500).jpg (553.84 KiB) Viewed 10349 times
006 (2000 x 1500).jpg
006 (2000 x 1500).jpg (611.97 KiB) Viewed 10349 times
cit 2.jpg
cit 2.jpg (583.47 KiB) Viewed 10349 times
Last edited by badlyworntoy on July 11th, 2012, 8:27 pm, edited 3 times in total.

Guy Croft
Site Admin
Posts: 5037
Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK

Re: Citroen BX 16v Rotrex compressuer

Post by Guy Croft » July 11th, 2012, 2:14 pm


A beautifully crafted article on an equally well developed setup, we done indeed!

Sorry but the technicalities are beyond me today....! My mind is on other things.

Re: "Now I’ll apologise in advance as I know most people that use this forum are bias towards the Fiat/Lancia products, but for the sake of the supercharger conversions please bear with me..."

FWIW I think your feature will have wide appeal and will inspire many thru the techniques and workmanship exhibited (as do many topics on this forum). No apology is needed - not at all - this forum was never intended to be exclusive to any particular model though I am conscious that I am best known for my work with F/L and it's natural that a fair number of enthusiasts gravitate here because of that.

Guy Croft, owner

Posts: 87
Joined: August 24th, 2010, 11:58 am

Re: Citroen BX 16v Rotrex compressuer

Post by Nobby » July 11th, 2012, 3:41 pm

Looks really good.

Is the last pic the finished car? I'd be wary of the hose which ends near the cam pulleys - especially if its for venting oil vapour to atmosphere (which is an MOT failure by the way) - you don't want oil on the cambelt. I assume its not finished though and its supposed to go the catch tank.
Chris Burgess
GC 01


Re: Citroen BX 16v Rotrex compressuer

Post by badlyworntoy » July 11th, 2012, 3:54 pm

Not so much finished, more as far as I've got.

Yes good observation about the breather system...but your right its not finished and does indeed need engineering up to the catch tank.


Posts: 116
Joined: March 9th, 2011, 5:20 pm

Re: Citroen BX 16v Rotrex compressuer

Post by timinator » July 11th, 2012, 4:34 pm

Nice work Ian. Just curious if the valves sizes you list are correct. I know that with low speed engines with blowers installed can have a valve ratio of near 1:1, but I have not seen it in small transportation vehicles. The picture you posted of a cylinder head shows two different size valves. Not sure if this is the head you installed. Definitely a fine effort in arranging all required elements under one small bonnet.



Re: Citroen BX 16v Rotrex compressuer

Post by badlyworntoy » July 11th, 2012, 8:16 pm


well spotted It was just a typo and I've corrected it.

Posts: 205
Joined: March 20th, 2007, 1:20 pm
Location: Midlands UK (A)

Re: Citroen BX 16v Rotrex compressuer

Post by 4v6 » July 12th, 2012, 1:45 pm

Its been rather a long time since I saw a BX on the road, I did see one on another site a few days ago the group B version going for some daft amount of money.
I've got to say Ian, I'd quite happily have yours and I'm hardly a "Citroen guy" even though I've worked on them for a dealer in the past.
Very nice indeed and good to see people still keeping them going.
Tony Warren. GC #96.


Re: Citroen BX 16v Rotrex compressuer

Post by badlyworntoy » July 12th, 2012, 3:38 pm

yes (In my Opinion) one of the most under rated performance cars ever made,performance figures were staggering when it was launched compared to all the other manufacturer's efforts at the time. The handling,power and brakes are better than anything else FWD I've ever driven from the 80's,and I've had everything trust me. They is only around 90 and about 30 on the road BX 19 GTI 16v's left in the Uk and they sold 9000 odd between 1987 and 1994. Being one of only 2 models sold in the UK with the all alloy 1.9 16v XU9J4 engine (Peugeot 405 Mi16) being the other,they didn't take long before they were hot targets for the Peugeot 205 GTI tuners to start buying them and culling them for their engines. That and having a very complex Hydraulic system which most back street garages and wallets couldn't cope with didn't help survival rates much either.

Me well I just love the complexity of it and the fact its different, I suppose you could call me a guardian of faith when it comes to Hydraulic Citroens, I just love the fact everybody thinks they were/are poor when they are just so not!! The French designers and engineers should have been given every design council award going for being brave enough to take the standard concept of the car and throw it in the bin and say they is other and better ways of doing things,that in my opinion is reason alone to preserve it.


Guy Croft
Site Admin
Posts: 5037
Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK

Re: Citroen BX 16v Rotrex compressuer

Post by Guy Croft » July 12th, 2012, 4:11 pm

I'd like to make an oblique point here.

Anyone can see how skilled Ian is. Even my 3 year old (where is she...?!)

Here we have an installation executed to an exceptional standard by an experienced enthusiast. But never forget that whilst there are many costs, time spent on one's hobby is not an 'overhead' cost.

I've been in racing engines for over 2 decades altogether and I have seen installation horrors at near-all levels of motorsport.

If you cannot execute the installation to a robust and effective standard the whole powerplant is at risk. If that engine has been built by someone like me you're putting an awful lot of cash 'on the line'.

I have been working thru a client's installation with the help of Wellingore Garage and the hook-up for the engine is a very very long list. When the client asked me to take on the car I simply said I want your credit card and no questions. I meant it and he understood and he's been as good as his word. Today I just paid the bill for the exhaust system which was formerly a pile of junk and would have knocked my engine to pieces. A fair price but not sixpence. Same went for the airbox, oil system (ie: NONE), fuel system, throttle cable, instruments etc etc.. Never mind the suspension which to be completely rejigged with a ton of expensive new parts. Even when the engine goes back in it needs firing up, bedding-in (prove out not a 5 minute blast down the road) suspension setup by a pro, MOT etc etc etc.

My point is this: If you consider a 'new' 'uprated' engine - be prepared for the costs as a whole package. The more sanguine will do that. Unfortunately most folk who ask me about doing work for them don't have a 'blind clue' about any of this. And in these days of 'comebacks' (ie: it's always the engine builder's fault) - we - inevitably will not work for them. The list of units with which we are prepared to work at all had shrunk very considerably in the last two years almost solely for this reason.

Guy Croft, owner

Posts: 459
Joined: August 13th, 2010, 8:05 pm
Location: Amsterdam, the Netherlands

Re: Citroen BX 16v Rotrex compressuer

Post by WhizzMan » July 21st, 2012, 9:26 am

I have a very small question, compared to the rest of this thread. Will those hose clamps take the pressure of the inlet charge?
Book #348


Re: Citroen BX 16v Rotrex compressuer

Post by badlyworntoy » July 23rd, 2012, 12:05 pm

yes easily its only 0.7 bar maximum

Posts: 89
Joined: May 31st, 2011, 2:40 pm

Re: Citroen BX 16v Rotrex compressuer

Post by Rallyroller » July 26th, 2012, 11:32 am

Very interesting project. On the point of not being a Fiat, I had a BX diesel as a company car a few years ago. A very underated car. You need to drive a good diesel to understand tourqe. Until you have driven a Citroen for a while you do not realise how good that suspension is. I also had a Zantia and C5- still miss the ride quality. Surely some room for mods in that area as well for a more sporting ride?

The BX is very light, ( half the body is plastic.) so performance from your modified engine should good. It is always good to see something different and learn from other peoples experiences. I look forward to seeing the completed results.

Guy Croft
Site Admin
Posts: 5037
Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK

Re: Citroen BX 16v Rotrex compressuer

Post by Guy Croft » July 28th, 2012, 11:11 am

Leftcoaster has some tech Q's on this one which I have respectfully deleted (after talking to him by email). I have archived these and imagine that when he is less busy he will post his points for me to examine in GC Q&A.

Members - try not to clutter Readers' Cars please. It's about cars. If you want to get stuck into deep technical ask the original poster first please by pm. Otherwise start a related discussion in GC Q&A or one of the OTHER engine discussion areas (competition or road as appropriate).

I have no intention of writing a Protocol over this BTW!

Thank you.

Guy Croft, owner


Re: Citroen BX 16v Rotrex compressuer

Post by badlyworntoy » November 27th, 2012, 8:39 pm

Ok folks a technical update with regards to “actual” performance rather than “speculative” performance (which I hate talking about anyway) with regards my old Citroen BX 16v. It was rolling roaded twice at Chipwizards in Rochdale Lancashire UK back in September and October. I had spoken to several reputable rolling road establishments in northern England and all seemed either non committal due to the amount of modifications on such a complex older vehicle. They were therefore demanding that I fit a powerful stand alone live ECU and engine management system before they began work or simply were not competent enough (in my opinion) to do what I wanted. I was loathe to go down the “stand alone” route as this meant heafty amounts of work in changing looms and re-calibrating engine sensors etc etc, I didn’t want to go down this route. Anyway after speaking to Wayne Scofield ( Chipwizads) in depth late one Friday night in the pub over the phone, this is two month after I left him a voice mail to contact me (it’s easier getting hold of the A-Team) he explained that you can “If” you know exactly what your doing and have designed your own software and hardware you can using an what’s called an emulator live map the standard Bosch 3 row ECU….dead easy says he, you just need bigger injectors, that’s all!

He then said something to me I’ll never forget. I said to him :- “Wayne are you sure you can do it?” he said back “Ian people tell me all the time things I do can’t be done, I just say to them I’d better stop doing it then, I’ve only been doing it 20 years! “

All I needed to know  and this is also bearing in mind this bloke map’s all the Arial Atom’s in a wooden shed in Rochdale 

The initial plan was at the first session we were to asses the capability of the hardware, reconfigure and do any remedial modifications to my design and simply get a Map on it that would get it on its feet and allow it to be driven so the old Mi16 engine can be ran in. The second was obviously designed to gain maximum performance from it. However it didn’t turn out the way we’d planned. It was quickly determined on arrival that the engine suffered lack of compression on number 4 cylinder, it was suspected that we had a bent inlet valve! We’d set the engine at TDC and attached a leakage gauge via the spark plug inset…..Game over for the day. All the way home in the Van I was racking my brain as to what I could have done wrong? Back at the rolling road we’d removed the inlet manifold and investigated with a micro camera and it looked as though only one valve was actually open the one next to it (in the same port) was sealed…spurious as usually if any mistake has been made during the build you expect them both to be bent.

Upon arrival home she was swiftly stripped down…..and this people is something that really pisses me off, you know when you pay £thousands for a so called specialist to do you one of their Stage 5 super dooper force inducted heads and then they can’t be arsed checking that the valve clearances are correct!!! That’s assembly time, 2 complete gasket sets, re-assembly time, logistics costs and rolling road booking fee’s all down the toilet all because they can’t be bothered to check the detail!!….the fcuker would have noticed if I’d have paid’ him a pound short though! Looks as though that the offending valve either had its new larger valve seat set too deep in the combustion chamber or it had been re cut too deep causing it to stay open when the cam had it shut. I had to take 0.7mm off it to correct it. Anyway once reassembled we had a solid 160psi across all 4 cylinders..some relief.

I returned to Chipwizard’s on the 4th October and Wayne quickly set to work. We had the whole day (and night) booked to run it in and performance map the engine. We had from 9am-11pm at night to do it, that’s what you call service, and his prices are very reasonable too! he even throws your supper in as well vial the local chip-oil. Anyway within an hour of arrival we’d gone from an engine popping and banging and generating loads of white smoke and sooting up spark plugs within minuites to a beautiful idling smooth sounding instrument, all courtesy of a bit of black electronic magic sent to the ECU via the roughest looking laptop you’ve ever seen. The next 3 hours consisted of 3 hours of driving going up and down the gearbox up to 4000rpm…think we managed to get around 250 miles on the clock in that time, which I know isn’t nearly enough time to run it in efficiently but Its all we could afford on a slashed budget thanks to “that Cylinder head specialist”

So power was estimated to be around 240bhp, we got this figure buy assuming from information given by the supercharger manufacturer that we would see somewhere around a 50% increase in performance on an engine between 1.6 and 2.5 litres given that the Peugeot/Citroen XU9J4 engine is 160bhp quoted in factory trim, so adding 80bhp (50%) should give a power figure somewhere near 240bhp. They was some debate over the design of the Siamese exhaust ports on my engine some months ago but I’m pleased to report that this mod along with its counterpart Richard Longman exhaust header seemed to have massively improved exhaust flow. Wayne Scofield the proprietor and head Rolling Road operative and my now approved engine calibrator says he’s mapped more Peugeot/Citroen Mi16 engines than you can shake a shake a stick at and they always respond to decent exhaust modifications regardless of what level of tune they’re at. I wish I could provide some scientific proof to support this but its difficult to separate the performance of each element of tune on the rolling road without changing parts and doing comparison tests, all we can do in this case is look at it as a whole unfortunately.

One of the biggest things I learned from this experience is how much difference in performance can be made with adjustments to cam timing. As much as 40bhp was found that day from cam timing alone, Wayne explained that cars like the BMW M3 E36 and Honda Civic Type-R V-Tec have around 60 degree’s range of variable cam timing over the whole rev band which is why they’re as docile as a cat at the bottom end and then can outrun the Millennium Falcon at the other. They certainly is no such thing as optimum cam timing, which stands to reason when you consider why the latest generation of performance engines no longer have static cam timing. In this case therefore it’s only about finding the best compromise. after all it’s a road car and has to be capable of being driven at both ends of the rev range.

I’d also know that the supercharger had been geared to something near it’s warranty limit once the engine reached 7000rpm I’d worked this out from the table below. I knew that I had only 5000 rpm change on the charger speed with a quick calculation. If we upped the rev limiter from 7000rpm to 7400rpm we would reach 120,000 rpm supercharger speed which would increase the maximum boost pressure from 7.5 psi to 9 psi which was too tempting not to try….power peaked at around 290bhp @7400rpm but since the engine wasn’t properly ran in I only gave permission for one power run in this configuration as we didn’t want to be taking the engine home in a plastic bucket. One point to note was that with doing this the supercharger oil (Rotrex SX 100 traction fluid) heavily frothed and leaked out of the breather on the reservoir filler…..pushing our luck I think. Rev limit was reduced back to 7000rpm for reliability and cam timing compromised and fuel and Ignition map’s burned.

Final Figure 247.5…..job done! Drove it off the rollers at 10:30pm that’s 11.5 hours work.

supercharger calc.jpg
supercharger calc.jpg (68.19 KiB) Viewed 9351 times
comp.jpg (135.54 KiB) Viewed 9351 times
299352_530127847001744_1665909492_n.jpg (81.64 KiB) Viewed 9342 times
46562_531232493557946_1082847520_n.jpg (131.21 KiB) Viewed 9342 times

Post Reply

Who is online

Users browsing this forum: No registered users and 4 guests