Strength of Citroen verses Renault UN1 transmission

[StevenD57]

I have heard differing stories from various people. Is one transmission inherently weaker than the other? I have recently had someone who is supposed to be knowledgeable tell me that I should not try to get too much power out of a MY1988 US edition Esprit because the Citroen transmission is so much weaker than the "better" Renault UN1 transmission.

Anyone care to offer opinions on this question?

--

Steve

[lotus4s]

Steve,

Don't know why no one else has chimed in yet but here's my 2 cents worth.

There really isn't much debate about which gearbox is stronger, the UN1 wins hands down. Back in the 80's as Lotus was increasing HP with successive models it became clear that the Citroen box was not going to be up to the task so the Renault UN1 was chosen as the gearbox successor due to it's higer rated HP capacity, availability, cost and ease of fitment.

It's not so much that one is "better" than the other as it is that one is able to handle more horsepower and torque before it fails. And with the Citroen parts becoming more scarce and expensive each year, it just doesn't make sense to push it to the limit.

Jim

[Quicker]

what little info I can find...

The highest torque rating of a engine mated to a Citroen gearbox is 200ft/lbs...

for Renault

- original UN1 – 016 with a Torque limit of 300 Nm (221 ft/lbs)

- the latest V8 version – UN1 – 027 at 400Nm (295 ft/lbs)

[StevenD57]

Thanks for the info guys.

[MrDangerUS]

Steve,

350HP is the limit for beefed up C35 tranny and it shifts better than UN-1.

[Quicker]

John - do you know the torque rating? that's what we really need for transmission limits...

[mike_sekinger]

If your moving away from stock, then a modified UN1 can cope with significantly more torque than 295lb/ft.

Late last month, we performed a series of WOT dyno runs ranging from 380-440 lb/ft. At higher outputs it would be wise to consider some transmission oil cooling, but as Jim states - there is no real debate which transmission is stronger.

Cheers,

Mike S

[DarrylV8]

Steve,

We have a 1981 Esprit (Turbo) with a Rover V8 not huge on power but higher on torque than the original 4pot engine.

The Citroen box started loosing gears and we would need to hold the gear lever to keep 1st; we changed the box a few years back; the Renault unit form a dead (Lotus V8) along with drive shafts, hubs, wheels and gear change cables, and since then upped the power with no issues with the UN1 box

[MrDangerUS]

One can convert to UN-1 preserving inboard brakes, which is much simpler/easier. Look here: http://www.facebook.com/photo.php?fbid=179165288761382&set=pu.178516808826230&type=1&theater

Edited December 8, 2011 by MrDangerUS

[StevenD57]

hmm, Interesting photo & conversion but that would mean I would need to find a spare UN1 laying about somewhere. I have to get the 1988 car running first and then we will see where to go from there.

Edited December 8, 2011 by StevenD57

[Günter]

Steven:

isn't there a notable difference in the outer bearings/support of the differential outputs on UN-1 & Citroen

...so how does this *inboard-brake mod* work, if for example there is some load sideways [seized callipers/pads] acting ?!

other point, if it is aimed to be a general modification, why not use any type of UN-1/386 Renault box -those are fittet in more than just Lotus/Delorean ..as most of us will know

[DarrylV8]

We had already moved the brakes out board before the g-box finally packed in; but i'm sure the hubs on the later (Esprit) cars have the calipers mounted on.

Before we changed the drive shafts we did discuss modifying the original drive shafts with a couple of (DS) manufacturers they did not consider this as a feasible option and we progressed with the later Esprit drive shafts etc

[Günter]

what I meaned is -as a natural way of wear, the bearings (and as such the previous 'drive shaft joints') would have some slight 'run out'/free play) over time ..so after a period of time the mounting flange for the brake disc would not run plane and could "wouble" around (as for the additional mass of material now there on the UN-1 differential output)

Therefore it does not sound like a well engineered solution, or at least is it only my misinterpration of the arrangement (shown in the picture link..) ??

[910Esprit]

_{Well I don't know if this is bad luck or an issue with mildly incresed performance, but I have just lost my 2nd citroen crownwheel in a couple of thousand miles. Latest was a low mileage 2nd hand unit thats showns very little wear, ironically the condition of the undamaged surfaces of the CWP are the best I have seen! I'll post a picture in the next daya or two.}

Edited December 10, 2011 by 910Esprit

[MrDangerUS]

Steve,

Could you please share w/us how far are your modifications and what you mean by " _{mildly incresed performance"? How did it fail? Tooth?}

[910Esprit]

Hi John - Yes a single tooth disappeared from Crown wheel (haven't dismantled the first box yet but the sympoms identical - car still runs OK, but an on ominous chunk of metal drained from diff). I assume performance similar to an HC+. HC head with multi angle valve seats (to widen the throat). Compression slightly increased to around 8.25. Boost to peak 9.5-10lbs. running DHLA 40's with modified jetting.

Not a pretty sight...

[dsvitesse1]

Hi guys

I still and really believe that an LSD or Quaife ATB would protect these acient cwp sets.

Just for your infos, i still have brandnew ( cyro possible) and very ,very good used sets.

With kind regards

Harry Martens

[MrDangerUS]

Harry is right, Cryogenic treatment is a MUST for this CWP C-35 box was designed for an aspirated car producing a maximum of 170 ft lbs of torque. The HCI Esprits made 220 ft lbs before bumping the boost. The C-35 gearboxes came in an early version and a late version. S3 car has the late version, the one on Craig list is the early version. It probably came out a Citroen; the gear change isn't on the (RH) correct side; the rear cover appears different (the early boxes used smaller studs), it has a different bell housing and appears to have different output shaft housings and who know what ratios it contains. The problem with the transaxle is that it was never designed to handle the power of the Turbo 910. Lotus first used it in the 160 Hp 907/ 912 powered Esprits, and that was a good match. But when they stepped up to the 910, they took the Citroen beyond its original design intent.

***** Article from my files:

Cryogenic Treatment Enhances the Performance of Metal Parts Deep Cryogenic treatment, also known as cryogenic tempering or freezing, utilizes ultra-cold temperatures (-317F) to modify the micro-structure of metals and other materials. The cryogenic treatment promotes additional crystalline transformations in metals. Deep cryogenic freezing ultimately improves the performance of the metal parts. Improvement in durability is around 100 percent (that's double). The typical increase in strength is 30 to 50 percent. As you might imagine, these results can dramatically reduce broken part expenditures. Technical Information for Cryogenic Treatment We provide cryogenic treatment services to improve the performance of the metal components like crankshafts, drive shafts, con rods, disc brakes and crown pinion gears. Cryogenic Treatment promotes three transformations in heat-treated steels and cast irons: a. Metal crystal structure (grain structure) becomes homogenous through the conversion of austenite to the desired Martensitic structure. After heat-treating, nearly all steels have a certain percentage of austenite that was not fully transformed into Martensite. This is what is called “retained austenite” or “RA”. It is widely accepted in the heat-treating industry that all heat-treated steels will have some percentage of RA and heat treatment recipes routinely specify that RA will “not exceed” a certain percentage. Cryogenic treatment transforms RA into Martensite. By eliminating retained austenite (or RA), imperfections in the steel microstructure are eliminated. b. The carbon structure of steels is modified through the precipitation of eta-carbides. The population of these eta-carbides – both brilliant ones (white ones) and dark ones (black ones) – is dramatically increased after cryogenic treatment. . c. All metals – not just steel, but also aluminum, copper, cast alloys, etc. – benefit from the residual stress relief that deep cryogenic treatment offers. All metals have residual stresses; As heat is extracted through cooling, crystalline dendrites form from the coolest areas first. Typically, these are the surfaces and edges. This irregular freezing results in natural stress lines where the dendrites collide, or along the boundaries of the remaining liquid (molten) metal and the solid metal. After the metal is cast in it’s raw stock form, (e. g. block, billet, plate, round, etc.), it is heat treated to normalize the material and modify its properties (e.g. hardness, tensile strength, etc.). Once the raw stock is further modified, additional stresses are added through machining, cutting, grinding, forging, etc., in the manufacturing process. When combined, all of these stresses form weak areas that are prone to failure through propagation of the stress lines into cracks. These are often characterized as fatigue failures or more simply “metal fatigue”. By relieving the residual stresses cryogenic treatment greatly reduces or eliminates fatigue failures or cracks in metal components. Many engineered components benefit simultaneously from two or more of the transformations discussed above. Cryogenically treated materials demonstrate better thermal properties, including improved heat dissipation and less warping or distortion. Stress relief means less cracking and the modification to the carbon microstructure means increased wear resistance. So you can see how brake rotors, for instance, will last longer and perform better after cryogenic treatment. Summary: Cryogenic treatment is an extension of the heat-treating process that further enhances metals in the following ways: 1. Relieves residual stresses 2. Promotes a more uniform micro-structure 3. Precipitates eta-carbides in steels for increased resistance to wear Cryo treated metals enjoy the following benefits: 1. Longer life due to reduced wear 2. Less failures due to cracking that result from the propagation of stress lines 3. Improved thermal properties 4. Better electrical properties with reduced electrical resistance 5. Reduced coefficient of friction on polished metals 6. Less creep & walk, and improved flatness for critical tolerance parts 7. Easier machining, polishing and grinding for better edges and finishes Cryo treating can make a major contribution to solving these problems: 1. High abrasive wear in cutting tools, gears, engine components, etc. 2. High corrosive wear in chemical, food, and oil equipment applications. 3. High erosive wear from, water, slurries and other abrasive grit carriers. 4. Distortions induced by design, forming, machining or environment. 5. Stress relief in complex tools, components, and welds. 6. Stress relief cracking of weld zones. 7. Surface finishing in any application where long life is needed. 8. Stabilization in parts and components as a result of stresses. 9. Machineability in steel, aluminum and copper parts.

Edited December 12, 2011 by MrDangerUS

[MrDangerUS]

Next... You may have all the gears treated with Isotropic Super Finish process by these guys(UK):

http://www.remchem.c...CFQse4QodXlXzoQ

and http://www.frozensolid.co.uk/

If you are in US, see: http://www.remchem.com/

In 1987, the REM Europe facility was established and by 2005 had tripled in size. The original Connecticut facility was expanded in 1991 and again in 2008. Also, in 1994, REM opened an additional facility in Brenham, Texas. All three REM facilities produce the highest quality surface finishing results in their operating space.

Isotropic Superfinishing is a chemically accelerated vibratory finishing process that is capable of generating surface finishes with an Arithmetic Mean Roughness (Ra)< 3 min. This process was applied to the third stage spur bull gear and mating pinions along with the second stage bevel gears of a Sikorsky S-76+ main gearbox. The gearbox completed the standard Acceptance Test Procedure (ATP) and a 200-hour endurance test. During these tests noise, vibration, and operating temperatures were shown to be significantly reduced due to lower friction.

Edited December 14, 2011 by MrDangerUS

[MrDangerUS]

ADDING LSD IS IMPORTANT.

Lotus 2.2 Turbo engine is capable of delivering in excess of 300BHP. However, the Citroen gear box CWP can not handle increased torque generated by modified engines. C-35 Gear Box was used on all Lotus Esprit models from 1975 to 1988. There are two versions, the early one with shift mechanism on the left side and late, w/shift on the right.

S3 car, as well as 1988 Turbo have the late version

C-35 box was designed for an aspirated car producing a maximum of 170 ft lbs of torque. The HCI Esprits made 220 ft lbs before bumping the boost.

Lotus was already pushing it going up to 220 ft-lbs. At 250ft-lbs plus, the rear tires will break loose or hop over ripples or "stutter bumps" under hard acceleration in 1st and 2nd gear. Between transaxle case flex and these internal shock loads, you'll quickly break down the case hardened gear surfaces on your ring and pinion until "BANG", and you're missing several teeth off one or both.

In order to avoid expensive break downs, or equally expensive conversion to the Renault UN1 gearbox (outboard brakes!), some Lotus enthusiasts successfully applied Cryogenic Treatment to CWP gears and installed QUAIFE ATB Helical LSD Differential. Quaife, due to design of its internals minimizes the internal inter-gear shock. Ask Harry Maartens!

European Clubmen reported several instances where modified Citro box was able to handle up to 350BHP, (not clear for how long, though).

[R33S GT]

I've been looking about for info on moving brakes to outboard, but can't find much out there. What exactly is involved? I'd definitely consider fitting a UN1-027.

[MikieP]

I am installing a Lotus Motorsports LSD into my UN1 on my 1993 SE. Would cryo still better the box's strength? If so, all or some of the parts, and approx cost as it is going to be cracked to do the LSD shortly. Each gear by itself required? Much thanks for any thoughts.

MikieP

[MrDangerUS]

Good article http://www.thesmokingtire.com/2010/addressing-the-lotus-esprit-v8s-weak-gearbox/

http://hollowayperformance.com/

You may check this http://www.lotustalk...ressions-79330/

and

http://hollowayperfo...gust%202011.pdf

Edited February 20, 2012 by MrDangerUS

[MrDangerUS]

Cryo-treating drive shafts and beefier Porsche 930 CV joints would be a great improvement over stock parts.

[Günter]

John, you may say it's a good article ..however, it only sums up what is already said in forums by owners, or those who believe to know.

Anyway -what I find curious is that the 'Derek Bell' in the *UN1 videos* in YouTube and on his old *UN-1/Lola T70 replica* webpage never looked like the mentioned 'Derek Bell -the LeMans driver' from the article

 

..so should we trust a automotive-journalist who pretends to know his stuff like this  ?  

Edited January 24, 2013 by Günter