Citroen 5th Gear Help.

[AZ88Turbo]

I notice that Moto Concepts has a 5th gear conversion for the Lotus 88 on to change the ratio and bring the RPM’s down. I’m assuming that it is for the Renault gearbox so I would like to do the same thing with my Citroen box (bring the RPM’s down when driving at speed), does anyone know where I can purchase anything like this for the Citroen box? Or even if there is anything like this available?

Thanks,

AZ88Turbo

[lotusguy]

Hi,

Why are you planning to do this? What benefit do you hope to derive? Happy Motoring! Jim'85TE

[AZ88Turbo]

Hey Jim, how are things?

In Arizona the speed limit on the freeways is 75mph, so naturally the normal crusing speed is around 80-85. I find that at this speed my RPM's are around 3500 and my turbo boost is constantly on. I figured that if I could keep the RPM's lower and the turbo boost down that it would be less wear and tear on the car and also perhaps a little better fuel consumption?

Any advice you have is valued as allways.

Mark (AZ88Turbo)

[Lasse]

Hi!

Having turbo boost at this RPM has nothing to do with the RPM in case your engine has an ECU. The ECU will supply adequate boost to give the engine enough power to maintain the cruising speed. Since there is significant air resistance at this speed the engine simply has to work harder...

Best Regards /Lasse

[madmike]

Ummm, I was under the impression that keeping the 5th gear cruising revs low to avoid boost circa those speeds was the main purpose of the MotoConcepts 5th gear swap out.

I don't believe the wind resistance difference between 70 and 80 Mph is significant enough to cause your ECU to engage boost at the lower RPMs.

[cnh1990]

I don't know about the 88's but at those speeds the turbo is only boosting a tad. Although if you are putting on a lot of hwy miles I can see why you would want to bring the rpm down. The engine would last a bit longer but that difference would be years down the road. It will give you a higher top end too.
Calvin

[AZ88Turbo]

The 88 has the Bosh fuel injection and ECU which I don't think manages the boost that finely. There are also reasons such as; cabin noise and just a general feeling of "not liking to see the engine revving so high for hours at a time when it really doesn't need to". As mentioned earlier; that is part of the idea of the Moto-Concept 5th gear (or so they say on their web site). Anyway, the conversation of "why" is interesting but I have made my mind up already, unfortunately still no mention of "where to buy". Does anyone have any ideas please?

Thanks,

AZ88Turbo

[lotusguy]

AZ88Turbo said:

The 88 has the Bosh fuel injection and ECU which I don't think manages the boost that finely. There are also reasons such as; cabin noise and just a general feeling of "not liking to see the engine revving so high for hours at a time when it really doesn't need to". As mentioned earlier; that is part of the idea of the Moto-Concept 5th gear (or so they say on their web site). Anyway, the conversation of "why" is interesting but I have made my mind up already, unfortunately still no mention of "where to buy". Does anyone have any ideas please? Thanks, AZ88Turbo

Hi,

Very interesting thread. Fact is, there's very little benefit from fitting this gear, but a couple concerns in doing so.

Cabin noise levels may marginally improve, but the Esprit is a loud car due in no small part to it's design. The suspension transmits a lot of Db to the cabin as does the engine at anything above 2500RPM. I doubt any reduction will be noticed or appreciated on a long haul high speed run.

Top speed will not be increased as you will be operating at RPMs where the power curve has already fallen off. The stock gear ratios are designed to have the engine spinning at the optimum point in the power curve.

You will not save any appreciable wear to the engine. With gears, as often in life, there are no free lunches. Staying off boost will work the engine more as you will be trying to make a high percentage of the previous power you made, but without the benefit of boost. Boost is a good thing!

Running the engine at say 3200 rather than 3500 won't appreciably add life to the engine. In fact, since both your oil flow and coolant flow are crank speed dependent, it may actually lower the life expectancy of the engine.

You will not be saving any gas. The Bosche K-jetronic ECM is designed, and pre-set to increase fuel flow at 3200 - 3,250RPM to correlate with the onset of boost. Without the additional air from the turbo, this fuel will not combust as fully, wasting potential energy and increasing pollutants. You'll see almost no discernable mileage increase.

Finally, and this may be my biggest concern is the actual spec of the new gear. What is the gear ratio? Currently, your fifth gear has an internal ratio of 0.76:1 which translates to 22.586MPH/1,000RPM. If it varies significantly, two things will happen. You will lose torque (passing ability) which will probably result in a change in shifting patterns which is likely to increase the use of your 4th gear, hastening it's useful life. This will negate any effects of the 'improved' 5th gear.

Also, what is the composition of this new gear? What is it's hardness? If it differs significantly from the hardness of the old gear, it will either wear, or cause wear with the gears it is meshing with resulting in a shortened tranny life. I'm not too keen on bedding in a new gear against the internals of a 15 yo gearbox. You may save a couple hundred bucks at the gas pump only to have to pay a few thousand at the Tranny Shop, a false saving really. Also, what effect will it have on transmission heat? Heat is the #2 killer of trannys.

Swapping gear ratios is best done by the pros who race. They are continuously rebuilding and adjusting their boxes and so don't suffer the effects of any long term detriment. If this 'miracle' gear were to keep you out of the 6k RPM range, the benefit would be better realized, but swapping a few hundred RPM in the mid-range will have few practical benefits. Just my humble $0.02. Happy Motoring! Jim'85TE

>> Edited by lotusguy on Tuesday 22 July 15:39

[madmike]

Thanks Jim, this response I liked.

I never was intending to do this mod myself (realized benefits wouldn't offset the cost, IMO) but was curious to see if anyone else had. I liked your points.

One question though, didn't Lotus alter the 5th gear ratio for the V8 Esprits? (Granted, your torque arguments still apply to unmodded cars, but for those with the hotter chips, bigger turbos, etc would this not lend itself to better use of a different 5th gear ratio?)

Also, most of us don't use 5th gear for passing...there's not much 'oomph' in it. Unless of course you have the high-torque chip installed, and we know that you love those mods

Thanks for the great detailed response!

Edited to add: Jim any chance you are heading to LOG 23 this year? Would love to meet you and try to siphon off some of that Lotus knowledge...

>> Edited by madmike on Tuesday 22 July 16:11

[lotusguy]

madmike said:

"... One question though, didn't Lotus alter the 5th gear ratio for the V8 Esprits? (Granted, your torque arguments still apply to unmodded cars, but for those with the hotter chips, bigger turbos, etc would this not lend itself to better use of a different 5th gear ratio?) We are talking about the Citroen C-35 box here, but, yes, Lotus did alter the 5th gear on the V8 w/ the Renault gearbox. This was to better compliment the different power/torque curves that engine has, which is really the primary issue in determining gear ratios - that at any given speed, there is a gear which allows the engine to rev in it's 'sweet spot'. Also, most of us don't use 5th gear for passing...there's not much 'oomph' in it. Unless of course you have the high-torque chip installed, and we know that you love those mods I'm surprised to hear that, I pass in 5th all the time on the expressway. Once the boost rises, it has just as much 'oomph' as 4th gear, but at lower RPMs. Thanks for the great detailed response! Edited to add: Jim any chance you are heading to LOG 23 this year? Would love to meet you and try to siphon off some of that Lotus knowledge... Sorry, the Missus (lotusgirl ) and I have two weddings to attend this year out of town, as well as a planned vacation to Charleston, SC (our favorite US city) in December. This precludes attending LOG this year I'm afraid, I would have liked to meet you as well. Happy Motoring! Jim'85TE >> Edited by madmike on Tuesday 22 July 16:11

[madmike]

Ahhh, thanks...I forgot we started this by talking about the Citroen box.

I suppose the 4th vs 5th gear highway pass depends on how quickly you need to do it, and what speed you are doing at the time. If you need to change quickly, dropping to 4th is a good option...staying in 5th does give you the juice eventually, but it takes a few seconds to get there.

DC beltway driving usually lends itself to the 4th gear...that and it's fun.

Too bad about LOG...have fun in SC!

One of these days I'll make it up your way...still dream of seeing my Vikes play in a home game.