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5 gear gearbox with overdrive conversion.


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My name is Wouter and I am from Belgium, I was looking to do a conversion to 5 gears with overdrive for my c303.
i would like to drop some engine speeds for cruising and also for noise reduction and fuel economy.
I'll try to show you what I do.

I started with a gearbox from a c306, these have a 5-speed shift pattern type: ZF S5-18/3
Then I also bought a gearbox from a Maserati Biturbo, which then had an overdrive of 1-0.791


I got both apart. the difference between the gearboxes is: 5 the gear is in a different place.


they use a different syncromash lever used on the top instead of the bottom. The 3rd and 4th of the c303 and 3rd and 5th of the maserati have swapped places. for that I will have to convert the shift arm and syncromashes from the maserati box to the TBG box. I'll try to rebuild the front half of the gearbox so I don't have to do the tapping again. There is also a spring-loaded ball that holds the shift levers in place. this ball is on the other side of the box.



All I have to do is level the top of the maserati box once. there is room for this. to be continued.... Wouter



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  • 3 weeks later...
  • 2 months later...


I removed the engine and gave it a fresh layer of paint.
because the 5speed gearbox is 63mm longer i had to make new engine mounts.
I used the original ones and extended them.
also the gearstick had to be adjusted. 
While the engine was out i also did a conversion to electric fan.
i bought  this one: https://www.spal-vertrieb.de/en/product/va33-bp91-ll-65a/
I didn't want a on/off switch so therefore i needed a pwm regulator: https://www.kemo-electronic.de/en/Transformer-Dimmer/Controller/Modules/M195-PWM-Power-control-9-28-V-DC-max-20-A.php

this also as the option to put in a 0-5V regulation signal. 
I added a PT100 in one of the connection tubes from the radiator. I'm currently working on  the PID regulator .
The advantage is that the fan wil only run when its needed on the correct amount of rpm.

Because the engine is going 63mm forward i had to relocate the brake booster and fuel filter.

because i had the engine out I also added a K-cam wich i hope will give me a little more power to encounter the 5th gear.
If ist not worging i can always switch back to the old A-cam.

bacause i also had an steel distribution gearset i also added it in place of the fiber one.

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  • 3 months later...

little update,

engine is back in the volvo! :wub: While the flywheel was off i resufaced it on the late. 
To take the ballbearing in the cranckshaft i used bread that i punched in with an allan key. works realy well!
Then i connected all hoses and started it up for the first time.
Everything was fine! 

I also added a temp meter for the gearbox. It is a standard BBQ meter wich you can buy on amazone or ali express. ist comes with 4 probes.
So i added one on the 5speed gearbox and one on the transfer box.
The bbq meter comes with an app wich allows me to put in an alarm if the gearbox is getting to hot an also a nice graph.
I glued the probes on with loctite 3463 2k steel. 

Then i also had a broken CV boot so it was time to inspect the front axle.
I took everything apart en to get it sand blasted. then some fresh 2K paint.
I found some blue silicone cv boots from asli4x4: https://www.asli4x4.com/products/volvo-c303-c304-c306-silicone-driveshaft-c-v-joint-cover-boot

The leafsprings i put together wit some graphite grease in between the leafs.

to check iff the carbs where tuned correct i also placed a AFR meter. 
With the new cam it does need more fuel!
I also am thinking on trying a automatic adjusting of the choke valve to adjust the mixture on demand of the afr reading.

just did my first test drive als all is looking fine.


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