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What gearbox would be best to get a longer 1st and 2nd gear for a CA 1.6 SOHC?

Short of buying a competition dog box from Harry Hockley theres not a lot to choose from.

All F5M22 boxes share common 2nd, 3rd and 4th gear ratios with only the final drive ratio, 1st and 5th changing. The closest set of standard ratios are those in the Colt and Satria GTi's and the CA and Proton 1600s, these have the higher first of 3.083 and the lower 5th of 0.777. Other Mitsubishi's and Protons use the lower 3.363 and very slightly higher 5th of 0.756.

There are also a variety of final drives available from the Cyborg 4.592 to the diesel 3.752.

What are you trying to achieve? Just a longer 1st and 2nd gear (say for 0-60 time purposes) or are you just after better performance, in particular acceleration.

For the former you already have one of the longest 1st and 2nds available, the only real option is to swap the final drive for a diesel one 4.021 for 3.752.

If the later you would be better swapping the final drive to a lower (numerically higher) one from say a GTi or if you can find one a Cyborg (4.592 also available from Space Star). Whilst not "lengthening " the gears (in fact could be said to be shortened), the lower FD would increase acceleration in all gears but would reduce the therotical top speeds, in reality the 1600s are overgeared so it would actually give you a more useful 5th gear.

Its for my brother in his autocross colt, which ivolves a circuit style racing, needed 'longer' 1st and 2nd gears to allow him to keep the higher revs going instead of changin up a gear and loosing the revs.

We didnt really want to splash out on one of Harry hockleys were just wondering if there was a gear box that would be better for our use over the standard one. so are you saying a diesel gearbox could be better? or have i misread what you said?

we were trying to achive better acceleration as in this type of motorsport there is not enough track to worry about top speed

If you're looking to avoid the change to 3rd then yes a diesel final drive ratio would certainly help (so would larger wheels/tyres but I'm sure he's thought of that). With the diesel box you get the short 1st so ideally the diesel FD needs putting into a 1600/GTi box.

Another alternative might be to use a Space star gearbox with its lower final drive and use 2nd and 3rd instead of 1st and 2nd, perhaps with a shift to small tyres. I know some autocrossers and autograssers set off in 2nd to avoid wheel spin away from the line.