15 HP+ for CB400A Hondamatic?

[youngtimer]

Hey folks,

have anyone experience or know something about power increasing with a Honda CB400A Hondamatic from 27 HP to 42 HP (or whatever)  by the Cam of the CB400T? Any help is appreciated.

Regards,
Youngtimer

[dave wilkins]

the 400 t cam wsill drop straight in to auto engine but dont know of any power increase

[youngtimer]

Thanks for reply. Do someone know the reason why the Automatic engine have only modest 27 hp? Is it because of the clutch or of the torque converter?

[lxmlvll]

Hi,

There are probably many more differences than you think, like smaller ports and different carburation, compression and ignition timing.

With a choice of only two gears (I think) instead of 5 on the higher powered bike, the 400A needs more torque at lower revs.

You might need a tow to get going!

A.

[sono]

The best way to achieve what you want is to SWAP THE ENGINE altogether ! The CM400a (automatic) can accept any CM400T or CM400E engine (both 5 speeds) or a CM450 (6 speeds) engine, without modifications to the ignition and frame. Carburators are similar but jetting is different.

As it is often the case with cars, automatic transmissions on that motorcycle are not as efficient as manual transmissions. The fact you have 6 speeds on the CM450 is somewhat irrelevant because you may never use the sixth speed unless you ride it as a sport bike (over legal limit), which it is definitely not meant to be!

CM450 engines have been in production more recently, so there are still some available on the used marked (must be purchased locally with a compression test within specs and minimal warranty).

[Tinker]

Honda deliberately detuned them, mostly for fearful new riders that they expected would purchase them.

You need the cam and the carbs are also smaller.  I have no idea what else is different.

[sono]

Honda deliberately detuned them, mostly for fearful new riders that they expected would purchase them.

You need the cam and the carbs are also smaller.  I have no idea what else is different.

I agree, the automatic version appears to be deliberately crippled in many ways. Too many in fact to hope correcting it. I maintain the best way to make it alive is to swap the engine. You would be surprised how little it costs to get a spare one with a compression test and minimal warranty, if you can swap it yourself and change your carb jetting. By itself, swapping engines is a simple operation.  I would then get valves and clutch adjusted to be on the safe side an the carb balanced.

[sono]

Incidentally, As I was trying to identify a pair of NOS (new old stock) oversize pistons, it appears as if compression ratio on CM400A is lower than CM400T.

My CM400T pistons (I just had the engine rebuilt) show three depressions on the top to leave clearance for the valves. The picture of the CM400A piston I saw (out of an OEM box with part number) was PLAIN, so it can't go as high in the top position into the cylinder, so compression is probably lower.