Superdream 250/400

[Moo.]

Im just wondering whether putting 400 barrels and pistons on the 250 will work, i know the obvious, change the carb jetting.

But my wonders are, in the haynes manual the valve spring free lengths for the 250 are longer then the 400. Would the 250 head still work with the 250 head?

Also, the valve timings, on the 250 the inlet closes at 30 degrees but on the 400 they close at 35 degrees. Is this going to cause a problem?

And with the cdi, am i right in thinking this controls when the sparks ignite? And that this is to do with the valves open/closing. What would the 400 cdi do differently to the 250, considering all the timings are the same, apart from the 30/35 degree difference.

Many thanks.
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A2 Passed 18/6/10

[MattWadz]

The bottom end is only designed to run 250 pistons/barrels...
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[Moo.]

I know theres a difference in the strokes.. but theoretically, if i put the pistons and barrels on.. shouldn't it still work? And the timings be the same?

Or.. are the pistons longer as well as wider? Hence the longer stroke..



This is working on the theory i could create a 323cc engine..

Displacement is Pi/4 x bore(squared) x stroke x no of cylinders..

Making 323cc..

Would this work? Taking into account jetting etc..

Just like putting a big bore kit on something..
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A2 Passed 18/6/10

[Kickstart]

Hi

Fair chance that you would land up with a very low compression 323cc engine.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Teflon-Mike]

Theoretically, the bike would still run... ish.
The cam timings are close enough to not be such a big difference, 400 timings are basically a bit 'wilder' with a longer duration.
Your main problem would be, compression.
If you have 250 bottom end, and 400 barels, where will the pistons sit in the barel at TDC.
400's longer stroke, will mean it probably has longer con rods, and or a different gudgeon to crown height.
You could have piston skirt fouling crank counter balences at BDC or the piston crown fouling head / valves at TDC or you could simply have piston sitting half an inch from the combustion chamber at TDC giving no compression.
But, if that's all OK.... then you'll simply have a 75cc over bored 250 engine..... possibly need re jetting to suit new displacement, might, on 250 carbs run out of breath at the top end.... other wise, would run like a bored out 250....
Given availability of 250N motors and bits, and cost / effort of rebuilding one.... though, one does have to ask if its worth it.
New pistons will set you back about £70, a Gasket set £20, and a rebore or second hand barel, IF you can get a 400 barel, probably another £50.... then you have to set it up......
When you can buy a taxed, & tested mid 90's CB500 for a few hundred quid.... you have to question the ecconomics of muggering about with a thirty year old commuter, and trying to eek a bit moore oomph out of it.......
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[Moo.]

What if i were to say i have 2 250 engines, a full gasket set, and 400 barrels and pistons laying around

I was expecting it to run like a bored out 250, im pretty sure the crankcases are the same, the 400 is just bored out more, with a slightly different crank and longer cod rods. If the pistons are just wider and not any longer, it should fit in..

If physically the engines are no different, how can the 400 have a longer con rod? Would the head be different?

[Kickstart]

Hi

If the stroke is longer then either the head is further from the crankcases or the rod is shorter. However a longer rod would be preferred to go with the longer stroke which suggests the head is quite a bit further from the crankcases (ie barrels quite a bit longer).

You might be able to bore out the 250 barrels to take the 400 pistons, assuming the distance between the gudgeon and piston crown is the same.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Moo.]

[Kickstart]

Hi

Difference in stroke is about 9mm, so half of that required as extra height for the stroke. From memory of my 400 many years ago there was enough clearance for 4.5mm extra height.

That said looking at the parts lists the top mounting bracket and the frame are the same between the 250 and 400 (have the same part numbers).

The cod rods do have different part numbers. If the 250 has longer rods to go with its shorter stroke the you might be OK. If the rods are shorter (normally stroke is a multiple of con rod length for a certain engine characteristic - shorter rods tend to be used on more revvy engines) and then the height made up with a different distance from gudgeon pin to piston crown, then using the 400 pistons would give a lower compression.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Moo.]

Many thanks for looking into that Keith

Malc.
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A2 Passed 18/6/10

[MickC]

Didn't they originally just put an extra sleeve in the barrels for the 250. i know i had an xs250 one time and that is what they did with them
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