Rm 250 T / Pe 250 X barrel difference

[oldfart]

compare these then

[suzuki27]

Up until 1980, from 1977, I know the PE & RM 250 ran the same bore and stroke of 67mm bore and 70mm stroke. I reckon the PE 250 T/X are way underrated as a vintage/twinshock motocross engine. I have an RM250 T and sometimes put my PE pipe on to mellow it out- and it does help. I am going to try and drop the whole motor into the RM this summer for a test run- give me bottom end and mid anyday for MX.

[LWC82PE]

81 and 82 PE250 was also 67x70.

its a bit like the PE 400 which actually has mor HP/torque than the RM 400. this was discussed long ago on the old forum.

[Doc]

a plane wing or chopper prop isn't a good comparison Loz as all the above mentioned use the edges to create turbulance thus creating lift. A rocket is pointy and it's designed to travel as fast as practically possible the 2 stroke is an air pump, the faster the air can pass through it the better it will go (in theory) sharpening the tranfers and inlet bridge is par for the course in near any ported 2 smoker barrel I have seen. Doesn't make it right I suppose but it's overwhelmingly standard practice.

[LWC82PE]

i was thinking the same as you Doc and became confused by this. Arent plane wing edges tapered to a point?? they are not square and block shaped because its not aerodynamic.

[Lozza]

Wings generate lift because they are flat on the bottom, thus the air passing over the wing travels further and faster than the air going under thus giving lower pressure on top of the wing=lift.Trailing edges are sharp and a track cyclists helmet is the perfect example. Rockets are super sonic and that changes everything.Gas flows in engines are never super sonic and knife edges don't contribute to gas speed at all.Gas speed is created by bigger and faster pressure drops, in the cases/cylinder and pipe. Knife edging and case stuffing has long fallen out of favour with any serious/professional engine builder or factory. A KTM 125 makes 43HP out of the box,  same for a TZ and RS 125's but there around 50HP no knife edges to be seen. As I said amature work.

[Marc.com]

guess you have to move with the times, but knife edges used to be a given, like carbon reeds and cutting that little spirally thing out of your PE muffler.

[oldfart]

Mr Lozza ... I have been scrathing me head all day over this ( just about gone bald )  The Oldfart has come to the conclusion - cranks are not the same , Pe wieghs in at  3.9kg and the Rm 3.5kg   internal - crank case volumes are different - both share the same crank  Daimeter, but Pe measures 62mm across where as Rm is 58mm   rods the same length - share the same piston - and the barrels ARE THE SAME . Difference is chamber - stator - CDI  ?  Pe has a 6 speed box ( and much stronger )  Rm = 5 speed and has a tendancy to wear gears ( split 3 today and all had the same problem )
NOW ON THE HUNT for a decent gear box  set for a Rm 250 T

[LWC82PE]

so the PE has higher crankcase compression?. are the cases the same and the larger mass of the PE crank webs reduces the crankcase volume air space giving more compression?  so if you think the barrels are the same, why does RM 250 T and PE 250 T have different part numbers? i wonder

[Doc]

  Lozza! are you saying my TM isn't supersonic!! now you're really pulling my leg not arguing the fact but there are many other contributing factors to be taken into consideration when comparing a 2008 125 to a 70's technology item. 2 of these considerations being the powervalve and digital ignition which means they can build the motors to pump out mega HP without the dramatic loss of low end power the early models suffered. You could get over 30HP from the old too but it became unusable unlike the new generation stuff which is infinately adjustable.   

[oldfart]

Not too sure about extra volume ,crank holes in PE = 28mm  Rm 30mm  case volumes should be the  same??? . Tolerances each side of crank when in cases are similar . Cases are NOT interchangable due to the fact of extra gear and 4 mm difference to accommodate crank.

Lozza ! I'm more a 4 stroker man than a 2 smoker ...... im my language are we talking about Duration versus lift
 

[GMC]

I had some RM & PE stuff a few years ago & I compared the barrels, I think they were C models. The barrels & ports were identical except for the exhaust port height being lower on the PE, this would be enough for different part No's.

[Doc]

yep, I'd reckon even the rubber blocks in the fins would call for a new number. The factory didn't want us thinking they'd simply used an RM barrel. Takes me back to an old article in a Trail and Track from 'RM250N Gone Enduro' remember this pic? 

[Lozza]

Proving it's not to hard to confuse a plumber Case volumes in the PE will be smaller, lower case volumes bring the engine into phase lower down the rev range. A great engine would be the heavy PE crank in the RM cases with an adjustment in case volume, would rev and be very tractable/forgiving and very fast around the track.What am I supposed to be looking at with the barrels Stew
Pipes and ports account for most of the performance Doc a 70's 125 typicaly ran a 32/34mm carb a 08 mx 125 has 38/39mm carb.The engine does the pumping not the ignition.

[LWC82PE]

ok i think ive worked this out. if the ports of RM T and PE T are the same then its must be the barrel rubbers giveing reason for differnt part numbers

the RM had none but the PE did

i say this because ive seen an optional sound kit availble for RM 250 T in the parts book which included a spark arrestor, barrel/head rubbers and a different air box lid, all to quieten things down for offroad use in USA.

but just as i write this im wondering were replacment  PE barrels sold with the rubbers fitted? when you find NOS barrels, do they have the rubbers already fitted?

if they dont that throws my theory out the door.