Porting and Head Mods for a Stroked Engine

[crawl9]

I've got a 2001 Rm 125. I've Gotten the Crank Stroked 5mm. I'm using a 100mm Rod from a 1997 Rm (102 is stock on 2001) The bore is stock, 54mm. I've gotten eveything Cleared for the Crank. I need some Advice on port timing and area and Exaust timing? Their's 3mm I'm still playing with. I really don't want to use a spacer plate, but may use more that one gasket. Also, I know I could Leave the 3mm alone and take it out of the head, But this would really change the Stock Timing. Could it be ported back in? I've been Leaning Toward taking .05 Off the Top of the piston, using two extra gaskets, (.020 a peice) So 3mm= .119 and minus .05 off top of piston head(timing edge)leave .069 and minus .04 for extra gaskets leave .029 . This will give me .029 more compression. If I go this way, What should I do with my Port and exhaust Timing? If I take .050 off top of my piston, it leaves .100 inches from ring groove to timing edge and a thinkness of .210 at top of piston. so That should be ok thier. Also, is their any good web site out their for learining porting and engine tuning or any good books.
Thanks

 

[crawl9]

also, I'm planning on using .035 squish cleareance with 10 degree angle. Want to run Race Gas.

 

[marcusgunby]

www.macdizzy.com has in depth porting info-may not have what you are after as its a unusual mod but they have some good info generally.

 

[EricGorr]

I use TSR's PORT2000 port-time-area calculation software. Its the best $200 you'll ever spend. www.tsrsoftware.com
You can jack around trying to get the info you need on the Internet and probably wreck $1,000 worth of parts with bad information. But in 10 minutes on TSR you can solve this problem for good.

 

[crawl9]

I have a copy of Bimotion 2-stroke Software, it gives Time-area targets, intake, exhaust, transfer, expansion chamber, cylinder head, and yamaha energy induction syst. Info. Has anyone else used this and is it a reliable way to tune a engine? What are some tips on using it?

 

[smarttoys]

If you increased the stroke by 5mm, you will only have to compensate for 2.5mm at the top of the stroke. You are already using a 2mm shorter rod so that leaves only .5mm to deal with not 3mm. For squish clearance anyways.

.5mm could be easily cut from the head or adjust your squish clearance with base gaskets. It depends on what you decide to use for your port timing.

 

[crawl9]

I stroked the crank 5mm. Went from 54.5 to a 64.5 MM Stroke

 

[marcusgunby]

Are you sure the gearbox will withstand the increased torque?the rm gearbox isnt the strongest when mated to a std engine.

 

[smarttoys]

That would be a 10mm stroke increase then. 5mm farther up and 5mm farther down, hence my confusion when you wrote 5mm stroke. It should be an interesting motor when it's done. I curious if many people have changed the stroke that radically in a dirt bike.

 

[crawl9]

RPM makes a Bigbore stroker RM125 that turns out to be a 167 and put's out close to 40 hp. So I'm gussing that the Torque will be a problem on stroked motor, it will be a 147. Also, Sorry about the Confusing Smarttoys, I should Have written that I moved the Pin 5mm.

 

[crawl9]

I meant to say the torque won't be a problem in the last reply.

 

[DEANSFASTWAY]

Cometic will make you alloy base spacer plates for cheap$ .I have them make them for me sometimes . I guess you made a longer PV linkage rod too Eh? 5mms sound pretty far , Think it will shake ? GOOD LUCK

 

[Brianc]

Originally posted by smarttoys
That would be a 10mm stroke increase then. 5mm farther up and 5mm farther down, hence my confusion when you wrote 5mm stroke. It should be an interesting motor when it's done. I curious if many people have changed the stroke that radically in a dirt bike.

We are having Powroll incease the stroke of our crank 9.5 mm on my son's CRF150. I know there are other companies stroking the 4 strokes a similar amount for more torque and h.p. through a broader rpm range. Stroking typically works best on an engine that is already oversquare, (the bore is larger than the stroke)

 

[fastwes]

What everyone is telling you is correct. The other thing to watch out for with so much stroke is that the piston skirt will travel above the bottom of the exhaust port at tdc. Play with different spacers(or multiple base gaskets) on the workbench looking in the exhaust port to see where you must go. Good luck.
Wes Gilbert

 

[DEANSFASTWAY]

Sometimes I think you may need to play with the ign timing a bit also.

 

[crawl9]

Hey DEANSFASTWAY,
Thanks for the Tip on Cosmetic Gaskets spacers. I got a spacer plate for less than $25 with Shiping. When I called, they didn't even have one in stock but I got it in less that a week.