Porting a 70 barrel

ok.. now your getting somwhere.

firstly dont raise anything by a full mm... start by FLATTENING the top of the exhaust by grinding ether side/corner of the top edge to the level of the middle.. (put the piston in so it just closes the port then draw round the barrel to get a straight line) then get 1/2mm crescent by raising the middle and blending it to the edges.. you want progressive but immediate opening of the port (blowdown) this releases the pressure nice and quickly meaning more rpm..

now widen the transfer ports to within 3mm of the ring end on the poston.. (marjk on the outside wall then straight edge the inside of the barrel for a marker of where the ring end is) then the other (primary transfer) point the lower edge towards the exhaust.. if you look at a few high spec portmaps close you will see the difference. there is no need to move the hight/placement of the transfers other than to blueprint at this stage as wider ports with equal durations mean a nice power spread without loss of low end power. it wears out quicker due to less ring support.

once you have this at a level your happy with (power vs life) then you can start raising the transfers by POINT ONE of a mm... thats 0.1mm at a time.. making sure you only grind one pair (primary or secondary) at a time an make sure there equal.. the primarys (neerer the exhaust) can open up to .5mm earlier than the secondarys but the secondarys must run behind or equal hight for it to not waste mix.. also like i said the oposing port of the same type MUST be equal or the piston will have an uneven load and bend the conrod due to one sided detonation...


reading it's easy.. understandings a bit harder but its actually doing what ive said properly thats the hard bit.

not getting messurements out of any of us i dont think.. as for my timings LOOOOOOOOOOOOOOOL

 

yep... widen the top edge too if you can so the exhaust port gets wider bottom to top. look at the port maps above and find one thats similer layout (same ammount of ports in the same ish places) and grind it to that. you can "blueprint" evo ports on to mhr reps if you like...

 

c16 all the way... work well with a ported iron.

basicly the chamfering is a bit two sided.. i do it for barrels i want to last a long time but for the top revving port you cant really chamfer the edges as it pisses the exact messurements up.. that or you make the chamfer edge where the port should be so you loose area of the port. for life of any barrel they should be chamferd tho (most barrels are piss poor port wise... always need work be it a £12 minimoto barrel or a full spec mhr team)

just remember to keep the botom edge of the ports perfect to the piston AFTER youve set the squish (piston to head clearance) as if you machine down the top of the barrel or use thicker/thinner gasgets to attain the correct squish after porting the tops and bottoms of the ports wont be where they should be.

so flow the case, widn the feeds, match the barrel, build it to set tollerences (squish at correct torque using a metal gasget) THEN strip the top end an butcher the ports.

you want to use a stock gasget and machine the top down so a stock gasget can be used rather than a 0.13mm metal one as there expensive and dont work too well.. where as set it up with one you have lying about.. messure the squish then work out how much more you need in there with the metal.. minus this from the orig gasget and you got spot on squish with a stock gasget.. every time. start big with paper gasgets tho as they shrink up to 1/5th of there thickness wich is why you need to retorque the head after 500 or so miles.

 

benchmark is 1mm... it will run and wont be over compressing by a long shot. most manufactors state smaller than this altho some use upto a 1.8 clearance for piston material type and head material (usually older engines)

 

also that is how most people messure the squish.. some people use up to 4 peices of solder tho.. 2 usually reads close enuff to one to not bother.

this is crutial for performance tho as you need this as close to specification as possible. depends alot on fuel type and any work done to the barrel obviously but kits will have the figure in the book somwhere.

 

fir higher compression ratios you need a higher octain not more fuel.. throwing a richer mix at somthing will only cure a problem youve created. doubt a bike running rich with a lower squish would amke more power.. would deffinatly deliver it differently tho but with these head profiles i think pre detonation would be the main concern

 

nah, higher octain added to road fuel is a slight increase and better running at the right comp ratio...

this years bikes will be run on race formulation fuel rather than road fuel at the pod so should be some interesting results