Stroking

E

Eoin70

Well-Known Member
ok if i wanted to place in a crankshaft who has 4mm extra stroke in my engine
i was thinking i need a 4mm space under the cylinder
but now i ask can the cylinder still support the piston with the extra 2mm of travel downwards?

And what about the port durations, are they now much too small?
 
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You need a 2mm spacer under the cyl (if the rod is the same lenght). However port durations would increase too much on a race cyl. To preserve more or less the timings you would have to add a spacer on the head.
 
It's not that easy.

-You have to put, a 1.85mm spacer or something, because you need a extra "pakking - i don't know the word in english"

-You also have to change the timing's etc.

-You have to create a spacer on the head.

And sure there is more, that you have to change. But i'm not a tuner, so don't ask me what:p .
 
BartR. said:
It's not that easy.

-You have to put, a 1.85mm spacer or something, because you need a extra "pakking - i don't know the word in english"

-You also have to change the timing's etc.

-You have to create a spacer on the head.

And sure there is more, that you have to change. But i'm not a tuner, so don't ask me what:p .
Click to expand...


I PM-d to you! :) Did you get it? If you don't answer isn't problem!
 
its minarelli !!
and yes the spacer needs to be a little less than the stroke difference/2 becuase of the gaskets
but i thought the port durations are now much less becuase the piston travels a longer distance
 
if your stroking the minarelli and youve not got tuning experience and facilities then its best to start with a cylinder kit that has street port timings as standard then when you add the stroker crank/packer it will increase the port durations up to full race port timings ,at bottom dead centre youll not have the lower exhaust and transfer area open but you can port this if required or leave it as it wont harm the set up too much. The increased stroke also affects the compression ratio calculation so using a street head then setting the squish at 0.8 instead of 0.5 will keep it safe
if you use a kit such as the mk sp or pro which uses a head thats recessed into the cylinder you can remachine the dome/squish band to allow the cylinder to be lowered further down to get lower port timings or you can use a malossi mhr rep head on a mk sp or pro to do the same
althogh the best thing may be to just buy an athena/simonini or similar barrell thats available for use with stroker cranks and all the hard works done for you
 
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longer stroke will increase duration you have to calculate how much stroke you add with the current port timing on the cilinder, i tuned my spare MHR Team milder than the previous one i plan to increase the stroke to 40,2mm using custom stroker crankpin i know a machine shop that makes those for 4stroke bike applications.

with 1mm increase it will affect timing but not much b'coz i have run much higher durations on my other cilinder compared to the new cilinder with 40,2mm crank.
 
can you please explain why it will raise the durations?
i thought it would lower them,im just a little confused

and now your saying also that the compression will be way too high
i think i can agree with that !! what about using out and out race fuel,will that help with the compression ratio so high.
if needed i can machine my head for much lower compression
 
the increased stroke provides a longer opening duration ,i guess you must be thinking how can this be so if the port has not been raised, but the port has been raised when the barrell is lifted to accomodate the extra stroke. although this will only effect the port closing duration as in reality the lower edge will be closed unless you port it to match the piston crown at bottom dead but the overall duration is still measured from bottom to when the port closes
best to stick a degree wheel on the end of your crank and see whats happening and it will all become clearer.
the cr is easier to understand if you have say 4.5cc on a 70 (approx 16.5)and you stroke it to 80cc (then it would be approx 18.7) then the % of head volume needs to be increased accordingly to the new cyl volume,approx 5.15cc would be back at 16.5 on the 80.these are rough simple calcs only
 
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i use a simple port duration calculator software i downloaded years ago, you just input data like bore, stroke, conrod length (center to center) & exhaust port height from deck to top edge of port.

every serious tuner should/must have one.
 
thanks guys you have really have cleared some things up for me !!

do you think it would be possible to run my Mhr TS 70cc with a 43mm stroke?
then i will have 76 cc of displacement

i have the spacer almost in production ive given a gasket to a friend in an engineering shop and he's about to make it !! and my head that can be machined at home on a lathe

now the next question is what about the port's!!
what do i need to do to them?
 
Eoin
im in england and have approx 4 different thickness of spacers and loads of shims for the minarelli and also have 3x 43mm cranks
the spacers i had laser cut for 43mm conversions
on a mhr ts thats gonna have some pretty high timings.

i use tsr tuning software from the states,very expensive but worth every penny but its still only as good as the final build and set up and dyno work.
its almost impossible to get all the combinations correct without one,i would stick my neck out and say that any tuned scooter motor that has not been set up and developed on a dyno will be around 20% away from its potential output unless its been built to known settings,malossi are one of the only companys who provide full specification to aid set up of the whole engine therefore these motors have the best chance of being good out of the box than most other manufacturers
thats the problem with non compatable stroker kits,everything needs fully re adjusting(compression,squish,msv,port durations,exhaust tuned length,ign timing/transmission set up / jetting) to get them to run anywhere near as good as the original kit does you may gain 6cc but you potentially get all the rest completely wrong which could lose more power than you hoped to gain , get it right and youll possibly have something as good as the 2 fast kit
 
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50cczip racer said:
Eoin
im in england and have approx 4 different thickness of spacers and loads of shims for the minarelli and also have 3x 43mm cranks
the spacers i had laser cut for 43mm conversions
on a mhr ts thats gonna have some pretty high timings.

i use tsr tuning software from the states,very expensive but worth every penny but its still only as good as the final build and set up and dyno work.
its almost impossible to get all the combinations correct without one,i would stick my neck out and say that any tuned scooter motor that has not been set up and developed on a dyno will be around 20% away from its potential output unless its been built to known settings,malossi are one of the only companys who provide full specification to aid set up of the whole engine therefore these motors have the best chance of being good out of the box than most other manufacturers
thats the problem with non compatable stroker kits,everything needs fully re adjusting(compression,squish,msv,port durations,exhaust tuned length,ign timing/transmission set up / jetting) to get them to run anywhere near as good as the original kit does you may gain 6cc but you potentially get all the rest completely wrong which could lose more power than you hoped to gain , get it right and youll possibly have something as good as the 2 fast kit
Click to expand...


My friend has a tsr and a MOTA too! He said the MOTA is better, but the tsr is quite good!
 
yes Tiba but then wont it increase my port durations with the larger squish :(

but yes it would be a shame to ruin the head
and also do you know where i can get an MHR 85mm conrod from?
 
you can get conrod from a Malossi Tech center i'm sure the 85mm conrod kit from the Piggio MHR Team crank is the same as the 85mm MHR Team Cross crank for minarelli.

btw with 43mm stroker the port timing increase will be huge you'll definitely would weld aluminum on the roof of those ports to bring back timing on sane levels, i backed out on making a 40,2mm ministroker MHR Team crank b'coz my timing came out with 135degrees on the transfer port & that's with untuned cilinder i want mine below 134 degrees with tuned cilinder.

your best bet with 43mm stroker crank is to use a very low port timing cilinder like MK SP then use a modular head from Polini EVO & then cover the water jacket with PR2 outer head or you can just buy 2F4R 80cc kit (sold out) or Fabrizi 84cc kit.
 
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