Arthur`s Answer ?

I could have taken the easy way to answer this question by quoting a number which has been used in the past ! This could be seen to bestow a measure of legitimacy by virtue of previous use , but this would almost certainly be miss-leading and a touch disingenuous .

All of the cylinder events are inseparable from each other , from the instant the transfers open to the point they close , and to the exhaust closure . Upon subsequent transfer re-opening it is the action of the exhaust that determines cylinder pressure and then mass transfer flow ! It might be argued that the crankcase could just be separated by virtue of the time lag from inlet closure to transfer opening . Here the element of time has been introduced , time areas . angle area and specific time area , and by time units in engines , millisecs .

There needs to be enough transfer time to adequately clear away stale gas and fully scavenge the cylinder , this is the first problem , if you are stuck with transfer ports that are not modifiable , either in width and / or height then in turn this will pre- determine the ex timing !
For most Bantam race applications 30/32* of blowdown , with revs under 10,000 , will get the job done , one thing is absolutely certain , excede a common sense blowdown number and the power band will disappear along with fresh charge down the ex port as torque rapidly falls , and with 3 speeds that is a great disadvantage and will be realised by poor lap times .
A coherent approach to porting phase orchestration will always make the power and provide for an easy bike to set up , ride and manage .

Determine the transfer port timing add blow down and you have your ex timing , my current w/cooled engine uses the lowest ex port timing i have ever used in a shorter stroke 125, but specific time area is more than adequate to get the job done , so bigger is not always better !
None of this includes port width , corner radii , duct proportions and outlet diameter and so on , but that is a different matter ?

Hope that helps you a little , but keep asking !

cheers Trevor

Hi Trevor
Thanks for that, not wanting to add any legitemcy to your post or mine for that matter, but after it Idid notiiice I had indeed genuinly done a typo !!!!!!!!!! error!!!!!!!!!! on my exhaust timing, by this ?

You see im conciouse I understand the OTHER CAMP is looking in,?????????? but based on you giving all away on a plate "I have corrected my typo", but I would always advising a new starter building a new motor, to start off lower "you can always lift it",

Talking about the exhaust cycle "that seems so dependent on the pipe" doing a half decent job, in evacuating and recharging, just for the new 175cc builder,!!!!!!!!!!!!!!!!!!!

if you were designing a new pipe for our 175cc formula what would it look like, the items that have been copied all produce light switch power bands. ? so just as a starting point, could you post what your design would look like, I dont think we have a good formula pipe or one that can be of use to a new starter, please include as you did for me on the 125 pipe you designed for me, with sizes and lengths, this may be a good place to post it ? especially if arthur is building a 175cc to sprint or race, Like my dad, at young age of 79.

will post some pictures of it soon.

P.S. thats if your head is not too bad after your brave night out on the town, lol
Derek

Derek ,
As you know i am always ready to help any one at any time , but maths equations needs some raw numbers to be inserted to be able to come up with an answer for a pipe design .

Maybe this is an area where the Bantam Rcing cognoscente could offer expert advice on a simple , start up specification based on an iron 175 barrel . Nick was outlining just such a suggestion at the Midlands meeting , and it recieved unanimous approval , and , if Nick can , comfortably , win the 125 championship using an iron barrel then it would augur well for an entry level set up !

It would be good to have the opinions of all of you out there , and such suggestions as you feel appropriate , it might be worth pointing out that in these very difficult financial times a low cost , basic , strictly enforced formula may just be the saviour of Bantam racing ?

Yes John , just like the good old days , electric drill , riffler files , slide rule and log tables , and every one in with an equal chance of success !

Trevor