inlet duration

Hello all, just wondered if there's an accepted figure for a 175 ?

Hello Dan ,
The short answer , and by definition , there can`t be one . Do you go for the longest you can get away with or the shortest , experience has shown that for the former , power may increase and torque reduce and the reverse also applies . The balance will be made with what you are comfortable with when racing and the type of power band to suit your riding style . It`s no good having 30bhp with a spread of 1500 rpm and then dolloping around at the back of the field always playing
a , disspiriting , game of catch up , 3 gears won`t cope with that .
One of the major drawbacks of the symmetry of piston porting is that during the crucial period of port closure , there is an equal opportunity for gas to pumped out of the port , by flow reversal , as there is for gas to be inducted in . With a disc valve engine all hell lets loose if the inlet is shut much after 80* atdc , once case pressure exceeds atmostpheric you shut the port and use what you have trapped. Sadly , piston port motors with long timimgs just can`t do that , but if you can balance case pressure , timing and duct length to provide a bit of ram charging then torque spread can be aided . But as inlet pressures rarely exceed 1.2-ish atm then you won`t gain too much on a Bantam set up .
Inlet timing is a curious topic , on the very last of the works 500 Yamahas , that KR campained , the inlet perid was 204* , occationally rising to 206* , contrast that with the Suzuki , wih disc valve , at 190* ( non symetrical of course ) . The Suzuki was such an animal to ride with extended timing that power delivery became a light switch affair that defeated even Bazza , the one feature the Yam had , was a power spreading exhaust valve .

Things just don`t stop with timing , there is little point in running a 38mm carb on a 175 /190 when a 125 with a 32mm carb makes more power , despite conceding 50% capacity , and the torque advantage that extra cc should provide , together with the inherent torquiness that a longer stroke imparts . Something else , somewhere , is going wrong and it is not just inlet timing , in pure racing terms 180* is a conservative figure and for a Bantam 200* is about the limit , take your pick,
but start low and work up untill you feel no gain ! Mind you , feeling good and lap times can often be in conflict !

Trevor

thankyou trevor,

i have read some tuning info but mostly its very broad in the engines it covers, maybe should have said the timings i have at present they are exhaust 191.39 , trans 126.82 and inlet i think a conservative 158.96 , i have a blow back figure of 32 degrees .

i have noticed i hardly get any fuel back out of the carb , and this is what got me thinking on inlet duration.

There you go Dan ,
Reasonable race spec exhaust and transfer timings and commuter bike inlet duration , simple to fix !

Do check that your inlet port area is compatible with your carb bore area , you could have a port 10mm high and chop the inlet skirt to get the required timimg, but that would be daft , i`m sure you know what to do and get the picture !
All the best with your tuning , let us know how you get on , help is always on hand !

Regards , Trevor

Trevor what should the inlet area be? A constant matching of area of the carb 34mm
or a gradual tightening to speed up gas ,currently the port is an oval 26mm high and about 42 wide, it has a pip to keep rings in on tje upper edge,

Hi Dan,

For comparison purposes -

I run an inlet timing of around 165 degrees the port measures maximum of 41mm wide by 22mm, if you look through the forum there are dimensions for the Tom Snow 175 (by far the quickest one out there) I think the port has maximum dimensions of 43mm wide and 33mm high which must be getting on for 180 degrees. Both of these are irregular rather than oval shaped ports.

A word of warning if you go to far the bike wont start and will only run at very high revs.

regards

James

Dan ,
The area of your 34mm carb is 908sq mm , the general rule is that the port should be slightly larger than this , say x1.1 , this will give around 1000sq mm .
A good rule of thumb for width is .7 of the bore dia , so with a 62mm dia this will be 43.5mm , to get the area , if you need it , lower the port then adjust skirt length
to arrive at your desired timing . You may well reach a point where extending duration brings no difference to performance , at this point you look else where to find some gain . I`ve been at this game since 1964 , and am still looking !?

Catch you again , Trevor

hello all again, i've increased the timing to 185 degrees , will need a test to see it all okay , looks like i might be roughly okay with the port area, mine i calculated at 1079.9 square mm so a little bit big for a 34mm carb.

thinking on the ports a little longer does anyone have the exhaust opening before the inlet has full closed? as if i need more duration i'll be getting close to this happening, been thinking it through and can't see any obvious (to my limited mind) problems that may arise

thanks to all dan

Dan,
Don`t worry too much about your port area , if it wasn`t for having to seal the port to get the engine to run , you don`t need a port ! RS hondas don`t have a port , the reed feeds straight to the case , and the piston hangs down in fresh air ?
It is worth remembering that with the exception of the dwell at tdc , to continuously varying degrees , the port is obscured by the piston skirt , and a bit of area could help keep meaningful flow going ! And of all of the ports , the inlet is the least critical , and transfer port area will become the most critical !

Trevor

Hi Dan

checking back I see the inlet timing was not given for the the Tom Snow 175

Its inlet port opens at 97 degrees btdc ie 194 degrees total opening.

This might be a little on the long side but not enough to affect starting and if you keep it on the pipe there's very little blow back.

Mick