porting/tuning newbie / guide to homebrew tune

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olliewtf
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Hi all,
the purpose of this thread is twofold; firstly i need all the advice i can get on tuning my barrel, but i also want this thread to act as a bit of a guide should anyone else be interested in undertaking such work; therefore if all replies could be over explained so anyone who is new to this can follow and understand the process that would be much appreciated.
Im running a sr175 kit which before i got it had been bored to 63mm with asso piston giving me approx 180cc. ive ran this in with 22m jetex and a clubman with a 4.9 final drive. running a standard mec gp crank.
I now have all the bits ready to fit a 28mm delly and underkickstart fresco. I also want to tune the barrel somewhat to take advantage of this. Ideally I want as much as i can get out of this barrel, whilst also remaining in a 'safe zone' as im sure some of my measurements might be slightly out andd other factors such as compression and timing etc im still no expert on so as said i need a bit of a safety margin.
The proposed work wont be carried out for a bit (need time off work first!) so i want to pool as much info before going ahead.
Ive started the process by creating a port map; for anyone reading this as a newbie like myself, a port map is essentially a transcription of the inside of your barrel onto paper. ttap it into google and theres a good guide, all you need is some stiff paper and a pencil.
My measurements so far are thusly, although i would give or take 1mm as my calipers are shite as is my ruler measuring!
exhaust: barrel top to top of port 39mm with ruler, 40mm with caliper. width, 40mm
transfers: 1 (right of inlet) 47mm ruler 46.95 caliper from top to top .... 2 46mm ruler, 47.8 caliper top to top
Inlet 73mm top to top 36mm wide.
Using the porting calculator i get 151.04 ex duration, 117.26 transfer
dont know inlet as i cant work out piston height inlet side.
I also did -1 for piston crown height to top of barrel as the edge of piston sits below top of barrel so i have for arguments sake assumed -1 though this could be wrong.
So can anyone advise on my maths and also explain the grey areas i have encountered and then we can go on to talk cutting metal!
cheers all, your help is much appreciated!
rondord
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As I understand it. Your -1 should be 1 and your inlet measurement should be top of barrel to bottom of the port.
why are you unable to measure the piston inlet skirt?
olliewtf
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very helpful cheers mate! well i stripped down top end yesterday as i had a leak and wanted to map barrel, after which i put it back together as the scoot is my everyday run around for work so cant be doing it too often. really i shouldve looked up everything i needed first. as far as my understanding goes however, if i knew the dimensions of a standard asso piston i could still work it out? italian li s3 125 block, so deck height should be fairly standard, with standard gaskets, as when i first built it i had no idea what squish was. so there are some anomalies due to inexperience but i have feeling these can be worked around with some maths? cheers
olliewtf
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so inlet from top of barrel to bottom of inlet is 93.5 mm... shouldve known that! cheers
eden
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rondor is correct..

The Piston Crown to Top of Barrel at TDC: is a plus number if the piston crown is lower than the top of the barrel at TDC and a minus number if the crown protrudes above the top of the barrel.

The inlet skirt should be measured really because every 225 asso piston I have ever got from AF have already had 2mm cut from the inlet skirt but others I have got from other places have not had anything cut from the skirt, having said that its been a long time since I've used an asso piston.

Its best to cut from the port than the skirt but cutting from the skirt is a great way of finding out what inlet timing works best with your set up and ridding style, once that's discovered you can always cut from the port after but remember its a lot cheaper to buy a new piston to alter the inlet than it is to have to buy a barrel lol

I doubt many people will be willing to just post all the info your looking for regarding port timings because if their anything like me it takes a lot of trial and error to find a combination that works how you want it to work and that trial and error is expensive.

A lot of it depends how YOU want your engine to ride, do things in stages and keep records so you can always revert back to how you liked it best.
olliewtf
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Thanks for your help! Im 75% sure my piston is intact, i cant remember noticing anything not even.
The next stage is to post my timings to see how conservative they are.
I appreciate what your saying eden, i think rather than simply posting 'x timing would be best' some info on how timings affect performance is more what i was after. Ive read and will reread graham bells book, which explains what affect timings have on engine performance, however i eant someone to telatevthese figured back to lambretta world as mbs tech site says theyre no good on lammies. Let me go away and do some reading and ill come back with something. Essentiwlly i want to propose my oen timings and for people yo clarify if theyre safr and decent if that makes sense. Anyway, thanks again and ill be back shortly!
olliewtf
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Right im back with some numbers and a better idea of what I want to achieve. to anyone following this who is as clueless as me let me recommend a. graham bells tuning manual for a very clear easy to follow guide to what all the different ports do etc. read this in conjunction with b. MB's porting tech site, lots of great info there.
After a it of a read I believe that the following port timings would be a very good starting point for me, allowing potentially for mistakes and possibly further tuning.
Exhaust: 170
Transfers: 118/129
Inlet: 140/150
These timings should hopefully create an easy ride with power through rev range and no major problems with flooding at low revs and unattainable powerbands. as ive had no experience with testing these timings im essentially talking out of my arse but the tech site convinces me this is a good set of timings and years of testing and experience has gone into understanding these things so credit where its due its a great article for someone like me! Do these timings sound good to you guys?
As for my problem with not having measured my piston skirt length; if anyone knows the standard length of an asso piston that would be great, for now im going to assume its 71mm (rightly or wrongly), and when I eventually get the time off to carry out the work im planning I can measure ad adjust if necessary using the port timing calculator which is another genius freebie online.
so therefore assuming my skirt length is 71 mm, my current timings are thusly:
exhaust: 157.53
Transfers: 122.57
Inlet: 135.73

So. a bit of metal needs cutting away it seems and luckily there is scope to do so.
The only parts I still cant get my head around are these;
with the exhaust port, if needed I will open up the outside to match the frescos manifold and carry through into the barrel... when I cut the exhaust port, am I following the shape of the top of the port? im sure I read in bells book that its beneficial to cut in a point, but is this relative to a lambretta? similarly, is it worth widening the port somewhat?
I may leave the transfers well alone as they seem in the right ballpark, does everyone agree?
and same again with the inlet, am I following the shape of the bottom of the inlet when removing material?

and lastly, I understand that the blowdown period is a relationship between exhaust port timings and transfers, and the timings im aiming for theoretically give me a 'good' blowdown period, but can anyone elaborate?
thanks again one and all!
rondord
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Tumble weed here. It looks like eden was right about no one wanting to give secrets away, either that or most people reading this have no clue.
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coaster
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I have had some very generous advice to tuning questions on here to be fair, Olliewtf it might help if you were a bit more specific about what you want to get out of the tune. Do you want a back lane screamer or is more of a touring tune to your liking? Which carb and exhaust will you be using?
Darrell Taylor
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coaster wrote:I have had some very generous advice to tuning questions on here to be fair, Olliewtf it might help if you were a bit more specific about what you want to get out of the tune. Do you want a back lane screamer or is more of a touring tune to your liking? Which carb and exhaust will you be using?

i agree coaster read it and too much info needed before good advise can be given and theres a books worth of info to provide to answer all ,its not as easy as it looks either ,i read fastfrogs post on tuning a cast barrell(great article) and be good for him to add some info of timeframes required to get to where he is now with it and share his experience level compared to a job for a newbie to tackle ,the cost of pistons heads etc and what tooling is needed to do the job in hand at that point most realise it would be cheaper to let a tuner do it or buy a pre tuned kit,commercially available performance kit.
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