Porting help

[netmech]

Not sure what area it should be in but as its a lammy cylinder im working with this time il C&P it here too.....


Im not too sure if this has been covered her previously, so here it goes, apologies if it has.

So, ive been practicing porting techniques on old iron cylinders, working with shapes, angles and just in general getting the right method down for the work i want to carry out.

After spending loads of time only reading jennings, bell, and the MacDizzy sites. Ive come to the conclusion that im kinda upto speed with the purpose of things like:

Timings
Mean Area
Time Area
Blowdown
Scavenging (Schneurle - loop)
Transfer Ducts
and so on.

My question is, (bearing in mind i cannot find any software to use). When deciding your cylinder characteristics ie. when and where you want it to make power and for how broad a range...i do i link this in with BMEP and then use results to determine the porting information required as listed above.

The way i see it, anyone can throw a degree wheel on flywheel up the transfers and exhaust timing, carv a bigger exhaust port and make a jump from 19 to 22 bhp. But there is obviously a lot more to it than that!

I cannot seem to find a link between it all. People say, 'We use BMEP to design a tune to your needs and engine characteristics' I cant work out from a BMEP calculation or the calculations to work out the list above how it ties in....! Im not too sure on peoples practical knowledge on here and im not asking for tuners to give up the secrets for their livelyhood, hopefully someone can help point a young gun in the right direction....

Again, sorry if its been covered already...I cant find much!

Info, threads, white papers and so on would be helpful.....

[netmech]

No one..... :wtf:

[Knowledge]

I think your question is too big. It will take some of the grown-up tuners too long to answer such a big question. Contrubutors don't like writing a post if it is going to take more than say 5 mins.

Start a new thread with a simple question, and then ask supplementary questions to develop the thread.

[Diablo]

I think Martin is right and that your question requires a very involved answer. The more you learn about 2 stroke tuning the less you realise you know. Its a bit like the government telling you that this or that food is alright one day then it gives you cancer the next. So the most important thing is to keep it simple.
Remember we have four gears,air cooled cylinders and the aerodynamics of a brick. Bear all this in mind when thinking about port timings. Also bear in mind that the Lammy crank is inherently weak and prone to twisting so the rev ceilings the books mention are unavailable to us.
Unless you are comparing improvements to your own motor BMEP won't really help you.
I think you should start basic and work from there. Lambretta barrels are limited regards port size because of the studs running through the barrel so the suggested timing figures in the tuning books don't always exactly relate to scooters. This especially applies to transfers.
Symetrical ports especially on Indian barrels will yield good results(ie especially opening and to a degree closing at the same time and having same area).Just reshaping the transfer exits(without modifying the roof angles) will make a difference. Changing the angles is difficult without the right tools so look to get timings by moving the barrel and then adjusting exhaust port to suit. If you build a motor with a long rod you can adjust timings easily without porting to see the effect. Learn how to measure your port durations accurately. There are a number of programs for this but I think the best way is still with a degree disk.
With regard to how a bike will ride on the road your blowdown timing is perhaps where you want to focus. Matching this up to an exhaust and gearing will achieve good results. Very generally speaking 25 degrees will give a broad spread at the expense of peak bhp but run longer gearing and 30+degrees for instance will give good peak bhp but a very focussed power band and will require short gearing. Don't forget that carb size,exhaust and gearing are all part of tuning.
Long stoking the crank will give you much easier port time area than enlarging ports will. Other gains come from reducing fin area on the flywheel,reducing drag in the drive train making the bike lighter or more aerodynamic etc. Always think of the big picture rather than just top end.
Don't be tempted to start porting large on the easy bits such as crankcase to barrel transfer flow as very often less is more. Peak bhp figures are easy with raised exhaust ports massive inlet timing etc but often a pain to ride in everyday use. You'll get the most gains from concentrating on the exhaust port. Invest in a decent piston with thin wire drawn rings and widening to 70% of bore diameter will show immeadiete gains. Pay attention to the exhaust stub and with an expansion think about flow out and back in. The cheapest way to do this is with a pattern jap piston. Buy Dave Websters tuning book for lists of pistons that work well but be prepared for some of them to be obselete.
Just concentrating on how the gas is going to flow through the motor and out the exhaust will reap benifits. Worry less about polishing and more about shape. Gas will flow round tight bends but definitely doesn't like obstructions. I always picture in my head putting your finger into the flow of a running tap to demonstrate how fluid reacts to an obstruction.
Piston port Lammys do like big carbs for peak horsepower.If power is all your after run the carb open without a filter. This will help low throttle openings.A big carb will only work though if you can achieve high revs through exhaust timing and gearing otherwise your just spewing fuel on your trouser leg. Longer inlet timing gives better peak hp as well. Experiment with cutting the piston skirt rather than the inlet port as you will soon find that the down side to this is very lumpy running at low revs and your scooter covered in fuel and oil. Gearing comes into it here as the quicker you run through the gears the less time you spend coughing and spluttering out of the power band.

[Diablo]

I was very drunk when I wrote that so its quite likely large portions of it are bollox

[drunkmunkey6969]

LOL.....I thought it was very good.

3am posting......its always awesome! ;o)

Sent from my HTC Sensation using Tapatalk

[netmech]

Hmm, ok, well i got a few things from yours Diablo! Cheers. Il crop up some other posts then along the way.

[paulmgreen]

I was very drunk when I wrote that so its quite likely large portions of it are bollox

And I bet it took longer than five minutes lol