Gs 160 stage four tune

[guygrrr]

kind of a limited appeal i think, but i'm having a go at tuning my buddies gs160. it's been bored out to take an ss180 piston and he's got a exhaust stub to take p2 pipes. it's gonna run the standard carb and a sito plus. so, i'm looking at some of the lammy port timing specs and i've set my sights on 125/170 and 145 intake, but i'm open to suggestions.

thanks!

[jonny snatchsniffer]

what pipe are you going to use ? it will be easier to chose a pipe and do the port timings for that particular pipe

[guygrrr]

the plan is to run a p2 sito plus. i figure it's close enough in theory to a lammy clubman, but i don't have years of barrel porting experience to say what port timings will work well.
when choosing the pipe/ port timings is there a reference somewhere that has recommendations?

thanks for the reply jonny, i thought for sure this question was gonna go unanswered.

[jonny snatchsniffer]

well first of all i would measure the port timings as it is now and see what you have, id say the transfers are a little high for the other timings you want and the inlet a little low
ive never had experience with piston ported vespas but the priciple is the same as lammys and i find something around 170 exhaust 120 transfers and 155/160 inlet makes a nice motor but as for with that pipe i dont really know
the p2 pipe is probably meant for something with lower exhaust timing tbh
when i had my barrel done it was to suit the pipe i had which likes around 185-190 exhaust and around 120 transfers but as its a rotary it has 205 degrees inlet which makes it very fun (polini evo smallframe)
all pipes are different and a race pipe would like the timing to be high and for it to rev
how are you planning to get these times ?

[Darrell Taylor]

as jonny said the transfers are too high in the example leaving insufficient blowdown
id measure std transfers duration (? how many transfers) and add between 50 and 55 degrees for the exhaust based on your parts list eg if transfers 110 160 ex 110 165ex the deciding factor would be the areas achieved from the ports
150 to 175 achieved from skirt not the port (if possible) on a small carb as a big inlet is not desirable

[Juan]

You could try contacting Max @ Curare Racing to bounce ideas around.

http://www.curareracing.com/71501/51601.html

[guygrrr]

well first of all i would measure the port timings as it is now and see what you have, id say the transfers are a little high for the other timings you want and the inlet a little low
ive never had experience with piston ported vespas but the priciple is the same as lammys and i find something around 170 exhaust 120 transfers and 155/160 inlet makes a nice motor but as for with that pipe i dont really know
the p2 pipe is probably meant for something with lower exhaust timing tbh
when i had my barrel done it was to suit the pipe i had which likes around 185-190 exhaust and around 120 transfers but as its a rotary it has 205 degrees inlet which makes it very fun (polini evo smallframe)
all pipes are different and a race pipe would like the timing to be high and for it to rev
how are you planning to get these times ?

cheers for that jonny. that's just what i was looking for. the timing i've measured is 120 t/ 150 x. i was gonna raise the the cylinder 1mm which would take the transfers to 125, but i'm not going to do that now. i like the sound of 120/ 170.

as jonny said the transfers are too high in the example leaving insufficient blowdown
id measure std transfers duration (? how many transfers) and add between 50 and 55 degrees for the exhaust based on your parts list eg if transfers 110 160 ex 110 165ex the deciding factor would be the areas achieved from the ports
150 to 175 achieved from skirt not the port (if possible) on a small carb as a big inlet is not desirable

yr going to make me break out my jennings. there are 2 transfer ports and they have been eaten up a bit with the rebore so their area has been reduced. i have no right angle tool to clean them up so the ports'll have to do. i would definitely need to reread jennings to get a handle on how the transfers and exhaust relate but i will take the intake to 150. thanks for the suggestion.

and jaun, that's a good site. i had forgotten about it. it's nice to see a tuner taking on some more ah, interesting work.

[jonny snatchsniffer]

you can widen the exhaust port to 65% of the bore too, that will give you more power thru the rev range, maybe darrel can help you with the transfers, as while you could get over a smaller area with duration its not the way to go in your case and area is always preferable if you need more gasses in

[Darrell Taylor]

ive only tuned 1 or 2 vespa cylinders but found a problem with widening the ex port due to the piston cut outs which would cause short circuiting of fresh charge straight out of the ex port ,i think 65% would be close to max on this type of piston but i generally base ex port width on the type of ring that is in use as a primary design
you might be able to basically widen the transfers with a ball ended burr,
so a simple tune up cut skirt/raise and widen ex/widen transfers/raise compression?

[jonny snatchsniffer]

yeah i said 65% as a max for the cast 2mm rings it probably uses , both my vespa barrels are at 70% tho they are not standard but kits and use wire rings, the p2 can be done to 65% but ive never seen a piston port vespa barrel and piston so dont really have any idea but i wouldnt have thought the piston has cut outs that large and probably has a bridged skirt ???? any pics ?