Topics for 2013

Hi all,

As the 2012 season draws to a close thoughts are turning to topics for off season development for this winter one of the areas I am planning on investigating in much more detail is cylinder head design , I remember the hype of Suzuki's TSCC (Twin swirl combustion chamber) and feel this is an area I have not really looked at. An addendum by Gorr in Jennings Two stroke tuners handbook states

"generally speaking, a cylinder head with a small diameter and deep combustion chamber, and a wide squish
band (60% of the bore area) - combined with a compression ratio of 9 to 1 is ideally
suited for low to mid range power. A cylinder head with a wide shallow chamber and a
narrow squish band (35-45% of bore area) and a compression ratio of 8 to 1, is ideally
suited for high rpm power."

Obviously this element cannot be taken in isolation. However, have other riders / builders any experience in this area and does the Hypothesis of Gorr hold true in your experience ?

Hi 175 ,
If nostalgia is your thing then continue with Jennings and his 40yr old theories , could i draw your attention to the last paragraph on page 99 ?
Nothing speaks more eloquently than that statement , leave that stuff to its place in history and seek out current developments , you`ll go faster
for it !

Swatting up on a valve radio hand book won`t qualify you to construct a tripple mosfet super amp !

Regards , Trevor

Trevor,

Thank you for the response , if we are disregarding old works what about 1662 and pV = K.

Now how about the actual question asked -
- what in your experience is the best shape for a combustion chamber and what % squish band ?
- is there an optimum in your view ?
- or is a hemisphere with 50% squish band width as good as it gets ?
-Or provided there is reasonable squish is the shape largely irrelevant ?

Hello again 175 ,

The state equation with pressure , volume and temperature as its constituent values is fine in the static world in which it was conceived .
Were you attempting to draw that into the combustion chamber topic , you will be chasing your own tail on that one , for all values will
vary with every step change in rpm ? It might just be relevent in steady state running , at peak rpm , on a long straight where stability
is achieved , but those conditions are few on the circuits Bantams use !

As to the " actual question asked " !

Isn't the problem - there isn't any modern 2-stroke tuning availible at least not in book
form? Racer175 try pit-lane.biz but its not all relevent as mainly related to Aprilla RSW / RSA.
I believe there's a Dutch 2stroke tuning site but i don't know its web address...

Heads i've tried a few in the last 30yrs - narrow squish, wide squish, no squish-low chamber haven't
tried top hat-wide squish or bell chamber head. But feel a lot of it depends on fuel/oil used and ratio
as to what may work? With a 175 i'd stick to 50/50 squish/hemisphere and 9:1 as Snowy's motor?

Regards Mike

this is the link to the bit on cylinder heads on pitlane.biz -

http://www.pit-lane.biz/t3173p220-gp125-all-that-you-wanted-to-know-on-aprilia-rsa-125-and-more-by-mr-jan-thiel-and-mr-frits-overmars-part-2

You need to join the forum and login to see the pictures but can read it without.

Its probably the best guidance there is on tuning two-strokes from people that really know their stuff and are happy to share it with everyone.

The thread has two main contributors - Jan Thiel and Frits Overmars both of whom were in the Aprilia GP racing department developing world championship winning two-strokes for many years and there are other contributors that have developed championship winning karts, snowmobiles and 50cc racers that also comment.

The main points they suggest from experience and testing for good cylinder head design are -

1) a 50% squish area which is about a 7mm wide band on a 54mm bore
2) as tight a squish clearance as your motor can handle (probably about 0.7mm) and parallel to the piston crown
3) no radius on the inner edge of the squish band
4) a hemispherical chamber that is slightly flattened in the region of the plug.

Testing also showed offset chambers were no better than central ones.

This particular link is to page 13 of part 2 of this thread but there's another 50 pages in part 1 and there's lots more useful tuning info there

Mick

Hi 175 are you going ahead with pit-lane.biz spec heads? If so lower
your compression ratio to something sensible for a 175.

I'm going to try some new heads next season based on info gleaned
off pit-lane.

Any other plans ideas?

Mike

HI

Its been interesting reading and will lead to a couple of minor changes one to the squish which will be reduced sightly and the second to the squish band in line with the following advice

A very important detail: even the smallest radius on the end of the squish-band gave a power loss.

Certainly will not be going to 16:1 compression ratio, will look somewhere between 10 and 12

Other than that for next year there are two new exhaust , so far designed and built to be fitted and tested and a couple of different carburettors if all that gets done and we find a reasnoble setup then its inlet tract length next.

The target is to have at least 20hp , 12 lbsft and a powerband of 3000 rpm quite a big ask as this year with a number of issues we had 12hp, 11 lbsft and about 2500 rpm but it was the easiest bike in the world to ride.

Oh and the bike will need its annual clean with a hankie with some spit on !!!

All thats needed now is the time to do some of it

You mean 175 Racer! 20 horsepower at the rear wheel, of course!!

Does nobody measure power output at the crankshaft anymore?

I would have thought people would be interested in transmission losses.
Certainly by what I have seen of ball bearings replacing the original plain bushes in the gearbox does suggest the losses are being taken seriously yet there may be other things dragging down the power... Derek Best reported on here that he had done a "Coasting Down" test which sounded interesting but Our Derek didn´t say how much, what he found out and how he actually did it ...

...or did I miss it all when my PC went belly-up from viruses...?

And Derek! you are sleeping -- are you? Sorry -- I remember now you did commment on tyre-pressures having an effect on Coasting Down times.... Which reminds me that when fuel consumption tests are made the tyres are very very hard. And there is data on how much tread-pattern affects road drag -- isn´t there??

And 175 Racer get yourself a good torque curve over a wide speed range and it won´t matter so much about the power....
Cheers!

JayBee -- for John-Boy celebrating his new front teeth.