Port Timing Figures

[rahtreelimbs]

Originally posted by dbabcock
Rich,

The length of the divergent section of the pipe

What is the " divergent section? "

 

[dbabcock]

The section of the pipe that is between the exhaust header coming out of the cylinder and the maximum diameter is called the divergent section. After the exhaust gases pass the point of maximum diameter, they travel into the convergent section of the pipe where it's diameter decreases. At the end of the expansion chamber is a small pipe where the exhaust comes out. This is called the stinger. It really doesn't matter where the exhaust gases exit (I've seen pipes where they exit at the point of maximum diameter as well), as the purpose of the divergent and convergent sections is to allow pressure waves (sound) to propagate out the divergent section and then get reflected back by the convergent section to pack some of the lost, unburned mixture back into the cylinder. The shape of the pipe has to do with sound wave propagation, not exhaust gas flow, per se.

 

[Gypo Logger]

Hi Rick, although I cant speak for others, Doug doesnt have the proffesional licence to disclose more than what is required to feed us wanabees into a higher tuned world of 2 cycle modification.
Since honey is honey sweet, however the hiving, let the chips fall where they may.
Ramblin Jack Lambert

 

[treeclimber165]

Dang, John!

54* INSIDE your house???!!! I think it got down to 54* outside last night.... I was in a T-shirt by 11AM today though. Average daily highs of 70*-80* from now till April really sux.

http://a.orlandoweather.com/

 

[dbabcock]

I would have noticed the thermometer had my gaze not been diverted by those killer speakers. I just have to get a hold of a pair of Samaurai's to complete my system!

 

[bwalker]

Saw gurus, Just for grins what do these figures look like for a 70cc saw. .5-.7 squish clearance, 200 degrees of exhaust duration,140 degrees of transfer duration, 200degrees of inlet duration. I got these figures form A Graham Bells book Two Stroke Performance Tuning. Its probaly a little on the radical side for a aircooled saw as the figures are roadrace biased, but am I close?

Nevada, the reason tuners do not mess with crank case volume today is becuase as you stuff the crank case you raise primary compression, as this goes up tranfer port flow velocity increases. This intern causes loss of fresh charge out the exhaust port because of the increase in transfer flow velocity. To combat this tuners in the past decreased the amount of transfer duration. This works fine at reducing charge loss at high rpm, but it still goes out the exhaust port at low rpn leading to the thin powerbands I mentioned. Add to that the impact of less transfer duration has on HP and its easy to see why no many people mess with it today. It also important to realise that engines are built with crank case volumes more intune with there intended use today.

 

[bwalker]

The one thing I can't figure out is how engine tuners get their specs for expansion chambers. I basically understand how they work. Is it all trial and error?

Rich, Go to your local library and check out these books. Two Stroke Performance tuning by A Graham Bell, and design and simulation of a two stroke engine by Dr Gordon Blair. Gordon Jenings also has a great two stroke book that someone uploaded to this site. Do a search under Gordon Jenings

 

[rahtreelimbs]

Thanx, Bwalker!!!

 

[dbabcock]

This is my new saw mod project.

 

[dbabcock]

Sorry about that

 

[Gypo Logger]

Sorry to interupt, dont know about you folks, but I'm in the mood to cut up some household furniture. Will keep you posted with regards to pent up energy stuff.
Seattle Red

 

[Gypo Logger]

Hey Doug, Fook the Craftsman, it's those 14,000$ speakers we want to rock out on! Have you got any boring Jazz music we can simulate a paint drying episode, or do you want to make your 3120KD get up and talk at the Stabbin Cabin?
Spambert

 

[Gypo Logger]

Sorry forgot the picture of appox. 157 hp of saw. Feel fortunate that you have one or two of these. Keep the big chips flyin'.
Ramblin Jack Lambert

 

[Gypo Logger]

Sorry bout that, I forgot the 385KD which will kick butt from the Eastern slope of the Rockies, to the Western edge of Quebec, Canada.
Hank the Finn

 

[NevadaWalrus]

Thanks Walker. That makes some sense to me. I suppose if there were a way to close the exhuast port after exchange was made but keeping the transfer ports open a bit, that adding primary pressure would then become effective. As it is it gets wasted out the exhaust.

 

[bwalker]

It would be possible with a motorized traping valve like the one Boyessan produced a few years ago.

 

[rahtreelimbs]

Bump!!!

 

[David Young]

bump