Discussion Starter

[Loony661]

Only if someone could print up a few clear cylinders to watch what is going on in there in super slow motion

They’ve done it in drag racing engines - I’ve seen some awesome high speed photography of valve train motion at over 9000 rpm. You wouldn’t believe how much the valve stems and springs “walk” back and forth..

 

[huskihl]

First motor I ever saw with the transfers flat and primaries opening first was an Echo. Makita/Dolmar 7900 may be similar but I don’t remember. Both those companies skirted epa regs for years by doing it. I figger there’s something to it

 

[Ketchup]

Ok I'm wondering about trans port lengths. This design being very short and able to move a short column of air very fast compared to a bottom of cylinder feed. Has there been a design that consistently yields mobedd,r?

The general trend is towards longer bottom fed transfers. Stock saws make more power than they used to…

 

[srcarr52]

The general trend is towards longer bottom fed transfers. Stock saws make more power than they used to…

Most of the longer transfers are for back filling with the strato ports.

 

[Ketchup]

Most of the longer transfers are for back filling with the strato ports.

Yeah. Strato technology really muddies the water. There was already a trend towards quads and bottom fed tunnels before the EPA regulations but the tunnels certainly got longer again with stratos.

Another thing to consider is what models do best when modified. There are strong side port duals, bottom fed quads and now some powerful strato duals. Not many open port models can hang. Personally, bottom fed quads are my favorite.

 

[pbillyi69]

why would the transfer opening flat keep the epa appeased? i dont understand? do they make for a more complete burn? with them flat how does that affect the performance of the engine? does it make more power or less?

 

[pbillyi69]

i am enjoying this thread it is thought prevoking for sure

 

[drf256]

Ok I'm wondering about trans port lengths. This design being very short and able to move a short column of air very fast compared to a bottom of cylinder feed. Has there been a design that consistently yields mobedd,r?

I think the MS461 was one of the first Stihl models with the long transfer tunnels. I believe they considered it a ”poor man’s strato”. The saw was designed to backstuff the uppers so that the first part of the loop was exhaust gas. So over scavenging would result in loss of burnt charge and not raw fuel.

We think of the path of the charge from the case to the uppers as one event, but it’s just little “blips”. The entire tunnel isn’t emptying with each revolution. Kinda doesnt matter if the tunnels are an inch or a foot long once they are filled with charge and have pressure. Certainly some laminar flow issues as the get longer, but the radius is more important than the length exponentially.

Anyone ever consider the fact that exhaust pressure might play some role in moving the transfer charge? If one opens the exhaust side of the transfers first, the exhaust pressure may be more than the transfer pressure at that moment. If remnant combustion pressure finds its way into the exhaust side of the upper, it may aid in shifting the transfer flow towards the intake side of the jug.

 

[drf256]

why would the transfer opening flat keep the epa appeased? i dont understand? do they make for a more complete burn? with them flat how does that affect the performance of the engine? does it make more power or less?

I think Kevin was referring more to the fact that the primary and secondary uppers were staggered in their opening degrees. The primaries opened first and pushed the charge away from the exhaust port and then the secondaries chimed in and pushed the charge upwards. Opening flat is thought to sweep the piston top clean first.

Flat vs angled transfer opening will always be debated here. Some saw motors like certain things and some don’t. There are general rules on what will make an engine run well/better, they just don’t apply to all engines.

If you take a look at the Dolmar 7900 cylinder you‘ll wonder how it runs so well. Long tunnels, tight case and tiny uppers. The motor runs by a lot of primary compression. Make those uppers nice and big and the saw loses power.

 

[Loony661]

The best, most efficient, most powerful 2 cycle engine is the one that can scavenge 100% of the exhaust gasses, and 0% of the incoming charge... How you get there is the mystery.

 

[Ketchup]

why would the transfer opening flat keep the epa appeased? i dont understand? do they make for a more complete burn? with them flat how does that affect the performance of the engine? does it make more power or less?

Companies have done lots of stuff to transfers to reduce emissions. Many of the EPA non stratos just restrict flow or shorten duration. The angles in those saw’s usually just show what’s necessary for the starved engine to run. Typically flat exhaust side (primary), very angled intake side (secondary), with low duration and often with restricted lowers. Not to mention choked up exhaust. Porting changes all that and what works best depends a lot on where you start.

Those saws have influenced me a lot though. The transfers have to be fairly efficient.