460 and 660 work saws double build

[Martinm210]

Wow, that's amazing work. I never did quite understand how the port mapping process works, that's very helpful.

I'll try to do the same with my 066BB jug coming. I'm just not completely sure I know how to do the calcs yet.

I did find this online calculator though that may be helpful.

http://www.modelenginenews.org/design/tcalc/TCalcForm.php#gen

Awesome

 

[Carlyle]

Think I will go back to the drawing board on this one, have 066 numbers worked out, have 066bb numbers worked out, but there is just no way as a work saw this jug can be modified to get close to those numbers.

Brian,
In your spare time could you explain what you mean by this statement, it is saw building 101 after all. Feel free to explain it after it comes off the drawing board. Santa is only bringing coal down here, so please wait until after the weekend and use snail mail. I trust them over a fat guy in a Red suit with 9 reindeer.

Carlyle

 

[CHEVYTOWN13]

Yo Professor!

Late to class but damn is this class kick ass!

 

[timberwolf]

just curious, why not widen the transfers over across towards above the intake????

interesting exhaust duration on the stock 460. I've never seen anything on a saw above even 165. not to be critical, but any chance you measured wrong????? That just seems wild to me.

The transfers have been widened a bit on the back side, did not go crazy with that, go too big and velocity drops and scavenging is then hurt. The saw will rev well but will be hurt torque wise. Also the rear side of the transfers should be aimed so the two streams flow into each other at the back cylinder wall so they then rise up the back wall of the cylinder, aimed too much towards the center or too far apart and the scaving loop is not as effective. Thats the way I read it anyway.

The porting numbers on these new stihls is high, it did not seem right to me, tripple checked and it is indead that high. 660 is higher than the 460 yet at 180 deg. Thats higher than I would port a work saw, more where a stock appearing racer would be.

For determining numbers I use a few pieces of computer software, some I came up with myself, to look at predicted port velocities, cylinder preasures, temperatures, even sonic wave action, then work up changes experimenting in theory with different changes to port and duct areas, heights, angles, timing, compression... It's fairly time consuming, takes 3-4 hours just to go through a saw and measure up everything, then each simulation takes 10-20 min depending on how complex the motor is, more ports, more detailed port and duct profiles and it takes longer, pipes or tuned exhausts also add time to the number crunching..

I'v been attacked before on using computers to simulate or predict what in theory could work, and I conceede it's very true that a simulation or numarical modeling is just that and it can never be 100% accurate on a complex and dynamic real word situation. But from trying maybe hundreds of theoretical designs on dozens of real motors and finding them all to have come out within 10%, and most with in 5% of the expected result gives me a fair degree of confidence that it's a valid method.

Alternatives are to reclcle a lot of aluminium trying different designs or just guess.

 

[timberwolf]

Here are a couple plots, stock 460 vs mod.

Keep in mind the simulations don't take in friction or limitations in flame speed so real world you would see more roll off over 14,000 RPM.

showing a gain in hp from just over 6 hp to about 8.5 (up 42%) and loaded RPM should also go from ~11,000 to ~12,500 a gain of 14%. With those changes I would expect cutting times to improve about 61% by the numbers, improvements were 68% real world, but the clean up of castings, piston finish and weight reduction not part of calculations likely would account for the rest.

 

[046]

so if I understand properly... both saws had pistons turned to form dome, cylinders turned to lower hight and ports smoothed out. did the porting change timing?

goal is a reliable work saw!

The porting numbers on these new stihls is high, it did not seem right to me, tripple checked and it is indead that high. 660 is higher than the 460 yet at 180 deg. Thats higher than I would port a work saw, more where a stock appearing racer would be.

 

[CHEVYTOWN13]

Green question

Here is the undersides of the pistons cleans up. Nothing radical, just improving flow where possible without hurting strength. Opened up the windows a hair but on this new 660 piston there is not much room to make them bigger.

Sorry for the green question, but do you have to take the piston/crank to a machine shop to have the rotating mass balanced after removing material from the piston?

 

[drmiller100]

The transfers have been widened a bit on the back side, did not go crazy with that, go too big and velocity drops and scavenging is then hurt. The saw will rev well but will be hurt torque wise. Also the rear side of the transfers should be aimed so the two streams flow into each other at the back cylinder wall so they then rise up the back wall of the cylinder, aimed too much towards the center or too far apart and the scaving loop is not as effective. Thats the way I read it anyway.

The porting numbers on these new stihls is high, it did not seem right to me, tripple checked and it is indead that high. 660 is higher than the 460 yet at 180 deg. Thats higher than I would port a work saw, more where a stock appearing racer would be.
.

180 is huge, and is actually my goal. on the 346, i couldn't get anywhere close to that unless i was willing to shim the jug up.

i wish i could figure out an efficient boost port on a piston port engine, but i suppose everyone else has the same wish!!!

TW, thanks for posting this. I really enjoy it, and I sure learn a lot and enjoy the discussions!!!!
Doug

 

[scotclayshooter]

Sorry for the green question, but do you have to take the piston/crank to a machine shop to have the rotating mass balanced after removing material from the piston?

If you were to convert your 6401 to a 7900 you would use the same crank but just put the bigger jug and piston on without any problems.
The crank is already balanced.
Seems that single cylinder engines dont suffer from the same problems as multiples.

 

[timberwolf]

In single cyl 2 strokes there is no simple way to balamce the motor. Without a second driven counter shaft no matter what balance is added or subtracted the vibs are just going to occure in a different direction.

General rule of thumb seems to be that half the piston and rod weight is balanced to the crank, so the crank bells cancel out 1/2 vertical viberation of the piston going up and down, but as there is no way to balance the crank bells as they themelves move from the back of the saw to the front then that makes for a new hrizantal viberation.

So if you end up with a little extra weight on the piston you get a little more up and down, less weight on the piston is no problem, just less up and down shaking but the same front to back.

I did not go raising the exhaust port much on 460, it was at 160 deg and can stand a little more exhaust durration, intake durration is lengthened a bit and transfers very close to the same timing wise. But on the 660 I will try to bring exhaust back down as much as I can to get some compression and trapped volume.

 

[timberwolf]

Finished off the muffler, tigged on an extention to the deflector and went back inside the saw to increase intake durration and time area a little. RPM up closer to 14,800 Here are a few more pics.

Cutting 3.38 s now, over 75% better than stock same bar and chain and about 110% with better chain.

http://www.youtube.com/watch?v=BeWBNG6z7f8

 

[Metals406]

Finished off the muffler, tigged on an extention to the deflector and went back inside the saw to increase intake durration and time area a little. RPM up closer to 14,800 Here are a few more pics.

Cutting 3.38 s now, over 75% better than stock same bar and chain and about 110% with better chain.

http://www.youtube.com/watch?v=BeWBNG6z7f8

Looks and sounds great!

 

[CHEVYTOWN13]

Bad Ass Work

Would it be worth it to gasket match the bottom of the port?

Or will removing too much material cause it to loose too much torque?

You do fine work TW. Your attention to detail I'm sure is second to none.

 

[timberwolf]

The gasket is hanging on the slack in the bolt holes. So lifted into place It's pretty close, and if anything I'd want the gasket a little bigger than the hole rather than projecting into the flange opening.

Need to be very carefull not to lower the actual exhaust port on these if you are boosting compression, Piston skirt will clear the floor of the exhaust port. This saw is walking the line on that issue.

 

[Peacock]

Sure makes it easy to line up the exhaust ports on those 2 piece mufflers, huh?

 

[timberwolf]

You got it, if the gasket moves a little one way or the other while bolting it up it still does not end up sticking out.

 

[CHEVYTOWN13]

Fuel mixture pooling

Out of curiosity, do these small carbs spray the fuel mixture like a fuel injector or more like a traditional carb, say a Holley?

I was just wondering about the fuel mixture pooling from a polished intake. I remember reading about how leaving the port a little rough helps break the fuel apart so it doesn't pool if one was using a carb.

Is there a concern for that in 2 strokes?

After seeing your saws run though, my guess would have to be no.

 

[Metals406]

Out of curiosity, do these small carbs spray the fuel mixture like a fuel injector or more like a traditional carb, say a Holley?

I was just wondering about the fuel mixture pooling from a polished intake. I remember reading about how leaving the port a little rough helps break the fuel apart so it doesn't pool if one was using a carb.

Is there a concern for that in 2 strokes?

After seeing your saws run though, my guess would have to be no.

Yes, leave the intake port rough, and polish the exhaust.

 

[scotclayshooter]

Yes, leave the intake port rough, and polish the exhaust.

I take it the polishing is only done to slow down/stop carbon build up?

 

[Metals406]

I take it the polishing is only done to slow down/stop carbon build up?

Build up, and to improve flow. You don't have to worry about mixing air and fuel once combustion has completed... But you do want improved flow, thus the polishing.

Some have even gone as far as to coat the exhaust port in ceramic, to aid in flow and retard buildup.