262XP Day

[cuttinties]

You guys are just TOTALLY over thinking! Trial and error is the only way you can make engines run better.

It's not necessarily over thinking. Piston speed affects everything that happens in a 2 stroke. Give me a couple of days and I'll post up my notes I've found playing with piston speed formula.

 

[skippysphins]

So Randy
Good morning did you have your java and grits ? Where is the thread going to today ? This has been very informative thus far going back together is harder than taking apart right ? Keep up the good work thanks .
Skippy

 

[old-cat]

It's not necessarily over thinking. Piston speed affects everything that happens in a 2 stroke. Give me a couple of days and I'll post up my notes I've found playing with piston speed formula.

All the thinking is fine, let your imagination roll! But it's only by making the mods that you can learn what works and what don't.

 

[drf255]

Unless you change the stroke piston speed is not changed. What you're seeing is affected more by crank angle than piston speed. The longer the stroke the higher the piston speed. This is why Randy's exhaust height guidelines work so well for him. Higher piston speed allows for higher durations. What appears to make the piston slow or dwell isn't slowing the crank down. For the time you're considering the port open the opposite is closed. So the intake isn't open longer than the exhaust. The durations of the transfers are affected because of the fact the location on the cylinder.

Look at the intake or exhaust ports and you'll notice that they reset (repeat their cycle) very much sooner before the transfers. Another thing to remember is that a smaller port is covered faster than a larger port. Which means that the durations will be affected quicker.

Yes. The longer the stroke, the more the opening points will vary with a change in height.

We're saying the same thing here. A degree of rotation will move the piston a greater distance near the 90* mark than at 0*. So the speed (distance/time) will not be static for the entire stroke.

 

[redfin]

A degree of rotation will move the piston a greater distance near the 90* mark than at 0*. So the speed (distance/time) will not be static for the entire stroke.

You have this backwards, the rod being further from center does not change speed just the relation of distance traveled.

 

[cuttinties]

All the thinking is fine, let your imagination roll! But it's only by making the mods that you can learn what works and what don't.

Trial and error work yes. But if you feel like looking deeper into how things work and applying it you'll never get optimal results. Trust me ruin a couple of cylinders and make a bad runner you'll look past what you think and dig for solid facts to find the cause. I built a saw that no doubt in my mind will be faster than better than half of the 385/390 build off saws (not even a Husqvarna). I want to know why I make the gains I do on that model. It only matters if you know where you're going and why.

 

[redfin]

You have this backwards, the rod being further from center does not change speed just the relation of distance traveled.

And those two points that we are relating are the piston and degrees of crank turning. This is clear in my head but prolly isn't making it out here.

 

[drf255]

And those two points that we are relating are the piston and degrees of crank turning. This is clear in my head but prolly isn't making it out here.

Mine either.

When the crank throw is positioned 90* to the left or right of the centerline, looking at the crank dead on at the end, the amount of movement of the rod or piston should be the greatest for one degree of rotation than at any other point.

When it's at 0 or 180*, one degree of crank movement will move the piston very little.

With 4 strokes, we want the longest rod available because it will slightly increase dwell time and allow more combustion pressure to build before the downstroke. Hence using 5.7" connecting rods in a 383 Chebby over the stock 5.65" 400 rods.

 

[drf255]

Anyone who's hooked a degree wheel up to a saw has seen this.

Try to find no piston movement at TDC. You can rotate the wheel 10* before you see movement either way. Loot at the piston through the exhaust port. You can see 1* of rotation.

I'm just trying to understand the relationship as it affects port timing and power.

I'm wondering if the difference between piston vs. Crank speed may be part of the "black magic" of saw performance porting.

I could be very wrong here though. The rod angle changes as well, so the moment of the prison based on crank oration may be geometrically changed.

 

[Mastermind]

You guys are just TOTALLY over thinking! Trial and error is the only way you can make engines run better.

Not the only way.......but they are overthinking the whole thing I believe.

I was mistaken about how much difference there was.......it's more like 5 degrees. I had a couple of odd jugs in the mix. The transfers were lower on those jugs.

When I checked the after machined heights I grabbed the wrong jug.

Al, you are right though.......at TDC and BTD the crank has to turn further to produce piston movement, so piston speed has to be faster at mid point, or 90 degrees than at top or bottom......a sine wave pattern. After all the piston has to stop, and change direction at each end of the stroke. It better slow down some first.

The transfer ports are at 120 opening point on average, and are fully open at BDC. That explains why they move more degrees in crank rotation per .001" of cylinder drop.

 

[redfin]

With 4 strokes, we want the longest rod available because it will slightly increase dwell time and allow more combustion pressure to build before the downstroke. Hence using 5.7" connecting rods in a 383 Chebby over the stock 5.65" 490 rods.

I must like over thinking but does the distance from crank centerline to rod bearing have more affect on dwell than rod lenght?

 

[drf255]

I must like over thinking but does the distance from crank centerline to rod bearing have more affect on dwell than rod lenght?

Same thing bud. The longer the rod, the longer the distance.

 

[redfin]

Same thing bud. The longer the rod, the longer the distance.

I was referring to the rod bearing at the crank and diameter of crank.

 

[Mastermind]

I've been fingering these jugs.

Am I doing it wrong?

 

[redfin]

I've been fingering these jugs.

Am I doing it wrong?

We shall need pics to determine.

 

[Paragon Builder]

I've been fingering these jugs.

Am I doing it wrong?

Depends. One or two fingers?[emoji13]

 

[Mastermind]

Depends. One or two fingers?[emoji13]

Yes.

 

[Mastermind]

They are all roughed in. Now to get the top dead on in height, and make the depths all uniform.

 

[srcarr52]

Not the only way.......but they are overthinking the whole thing I believe.

I was mistaken about how much difference there was.......it's more like 5 degrees. I had a couple of odd jugs in the mix. The transfers were lower on those jugs.

When I checked the after machined heights I grabbed the wrong jug.

Al, you are right though.......at TDC and BTD the crank has to turn further to produce piston movement, so piston speed has to be faster at mid point, or 90 degrees than at top or bottom......a sine wave pattern. After all the piston has to stop, and change direction at each end of the stroke. It better slow down some first.

The transfer ports are at 120 opening point on average, and are fully open at BDC. That explains why they move more degrees in crank rotation per .001" of cylinder drop.

You all know the math for this is not terrible and it's all laid out in Bell's book pretty well. There are also a couple of websites that have port opening/duration calculators if you have an affliction with calculators or excel.

 

[Mastermind]

I'll just keep measuring. lol