Four-stroke performance tuning?

[TZ350]

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Thats very interesting reading. Thanks.

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[speedpro]

Long rods in a 2-stroke may be a benefit due to the extended dwell at BDC. The time/area of the ports is increased whilst retaining the area of the ports.

Back to the dark side . . . . . .

I stopped in to see the guys at a cylinder head shop in the USA a few years ago. They supplied heads to people like Mercury Marine and a number of top drag racers. The information they wanted to ensure the correct head specifications was amazing and included things like rod length. The top guys were changing heads with cam or pistons and rods. They were doing CNC porting and cutting heads in 2 and then welding them back together with the front pair of cylinders spaced further away from the rear pair to fit on mountain motors. I got the guided tour from the main man and learnt lots from that couple of hours.

[Ocean1]

Long rods in a 2-stroke may be a benefit due to the extended dwell at BDC. The time/area of the ports is increased whilst retaining the area of the ports.

Dunno about rod length ratios on 2 strokes.

But .

A 2 stroke will create an intake pressure wave as it accelerates away from BDC. At BDC you get a much less less pronounced acceleration plot compared to that at TDC, but more to the point the effects of rod ratio on piston acceleration are the inverse near BDC.

On a 2 stroke I'd be interested in how that affected squish velocity. Mebe not a lot, not much variation that close to TDC...

[speedpro]

I just did the numbers for a stock MB100 and got 14.33deg rod angle and for the new high revving Wobbly spec engine with reduced stroke and longer rod I got 13.213deg.

Interesting to see that so far both 2-stroke examples are down one end of the range from Ocean1 and they aren't high spec engines. A couple of examples from the "dark side" would be interesting for comparison.

[speedpro]

but more to the point the effects of rod ratio on piston acceleration are the inverse near BDC....

Obvious when it's mentioned, simple geometry and all that, but I had never thought of it. Do you have any graphics you can post plotting velocities and various crank positions versus rod ratios?

[speedpro]

Perhaps Ocean1 could comment on "offset boring" cylinders as well. Fairly obviously it will affect rod angle in opposite directions depending on crank position.

[Ocean1]

Do you have any graphics you can post plotting velocities and various crank positions versus rod ratios?

I don't, much as radial graphs would be quite illuminating. Guess someone could write a wee macro so we could dump experiments in and get comparitive graps out. Unfortunately I don't have time.

Perhaps Ocean1 could comment on "offset boring" cylinders as well. Fairly obviously it will affect rod angle in opposite directions depending on crank position.

Ocean1 is on his sixth dram and will be making no comments involving quantifiable conclusions. Is a good idea though, if for no other reason than ideal piston velocity profiles from 180-0 are always going to be different to ideal velocity profiles from 0-180. Don't know why it's not more common.

[Kickaha]

when we had a 125 limit I had a bore and stroke of 57 x 48.8 and a 106.4 long rod,the rod/stroke was 2.18 and the bore/stroke 1.16

when we went to 140cc we stroked the cranks to get 57 x 54.6 so the rod/stroke stayed the same but the bore/stroke changed to 1.04

Then with the 150cc limit we went to 59 x 54.6 so still the same rod/stroke but bore/stroke changed again to 1.08

The next engine is going to be 66 x 44.8 with the same length rod so it will end up rod/stroke 2.37 and the bore/stroke 1.47

I had a few goes at working out the rod angle but fucked it up every time, must be why I failed school c maths somone help me out ta

[TZ350]

rod is 106.4mm and stroke is 48.8mm.

Sine of rod angle = 48.8 / 106.4 * 2)

Sine of rod angle = 48.8 / 212.8

Sine of rod angle = 0.229

..........rod angle = 13 1/4 degrees for Stroke = 48.8mm


rod is 106.4mm and stroke is 44.8mm.

Sine of rod angle = 44.8 / 106.4 * 2)

Sine of rod angle = 44.8 / 212.8

Sine of rod angle = 0.210

..........rod angle = 12 1/8 degrees for Stroke = 44.8mm


For Sine of rod angle = 0.210 use the Sine-1 function on your calculator to get 12.122 Degrees. More or less 12 1/8 degrees. Also instead of 1/8 could use minutes of a degree.

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[Kickaha]

Ta and the 44.8 stroke would be rod angle 12.12

I'm fucking sure you didn't have that up there originally or am I going blind?

[TZ350]

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There is another way to state the size of an angle, one that subdivides a degree using a system different than the decimal number example given above. The degree is divided into sixty parts called minutes. These minutes are further divided into sixty parts called seconds. The words minute and second used in this context have no immediate connection to how those words are usually used as amounts of time.

In a full circle there are 360 degrees.

Each degree is split up into 60 parts, each part being 1/60 of a degree. These parts are called minutes.

Each minute is split up into 60 parts, each part being 1/60 of a minute. These parts are called seconds.

The size of an angle could be stated this way: 40 degrees, 20 minutes, 50 seconds.

There are symbols that are used when stating angles using degrees, minutes, and seconds. Those symbols are show in the following table.

Symbol for degree: 0

Symbol for minute: '

Symbol for second: "

Bit of info from id.mind.net
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[TZ350]

I'm fucking sure you didn't have that up there originally or am I going blind?

Your right, I did it in two bits.

[speedpro]

The thing I've been wondering about lately is - if you had a 180deg twin what would be the best thing to do with the exhaust going to the turbo. It seems like some sort of plenum chamber, but what length headers - tuned or not, what volume chamber, length of pipe from chamber to turbo, design of chamber, or no chamber??? More or less same questions for the intake. I did read somewhere about this but it was a bit vague on things like lengths and how to determine optimum volume.

[Yow Ling]

The thing I've been wondering about lately is - if you had a 180deg twin what would be the best thing to do with the exhaust going to the turbo. It seems like some sort of plenum chamber, but what length headers - tuned or not, what volume chamber, length of pipe from chamber to turbo, design of chamber, or no chamber??? More or less same questions for the intake. I did read somewhere about this but it was a bit vague on things like lengths and how to determine optimum volume.

my understanding is you need to have the volume of at least 1 cylinder for an uneven firing enginein the inlet, I have a 550 cc twin turbo engine in a car it has 360 deg crank and has a plenum volume of more than a litre s maybe just the bigger the better. Dont think the exhaust is so critical

[Skunk]

The thing I've been wondering about lately is - if you had a 180deg twin what would be the best thing to do with the exhaust going to the turbo. It seems like some sort of plenum chamber, but what length headers - tuned or not, what volume chamber, length of pipe from chamber to turbo, design of chamber, or no chamber??? More or less same questions for the intake. I did read somewhere about this but it was a bit vague on things like lengths and how to determine optimum volume.

What are you building now?...