Random thoughts thread

[serialk11r]

gThe formula is nasty, just plug into the piston motion equations Want.FR-S linked and remember that the difference between it and the usual sine/cosine graph is essentially the difference between the crank pin's "height" from some reference crank angle and the wrist pin's "height" from the same reference crank angle.

However these days cranks are offset so that's not even precise.

The easy way to think about it is just with simple geometry. The rod constitutes a circle with center at the wrist pin, radius = rod length. If the rod length were "infinite", then the position of the piston along its travel corresponds directly to the same vertical position at the crank (that would be like, 90 degrees after bottom dead center = piston halfway through its travel). Higher rod /stroke ratio approaches this. Low rod/stroke ratio means piston is moving faster at any point around TDC, slower at any point around BDC. It's SUPER easy to visualize, and I'd draw you a picture if I weren't so lazy. It is literally like, take 2 circles with respect to an axis, look at the distance between them along some line parallel to the axis.

The reason high rod/stroke ratio is good besides less friction and forces on the reciprocating components is that the piston is moving away from TDC slower at the same engine speed, so there is effectively more time for the charge to burn. Now this (just like ultra high compression ratio) cuts down the usefulness of the engine at low rpm (I'm talking like well under 2000 in a car sized engine) since you end up with more heat rejection.

Yes the air does lag behind the piston, that's the point of late closing intake valves (other than bigger lift for less restriction). With shorter rod, the piston moves slower at the bottom of the stroke. Since the piston moving up counteracts whatever volumetric efficiency increase you get from late intake valve closure, in theory short rods can give you more VE (there's also the fact that the piston is moving faster most of its way down).

In real life though, a 1.4 rod stroke ratio and a 1.8 rod stroke ratio don't look all that different on the piston velocity graph, and whatever friction savings you get from the long rods get partially lost to the heavier rod causing more friction, and likewise whatever volumetric efficiency and torque you pick up with the short rod probably disappears to the higher friction and lower burn rate to some extent.

As for the air itself, how long it can flow against the rising piston is probably when the peak of the wave hits the piston itself. Knowing just how much air gets trapped in the cylinder is probably way too complicated for anyone to figure out, but you can know the maximum.

[IntotheOcean]

New pick up line: I don't need ios 6 to get lost in your eyes.

http://www.vicpolicenews.com.au/more...ng-system.html

Front Page on reddit... Apparently you can get really screwed being out there

[Spd229]

Originally Posted by IntotheOcean

4th time seeing frs in person on my way to work today!!! Had to switch a lane, speed up, than slow down, than kept going only to realize I wanted to look again lol, so slowed down again and just eye balled his car for the longest time. Haha I shook my fist in the air while smiling thinking to myself 'dammit why aren't you mine right now'
Haha wonder what that guy was thinking 'wtf is up with that chick'
Maybe next time a simple honk will do

I do stuff like this...and I even drive one XD

Pretty sure one guy in a WRB BRZ was like, da fuq is this FR-S doing?

[Spd229]

Been at this company a year and a half now. They bring me in to interview incoming employees on their level of technical knowledge, yet they also don't mind if they aren't specialized in the field and are willing to learn as generally it's for a trainee position.

We've got someone coming in soon that sold me on their drive and willingness to learn, they've been exposed to some of the surrounding technology but by no means are familiar with it as I was when I graduated.

I surely hope I have not been duped. If so, I guess I just can't be so generous again.

[Jayde]

I'm getting the "I need to go to car meets" urge. Thankfully there is one is Friday! Hopefully two more 86s from here will be there!

[IntotheOcean]

Originally Posted by Spd229

I do stuff like this...and I even drive one XD

Pretty sure one guy in a WRB BRZ was like, da fuq is this FR-S doing?

Haha yeah, the best would be if your reaction on the road was so weird it keeps that other guy up at night.
Next time I'm gonna stick with beeping-maybe a thumbs up

[Want.FR-S]

Originally Posted by Dimman

What I'm looking at is the different ways that flow in a head is controlled. Most port work on a flow bench just chases a big cfm number since they just measure pressure differences on a cylinder head attached to a giant vacuum, but I want to look more closely at how velocity affects power. The nature of the piston motion and its velocity curve pulls in air, plus how a cam's profile affects the airflow and velocity.

What flow benches can't account for is how long it takes the air to start moving, plus how long it will continue, even after the piston stops 'pulling'. All rpm dependent, too.

I'm thinking that my old Supra could benefit from slightly smaller porting, but larger valves and longer duration cams. It has a longish 91mm stroke, so chasing big flow with rpm will be more stressful on the rods.

I am thinking out loud here. I think this has something to do with fluid dynamic, and I can think of an analogy is by using needles or things like that. Think your cylinder as a needle. Empty the air inside and put it into the water. When you pull the handle, it creates a vacuum and then eventually the water fills the vacuum. Observe when the water starts to flow after the vacuum has been created, you should see that there is a delay caused by the size of the needle (port size in engine) because the water cannot flow fast enough.

However, if the needle size is bigger, this lag of water flow should be decreased since water flows more freely. So bigger water flow reduces the lag.

On the other hand, with higher RPM, the faster the vacuum is created. The pressure differences between the port and cylinder is greater when the intake valve opens. Therefore, the air should be sucked in faster so the cylinder should be filled quicker.

So now, back on topic, there are parameters that you are playing in this case: port size, valve size, cam profile (duration of opening), and rpm. So if we fix the rpm first, if we open the cam more, it may allows more air into the cylinder, but what if before the piston reaches BDC the pressure between the port and cylinder reach equilibrium, by opening the valve a bit longer could the piston actually pushing the air out back to intake port?

I think the benefit of opening the valve longer is based on the thinking if the equilibrium of air pressures between port and cylinder cannot be reached fast enough. I would think this happens at the high RPM range.

As to port size and valve size, I am thinking if you reduce the port size, the air flow would be restricted, just like that experiment with the needle with smaller size. If you keep the port size the same while increase the valve size, it should allow more air into the cylinder (better flow) and thus reduce the time required to fill the cylinder. However, with more air into the cylinder with potentially more power, this does put more load on the longer rod.

This is getting really complicates....

[Spd229]

Originally Posted by IntotheOcean

Haha yeah, the best would be if your reaction on the road was so weird it keeps that other guy up at night.
Next time I'm gonna stick with beeping-maybe a thumbs up

I try to just wave or give a thumbs up, but driving on I85 or GA400 around here it ends up being a quick, awkward motion.

I think the weirdest thing I've ever had happen so far while in my car is this Toyota Solara convertible with 3 bros in it. Their speed was horribly inconsistent so I'd pass them, soon as I did, they'd fly back by....while bouncing around with each other. No idea what it was supposed to mean.

[Dimman]

@serialk11r @Want.FR-S

What I'm looking at is maybe getting an increase from faster intake flow, rather than more.

Because the port itself isn't really the limiting factor, it's the complete interplay of port, cam, valve and piston. So if we shrink the port so the flow matches the capacity of the cam/valve better we can get more velocity, and possibly better low rpm response and some top end as well.

[Jayde]

Finally remembered to put my new insurance card in my wallet. My old one expired Dec 1st.

[M-17]

Figuring out which girl I'm going to take out this weekend isn't as easy as I thought it would be. It's like asking a fat kid to choose one pie that is not his favorite.

[Jayde]

Originally Posted by M-17

Figuring out which girl I'm going to take out this weekend isn't as easy as I thought it would be. It's like asking a fat kid to choose one pie that is not his favorite.

I don't have that luxury. :foreveralone:

[M-17]

Originally Posted by Jayde

I don't have that luxury. :foreveralone:

Reminds me of this: All the desirable things in life are either illegal, expensive, fattening or married to someone else.

Cheer up though, its all do in time.

[Shagaliscious]

Originally Posted by Jayde

I don't have that luxury. :foreveralone:

Gotta look at the bright side though
[ame="http://www.youtube.com/watch?v=Ji01gRYQcHo"]It's Worse Way Worse - Tosh.O - YouTube[/ame]