Engine technology thread.

[serialk11r]

Hey you guys so I was wondering, a lot of pages back Exage I think was talking about engine geometry, and rod length and whatever. I was wondering, how essential is it to have longer rods with higher rpm? Longer rods create less friction and loads, but friction is not even linearly related to stroke/rod ratio (the inverse of the rod/stroke ratio), so clearly that's not the big issue...the piston acceleration is decreased but again it's not a very big difference either... could the main issue be that the piston spends less time near TDC?

I made "measurements" based on pictures that we have and it looks like the rods are as I predicted, somewhere around 1.5 times the stroke, which seems a bit short as every high rev engine made before has 1.6 or higher.

[Dimman]

Originally Posted by serialk11r

Hey you guys so I was wondering, a lot of pages back Exage I think was talking about engine geometry, and rod length and whatever. I was wondering, how essential is it to have longer rods with higher rpm? Longer rods create less friction and loads, but friction is not even linearly related to stroke/rod ratio (the inverse of the rod/stroke ratio), so clearly that's not the big issue...the piston acceleration is decreased but again it's not a very big difference either... could the main issue be that the piston spends less time near TDC?

I made "measurements" based on pictures that we have and it looks like the rods are as I predicted, somewhere around 1.5 times the stroke, which seems a bit short as every high rev engine made before has 1.6 or higher.

Something to do with the acceleration forces. With rpm and stroke we can calculate mean piston speed, but when we add in the rod:stroke ratio we (well people better at math/physics than myself) can calculate the peak acceleration forces of the piston and rod. Gives a more accurate view of the stress on the rod.

[serialk11r]

I know, I already thought about this and did some calculations. The difference in acceleration is there, but not very large. There has to be another reason IMO, otherwise it wouldn't make sense to throw in long rods and increase engine dimensions. Or maybe it is just to pick up that extra few % more power from the friction. I feel like the slower acceleration at TDC allowing slightly more time for the charge to burn, and slightly increasing the effective expansion is probably it (although the greater acceleration at the bottom of the stroke causes a decrease in expansion because the valve has less time to open compared to the stroke travel, but it's probably more efficient to have the pressure peak earlier, I dunno).

EDIT2 (sorry lol): Yea at the bottom of the stroke, having the valves open a tiny bit earlier represents a very small loss of power, while having to ignite the mix at a higher volume and lower pressure makes a bigger difference. This is probably why compression ratio increases actually see benefits when under ideal circumstances the differences look pretty much negligible.

[Dimman]

Originally Posted by serialk11r

I know, I already thought about this and did some calculations. The difference in acceleration is there, but not very large. There has to be another reason IMO, otherwise it wouldn't make sense to throw in long rods and increase engine dimensions. Or maybe it is just to pick up that extra few % more power from the friction.

Could be. They're kind of at the point where they really have to go after the little details, and hope enough of them add up to good gains.

[serialk11r]

Another question for people who might be able to answer it, what are the electronic limitations to raising revs? I imagine there may be issues with the ECU as they would only be programmed for stock rev range from the factory.

[Dimman]

What do you mean 'electronic limitations'?

How the rev limit is activated? Usually just cuts ignition/pulls timing based on rpm signal, I believe.

It will probably have a 3d ignition map timing x rpm x load and in the upper rpm cells they will just cut it, or retard it gradually. Something like that.

[serialk11r]

I mean like, is the ECU going to be able to operate at all above the factory rev limit? You upgrade the components so they can spin at 8500 rather than 7500, but is the ECU going to even have cells available for you to give it values or anything? Is it even possible to tell the ECU not to freak out over 7500? Needing a whole new ECU for that would really blow...especially with this timing sensitive direct injection business.

[Exage]

Originally Posted by serialk11r

Another question for people who might be able to answer it, what are the electronic limitations to raising revs? I imagine there may be issues with the ECU as they would only be programmed for stock rev range from the factory.

"The car is running a Power FC so the CEL light does not work as it would on a stock ECU. What flashing meant was the knock sensor readings where over threshold, meaning possible knock."

[SkullWorks]

Generally you can Re-Scale the tables in an ECU to deal with the RPM you desire, you don't get more cells you just re distribute them, this is dependent, however, upon how much control you really have with the programming suite(s) that will become available,

[Dimman]

Hmmm... hadn't thought of that. On standalone ECUs you often have a set number of cells and you can define the ranges (ie 64 cells but you can put any rpm, ignition, boost values you want). Don't know about the factory ones.

Optimistic that there is room for this motor to grow, though.

[serialk11r]

Are stock ECUs ever reprogrammable that way? I guess we'll have to wait and see what this car has. The more I think about it, the more I want Subaru to actually do what they suggested could happen for the STI, drop a few hundred pounds, raise the rev limit. This car modding business sounds like a total pain in the ass

[Dimman]

Originally Posted by serialk11r

Are stock ECUs ever reprogrammable that way? I guess we'll have to wait and see what this car has. The more I think about it, the more I want Subaru to actually do what they suggested could happen for the STI, drop a few hundred pounds, raise the rev limit. This car modding business sounds like a total pain in the ass

It is a joyous and expensive pain in the ass...

[Grip Ronin]

so are they planning to use toyotas variable valve timing? vvti i belive they call it

[old greg]

Originally Posted by Dimman

How the rev limit is activated? Usually just cuts ignition/pulls timing based on rpm signal, I believe.

Fuel cut is a lot more common, as it's much kinder to catalytic converters. It doesn't sound as cool though.

Originally Posted by Dimman

On standalone ECUs you often have a set number of cells and you can define the ranges (ie 64 cells but you can put any rpm, ignition, boost values you want).

Originally Posted by serialk11r

Are stock ECUs ever reprogrammable that way?

Yes. OEM ECUs work the same way, assuming you have access to them.