fahrvergnugen

This thought occured to me after reading the 'compression ratios and forced induction' thread.

Years back, I read what Gene Berg decided was the best way to make power with an air-cooled motor; increase bore, increase stroke, and decrease compression ratio. Now obviously an A/C motor is much more sensitive to knock than a W/C motor, but that is not my point.

What I want to know is this; would you make more power with a Miata motor if you increased the stroke, therefore increasing the EGVs? Compression ratio, specs of the turbo, etc. are all up in the air, along with the possibility that you might or might not be able to alter stroke feasibly. Theoretically, would this increase power? If so, what might the hp curve look like?


Thanks...

samnavy

iTrader: (7)

AOTBE, longer stroke=lower absolute RPM potential.
AOTBE, lower RPM= lower horsepower.

Gasoline "truck" motors and diesel motors work off this principle. Longer stroke with larger displacement = more torque but less RPM so less absolutel horsepower.

patsmx5

iTrader: (16)

Generally speaking, big bore, long rod, short stroke is the magic recipe to make big power. Long stroke, small bore, long rod is the magic recipe to build a torque motor. There are many many reasons for this.

Our motor is already undersquare. That is, it has more stroke than bore. It's no surprise why these motors don't make the power in the high RPMs. The motor was not built for it. Ideally, we would have a larger bore, longer rod, less stroke 1.8L engine. It could have a larger cylinder head with larger valves and less shrouding.

Stroking it would increase VE down low, and therefore it would make more power down low. But as RPMs increase, it's VE will taper off quicker than it used to and power will not fade off to about what it was before you stroked it.

Simple example: Harley's have long stroke motors for torque. Sport bikes don't.

iWeasel410

Just to prevent any misinformation here, generally speaking can't go with a long stroke and a long rod unless your piston has an extremely short compression height. Figure it like this:

Block Height = Stroke/2 + Rod Length + Compression Height

If you increase the stroke, you gotta decrease the rod length to keep the same compression height. Of course this depends on what the OEM compression height is, and how much of a stroke you add.

Now if you want torque, consider the geometry created by your crank centerline, the piston pin centerline, and the centerline at the connecting rod journal on the crank. If you put a point on each of these centerlines, you'll notice you have a triangle (real complicated stuff here). Now suppose you have a constant cylinder pressure (thus constant downward force on the piston crown). To maximize torque, you want to create the LARGEST torque arm about the crank centerline (again, real complicated stuff here). So here you have to consider the angle your rod makes with the crank, which is depending on stroke/2 and the rod length, it's just a triangle.

Also, don't forget your rod to stroke ratio and side loads. Long rods with short strokes generally have much lower side load, and thus can rev much higher (ala F1).

patsmx5

iTrader: (16)

Yeap, I agree. I was just saying in an ideal world. But indeed there are constraints such as the ones you mentioned that will ultimately dictate what rod length can be used.

kotomile

iTrader: (24)

Exactly, so simple yet so often overlooked.

944obscene

So I just thought of something that could make the Ferrari boys and the Chevy boys happy. A variable length piston throw-crankshaft. Lots of stroke down low and shorter stroke up high. I just haven't thought about one thing... Actually pulling that off. I'm guessing lots of oil hydraulics and solenoids and completely impractical stuff that I could never make myself. I could sit down and draw some things up, but it'd be shot down by a real engineer in seconds (which is why I want to stop messing around and finish my schooling).

fahrvergnugen

True, which is probably why it is possible to do just that with a VW T1 motor; there is tons of meat on the top of the piston, and it is a piston/sleeve motor as opposed a piston/block motor.

944obscene,

Saab actually has a variable compression motor that was due out about now IIRC. Instead of adjusting the rod length, they adjusted the distance of the head from the top of the piston @ TDC. IMO, I am sure this is more complicated than it is worth, and I would bet that the longevity of such a motor would be compromised.

Pat,

That makes sense. It works for VWs since boxer motors are torquey from the get-go, and they only have 2-valves per cylinder. The setup for the Miata is probably not far from where it needs to be as is.

Thanks all.

944obscene

True. I wouldn't expect initial attempts of this kind of technology to last for long periods of time without some sort or tear down and inspection or general failure due to wear. But at the same time, the Vtec system developed by Honda seems to have been quite a success, and that's coming from a guy that really really REALLY isn't a Honda fan.

I'd be interested in seeing the system in action and read about its development.

iWeasel410

All you need is access to the SAE Tech Select database and you'll find reports covering variable geometry and variable compression engines. Of course that costs money though

944obscene

:edited: I type too much when I'm up late.

Ah, see? The whole "money" thing. That's the kicker for me.