Stupid but need Explanation please

[Wild Cobra]

Need an explanation please regarding high RPM?
Lets take a look at a Ferrari or Lambo, and I might be completely of on this, but both cars have 12 cylinders, ok some do. Both cars are 5 liters, right?
So the 12 cylinders got shorter stroke and a lot smaller bore, but it will rev close 8 grand. How?
any explanation is grateful.

[rdorman]

You don't want much do you

How about the new BMW F1 engine? It hits 19,000 rpm. How about that for a four stroke piston engine!

The short answer (all I can give here) is engineering. Parts engineered and built specifically to operate at high RPM.

Here is some generalities

Why? Power. Through in some cool factor. An engine is an air pump. To produce more power it must pump more air. You can do this by increasing efficiency, RPM, or size of the pump. Of course you must engineer the whole package to work together. Packaging, target market and price, usage, etc all figure into how much do you want.

Higher RPM = lighter, stronger, more careful design and assembly, more exotic materials = Bigger bucks.

Lambo = $200,000, Mustang = $20,000. So you can afford to 'play' a lot more with the engineering in a Lambo than you can in a Mustang.

So why do most of our motors not run 8000 RPMs all day? For the most part we are all using engines that where originally designed for mass produced cars and trucks at a relatively low cost point. Why do you think you hear 'there is no substitute for cubic inches' all the time? Well actually there is. It is MONEY!

[Jeff Frigo]

Nascar sb: 9,000 rpm

NHRA bb: 10,000 rpm

F1 engines: 19,000 rpm !!

[cobish]

19000 rpm, it is the size in diameter of the piston.
Large diesel engines that turn at 300 rpm, that drive generators, the piston is huge . How many poles on generator at sync speed of 300 rpm.

[Excaliber]

It has actually has more to do with the stroke than the piston size. Those really large diesels have a really long stroke and a smaller piston size.

High rpm is much easier to achieve with a short stroke. Classic drag racing "formula", short stroke BIG bore builds rpm faster and revs higher than vice versa. Porsche is famous for short stroke big piston engines. The down side of short stroke? You loose torgue, which is the "leverage" effect you get from a longer stroke/rod combination.

Torque falls off with high rpm, but horse power increases! H.P. is a measure of "time to distance", torque is more a measure of "pulling power", slow but more "force". A Mack Truck is in no hurry, but can pull a 80,000 pound load up a mountain road with a mere 300 horse power, but might have 700 ft lbs of torque at 1800 rpm!

In a Cobra, your probably in a "hurry", need more horse power!

Ernie

[KobraKarl]

bigger thestroke= larger diameter rotating force= more centrifical energy=more mass to reverse on stroke = difficult to keep together with high RPM...........................

shorter stroke =less mass to reverse = less leverage=more RPM without engine destruction, but less HP/torque................

ADD PISTONS !! problem solved

[petek]

Cobish,
Number of poles? Depends upon the desired output frequency.

Let's aim for 60Hz.

300rpm = 5 rps (revs per second),
want 60 cycles per second,
60/5 = 12 poles

right?

[Excaliber]

well how many poles does it take to make a Kirkham?

ernie

[btsai]

Ernie -- ROTFLMAO!



Good one!

Ben

[Wild Cobra]

ok I do understand exotics verses the regular production.
BUt, the above nascar including wich not for the street but the rest of the exotics do have a very short stroke including the porcshe. but besides been the short stroke what else?
THe valve train that can breathe the induction to help it breath, and oh yah the whole concept? Hmm.
Take a look at a kawasaki bikes they are four strokes, and do rev all the way up to 14-15 RPM range, is it the balance? or what.
Thanks
for all the replies.

[petek]

Poles, Kirkham, LOL!

[wilf leek]

300rpm alternator would need 20 poles for 50hz, 24 for 60hz. (remember that synchronous speed for a 2 pole machine is 3000rpm for 50 hz, 3600 for 60hz).

And it would be a big old thing too - I remember standing inside the stator of a 20 pole synchronous motor on an oil rig once.

[Excaliber]

Smaller motors, less mass, easier to rev high. Kawasaki bike has little torque compared to horse power.

Balanced? Extremey balanced. That doesn't "make" high rpm as much as it keeps the motor from tearing itself apart AT high rpm.

BUT you've touched on one of the more fascinating aspects of high rpm. How in the heck can the valves keep up at extreme rpm? Through out automotive history THAT has been the rpm limiting factor. And for MOST cars today, remains the MOST limiting factor to rpm.

Picture this: The lobe on the cam pushes down on the valve, which then opens. Cam turns, lobe moves away form the big part to the little part and the spring on the valve pushes it back toward the lobe of the cam and closes the valve. Cam pushes, valve spring pushes back, they stay happily married, valve up valve down.

High rpm:
The cam pushes on the valve SO fast it "throws" the valve open with such violence the poor valve spring can't push back fast enough. The valve has been accellerated open SO quick the mass/weight of the valve want it to keep going, right through the top of the piston if you would! The spring is TRYING to push the valve back, but it cannot. For a very brief moment in time, the valve is NOT in touch with the cam, which has moved on with it's life. There is the valve, hanging open waiting for the kiss of death from the top of the piston, which is being shoved up it's hole by the brutal and unforgiving cranky shaft.

This is "valve float" (all though it's beginning to sound a LOT like my divorce years ago, I was the poor valve, ).

So extreme valve SPRING PRESSURE is required to to keep the valve in it's place. And the lobe of the cam has to be shaped "just right" so it's not SO violent as to abuse the "poor valve". The cranky shaft don't care about all that, it's going to shove the piston around with no thought about the poor valve.

Ernie
(who now needs a drink, as I contemplate how my life is like a valve and lawyers are a cranky shaft).

[Mr.Fixit]

Your shortblock's limiting RPM is the critical piston speed. The longer the stroke, the further the piston must travel per rotation, so it accelerates faster at any given RPM compared to a shorter stroked motor. The radpid changing of direction of the piston puts big loads on the conrods, which will stretch and try to oblong the big end's bore. If you get the shortblock to stay together with high qaulity parts, then the next obstacle is the valvetrain.Reduce it's mass as much as possible, so there is less inertia to resist the valve's wanting to change direction so rapidly. This is where overhead cams have an advantage, no pushrods, rockers and a tiny lifter. This is also where titanium valetrain parts become more necessary, they weigh less. But, then again, i have driven several bigblocks with over 4.3" stroke that readily see 8000 rpm in every gear, with nothing exotic at all. mechanical tappet cam, steel rods, all 60's stuff. Your odds of sawing your motor in half do go up dramatically with RPM, you are pushing your parts closer to their breaking point.

[KobraKarl]

Ernie ,Who was the "other " valve.............. or was she the cam

[Wild Cobra]

You guys are correct on the fact that their is a problem with the VALVE float, but yet the 327 Chevy motor (OH OH how terrible of me oh well) that block can easily see the upper of the 8000 RPM range and seen even go further then that. Is it the valve train that allows it or is it the short stoke short bore?

Now lets bring another one a Mazda rotary engine no valve train at all and barely can see the 8500 mark so what gives?

Yes the Kawasaki is well balanced, so is the 302 ford.
That block dates to the middle of the sixties, and yet still having a hard time to go past the 7000 mark.

The titanium valves are great reduce weight and all but like you guys mention at such a high rpm it will tear it self to pieces.

[Excaliber]

No way a "stock" 327 (and that is a bad word) can turn 8000. Modified valves, springs and cam, then it can! Heck lots of engines can turn 8000 with the right "valve stuff". The question is: Will they live? And 327's WILL blow up if you keep doing that high rpm.

"Rotary valves" on a piston engine. They took a stock 302 Ford, bolted on the rotary valve heads and spun it to 10,000 rpm with no other mods! As was mentioned, piston "speed" as it relates to stroke length is also a limiting factor. But I still say, VALVES are the primary and first problem area.

Web site on "rotary" valves. First tried WAY back in the early 1900's. http://www.coatesengine.com/products.html

The "wankel" rotary engine COULD turn higher rpm, BUT consider this. This is a "stock" engine with a warranty in a "regular" car and it turns 8000 plus rpm???? DAM, name ONE other "stock" showroom car that will turn CLOSE to that range! AND still be "factory" warranty covered? Thats awesome rpm!

Only ONE car comes to my mind. The new Honda S2000, with it's tiny motor (think theres a connection there with small enigne\high rpm?) I THINK tjhat motor comes close to 8000 in it's "stock" form. Therefore, with mods, I bet it would turn 10 plus!

Ernie

[speed220mph]

It's as simple as three little words: mass, inertia and material. Reducing mass reduces the inertia that tears apart an engine. Make the parts out of lighter, stronger materials and mass and inertia are reduced. Maintain displacement, but reduce stroke and bore size by increasing number of cylinders and mass and inertia are reduced further. And to really allow rpm to be bumped up, remove the cam and followers and replace them with electrically actuated valves and, thus, the quantum jump in revs to who knows where.

[Sheppard]

Hmmm....I asked a friend of mine who has some expereince in these matters...mass inertia...length of the stroke.....piston diameter, bouncing valves on their seat etc......

She tells me that generally speaking a long stroke is a good thing, but that given the choice....a large diameter just seems to get the job done ....something to do with the conversion of mechanical energy, friction and creation of additional heat.....I didn't understand everything she was telling me but will I'll see if maybe we can get it all sorted out later today....I'll report back later!

[brizz]

As you read this I will just be finnishing filling the trunk, passanger seat and all other vacant cavities of space in my cobra with moltent lead. This I'm doing to get the weight up to about80,000 lbs for traction.I have also thanks to a truck load jb weld,kawasaki and cummings created the first hybrid cobra engine in an attemt to set a new preformance record!!! see you at pikes peak!