Limiting factor for RPMs?

[TheSilverBuick]

RPM being the same?? as in all things being equal??...... That's the problem with his logic, he's trying to argue a different point than I am, I'm talking about an acceleration from idle to 8000rpm and how fast the motor will get there, not how fast a piston is moving at a certain RPM........

The problem with my logic or the point you keep failing to recognize?

[squirrel]

RPM being the same?? as in all things being equal??...... That's the problem with his logic, he's trying to argue a different point than I am, I'm talking about an acceleration from idle to 8000rpm and how fast the motor will get there, not how fast a piston is moving at a certain RPM........

You can't make two engines with different strokes, and keep every thing else equal. when you change the stroke, the displacement changes....unless you also change the bore, then the combustion chamber changes...and when you change the stroke, the piston speed vs rpm changes.

Also, if you're going to make a statement about changing one variable, you need to specify what happens to all the other variables--what are you assuming stays the same, and what are you assuming changes.

[TC]

As Squirrel says, I know that rpm is the same. Pray tell, how to build a stock 350 and a 350 like you're building and keep the bore the same? Wouldn't want the "potential of fantasy valve shrouding" to possibly skew the results.

Ok my bad the bore would have to be different to, but you knew that, and I knew it to just forgot to add it in my post......

[TheSilverBuick]

Ok my bad the bore would have to be different to, but you knew that......

And plays a significant role in air flow into the engine and HP production.

[TC]

You can't make two engines with different strokes, and keep every thing else equal. when you change the stroke, the displacement changes....unless you also change the bore, then the combustion chamber changes...and when you change the stroke, the piston speed vs rpm changes.

Also, if you're going to make a statement about changing one variable, you need to specify what happens to all the other variables--what are you assuming stays the same, and what are you assuming changes.

Ok I'll rephrase what I said and just say with minimal changes to the engine, like bore, stroke, etc...... But the end result would be 350 cubic inches....

Also how does the combustion chamber change??... you run zero deck on both blocks with flat top pistons with the same head gaskets, the combustion chamber in the head stays the same, though the quench area would be a little bigger with the bigger bore, but I don't think that would make a noticeable change in the result.....

[TC]

And plays a significant role in air flow into the engine and HP production.

But you got piston speed and a smaller bore with less ring drag.....

[TheSilverBuick]

but I don't think.

The only relevant part of that statement. With out extensive quantifiable testing, how can you call any change minimal? Just like guys that port heads an intakes, it doesn't take much physical change to make some big differences.

[TC]

Well it seems DF did an article all about this very subject......

http://books.google.com/books?id=Y98Smxj3XREC&pg=PA54&lpg=PA54&dq=short+stroke+vs+long+stroke+acceleration+dyno+test&source=bl&ots=2QKehFrzWV&sig=9yfT9EcHbtaZm9KElg8lIab7oRo&hl=en&sa=X&ei=jmj6T9j9A8K82wX4u4zcBg&ved=0CGIQ6AEwBA#v=onepage&q=short%20stroke%20vs%20long%20stroke%20acceleration%20dyno%20test&f=true

[anotheridiot]

you build a 400 block into a stroker motor and end up with a 383 by using a 350 crank and the walls are so thin on a 400 block you really cant make up the difference with pistons, so a standard stroker motor will always have a smaller CID. You can sneak a little stroke out of any small block by using a small journal crank in the place of the large journal. As far as RPM'ing faster, the stroker motor is always ahead of the stock stroke from the very first cycle, its already on its way back up the cylinder while the regular motor is still on its way down, by the first few hundred revolutions, the stroker motor already made 15% more. So the simpleton in me realizes the stroker motor will rpm faster, just because it takes less time to make one revolution. Thats one side of it. But if you are building an elephant motor and adding stroke, then its the opposite. I just wonder if you are all arguing about two different directions of stroke.

[Beagle]

instead of reading all 22 pages, I'll assume this has already been said and repeat it anyway.

The limiting factor to RPM is money.

Honda did some pretty funky research in the late 70's iirc, oval pistons with 8 valves per hole. Even with their seemingly bottomless pit of R$D money, they gave up on it. I think that was a search for more RPM for more HP to keep up with the smokers.

I had to go look - found this. It's an interesting story if you give a crap about that sort of thinking.

http://world.honda.com/history/challenge/1979pistonengine/text/02.html

[JeffMcKC]

Lets try to turn this into useful information. The OP should know the limit of his parts. Parts limit RPM thats it and ONLY IT.

The engines Power Band is not at Max RPM. The car runs best in its Power Band. RPM not making power is usless, this is why a lot of guys will be quicker if they "Short Shift". You see this problem a lot with guys building 434s and using the same 23*heads and cam as they had on their 383, yeah they will turn 8000 RPM but they quit making power at 6000 and made 100 hp less at 8000.

As far as this quicker to RPM its crap, the crankshaft on a 383 (any engine) RPMs the quickest at peak TQ, so the same 383 with a peak TQ at 3000 RPM and one at 6000 RPM with have the fastest accelerating crank in different RPMs even when the are the same engine. So like most things, its relitive not absolute.........

[TheSilverBuick]

Well it seems DF did an article all about this very subject......

http://books.google.com/books?id=Y98...%20test&f=true

There are a lot of "Maybe's" in that article AND it says it took 4HP more to run the short stroke engine, which would be 4hp less being used to accelerate the engine and car. And it mentions that the short stroke for slower piston speed is to make the engine last 500 miles, not about making more HP or being more efficient. Also says that torque is about engine size and volumetric efficiency, like OldMachinist was saying, more cube's. In class limited fields, short strokes win out over long strokes to get big bores for more airflow on the same capped displacement. Of course all this is just a repetition of what's been said over the last 150 posts. The last line, "If you can afford it, a combination of big bore and long stroke is the ultimate answer. There will never be a replacement for displacement." And you are shorting your build on displacement.

[Bob Holmes]

There will never be a replacement for displacement."

Except forced induction.

[TheSilverBuick]

Except forced induction.

Shhh....you....you.....Ffoooour cylinder lover

[JeffMcKC]

Now we get to whats the quickest ET this gets talked about on every board all the time. Mike Jones put it very
well from Jones Cams

I really can't believe some people can't understand such a simple fact.

The only torque that matters is Wheel Torque.
Wheel torque is engine torque multiplied by the gear ratio.
To be able to run a gear that is a higher multiplier of torque, you need to be able to make that torque at a higher RPM.
So the two things that can increase wheel torque is Engine torque, and RPM.
We measure the combination of engine torque and RPM as Horsepower. That's why we look at Horsepower.
If you can move the power curve of the engine up 20%, you can run change to a gear that'll increase the wheel torque by 20%, so even if the engine torque is down 15%, you'll be faster.
We look at HP because it is Torque and RPM.


I would add the TQ on a Automatic is a fine multiplyer of TQ if its a poper converter for the job