Piston speed?

  How much piston speed do you push your engine to?   Are your pistons stock,   forged ?  Aftermarket? 

I rev the Datsun A15 to 8200 rpm, it has an 82mm stroke. I rev the A12 motor to 8600rpm it has a 70mm stroke. I use stock cast pistons in both.

Back when F1 cars were running 18,000 rpm, a Winston Cup motor ran higher piston speeds, causing F1 engineers to visit Yates.

That's all I got.

I don't think you should be worried about piston speed as much as "do I have a valvetrain that can handle that RPM?" or "do I have an induction system that can flow enough air to make that RPM worthwhile?".

In reply to Patientzero :

The factory ran the engine up to 8300 rpm without issues such as valve float.  
     However I'm looking at aftermarket pistons. Cheap, aftermarket pistons that the manufacturer specifically says not for racing.   With my abundance of spares it's not a disaster if it goes whoops. 
    Stock redline is 6500 rpm. the cams I'm planning on using continue really strong up past 7000 ( also highest numbers on the tachometer).  
     Looking at guidelines 4000 ft/per/sec  is the drop dead zone.  I'd be just slightly past that @4127 FPS @ 7000 rpm and at 6500 rpm I'm at 3755 FPS. 
     

In reply to Tom1200 :

The piston speed on your A15 is 4412  FPM 

your A 12  is only 3930  FPM  

  Are they limited due to valve train? Camshaft? Other?  

Most I've heard a 300 run is 6000RPMs.  Mean piston speed there is 3980 ft/min.

Probably forged pistons, but I can't remember 

Bone stock VW 3.6L 8300 rpm with cast crank, cast pistons, factory rods, factory springs and all my stupid. Works out to 5239 fpm.

Amazingly it still makes power up there. Sometimes the roller rockers go for a short trip around the valley, sometimes the flywheel bolts come out. 

Ran some numbers on various cars I own.  Highest is my M3 at 4736 (8300 RPM stock motor).  Lowest is my F-250 (diesel) at 2621. 

RX-7 comes out as NaN due to divide by zero errors. :)

Streetwiseguy said:

Back when F1 cars were running 18,000 rpm, a Winston Cup motor ran higher piston speeds, causing F1 engineers to visit Yates.

That's all I got.

The early V8 era of F1 had the cars turning just over 20,000 RPMs.

Fiat 850 H-Production:

bore 68.25 mm, stroke 68 mm. Forged Arias pistons

I generally took it up to 8500 rpm

Think the ring pack my have something to do with this?

The question as I see it is, why does piston speed matter? what goes wrong when you spin a motor too fast? I think that ring flutter is identified as one of the early failure mode. So I'd be looking real hard at your rings

In reply to twentyover :

Remember every piston goes to a full stop at the top of the stroke and again at the bottom of the stroke. Remarkably peak speed is not when the crankshaft is 90 degrees but slightly past it. 
   That's a lot of energy pulling the piston apart on the intake stroke.  Boost actually lowers the amount the piston has to use to resist "pulling apart" 

frenchyd said:

In reply to Tom1200 :

The piston speed on your A15 is 4412  FPM 

your A 12  is only 3930  FPM  

  Are they limited due to valve train? Camshaft? Other?  

 

Bit of an open ended set of questions but I'll try and answer.

The stock bottom end is good for 9000 RPM (no extra balancing, stock rods and rod bolts)

These are non interference engines in stock trim, the valves float about 7000 RPM in the US market cars.

The GX dual valve springs will go to 7800-8000 RPM (Japan got a twin carb motor and a cam with slightly more duration and dual valve springs called a GX motor)

The GX cam will pull to 8200 RPM, the Isky valve springs will let the motor rev to 9100 RPM.

An after market cam with 280 degrees of duration and .420 lift will pull to 8600 rpms.

If you keep the motor to 7800  RPM it will do 10 seasons of racing before the rings finally wear out.

At 8200 rpm the rings make it 5 seasons

At 8600 rpm the rings will do 5-6 weekends.

Past 8600 rpm you are looking at rebuilding the motor every 6 hrs of run time.

The question is what failure mode are you concerned about?

If you're worried about component strength, it's not piston speed but inertial force that you need to worry about. So piston acceleration and piston mass would be more important than mean piston speed. Look at a 4.8 vs 5.3 'LS' engine. They can use the same block with the same pistons. The only real difference at that point being the crank stroke and rod length. The 'long' 3.622 stroke 5.3L (as well as larger bore/heavier piston variants) are regularly run to 7000 rpm on the stock bottom end. This equates to a 4225 mean piston speed. The same max acceleration acting on the piston in a 'short' 3.268 stroke 4.8L is at ~7400 rpm, but only has a 4030 mean piston speed. Mean piston speed is very rough approximation that, to the best of my understanding, is only so widely used because it was the easiest thing for math-fearing people to calculate and understand. So don't get too caught up in the exact number.

If you're worried about other piston speed effects, I'd think max piston speed would be more important than mean piston speed.

https://www.enginebuildermag.com/2018/03/how-a-stroker-crankshaft-affects-piston-speed-and-inertia/

In reply to Tom1200 :

So the "weakness" is the rings?    Hmmmm interesting.    I had a set of Forged pistons  that I took to 5400 FPS regularly and occasionally to 6150 FPS. after 20 years of racing the rings were the only significant area of wear.  
  A fresh set of rings and a light redressing of the valve seats was all it had done to it the rest of the time until it went into the Packard museum about 2016. 

z31maniac said:

Streetwiseguy said:

Back when F1 cars were running 18,000 rpm, a Winston Cup motor ran higher piston speeds, causing F1 engineers to visit Yates.

That's all I got.

The early V8 era of F1 had the cars turning just over 20,000 RPMs.

And a Cup motor still had higher piston speeds.

5220 feet/min or 26 meters/sec. 

Stock internals F22C Honda S2000 revving to 8800 rpms and running 12#s of boost. 

codrus (Forum Supporter) said:

RX-7 comes out as NaN due to divide by zero errors. :)

Even so, there has to be a way to calculate rotor tip speed.  Who's the resident math whiz?

In reply to Driven5 :

Yes I'm math challenged and went for the easy number.   But that's also the number used as guidance for the relative strength of cast vs Forged. 
     But you're right. Factors such as weight, expansion rate,  and design, play into it. Not just cast vs Forged. 
    The numbers I'm getting tell me that 4000 is not a legitimate failure number.           Since I'm only exceeding 4000 by a relative trivial amount. I'm going to go for it.  Worst case I've got spare engines. 

1988RedT2 said:

codrus (Forum Supporter) said:

RX-7 comes out as NaN due to divide by zero errors. :)

Even so, there has to be a way to calculate rotor tip speed.  Who's the resident math whiz?

Since the limits of rotor speed are so well explored and the weaknesses so well known.  I'd think it's a moot point? 

My race car engine = 89 mm stroke and 7800 rpm red line.

One of my street cars = 91 mm with stock red line of 7900 rpm

Another had an engine with a 7500 rpm red line but it only had a 62 mm stroke, so not as much stress there.

soooooo, as someone with nearly zero knowledge of modifying my average piston speed, I gotta say the thing everyone is leaving out of this picture entirely is piston mass, or maybe even mass/area.

To me, it seems like average piston speed is nearly useless by itself. 

Also, while stroke increases piston speed, doesn't it ALSO increase piston acceleration? (and the acceleration is why the mass matters so much). 

In reply to Robbie (Forum Supporter) :

Mass matters but also strength. Forgings are heavier than castings. But they are also many times stronger.  
    Then there is the matter of pins.  I've seen really thick wall piston pins  and pins with tapered walls and relatively  thin walls inside at the rod end. Plus every combination between. 
Just my observation, but I've never seen a pin failure. 

frenchyd said:

In reply to Tom1200 :

So the "weakness" is the rings?    Hmmmm interesting.    I had a set of Forged pistons  that I took to 5400 FPS regularly and occasionally to 6150 FPS. after 20 years of racing the rings were the only significant area of wear.  
  A fresh set of rings and a light redressing of the valve seats was all it had done to it the rest of the time until it went into the Packard museum about 2016. 

There are no weaknesses in the motor; purely a case of the rings wearing first. They don't actually break things till you regularly spin them past 9000 RPMs. From memory: I think the crank doesn't start breaking till you spin it past 9800, the stock rod bolts around the same RPM.  I've also been using the stock push rods but people running them past 9000 are using titanium push rods. The motor is really over engineered.