Reading Rat's blog below👇........was curious since we're seeing some higher redlines with Loi's tune and the locking of the transmission. What would be the 80 ft/sec of our engines to see when we would be approaching that with a tune?

27. Maximum Safe Piston Speed Redline

NOTE: The redline calculation and information given in this Tech Article, is in reference to “short block” or “lower end/bottom end” maximum safe piston speed redline, unless otherwise specified.

Bottom end maximum safe piston speed redline involves the crankshaft, connecting rods and piston/ring assemblies. So, the valve train capability is not involved in this calculation or information.

Valve train capability can vary WIDELY from engine to engine. So, a given valve train may not be capable of revving to as high an rpm as the bottom end. Therefore, a given valve train must be considered separately on an individual basis.

OEM’s typically consider 80 ft/sec as their “max safe ave piston speed redline”. Of course a piston’s speed varies between zero at TDC and BDC, and max when the crankshaft is at 90* to the cylinder centerline. So, the “average piston speed” is simply the “average” of those different speeds, as the name implies.

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Here are some stock factory production engines’ specified tachometer redlines for their total overall engine assembly (not just piston speed redlines), and the resulting ave piston speeds at those specified tachometer redlines:

2011 Ford 5.0 Mustang GT “coyote” V-8 tachometer redline = 7,000 rpm = 71 ft/sec ave piston speed

2010 Harley XR-1200, 2 cylinder motorcycle tachometer redline = 7,000 rpm = 74.1 ft/sec ave piston speed

As you can see, both of these factory engines fall below the OEM recommended maximum safe piston speed redline of 80 ft/sec.

Now let’s look at a stock factory production engine that spins sky high, and see what it shows.

2011 Yamaha YZF-R6, 4 cylinder 600cc motorcycle tachometer redline = 16,000 rpm = 74.4 ft/sec ave piston speed

WOW, this one is STILL under the safe 80 ft/sec limit for production engines. How can that be? The stroke length makes “ALL” the difference. Take a look…….

Their stroke lengths:

5.0 Mustang = 3.650 in.

Harley XR-1200 = 3.812 in.

Yamaha 600 = 1.673 in.

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But High Performance Hotrod and Racing engines that use good quality aftermarket forged parts, can generally use a more aggressive 90 ft/sec. as their “max safe ave piston speed”.

So, for High Performance Hotrod and Racing engines, you can calculate the max safe piston speed redline, using that 90 ft/sec figure and an engine’s stroke length. For example:

Big Block Chevy (BBC)
For a 540ci BBC with a stroke of 4.25”

At 90 ft/sec ave piston speed, its max safe piston speed redline would be 7,624 rpm

Using the OEM’s 80 ft/sec, you’d get a max safe piston speed redline of 6,776 rpm, which is 848 rpm lower.

Small Block Chevy (SBC)
For a 383ci SBC with a stroke of 3.75”

At 90 ft/sec ave piston speed, its max safe piston speed redline would be 8,640 rpm

Using the OEM’s 80 ft/sec, you’d get a max safe piston speed redline of 7,680 rpm, which is 960 rpm lower

Small Block Ford (SBF)
For the 221, 260 and 289 V-8’s, with their very short 2.87 inch stroke, their max safe piston speed redline looks breathtaking for a traditional old-school American V-8.

Using the OEM’s 80 ft/sec, its max safe piston speed redline would be an eye popping 10,000 rpm.

That is similar to the Yamaha 600cc motorcycle above, which has a super high max safe piston speed redline because of its very short stroke.

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Factory-based engines will fall within the normal recommendations. But, if you build a non-factory stroker motor, it would be good to know where your engine stacks up. So, you can use the following equation to check your own engine:

MAX SAFE PISTON SPEED REDLINE = (max safe average piston speed in ft/sec x 360)/ stroke in inches

You can plug OEM’s 80 ft/sec, or High Performance/Racing engine’s 90 ft/sec into the equation, along with your engine’s stroke in inches

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540 RAT

 

I’ve asked around about this myself, and haven’t found a single technical answer other than… it’s safe. I too am skeptical. Notably, the RR tune has had a 7200 rpm limit, and even made a 7400 rpm limit available, and I haven’t seen any evidence of failure from it. Some of our members bang the 7200 rev limit on road courses with no bad reported effects. We also have a member that runs it to 7500 at the drag strip.

Novel ruined an engine at 8k rpm.

I believe it shortens service life, so will
only use that extra RPM at the drag strip. The rest of the time, I’ll aim for around oem shift points.

Notably, revving a bit higher and not banging the rev limiter is healthier than banging the rev limiter, or so I’m told. Riding a rev limiter is not healthy. All tuners have stated 7200 rpm is safe (RR and Loi).

I would love a more defined, technical answer. I just haven’t found one. The above is a summation of all my research and conversations.

 

We can plug in the equation above if we know the stroke in inches to see what it comes out to? If anybody has that info handy, would be interesting to see what the engineers gave the 2UR-GSE for a safe redline and what was left on the table🤔

Yea the YT video on Loi's tune from one of our members running to 7,500 got my attention!

 

​​​(80 ft/second x 360) / 3.52 = 8181rpm
(90 ft/second x 360) / 3.52 = 9205rpm

Stock piston speed
(6800rpm x 3.52) / 360 = 66.5 ft/second at redline ISF
(7300rpm x 3.52) / 360 = 71.4 ft/second at redline RCF/GSF

3.52 is the 2urgse stroke in inches

All numbers rounded to nearest decimal

 

Very helpful. It’s nice to know the MPS is well
within safe limits.

 

Yea thanks for that! Seems like they (engineers) left a pretty big cushion for safety in our motors😎 Even upped to 7400 on our motors I think would still be within a safe tolerable piston speed if not abused and used properly when needed.

 

Now we need some cams to see if we can make a little more power up top

 

This table is an extract from a spreadsheet I built a long time ago calculating mean piston speed and volumetric numbers for the 2UR-GSE. Mean piston speed over 20 m/sec is considered very high for a production engine. The Integra Type R came from the factory with a 24 m/sec mean piston speed which at the time was considered radical for an OEM. Keep in mind, mean piston speed is a rule of thumb. In order to properly assess the configuration, you need to work up the full kinematics which includes piston mass, wrist pin mass, rod length and mass, and stroke. Mean piston speed is just a rule of thumb because it doesn't account for all the variables as a full kinematic assessment does. Finally, the issue here is service life. The faster you spin an engine, the sooner it either fails or needs a rebuild because failure is imminent. A Top Fuel engine runs about 4 seconds at redline before it needs to be rebuilt, but it make north of 10k hp. The faster the engine turns, the sooner it needs new rods, bearings, and pistons with maybe valves and valve springs. Decide how important it is to make power, and adjust your budget to suit your choice.

 

How do we reconcile these numbers? Per Rat’s designation, we’re good to go. By Lobux’s numbers, we’re pushing it at oem redline.

Lobux, are your numbers different because you took everything into account, such as rotating assembly mass, etc.? Feel free to address me like I’m 5 years old.

 

Edit.........

 

Lobux,

Nice chart/spreadsheet you put together! But like Jwconeil says above how does your spreadsheet and the basic formula that Rat gave coincide with each other for our motors?

Here are the conversions for reference to compare in ft/sec:

6750 rpm = 20.14 m/sec or 66.1 ft/sec
7000 rpm = 20.88 m/sec or 68.5 ft/sec
7250 rpm = 21.63 m/sec or 71.0 ft/sec
7500 rpm = 22.38 m/sec or 72.2 ft/sec
7750 rpm = 23.12 m/sec or 75.8 ft/sec

Those were also rounded off to the nearest decimal

 

In SAE units, mean piston speed is measured in feet per minute, not feet per second. 4000 feet per minute is the "accepted" limit, but as I said in my post, the limits are all based on service life. Drag racers spin 2JZ engines over 10k rpm which completely violates all the rules of thumb for mean piston speed, but they expect to rebuild the engine in short order, not run for 200k miles or more between rebuilds. The rule of thumb has also increased over time with better materials and better engine designs.

RAT's blog is assuming 4800 ft/min is acceptable. That's not what I've been told over the years. RAT using ft/sec is also non-standard and one of the ways he brings arguments to the table - he does things his own way, and when the real engineers show up, they look at him funny because he's not using standard units for anything.

To ice the cake, these mean piston speeds are based on rules of thumb, not actual piston kinematics which would take acceleration into account, and acceleration is what breaks pistons and rods. I am not the original author of these spreadsheets, but I have definitely modified them to suit a purpose.

 

According to Rats data, we should be able to rev to around 8k… Novel has already proven that is not possible without catastrophic failure. So I’m inclined to take Rats data with a grain of salt.

I want to know more about the novel failure. I’ve read that 4000 FPM is generally accepted across the board as safe for oem autos. I’ve also read that with the use of forged parts, that limit raises to 4600-4800.

 

I'd like to know the backstory to Novel and the failure they had @ 8,000 (+?).....do we know exactly what rpm the failure happened at, or the length of time it was spun at for that rpm speed?

 

If I could log into his blog in the Q & A session I would pose more questions for him to elaborate on concerning our discussion here, but I don't have Facebook or Twitter or Wordpress. I think those 3 ways are the only way you can log in to ask him a question or else I would. I'm curious as to what he could expound on in his own words and why he came to those conclusions.

BTW I'll check out those links you posted above