How do you determine maximum safe RPM for a race engine?

[novafornow]

Strong short blocks are available everywhere anymore. I feel it is valve train/budget that sets this. SS valves or titanium. Shaft or pedestal rockers. 904 lifters. These set the parameters. Then the head to match the CI and rpm. Myself, SBC with SS valves and titanium retainers, 7500 limit. Titanium valves, 8500 or more. Without spinning, experience tells me to stay in these ranges. This seems to be confirmed when talking to cam guys that this is an appropriate range.

[CamKing]

If you need it to last a few thousand miles between rebuilds, 4,500fpm ave piston velocity.

[digger]

I must admit I'm not fully up to speed why MPS matters I mean surely it only pertains to wear of rings or skirts etc and friction in the reciprocating bits. Materials and processes have come a decent way from when some guidelines were developed

[modok]

keep running it higher till it blows up, then back off a ways.

[wriggley monkey]

Or improve the part that broke and repeat............

[MadBill]

If you need it to last a few thousand miles between rebuilds, 4,500fpm ave piston velocity.

And if you don't, Jon Kaase's BBF-based 826" turns 8600 RPM with a 5.75" stroke, achieving a piston speed of 8,421 fpm. (42 m/sec.)

On the other hand, I saw a shiny new piston from a 672" 5.0" stroke Hemi that had had the pin ripped out of the bosses on maybe its first run. The owner hadn't changed the gearing after stepping up from a much smaller engine and buzzed it to ~ 8400 RPM, a mere 7,000 fpm...

[pcnsd]

There is of course a difference between average piston velocity and maximum piston velocity. Camking's 4500fpm is about 7770 RPM in a SBC 350. Maximum piston velocity in this combo at the same rpm is about 7400FPM. Don Terril's The HP Chain offered a review of this and suggested that the bottom end was statistically limited due to material stress to a maximum piston velocity of about 10,000 FPM. The exact number depending on rod/stroke ratio and perhaps another factor or two. Anyways this proposed 350 would have a maximum RPM of about 10,400.

https://i.postimg.cc/xT9y6SdB/Piston-Motion.png

[MadBill]

land speed racing my vintage bikes is a touchy deal concerning maximum RPM...Modern pistons and quality forged steel rods allow safe high rpm but the cranshaft is a problem. My budget doesn't allow $2200 custom made cranks so I make do 60 year old cranks and hope than nitriding will add some durability..But if the crank breaks the engine will lock up and it's possible the rider will thrown off the machine onto the track ...Not a good thing...So I limit RPM to a known safe limit based on experiences and then tune the engine to get most power within limits....This worked for about 4 years but now the competition with expensive crankshafts is up my ass... so I retire the machine or do what it takes to raise the RPM limit to make more power...

Maybe a sprag clutch in the crank sprocket...

[ptuomov]

I think it’s really the peak negative acceleration that matters. MPS is just a quick shortcut to rule out obviously implausible combos.

With the peak piston negative acceleration, one then can look at the reciprocating weight, which gives one the tension load. The rod cross-sectional area in turn gives one the load/area which is in pressure units. That has to match the properties of the rod material.

In practice, one needs to just ask what the rod and piston manufacturer’s software says about the RPM limit.

[MadBill]

There is of course a difference between average piston velocity and maximum piston velocity. Camking's 4500fpm is about 7770 RPM in a SBC 350. Maximum piston velocity in this combo at the same rpm is about 7400FPM. Don Terril's The HP Chain offered a review of this and suggested that the bottom end was statistically limited due to material stress to a maximum piston velocity of about 10,000 FPM. The exact number depending on rod/stroke ratio and perhaps another factor or two. Anyways this proposed 350 would have a maximum RPM of about 10,400.

https://i.postimg.cc/xT9y6SdB/Piston-Motion.png

Piston speed is mostly just a stand-in for negative acceleration ~ TDC.

[digger]

There is of course a difference between average piston velocity and maximum piston velocity. Camking's 4500fpm is about 7770 RPM in a SBC 350. Maximum piston velocity in this combo at the same rpm is about 7400FPM. Don Terril's The HP Chain offered a review of this and suggested that the bottom end was statistically limited due to material stress to a maximum piston velocity of about 10,000 FPM. The exact number depending on rod/stroke ratio and perhaps another factor or two. Anyways this proposed 350 would have a maximum RPM of about 10,400.

https://i.postimg.cc/xT9y6SdB/Piston-Motion.png

The thing is MPS you little to nothing about accelerations which is where loads ( excluding cylinder pressure) and hence stress come from.

Max piston speed tells you very little as well but doesn't add anything over what MPS tells you as rod ratio effects of normal configurations are minimal in the overall scheme.

[rustbucket79]

So let's move the topic on a bit of a tangent, and focus on bracket race domestic V-8 engines.

Tell us about a build that had unintended consequences in terms of horsepower peaking much higher (in RPM) than anticipated.

I'll start, my own personal small block, typical 406 with decent parts (Dart block, Wiseco pistons, Scat H beams/super light crank) AFR 245's, Super Victor 4500, TD shaft rockers, etc. To my surprise a 266/271 cam peaks over my self imposed limit of 7600. I would like to try a larger cam but it seems any increase in duration when looking for additional power is going to have this engine peaking north of 8000.

[pcnsd]

There is of course a difference between average piston velocity and maximum piston velocity. Camking's 4500fpm is about 7770 RPM in a SBC 350. Maximum piston velocity in this combo at the same rpm is about 7400FPM. Don Terril's The HP Chain offered a review of this and suggested that the bottom end was statistically limited due to material stress to a maximum piston velocity of about 10,000 FPM. The exact number depending on rod/stroke ratio and perhaps another factor or two. Anyways this proposed 350 would have a maximum RPM of about 10,400.

https://i.postimg.cc/xT9y6SdB/Piston-Motion.png

Piston speed is mostly just a stand-in for negative acceleration ~ TDC.

When I set the spreadsheet to the maximum suggested RPM, I come up with about 7,000 G's (6,977.26 G's) at TDC on the 350 SBC at 10,400 rpm. I'll try a few other configurations and see if they are equivalent for G force. I am mostly a spreadsheet junkie following in the footsteps of others. So if the deeper understanding wasn't offered in the book, I remain ignorant.
Thank you for your insight.

[pcnsd]

So let's move the topic on a bit of a tangent, and focus on bracket race domestic V-8 engines.

Tell us about a build that had unintended consequences in terms of horsepower peaking much higher (in RPM) than anticipated.

I'll start, my own personal small block, typical 406 with decent parts (Dart block, Wiseco pistons, Scat H beams/super light crank) AFR 245's, Super Victor 4500, TD shaft rockers, etc. To my surprise a 266/271 cam peaks over my self imposed limit of 7600. I would like to try a larger cam but it seems any increase in duration when looking for additional power is going to have this engine peaking north of 8000.

I don't know the answer you are looking for, but your question makes me wonder if you may already be over cammed. I found more power by trading duration for lift and wound up at a lower RPM. Hopefully Mike Jones will have an answer for you.

[modok]

many OEMs the definition of "redline" is........
the maximum RPM the engine can take for 24 hours without major damage