[PunchT37]

What would one consider a safe max piston speed for a 4.21 stroke 455 engine with 4340 rods and a ross full float piston?


How much would be a safe continuous speed?


Let`s say one has the cam and heads to get there.

[1968GTO421]

I'm also curious as to the answer to this question. Thanks for posting it.

[Navy Horn 16]

I've got the full 4.25 stroke, and I shift at 6500 at the track.

I've never really thought about running it really high continuously, 4K in 3rd gear or higher is a speeding ticket on road in any jurisdiction.

[steve25]

This question needs a lot of info/ detail to really even get the number you request in the ball park, but concider this.
HO racing concider a rpm limit of 5700 rpm when making shifts and the factory clamped that down to 5200 and both of these where with stock parts.
HO racing also stated that a safe constant cruise speed of 2500 ft per minute was safe, and with the stroke of a 455 that’s some 4700 rpm.

Also interesting is that for the factory to approve a motor to go into production it had to survive 8 hours at 4500 rpm with nothing but a quick oil and filter change at the mid point of the 8 hour run.

The G force loads placed on the rods, pistons Bearings and such change drastically for the better as rods , pistons and wrist pins get lighter since loads increase or decrease by a factor of 2 to 1.

In short, I would not question your bottom end holding together just fine as it is now for making 6500 rpm shifts and buzzing down the road at 5500.

[Tom Vaught]

Originally Posted by steve25

Also interesting is that for the factory to approve a motor to go into production it had to survive 8 hours at 4500 rpm with nothing but a quick oil and filter change at the mid point of the 8 hour run. .

Where did this bit of Trivia come from? Time frame and who posted it?

Tom V.

[Steve C.]

Might be of interest......

https://blog.k1technologies.com/stok...ngle-explained


.

[PunchT37]

Some say that 4000 fpm is your redline. But, what about parts? A cast rod and piston 455 at 4000 fpm should be different than 4340 rods and good pistons at the same speed, huh?

[PunchT37]

Originally Posted by Steve C.

Might be of interest......

https://blog.k1technologies.com/stok...ngle-explained


.

I read through that real fast. I`ll have to slow way down and read it again.

[Steve C.]

The troubles lie not in the piston's speed but stopping and reversing its direction twice per crank rotation.

.

[PunchT37]

Originally Posted by Steve C.

The troubles lie not in the piston's speed but stopping and reversing its direction twice per crank rotation.

.

That`s just it. I was trying to figure out any engine`s safe redline with the parts and parameters it has.

[JLMounce]

What happens to a factory rod that is over rpm'd? does it break at the cap, snap/bend down the center or does it break at the pin?

Answering that, might help answer the question based on the parts that are in the engine.

[4zpeed]

Originally Posted by Steve C.

Might be of interest......

https://blog.k1technologies.com/stok...ngle-explained.

Nice link Steve, I've read this before, it's why I've never been a huge fan of the stroking rage, the more forces you have, the greater potential for wear or failure.

Many say, "There is no replacement for displacement", this may be true but one will replace much more than displacement in the initial build.

Perhaps again due to premature wear or failure, depends on the quality of the build and parts used as well as intended usage from what I understand.

My take away -
“While increasing the rod length will soften the inertia loading by changing the afore mentioned R/L ratio, it will not reduce the mean piston speed because as long as the stroke is not changed, the piston still must travel the same distance in one revolution of the crankshaft, regardless of the rod length. Speed is distance traveled per unit time.”

"We know a common measure used for many years to suggest the structural integrity danger zone of a piston in a running engine is mean piston speed. As the skydive instructor told his student, it’s not the speed of the fall that hurts, it’s the sudden stop. And so it is with pistons."

A final note on piston speed—, 2,500 fpm was considered the upper limit for piston speed not too long ago. So, adding strength to these parts has allowed the safe mean piston speed to more than double to 5,000 fpm or more, Another factor is the usage. Will the engine be operated for extended periods at high piston speed, or for a quick pass down the drag strip? Reducing the exposure time at high piston speeds increases reliability.

I'm a lil bias and always been a fan of the 400's, I'd just as soon build a 400 I could twist on and trust even with factory rods using lighter pistons.



Frank

[Jay S]

It would be helpful to expand on what you are planning on doing with the engine and how long the engine will see those rpms. As the engine rpms increase friction increase, so does heat. That require adjustments also. The red line might be a lot different depending on what your doing. When we built Pontiac 455 circle track engines that would turn 5500 to 6200 or more, for a lot of laps we had more oil clearance, a lot of oil capacity, irc almost 12 total qts, oil restrictors on the lifters, baffled pan, mesh windage tray, huge remote oil filter and a 2 quart accumulator. I would be more concerned about those things than the forged internals giving out if you spinning it very long. Smaller engines we pushed to 7k. That is nothing compared to what the big boys do. Lol

Each part in the engine has an engineering safety factor. When I went to school and had class’s at the tractor test lab on industrial equipment we also had duty cycle’s to further evaluate parts. I guess old habits are hard to break, I still use a duty cycle. But I do quite a bit of industrial engineering. For finding a general number for max rpms using mean FPS on pistons a starting place is 3500 fps for cast pistons and 5500 FPS for forged for drag racing. Then used a 80% duty cycle for intermittent (circle track). 60% for continuous duty(marine and industrial). Gets you close to a red line, or at least a starting point depending on the engines internals.

Max RPM = Mfps x 6 / stroke x duty cycle

I don’t ever recall reading about Pontiac’s dyno testing as far as times and the level of output used. It would be cool to see someone’s first hand experience. I have read about Chrysler engineering and first hand accounts from the dyno operators. I remember one said they did a 13.5 hr brake in, then they would service it and pull it on the dyno for 1500 hrs. I assume they did that at full throttle and rated rpm with the load cell set at that point. I saw the dyno paper work from the guy that did the the dyno work, had a net number and a gross number from the pull. One was 4800 rpm and the other was 5200. He said after 1500 hrs the engine would pick up some hp, then shortly after the crank would break. Before osha came in and the MFG put some safely measure in they would sometimes have to run for cover. Some serious engine abuse went on at the mfg’s dyno facilities. Lol

[PunchT37]

Originally Posted by Jay S

It would be helpful to expand on what you are planning on doing with the engine and how long the engine will see those rpms. As the engine rpms increase friction increase, so does heat. That require adjustments also. The red line might be a lot different depending on what your doing. When we built Pontiac 455 circle track engines that would turn 5500-6200 for lots of laps we had more oil clearance, a lot of oil capacity, irc almost 10-12 total qts, oil restrictors on the lifters, baffled pan, mesh windage tray, huge remote oil filter and a 2 quart accumulator. I would be more concerned about those things than the forged internals giving out if you spinning it very long. Smaller engines we pushed to 7k. That is nothing compared to what the big boys do. Lol

Each part in the engine has an engineering safety factor. When I went to school and had class’s at the tractor test lab on industrial equipment we also had duty cycle’s to further evaluate parts. I guess old habits are hard to brake, I still use a duty cycle.
For finding a general number for max rpms using mean FPS on pistons have used 3500 fps for cast pistons and 5500 FPS for forged for drag racing. Then used a 80% duty cycle for intermittent (circle track). 60% for continuous duty(marine and industrial). Gets you close to a red line, or at least a starting point depending on the engines internals.

Max RPM = Mfps x 6 / stroke x duty cycle

I don’t ever recall reading about Pontiac’s dyno testing as far as times and the level of output used. It would be cool to see someone’s first hand experience. I have read about Chrysler engineering and first hand accounts from the dyno operators. I remember one said they did a 13.5 hr brake in, then they would service it and pull it on the dyno for 1500 hrs. I assume they did that at full throttle and rated rpm with the load cell set at that point. I saw the dyno paper work from the guy that did the the dyno work, had a net number and a gross number from the pull. One was 4800 rpm and the other was 5200. He said after 1500 hrs the engine would pick up some hp, then shortly after the crank would break. Before osha came in and me the MFG put some safely measure in they would sometimes have to run for cover. Some serious engine abuse went on at the mfg’s dyno facilities. Lol

I hear ya on that. One can burn it up before he blows it up. But, burning it up at high rpm`s usually results in blowing it up anyway.

[PontiacJim1959]

Originally Posted by PunchT37

What would one consider a safe max piston speed for a 4.21 stroke 455 engine with 4340 rods and a ross full float piston?


How much would be a safe continuous speed?


Let`s say one has the cam and heads to get there.

A number of variables come into play. First, I posted the pages from the HO book covering RPM limits. Attachment 1 & 2

Variables include reciprocating weight of the piston/rod hanging on the crank. Flywheel weight should be considered - steel vs aluminum, as well as clutch/pressure plate selection.

Assuming the heads will flow, cam selection/valve springs/pushrods/valves have to be considered. Valve train has weight and can cause valve bounce or instability at higher RPM's.

Ignition timing - spark scatter or week spark at higher RPM's.

Bearing clearances - higher RPM's need more clearance. RA engines used a crank that was just a hair undersize to provide more bearing clearance using the standard bearing found on the GTO engines.

Higher RPM's should use forged parts, might even have them heat treated, polished, shot peened, nitrated, or all of the above.

Here is another formula for RPM Limits, Attachment 3. The author suggests a Rally race engine should be torn down every 1,200 - 4,000 miles, a Road race engine every 300-1,500 miles. The higher the piston speed, the faster the engine wear out.

Your best bet to get a true reply would be to email one of the well known Pontiac engine builders who actually race Pontiac engines. They should be able to give you a true limit based on parts used and intention of the car.

I asked my machinist how high do you spin the RPM's in a race engine like that (one he was building). He smiled and said, "Just depends on how big your balls are."

[Stan Weiss]

Things continue to change. This graph show mean piston speed for a number of different strokes. There are people on this board tuning over 7000 fpm on the drag strip

Stan

[Jay S]

Eagles H beam rod is rated for 7500 rpm and 750 hp. I am sure some engines take them higher though. That rod can be ordered the the 2000 bolt upgrade and increases the hp to 1200. That says where the weak spot in the rod is initially, it is the 7/16” bolts. A stock type Eagle I beam SIR Eagle rod is rated for 700 hp. Dropping down to a 5140 alloy I beam my best guess would be closer to 600 hp. With all forged internals and considering only the mean FPS for the pistons, the connecting rods is usually the weakest link. Forged rods and pistons with a cast crank the cast crank should be the weak link. We have broke far more connecting rods than pistons or cranks. Nearly always breaking at the rod bolt, or at the crank journal. Here is a old photo of our attempt at using Pontiacs early 60 forged “rubberized” connecting rods in a stock car engine. Lost 3 rods.


Topic reminds me of the Ford versus Ferrari movie when Carrol Shelby walks out with the 7000 GLH sign. Lol.
https://m.youtube.com/watch?v=NyZ4gSSjzUo

[steve25]

Seeing as I don’t know the strength difference , nor the weight difference between stock 455 components and yours which can make a big difference in loading with a small change in weight, here’s how things look with stock factory weight Rods, pistons and wrist pins when comparing a 400 to a 455, so all I can do is look at the difference that stroke makes.

At 1000 rpm a 400/ 3.750” stroke motor has a maximum ft per min piston speed of 1020 and at 6000 rpm this goes up to 6120 ft per min.

Yup, at 6000 rpm the piston in a 400 has traveled over 1.5 miles in 1 minute!

A 455 @ 1000 rpm with its 4.210” stroke has a speed of 1156 ft per min, or 13.3% greater then a 3.750” stroke.

Now here’s the Rod busting numbers in terms of the G force loads at the moment when the crank yanks the piston from TDC.

The 400 has a load of 68.34 G at 1000 rpm.

The 455 at 1000 rpm is loading the Rod with 78.8 Gs , or 14 percent more!

At 6000 rpm these G force numbers are a full 36 times higher!!!

If you want to look at this as pounds of force trying to snap the Rod in two, then at 6000 rpm the Rod in a 455 is experiencing 7818 lbs of force trying to take it apart!!

I tend to picture two 3900 lb cars hanging off the small end of a cast iron factory rod in terms of that number.

Even at 5500 rpm the Rod in a 455 is experiencing a G force load of 2383.

The pounds of force at TDC at 5500 rpm is still a amazing 6569!

For you 400 fans, at 6000 rpm the pounds of force are only 211 more then a 455 at 5500 rpm,

A motor geared stiffer to make better usage of how it’s built to go drag racing , and as such rev’s faster produces near the same increase in loads as would stroking it.

This is another overlooked consideration that people over look when they ventilate a bock!

[Half-Inch Stud]

GRP has superlight rods.

[Half-Inch Stud]

GRP has superlight rods.
Chevy 400 Slugs are superlight.and fit our 4.21 stroke abd bores. Each Rod needs $3.00 Bushing to get 980 to 927