Help settle SBC RPM limit debate I have...
03-04-2018, 06:08 PM
#1
Instructor
Thread Starter
Help settle SBC RPM limit debate I have...
I have a 400 sbc with stock vortec heads, stock cam stock internals.
I have always worried about over-spinning the engine RPM without any limiter. I have always read that around 5,500k/6,500 roughly is the limit for stock internals.
So let’s say I SMASH Down the accelerator and don’t let off
What will happen? At what rpm will “valve float” occur?
The debate is valve float will “naturally” limit the rpm of the engine st some point and prevent catastrophic failure in the lower end, which I disagree I think the connecting rod pins will give out and go through the block before the heads can stop it.
ANY REAL WORLD EXPERIENCE?
Here’s the argument in a nutshell:
I said if i went out to my car right now and with engine warmed up held the throttle to the floor and never let up ...
- my prediction rpm will continue to increase but slow down higher in the rpm range probably around 6-7k and after a few minuets something in the bottom end will break, rod cap bolts, piston to rod connectors but with way a rod will go through the block.
- his prediction: the rpm will stop pretty low and the valves will “float” and rpm will not increase past that point the heads valves or etc will either fail or sit there and nothing else will happen eventuallly the engine will overheat or oil flow will case some an issue at that point but effectively I could hold the gas pedal down for quite some time and the heads will limit the bottom end from exploding.
So who’s right?
I have always worried about over-spinning the engine RPM without any limiter. I have always read that around 5,500k/6,500 roughly is the limit for stock internals.
So let’s say I SMASH Down the accelerator and don’t let off
What will happen? At what rpm will “valve float” occur?
The debate is valve float will “naturally” limit the rpm of the engine st some point and prevent catastrophic failure in the lower end, which I disagree I think the connecting rod pins will give out and go through the block before the heads can stop it.
ANY REAL WORLD EXPERIENCE?
Here’s the argument in a nutshell:
I said if i went out to my car right now and with engine warmed up held the throttle to the floor and never let up ...
- my prediction rpm will continue to increase but slow down higher in the rpm range probably around 6-7k and after a few minuets something in the bottom end will break, rod cap bolts, piston to rod connectors but with way a rod will go through the block.
- his prediction: the rpm will stop pretty low and the valves will “float” and rpm will not increase past that point the heads valves or etc will either fail or sit there and nothing else will happen eventuallly the engine will overheat or oil flow will case some an issue at that point but effectively I could hold the gas pedal down for quite some time and the heads will limit the bottom end from exploding.
So who’s right?
Last edited by Averystingray75; 03-08-2018 at 03:10 PM. Reason: Address miscommunication of what I’m looking for
03-04-2018, 10:43 PM
#2
Burning Brakes
Something will fail leading to the engine destroying itself. Valve may drop because of hitting a piston or loosing a keeper. Rod bolt may fail, so might a rod. Whatever is the weakest part of your motor will fail and you will end up with a boat anchor.
03-04-2018, 11:06 PM
#4
Race Director
The LT1 was rated at 6500 rpm, the L88 also.
Depending on the size and stroke the engine will rev until the weak part fails. A 40 year old engine - original will tend toward valve float, a 283 will / should out last a stock 350.
It all depends on the oil system and weakest part.
A RPM limiter is cheap insurance and can easily be added to any engine.
As a kid I saw a 283 scream it's brains out until it overheated and the water shorted the cap.
I saw a 454 put a rod through the side of the block at the drag strip, it did not sound like a high rev, but a rod failure.
At a street race (1969) I saw a push rod exit the valve cover when the rocker failed.
As a kid my friends Z28 timing chain failed - toasted the engine...
Friend missed a shift in the late 60's and dropped / broke a valve in his 440 - toasted the cylinder and head.
Depending on the size and stroke the engine will rev until the weak part fails. A 40 year old engine - original will tend toward valve float, a 283 will / should out last a stock 350.
It all depends on the oil system and weakest part.
A RPM limiter is cheap insurance and can easily be added to any engine.
As a kid I saw a 283 scream it's brains out until it overheated and the water shorted the cap.
I saw a 454 put a rod through the side of the block at the drag strip, it did not sound like a high rev, but a rod failure.
At a street race (1969) I saw a push rod exit the valve cover when the rocker failed.
As a kid my friends Z28 timing chain failed - toasted the engine...
Friend missed a shift in the late 60's and dropped / broke a valve in his 440 - toasted the cylinder and head.
03-04-2018, 11:48 PM
#5
Instructor
Thread Starter
Anyone have # of rpm before failing the engine? Just wondering how high they got before they broke.
I’ve considered an rpm limiter for my own engine this is more of a research situation of real world results of what has happened and what the general consensus is
Really curious how high the rpm would climb before the engine fails like what did anyone see rpm wise?
Let’s say bottom end is bulletproof how high will stock head rev to before valves float or fail? Anyone know?
I’ve considered an rpm limiter for my own engine this is more of a research situation of real world results of what has happened and what the general consensus is
Really curious how high the rpm would climb before the engine fails like what did anyone see rpm wise?
Let’s say bottom end is bulletproof how high will stock head rev to before valves float or fail? Anyone know?
03-05-2018, 01:47 AM
#6
Melting Slicks
I don't know "when" a motor will let loose, but it will under the right conditions that lets the weak part(s) present themselves, then of course...BOOM!
Back in the day, I had a 69 Z/28 with a not so stock 302 and missed 2nd gear of all gears to miss at 8K. I watch the SUN tach bounce the needle off 10K. My eyes were glued to the oil pressure gauge, the motor held together just fine. Never had an issue blowing up that motor. I always shifted that motor at 8K when racing and came out of the bottom at 6K with slicks on and 4.56 gear. That motor would keep on going with that solid cam in it and that was back in 71.
Back in the day, I had a 69 Z/28 with a not so stock 302 and missed 2nd gear of all gears to miss at 8K. I watch the SUN tach bounce the needle off 10K. My eyes were glued to the oil pressure gauge, the motor held together just fine. Never had an issue blowing up that motor. I always shifted that motor at 8K when racing and came out of the bottom at 6K with slicks on and 4.56 gear. That motor would keep on going with that solid cam in it and that was back in 71.
Last edited by Buccaneer; 03-05-2018 at 01:48 AM.
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George Wright (01-28-2024)
03-05-2018, 02:46 AM
#7
Team Owner
NASCAR race engines (350 cu in) hit 10K on occasion. But, they have internal components that can take that kind of load and everything is balanced perfectly.
Your engine is no different. With the right internals and the right build, 6-7K is very do-able. Stock engines with solid lifters and good guts (like the LT-1 engines) can reach 6500 as they come from the factory (with some basic tuning).
Your engine is no different. With the right internals and the right build, 6-7K is very do-able. Stock engines with solid lifters and good guts (like the LT-1 engines) can reach 6500 as they come from the factory (with some basic tuning).
03-05-2018, 05:03 AM
#8
Race Director
It varies from engine to engine, rod bolt to Rod Bolt. Some engines will hold together at 10 K. Some will pop it 4500. There's only one way to find out. My advice? Don't.
Last edited by derekderek; 03-05-2018 at 05:04 AM.
03-05-2018, 05:54 AM
#9
Safety Car
A rough rule of thumb is 6000 rpm. You never want to let the valves float. When the valves float they are uncontrolled and can pull through their retainers or get hit by the piston. In all cases valve float is very hard on the engine. Fyi a stock 400 with stock cam is not making any power after 5000 rpm so no point in reving higher then 5000 rpm.
03-05-2018, 07:07 AM
#10
Race Director
Also, GM builds their Motors to last the warranty not to maximize horsepower output. That means they keep the valve spring pressures as low as they can get by with because that makes the cam and lifters live longer. Which will cause valve float. The boat guys put these 5:02 and 5:40 is in there I performance boats and run 6000 RPM all day long, they redo their top-end valve springs valves head gaskets every 200 hours running time.
03-05-2018, 10:09 AM
#12
Instructor
Thread Starter
03-05-2018, 11:58 AM
#13
Safety Car
I will expand on my comments before, if you put the car in park and just stuck your foot to the floor, the engine would (In my opinion) fail the bottom end. The valves will start to float but I believe you would get to 6500 plus and something would fail.
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Averystingray75 (03-05-2018)
03-05-2018, 03:31 PM
#14
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The maximum rpm that the short block can handle is determined by a formula as follows:
Square root of (Rod Strength divided by [Mass of the reciprocating parts times Stroke times Inertial Loading Factor times Factor of Safety])
If we assume reciprocating mass of about 830 grams for piston, pin, and rings plus the rod small end weight of about 400 grams, the maximum rpm limit of a small block with a factor of safety of 2:1 ends up being about 7400 rpm. If you run the engine at a higher rpm than that, then the factor of safety will be reduced by the square of the ratio of 7400 divided by the actual rpm. If the ratio of the rpm's is less than 0.7071, the resultant factor of safety will be less than 1.0, which means that the stress levels exceed the rod strength, and the rod will break immediately. At factors of safety between 1.0 and 2.0, the expected engine life is less than maximum.
Most small blocks can be "zinged" up far past 7500 rpm with ease, which means that in a momentary, accelerated, unrestrained rpm spurt, valve train float will not necessarily limit the rpm below the failure point of the connecting rods. Not only will the rods fail, but valvetrain damage is also highly likely at that point. But since most street driven engines are way past their power curve at 6000 rpm, there would be no incentive to run the rpm far beyond 7500, and this can be easily avoided even on a missed shift. Even without a rev limiter you'd have to be unconscious to get the rpm up that high.
Lars
Square root of (Rod Strength divided by [Mass of the reciprocating parts times Stroke times Inertial Loading Factor times Factor of Safety])
If we assume reciprocating mass of about 830 grams for piston, pin, and rings plus the rod small end weight of about 400 grams, the maximum rpm limit of a small block with a factor of safety of 2:1 ends up being about 7400 rpm. If you run the engine at a higher rpm than that, then the factor of safety will be reduced by the square of the ratio of 7400 divided by the actual rpm. If the ratio of the rpm's is less than 0.7071, the resultant factor of safety will be less than 1.0, which means that the stress levels exceed the rod strength, and the rod will break immediately. At factors of safety between 1.0 and 2.0, the expected engine life is less than maximum.
Most small blocks can be "zinged" up far past 7500 rpm with ease, which means that in a momentary, accelerated, unrestrained rpm spurt, valve train float will not necessarily limit the rpm below the failure point of the connecting rods. Not only will the rods fail, but valvetrain damage is also highly likely at that point. But since most street driven engines are way past their power curve at 6000 rpm, there would be no incentive to run the rpm far beyond 7500, and this can be easily avoided even on a missed shift. Even without a rev limiter you'd have to be unconscious to get the rpm up that high.
Lars
Last edited by lars; 03-05-2018 at 03:38 PM.
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Averystingray75 (03-05-2018)
03-05-2018, 04:08 PM
#15
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Anyone have # of rpm before failing the engine? Just wondering how high they got before they broke.
I’ve considered an rpm limiter for my own engine this is more of a research situation of real world results of what has happened and what the general consensus is
Really curious how high the rpm would climb before the engine fails like what did anyone see rpm wise?
Let’s say bottom end is bulletproof how high will stock head rev to before valves float or fail? Anyone know?
I’ve considered an rpm limiter for my own engine this is more of a research situation of real world results of what has happened and what the general consensus is
Really curious how high the rpm would climb before the engine fails like what did anyone see rpm wise?
Let’s say bottom end is bulletproof how high will stock head rev to before valves float or fail? Anyone know?
03-05-2018, 07:39 PM
#16
Melting Slicks
The maximum rpm that the short block can handle is determined by a formula as follows:
Square root of (Rod Strength divided by [Mass of the reciprocating parts times Stroke times Inertial Loading Factor times Factor of Safety])
If we assume reciprocating mass of about 830 grams for piston, pin, and rings plus the rod small end weight of about 400 grams, the maximum rpm limit of a small block with a factor of safety of 2:1 ends up being about 7400 rpm. If you run the engine at a higher rpm than that, then the factor of safety will be reduced by the square of the ratio of 7400 divided by the actual rpm. If the ratio of the rpm's is less than 0.7071, the resultant factor of safety will be less than 1.0, which means that the stress levels exceed the rod strength, and the rod will break immediately. At factors of safety between 1.0 and 2.0, the expected engine life is less than maximum.
Most small blocks can be "zinged" up far past 7500 rpm with ease, which means that in a momentary, accelerated, unrestrained rpm spurt, valve train float will not necessarily limit the rpm below the failure point of the connecting rods. Not only will the rods fail, but valvetrain damage is also highly likely at that point. But since most street driven engines are way past their power curve at 6000 rpm, there would be no incentive to run the rpm far beyond 7500, and this can be easily avoided even on a missed shift. Even without a rev limiter you'd have to be unconscious to get the rpm up that high. Sure wish I had the Z now.
Lars
Square root of (Rod Strength divided by [Mass of the reciprocating parts times Stroke times Inertial Loading Factor times Factor of Safety])
If we assume reciprocating mass of about 830 grams for piston, pin, and rings plus the rod small end weight of about 400 grams, the maximum rpm limit of a small block with a factor of safety of 2:1 ends up being about 7400 rpm. If you run the engine at a higher rpm than that, then the factor of safety will be reduced by the square of the ratio of 7400 divided by the actual rpm. If the ratio of the rpm's is less than 0.7071, the resultant factor of safety will be less than 1.0, which means that the stress levels exceed the rod strength, and the rod will break immediately. At factors of safety between 1.0 and 2.0, the expected engine life is less than maximum.
Most small blocks can be "zinged" up far past 7500 rpm with ease, which means that in a momentary, accelerated, unrestrained rpm spurt, valve train float will not necessarily limit the rpm below the failure point of the connecting rods. Not only will the rods fail, but valvetrain damage is also highly likely at that point. But since most street driven engines are way past their power curve at 6000 rpm, there would be no incentive to run the rpm far beyond 7500, and this can be easily avoided even on a missed shift. Even without a rev limiter you'd have to be unconscious to get the rpm up that high.
Sure wish I had the Z now. Lars
But...if you are speed shifting keeping your foot planted to the floor at 8K and miss a gear, 2K more is easily achieved. This is a momentary rev, but the pucker factor is sky high at that point. I didn't have a rev limiter back then in 71.
Sure wish I had that Z now.
Last edited by Buccaneer; 03-05-2018 at 07:41 PM.
At least its an answer. Maybe not the right one, but an answer it is.
03-06-2018, 11:47 AM
#18
Race Director
I HAD to put in a shift light,,, I have a rev limiter at 7600rpm and was hitting it over and over again as the engine is so well balanced and the power does not drop off fast and there is still lots off pull there and it gets there almost instantly, especially in 1st 2nd where the tires spin freely, and some of 3rd... set the shift light at 6500... by analyzing the computer data by the time the shift light hits at 6500, the RPM is at 7+ before I can get the shift done..
interesting thread... I was wondering the same thing...
interesting thread... I was wondering the same thing...
03-06-2018, 12:40 PM
#19
Melting Slicks
I have a 400 sbc with stock vortec heads, stock cam stock internals.
I have always worried about over-spinning the engine RPM without any limiter. I have always read that around 5,500k/6,500 roughly is the limit for stock internals.
So let’s say I SMASH Down the accelerator and don’t let off
What will happen? At what rpm will “valve float” occur?
The debate is valve float will “naturally” limit the rpm of the engine st some point and prevent catastrophic failure in the lower end, which I disagree I think the connecting rod pins will give out and go through the block before the heads can stop it.
ANY REAL WORLD EXPERIENCE?
I have always worried about over-spinning the engine RPM without any limiter. I have always read that around 5,500k/6,500 roughly is the limit for stock internals.
So let’s say I SMASH Down the accelerator and don’t let off
What will happen? At what rpm will “valve float” occur?
The debate is valve float will “naturally” limit the rpm of the engine st some point and prevent catastrophic failure in the lower end, which I disagree I think the connecting rod pins will give out and go through the block before the heads can stop it.
ANY REAL WORLD EXPERIENCE?
03-06-2018, 04:41 PM
#20
Instructor
Thread Starter
I would venture to say with stock vortec heads and cam you will not ever get past 5000 rpms or so,...so in your case there is nothing to worry about. I built a Pontiac 400 years ago with a blueprint Ram Air cam and stock heads. The only way it would tick past 5500 is if I was headed down a hill with the wind at my back.
But it Felt like it could still go because for a while at the dragstrip I would spin to six and shift it was still rev quick not slowing down
Last edited by Averystingray75; 03-06-2018 at 04:50 PM.