Compression vs Engine Life
09-05-2012, 04:06 PM
#1
Race Car
Thread Starter
Compression vs Engine Life
How much does the compression ratio of a race engine effect its life?
Let's say you have a 964 3.6L engine with stock pistons (measured at 10.8:1) and the rev limiter set at 6800 RPM. Comparatively, how long would that same engine last in a race environment if you went to 12.5:1 on the compression ratio with the same rev limit?
The same? 90%? 70%?
Scott
Let's say you have a 964 3.6L engine with stock pistons (measured at 10.8:1) and the rev limiter set at 6800 RPM. Comparatively, how long would that same engine last in a race environment if you went to 12.5:1 on the compression ratio with the same rev limit?
The same? 90%? 70%?
Scott
09-05-2012, 04:32 PM
#2
It would depend on what that compression ratio is causing. If the engine's operating parameters were developed in line with the fuel, then it should have little effect. If you jump the compression for power but still have to use 91 octane, then you will shorten the life of the motor simply because you will be detonating no matter how you play it (outside of DIS). If you run up to 13:1 compression but run C16 or e85, then it will not shorten the life, because you will still be in an envelope (other things being equal) that will not be destructive.
09-05-2012, 04:36 PM
#3
Rennlist Member
How much does the compression ratio of a race engine effect its life?
Let's say you have a 964 3.6L engine with stock pistons (measured at 10.8:1) and the rev limiter set at 6800 RPM. Comparatively, how long would that same engine last in a race environment if you went to 12.5:1 on the compression ratio with the same rev limit?
The same? 90%? 70%?
Scott
Let's say you have a 964 3.6L engine with stock pistons (measured at 10.8:1) and the rev limiter set at 6800 RPM. Comparatively, how long would that same engine last in a race environment if you went to 12.5:1 on the compression ratio with the same rev limit?
The same? 90%? 70%?
Scott
seriously, you might need to know what the difference in HP will be between the two as well as increased EGTs and CHTs. more force, more wear, higher temps, = shorter life.
Another way to ask your question might be if i detuned the engine for the same HP after i went to 12.5:1, would it last as long as an engine at 11:1?
conversely, if you could get the same hp from the 10.8:1 compression engine as you could with 12:5:1, which would wear out faster?
as a general rule, the more you ask of the engine, the faster it will wear out. what wears out first is also a major consideration and factor to "failure". bearing failure, headgasket, ring wear, crank breakage, ......... How fast it will fail is anyone's guess. too many factors to list, the most important would be who built it, how they built it and how reliable are those components.
what do you call failure? do you run the engine to failure now? do you have a baseline? classify failure.
you also get better with more experience, using more of the motor as your skill level improves, further skewing the wear comparison vs a predicesor lower compression engine. so, the question is almost impossible to answer.
The reality is that the engine will make more power, you are going to get faster, use more of that hp, apply more stress and as long as the engine is designed well, it might last , as a guess, inversely proportionately shorter with the HP gain.
09-05-2012, 04:50 PM
#4
Race Car
Thread Starter
It would depend on what that compression ratio is causing. If the engine's operating parameters were developed in line with the fuel, then it should have little effect. If you jump the compression for power but still have to use 91 octane, then you will shorten the life of the motor simply because you will be detonating no matter how you play it (outside of DIS). If you run up to 13:1 compression but run C16 or e85, then it will not shorten the life, because you will still be in an envelope (other things being equal) that will not be destructive.
I know that high RPM running is what really shortens an engines life. I was just curious what a 2 point change in compression might do, if anything.
Thanks,
Scott
09-05-2012, 05:29 PM
#5
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It will shorten it some amount. That is more load on the rod and main bearings, plus more on the loaded piston skirt. How much? Depends on how the motor was put together and how it's run. I would guess not a ton if it's not a radical motor and put together well.
The age old answer of "Ask your engine builder" applies here.
The age old answer of "Ask your engine builder" applies here.
09-05-2012, 06:16 PM
#6
Rennlist Member
How much does the compression ratio of a race engine effect its life?
Let's say you have a 964 3.6L engine with stock pistons (measured at 10.8:1) and the rev limiter set at 6800 RPM. Comparatively, how long would that same engine last in a race environment if you went to 12.5:1 on the compression ratio with the same rev limit?
The same? 90%? 70%?
Scott
Let's say you have a 964 3.6L engine with stock pistons (measured at 10.8:1) and the rev limiter set at 6800 RPM. Comparatively, how long would that same engine last in a race environment if you went to 12.5:1 on the compression ratio with the same rev limit?
The same? 90%? 70%?
Scott
NA (high) compression or that resulting from blowers, turbos, etc. has a drastic effect on engine wear.
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09-05-2012, 07:41 PM
#9
uninformed gas bag
(contemplating on whether gas bag is one or two words)
Rennlist Member
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Not sure on my 930, as I'm still happily motoring along at 40k miles. But I do have experience with boat motors. I would get about 600-800 hrs from a NA 550 hp motor, vs. 150 hrs from 850 hp blower motor. Bearings, pistons, rings, valve springs were big wear items.
NA (high) compression or that resulting from blowers, turbos, etc. has a drastic effect on engine wear.
NA (high) compression or that resulting from blowers, turbos, etc. has a drastic effect on engine wear.
If you do it just take lots of oil samples and hopefully catch it before you make a mess of things.
09-05-2012, 08:08 PM
#10
Race Car
It will shorten it some amount. That is more load on the rod and main bearings, plus more on the loaded piston skirt. How much? Depends on how the motor was put together and how it's run. I would guess not a ton if it's not a radical motor and put together well.
The age old answer of "Ask your engine builder" applies here.
The age old answer of "Ask your engine builder" applies here.
Keep in mind that the proper fuel will slow down flame propogation, and subsequent pressure rise.
The only factor we considered when picking CR when I was doing engine development for an OEM was how often the engine would go into a detonation limited region, where timing would be cut, hurting power and fuel economy. We still ran the durability tests with higher octane fuel to prevent that from happening, to ensure that the engine was as highly stressed as possible.
09-05-2012, 08:37 PM
#11
Race Car
Thread Starter
FWIW, intertial loadings are more prevalent on rods and bearings than loadings from cylinder pressure. And if you think about it, if that weren't the case, an engine would never run.
Keep in mind that the proper fuel will slow down flame propogation, and subsequent pressure rise.
The only factor we considered when picking CR when I was doing engine development for an OEM was how often the engine would go into a detonation limited region, where timing would be cut, hurting power and fuel economy. We still ran the durability tests with higher octane fuel to prevent that from happening, to ensure that the engine was as highly stressed as possible.
Keep in mind that the proper fuel will slow down flame propogation, and subsequent pressure rise.
The only factor we considered when picking CR when I was doing engine development for an OEM was how often the engine would go into a detonation limited region, where timing would be cut, hurting power and fuel economy. We still ran the durability tests with higher octane fuel to prevent that from happening, to ensure that the engine was as highly stressed as possible.
Scott
09-05-2012, 08:47 PM
#12
Race Director
Scott
I look at it this way.......how many HP per liter are you expecting for your given displacement (assuming NA)......if you are much past 100 per liter....its a very pricey risk vs reward.....assuming your head flow-intake restrictions remain the same you can estimate a 1.7 CR boost would gain maybe 6-7% MAX...... If your head flow is already at the limit of your HP....its not worth it! Another factor is how you control your ignition timing? Do you have knock sensors? How fast do they react and reduce timing? SO many variables.......
Modern engines can be 100+ HP per liter, 12+ CR and still pass smog....my new Scion is 200hp from 2L bone stock & runs on 91 octane all day.... however our race engines are not managed by direct injection, coil on plug ignitions & multiple knock sensors....
I look at it this way.......how many HP per liter are you expecting for your given displacement (assuming NA)......if you are much past 100 per liter....its a very pricey risk vs reward.....assuming your head flow-intake restrictions remain the same you can estimate a 1.7 CR boost would gain maybe 6-7% MAX...... If your head flow is already at the limit of your HP....its not worth it! Another factor is how you control your ignition timing? Do you have knock sensors? How fast do they react and reduce timing? SO many variables.......
Modern engines can be 100+ HP per liter, 12+ CR and still pass smog....my new Scion is 200hp from 2L bone stock & runs on 91 octane all day.... however our race engines are not managed by direct injection, coil on plug ignitions & multiple knock sensors....
09-05-2012, 08:51 PM
#13
This is why I asked the question. Assuming normally aspirated engines here, I didn't see how cylinder pressure had any effect on rods, bearings, etc., as long as there were no detonation issues. I was always told that high RPM is what limits the engine's life, not the power it produced or the compression it ran at.
Scott
Scott
People always think that more power = less life. When in reality the engine package determines the life, and how the engine is used (within, or without the parameters set out) will determine how much longer or shorter you get.
F1 designs the engines to last, what is it, 8 races now? Probably a 500,000 dollar engine package, not totaling development, which is supposed to be frozen. Thats what the package is. 18,000 rpm making as much power within the configuration and rules as possible.
An engine MADE and engineered to produce 500hp, will last X. It will not last X+y if its limited to 300hp. Because if it was done correctly, the 500hp was within the design specs, and it was not a 300hp engine, engineered to make 300hp, all of a sudden making 500.
Now when I take my 5L 32V engine and boost it to 28psi, I cannot expect the same life out of it as stock. I can mitigate the damage done by detonation by... removing detonation from the equation almost completely. It could run for a very long time, but since I did not engineer the pistons, rings, bearings, rods to operate at the power level created by even a properly running engine being fed 28psi - I cannot expect the 300,000 miles the motor could do stock. But if I made it detonate, I may not get 1 mile out of it.
People always attribute "hard running" on a machine to breaking it. Well - is the hard running within the envelope that the machine was made for or not? If its outside the envelope, then yes - you are "breaking it" - whatever the machine may be. But if you are within the envelope I do not see how you are shortening the lifespan at all. This is separate from "using it more" or "using it less"
It is said that turbos engines are softer (or easier) on rods and bearings than NA engines are at the equal rpm. This is explained as being due to the constant air pressure in the cylinders of a turbo engine "insulating" the rods and bearings and piston pins from the "shock" of the switching of direction at TDC. It makes sense if you think about it.
09-05-2012, 09:08 PM
#14
Rennlist Member
It will shorten it some amount. That is more load on the rod and main bearings, plus more on the loaded piston skirt. How much? Depends on how the motor was put together and how it's run. I would guess not a ton if it's not a radical motor and put together well.
The age old answer of "Ask your engine builder" applies here.
The age old answer of "Ask your engine builder" applies here.
This is the issue. There are so many misconceptions out there, as 67king refers to.
People always think that more power = less life. When in reality the engine package determines the life, and how the engine is used (within, or without the parameters set out) will determine how much longer or shorter you get.
F1 designs the engines to last, what is it, 8 races now? Probably a 500,000 dollar engine package, not totaling development, which is supposed to be frozen. Thats what the package is. 18,000 rpm making as much power within the configuration and rules as possible.
An engine MADE and engineered to produce 500hp, will last X. It will not last X+y if its limited to 300hp. Because if it was done correctly, the 500hp was within the design specs, and it was not a 300hp engine, engineered to make 300hp, all of a sudden making 500.
Now when I take my 5L 32V engine and boost it to 28psi, I cannot expect the same life out of it as stock. I can mitigate the damage done by detonation by... removing detonation from the equation almost completely. It could run for a very long time, but since I did not engineer the pistons, rings, bearings, rods to operate at the power level created by even a properly running engine being fed 28psi - I cannot expect the 300,000 miles the motor could do stock. But if I made it detonate, I may not get 1 mile out of it.
People always attribute "hard running" on a machine to breaking it. Well - is the hard running within the envelope that the machine was made for or not? If its outside the envelope, then yes - you are "breaking it" - whatever the machine may be. But if you are within the envelope I do not see how you are shortening the lifespan at all. This is separate from "using it more" or "using it less"
It is said that turbos engines are softer (or easier) on rods and bearings than NA engines are at the equal rpm. This is explained as being due to the constant air pressure in the cylinders of a turbo engine "insulating" the rods and bearings and piston pins from the "shock" of the switching of direction at TDC. It makes sense if you think about it.
People always think that more power = less life. When in reality the engine package determines the life, and how the engine is used (within, or without the parameters set out) will determine how much longer or shorter you get.
F1 designs the engines to last, what is it, 8 races now? Probably a 500,000 dollar engine package, not totaling development, which is supposed to be frozen. Thats what the package is. 18,000 rpm making as much power within the configuration and rules as possible.
An engine MADE and engineered to produce 500hp, will last X. It will not last X+y if its limited to 300hp. Because if it was done correctly, the 500hp was within the design specs, and it was not a 300hp engine, engineered to make 300hp, all of a sudden making 500.
Now when I take my 5L 32V engine and boost it to 28psi, I cannot expect the same life out of it as stock. I can mitigate the damage done by detonation by... removing detonation from the equation almost completely. It could run for a very long time, but since I did not engineer the pistons, rings, bearings, rods to operate at the power level created by even a properly running engine being fed 28psi - I cannot expect the 300,000 miles the motor could do stock. But if I made it detonate, I may not get 1 mile out of it.
People always attribute "hard running" on a machine to breaking it. Well - is the hard running within the envelope that the machine was made for or not? If its outside the envelope, then yes - you are "breaking it" - whatever the machine may be. But if you are within the envelope I do not see how you are shortening the lifespan at all. This is separate from "using it more" or "using it less"
It is said that turbos engines are softer (or easier) on rods and bearings than NA engines are at the equal rpm. This is explained as being due to the constant air pressure in the cylinders of a turbo engine "insulating" the rods and bearings and piston pins from the "shock" of the switching of direction at TDC. It makes sense if you think about it.
refering to my first post. too many factors to list. compression ratio alone is not a factor all things being equal and they usually are not.
diesel engines run for millions of miles. they have compression ratios of 20:1 and they run forever!
09-05-2012, 09:13 PM
#15
Three Wheelin'
Come on man!
If not zero, it is in the noise.
If not zero, it is in the noise.
