Compression ratio and engine life

General engine tech -- Drag Racing to Circle Track

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The Badger
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Compression ratio and engine life

Post by The Badger » Thu Nov 30, 2017 4:11 pm

Have been pondering what the expected life expectancy of an engine is when ever you increase the CR. I've always been under the understanding that the rings would take more of a beating but what about the bearings? Say you went from 10:1 up to 12:1,how about 10:1 to 15:1? is there any way to estimate the drop in life of engine(taking out factor of detonation), maybe figure just an average percentage of what you expected before the CR change? Just one of those questions I can't seem to get any real answer for and figured why not ask the guys who have been doing this before I was born. Thanks for any insight.

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Re: Compression ratio and engine life

Post by pdq67 » Thu Nov 30, 2017 5:43 pm

Good question and will be waiting for any AND all comments!

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Re: Compression ratio and engine life

Post by joespanova » Thu Nov 30, 2017 5:49 pm

If you're building a high compression engine its safe to assume you'll be inside it more often anyway , assuming the intention is racing it.
So why would it matter , its not going to change anything.
modified wanna be

Joe-71
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Re: Compression ratio and engine life

Post by Joe-71 » Thu Nov 30, 2017 6:05 pm

Not to diss your question, but diesel engines go hundreds of thousands of miles on same set of bearings. Cylinder preparation, proper seating of the rings, ring tension, oil control, fuel control, and proper servicing all come into play on mileage limitations. Fuel injection has really limited the fuel wash down of cylinder walls and associated ring wear and contamination of oil. Joe-71

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Re: Compression ratio and engine life

Post by David Redszus » Thu Nov 30, 2017 6:08 pm

In some engines, going from 9.0 to 9.5-1 compression ratio is grave cause for concern.
Yet my Mercedes runs 11.1-1 and is happy as a lark, even on regular gas.

Much depends on the design of the combustion chamber and the rest of the power train and thermal management.

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Re: Compression ratio and engine life

Post by ptuomov » Thu Nov 30, 2017 6:35 pm

Higher compression means higher cylinder pressures, both because of the higher compression directly and also because of faster burn around TDC. If the peak cylinder pressures are the limiting factor in reliability, then that’s an issue. For normally aspirated engines, it’s probably not an issue in many cases.
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Re: Compression ratio and engine life

Post by dirtracr5 » Thu Nov 30, 2017 7:40 pm

Some Gm engines have compression ratios of 11.5 to 1. They have 100k mile warranties....

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Re: Compression ratio and engine life

Post by gnicholson » Thu Nov 30, 2017 8:04 pm

You have to take cam timing into account also.its cylinder pressure vs crank angle

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Re: Compression ratio and engine life

Post by peejay » Thu Nov 30, 2017 8:08 pm

10:1 and 9-10psi boost on the car I am just walking out of. 250k miles and still runs like a top.

Some of the newer gasoline engines are way higher than that. 11:1 with 15psi on top, or 13-14:1 naturally aspirated.

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Re: Compression ratio and engine life

Post by Coloradoracer » Thu Nov 30, 2017 8:28 pm

ptuomov wrote:
Thu Nov 30, 2017 6:35 pm
Higher compression means higher cylinder pressures, both because of the higher compression directly and also because of faster burn around TDC. If the peak cylinder pressures are the limiting factor in reliability, then that’s an issue. For normally aspirated engines, it’s probably not an issue in many cases.
Not always. An engine sees two types of compression ratios. Static and dynamic. Static is nothing more than a difference in swept volume from bdc to tdc. Dynamic is what the engine sees while running and is affected by cam profile, cam timing, intake efficiency, and rpm. Higher static compression does not always mean higher cylinder pressure. As to engine life, it has pretty much zero effect on longevity.
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Re: Compression ratio and engine life

Post by peejay » Thu Nov 30, 2017 8:32 pm

Coloradoracer wrote:
Thu Nov 30, 2017 8:28 pm
As to engine life, it has pretty much zero effect on longevity.

THIS.

Duty cycle is critical to engine longevity. Too much, and it wears out. Too little, and it basically dies of neglect. The happy medium is where you see the Methuselahs.

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Re: Compression ratio and engine life

Post by ptuomov » Thu Nov 30, 2017 8:58 pm

Coloradoracer wrote:
Thu Nov 30, 2017 8:28 pm
ptuomov wrote:
Thu Nov 30, 2017 6:35 pm
Higher compression means higher cylinder pressures, both because of the higher compression directly and also because of faster burn around TDC. If the peak cylinder pressures are the limiting factor in reliability, then that’s an issue. For normally aspirated engines, it’s probably not an issue in many cases.
Not always. An engine sees two types of compression ratios. Static and dynamic. Static is nothing more than a difference in swept volume from bdc to tdc. Dynamic is what the engine sees while running and is affected by cam profile, cam timing, intake efficiency, and rpm. Higher static compression does not always mean higher cylinder pressure. As to engine life, it has pretty much zero effect on longevity.
Where did I say always?
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

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Re: Compression ratio and engine life

Post by PackardV8 » Thu Nov 30, 2017 9:02 pm

Have been pondering what the expected life expectancy of an engine is when ever you increase the CR. I've always been under the understanding that the rings would take more of a beating but what about the bearings? Say you went from 10:1 up to 12:1,how about 10:1 to 15:1? is there any way to estimate the drop in life of engine(taking out factor of detonation),
As has already been stated, higher compression doesn't hurt anything. OEMs today are warranting 12:1 engines. But, as in your (), detonation kills, whatever the compression.
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Re: Compression ratio and engine life

Post by digger » Thu Nov 30, 2017 9:36 pm

If I take a given engine and increase compression the temperature and pressure go up so do the loads and thermal stresses. It's extremely difficult to quantify what effect this have on life You won't find the answer here though as with everything it depends and why OEM spend millions trying to find out

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Re: Compression ratio and engine life

Post by joe 90 » Thu Nov 30, 2017 9:52 pm

Higher compression = more detonation when tuned wrong= hammered bearings and blown head gaskets.




The important part is.......when tuned wrong.

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