Don Terrill’s Speed-Talk

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.

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

pdq67

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.

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

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.

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.

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

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

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.

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.

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.

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?

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.

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

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




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