Safe compression ratio?

[stovie]

Ok what's the highest compression ratio you can hit with a stock engine before needing reinforcement?? I was looking on line about ethanol and other fuels like it, and they say that if you can hit like 14.5-1 cr you'd get better FE out of them then with gasoline but I've been told that if you did that to a stock engine you'd blow the pistons out the bottom or else destroy your main bearings really fast. Any ideas??

[Sean.Heihn]

Depends on your engine. Try looking on a high performance/tuning website for your vehicle.

[some_other_dave]

The compression that you can run depends on a whole myriad of things. From stuff like ignition timing and fuel octane rating, to combustion chamber shape and cam grind, to cooling efficiency, and more.

The real problem with high compression isn't blowing the head off the motor, it's pre-ignition. AKA "pinging", "detonation", or "knock", that is where the mixture lights off in the combustion chamber before you want it to. The high temperatures and uneven burning can lead to hot spots on the valves, or on the pistons, or on the head, and those can over time lead to parts failure. I've seen pistons with holes burned right through them from detonation.

Most modern cars have knock sensors and will retard the ignition if they detect detonation. Most older cars do not, so they tend to leave more safety margin before knock would occur--so you can make more gains in them by increasing the ignition advance or increasing the compression than you can with the newer cars.

-soD

[Allch Chcar]

It has been done before with E85 in a built engine. I've heard of at least one big block hemi with 14.5 SCR on E85. I believe it was an old carbed engine too. DOE did some testing in a Diesel engine converted to a one cylinder and tested both Ethanol and Methanol. The test engine was ~19:1 Static compression ratio. There is a study on E85 that used another one cylinder engine, Direct Injection, and static compression ratios from 9:1 to 13:1. Needless to say, E85 was fine at any ratio while Premium Gasoline was knock limited.

It's hard to find an off the shelf set of pistons to go as high as you're talking. But it depends on the engine. I have heard of a few builds that used moderate compression with turbocharging. We're talking 12:1 up to 13.5:1 on 2.0L Honda engines.

Pre-ignition/knock/detonation is public enemy number one.

[Big Dave]

Would the honest 120 octane of CNG allow 15:1 compression? Or higher?

[stovie]

I believe so sense ethanol has a octane of 116! So I was looking into it if I get a 13.5-1 cr piston then mill 2mm(I believe that was the measurement??!?) I should then have 14.25-1 cr. Then if I get a 4.06" crankshaft, I should then be just over 15-1 cr. I believe there was a head for my jeep that went from(I believe) 25cc's to 18cc's, what would my cr be then?? And would the inline six be able to handle it???

[IsaacCarlson]

14:1 seems to be the max for gas engines. 12:1 is the assumed safe limit.
10:1 is considered good.

[ksa8907]

You will probably want forged internals, ie crank shaft, rods, and pistons.