As you increase compression what is the result. 10:1, 11:1, 12:1. What is the difference, what are some problems and what has to be done to the fuel?

 

The higher your compression, the faster your fuel will burn. Therefore, you must also use a higher octane fuel with higher compression ratios. Too low of octane fuel will pre-detonate, or ping, because of the higher cylinder pressure causing the fuel to ignite too soon, and you'll quickly ventilate the top of your piston, or worse. If you don't have experience with jetting, and access to hi-octane fuels, you probably shouldn't raise your compression to high.

 

If it were my engine I would use a minimum of 100 octane at 10:1, which means aviation gas, or failing that, a mix of 92 octane pump gas & race gas, which would give you a little better than 100 octane, in my opinion. There are additives on the market claiming to increase octane numbers but I know of no one who has used them or whether they are just snake oil.

 

Fuel's been covered; you asked in general about "problems." Although higher power is produced by higher compression engines (I won't explain why here), "there ain't no such thing as a free lunch!"

Higher compression/power means additional heat load; most engines can dissipate it; if not, uh-oh!

Higher compression makes valve function more critical; a valve that seals at 9:1 might not at 11:1; valve reconditioning time can come sooner.

The rod/pin/crank/bearings/drivetrain ALL mechanical components are stressed more by increased compression (Newton said something about this, although engines were past his time).

Now, having pointed out these factors, reducing reliability and durability as an expense of more power, do I try discouraging you from hopping up your mill? Nah! Go for it, man! Dig! Dig!

As the legendary racer, Tom Slick, often said, "There's no such word as "fail" in motor racing, Marigold!"

Tree Farmer

 

A few slight misconceptions here. First you must specify 2 or 4 stroke. Compression ratios are a little different for either. On a 4 stroke 11:1 is usually ok on 92 octane, it really depends on psi of the compression stroke and piston and head configuration. On a 2 stroke with twice the power strokes and higher revs. 9.5:1 or 10:1 is pushing it on 92 oct. Also if you run a higher compression on 92 then switch to race fuel you will usually have to rejet.
I do agree if you do not know how to keep a modded motor tuned, you may want to leave well enough alone :-)
Hope this helps
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