## Octane vs. Compression Ratio/PSI Chart

So I read that you need higher than 87 (octane of regular gas in va) once you get higher than 12.5CR, which is roughly 180 PSI. (SteveW I think that was you)

anybody have any sort of chart that displays roughly as your CR goes up your octane needs do as well.

I milled some heads yesterday. We had a bike up to 210psi yesterday, but definately started pinging with the 87 that was already in there.

So basically I want to know whatever it is; for example higher than 180psi you need 89; higher than 210 you need 93 etc.

## Re: Octane vs. Compression Ratio/PSI Chart

i run 91 on my maxi and its got almost 2400psi of compression. works well i guess.

I'm also super curious as to how one determines this accurately.

## Re: Octane vs. Compression Ratio/PSI Chart

Good luck finding a detonation chart thats accurate. too many variables. I guess If you had a chart for one specific engine it would be semi accurate.

Just run premium. Its not going to cost you a whole lot more(we are talking mopeds here).

My rule of thumb has always been 10:1-13:1 premium, higher than 13:1, race gas or premium with an octane additive. I know there are a lot of motors out there that can get way with that compression without race fuel, but better safe than sorry especially on an aircooled engine. Just FYI, just cause you're not hearing "pinging" doesnt neccesarily mean your engine isnt detonating.

## Re: Octane vs. Compression Ratio/PSI Chart

holy crap 2400 PSI!!

## Re: Octane vs. Compression Ratio/PSI Chart

Too many variables to make such a chart, joseph.

Lol at 2400psi. Mine is over 9000.

## Re: Octane vs. Compression Ratio/PSI Chart

And I don't think Elliot was kidding. He just hit the zero key too many times. As Mr Snider says there are a lot of variables. Error on the high octane side for that rare situation like you are pulling up a steep grade on the pipe on a real hot day. I run 230 psi on 92 octane with an occassional splash of octane booster.

Over 9000!?

## Re: Octane vs. Compression Ratio/PSI Chart

engine compression (psi) and compression ratio are almost completely unrelated. depending on exhaust port duration and positioning, engine compression (psi) values can vary greatly on the same cylinder/head combination. dont count on compression (psi) values to determine if you're compression ratio is high.

with that said however, milling your head will give an increased engine compression AND compression ratio if the cylinder port timings are unchanged. so in this case your CR has increased leading to detonation. i would probably just go up in octane ratings until your detonation is inaudible, then perhaps up another octane rating once you stop hearing the pings just to be safe (at the expense of small loses in power because of unburned fuel).

## Re: Octane vs. Compression Ratio/PSI Chart

Also for note, because the exhaust is open on a 2-stroke for most of the actual compression stroke they have a seperet rating called corrected compression ratio, which is the volume of the combustion chamber at TDC divided by the volume in the cylinder when the exhaust port is closed. But it all kind of goes to hell with a pipe anyways due to the charging effect of the waves. I've read that some scooter race kits run as high as 15.8:1 but they also have an exhaust duration of about 190 degrees, and only do about 140 psi, on the stand with me kicking it, the pipe will make it much higher at running speeds. Also read that a 125cc with a proper pipe can have as much as 144cc of gas in it due to the pipe, (something like that).

## Re: Octane vs. Compression Ratio/PSI Chart

Some people figure cc displacement from the top of the exhaust port to the top of the cylinder

## Re: Octane vs. Compression Ratio/PSI Chart

It is the compression ratio.

and that AKI

## Re: Octane vs. Compression Ratio/PSI Chart

what other variables are there?

can we narrow them down a little for mopeds?

## Re: Octane vs. Compression Ratio/PSI Chart

Volume of the head

Exhaust Timing

Squish Band/Piston Clearance

those are the three variables for compression ratio.

## Re: Octane vs. Compression Ratio/PSI Chart

Yup. And variables for detonation:

Temperature

Octane-number

Compression-ratio

Dunno if those are all the variables, but they are the major ones.

## Re: Octane vs. Compression Ratio/PSI Chart

Don't forget timing

and the majik.

## Re: Octane vs. Compression Ratio/PSI Chart

all i know is my puch that read 24:1 on the compression tester, would not idle. and basically ran like crap.

spark does not like jumping the gap under pressure.?

~SABAT!

## Re: Octane vs. Compression Ratio/PSI Chart

The spark doens't need to jump the gap under pressure, as the engine would detonate the fuel b4 the spark occurs when you're running 24:1.

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