High compression, Octane?

[p26575r16]

I am almost done building my 12:1 350x and am wandering what octane fuel should I run? I am at 4500 feet. 93 is the highest pump gas around here, but 104 and 110 is available (but expensive, $6\gal). Wiseco and other builders reccomend high octane but no one say's which. What are you high compression guy's running? Thanks

[84honda200s]

i run 93 in my 200s 12:1 it runs good. but its just a 200s and im in ny.

[SWIGIN]

start with the 110 at least for break in...you dont want to mess it up right away.

the last 12-1 i ran i had to run 110 octane

[DefEddie]

93 octane should be fine without pulling any timing.I'm not sure if you can adjust timing on that,but if you could rig some way to do it then it would be a good idea.adjusting timing a few degree's back and forth would be a serious asset,you could pull timing when you need to run lower than 93 and add a few degree's if you feel like putting the good stuff in it.
It would also help fine tune for very hot/cold day's...

I'm going off auto experience though,so take that into account.But you should be okay on 93 with 12:1 on a car so...

[SWIGIN]

the 350x motor im running now has a 12-1 that i milled down a little to about 11-1 and that will run on 93...if your head is still off you might give that a try

[p26575r16]

swigin, I just assembled the engine, so the head is no longer off. I want it to be 12:1, I have a little grudge match between me and 2 co-workers, so I need every bit of power I can get. I also ported the head and had the flywheel lightened.

[ProCarbine2k1]

theres no reason you shouldnt be able to run 93... my race quad is 12:1 and i normally run 93 or sunoco 94...

[MTS]

theres no reason you shouldnt be able to run 93... my race quad is 12:1 and i normally run 93 or sunoco 94...

Yes, But thats in a Highly Advanced Liquid Cool'd Motor....FAR FAR Better Heat disapation Than an air coold 350x Motor, Run at least 110 for the X.....Or your gona start Burnin up pistons...and when them wisco 12:1's come apart from pre-det...they come apart GOOD!!......Firsthand experiance......200m with a 12:1 and 93 octane pumpgas.......lasted 2 days......

[NOS_350X]

I run 91 on a 13:1 in my 350x. No problems. (ive torn my enigne appart several times sicnce ive beend doing that) IMO the new trx450's are 12:1 and say to run premium, why cant the 350x, compression is compression its the same thing just diffrent enignes. The biggest thing is to make shure its jetted properly and you shouldnt have any problems. Also a cheep alturnative is AV gas AV101LL feels better than the race fules ive tryed and is half the price. But with AV gas you have to jet WAY UP.

[GPracer2500]

Air-cooled vs. liquid-cooled is apples to oranges when talking about an engines octane requirement.

Also, dynamic compression is the important measurment. 10:1, 11:1, etc are static measurements that don't take into account timing of valve events. It's ok to use static when your talking about the exact same model engine--or at least one that's very similar--because they'll tend to have the similar ABDC intake valve closing specs. Dynamic is always better, but static is more convienent because that's all we usually have available.

Here's some 400EX dyn comp info that probably pretty closely matches 350x:

Taking into account the valve specifications:

Stock cam cranking compression at 1000ft = 8.2:1
HotCams stage 1 cranking compression at 1000ft = 7.8:1
HotCams stage 2 cranking compression at 1000ft = 7.6:1

If you go to a 11.0:1 piston (instead of the stock 9.1:1 piston) then the numbers change to:

stock cam: 9.9:1
HotCams stg 1: 9.5:1
HotCams stg 2: 9.2:1

[figures may not be absolute but the comparison is accurate]

The thing that's responsible for the different compression ratios is the timing of the intake valve closing--obviously this is controled by the cam. So, different cams get you different amounts of cylinder compression.

Usually cylinder pressure measured in PSI with a comp tester is a better way to make decisions on required octane. In my experience with air cooled Honda ATV engines, something beyond premium pump may be needed after ~175psi. The auto decompression mechanisms on many of these engines makes using a compression tester an issue. But many aftermarket cams don't utilize the auto decomp mechs so you can get good data in those cases.

Elevation makes a big difference too. 5000ft of elevation is about the equivalent of 1.0 points of compression (i.e. 11:1 at 5000ft will perform like it was 10:1). And don't forget the regional differences in fuel (two fuels may both measure 91 octane in a Waukesha test engine but perform differently in our engines). Typical riding temps, typical engine loads (sand dunes?), jetting preferences (rich a/f ratios are more resistent to deto), and even what gaskets are used all make a difference. That's how one guy's engine runs fine for him at 13:1 on premium while another guy is deto'ing his engine to death at 12:1 on his premium.


My hands on experience that might be relevent is with a 400EX. I had a JE 10.8:1, HC stg 2, stock base gasket, unkown thickness head gasket (stock I think), and it pumped 180psi cold at 1300ft. It knocked on the crappy AZ 91 pump. For trail riding it wasn't bad enough that I couldn't ride around it but in the dunes it was a recipy for disaster. I ran a 50/50 mix of VP110 and 91 pump. That was fine.

[ProCarbine2k1]

although the x motor is not of space age design like some of the newer liquids, i still say that you should be able to run it on premo gas... atleast give it a shot, and be extra cautious of overheating... if she starts showing any symptoms, shut her off drain it out, and shell out the bucks for 110...