Topic: OCTANE ??

[MATCODUDE]

 I built a 31avs engine last year. Eng had stock avs piston, arc billet rod, precision cam, billet fly wheel, crank was balanced and welded,  billet timing gear, ec dominator carb, and head was shaved .130, pushrods cut to length, and ec roller rockers, compression was 190 lbs on warm eng, ( was checked with 2 different gauges(new)),  I never did check timing, just lined up with second notch. Eng ran great last year, always keep exhaust temp around 1200 F. Always ran Sunoco 110 race fuel, took it out this year ran great for three races then out of know were exploded the piston at 5900 rpm doing severe damage to most components. The only thing I did different was mixed my 110 fuel with 93 octane fuel with a 50/50 mix. So just built new 31 eng, stk piston arc rod, ec billet cam, billet flywheel, 28cu billet crank (with bushing in pan), ec dominator carb, head shaved .100, chrome molly pushrods to length, ec roller rockers. Set timing to 30 degrees, (degree wheel and timing light) checked compression after a little running and was 195 lbs. I know my figures are off some but im figuring that's about 13:1 comp ratio. My question is did my old eng explode because I mixed the fuel 110 and 93 (lowered octane)or was there another problem that I had. While inspecting my old eng for evidence I also found that if you rubbed your fingers around the cylinder wall in felt like a wash board (rippled) all the way around the cylinder and the full length of travel of the piston. Should I continue to run Sunoco 110 straight or should I mix it 50/50 with 93 octane. Any ideas on the rippled cylinder wall, guessing that didn't happen all at once. Any suggestions or comments welcome Thank you

[larry reddinger]

at that compression imnot well versed in the avs engines but I would think running straight 93 is all that would be necessary

[Jeff McKelroy]

You raise an interesting question, I have seen several AVS engines "ripple" like you speak of.  I am thinking higher compression vs. thin cylinder wall, or poor sleeve material, something like that.  But at the same time, Briggs pistons aren't exactly made for what we do.  Personally, I don't think fuel had anything to do with it.  I think it's taking a light-weight production mower engine and lighting that candle on both ends.

[foolnhismoney]

I agree with Jeff for the most part. We put all kinds of money into billet everything and then ask the stock cast piston to hold up its end of the deal. But the cylinder condition you talk about has happened to one of my motors also and would be really interested to find out what causes that problem. I would like to bore it .020 but not sure it wont happen again.

[Todd99]

We we're told to run 87 octane in our motors by our engine builder he told us we weren't going to get anymore performance out of the motor running high octane fuel. So that is what we do and have no problems and also have no problems beating or running with guys we know are running high octane fuel.Oh it's Tony i guess you already know what we run!!! LOL!!!

[PJG56]

At 13-1 static comp ratio, dynamic comp is roughly 9-1 depending on IVC closing point so no need to run any more than pump gas.

[berthyd]

Honest answer from an Indian guy with first hand knowledge of the B&S single...that is an absolute junk piston. You did nothing wrong. It's not the first (nor the last) Briggs piston to disintegrate at 5900rpm. Fast Eddie (Redline) can tell you all about it.

Bert

[Darkviper]

On website I bought my gx440 at  said only thing under 10.2:1 ration Max  is 87 octane. Max for 91 octane is 10.8:1. After that is race fuel or alky. Also any over 6000 will snap crank quick sorry about off piston topic.

[Jeff McKelroy]

And after all, octane is resistance to knock only.  There are many, many more attributes to a racing fuel than just octane.  But you guys are correct by saying high-octane fuel is not always necessary.

[Johnson Mowersports]

Timing and compression. Are what depend on what fuel is needed you need to know exactly what you have. Anything over 11:1 compression and 32 degrees timing should be run on race fuel and the more of the two you have the higher octane you will need. You would rather have more octane and not need it than not enough and cause detonation

[BIG AL 202]

I AGREE WITH ALL THEE ABOVE, THE COMBINATION OF 93 AND 110 SHOULD HAVE BEEN PLENTY TO PREVENT DETONATION. REMEMBER BRIGGS ONLY RUNS TO 3600rpm's. I ALSO BELIEVE JEFF IS COORECT IN THE CYLINDER WALLS ARE THIN. TO PREVENT THAT FROM HAPPENING YOU WOULD NEED TO INSTALL A BETTER SLEEVE THEN YOU WILL FIND OUT WHAT THE NEXT WEAK LINK IS!

[redline]

The only way to keep a Briggs single 31 piston from exploding is to throw it away before installation.

[Rooster]

The only way to keep a Briggs single 31 piston from exploding is to throw it away before installation.

...or NOT turn it 6k?

[redline]

...or NOT turn it 6k?

That's simply

[George Herrin]

problem is in the timing, you aint running 13-1 compression. Not with 190lbs of cranking compression. Unless you used a degree wheel with head off using dail gauges and physically set the timing your absolutely guessing. Use a degree wheel on a avs even with an avs wheel at full negative then tell me what your timing is. Once you determine this truly you will uinderstand. I run 87 octane in my V-twins and turn em as hard as 9500 go figure.