Kredz28

Can a 383 with forged pistons, rods, and crank with a ballpark compression of 11.8:1 be able to avoid detonation on 91 octane? The guy who currently owns the car lives in texas and runs 93 in it all day, but in Colorado, we can only get 91 ;(

It does have an MSD timing controll unit on the car, so would I maybe need to manually pull a couple degrees of timing out of it?

dangalla

if you want to avoid compression just pull out the spark plugs, that ought to do it

if nothing else just throw some octane boost in it, to avoid detonation of course

jasonface

Probably not, but if it has the right cam and you pull a bunch of timing, you might just squeak by. It will be right on the ragged edge though. Is this going to be a daily driver? If not, plan on octane boost or maybe mixing some 100 octane racing fuel and have fun with it the way it is.

96capricemgr

Depends on the cam. The reason you get low octane like that is at altitude there is less air density so cylinder pressures are lower and you need less octane.

Octane boosters are useless, when they say "10 full points" they mean from 91 to 92 octane not from 91 to 101 octane. xylene and Tolulene are common paint thinners with octane ratings up around 115, guys have used them to mix and gain octane. I think Tolulene is used in making some illegal drugs though so I think it has gotten harder to buy it by the 5 gallon pail.

Injuneer

As pointed out... when you are at 6000-ft altitude, 91-octane is like racing fuel .

I've seen numbers published that indicate the octane requirement will drop anywhere from 0.2 (R+M)/2 units, up 2.0 RON units for every 1000-ft of altitude. If you just assume one octane number per 1000-ft altitude, your 91 will work like 97 would work at sea level.

Another way to look at it would be to consider air density. The density of air at 6,000-ft is only 80% of the density at sea level. Multiply 0.80 x 11.8 = 9.45

dangalla

that stuff works great, my buddy used it on his 03 cobra with a whipple and 28# making just over 700 at the wheels...... as a daily driver

it is kinda pricey though at one bottle per fill up

Me>you

ballpark compression??? Like does it plump when you cook it???

Should find out for sure what the static and dynamic compressions are...You could be alright, i ran a similar number on an LS1, and had no issues...All in the tune...

SS MPSTR

As said, the valve timing events, DCR, tune and quench will all determine how much SCR the engine is going to like with the fuel you have.

Kredz28

The cam its got in it is a 226/234 114 lsa .560 int/ .580 exh

Kredz28

Thanks for the info...reason i posted this thread is because I am in the process of buying a car with this engine from a guy in Texas and am unsure how it will run at my elevation. When I get it here, should I for sure get it on the dyno for tuning or can I sneak by on the existing tune?

WS6T3RROR

Should be able to get by on the existing tune just fine, depending on how it is set up, if its open loop then you'll have to tune it or it will be rich everywhere. Closed loop should be fine though, and if anything wot may be a bit rich.

Kredz28

See I heard the same thing! I know that the computer goes into closed loop once the O2 sensors reach a certain temp right?

Injuneer

The PCM goes into closed loop when 1) O2 sensors get hot enough to work (~600+*F), 2) the coolant reaches minimum required temp (~120-140*F) and 3) a timer times out (typically about 200+ seconds).

Steve in Seattle

11.8:1 in Colorado? I'd be shocked if you couldn't make that work.

I'm running 12.5:1 at sea level on 91. Just pick a big enough cam and you'll be fine (shoot for less than 9:1 DCR )

sbs

With a stock tune, yes. Presumably this car doesn't have a stock tune.