11.8:1 on 91 pump gas

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?

Can a 383 with forged pistons, rods, and crank with a ballpark compression of 11.8:1 be able to avoid compression on 91 octane?

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

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

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.

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.

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

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.

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 :eek:

it is kinda pricey though at one bottle per fill up

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...

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.

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.

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

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
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?

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.

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.

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

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).

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 ;))

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

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

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

That likely wouldn't matter, unless this car was purposely made to run open loop (very rare, and for extreme situations... and the one time I did hear of a guy trying this it was a speed-density, boosted application and was having problems... until the BLM locker for OBDII applications came out). I also heard of people tossing this idea around for 4/7 cam swap ideas (bank-specific O2 feedback can cause fueling problems), but I've never actually heard of someone using it for that (the only 4/7 swaps on LT1's I've heard of were in Carb'd or FAST PCM situations).

Even the most radical NA builds (way beyond this run of the mill 383) just adjust lookup tables and switch DTC's on/off, there's typically no "coding" or algorythm adjustments needed (the OEM PCM is VERY robust and makes table adjustments cover nearly every possibility). There are a few exceptions of course (like a BLM-locker), but most everything is "just" a matter of changing the right table value, flag.

There are very few advantages to disabling the O2 sensors... my money's on this 383 just having a normal spark/fuel change, no change on O2 operation.

As mentioned, you should have no problem with detonation. higher altitudes can use lower octane, and even if he DID drive it to sea level on 1 tank :p he HAS knock sensors... they can adjust spark to compensate until the next fill up. (but honestly my build probably has more compression than this 383 and we run 91 octane at sea level ALL the time. Nothing spectatular if you have functional O2's and knock sensor (and a big enough cam :))

That likely wouldn't matter, unless this car was purposely made to run open loop (very rare, and for extreme situations...

Tons of people run open loop tunes. And most of them are not for "extreme situations", they are simply due to tuner ignorance or incompetence.

I prefer to not assume how the car in question was tuned, especially since that could lead to the OP getting screwed.

Tons of people run open loop tunes... due to tuner ignorance or incompetence. Ok, iff you say so. Most incompetant tuners (I'm not gonna name someone specificly mind you) wouldn't know HOW to set up a open loop tune properly, but that JMHO.

I prefer to not assume how the car in question was tuned, especially since that could lead to the OP getting screwed. ummm... ok, apparently you didn't read the thread or know what open-loop tuning requires. You honestly just can't do an open-loop tune at 14.7:1 A/F like you can in closed loop with O2 feedback. An open loop tune almost universally requires the tuner shoot for a richer A/F to avoid lean-out in the various conditions it can see. If the car happens to be a open-loop tune it's already rich.

This is all beyond the point however... if it's fine in texas with 93, it'll be fine in colorado with 91. Like I said, we run more compression at sea level in this area without issues.

:rolleyes:

That cam isnt too compression friendly woulkdnt you say.