Ive come across a motor build up thats a 700hp/700tq 427 that has 13.5:1 compression and runs on 100 octane. due to my location in the midwest, 100 octane is hard to find. so if i knocked the compression down a point to 12.5:1 will i have detonation problems? wold i be better off at 11.5 or 12.0:1? i may run a 100 shot on top of the motor, so compression will play an important part for me, thanx for the help.

depending on the cam your probably looking to be below 10:1

Depending on cam and altitude you might be able to get away with more or you might have to drop it even more.

Big bore motors like the 427bbc are not very octane tolerant. For reliable everyday performance on 91 octane you'll need an actual cr of less than 10:1. Race motors don't make good street motors. The motor will no longer make 700hp.
I'm suprised it runs properly on 100 octane.
Most pro engine builders build their true "pump gas" big blocks to 9.6:1. 91 is all that is available at most marina's so most hi perf marine motors are 9.6 or less.

You might get it to run on E85 fuel. Thats about your best practical choice.

here is a list of E85 gas stations in Wisconsin
E85 at the pump

for nitrous, anywhere around 10:1 to 10.2:1 should be the highest.. i'm running at about 10.1:1 right now and my machinist said the motor should be able to handle a 150 shot on 92 octane.. but i dont use nitrous so i'm going to up the comp ratio on my motor to 11.2:1.. and still run 92 octane.. my cam is a monster in the duration department so thats why i can..

im running 11.0:1 compresson on my 385 and spraying 300 worth of nitrous on a 2 stage (110 octane in stand alone fuel cell) and it runs off 93 octane just fine and drives on the street with nice manners as well

Unless your block is "0 decked", your cr with flat tops is only 10.49:1 could be even less. which would cruise around town pretty good with a few degrees ignition retard dialed in. But give it the proper amount of octane and max the timing and air fuel ratio out for max power and you'll see what your missing. A true 11:1 compression motor that is not silly over cammed to dump horsepower (cylinder pressure) right out the exhaust valve will need about 98 octane +/-, depending on the weather.

And when that doesn;t work quite as planned what will you do? go to an even bigger, way too big cam? Why not leave the cr as is and put in a better more powerfull cam overall and pick up lots of power and torque and make your car faster with reliability. YOu'll only pick up at best a 4% gain in power (more likely less) by going from 10:1 to 11:1 assumeing it does not ping at 11:1. You will easily pick up that much power and more by properly camming the motor based on what it needs to make the best overall mean effictive cylinder pressure over the usable power band ( (make the most power) within the limitiations of the available octane. Start by flowtesting the heads.

i know this is not chev, but my dad has a 466bbf engine. we run about 660 hp, right now with 91 or 93 octane. with our old heads we ran about 750 on 93. i drove it once when he was out of town (he wont let me drive it) at 660 hp, and it was a beast. why do you want that much power on the street anyway, its fricken dangerous, and i cant imagine 750hp. i know its totally tuned to the max for 93 octane, and it took him like 2 weeks of tuning to drive it on the road like this, but he did something.

^^

True, but you gotta have some power!

Even a lightly moded 350 is plenty to blow the doors off a r!cer, but y'know, everyones gotta have a 6 second car!

oh man, power is so awsome, but i dunno. my dads car gets about 4 mpg feathering the gas. it is not a daily driver. sunday night cruse night, il drive my trans am, and he will drive the ford, and he stops for gas every 40 min, and puts in 60L. my friend works at a gm dealer, and he took out a new zo6. at the end of the summer, when we had the 750hp heads on, my dadio and i to took a race. we dominated that car so bad it was embarassing. in fact, on sunday, for cruse night (where i live, sunnday is cruse night) we even raced a R1, suzuki hyabosa (spelling) and a blown 1932 for roadster. the only thing that came close was the suzuki. we changed the heads because it was to fast. and we get about 8 mph now . i live in canada where there are no freeways, i could not imagine driving the mustang on the freeway. like this car is so scarry, with canada legal street tires, if my dad floors it, it wont get traction until late late 3rd gear. its just scarry. i dont want to be a downer, even though i am, but i dunno, dont expect it to be much of a daily driver.

to answer your question, we run 11.5 cr, my dad told me we have a highy tuned msd spark box or something. i forget the name, anywho there is a computer on board or something that retards the time or advances it so it wont nock like crazy, plus the timing is tuned perfectly. we dont drive it very much because it uses 30L of gas to go anywhere. he usualy uses 100 octane or 110 from work (he a pilot) but i know it can be filled with 93. i dont know to much about it because i never drive it so i dont care to much about it. but something like that.

my 350 gets worse mileage...

(i just found out!)

well if that doesnt work, then i'm going roller.. with the 11.2:1 cr.. and i cant really flow test the heads because no one on this little island is that "all out racer" to have a machine for it.. but i do have the numbers from brodix.. i didnt really touch the heads after i got them, i only cleaned out the combustion chambers and cc'd then.. plus, the cam i have recommends 10.5:1 cr.. i initially thought i had the cr but i found out later i was low..
this is what comp has for the despcription:
SOLID: Street / strip. 3500+ stall, 10.5:1 compression. Low gears, headers and intake. Racy idle.
i'm just gonna stick with this till i think i need more..