Easy enough question.

With cast heads. What compression should you really start useing race fuel instead of pump gas?

 

Boy,, this is going to start some arguments!!

I would say as a general statement, with cast heads and fuel inj, anything over 11 to 1 would need race gas. Of course there are lots of variables like ign timing, combustion cyl temps, head design, coolant flow direction, and abt a million other things.

 

Non EFI - When you can't do a hard accelleration without the engine pinging.

EFI - When the knock count is too high that the computer can't compensate for the pinging. With a knock sensor, the computer will retard the timing until the knock count decreases. This retarted timing will only be felt as a loss of power. The only way you'll know you're losing power is to use a higher octane. The amount of power the engine produces will be the same however if the higher octane doesn't let the engine ping, the computer won't retard the timing and you'll feel more power.

You only need enough octane to eliminate pinging/knock. Any more octane and you're spending more money for something that you don't need. A 12:1 engine can still drive around on 87 octane even with moderate accelleration. It's when the engine goes under a heavy load that low octane and high compression stops working.

Under safe conditions 10:1 is normally the maximum accepted compression ratio for regular 87 octane pump gas. It's really a debatable number. Many might say 9:1 is the best safe number. If the engine is designed properly, 12:1 could survive on 92 octane.

Since no two engines are exactly alike, what works well in one engine may not work in another.

 

:whathesaid:

 

stephen, here's a personalized variant, you think i'll be OK at our altitude, with a 10:1, on 91 octane?

 

Higher altitude can get away with a bunch of things

High altitude tuning:

Lower octane is required for higher compression
More timing can be used without detonation
Smaller jets to lean out the mixture due to less air to burn

In high altitude, I'm usually refering to over 4000 feet in density altitude. Here in Calgary, during the summer, the density altitude normally doesn't go below 5000' in the day time. In Denver it's closer to 8000'.

When I ran my old 383 stroker, the compression was about 10.2:1 and I only used 92 octane.

 

Hi guys. i have sort of the same question. I will be running 99 octane fuel (standard where i live) with a 377sb iron heads and voodoo 233 241 dur @050 and 504 525 lift does anyone have an idea of how high compression i could go with this combo.
Thanks

 

in sweden you can buy 99octane fuel??? geez, theres one more reason for me to go there... (aside from swedish girls )

Thanks Stephen, that's kinda what I thought, I just wanted to confirm it. (never knew about the jets... hmmm)

 

well the bas price for 95 octane (cheapest stuff you can find here) is $5 a gallon and this will run you like $5.50 so.....................
But the girls still look great LOL!!!!!!!!!!!

 

Five dollars a gallon wow, glad I live in the 'States.

 

Your minimum wage must be $25 an hour

 

its more like $10 an hour and yes the nissan micra is quite popular lol.

 

Whenever my girlfriend calls & I'm on the phone with someone else, she'll ask me who I was talking to. I always say "the Swedish bikini team".

 

sorry. did´nt meen for this tread to take this side step, not that i dont appriciate the comments on our lovely ladies but i think somewhere in the begining we were talking aboute compression ratios.

 

haha, swedish girls, I guess it's something about blonde girls

ok ok, lets try and focus here. I think with 99 octane, you're close to race fuel there, assuming there isn't something funky in that to make it low quality ( ), you should be fine with 10.5-11:1. That cam is the 60104 cam right? Decently large.

 

the fuel is called shell v-power and is developed with the race techs of scuderia ferrari (the formula one team!) so i sure hope its good quality gas. yes the 04 cam and 1.6 rollers

 

My LT1 had 10.5:1 compression and I had to use 91. Less and it pinged at me.

 

is the advertized octane of your fuel in Sweden that same as ours here?

Ours: Pump Octane = research octane + motor octane /2

if your "99 octane gas" is rated the same as ours you should be good for 11.5:1.

But if your pump rating is "research octane", its about the same as our 92. good for 9.5-10:1

 

need more specific info.

What motor, what heads, what purpose, what pump gas?

Detonation is like a bomb. Until you light the fuse, its not a problem. You can raise compression up to the point where the fuel self ignites in the motor or explodes just after normal spark ignition instead of burning. up to that point you don't have a problem but suddenly with just a little more compression ratio it starts to detonate for just a little too much heat.

In the real world the operating conditions that your cars sees change. temp, under hood temp, water temp, humidity air pressure, etc etc.
Ya want to get the cr up there to get good performacne but you don't want to exceed what can be tolerated on the worst day or condition. Its a slippery slope.

the power increase you get form increasing the cr by 1 is not linear. its about 4% when going form 7 to 8:1 but less
than 4% when going from 10 to 11:1. sometimes insignicant. But when the motor is detonating the power goes way down along with engine life.
i've found significant difference between different brands of gas as to wether the car will ping or not.
Sunoco seems to be about the best here.

if you give the specifics I'll recomend a safe/usable but aggressive compression ratio to shoot for.
the power difference between a safe/usable but aggressive cr and the absolute maximum optimiun cr for your motor is very very small. A motor is no good to you when its all in pieces. A motor that you have to constantly fiddle with the ignition timing to keep it out of detonation on different days is a pain and gets old in a hurry.
The more specific the info the better.

 

Hold your horses guys. This is not correct. I`m from Norway and Sweden is my neighbor-country.

99 octane he is talking about is NOT what you think. In Europe we measure octane in a different way. 99 octane here is equal to 95 octane in your country/USA.

This has been discuissed in an another tread here. And also how you can calculate difference between US and Europe octans.

And again: The price for this 95octane (here 99 octane) is horrible 7$ gallon. 95 octane here (91 octane in US) is only 25 cent cheaper.

So there is no reasons to move here....... :/