I have a 383 running about 11.5:1 compression and I wanted to know what gas I could run it on. Someone told me pump fuel won't run at higher than 10.3:1 compression but I've ran 87 octane on 10.5:1. I was thinking that the 93 would run at 11.1:1 but I don't know about any higher, I could burn LS1's before in my 360 on 87 octane. So what are some alternatives if the 93 won't work? Octane booster or 110 octane fuel? Where can I get it and how much does it cost, or could I mix in some jet fuel or diesel fuel with 93 to raise the octane? PLEASE HELP!!!!

 

93, no more no less

 

11.5:1 static isn't the number you need, you need to determine the dynamic compression, and if it is ~8.5, pump gas will be fine


search for dynamic compression, and you should find links to calculators

 

i agree. 93 octane is good to about 11.6 - 11.7 compression. anything over u need higher octane.

 

Ya need to figure your DCR which is cam related.
Static don't mean s*** as far as fuel octane. Don't let anyone tell ya any different. It's just a reference number.
I have some running 12.5-13.3 static and still running 92 octane.
Figure it up and stay below 9.0 DCR and ya can run 92 with a good tune and tight quench.

 

i run 93 in mine and i'm runnin about 11.7:1. does just fine

 

So your saying 93 octane will support the 11.5:1 compression?

 

Thanks guys, had a old timer tell me today that it pump fuel won't support anything over 10.1:1 -10.3:1. Nothing against old people, they builld fast *** carbed cars. Thanks

 

Well,I'm an old fart and didn't tell ya that. Ya got to take all these factors into consideration when building the engine.
92 octane will support 13-1 built right and the right cam,Again the DCR is what counts.

 

Thanks, I guess...

Anyway, the DCR will tell you if it can run on 91/93 fuel. It really depends on how the engine was designed. The generally accepted rule of thumb is that aluminum heads are worth a full compression point over the old standard of 10:1 on pump gas and if the cam/heads combo is spec'd right, you can run a lot more compression than you think.

As a comparison, my old '89 XJS ran 11.5:1 compression in its 5.3l V-12 (all aluminum block/heads) and the owners manual called for 87 Octane. Although it made me wince the first time I filled it up with the cheap stuff, it ran great and had no pinging whatsoever. As it got carboned up over the years, I had to switch to 89 Oct. fuel, but that took over 110K miles to get to that point.

 

So I need to find my dynamic compression ratio???? okay.

 

Do that.

 

http://www.wallaceracing.com/dynamic-cr.php is a calculator but I don't know everything needed.


Bore in Inches : 4.060
Stroke in Inches : 3.750
Rod Length in Inches : 5.7
Static Compression Ratio : 11.7 (:1) (not sure http://store.summitracing.com/partde...60%2D8&FROM=MG)
Inlet Valve Closes ABDC : ????? (0 to 90 Degrees @ .050" Lift)
Boost Pressure in PSI : 0
Target Altitude : (Feet) 0

So what is the Inlet Valve Closes ABDC : (0 to 90 Degrees @ .050" Lift)

my cam is .584/.579 w/1.6ratio rockers

If i use 60 degrees it says

Your engine summary is as follows:
Bore - 4.060 inches
Stroke - 3.750 inches
Rod c-c length - 5.7 inches
With a static compression ratio of 11.7:1.
Your camshaft specifications call for an inlet valve closing of 60 degrees ABDC (after bottom dead center).
Your chamber volume is 74.35 cc's. With this camshaft your dynamic, or effective stroke is 3.05 inches.
Your dynamic compression ratio is 9.70 :1 corrected for cam timing, altitude, and rod length.
Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 203.63 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 9.70 :1.

 

Ya have to use the ADVERTISED duration spec's as the valve isn't closed @50.
If ya don't have them on your card ya will have to call your cam manufacturer and get them.
Then,if ya send me the numbers I will figure it for ya.

Old farts are good for more than one thing,ya know.
Ya know ya are old when ya take a nap and other people wonder if ya are dead.

 

Here's the cam. http://store.summitracing.com/partde...68%2D8&FROM=MG Thanks for the help lol.