Blake-73

I just stroked out to 383 which raised my CR up to 11:1. Any suggestions on
what octane I need for this to run ok? I am thinking I might need something better than pump premium.


Blake - 73

63Banshee

Assuming the true compression is 11:1 it is safe to say that your days at the pump without 2 cans of octane buster or 3 gallons of 108 octane racing fuel are over. Or back the timing down to 32 or 33 total and use one can buster.

rainman69

Boy, it's times like this when I wish I knew what my compression ratio is...Stock it was 11:1 (advertised). Now, who knows, but I don't seem to have any problems out at 36 total advance with 91 pump gas. (at 6000' above sea level).

:smash:

QuickVet

i would use sunoco 94. :yesnod:

gearheadz

I have 11.0-1 and have no pinging w/93 octane pump gas & 36 degrees total advance (all in by 3000). Aluminum heads & a good cooling system helps alot.

Mark

rainman69

Well, I have stock 186 heads (I think, I know they're 186, but I don't know about porting) and a good cooling system (stays at 190* all the time)...

So, do I have 10:1, more or less...ARGH! I wish I knew more about the internals!

:smash:

Les

Mark,
You pretty much addressed the key areas. If he has aluminum heads on his 383 he's got a better chance of running an aggressive curve, and better yet if he's got an improved cooling system. I'd also emphasize the importance of dialing in the jetting on his carb to make sure he's not lean. After that, it would be time to increase octane if he's having problems.

Hib Halverson

To properly answer this question the following information is need.

Cylinder head material
camshaft specs.
spark timing curve
altitude in the area the car will be operated at WOT
approx. air temperature in the area the car will be operated at WOT.

Why all this?
Well, an engine with cast iron heads, a short duration hyd. roller and 38 deg. adv that comes in quickly that is run in hot weather at sea level is probably going to need 95-97 oct. But an engine with aluminum heads, a long-duration flat tappet cam and 34 deg. adv that comes in not too fast that is run in Denver in the winter will probably run ok on 90-92-oct gas.

mountainmotor

Yes and much more including rod length,combustion chamber design,car weight and final drive gear.Torque converters and on and on.

Blake-73

Thanks everyone....here is some more info:

Cam: 270H Comp
Location: Atlanta, GA (temp in winter 40-60 degrees, altitude 500-1000FT)
Heads: 041 Cast heads from 1969 (64cc)
24 degree mechanical advance

So far it hates any initial over about 2 degees, pings like hell at WOT at anything higher. Either I am lean or low on octane, or both. What to do?

mountainmotor

Not saying this is entirely your problem but that 600 carb is hurting you assuming the jetting is stock.Even if it is not a Holley 750 Double pumper is in order performance wise and it has a different fuel curve than the 600 which will help by that point alone.If you get one,square up the front and rear jets as a starting point with 72's.
What fuel pump do you have and what is the brand and part# of the pistons?The cam is not put in grossly retarded is it?Like a mistake made by using a multi key bottom cam chain gear?Any vacumm leaks been checked for?Tight valves?


[Modified by mountainmotor, 8:39 PM 11/26/2001]


[Modified by mountainmotor, 8:43 PM 11/26/2001]

Blake-73

Pistons are Claimer part #H3427 for 4.03 bore, and the 600 cfm is stock setup.
http://www.kb-silvolite.com/Claimer/...20383%27s.html

mountainmotor

Are you running 6 inch rods with that piston set?

Blake-73

Yes, 6 inch rods....

Blake

Blake-73

ALso, cooling system is good. Weiand water pump, freshly reconditioned
radiator, 180 degree thermostat.

mrvette

I dunno there, it seems to me running anything over 9.5 to one on the street in average driving is nearly impossible with carbs and today's gas and iron heads.....detroit didn't reduce compression down to 7.5 just for their health in the 70's....the gas is about 1/2 diesel these days....even worse than available in the 70's.......
now switch to DPFI and aluminum heads, you should be fine....
mostly it's dependant on the cam like other's have said......a wild cam will blow off alot of that compression, but won't idle worth crap and is hell to live with on the street....a mild cam idles nice and is friendly to drive, but with all that compression it will tend to run HOT in the 100f summers and ping like a bag of marbles....

GENE

mountainmotor

Among the above other things to check it would be good to get a cranking pressure reading from one cylinder.

gkull

How did you get 11-1? I was able to run 10.7 with iron heads and 92 octane. You have to polish out the chambers, tight .040 ish quench, run a slow advance curve, and have a cam even timing to keep dynamic C/R within limits.

Mountain Motors question is very valid. Once you get cranking pressures up in the 185+ range the dynamic c/r is requiring fast burn chambers and thermal coatings to run 92 octane.

Matt Gruber

blake
try an air gap mainfold with cold air intake.

or just get some D dish pistons

u need some good heads- like ALUMINUM AFR
y run antique heads on a 383?


[Modified by drives61, 3:31 PM 11/27/2001]

Blake-73

Will the air gap fit under the stock L-48 hood? I heard it would not.
I wish I had gone with Alum heads now. The 041's big valve heads are nice,
but I do need the alum's. Will the air gap fit?

Blake