Compression for aluminum heads 91 oct

[Katz427]

What compression ratio with a fairly streetable cam using either BBM or Survival heads is reasonably safe. In this neck of the woods 91 octane non ethanol is it. Wondering how far some of you are pushing this. ?? 445 stroker

[plovett]

I'd say 10:1, possibly 10.5:1 depending on the specifics.

More info would help.  The definition of a "fairly streetable cam" varies greatly.  Also, displacement, car weight, tranny type and stall speed (if auto), and rear gear ratio would help.  Even how efficient your cooling system is will make a difference.

I run a 10.5:1 FE on premium pump gas with no trouble.  The cam is 279/287 and 252/260 @ 0.050, installed on a 105 degree intake centerline.  I put 92 or 93 octane in it most of the time, but it runs fine with 91 octane. 

JMO,

paulie

[blykins]

No reason to go over 10:1.  The horsepower increase in a half point or a full point at that level is going to be negligible.  I'd much rather have the ability for the engine to run at a lower octane rating if necessary. 

[Katz427]

Thank you ! That is what I need to know. The car is a 1966 galaxie with a wide ratio toploader. So a fairly heavy vehicle. I do have access to 98 octane Sunoco about 12 miles away but it is laced with MTBE  for oxygenator. Kind of pricey though. $10 / gallon

[e philpott]

I have no experience with cars , but on MX dirt bikes you have to re-jet for the oxygenated stuff

[Falcon67]

As noted, a lot depends on the intake duration and other factors.  There is a myth that aluminum heads will handle more compression before pre-ignition but testing shows that as a false premise.  I've run 10.5:1 with iron heads on a 351C, 238 @ .050 intake duration, 38 ignition lead, 91 octane and one step colder plug.  Same heads on a roller cam motor with similar .050 duration but shorter seat-to-seat timing requires 4:1 mix 91 pump with 110 VP racing fuel to keep the plugs clean.  10:1 with a decent cam and 91 should be doable.  Watch the plug insulators, they will tell you real quick what's going on in the chamber. 

Little surprised on the MTBE, that should be pretty much gone from everywhere.  The supplier has no shield from law suits for polluted ground water so they are taking a big chance that somebody's tanks won't leak.  Nothing wrong with 10% ethanol, got plenty of that around here.  Most pump octane here is 93 at a few - 3 - stores, rest max 91. 

[jholmes217]

I have 10:1 with iron 428 Cobra Jet heads, and have been running 91-92 octane pump gas with lead substitute with no issues. 

[Katz427]

Yes! MTBE is outlawed for street use but for some reason they allow the cars at the local dirt track to use the stuff. I never liked the stuff as it ate up the power valves after a summer. Lots of small engine troubles too.

[afret]

Got Edelbrock heads with 11.2:1 compression.  Car weighs about 3450 lbs with me in it.  Converter has about 5K stall, 4.30 gears, and 32-34* initial/total timing.  285*/293* advertised,  255*/263* duration at .05,  108* LSA. Seems to run OK with 91 octane non ethanol gas just taking it easy around town.  Definitely will run race gas at the track.

[JamesonRacing]

Got Edelbrock heads with 11.2:1 compression.  Car weighs about 3450 lbs with me in it.  Converter has about 5K stall, 4.30 gears, and 32-34* initial/total timing.  285*/293* advertised,  255*/263* duration at .05,  108* LSA. Seems to run OK with 91 octane non ethanol gas just taking it easy around town.  Definitely will run race gas at the track.

Pretty much the same for me, 11.1CR, 254/260 solid roller on 110 C/L, 3500 lb car, TKO600 with 4.00:1 gears.  Run 34 degrees timing, drive everywhere on 91 octane; leave it in at the track.  Has gone 121mph through the lights in street trim.