[luca_brozzi]

How much CR is safe to run with pump gas? Can you put toulene in to boost octane, and if so how
much.

[luca_brozzi]

How much CR is safe to run with pump gas? Can you put toulene in to boost octane, and if so how
much.

[Goatman]

Great Name! Sleeping with the fishes!

About 9.5:1 with cast iron heads is safe as long as your octane is what it says it is.

Most octane boosters are junk. Tolulene is a dangerous substance and I'm pretty sure its illegal to add to your gas.

[Tom Vaught]

I agree with Goatman, on the additives. Most of the stuff out there will kill you if you make any kind of a mistake adding it to your tank. There was a time when Gale Banks was very big on the stuff in the early 80s with turbo stuff. I talked
to him at Bonneville this year and he was using
other stuff now. Alcohol!! Rather than mess with
the octane.

[Phil69GTO]

Although tuning can make a big difference, I think anything much above 9.5 or so with iron heads is getting into dangerous territory. Gasoline quality isn't getting any better these days. Over-the-counter additives are worthless in my opinion. The small incremental horsepower increase from moving to 9.5 to 10.0 for example just doesn't make sense for a street application.

[Dave A]

The above advice sounds sensible to me so far. There is no hard and fast 'limit' to CR to run on pump gas, and folks certainly have ventured higher on pump gas with iron heads than 9.5:1, but 9.5:1 is a very sensible number to work with.

The exactly danger point will be a function of many other factors, such as cam timing, exhaust system, intake system, combustion chamber design/mods, vehicle weight and gearing and converter, engine materials, cooling system, timing curve, etc etc etc.

[Brian Baker]

.030" over 455 with 6X heads (measuring 86cc), deck height = .013". CR is therefore computed to be 10.12:1. Total timing = 34 degrees (all in by 2600 rpm). AC-R45TS plugs gapped @ .045".

Been running it this way for 2.5 years now with either Shell 93 or Amoco 94. No signs of damage to engine by detonation. Shortblock is still holding up strong after countless hours of street driving (some of it commuting to and from work) and over 125 passes at the dragstrip.

I think cam selection does play a small role in whether CR's above 9.5:1 are safe to use with pump gas. Cylinder head design and material also plays a role (how else could Chevrolet have over 11:1 CR in their new smallblocks?).

--------------------------------------------------

79 T/A, TH400, 3.23, 4150 lbs. race weight:
12.78 @ 106, 1.81 60 ft. "on motor"
11.91 @ 115, 1.75 60 ft. "on 150-HP n2o"

[Brian Baker]

Forgot to mention, ET's I listed in previous posting are on DOT tires and through the exhaust. I do however mix race gas when spraying n2o, just to play it safe.

[Tim Corcoran]

Most of us do not have 93 and 94 octane fuel available at every station around town. The highest I have seen here in Arizona and California is 92. When it comes to compression in a street car it just isn't good sense to push the compression limits for the small gaim in performance. I'd say 9.5 MAX for a street car with iron heads.

Tim Corcoran

[Brian Baker]

Not disputing that point, Tim. Just sharing info based on my own personal experience.

[Dave A]

Hmmmm, the octane of available fuel seems to go down with increaseing altitude, such as those found in Az, but so do octane requirements.

I also run about 10:1 on 93 octane, and have done so for 10 years with no evidence of detonation after teardown, but my preference would be for slightly less static compression and slightly more safety margin for tuning.

The new LS-1's also benefit from knock sensors, electronically controlled fuel injection and spark, smaller cylinder bore, efficient combustion chambers, CFD designed cooling system, in addition to the aluminum heads already mentioned.

[Skip Fix]

Brian from the factory my 78 6X motor took R45TSX (extended gap) and used 0.060 with the factory HEI.

[Brian Baker]

I'm running a 462. HEI triggered by an MSD-6AL (which is only good for improving idle quality and having a built in rev limiter). I've tried .060, .050, .045, .040, and .035. Noticed no difference in performance from .045 to .060, so I settled on .045