What compression ratio range will work with 87 and 89 octane fuel with Vortec heads and a mild cam. I am building a daily driver 350 that will see an occasional trip to the track.

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1986 IROC "Bone Stock" (for now)

 

too many varibles to say for sure and some engines bulit alike have differant fuel needs. best thing to do is buy some 87 and see if you have any knock. you'll need a scanner to look at knock counts. if you have knock step up to the next higher fuel rating.

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ICON Motorsports
1st & 3rd

 

Stay in the low 9's, like 9.1 to 9.4 and always run 92 octane when you go to the track.

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1991 Camaro Z28
5.7L 5-Speed (originally 305)
13.25 @ 107.18 MPH
Southern California
Member: SoCal 3rd Gen F-Bodies
Webmaster: SoCal F-Bodies
-=ICON Motorsports=-

 

I guess Vortec heads are less likely to cause pinging than older heads. In the mid to late 1970's when the only unleaded was 87 octane rotgut, 7.6-8.0:1 was common. 8.5:1 was about as high as you could get. There are some advantages to running a lower compression ratio if you want to get the most out of 87 octane rotgut.

Lower compression will give you lots of flexibility in spark advance profiles. You can put in a really high lifting RV cam without worrying about uncontrollable pinging. A little bit of richening up will solve just about any detonation problem you'll have.

9:1 and above and you'll really have to walk a tightrope with spark advances, the cam you pick and richening up may not fix a detonation problem. As soon as carbon builds up you'll have a constant pinging problem w/low-octane gas.

If you have all that computer stuff with the knock sensors, then it's no big deal. I'm just assuming that you might be building a straight-forward non-CCC 350.

 

Well, Before I had to switch carbs here, I had Mine Timed To just the brink of Ping, That is to Say that IF I used 87, It pinged, If I used 92, It didnt. I was running 8* BTDC with a vaccum advance dist, But That is timed off a 150k mile Dampner So your guess is as good as Mine.



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60 Ranchero - Project ( Money Hole )
85 Sport Coupe LG4 - Daily Driver

Livin' the Stereotype

 

I used to be able to use 87 octane. Now if I use 87 I get a lot of gas pinging when I accelerate. I guess my car is timed for 93 octane.

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Zepher
'86 Pontiac Trans Am WS6, LG4 305, 5 Speed, CF DF Clutch,
Edelbrock 1406 Carb, Edelbrock Performer RPM 7101 Intake Manifold.
Edelbrock Open Element Air Cleaner: Flowmaster Exhaust, 160* Thermostat,
Grant GT Steering Wheel, MSD HEI Coil, Taylor 8mm Wires, Leather 96 Camaro SS Seats,
Alpine CD+Changer,Alpine V12 Amps, Premier TS-1040C Subs, Polk Audio highs/mids.
Best ET: 14.94@91.67
High Speed Run: 157mph
WS6 Trans Am

 

Zepher, I'll bet you have a heavy carbon buildup. Run some GM carbon remover stuff through and then get it out on the highway and run it like hell for a while. That ought to bring the octane requirement back down.