- BB Mopar 452 c.i. - 4.380" bore x 3.75" stroke w/ 6.76" rods (1.8 rod ratio)
- 3800# street car w/ driver
- 4.10 gears & 275/60R15 DOT drag radials
- 3-speed auto trans & 4500-flash converter
- Cruise RPM @ 65 MPH approx. 3500
- Expected shift RPM ~ 6500
- Expected peak RPM 1/4 mile ~ 7000
- 160* thermostat w/ hi-volume water pump (although subject to higher temps if stuck in summer traffic...)
Operating environment:
- Mid-Atlantic region (Washington, D.C., metro area)
- Wide range in temps during race season (50s to mid-90s) & high humidity levels prevalent in summer
- Local tracks are generally close to sea level up to ~ 1000 ft. elevation
- Driving distance to local tracks ranges from 60+ to 100+ miles (one way)
#1. Same short block, etc., but two versions of the same head casting w/ similar (not identical) levels of porting, but different chamber designs & chamber volumes that impact the quench area & CR. (SEE PICTURE AT BOTTOM OF POST)
IF they were the same volume (and they're not, but that's another question), would the "small" chamber w/ the increased quench area typically result in:
a) ability to run a higher CR than "big" chamber with same octane fuel, or...
b) needing less total timing, but still requiring the same octane, or...
c) some to-be-determined combination of both 'a' & 'b'
#2. The previous combination on the same short block used a different head that had a 78 cc chamber (which was actually close in quench area to the "small" chamber version of the new head), with a comparable-duration cam. Here's a comparison of the calc'd CRs:
a) Original heads w/ 78 cc chambers & .040" quench ==> 10.70
b) New heads w/ "big" 76 cc chambers & .040" quench ==> 10.91
d) New heads w/ "small" 71 cc chambers & .045" quench ==> 11.32
d) New heads w/ "small" 71 cc chambers & .040" quench ==> 11.47
(I won't bother w/ the "Adjusted Compression Ratio for Intake Valve Closing Point" calcs after reading through some of the old & lengthy threads here re: "Dynamic Compression Ratio."
)Combination '2.a' ran fine on the street & track w/ the local 93 E10 pump fuel at 35* total timing. After doing some research on what a couple of the race fuel companies provide as guidelines for using some of their fuels, I put this s.w.a.g. chart together for expected octane requirements:
If 10.7 = 93 min
...10.9 = 94 - 95
...11.5 = 97 - 98
...12.0 = 99 - 100
Anything above 93 E10 is going to require me to blend it w/ higher-octane fuel, at least when the car's being run at the track. I have a local Sunoco race fuel source, in addition to what the local tracks sell on site, so that's feasible... just more expensive to do it.
Sorry to meander so much before getting to the final questions:
==> Do my CR-to-Octane estimates looks reasonable for the described build & application?
==> Would you expect the differences in the chambers used in the 10.9 CR build and the 11.5 build would change the octane requirements? I.e., would the "better" chamber in the higher CR build offset some of the need for increased octane?
