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Discussion Starter #1 (Edited)
Can i run pump gas (93 octane) or not, if not why?

i have a 347 with aluminum heads with 10.432:1 Static compression ratio.
Using a Calculator i get...

"With this camshaft your dynamic, or effective stroke is 3.04 inches. Your dynamic compression ratio is 9.33 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 193.77 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 10 PSI is 15.68 :1."

To me that sounds like race gas territory.

BUT!!! I see several people running static compression ratio of 9.1~9.2:1 and an f cam (42* ABDC) running 15 psi on 93 octane and ~18* timing.
If i do the same calculations on my engine, so 9.1:1 static compression instead of 10.432, and 42* ABDC instead of 45* and 15psi instead of 10psi i get...

"With this camshaft your dynamic, or effective stroke is 3.09 inches. Your dynamic compression ratio is 8.26 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 165.70 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 15 PSI is 16.69 :1."

So how come i keep being told i cant run pump gas, but i keep seeing people with a higher Dynamic boost compression ratio than me running pump gas?


I am by no means a expert, just trying to keep from blowing my engine up so somebody PLEASE school me on this.


Thank you,
Garret
 

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Discussion Starter #2
I am not opposed to running E85, my problem is that there is only one station with in 15 miles of me, and it is not on or around my daily route. I will run it if i have to but i would like to run 93 if possible.
 

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Look at my sig. 9:1 comp. 3.40 stroke, 5.315 rod. All 93 pump.
 

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Discussion Starter #6
I would assume you have flat-top pistons. What size are the chambers on your heads?
Yes flat top pistons (-4cc) 54cc chamber heads, pistons are .008 in the hole, gasket is .075 thick compressed, 4.030" bore and gasket bore.
3.4" stroke, and 5.315" rod.
 

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The reason you're in race gas territory is because of where your initial DCR is. You're going to generate more heat, and in turn, you're going to detonate. Dynamic boost compression is a direct function of pressure ratio - for example, take my setup. I'm running a DCR of 8.30:1, and a total of 15psi (although I've not turned it up that far yet). 15 psi is a pressure ratio of 2.02 (15 + 14.7 / 14.7) ; so, dynamic boost compression is 8.30 * 2.02 = 16.77:1 ; But, at the same time, if you take one of these hopped up 4 bangers running 8.8:1 static and 30psi are running a dynamic boost compression of 20:1. They can do that because their DCR is down to like 6.52:1.

As a general rule, less compression = more boost = more power ; however, you sacrifice off-boost response. You have to find a happy medium to make it work well together. If you want to use you current setup with pump gas and boost, then you're going to have to ditch your cam and get a new one that bleeds off more initial cylinder pressure.
 

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Discussion Starter #11
Thank you for that explanation, i had a feeling the DCR was more important in figuring out which fuel to use than DBCR.
 
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