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So... I have this 427 bbc 11.5 to 1 alum head motor........... man this is going to be harder than I thought. I guess what I'm asking is what is dynamic compresion and how would you calculate for that . In other words what is the real/best way to know your octaine needs. Oh I'm at altitude also...I guess I need some dyno software or something.:D
 

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you can go to kieth blacks website they have a very easy to use compression calculator and figure your static and dynamic comp.out very quickly need to know the ivc.intake valve closing on your cam and youll be good to go,i use it all the time,hope this helps.travis:)hand
 

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Discussion Starter #3
Thanks hotbo. I'll check that website out. BTW I didn't know the term static compression was used on a N/A motor, I thought it was more for the blower/turbo guys.
 

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More altitude can effectively use more compression.\
What elevation for what compression ? I dunno - I'm near sea level.
However I do know that I'd be asking builders in your area and or those in your exact altitude.
Less air = less effective pressure.
 

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Discussion Starter #5
So.... what's the magic #(dynamic), say for 91 octaine, 9.8-1ish? I can't get 93 here at all. Maybe later in the season I'll get brave and try some e-85.
 

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lots of people safely running 91 at over 11:1 with aluminum heads.
 

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In a jet boat with a cold water supply(like in your avatar) I bet you could get away with 11:1 on 91. The altitude will help and you will need to be jetted real good. learn to read plugs next:)devil
 

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I ran an 11:1 motor at 4500 ft elevation no problems on 92 for years. By my basic back of the envelope calcs at 4500 ft you loose 1 point of compression... making a 11:1 motor here equal to a 10:1 motor at sea level.

Altitude Sucks :mad:

As far as mixing goes, I have mixed Av Gas and pump gas on a 12:1 motor. It actually is a good thing to do to help the AV Gas get back the molecules its lacking The only thing you got to watch out for is Av Gas needs a larger jet, so as you mix you will have re-jet. Now the hard part... how do you know you have the same ratio from tank to tank all the time?
 

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When you come down from altitude, would that 11:1 motor turn into a 12:1 motor? Or is that just crazy talk?:D
 

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When you come down from altitude, would that 11:1 motor turn into a 12:1 motor? Or is that just crazy talk?:D
:)hand a 11 to 1 compression motor would be figured at sea level, not at the bottom of the sea:p
 

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:)hand a 11 to 1 compression motor would be figured at sea level, not at the bottom of the sea:p
:)LegHumpjust cant get a break can ya H.J.:D


i would venture to say 11.1 would be okay,but it also comes into play with your cam profile;)
 

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Wow!!!! I re-read Bigblockbill's post, and now I feel like a complete idiot for asking that!:)sphss I think I need to shut-up for a little while!
 

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Discussion Starter #14
As far as reading plugs, I give up on that. Supposedly I have a set of dry headers on the way I'm going to set up with both an a/f and a pyrometer. When they arive, I'm sure I'll be asking more Q?'s. I assume the pyrometer is going to be the most help with the octaine thing??
 

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This post has got me thinking, about my motor if my motor was built in california on the coast and it was 12.5 to 1 there and i have brought it to Oklahoma are you saying my compression ratio is no longer 12.5 to 1?? Any input and correction would be greatly appreciated........Confused
 

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Discussion Starter #16
I have no idea what the altitude is in okla. less air= less power= less cyl. pressure.
 

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So... I have this 427 bbc 11.5 to 1 alum head motor........... man this is going to be harder than I thought. I guess what I'm asking is what is dynamic compresion and how would you calculate for that . In other words what is the real/best way to know your octaine needs. Oh I'm at altitude also...I guess I need some dyno software or something.:D
Okay this is going to give me a head-ache but the formula takes longer than getting rid of a wife you don't want.Cobustion chamber volume plus compressed head gasket volume plus piston deck height plus dish or dome volome minus cylinder volume at bottom dead center, That would be actual chamber compression ratio. I need a tylonol. M
 

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Discussion Starter #19
As far as head cc's they are broxix bb2+ they were suposed to be 118 cc like the 990's I pulled off but the only cc'd to 116. I pinged the same set up to death the 1st time I put pump gas in it, just now I have alum. heads, and 3cc's less. I'm done guessin fer dollars - I need an a/f and a pyro:)devil:)grn
 

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So.... what's the magic #(dynamic), say for 91 octaine, 9.8-1ish? I can't get 93 here at all. Maybe later in the season I'll get brave and try some e-85.
From what I have gathered, 91 octane is good for 11:1 (near the edge) for a boat motor on 91 octane with raw water cooling run the cooler non-thermostatted setup on aluminum heads.
Iron heads arround 10.5 or so.

If you are really 11.5 or so, IMO you may need to mix in some race gas or av-gas to get 91 up to a 93+ figure at least.

Realize that teh :eek:ctane boosters" you buy (like "104+" boost about 1 point, which is NOT from 91 to 92, but from 91 to 91.1. Label says so if you read closely.
 
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