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Octane requirements #2625719
02/25/19 01:21 AM
02/25/19 01:21 AM
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Great Neck,LI,new york
hemi-itis Offline OP
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1. what is the max allowable CR for a 2nd gen hemi with Iron heads with pump gas?How much more with aluminum heads?
My problem is with a 545 inch 2nd gen with a 1071 stage 3 BDS roots.According the general charts available I can go to 6 lbs of boost before more octane is needed.My static CR is 9:1.I have seen intake air temps as high as 160 when it's 90* out.
I pm'ed back and fourth with a w/m expert and he said his witches brew will support 110 octane.According to this chart my CR goes up 1 point per lb of boost.I also have an MSD box where I can plot the timing every 100 degrees and a boost retard programmable by 1/2 lb increments.
How does lowering the intake air temp effect timing & boost.

I have been thinking too much,but I want my 8.50 timeslip this season!
Also,I dont see how CID effects all ths..........

Compression-Ratio.jpg
Last edited by hemi-itis; 02/25/19 01:38 PM.

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Re: Octane requirements [Re: hemi-itis] #2625757
02/25/19 05:29 AM
02/25/19 05:29 AM
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astjp2 Offline
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Google a dynamic compression calculator, that will give you a better indication of what you can really run. Cam LSA can really change the amount of boost an engine can tolerate. Tim


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Re: Octane requirements [Re: hemi-itis] #2625812
02/25/19 12:50 PM
02/25/19 12:50 PM
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polyspheric Offline
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All that table does is multiply boost pressure + atmospheric pressure (presumed to always be 14.7 psi) ÷ atmospheric pressure × static CR.
Doesn't work like that.


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Re: Octane requirements [Re: hemi-itis] #2625819
02/25/19 01:17 PM
02/25/19 01:17 PM
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hemi-itis Offline OP
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Good question Jeff! I do know that boost sensor will read between 14.5 & 14.7 depending on the atmospheric pressure and must be taken into account when programming the boost retard.
The way these charts are presented I would think the 14.7 is taken into account work


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Re: Octane requirements [Re: hemi-itis] #2625826
02/25/19 01:26 PM
02/25/19 01:26 PM
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As intake temp goes up air expands. The air charge is confined to the intake track so it will show a higher boost number. Not real usable boost. In my testing more "actual" boost makes more power than more timing up to the point where the compressor becomes inefficient. E85 and alcohol are your friend. To much timing, low octane and heat are not. Alcohol can increase boost numbers as well. Far more liquid that is non compressible in the confined charge area.
Doug

Re: Octane requirements [Re: hemi-itis] #2625830
02/25/19 01:43 PM
02/25/19 01:43 PM
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hemi-itis Offline OP
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I have been looking at W/M inj for while.I have seen claims that they can support octane requirements to 116 realcrazy
Had word with one of the major W/M manufacturers and he said 110 octane.
With where I want to go 114 might cover me.
I know lowering my IATs will be helpfull,just want to be safe and not melt anymore pistons!!


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Re: Octane requirements [Re: hemi-itis] #2625880
02/25/19 03:10 PM
02/25/19 03:10 PM
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GTX MATT Offline
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Originally Posted By hemi-itis
How does lowering the intake air temp effect timing & boost.


The boost gauge will read higher at the high intake temp but the engine will be moving less oxygen.

Low intake temp NEEDS less timing for MBT vs higher intake temp.

Low intake temp will tolerate MORE timing without knock vs higher intake temp.

Your setup is way different, but modern EFI boosted stuff will typically run 2-4 degrees more timing below 50 degrees than it will at 100 degrees. Its hard to get IAT that low unless its cold outside and you're moving though.

Something like your setup without any knock sensor you are just looking for margin of safety (I assume), I would give it the coldest air I could.

I wouldn't base anything off of that table...

Last edited by GTX MATT; 02/25/19 03:12 PM.

Now I need to pin those needles, got to feel that heat
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Re: Octane requirements [Re: hemi-itis] #2625928
02/25/19 04:50 PM
02/25/19 04:50 PM
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polyspheric Offline
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The inside of a combustion chamber is not a math problem, that formula isn't remotely accurate.


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Re: Octane requirements [Re: polyspheric] #2626131
02/26/19 12:17 AM
02/26/19 12:17 AM
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Originally Posted By polyspheric
The inside of a combustion chamber is not a math problem, that formula isn't remotely accurate.

That's great Jeff eekCould you help this old guy learn new tricks?

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Re: Octane requirements [Re: hemi-itis] #2626261
02/26/19 12:22 PM
02/26/19 12:22 PM
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polyspheric Offline
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