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Discussion Starter #1 (Edited)
Ok, another stupid question, I know, I know I am full of them :D But anywho....What is the max compression ratio you can use and still run pump gas? Lets say 92 octane. BTW is it the dynamic compression ratio that you have to be concerned with? If so what is the max Dynamic compression ratio you can run and still use pump gas?
 

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Typically 8:1 dynamic compression is acceptable with iron heads and 8.5:1 with aluminum heads.

Kev
 

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Correct me if I may be wrong but I've also heard it's also in the cam profile that determines what octane you'll need.
 

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Discussion Starter #6
DriveWFO said:
David Vizard recommends a DCR of 7.5:1 - 8.5:1 for pump gas.

When they say pump gas which octane gas are they talking about? :confused:
 

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Discussion Starter #7
Dawg I posted a link the other day in the weblink library that has a calculator that helps you determine both your dynamic and static compression ratios.
 

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I thought we had a post about this somewhere. Need to find it and put it in the FAQ before it gets deleted.

I'm reluctant to draw a line to the limits of compression vs octane. A long time ago 6:1 was considered High Compression.

I was talking the other day to Scott at King motorsports and he claims to have street Honda's running 12:1 :eek: with 93 octane. These have VTEC so the DCR is variable as well.

There is much more to octane tolerance than DCR, which btw most of those online free calculators don't figure it correctly or even the same way. If the calculation isn't correct then comparing DCR numbers will be misleading.

More pressure = more power but also puts you closer to detonation. The closer you stand to the brink the better your setup has to be.

Minimum squish is essential. Spark curve must be custom tailored. Air fuel ratio must be smack on and even on the rich side. Air inlet temperature must be minimized or at least controlled. Piston speed rate of change near TDC can make or break a borderline detonator. Atmospheric pressure changes will effect an engines octane tolerance.

The last Engine Masters challenge it was noted that a lot of the competitors who pushed it to the edge were pinging in the dyno cells.
 

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Pump Gas on 11:1 Comp.

Just a quick question, can you run pump gas on 10.5 to 11:1 Compression with Brodix Aluminum Heads?
 

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well yes and no. i have 11.1 with some vic jr heads. depends on how much timing you go with. i have had problems at 32* but i add ocatne in a bottle and can squeeze a little mo timing.
i switched over to the aluminum heads to run pump gas i dont think it was worth the effort.
 

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10.5 compression on pump gas is easily done with the right cam, quench, combustion chamber, etc.
 

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Do a net search for "dynamic compression ratios". Your static compression ratio is only part of the equation for your engines ability to run on lower octain fuels.
 

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re

absolutly....i run 11.1:1 compression on 89 octane. And with the Race Rite heads. I get my compression from 5cc valve reliefs in my SRP'S, 60 cc chambers {cnc option from brodix} a 4.040 bore and a .040 gasket, now this is actually a .033 quench because my machinist ran my 0 deck a little past 0 and my pistons actually stick out about .007. I also dont run over 165 degree temps. I run a stock car Howe aluminum radiator, aluminum intake, water pump, and heads. Now for the kicker i have WAYYYYYY too much cam at 268/[email protected] Im switching to somthing in the 250's @.050 and im still confident i can run 91 octane dependably.
Eric68 runs 11.3:1 on pump gas with a comp Street roller that specs at somthing like 248/[email protected] .050. It can be done fairly easliy, but not with a small cam, the correct cam choice is CRITICAL!!
 

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I am running a comp cams xtreme energy.507/.510 lift with 1.6 rockers which bumps it up to around .540 lift
 

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chevytalk.com is down until monday morning, but if you go to their performance section there is an awesome link called "understanding dynamic compression" stickied there.

due to the site being down i can also not post the link to pat kelleys awesome compression ratio calculator which a very good description on compression ratios as well as a sweet downloadable calc.
 

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Yeah, lot's of duration will bleed of cylinder pressure enabling a high static compression motor to run on lower octane.
 

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DriveWFO said:
Duration doesn't "bleed off" cylinder pressure. The cylinder doesn't begin to build pressure until the intake valve closes.
it has to do with the overlap,something long duration cams usually have a lot of :)
 
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