Re: How high a compression on pump gas
[Re: cudaman1969]
#3003411
01/09/22 12:02 PM
01/09/22 12:02 PM
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Joined: Jan 2003
Posts: 10,696 North Dakota
6PakBee
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Talk to your cam supplier. If you give him your engine particulars and what octane you want to run, if he's any good he can tell you what compression ratio to run. On an AMC 401 I went this route and with iron heads, stock intake and exhaust, running 87 octane pump swill, the recommendation was absolutely no more than 9.2. How did it run? Still putting it together so we shall see.
"We live in a time when intelligent people are being silenced so that stupid people won't be offended".
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Re: How high a compression on pump gas
[Re: mopar dave]
#3003424
01/09/22 12:40 PM
01/09/22 12:40 PM
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Joined: Jan 2003
Posts: 20,726 A collage of whims
topside
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The Hemi chamber is actually pretty detonation-resistant; one of the reasons it was employed in the Gen1 engines. I had an iron-head 426 Hemi @ 10.8 measured, never an issue @ 36 degrees total, + vac advance, even on CA 91 octane. It did have a fairly big-for-the-time old Crower FT cam, IIRC 247 @ .050, lift in the .540s, which bled off some dynamic CR (and idle vacuum). With aluminum heads, that engine could have gone more like 11.3-11.5 CR, I'd bet. 4-speed, 4.10s, 3700-lb RR. I wouldn't push a wedge that far, from personal experience.
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Re: How high a compression on pump gas
[Re: cudaman1969]
#3003805
01/10/22 01:46 PM
01/10/22 01:46 PM
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Joined: Mar 2003
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EvilB1Dart
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Depending on the overall setup of the build, where the compression/quench zone ends up at on the engine, and to remove some worry to make things a little more forgiving, you probably should look into having the chambers softened on the heads.
"Any fool can know. The point is to understand"
- A. Einstein
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Re: How high a compression on pump gas
[Re: jwb123]
#3003806
01/10/22 01:46 PM
01/10/22 01:46 PM
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Al_Alguire
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The Hemi I just sold was 12.3 and had zero detonation issues on pump swill..Steel block aluminum head.
"I am not ashamed to confess I am ignorant of what I do not know."
"It's never wrong to do the right thing"
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Re: How high a compression on pump gas
[Re: cudaman1969]
#3003887
01/10/22 05:56 PM
01/10/22 05:56 PM
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Joined: Jan 2003
Posts: 17,838 S.E. Michigan
ZIPPY
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So you really know where you're stating from, I'd advise to measure the actual 1" down volume subtracted from theoretical flat (to manually calculate your dome volume) along with chamber CCs, and use these to manually calculate your CR....before you order gaskets.
Rich H.
Esse Quam Videri
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Re: How high a compression on pump gas
[Re: cudaman1969]
#3003894
01/10/22 06:35 PM
01/10/22 06:35 PM
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polyspheric
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Closing the intake valve later to reduce CCP works at cranking and low RPM, but its effect on anti-knock is gone well before your torque peak.
Boffin Emeritus
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Re: How high a compression on pump gas
[Re: EvilB1Dart]
#3003983
01/10/22 11:33 PM
01/10/22 11:33 PM
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Joined: Feb 2006
Posts: 288 no mans land
racerbychoice
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Depending on where the compression/quench zone ends up at on the engine, and to remove some worry to make things a little more forgiving, you probably should look into having the chambers softened on the heads. ^^^This is what we did with my dads 528 pump gas HEMI. 12.5 on 93 and he beats the hell out of it and still going after 8 yrs.
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Re: How high a compression on pump gas
[Re: polyspheric]
#3004082
01/11/22 11:51 AM
01/11/22 11:51 AM
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451Mopar
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Closing the intake valve later to reduce CCP works at cranking and low RPM, but its effect on anti-knock is gone well before your torque peak. Controlling cranking compression can help with pre-ignition. Compressing a gas increases its temperature. Starting with a colder intake charge also helps. Detonation is an uncontrolled combustion event which occurs after the spark event. Chamber / piston shape, spark plug location, and quench distance can help or hurt. Either way, you want to avoid hot spots in the chamber and on the piston. There are a bunch of dynamics that I don't understand. As RPM goes up in may reduce the chance of detonation, but not sure how the relationships of Time (decreasing with RPM) vs Air/exhaust flow (increasing with RPM) vs Engine load (how fast the RPM is changing.) I think the slower reving engine (think heavy truck) us going to build more chamber heat than a light vehicle with the same power. So it makes it hard to compare just compression ratio vs. octane as there are a bunch of unknown variables.
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