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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: 10,735
North Dakota
6PakBee
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 10,735
North Dakota
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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: 20,818
A collage of whims
topside
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 20,818
A collage of whims
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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
Posts: 6,334
Heaven
EvilB1Dart
master
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master
Joined: Mar 2003
Posts: 6,334
Heaven
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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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Joined: Jan 2003
Posts: 19,375
Las Vegas
Al_Alguire
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 19,375
Las Vegas
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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: 17,876
S.E. Michigan
ZIPPY
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 17,876
S.E. Michigan
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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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Joined: Oct 2007
Posts: 4,209
New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,209
New York
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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
enthusiast
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enthusiast
Joined: Feb 2006
Posts: 288
no mans land
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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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Joined: May 2008
Posts: 5,399
Aurora, Colorado
451Mopar
master
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master
Joined: May 2008
Posts: 5,399
Aurora, Colorado
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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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Re: How high a compression on pump gas
[Re: Cab_Burge]
#3004427
01/12/22 03:54 AM
01/12/22 03:54 AM
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Joined: Feb 2006
Posts: 288
no mans land
racerbychoice
enthusiast
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enthusiast
Joined: Feb 2006
Posts: 288
no mans land
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“chambers softened” Not sure what this means Probably not applicable to Hemi heads   Definitely out of the norm for HEMI heads, but was applicable, although an experimental gamble with someone who knew what they were doing. My dad had a well known big name shop in N. Carolina do them. It worked out. DJ
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Re: How high a compression on pump gas
[Re: jwb123]
#3004472
01/12/22 09:58 AM
01/12/22 09:58 AM
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Joined: Jan 2006
Posts: 8,162
USA
360view
Moparts resident spammer
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Moparts resident spammer
Joined: Jan 2006
Posts: 8,162
USA
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Performance trends engine program will make a prediction on spark knock and will generate a spark curve to use if knock is predicted, very cheap insurance when designing an engine build. I have built pulling truck engines that had to run on pump gas with 13 to 1 static compression, and it did not detonate. Kevin at Performance Trends has greatly impressed me. After buying two Engine Analyzer versions and one Fuel Economy Calculator program his on the phone support was top notch, and twice when I mentioned “It would be great if” he did a software mod at no charge.
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Re: How high a compression on pump gas
[Re: B1Frank]
#3004538
01/12/22 01:33 PM
01/12/22 01:33 PM
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Joined: Aug 2011
Posts: 2,864
Pattison Texas
CSK
master
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master
Joined: Aug 2011
Posts: 2,864
Pattison Texas
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I have seen the new motorcycle engine's have 11.0 on the the compression, it is all about the volume you have in the engine . My new 6.4 Challenger is 10.9
1968 Charger COLD A/C Hilborn EFI
512ci 9.7 compression, Stealth heads, 4.10 gear A518 ODtrans 4100lb,10.93 full street car trim
2020 T/A 392 Stock 11.79 @ 114.5
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Re: How high a compression on pump gas
[Re: B1Frank]
#3004547
01/12/22 01:52 PM
01/12/22 01:52 PM
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Joined: May 2019
Posts: 713
Lake Villa Il
INTMD8
super stock
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super stock
Joined: May 2019
Posts: 713
Lake Villa Il
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I have seen the new motorcycle engine's have 11.0 on the the compression, it is all about the volume you have in the engine . Some are 14.0-1 Lots happening for that to work
69 Charger. 438ci Gen2 hemi. Flex fuel. Holley HP efi. 650rwhp @7250 510rwtq @5700
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Re: How high a compression on pump gas
[Re: John Burdine]
#3004590
01/12/22 03:00 PM
01/12/22 03:00 PM
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Joined: Jan 2003
Posts: 8,028
Tulsa OK
Bad340fish
master
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master
Joined: Jan 2003
Posts: 8,028
Tulsa OK
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“chambers softened” Not sure what this means Anyone? I'm going to go with this, softened means they have been deburred and any sharp edges that could be a hot spot removed.
68 Barracuda Formula S 340
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Re: How high a compression on pump gas
[Re: CSK]
#3004607
01/12/22 03:34 PM
01/12/22 03:34 PM
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Joined: Jan 2005
Posts: 12,419
Kalispell Mt.
HotRodDave
I Live Here
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I Live Here
Joined: Jan 2005
Posts: 12,419
Kalispell Mt.
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I have seen the new motorcycle engine's have 11.0 on the the compression, it is all about the volume you have in the engine . My new 6.4 Challenger is 10.9 I had one of those same 6.4 SRT8s in my 2011 ram 1500 4 door 4wd but I had more compression, I had eagle heads swapped onto it that are about 10CC smaller, ran great on 91 octane (3000 foot elevation) but would knock if it even sniffed anything lower. I don't remember the exact number but I think it was around 12to1 actual.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: How high a compression on pump gas
[Re: John Burdine]
#3004677
01/12/22 06:32 PM
01/12/22 06:32 PM
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Joined: Nov 2015
Posts: 879
Missouri
jwb123
super stock
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super stock
Joined: Nov 2015
Posts: 879
Missouri
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“chambers softened” Not sure what this means Anyone? Softening the chambers is something that is done usually to nitrous engines to keep from melting pistons. I have never seen it done on a Mopar engine, I have seen the mod on a few Chevy engines. The last one was a big block chevy with raptor heads on a mud racer. Basically they CNC the chamber to give it a taper from the quench to the valves. Reher Morrision did the work.
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Re: How high a compression on pump gas
[Re: John Burdine]
#3004740
01/12/22 09:39 PM
01/12/22 09:39 PM
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Joined: Mar 2003
Posts: 6,334
Heaven
EvilB1Dart
master
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master
Joined: Mar 2003
Posts: 6,334
Heaven
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“chambers softened” Not sure what this means Anyone? John, Keep in mind this procedure is mostly done in high performing race engines. In the OP's case it could be purely unnecessary, but has been utilized on HEMI heads. Chamber softening is where the chamber is laid back and cuts out sharp areas that create hot spots and slows the flame travel down. We all know each combustion chamber design will vary with its efficiency, but from talking with Monte he said its great if you can start out with a head that already has efficient chamber design. Removing area's in the chamber will allow heat to soak in and give more area for expansion. Of course quench & chamber shape are important when doing this. Most importantly its finding someone who has a good program and knows what they're doing. This is typically done with nitrous or other power-adder applications, and "some" N/A setups. My current heads chambers are softened by Slawko. There are several dudes who know what they're doing that have programs for Mopar heads. It all comes down to heat. As we know the whole deal in an engine is the overall combination and managing heat. In my conversations with Monte, he stressed emphasis on being all about timing, valve events, combustion efficiency, ability to fill the cylinder, etc. Man that dude is missed. Wes
"Any fool can know. The point is to understand"
- A. Einstein
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Re: How high a compression on pump gas
[Re: EvilB1Dart]
#3004933
01/13/22 03:03 PM
01/13/22 03:03 PM
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Joined: Sep 2003
Posts: 19,480
north of coder
moparx
"Butt Crack Bob"
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"Butt Crack Bob"
Joined: Sep 2003
Posts: 19,480
north of coder
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for sure, Monte knew his stuff and is missed by many here ! if a guy were to knock off the sharp edges by hand with sandpaper only, such as around the plug hole, and any other machined defined edges, would that be a waste of time, or just something that couldn't hurt ? kind of like dressing the sharp edges of the valve reliefs on pistons. 
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Re: How high a compression on pump gas
[Re: EvilB1Dart]
#3005020
01/13/22 07:34 PM
01/13/22 07:34 PM
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Joined: Feb 2013
Posts: 154
Canada
cuda499
member
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member
Joined: Feb 2013
Posts: 154
Canada
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“chambers softened” Not sure what this means Anyone? John, Keep in mind this procedure is mostly done in high performing race engines. In the OP's case it could be purely unnecessary, but has been utilized on HEMI heads. Chamber softening is where the chamber is laid back and cuts out sharp areas that create hot spots and slows the flame travel down. We all know each combustion chamber design will vary with its efficiency, but from talking with Monte he said its great if you can start out with a head that already has efficient chamber design. Removing area's in the chamber will allow heat to soak in and give more area for expansion. Of course quench & chamber shape are important when doing this. Most importantly its finding someone who has a good program and knows what they're doing. This is typically done with nitrous or other power-adder applications, and "some" N/A setups. My current heads chambers are softened by Slawko. There are several dudes who know what they're doing that have programs for Mopar heads. It all comes down to heat. As we know the whole deal in an engine is the overall combination and managing heat. In my conversations with Monte, he stressed emphasis on being all about timing, valve events, combustion efficiency, ability to fill the cylinder, etc. Man that dude is missed. Wes Totally agree and great explanation, but hemi heads dont work like wedge heads..... There is no quench or 360* of quench, depending how you want to look at it. I believe chamber softing isn't supper applicable to a hemi's chamber. Works great on a lot of wedge heads, and have also seen it done on pistons. Fwiw, ive been 11.3:1 on pump gas and made 986hp with 572 cubes and a indy 1RA head. As for Best machines theory on compression....... I took a motor out of my racecar and changed pistons and cam. Went from 15:1 to 11.3:1 and made more power. Compression doesn't work the same in hemis.
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Re: How high a compression on pump gas
[Re: cuda499]
#3005031
01/13/22 07:53 PM
01/13/22 07:53 PM
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Joined: Mar 2005
Posts: 1,404
Chesterfield Twp. Mi.
John Burdine
pro stock
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pro stock
Joined: Mar 2005
Posts: 1,404
Chesterfield Twp. Mi.
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“chambers softened” Not sure what this means Anyone? John, Keep in mind this procedure is mostly done in high performing race engines. In the OP's case it could be purely unnecessary, but has been utilized on HEMI heads. Chamber softening is where the chamber is laid back and cuts out sharp areas that create hot spots and slows the flame travel down. We all know each combustion chamber design will vary with its efficiency, but from talking with Monte he said its great if you can start out with a head that already has efficient chamber design. Removing area's in the chamber will allow heat to soak in and give more area for expansion. Of course quench & chamber shape are important when doing this. Most importantly its finding someone who has a good program and knows what they're doing. This is typically done with nitrous or other power-adder applications, and "some" N/A setups. My current heads chambers are softened by Slawko. There are several dudes who know what they're doing that have programs for Mopar heads. It all comes down to heat. As we know the whole deal in an engine is the overall combination and managing heat. In my conversations with Monte, he stressed emphasis on being all about timing, valve events, combustion efficiency, ability to fill the cylinder, etc. Man that dude is missed. Wes Totally agree and great explanation, but hemi heads dont work like wedge heads..... There is no quench or 360* of quench, depending how you want to look at it. I believe chamber softing isn't supper applicable to a hemi's chamber. Works great on a lot of wedge heads, and have also seen it done on pistons. Fwiw, ive been 11.3:1 on pump gas and made 986hp with 572 cubes and a indy 1RA head. As for Best machines theory on compression....... I took a motor out of my racecar and changed pistons and cam. Went from 15:1 to 11.3:1 and made more power. Compression doesn't work the same in hemis. Like to here more details about that 572-1RA deal.
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Re: How high a compression on pump gas
[Re: cuda499]
#3005216
01/14/22 12:01 PM
01/14/22 12:01 PM
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Joined: Feb 2006
Posts: 1,051
The Great White North
RAMM
super stock
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super stock
Joined: Feb 2006
Posts: 1,051
The Great White North
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“chambers softened” Not sure what this means Anyone? John, Keep in mind this procedure is mostly done in high performing race engines. In the OP's case it could be purely unnecessary, but has been utilized on HEMI heads. Chamber softening is where the chamber is laid back and cuts out sharp areas that create hot spots and slows the flame travel down. We all know each combustion chamber design will vary with its efficiency, but from talking with Monte he said its great if you can start out with a head that already has efficient chamber design. Removing area's in the chamber will allow heat to soak in and give more area for expansion. Of course quench & chamber shape are important when doing this. Most importantly its finding someone who has a good program and knows what they're doing. This is typically done with nitrous or other power-adder applications, and "some" N/A setups. My current heads chambers are softened by Slawko. There are several dudes who know what they're doing that have programs for Mopar heads. It all comes down to heat. As we know the whole deal in an engine is the overall combination and managing heat. In my conversations with Monte, he stressed emphasis on being all about timing, valve events, combustion efficiency, ability to fill the cylinder, etc. Man that dude is missed. Wes Totally agree and great explanation, but hemi heads dont work like wedge heads..... There is no quench or 360* of quench, depending how you want to look at it. I believe chamber softing isn't supper applicable to a hemi's chamber. Works great on a lot of wedge heads, and have also seen it done on pistons. Fwiw, ive been 11.3:1 on pump gas and made 986hp with 572 cubes and a indy 1RA head. As for Best machines theory on compression....... I took a motor out of my racecar and changed pistons and cam. Went from 15:1 to 11.3:1 and made more power. Compression doesn't work the same in hemis. Thats because high compression requires a very tall dome which inhibits flame travel. J.Rob
2009 PHR\EMC Competitor
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2016 HotRod/EMC 3rd place SPEC Bigblock
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Re: How high a compression on pump gas
[Re: RAMM]
#3005458
01/15/22 08:12 AM
01/15/22 08:12 AM
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Joined: Nov 2020
Posts: 348
Isle of Sheeps
Gtxxjon
enthusiast
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enthusiast
Joined: Nov 2020
Posts: 348
Isle of Sheeps
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So after all this tech talk, what was the answer? I have run many engines on the street and at the track too. Iron/iron, iron/ali, ali/iron there's lots of variables here... But when folks ask me these days, I just say 10 to 1... Its safe and its old school, flat top piston closed chamber head.  But when it comes to hemi's its a whole new ballgame! I got a street 'all iron' hemi from a guy a few years back with 13 to 1 pistons, driving it on pump gas. Yes the pistons were worn, but it seemed to run ok?. 0.008 thousanths clearance in the bores and smoked when hot etc... He said it ran good, cos its got a RACE camshaft... Yep it was huge around 700 lift...  ''Bleeds OFF all the compression at low RPM's''! I heard that a lot over the years and maybe its TRUE???
Last edited by Gtxxjon; 01/15/22 08:13 AM.
Carpe diem, quam minimum credula postero, thanx Horace!
There’s no point trying to fix stuff that ain’t broke,,, 'but if ain’t broke',,, you is not trying hard enough...
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Re: How high a compression on pump gas
[Re: Gtxxjon]
#3006469
01/18/22 11:44 AM
01/18/22 11:44 AM
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Joined: Jan 2003
Posts: 13,374
Marion, South Carolina [><]
an8sec70cuda
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 13,374
Marion, South Carolina [><]
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FWIW...my 575" hemi is right around 11:1 and I run it NA on pump gas. On spray I do run race fuel.
CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax
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