CR on the street and your octane requirement...
#2688041
08/15/19 01:52 PM
08/15/19 01:52 PM
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Joined: Jan 2003
Posts: 15,439 Val-haul-ass... eventually
BradH
OP
Taking time off to work on my car
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OP
Taking time off to work on my car
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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Carry on.
Last edited by BradH; 08/15/19 06:45 PM.
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Re: What's the latest on your CR vs street octane requirement?
[Re: BradH]
#2688101
08/15/19 04:03 PM
08/15/19 04:03 PM
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Joined: Mar 2005
Posts: 403 Colorado front range
BcudaChris
mopar
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mopar
Joined: Mar 2005
Posts: 403
Colorado front range
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My EQ headed 408 Magnum came out at 10.3, runs fine on the pump 91 we get around here. I run a dual pattern Hughes 230/234, .52/.57" @.050" (as near as I can recall at the moment anyway). 750DP, air gap, 1 3/4 Super Comps, 2.5" exhaust, 904,2500 stall, 3.55 gears. I'm running 18 initial/36 total mech with a vac advance. Mech advance curve is slow (all in at 3200). I'm still fine tuning the whole package for bullet proof street driving. And I'm at 5K above sea level. When I get to the track, I'll spike my fuel with race gas. My 9.0:1 Brand X equine with over head cam and aluminum heads will start to knock on your 2nd or third run down the 1/4 on pump 91, even pure gas 91 (or 93 at sea level for that matter). Not that I would do this, but were I to wring out the 408 in all three gears (on a closed course with a professional driver), I am quite confident, it would show no signs of detonation in any gear. Were I to repeat that exercise, I'm pretty sure it would be in my best interest to spike the fuel with race gas or octane boost like I would at the drag strip. Normal street driving is a traction limited blast. You can run a higher-than-you-think static compression ratio for a given fuel octane, provided your dynamic compression remains within acceptable limits. Your local density altitude and the altitude range you operate the vehicle in will have an impact on dynamic compression. Cam selection is critical. http://www.enginebasics.com/Advanced%20Engine%20Tuning/Static%20vs%20Dynamic.htmlhttp://wallaceracing.com/dynamic-cr.phpWe'll see when I get to it, but I'll for sure I'll have to re-jet the carb and probably have to run octane boost or cut with race gas when I get this near sea level. Now, how much power is that extra 0.8:1 compression getting me vs. if I had kept it around 9.5?? Not much with my setup currently. But I'm a head gasket away from an acceptable forced induction static CR (would still have to open up the rings), or real compression requiring real gas. Durability and flexibility were where I was going here.
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Re: What's the latest on your CR vs street octane requirement?
[Re: BradH]
#2688104
08/15/19 04:05 PM
08/15/19 04:05 PM
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Joined: Dec 2007
Posts: 12,593 Great Neck,LI,new york
hemi-itis
I Live Here
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I Live Here
Joined: Dec 2007
Posts: 12,593
Great Neck,LI,new york
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There are so many variables,stay SAFE and read your plugs!
https://www.google.com/search?q=octane+required+for+compression+ratio+charts&tbm=isch&source=iu&ictx=1&fir=I5vBMm4xO5y2BM%253A%252C19J5ko9OrKSMqM%252C_&vet=1&usg=AI4_-kSJNUY5iVTnNmn9DMOptHP38VXfTA&sa=X&ved=2ahUKEwj02ryc1YXkAhUshOAKHV1VCs4Q9QEwEXoECAkQFQ#imgrc=lKy3u_EFLFr_SM:&vet=1
HEMI-ITIS has no cure. My condition is fully BLOWN!!
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Re: Mods - Please delete this
[Re: BradH]
#2688160
08/15/19 06:11 PM
08/15/19 06:11 PM
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Joined: Dec 2007
Posts: 20,328 Puttin' on the foil in Charles...
not_a_charger
Mr. Big Shot Moparts Moderator
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Mr. Big Shot Moparts Moderator
Joined: Dec 2007
Posts: 20,328
Puttin' on the foil in Charles...
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Mods - Please delete this Why would I do that? The post doesn't violate Tom's rules, it's not a duplicate, and it's in the correct forum.
Earning every penny of that moderator paycheck.
DBAP
"They don't think it be like it is, but it do." - Oscar Gamble
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Re: Mods - Please delete this
[Re: not_a_charger]
#2688170
08/15/19 06:43 PM
08/15/19 06:43 PM
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Joined: Jan 2003
Posts: 15,439 Val-haul-ass... eventually
BradH
OP
Taking time off to work on my car
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OP
Taking time off to work on my car
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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Mods - Please delete this Why would I do that? The post doesn't violate Tom's rules, it's not a duplicate, and it's in the correct forum. I'll change the title and y'all can keep it if you want. The intent wasn't to waste people's time, so my apology for that. I saw something brought up in one of the earlier replies and thought to myself: Nope, not going down that rat hole again.
Last edited by BradH; 08/15/19 06:44 PM.
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Re: Mods - Please delete this
[Re: BradH]
#2688194
08/15/19 07:30 PM
08/15/19 07:30 PM
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Joined: Aug 2011
Posts: 2,904 Pattison Texas
CSK
master
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master
Joined: Aug 2011
Posts: 2,904
Pattison Texas
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I went with 9.7 cr. on my 512, I have made the mistake more than once trying to run more CR on the street,pump gas, here in HOT, almost sea level Southeast Tx, all is good..038ish quench
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: CR on the street and your octane requirement...
[Re: BradH]
#2688215
08/15/19 08:41 PM
08/15/19 08:41 PM
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Joined: Oct 2007
Posts: 4,219 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,219
New York
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Dynamic compression ratio occurs at (and somewhat above) cranking speed, and disappears long before your torque range. The only factor in DCR calculation is reduced charge mass due to reversion ABDC - when that stops, DCR is not relevant. The 12.0:1 engine that only has 9.0:1 DCR due to late intake closure has 12.0:1 actual ratio by 3,000 RPM. Above critical RPM (when reversion is nearly stopped) you have the static ratio, and more than that, when your exhaust tuned length is working.
Running a later IVC than your engine wants (by too much intake duration, wider LSA, or retarded) to reduce knocking is not a good idea, it's not reliable or predictable, too many variables.
Boffin Emeritus
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Re: Mods - Please delete this
[Re: CSK]
#2688216
08/15/19 08:41 PM
08/15/19 08:41 PM
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Joined: Dec 2004
Posts: 10,607 Rittman Ohio
fourgearsavoy
I Live Here
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I Live Here
Joined: Dec 2004
Posts: 10,607
Rittman Ohio
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I'm just a tick over 10.8 with my 493 Indy head engine. I cant hear any detonation with the pump 93 here in NE Ohio and it runs very well. I haven't checked my cranking compression with my new cam but it seems to run good on pump 93 I limit my total advance to 34degrees mechanical with no vacuum advance. Gus
64 Plymouth Savoy 493 Indy EZ's by Nick at Compu-Flow 5-Speed Richmond faceplate Liberty box Dana 60
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Re: Mods - Please delete this
[Re: fourgearsavoy]
#2688280
08/16/19 02:04 AM
08/16/19 02:04 AM
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Joined: Aug 2003
Posts: 43,719 Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,719
Bend,OR USA
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I'm just a tick over 10.8 with my 493 Indy head engine. I cant hear any detonation with the pump 93 here in NE Ohio and it runs very well. I haven't checked my cranking compression with my new cam but it seems to run good on pump 93 I limit my total advance to 34degrees mechanical with no vacuum advance. Gus My 518 C.I. pump gas motor in my old Duster ended up having 10.78 to 1 mechanical compression ratio with the set of Indy SR with 76.0 chambers, it never acted like or showed signs of detonation(on the plugs) on Oregon 91 octane pump swill with no ethanol in it back then I ran the same timing as Gus (34 degrees)with no vacuum advance also
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Mods - Please delete this
[Re: fourgearsavoy]
#2688291
08/16/19 06:44 AM
08/16/19 06:44 AM
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Joined: May 2004
Posts: 12,476 Taxes & Virus's R-US, NY
Dragula
I Live Here
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I Live Here
Joined: May 2004
Posts: 12,476
Taxes & Virus's R-US, NY
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11.5:1 Hemi running on 93 with 10% Ethanol cause thats all I can get.....timing at 34* or less. Runs awesome. I run 34* on the street and turn it down to 31* at the track.....ran 9.90
512/440 running 38* degrees on the street or track. Indy EZ1 heads...Also 93 with E10%.....No issues. Ran 9.70....
It is looking like the 93 around here is quickly going away, and 91 will be all I can get.....I hate NY.
Last edited by Dragula; 08/16/19 06:45 AM.
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Re: Mods - Please delete this
[Re: BradH]
#2688324
08/16/19 09:02 AM
08/16/19 09:02 AM
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Joined: Jan 2003
Posts: 18,025 S.E. Michigan
ZIPPY
I Live Here
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I Live Here
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Posts: 18,025
S.E. Michigan
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I saw something brought up in one of the earlier replies and thought to myself: Nope, not going down that rat hole again.
I have an idea which one that might be, and we can just carry on and pretend it isn't there if that'll help. Being that potentially useful content is somewhat lacking these days, in my humble opinion I'd like to see some discussions kept alive if possible. TBH I forgot my intake closing point long ago, but on my 451 which has aluminum heads and .045 quench, 10.75:1 measured, I'm at about 175lbs cranking compression and don't hear or see any detonation on local E10/93. I have not noticed a huge difference in performance or tendency to detonate between 34, 36, and 38 total timing. Of every grade of normal street gasoline available, I feel 93 is the most likely to be discontinued/obsoleted in the future and this made me comfortable building my future 572 with 10.2:1.
Rich H.
Esse Quam Videri
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Re: CR on the street and your octane requirement...
[Re: SILVER67]
#2688754
08/17/19 02:17 PM
08/17/19 02:17 PM
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Joined: Jan 2003
Posts: 1,238 North Central, Indiana
Roughbird72
pro stock
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pro stock
Joined: Jan 2003
Posts: 1,238
North Central, Indiana
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There are a lot variables, but in my area I stick to Sunoco Ultra 93(tested at 5% eth) and Marathon 93( tested at 7% eth). I'm running a 10.6:1 static ratio in my 440 with no ill effects to date.
72 Plymouth Roadrunner 11.08@123.25
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Re: CR on the street and your octane requirement...
[Re: Twostick]
#2688896
08/17/19 08:51 PM
08/17/19 08:51 PM
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Joined: Oct 2007
Posts: 4,219 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,219
New York
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Thanks. I suspect that factory knock detection by frequency may be compromised in a race engine by changing torque of fasteners, adding/subtracting attached accessories, raising the RPM range etc. and both give false positives and fail to warn on new noises not within the planned range.
Boffin Emeritus
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Re: What's the latest on your CR vs street octane requirement?
[Re: BcudaChris]
#2688902
08/17/19 09:24 PM
08/17/19 09:24 PM
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Joined: Nov 2015
Posts: 896 Missouri
jwb123
super stock
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super stock
Joined: Nov 2015
Posts: 896
Missouri
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I hate to be a broken record, BUT they make engine programs that will predict what octane fuel an engine will need, and for example if you have to run pump gas, it will calculate an advance curve to safely do that as well. Program I use is less than $200 and I have been using it for many years. It may not be perfect but it gets you a lot closer than asking suggestions. It also predicts cranking compression, idle vacuum, and a bunch of other stuff. If you take the time to enter flow bench numbers, and calculate the REAL static compression, etc. it has hit every engine on a real dyno within just a few horsepower. I set up a pump gas procharged 410 LA engine, and simply ran the numbers in the program without boost and then with boost, plugged those numbers into the timing maps on the Fitech, and it ran perfect, just one example. I have a distributor machine, and when I build a street engine I calculate the advanced curve, set the distributor to that curve on the machine, and have yet to have an issue. I still look at the heat mark on the electrode, but they are always right where it needs to be.
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Re: CR on the street and your octane requirement...
[Re: ZIPPY]
#2689443
08/19/19 03:49 PM
08/19/19 03:49 PM
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Joined: Jan 2003
Posts: 15,439 Val-haul-ass... eventually
BradH
OP
Taking time off to work on my car
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OP
Taking time off to work on my car
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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I saw something brought up in one of the earlier replies and thought to myself: Nope, not going down that rat hole again.
I have an idea which one that might be, and we can just carry on and pretend it isn't there if that'll help. It doesn't matter; I'm transitioning into more of an observer here anyway.
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