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#1 |
Between Trucks...
Join Date: Mar 2002
Location: Los Angeles, CA.
Posts: 3,830
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Working on getting specs together for a new motor. What's the highest compression ratio I can get away with, for low octane, cheapo gasoline? I'll be running a pretty tame cam, not much more than .450 lift...
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#2 |
Senior Member
Join Date: Jan 2003
Location: daytonabeach
Posts: 22,956
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Re: Compression ratios for cheap gas...
about 9to1
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71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY |
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#3 |
Between Trucks...
Join Date: Mar 2002
Location: Los Angeles, CA.
Posts: 3,830
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Re: Compression ratios for cheap gas...
Cool,that's what I thought. Do you think 9.5:1 would be pushing it?
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#4 |
20' Daredevil (Ret)
![]() Join Date: May 2003
Location: Jefferson State
Posts: 13,647
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Re: Compression ratios for cheap gas...
^ Gotta agree with that. Of course modern engine control systems allow considerably higher ratios, but I would think 9:1 would be about right for our old engines.
Ack, you beat me with your post! Mmmm 9.5:1 would probably still be OK, but that would be about the top. The worst that could happen is you might need mid-grade fuel...
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- Mike - 1972 K20 LWB 350/350/205 RIP El Jay Last edited by Stocker; 08-25-2007 at 04:14 PM. |
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#5 |
Special Order
![]() ![]() Join Date: Nov 2004
Location: Mt Airy, MD
Posts: 85,859
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Re: Compression ratios for cheap gas...
Aluminum heads will let you run higher compression on pump gas.You could go close to 10:1.I`m building a 327 with 76cc heads and flat tops.I`ll be at about 9.75:1 and I`m planning on running regular most of the time.If you run premium the mileage will improve enough with a light foot to pay you back about 1/2 the extra you spent on premium.(in any motor)
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"BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~ |
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#6 | |
Do the Dew!
Join Date: May 2005
Location: Vernal, Utah
Posts: 191
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Re: Compression ratios for cheap gas...
Quote:
![]() BTW I run on midgrade pump gas which is 89 octane, 650 CFM Edelbrock, retarded the timing, and I am at about 5300 feet above sea level. ![]() |
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#7 |
Registered User
Join Date: Mar 2004
Location: Red Chute,LA
Posts: 62
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Re: Compression ratios for cheap gas...
Cam specs play a part in octane requirements with a given compression ratio. If you're stuck with 9.5:1 you can get a little leeway with cam selection. Some of the real tech guys can explain dynamic vs static compression better than I could. Call a cam company with what you need to do and they can help suggest the right cam specs.
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#8 |
Senior Member
![]() Join Date: Dec 2006
Location: Studio City, Calif.
Posts: 2,873
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Re: Compression ratios for cheap gas...
I always heard 8.7 for 87Octane and 9.5 for 92. My last engine was 9.0 and needed 89 octane or retard the timing or it would ping.
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'69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme. |
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#9 |
Sierra Grande Club
Join Date: Jan 2007
Location: New Mexico USA
Posts: 2,433
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Re: Compression ratios for cheap gas...
Altitude has a roll in compression ratios too. I’m in the 7000ft club. 10:1
s/t
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1971 GMC Sierra Grande, 1/2 ton short wide, original 4 bolt 010 020 block & heads. (matching #'s). 383 stroker, SMI q jet 750 cfm, Lunati Voodoo 60102 cam, Scorpion roller rockers, Spin Tech pro street mufflers with X pipe. ![]() |
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#10 |
LAIDOUT
Join Date: Jan 2005
Location: NW Montana
Posts: 928
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Re: Compression ratios for cheap gas...
Interesting, so less ambient pressure [altitude] means less cylinder filling, so less actual compression?
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#11 |
Sierra Grande Club
Join Date: Jan 2007
Location: New Mexico USA
Posts: 2,433
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Re: Compression ratios for cheap gas...
From what I hear, my 10:1 is = 9:1 at sea level. If I drive down in altitude I'll have to run premium.
s/t
__________________
1971 GMC Sierra Grande, 1/2 ton short wide, original 4 bolt 010 020 block & heads. (matching #'s). 383 stroker, SMI q jet 750 cfm, Lunati Voodoo 60102 cam, Scorpion roller rockers, Spin Tech pro street mufflers with X pipe. ![]() |
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#12 | |
blood type; Retumbo
Join Date: Jan 2004
Location: next to my reloading bench
Posts: 10,269
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Re: Compression ratios for cheap gas...
Quote:
CROWER HAS THEIR S!!T TOGETHER !! (grind # 00210) |
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#13 |
Senior Member
![]() Join Date: Mar 2006
Location: Clarkrange Tennessee
Posts: 1,786
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Re: Compression ratios for cheap gas...
Dang guys! first I want to know where to get CHEAP gas!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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#14 | |
Do the Dew!
Join Date: May 2005
Location: Vernal, Utah
Posts: 191
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Re: Compression ratios for cheap gas...
Quote:
I am not saying what you can or can't use gas wise, but at least keep in thought as to WHY that CHEAP GAS is so cheap. |
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#15 | |
Between Trucks...
Join Date: Mar 2002
Location: Los Angeles, CA.
Posts: 3,830
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Re: Compression ratios for cheap gas...
Quote:
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#16 |
Special Order
![]() ![]() Join Date: Nov 2004
Location: Mt Airy, MD
Posts: 85,859
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Re: Compression ratios for cheap gas...
We call it "spot gas".They just get it where it`s cheapest without any other consideration.
Hey Mtdewmaniac66,to answer your question..."Do you see a pocket protector and six pens? ![]()
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"BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~ |
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#17 | |
Do the Dew!
Join Date: May 2005
Location: Vernal, Utah
Posts: 191
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Re: Compression ratios for cheap gas...
Quote:
![]() I had to laugh at that one, it reminded me of that commercial with the two lab techs driving identical cars but using different brands/types of fuels. I am sorry for the misconception though, I am use to seeing stuff that is crappy "cheap" while forgetting that "cheap" also meant "economically priced". While I would love to use the better priced fuel, I am using the midgrade fuel due to my engine requirements. |
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#18 |
Formerly yellow72custom
Join Date: May 2001
Location: Austin, TX
Posts: 7,531
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Re: Compression ratios for cheap gas...
I have 9.7:1-ish in my truck, and it runs on 91 fine in the summer, and I can put 89 in it in the winter. I'm not exactly sure on the compression, but it has a 350 with stock-ish flat tops, with valve reliefs, stock Fel Pro head gasket, and 64cc double hump heads. The cam only has .440" lift.
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'72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg |
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