Compression ratios for cheap gas...

[Sinister]

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...

[cdowns]

about 9to1

[Sinister]

Cool,that's what I thought. Do you think 9.5:1 would be pushing it?

[Stocker]

^ 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...

[special-K]

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)

[mtndewmaniac66]

Originally Posted by special-K

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)

So, umm, What would my compression ratio be in my 327, using TRW Flat Tops w/4 valve reliefs, Steel head gaskets (replaces original) and GM Performance Parts Cyl Heads with 64 CC chambers, 170 CC intake runners?

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.

[72hotrod]

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.

[WorkinLonghorn]

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.

[Sport/Truck]

Altitude has a roll in compression ratios too. I’m in the 7000ft club. 10:1
s/t

[Ricks1971]

Dang guys! first I want to know where to get CHEAP gas!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[BigSky71]

Originally Posted by Sport/Truck

Altitude has a roll in compression ratios too. I’m in the 7000ft club. 10:1
s/t

Interesting, so less ambient pressure [altitude] means less cylinder filling, so less actual compression?

[Sport/Truck]

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

[mtndewmaniac66]

Originally Posted by Ricks1971

Dang guys! first I want to know where to get CHEAP gas!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Trust me, you do not want to run CHEAP GAS, It will do you more harm than good in the end. Trust-worthy stations such as Amoco, Conoco, Phillips, Sinclair, and Texaco, manufacture fuels that have been formulated to work most efficiently in any vehicle. Including our older GMs. By running an un-trustworthy gas in your vehicle permits unwanted contamination, unneccessary inert ingredients, and even a higher percentage of water. Ever wonder why the Cornerstop Pumps are way cheaper than the above mentioned? Is it worth the risk? I have owned a few built cars in my time, just to mention my two faster ones, both the '70 Nova, and the '82 TransAm. I have had to replace Carburetors, re-ring, decarbonize Valves, and replace fuel pumps due to CHEAP GAS.

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.

[Sinister]

Originally Posted by mtndewmaniac66

Trust me, you do not want to run CHEAP GAS, It will do you more harm than good in the end. Trust-worthy stations such as Amoco, Conoco, Phillips, Sinclair, and Texaco, manufacture fuels that have been formulated to work most efficiently in any vehicle. Including our older GMs. By running an un-trustworthy gas in your vehicle permits unwanted contamination, unneccessary inert ingredients, and even a higher percentage of water. Ever wonder why the Cornerstop Pumps are way cheaper than the above mentioned? Is it worth the risk? I have owned a few built cars in my time, just to mention my two faster ones, both the '70 Nova, and the '82 TransAm. I have had to replace Carburetors, re-ring, decarbonize Valves, and replace fuel pumps due to CHEAP GAS.

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.

Well, by cheap gas, I mean 87 octane. I usually use 76 or Mobile. My buddy rebuilds carburators for a living and agrees with your view on crap gas...

[special-K]

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?I could only guess.My machinist came up with that.I`m using the original 250 hp heads.It could vary according to gaskets and head planing affecting deck height.

[mtndewmaniac66]

Originally Posted by special-K

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?I could only guess.My machinist came up with that.I`m using the original 250 hp heads.It could vary according to gaskets and head planing affecting deck height.

Do you see a pocketprotector and six pens?
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.

[68C15]

Originally Posted by 72hotrod

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.

this is a VERY important factor. back in 98 I built a 355 with 0.125 domes under 72CC heads (882's shaved a bit) and 14* initial with 36 total (no vacuum). it ran great with 89 octane due to the cam bleeding off dynamic compression until it hit the 2300 RPM mark.

CROWER HAS THEIR S!!T TOGETHER !! (grind # 00210)

[lukecp]

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.