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SmokinVette.com Forums : Corvette Forums : C3 Corvette Forums : C3 Tech & Performance : comp ratio vs octane
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Old 10-24-2012, 03:18 PM   #1
buster
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Default comp ratio vs octane

OK, not a C3 question but, I read that the new C7 will have a SBC with a comp ratio of 11.5:1. How can that run on street gas of CA 91 octane?
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Old 10-24-2012, 06:03 PM   #2
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Computer control. Spark timing, fuel mix, etc. is all controlled by the computer and changes constantly based on the atmospheric and engine conditions from pressure to temps the computer is changing injection patterns, advancing and retarding timing, killing cylinders on the C7 and sense spark knock. Bottom Line, serious high tech engineering.

In addition to that I can only guess that they have machined the cylinder head combustion chambers smooth so no hot spots from metal burrs occur. All the things we have to manually do they have incorporated. Of, course this is part of the hefty price tag on these cars but for the most part they're turn key and worry free. No offense but look at most of the people who drive them, gasing up is the extent of their mechanical capability, not a bad thing just something that separates those who own old school from new school.
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Old 10-24-2012, 06:53 PM   #3
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direct fuel injection and chevy also machined the top of the pistons in a special way

http://www.ncminsurance.com/corvette-c7-lt1-engine.html

http://www.autoblog.com/2012/10/24/m...mall-block-v8/
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Old 10-25-2012, 04:23 PM   #4
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heres what GM says..they ve brought back the legendary LT1 nameplate...."The all-new LT1 engine also utilizes Active Fuel Management which is a first for the Corvette. This helps save fuel by very slightly, but gradually, reducing half the engines’ cylinders in light-load driving conditions. Also a first for Corvette is Direct Injection, which is the primary contributor to the greater combustion efficiency of the engine. This allows a more complete burn of fuel in the air-fuel mixture by precisely controlling the mixture motion and fuel engine spray pattern. Direct injection keeps the combustion chamber cooler, which allows for the higher 11.5:1 compression ratio"... theres your reason why it works Buster
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Old 10-29-2012, 04:22 AM   #5
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Quote:
Originally Posted by buster View Post
How can that run on street gas of CA 91 octane?
Camshaft selection can have a big effect on the true (DCR ) compression which is always lower than the static CR

http://members.uia.net/pkelley2/DynamicCR.html
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Old 10-29-2012, 05:51 AM   #6
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good article ..very informative
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Old 10-29-2012, 10:38 AM   #7
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Yes, that is a very good article as it clearly spells out why there is a huge difference between compression ratios and actual cylinder pressures.

That is why I advanced my 220/220 duration marine cam 4 degrees. By closing the intake valves 4 degrees earlier more of the air/fuel mixture gets trapped in the cylinder which then provides more torque down low where I needed it the most because of my overdrive 4th gear.

Cams will provide better low and mid-range power when advanced a few degrees and is the reason why most aftermarket timing chain/sprocket sets have the advance/retard keyways cut into the cam sprocket.
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Old 10-31-2012, 05:11 PM   #8
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remember Steve, back when we could build street motors and pop 12.5:1 domes in them? , buy 100 or 105 octane Premium gas at the Amoco on the corner for 79cents/gallon ...run for years.. ah, the good old days!!! now I ve got 10.5:1 CR and can just get by on 93 Octane E10 gas .. altho, I am still cranking out big power..but premium is $4/gal now .
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Old 10-31-2012, 06:05 PM   #9
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Quote:
Originally Posted by fishslayer143@yahoo.com View Post
remember Steve, back when we could build street motors and pop 12.5:1 domes in them? , buy 100 or 105 octane Premium gas at the Amoco on the corner for 79cents/gallon ...run for years.. ah, the good old days!!! now I ve got 10.5:1 CR and can just get by on 93 Octane E10 gas .. altho, I am still cranking out big power..but premium is $4/gal now .
Gee, I remember paying like 34.9 for Premium that I put in my '59 Ford Sunliner (about $3300.00 new) with the Interceptor 390. And, I remember 23.9 that I put in my '51 Pontiac (about $2000.00 new) before that. But, if you adjust up for inflation since then you wont be too many pennies away from the $4.00 for today's gas. In 1967 I was making about $80 per week take home and today, if I were still working, I would be making about 30+ times that. Inflation since 1970 to 2010 has been about 1800%. That is one thousand eight hundred percent. That is a multiplication factor of 18. In 1970, the price of a gallon of regular gas was about $0.36. Multiply that by the factor of 18 and you get $6.48. So, relatively speaking, gas is cheaper now than it was then. Therefore, IMHO, the "Good Old Days" were not that good.
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Old 11-01-2012, 02:26 AM   #10
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Quote:
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Gee, I remember paying like 34.9 for Premium that I put in my '59 Ford Sunliner (about $3300.00 new) with the Interceptor 390. And, I remember 23.9 that I put in my '51 Pontiac (about $2000.00 new) before that. But, if you adjust up for inflation since then you wont be too many pennies away from the $4.00 for today's gas. In 1967 I was making about $80 per week take home and today, if I were still working, I would be making about 30+ times that. Inflation since 1970 to 2010 has been about 1800%. That is one thousand eight hundred percent. That is a multiplication factor of 18. In 1970, the price of a gallon of regular gas was about $0.36. Multiply that by the factor of 18 and you get $6.48. So, relatively speaking, gas is cheaper now than it was then. Therefore, IMHO, the "Good Old Days" were not that good.
You make some interesting points Pappy. I started out at 3.60 an hour in '73, just 21yrs. old, thought I was steppin' in high cotton. I stayed for 37yrs, and when I retired I was up to 39.00. I guess everything is relevant, but those were the days being much younger!
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