max compression ratio on 89 octane - Page 2 - Hot Rod Forum : Hotrodders Bulletin Board
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  #16 (permalink)  
Old 06-25-2005, 11:39 AM
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Because you have a knock sensor and it retards the timing and has a computer processor.

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  #17 (permalink)  
Old 06-27-2005, 11:20 AM
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Quote:
Originally Posted by Rick WI
Because you have a knock sensor and it retards the timing and has a computer processor.
I hear you, but it pings on acceleration with 87 sometimes (took awhile to train my kids to stay off the 87). No hint of ping with the 89. I know a few of the design engineers for this motor and they told me the octane recommendation was on the conservative side, they think it should be ok with 89.
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Old 06-27-2005, 12:19 PM
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I suspect there is a limit to the amout of timing the computer can pull out. This is not unlike what will happen on say an LT1 motor that is run on 87, in addition to a large decrease in performance overall.
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  #19 (permalink)  
Old 06-28-2005, 12:25 AM
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How about my friend's Eclipse? It ran 11:1 stock with a turbo charger. He put a bigger aftermarket turbo on it and it runs fine with 91 octane.
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Old 06-28-2005, 12:45 AM
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Your not going to run 11:1 with a turbo on 91 pump gas with any kind of reasonable tuneup. I'm not even going to bother debating that point.
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Old 06-28-2005, 12:58 AM
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Quote:
Originally Posted by lluciano77
How about my friend's Eclipse? It ran 11:1 stock with a turbo charger. He put a bigger aftermarket turbo on it and it runs fine with 91 octane.
Your friend does not have a stock eclipse with 11:1.
No matter what he tells you.
They have around 8:1.
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  #22 (permalink)  
Old 06-28-2005, 12:59 AM
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i just put a 9.4,406 together and it uses a comp cams magnum 280 with 1.6 rockers and i am using 87 so far.
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  #23 (permalink)  
Old 06-28-2005, 08:25 AM
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Each engine design has it's own octane requirement. Comparing the compression ratios of late model, high tech engines to an old school V8 is not going to work.

Furthermore, old style V8's need a bunch of ignition advance to make power. Slow flame travel over a big piston needs a bunch of timing advance.

Old BBC's typically need over 40 degrees of timing to make max power! This makes them very prone to detonate. Even a semi-high tech vortec 350 has enough modern design in it's combustion chamber ("fast burn heads") to only require 32-34 degrees advance. Plus their "smaller" diameter pistons need less timing advance because it requires less time for the flame to travel across a smaller piston. Therefore a sbc with late model heads (vortec or aftermarket) can run a lot more cr as compared to a BBC.

Due to more engine tech, it is becoming common to turbo charge high compression engines. The new Audi A3 2.0T uses 10.5:1 cr with it's turbo engine! (Road and Track, July 2005, pp 74-79). However, when determining the max cr of an engine with a low octane fuel, it is important to compare apples to apples. So, an old school 400 Ford needs lower compression than most, especially if used with a mild cam in a heavy car with a low numeric gear.

Last edited by 454C10; 06-28-2005 at 08:30 AM.
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  #24 (permalink)  
Old 06-28-2005, 08:31 AM
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double post???

Last edited by 454C10; 06-28-2005 at 08:59 AM.
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  #25 (permalink)  
Old 06-28-2005, 08:42 AM
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It has 10.3 not 10.5 and it only has that because it uses a new method called direct injection, where the fuel injector is located in the combustion chamber to keep it cool, along with aluminum heads and with an Extremely low boost level, makes it possible
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Old 06-28-2005, 08:53 AM
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That is exactly my point. Can't compare new high tech engines to old school american V8's. Even late 80's engnes have more engine tech than those old 60's designed V8's.

10.3 or 10.5 is about the same thing. However, printed in black and white, Road and track claims 10.5:1 cr. Maybe R&T got it wrong.
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Old 06-28-2005, 09:01 AM
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camshaft duration also factors in big time, cranking compression is a great indicator of what fuel can be compatible. Burn rate also.
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  #28 (permalink)  
Old 06-28-2005, 11:59 AM
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there's always somebody who claims they run 12:1 on 87 octane. This could be for any number of reasons, but the fact of the matter is that its really hard to get a motor to perform on any kind of street gas with iron heads and more than 10:1 compression, and few people have luck with it. Some of the things in terms of stories you folks are going to run into are the following: new motors or EFI cars. Different ball game, different conversation completely. Folks running aluminum heads. You're talking about one less point of compression due to heat sinking, you really can't parallel the two. You'll also run into people who say "its an 11:1 motor" or "12:1" who have no exact measurements to prove it.

There's a lot of stories this way and that in hotrodding, don't believe all the hype, 99.9% of motors adhere to the rule of thumb, which is 10:1 for iron and 11:1 for aluminum on street gas.

K
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  #29 (permalink)  
Old 06-28-2005, 05:53 PM
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Quote:
Originally Posted by Rick WI
Your not going to run 11:1 with a turbo on 91 pump gas with any kind of reasonable tuneup. I'm not even going to bother debating that point.


He put a bigger turbo on then the original. It was running low 11s.
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  #30 (permalink)  
Old 06-29-2005, 09:24 AM
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Here is a site that gives turbo eclipse specs: http://dsm.misterdeedub.com/1gvs2g.html

7.8:1 cr for 90 to 94 turbo eclipse
8.5:1 cr for 95 to 99 turbo eclipse
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