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Old 03-31-2003, 08:52 PM
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Post highest compression on premium

What would be the limmit on compression ratio to run on premium without pinging?

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Old 03-31-2003, 08:56 PM
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9.5:1 is about what`s considered to be "safe" but there are some out there running 10:1 on 93. A small chevy with 6 inch rods can run 10:1 safely on 93.
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Old 03-31-2003, 10:08 PM
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Mike morgan who has been in a bunch of magazines is running over 14:1 compression in his 406 SBC. Now hes running more duration @ .050 than alot of you guys want to run actual duration. {280 somthing @ .050} Oh and he drives it on the street. But it all comes back to how much actual duration your cam has. If your running a little 262 advertised duration cam then your compression max will probably be around 9:1 or so but if you get up around 295-300 then you can run 11:1 and be fine. Duration bleeds off cylinder pressure so the more duration your cam has the more compression you need. This has NOTHING to do with lift, just duration. Hell my buddy who just joined the board has a 350 chevy hes running aprox 10.5:1 with a 312 advertised cam and he can run it on 87 without fiddleing with the timing or anything, just pumped the gas and drove. So dont believe that "9.5:1 is the safe max" because thats a load of crap. It all comes down to cylinder pressure. Correct me if im wrong here but 180 psi is ideal, i believe 190 psi is about the safe max. And 200 Is about the absolute limit for a 93 octane motor but dont quote me on that.
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Old 03-31-2003, 10:20 PM
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long duration bleeds it off at low RPM, as the engine climbs in RPM there`s less "time" for it to bleed off cylinder pressure. as I recently seen in a article, just because you can`t hear detonation doesn`t mean it isn`t happening.
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Old 03-31-2003, 11:01 PM
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Double vision...reread my last post as many times as necessary until it sinks in. My neighbor does not run 87 all the time, it was a one time thing cause he was in a pinch or somthin. The car obviously dosent detonate because its been running strong for 6 years. No i wouldnt run 14:1, but Mike does so hats off to him for that, i was just illustrating a fact that 10:1 is not the "limit" The limit is the limit, what the limit is what you have to figure out. Thats where Dynamic compression comes in.
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Old 04-01-2003, 10:06 AM
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these guys with compression higher than 10 are using electronic fuel injection on newer style engines as well with all the computerized gizmos imaginable
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Old 04-01-2003, 11:01 AM
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Cam duration is a key in the compression game. I ran a 14:1 GSXR for about 3 years and didn't ever get detonation, head removal verified it but the bumpstick didn't do much for the bottom end, with a 14,000 redline it made it up quickly once I got it above about 3500.
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Old 04-01-2003, 04:38 PM
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Im running 11.75:1 with aluminum heads on 93 octane without any problems.
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Old 04-01-2003, 08:45 PM
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Bullheimer no theyre not, you dont have a fricken clue what your talking about, shut up. Theyre using carborators and an aftermarket ignition system {mallary, MSD etc}, a good cooling system {aluminum radiator, heads, intake, and water pump} and aparently more resources than the typical hot rod subscription. And the key to it all for the last damn time is.....TADAA.. A cam with a longer duration. If you want to argue about this any more then we can start an arguement thread on this topic. If not then do some research on Dynamic compression and cranking PSI and learn a little
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Old 04-02-2003, 05:10 AM
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And I am running about 11.5 to 1 on a 400 pontiac 670 heads with 94 octane no problems
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Old 04-02-2003, 05:25 AM
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This seems to be an ongoing argument that seems to make it's way into many threads. Seems there are no definite opinions shared by everyone. IMO not running the max allowable comp is like giving away free HP, but I'd hate to have to pull the heads and change head gaskets if it were to knock.It would be nice to know if I were right there on max comp just for peace of mind.
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Old 04-02-2003, 06:44 AM
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I've never even heard of 16:1 on a gas engine but at popular hot roddings engine challange their was a engine that ran with a hint of detonation with 16:1 comp. and they were limited to 92 octane thats crazy
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Old 04-02-2003, 07:48 AM
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I personally run 11.5:1 compression on 94 premium in a "small" block 412 chevy. This is with a holley carb- no fuel injection. It has a big cam (264/ 270 duration at 0.050"), aluminum heads, big universal fit griffin Al radiator, stewart water pump, coolant hoses from pump to center of heads, etc..

The big cam bleeds off cylinder pressure, and the cooling system has to be up to snuff.

I checked the bottom end over the winter, no hint of detonation.

I have gone as fast as 10.91 @ 122 MPH with this combo in a 3500 lb street car on 94 pump gas alone. When the weather gets cooler and denser, I chicken out and mix in some race gas for "peace of mind."

It has about 210 lbs of cranking pressure. I _have_ run into detonation at 200 lbs of cranking pressure in a totally different (much milder) combo with iron heads.

Static compression ratio and camshaft profile (intake closing, etc...) go hand in hand to determine the dynamic compression ratio.

Do a web search for "dynamic compression ratio calculator." I think you can push the limits of the calculator. In other words it is a little conservative if anything.
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Old 04-02-2003, 08:03 AM
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People are getting better & better at running High Compressions on street engines with pump gas.

Aluminum Heads make a huge difference in Hi comp engines due to their cooling characteristics. Also the combustion chamber design means alot, it takes some really good head work to clean up all the Hot & cold spots in High Comp Street motors. I've seen 12.5 to 1 engines doing great at the track. On the other end of the spectrum, I've seen 10.5 to 1 engines with great gas in them loosing all power above 4k rpms due to Detonation, because of bad head designs & timing issues.

On fuel injected engines it's much easier to run high compressions due to the OBD1 & OBD2 systems having Knock sensors & computers that are capable of pulling timing (KR) when they sense detonation. Now that OBDII is a standard, people will be able to do some great things with these computers, until they finally get around to locking down the computers so that they can't be accessed.
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Old 04-02-2003, 08:42 AM
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[quote]Do a web search for "dynamic compression ratio calculator." I think you can push the limits of the calculator. In other words it is a little conservative if anything.<hr></blockquote>

Nice tip I found some free software that makes this too easy to figure out.

[ April 02, 2003: Message edited by: bigmark ]</p>
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