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Hey everyone I am building a 351 cleveland with 4v closed chamber heads. I had some flat top pistons on my first build and now want to try pop up pistons to raise the compression to around 12.5. I have heard that if you run a big enough cam and your dynamic compression is below 8:1, you can run pump gas. The cam has an intake closing ABDC of 80 degrees. With the calculators online it tells me it will be close to a 7:1 dynamic compression, and in theory will be able to run off atleast pump 93. Im in Alabama so elevation is basically sea level. Ill be running megasquirt fuel injection and EDIS. Does anyone have any experience in dynamic compression?
 

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Not possible! The only way it can be done is with direct injection where no fuel is present during compression. 12.5 mechanical compression will detonate 93 octane regardless of how much you try to fool it with aluminum heads and or big camshafts. With iron closed chamber heads ,11 is max on 93. Don't confuse (audible) pre ignition with ( silent) detonation or oil pan failure will be the result. My back ground comes from 50 years of dealing with them including one I still own that is all aluminum.
Randy
 

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ding a 351 cleveland with 4v closed chamber heads. I had some flat top pistons on my first build and now want to try pop up pistons to raise the compression to around 12.5. I have heard that if you run a big enough cam and your dynamic compression is below 8:1, you can run pump gas. The cam has an intake closing ABDC of 80 degrees. With the calculators online it tells me it will be close to a 7:1 dynamic compression,
Listen to GT350, the late intake closing only works to an extent. In theory, if you ran such a late and cam with no overlap, it could behave OK in the one parameter of octane tolerance, but the design of that cam would be so miserable in every other sense that it wouldn't make any torque or power, it would counter every single thing you are trying to do

What happens is, eventually, despite the late intake lobe, the engine becomes efficient at some RPM and actually can fill the cylinder more than atmospheric pressure can on it's own, at that point, you are at LEAST at the 12.5:1 compression, but could even be higher dynamically, and that is when it gets ugly if on pump gas.

Although keep in mind that could happen at 10:1 too with the wrong cam choice, so it all has to match

Something to keep in mind

1 - Make power with heads (and intake and headers)
2 - Pick cam for RPM range and desired behavior for that setup
3 - Pick compression for cam and fuel.

Very simplified, and it's not as sequential is it seems, but just going for more compression isn't the way to do it, you are backwards. Although pretty big in volume, the 4V head flows pretty well, so pick a cam that meets the use of the vehicle, and then make compression and quench (if possible) right. I am not a parts dealer, but Brent Lykins at Lykins Motorsports can help you with a cam and piston choice (if a compression change is even required), he does some good C work, so worth a call or email

You will be amazed at the power a cam change alone can give depending on where you are now. Things he will want to know is, vehicle, gearing, tranny, current heads intake, and compression, quench, and end use of the vehicle along with your EFI plans. Anything you assume (don't measure) or do not answer the way you will really use the vehicle, will work against you in the end

That being said, I am a DCR guy, and like to use the PKelley program, any program is likely good enough, but you need to have experience with the numbers over time. I use it as an extra tool to check my choices. DCR is a love/hate, but no compression ratio calculation ever really happens in an engine, they are just indicators of mechanical varaiables. I have MANY builds across similar engine families using that tool, so you get to know how to use it as one of many tools, and like other tools, use it wrong, it will hurt you

As a rule of thumb though, I tend to use it to take the right cam and compression combo, and then lean intake centerline a little to get it to be a little closer to what I expect, usually as a buffer for street fuel. This is usually after head, cam, intake, compression, etc are all pretty close to being locked in. Then I try to keep DCR (using that program) at 8 or under on iron, or 8.25-8.35 on aluminum with a good chamber. I lean lower for big chambers, bigger quench distance, and/or a flame path that is obstructed by domes, etc.

Hope this helps
 
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