Which CAM will give me 9.5+ Compression? - Pontiac GTO Forum
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post #1 of 18 (permalink) Old 12-29-2014, 08:36 PM Thread Starter
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Which CAM will give me 9.5+ Compression?

Need some help selecting a Cam. I have a 400 that is .030 over. 62 heads. flat top pistons, .042 head gasket thickness. I want to select a cam that will have sufficient valve overlap to bring my compression ratio into the 9.5 to 9.8 range. Street / strip use. Any suggestions?
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post #2 of 18 (permalink) Old 12-30-2014, 09:06 AM
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CR isn't dictated by camshaft. It's a result of the volume of the combustion chamber. There is a theory that a large cam with a lot of overlap will 'bleed off' static compression. This is true, at idle speed and low rpm cruise. But not in the powerband. And not at full throttle, when the cam does its job and packs the cylinders with fuel and air. With your current combo, you will need either dished pistons or 95-100 octane fuel. You MAY be able to squeak by with 93 octane if you have a properly jetted carb and a perfect ignition curve.....but it's a narrow edge.
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post #3 of 18 (permalink) Old 12-30-2014, 04:59 PM
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... in spades with chocolate on top.

This so called "pressure bleed off" approach only works when you're running a cam that's optimized for a high rpm range, and then only while the engine is running well below that range (in other words, when it's very inefficient and running like a dog). When you get up into the rpm range where the cam profile creates maximum VE (volumetric efficiency), this "pressure bleed off" phenomenon disappears.

Any detonation defense you get from doing this comes from placing the peak VE point so high in the rpm band that the engine doesn't have time to get into detonation because the compression/ignition/power cycles are happening so fast.

Can it work? Yeah, sort of ---- but to get there you have to build an engine that's only "happy" at high rpm, which means it's going to be a pig under normal street driving conditions.

For a street engine, it's much better to choose a cam that places peak VE in the rpm band where the engine will be living most of the time, and also build it with a compression ratio that is sensible enough to allow it to be happy on the fuel available. There are no short cuts.

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post #4 of 18 (permalink) Old 12-30-2014, 06:10 PM Thread Starter
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Thanks very much you guys.. so the next question is.. short of replacing my pistons, what if anything can I do with my 62 heads to get them to a CR that will allow me to run pump gas? I've seen some places that say you can remove material around the valves which increases flow and reduces compression to a usable level.
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post #5 of 18 (permalink) Old 12-30-2014, 06:43 PM
 
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Thanks very much you guys.. so the next question is.. short of replacing my pistons, what if anything can I do with my 62 heads to get them to a CR that will allow me to run pump gas? I've seen some places that say you can remove material around the valves which increases flow and reduces compression to a usable level.
The amount you will remove will only be about 2 cc's. Nowhere enough. Pistons are your answer-unless you choose to get a different set of heads which have larger chambers, this is what Pontiac did to lower compression, larger chambers.

But, depending on which heads, you may or may not get the screw in studs which are better, but can be fitted to any head for more $. So, new pistons would probably be the better of 2 evils.
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post #6 of 18 (permalink) Old 12-30-2014, 08:15 PM
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I've got an Excel spreadsheet embedded in a Word document that I've posted on here in various places several times. You can use it to easily play "what if" games on compression by seeing what effect making changes to various engine dimensions will have on compression ratio.
The bad news is that it's highly unlikely you're going to be able to make those '62's "live" on a street engine on pump gas without replacing pistons, even if you run super-thick head gaskets (which is a horrible idea all by itself).

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post #7 of 18 (permalink) Old 12-30-2014, 09:33 PM Thread Starter
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hmmm.. well.. if I do have problems...how about octane boosters? I see I can get a case of 12 bottles online for about $80. Assuming a bottle treats a full tank of gas, that would easily last me a full summer. I'd be perfectly fine with that.

Or this stuff: http://www.batterystuff.com/fuel-treatments/OScase.html

More expensive but sounds like it would do the trick. Just have to toy with the ratio until I get it to the point it doesn't ping but if I can start with 93 octane and add 6 points I think I should be good? Would still be one bottle per tank more or less. Couple hundred bucks for a summer of fun.. can't beat that.

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post #8 of 18 (permalink) Old 12-30-2014, 10:26 PM
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Better read up on that booster. To raise the octane rating of a tank of gas up to where it would need to be would require significantly more that just one bottle per tank. If you work it out you'll probably find that trying to get by with booster would be significantly more expensive than just finding a source for race gas and running that.

Trust us on this. There aren't any shortcuts.

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post #9 of 18 (permalink) Old 12-31-2014, 07:19 AM Thread Starter
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This stuff called "race gas" RACE GAS | Racing Fuel Concentrate claims to raise octane by 4 full octane numbers with 2 ounces per gallon. So 93 becomes 97. $163 for case of 6 @ 32 ounces per. Basically one can per tank full. That's getting a little steep. Hopefully I can walk that fine line that Jim was talking about with properly set up carb and careful timing control, and only need minimal if any booster. Don't really have much of a choice short of pulling my engine and doing a complete rebuild. Plus these were stock heads in 1969. I refuse to cave into the man giving us crap gas! lol
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post #10 of 18 (permalink) Old 12-31-2014, 07:40 AM
 
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Do you know any drag racers? They might sell you some left over fuel from last year for less than new stuff. Some use it all, some have leftovers which would be more than ample for use in an old school street car.
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