# Which CAM will give me 9.5+ Compression?



## EdR (Sep 2, 2014)

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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## geeteeohguy (Feb 2, 2008)

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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## BearGFR (Aug 25, 2008)

:agree ... 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.

Bear


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## EdR (Sep 2, 2014)

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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## PontiacJim (Dec 29, 2012)

EdR said:


> 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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## BearGFR (Aug 25, 2008)

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).

Bear


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## EdR (Sep 2, 2014)

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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## BearGFR (Aug 25, 2008)

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.

Bear


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## EdR (Sep 2, 2014)

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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## 666bbl (Apr 13, 2014)

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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## geeteeohguy (Feb 2, 2008)

Here's the reality: If you want to stick with your current combo, you can run a real, actual octane booster (like TEL130), race gas, or aviation gas. I know of no other 'octane boosters that actually do anything, other than turn the spark plug electodes funny colors. If you intend on putting realistic mileage on the car, you'll need to drop compression ratio, and do it with different heads or dished pistons. BTDT myself, over the decades. Run race gas in my '65 GTO, which sees less than 500 miles a year, and dropped the CR on my '67 GTO, which now runs on pump gas and sees about 3,000 miles a year. If your car can run on pump fuel, believe me, you will drive it _much_ more often.


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## RT-1 (Mar 21, 2012)

If you are anywhere near an airport, you can buy 100 octane low lead for your "Cesna". You have to bring tanks because they won't put it into a motor vehicle.


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## PontiacJim (Dec 29, 2012)

Here is an interesting PDF for a product said to increase octane rating. It talks of the 3 types of octane boosters used in fuels. The thing I thought interesting is: "Wondering whether the octane booster product you’ve been using contains MMT, or oxygenates? Here’s how to find out. Request a copy of the Material Safety Data Sheet (MSDS). Manufacturers are legally obligated to provide this information. Most importantly:Beware of octane booster products that contain oxygenates!"

You will also see a tab up top that tells about regulations and registering fuel additives.

Interesting was it also pointed out that the 90 octane rating simply had more ehtanol added. Hmmmm.:nonod:

http://www.epa.gov/oms/fuels/registrationfuels/

Also found this product that seems to show up on many forums of various muscle car types. Has a blending page to figure octane boost. http://wildbillscorvette.com/OctaneSupreme01.htm


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## geeteeohguy (Feb 2, 2008)

The TEL130 or Octane Supreme are one in the same, from Kemco. This stuff works excellently because it is Tetraethyl lead. It isn't cheap, but you get what you pay for. I've used it myself, and it works.


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## BearGFR (Aug 25, 2008)

..and real tetraethyl lead is a very nasty poison. NOT to be handled carelessly. You don't want to get this stuff onto your bare skin.

Bear


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## EdR (Sep 2, 2014)

666bbl said:


> 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.


Unfortunately I don't. I do know a pilot though that can get me airplane fuel. He said that's 100 octane. About a $1 a gallon more than pump gas. We'll see if I need anything first and then come up with a plan of action if I do. It sounds like if I'm careful with timing and carb set up, and run the engine cool I might be ok. If not I'll punt and come up with another plan. But thanks for the suggestion.


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## BearGFR (Aug 25, 2008)

Well Ed, sounds like you're bound and determined to go ahead with this anyway despite all the folks who've tried to warn you. Do yourself a favor and make very frequent examinations of your spark plugs with a magnifying glass. If you see shiny specs, that will be flecks of aluminum coming off your pistons from detonation --- regardless of whether you can hear it or not. When you see that, and you're almost sure to, stop running the engine and deal with the problem in the right way. Otherwise, you're very likely to damage the engine beyond the point where it can be repaired.

Bear


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## EdR (Sep 2, 2014)

BearGFR said:


> Well Ed, sounds like you're bound and determined to go ahead with this anyway despite all the folks who've tried to warn you. Do yourself a favor and make very frequent examinations of your spark plugs with a magnifying glass. If you see shiny specs, that will be flecks of aluminum coming off your pistons from detonation --- regardless of whether you can hear it or not. When you see that, and you're almost sure to, stop running the engine and deal with the problem in the right way. Otherwise, you're very likely to damage the engine beyond the point where it can be repaired.
> 
> Bear


Thanks Bear.. I'll do that.


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