i think i have figured out my compression to be 9.23 is this too much for a 6 psi supercharger on pump gas? 351w

 

It depends on how your set up. I am running 9..1:1 compression on a 6 psi Paxton. The shop that cut the cam for it said that it should run fine on premium fuel with the supercharger or regular gas without. They did convince me to set the quench distance the piston and heads to .045" instead of the .075" for a stock set up. I have a '69 351w with a .035 deck clearance. I bought keith black pistons that come to the top of the block so the quench distance is just the thickness of the head gasket which is supposed to be .043". I have not run the motor as of yet so I don't know if I got things correct or not. I did some research on quench distances though. Supposedly above about .050 the flame travel from the spark is erratic and causes predetonation from local hot spots.

Others can hopefully comment on this.

 

I was thinking of going with a .060 thick copper head gasket. by my calcualtions this would drop my compression .375 which would get me to about a total of 12.7? I have read that the"target" for pump gas is below 12.4:1 I have not heard of the quench distance-are you saying that my "thicker" head gasket puts the spark too far away from the fuel? I dont know how far below the deck the pistons are but they are a stock dished setup. I have never had a supercharged vehicle or "played" with a fuel injected motor before. all my other toys have been carburated and I never worried about compression. The lightning is not my everday vehicle but I dont want to have to run racing fuel. any more advice would be greatly appreciated - anyone?

 

The shop that I talked to was Elgin cams in Redwood city California. The guys there seamed knowledgable. They also turned me onto a local guy for other machine work that happens to hold the land speed record for a stock carberated gas engine. Don't rmember his name and I have the shop info at home. Your supposed to be able to run higher compression with a more controlled spark. The thing they did in the 70's was to reduce compression but it actually made the detonation worse becuase they did it by reducing the stroke or piston height opening up the quench distance. So the answer is yes to your question. Keep the chanber thickness between the piston and combustion chamber to .040-.050 thickness. If you are running pop up pistons your flame front is already going to be screwed up. If your have a lightening go to the many boards they have set up for it as those guys are pretty familiar with how to set up the newer motors. Running a supercharger is only going to increase your effective compresion ratio since your stuffing more air in the cylinders.

There is a good article in Automotive Industries Magazine this month that talks about the new motors from the big three and how they get such high horseposer and still meet emmisions. Most of it has to do with combustion design and spark control.

12:1 is really high for a supercharged engine. You would probably need premium fuel without a supercharger. I did an internet search on combustion and quench distance and came up with quite a bit of info on later model head and cylinder design. The newer engines are generally better due to the sophisticated chamber designs which control the spark better.

Good luck on your project.

 

THANKS FOR THE HELP. ON THE 12.7:1 CR I WAS MEANING THE COMBINED "TOTAL" CR WITH THE 6 POUNDS ALREADY FIGURED INTO THE EQUATION. I SHOULD ALSO MENTION THE HEADS I HAVE PURCAHSED ARE TRICK FLOW ALUMINUM "TWISTED WEDGE" - THE SPARK IS SUPPOSED TO BE CLOSER TO THE CORRECT AREA BY ROTATING THE VALVES A LITTLE ( THEY ARE NOT EXACTLY IN LINE LIKE A REGULAR SET OF HEADS.) WITH A NON SUPERCHARGED ENGINE- ALUMINUM HEADS ALLOW YOU TO RUN UP TO 1/2 CR HIGHER ON THE SAME OCTANE FUEL, DOES THIS HOLD TRUE IF THE MOTOR IS SUPERCHARGED? -----ANYONE??
THANKS JEFF MOON