AMC turbocharging - adjustable compression ratio!!!

[Holeshot]

I was posting data about compression ratios, preignition, and piston to head clearance in another thread....

LOW compression can CAUSE serious pinging & force the use of race gas!!!

I have a '69 390 that I pulled out of my BBO '69. It was FULLY modified & dynoed 518HP on the HotRod Magazine traveling chassis dyno. It pinged like crazy on anything but race fuel. Upon disassembly & measurement, I found that someone had turned a bunch of material off the piston heads (dished them), giving a .118" piston to head clearance. The math revealed a 9.1~1 CR.

.045" is what that clearance should be for most steel rod engines (including motocrossers & go karts). The excessive clearance prevented the mixture from getting "excited" (heated) by the rising piston that was trying to squish it.


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[Holeshot]

Blower & Turbo Yahoo groups:

http://autos.groups.yahoo.com/group/Carburetedblowers/

http://autos.groups.yahoo.com/group/...gsmallengines/

http://autos.groups.yahoo.com/group/...turbochargers/

[MrFurious]

I was posting data about compression ratios, preignition, and piston to head clearance in another thread....

LOW compression can CAUSE serious pinging & force the use of race gas!!!

I have a '69 390 that I pulled out of my BBO '69. It was FULLY modified & dynoed 518HP on the HotRod Magazine traveling chassis dyno. It pinged like crazy on anything but race fuel. Upon disassembly & measurement, I found that someone had turned a bunch of material off the piston heads (dished them), giving a .118" piston to head clearance. The math revealed a 9.1~1 CR.

.045" is what that clearance should be for most steel rod engines (including motocrossers & go karts). The excessive clearance prevented the mixture from getting "excited" (heated) by the rising piston that was trying to squish it.

The compression ratio isn't the problem, just the quench height. You can run a 7.5:1 CR if the quench height is correct. The design of the combustion chamber in the head will also make a differance.

[Holeshot]

That is correct - the quench ht is VERY critical to the combustion process!!!