Packard Forum Index » Modification Discussion

I found a head gasket on ebay that was reasonably priced. The thickness of the gasket is .100. Does anyone know what the compressed thickness is going to be? As I am planning out my 356 build, increasing compression is high on the list. With that in mind, the phenomenon of quench is one of the methods used to allow more compression without detonation. Quench is the distance between the top of the piston and the flat area of the head. The piston comes very close to the quench area of the head squeezing the air/fuel into the combustion chamber. If you can imagine stepping in a shallow puddle and the water sprays out from under your shoe. It has been found quench is nearly non-effective above .060" distance. .040" is pretty common target for suitable quench with a decent safety factor. Tightening the quench has provided improved performance, until the piston makes contact with the head, which is a problem. Building the 356, I'd like to target .035-.040 quench. If the available gaskets are .100 thick and don't compress much or any further, then I will have to spec a piston that tops out .060" above the deck surface to achieve the desired quench distance. If that makes the compression to high with an early 288 head, then I might have to use a stepped piston.
This is going to be a fun project. ARP has already mentioned $1000+ for a set of custom rod bolts. Not quite sure I need them as I'm not trying for high rpm, but new/better rod bolts would help to reduce worrying about it.

Mark

If I am recalling correctly, you can't go above the block with the piston stroke as the combustion chamber does not completely cover the bore and relying on the gasket thickness to provide the space is very risky at best. Remember that these were low-revving high torque engines where horsepower was more or less secondary.

Hi Don,
The goal is to get the piston closer to the flat area of the cylinder head, not into the combustion chamber.

Mark

FYI the V8's used a 0.045 quench distance.

FYI, it's usually not necessary to have $1000 custom rod bolts for two reasons:
1. I've always been able to find an existing part in the ARP catalog close enough to interchange.
2. Rod bolts are a high RPM problem; the 356" is a low RPM engine.

And yes, quench is a good thing. Back in the day, the flathead Ford V8 racers used pop-up and/or domed pistons into a matched recess in the cylinder head. The Harley-Davidson KR racers used a domed piston with the top edge facing the cylinder beveled at a 45-degree angle.

Finally, the Packard engineers knew what they were doing with the I8. Does anyone have the specs on the quench distance of the 8.7 compression '54 Caribbean? AFAIK, that's the highest compression ratio ever used on a production flathead engine.

One final thought; if I were building a high compression iron head 356", I'd definitely be planning to use water/alcohol] injection.

jack vines

The old used compressed gaskets hanging on my wall measure .066" thick, and the new ones in the cabinet average .080". Never seen any as thick as .100".

All the postwar straight eights had the pistons come flush to the top of the block within the range of normal manufacturing tolerances which seem to have been pretty good as I've never noticed any high or low. So the quench distance ends up being the thickness of the head gasket. To note is that the quench area is huge on these flatheads as the combustion chamber is just a wedge shape over the valves and quench covers more than half of the piston. Its a whole different ballgame to what is going on in an ohv. When the piston nears TDC there is a mad turbulent rush toward the sparkplug.

I'd note also that those Packard boys worked some very elaborate contours into the chamber. I'd love to learn their thinking on them as they changed from year to year. For my own cars I always look for the latest Packard head I can find.





My flathead Studebakers all run great but their combustion chambers are far simpler in form.

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Ross, FWIW, there are at least five different versions of the Champion head, with the last version being the highest compression.

jack vines