Compression Depression

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scottylack

scottylack

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ok ... big problem. Putting togetrher a 360 which will have a Paxton Supercharger on it.Decided on a 78 block... stock 4027569 heads, all redone.
I picked these heads as I figured the compression was low enough to run...well I cc'd the heads and they came out to 65 cc ... I figured they would be in the 70's at least, mid 70's. this brings my compression up into the 9.6 range which is borderline for this suercharger.
Why are the chambers so small? Figured with the compression they ran back then, they would be bigger .....??? Questions....
I did three cylinders... all were 65-66 ..ml's. I figure this is the same a cc's....
HELP!
 
My guess is they may have been shaved a bunch and/or the valves changed to a style that occupies more space in the chambers. When I rebuilt my X heads I cc'd them before changing valves and they cc'd an average of 70.8 cc's. I changed the valves to REV stainless valves, same size, and they cc'd an average of 66.2 cc's. Valves can make a big difference as you can see.

You know you can always use a thicker head gasket to lower the compression. Cometic makes them in most any thickness you want. Running a supercharger you need real good gaskets anyway.
 
I'd probably go with some Cometics. They can custom make a set about any way you want them. Probably cost around $200 but it'll be cheaper/easier than changing pistons or getting a different set of heads & having them made ready to use.
 
How much boost do you plan? The "little" Paxton on my Mustang only makes 6-7 pounds. The compression is about 9.5 (I don't really know, it's a stock '87 motor.) and I've never had a problem as far as detonation is concerned.
 
Speed Thrills makes a noteable mention. It is possible to get away with this. A bit of help would be in the timing.
 
as long as full boost is under 8 psi on 92 premium octane...9.5:1 is doable...its when you want boost levels over 8 psi/.7 bar that the compressor REally starts heating the charge air...and a Charge Air "cooler" (intercooler) is required for sustained pulls over 8 psi or so.

For a paxton supercharger setup...I would recommend a Air/Water intercooler for any boost level over 8.
 
I believe its because a stock 360 had dished pistons to increase the combustion chamber volume. And 66cc is correct for those late 70's 360 heads.
 
scottylack said:
anybody else?
Click to expand...

I have a new set of std bore 360 flat tops pistons w/o valve reliefs I decided not to use due to the low compression figures. These pistons sat .1092 in the hole @ tdc

I changed them out for a set of zero deck ht kb's. (more like .020 below but close enough)

The pistons I removed from my assy gave me around 8.6 static with the 66cc heads I have. Never ran them, they are new.

Maybe these pistons would be more appropriate for your build. Just food for thought.
 
with that paxton setup i highly recommend a a/f ratio gauge, boost gauge, and a fuel pressure gauge. reason being you'll be able to monitor that sucker really well. i have a paxton on my truck and without those gauges you wont know if the belt is slipping or not, if you're running rich, and if you have the right fuel pressure.

When i first got my truck it wouldn't pull over 4000rpm because the ecu for the fuel pumps told the second pump to come on and it would flood my engine basically.

the sound from the paxton is very nice although i think a roots/screw system would be more fun since the power curve is opposite of the paxton
 
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