more than likely they are .041 installed..

 

So do you think the 1.605 pistons would be acceptable, if block is not cut excessively?

 

1.605 will put the piston top right at zero deck with most blocks.The 1.585 pistons are .020 down the hole at TDC. That usually translates to a half point in the ratio. Std Ford small block head gaskets are .040-.042.

 

with a 302 Engine 1968-72 and 77-Later Block w/8.206 Deck Height

85 to 87 5.0L HO piston 1.619 it will be 0 deck
the 1.605 it will be .014 down the hole ...most common
the 1.585 it will be .034 down the hole....
the 1.560 it will be .059 down the hole .. they call this one the chevy piston

and if you have a 1973-76 Block w/8.229 Deck Height well do the math... add .023 to the above numbers

 

OK, so help me do some calculation. If my block stays standard (no decking) and I go with the 1.605 'compression height' piston and normal head gasket......should I be able to run regular with a piston listed as 9.08:1? Man, you all give a guy a lot to think about!

 

yes...............

 

Thanks, Hemi. I'll look to stay with 1.605's. I'm still not clear on compression ratio. It seems to me that the closer the piston is to the the head the higher the compression ratio and therefore the higher the octane rating gas needed to avoid pre-detonation. What am I missing?

 

got this from another site...
Excessive cylinder pressure will encourage engine destroying detonation with no piston immune to its effects. The goal of performance engine builders should be to build their products with as much detonation resistance as possible. An important first step is to set the assembled quench distance to .035". The quench distance is the compressed thickness of the head gasket plus the deck height, (the distance your piston is down in the bore). If your piston height, (not dome height), is above the block deck, subtract the overage from the gasket thickness to get a true assembled quench distance. The quench area is the flat part of the piston that would contact a similar flat area on the cylinder head if you had .000" assembled quench height. In a running engine, the .035" quench decreases to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, average the chamber temperature, reduce detonation and increase power. Take note, on the exhaust cycle, some cooling of the piston occurs due to the closeness to the water cooled head.

If you are building an engine with steel rods, tight bearings, tight pistons, modest RPM and automatic transmission, a .035" quench is the minimum practical to run without engine damage. The closer the piston comes to the cylinder head at operating speed, the more turbulence is generated. Turbulence is the main means of reducing detonation. Unfortunately, the operating quench height varies in an engine as RPM and temperature change. If aluminum rods, loose pistons, (they rock and hit the head), and over 6000 RPM operation is anticipated, a static clearance of .055" could be required. A running quench height in excess of .060" will forfeit the benefits of the quench head design and can cause severe detonation. The suggested .035" static quench height is recommended as a good usable dimension for stock rod engines up to 6500 RPM. Above 6500 RPM rod selection becomes important. Since it is the close collision between the piston and the cylinder head that reduces the prospect of detonation, never add a shim or head gasket to lower compression on a quench head engine. If you have 10:1 with a proper quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will create more ping at 9.5:1 than you had at 10:1. The suitable way to lower the compression is to use a dish piston. Dish (reverse combustion chamber), pistons are designed for maximum quench, (sometimes called squish), area. Having part of the combustion chamber in the piston improves the shape of the chamber and flame travel. High performance motors will see some detonation, which leads to preignition. Detonation occurs at five to ten degrees after top-dead-center. Preignition occurs before top-dead-center. Detonation damages your engine with impact loads and excessive heat. The excessive heat part of detonation is what causes preignition. Overheated combustion chamber parts start acting as glow plugs. Preignition induces extremely rapid combustion and welding temperatures melt down is only seconds away!

 

i have a 96 302 that will at some point need more power. i was wondering what kind of head work i could do to yeild the same results capper was looking for, except more low end torque. any suggestions are appriciated. thanks

 

Thanks Hemi, I understand the relationship between quench, compression ratio and detonation now.

 

I guess I should throw this in. I swapped a 95 302 MAf into my 87 F150. One thing is to be noted is the lower intake on 94 on up doesnt have a Air temp sensor on it. You can see where it would be. I drilled and tapped it 3/8 pipe thread while it was in the truck. I tried to keep as many aluminum shavings out of there as I could. The truck runs great from 1000rpm on up. Idle is ok but I am upgrading to MAF when I get time.