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Old 11-09-2012, 03:09 PM
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Quick CR question

Stock rebuilt "tbi" era short block bored .040 over with stock GM vortec cylinder head compression ratio? hoping for 9:1?

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Old 11-09-2012, 03:27 PM
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okay well looks like ive answered my own question and the CR would be lower then I want.
Can i get someone to refer me to the proper pistons? I would like 9:1 CR.

350tbi block, vortec heads, carb.. daily driver truck..
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Old 11-09-2012, 04:48 PM
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Originally Posted by 0trbo4myCHEVUICK View Post
okay well looks like ive answered my own question and the CR would be lower then I want.
Can i get someone to refer me to the proper pistons? I would like 9:1 CR.

350tbi block, vortec heads, carb.. daily driver truck..
Depends on a couple major things and a couple important but lesser contributors.

Numeral uno-A is whch head the 64-66 cc casting 14102193 or the 76-78 cc chamber head 14102191?

Numeral uno-B is the piston, GM uses a thing with a big round dish and 4 big valve reliefs that has considerable volume in the dish and greatly diminises squish/quench.

These two things establish most of what the compression will be. When rebuilding these things it is a great idea to replace the pistons with something that has a D shaped dish or cup as Keith Black calls them. This can be used to get the compression up and this configuration greatly controls and improves the combustion process making it more efficient and lowers the octane needs of the motor for the compression used.

The other impacts on both compression and squish/quench are the thickness of the head gasket and the distance between the crown (top) of the piston and the block's head deck. GM production specifies .025 inch. Not all come out this way as sometimes you find your the owner of a factory recovered damaged in production block that sometimes are milled to something less down to nothing. Or this milling operation sometimes called zero decking may have been done by your rebuilder to restore a flat and square head mounting surface. For best squish/quench you want the distance betrween the piston crwon and the heads squish/quench deck to be close to .040 inch, that's the sum of the crown to deck clearance plus the head gasket thickness.

This leads to the next observation which is the distance between the wrist pin centerline of the piston to the outside edge of the piston's crown. A production piston will have 1.56 inch here. If the block has been zero decked the piston may measure 1.535 inch to restore the .025 inch deck clearance. This causes another bad situation for that you can get into in that many rebuild pistons come in this more or less 1.54 compression height and if they are used on a standard clearance block the piston craon is now down the bore .050 inch and a lot of compression is lost, so you've got to know these numbers before you put things together.

Don't know where you want to go with the TBI engine, there isn't a lot in terms of camshaft that can be done before the computer goes "TILT". It is extremely sensitive to the relationship of RPM, throttle position, and manifold vacuum (GM calls this Manifold Absolute Pressure "MAP"). Edlebrock makes a flat hydraulic tappet cam for the TBI that is about the upper edge of what you can do without delving into the computers PROM chip; this thing is part number 3702 it along with some more compression will really wake up the TBI engine, at least to the limit of head flow with the Swirl Port.

So come on back with some details of where you wnat to go. Cruising the Keith Black site lets you look at piston choices and run their compression calculator. If you wna tot run the DCR which is important the 3702 cam closes the intake at 24 degrees after bottom center measured at .050 inch lift and the stock rods are 5.7 inches long. United Engine & Machine Co. Incorporated

I'm out of the shop this weekend, my wife has plans, I'll be back on Monday.

Bogie
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