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Old 09-02-2010, 08:13 PM
oldbogie oldbogie is offline
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Originally Posted by Uncle Buck
Here's the problem I keep running into: if I bring quench to something acceptable I drive CR up.

Maybe you can help me approach this from a different direction. I plan on using Vortec heads, and I know these heads are more compression tolerant- but how much? I'm rebuilding a carbureated 327 for use in low-geared 4x4 (Toyota FJ40). I plan to install a "torque cam," but haven't even started spec'ing one yet (does that help or hurt DCR?). Can someone give me a geneal idea of the max "safe" CR for this combination running plain old 87 octane?

Also, from a straight cost standpoint, is it cheaper to buy new pistons or have the block zero-decked?
Buying pistons over decking the block is something you'll have to look at since prices can vary quite a bit from place to place.

My tendency would be to go with pistons for two reasons.

1) Thinning decks whether head or block reduces strength and in this age of thin wall castings you want to preserve as much of that as you can to insure against gasket leaks and casting cracks. I don't mill OEM castings anymore than it takes to get a straight surface.

2) Piston selection when everything else is dimensionally known will let you dial in the compression and optimize the squish/quench. Flat tops as you have make for ideal squish/quench as their flat crown surface will close opposite the squish/quench deck of the head optimizing that area with the possible exception of the valve pockets (2 being better than the usual 4 but even 4 is way ahead of the OEM style circular dish. But as you see, you quickly get into CR trouble with tight chamber heads. The next best solution is choosing the so called D dish or D cup piston where the dish is kept under the valve pocket and a flat area not unlike that of a flat top piston is presented to the squish/quench deck. These let you maximize the squish/quench effect like a flat top while dialing in the SCR by selecting the dish volume. These type pistons can be had in cast or forged hyper-eutectic. Hyper-eutectic is nice stuff as it's very thermally stable which lets you run tighter clearances which reduces oil consumption, thus preserves the octane rating of the fuel you buy. you will want to hit a DCR between 8 and maybe 8.5. So you have to play with SCRs and the cam's intake closing event in the formula. One of the easier formulas to get at is at Keith Blacks house of pistons. So you'll have to get some data together on the dimensions that make the SCR and some specific cam candidate info and fit this stuff into the equations. Have link on me; http://www.kb-silvolite.com/calc.php Cam data you can get from the various cam stores like Comp, Edlebrock or Howards to mention a few. The KB link will want intake closing degrees After Bottom Dead Center (ABDC of BDC) and rod length which is 5.7 inch for 302, 327 and 350 Chevy's.

Go look at Chevy High Performance magazine, a few years ago they did an article titled "Agent 87". It gives a lot of clues as how to get a higher compression 350 to live on 87 octane while pumping out power in the high 300s to about 400. This was reprinted in a book by them titled "Small-Block Chevy Engine Buildups: How to Build Horsepower for Maximum Street and Racing Performance" This book has a lot of detail build info and dyno power graphs. While it's a lot more difficult to get an engine once in a vehicle to duplicate the kind of performance you can get on a dyno; it does lead you in some directions to max the amount and type of performance you're looking for without having to use your hard earned dollars on researching for yourself the effects of parts by guess and by golly. By performance between a crankshaft dyno and installed I mean more than the power numbers but how it reacts to detonation resistance, ignition timing, fuel ratios, power effects in different gears, etc. kind of stuff. This can be quite different once the motor has to haul around something with weight, gears and tires as opposed to exciting a generator or pump attached to the end of its crankshaft. I see people get all excited about numbers off a chassis dyno which are usually lower than expected. The purveyors of these things like to sell installed power looses as being around 15 to 20 percent. My experience says that upwards of nearly 50 percent loss isn't all that uncommon. So that 400 horse on the crank engine someone bought and had test feels cheated when the chassis dyno numbers read 260 corrected to 312. When in fact the installation may actually be loosing 50 percent off the crank which would make his 260 into 390, which is a lot closer to what was advertised by the seller of the motor.

OK its 7 something PDT and the ole' lady wants to do dinner so I'm out and down for tonight.

Bogie
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