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Old 09-01-2010, 05:15 PM
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Quote:
Originally Posted by oldbogie
PS you got me to thinking. I never, nor have I read about, a head swap to Vortecs that also included a piston swap to idealize the squish and quench characteristics of the Vortec head. It's always been a Vortec head strapped to whatever bottom end was already there whether the pistons had round dishes or not before and after. So your comment on greater compression tolerance got me thinking that I need to build a stock Chevy small block with those round dish pistons and pre Vortec heads, then put Vortec heads on it and redo the dyno run, this is what we usually see for data. Then take the engine apart and put D dish or flat top pistons in it, idealize the deck clearance to run the compression up and maximize the squish/quench of the head taking the SCR to the tolerance limit of the existing timing to see how far the SCR can be pushed before the chamber pings. Then test to see if the usually seen lost bottom end torque of these heads is recovered while the top end gains are either improved or at lest not compromised. I've never done it quite that way in back to back tests nor have I seen it done. hummmm!

Bogie
Yep, I need to start thinking about those details next. My big problem is that the whole SCR/DCR/squish/quench thing makes my head hurt... don't be surprised when I come back with more questions!!! I sure miss the days of just building to a target of 8.75 SCR and calling it good to go for regular pump gas.... but I guess I'm showing my age.

Right now I'm running stock style 327 flat top pistons with 4-D valve reliefs, but the compression height on these pistons leaves them something like .04 in the hole at TDC. Certainly not optimal from what I've been able to read about Vortec heads, but if the existing pistons are in good shape it all goes back to that cost/benefit analysis you discussed earlier, particularly since this is a truck engine and I don't intend to push the envelope anyway....

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  #17 (permalink)  
Old 09-01-2010, 05:41 PM
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Quote:
Originally Posted by Uncle Buck
Yep, I need to start thinking about those details next. My big problem is that the whole SCR/DCR/squish/quench thing makes my head hurt... don't be surprised when I come back with more questions!!! I sure miss the days of just building to a target of 8.75 SCR and calling it good to go for regular pump gas.... but I guess I'm showing my age.

Right now I'm running stock style 327 flat top pistons with 4-D valve reliefs, but the compression height on these pistons leaves them something like .04 in the hole at TDC. Certainly not optimal from what I've been able to read about Vortec heads, but if the existing pistons are in good shape it all goes back to that cost/benefit analysis you discussed earlier, particularly since this is a truck engine and I don't intend to push the envelope anyway....
See! There is an advantage to a low compression engine, fewer head aches.

Bogie
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Old 09-01-2010, 07:41 PM
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Quote:
Originally Posted by oldbogie
See! There is an advantage to a low compression engine, fewer head aches.

Bogie
We took down a similar engine a couple weeks ago. They guy who built it was very disappointed with it. He'd bought every bolt-on he could think of; hot ignition, big tube headers, big cam, nice intake, ported 461s, and more. First time I'd ever seen forged pistons sitting .043" in the hole. He had a machinist build the bottom end.

Our plan is to deck the block down to 9.004", leaving the piston at about .021" and use a coated Felpro steel head gasket.(That should give us a quench distance of about 039 or .040") Distance between top of piston and flat area of the head..
It's a small journal, and I'm sure came with steel gaskets anyway but the manifold is aftermarket and doesn't know that. So it will call for the intake surface of the heads to be cut about .022".
It will still take some window matching on that intake but that's just something we always do anyway.
With the right cam he should give him back a nice running engine. Total cost of machine work will be about 150 or 175 dollars.

Didn't consider pistons because it's a recently bored .040, but run long enough to warrant keeping the pistons where they are... Well we took them out and trued up the balance. IMO, a quench much over .040" is just asking for spark knock, and/or an engine that won't stay tuned.
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Old 09-01-2010, 10:26 PM
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Quote:
Originally Posted by Duntov
We took down a similar engine a couple weeks ago... IMO, a quench much over .040" is just asking for spark knock, and/or an engine that won't stay tuned.
That's something I've been thinking about. But with 64cc heads and flat top pistons (say 5cc for valve reliefs) I'm looking at a SCR of 9.65 if I keep quench to a max of .045. Haven't started cam shopping yet, so I can't even make an educated quess at DCR, but that sounds way too much compression for 87 octane in a carburated truck engine built for torque (even with "compression tolerant" Vortec heads).

I've hijacked this thread enough- but I will start a new thread when I actually start spec'ing this engine out. I'm going to need lots of help!
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  #20 (permalink)  
Old 09-01-2010, 10:51 PM
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Quote:
Originally Posted by Uncle Buck
That's something I've been thinking about. But with 64cc heads and flat top pistons (say 5cc for valve reliefs) I'm looking at a SCR of 9.65 if I keep quench to a max of .045. Haven't started cam shopping yet, so I can't even make an educated quess at DCR, but that sounds way too much compression for 87 octane in a carburated truck engine built for torque (even with "compression tolerant" Vortec heads).

I've hijacked this thread enough- but I will start a new thread when I actually start spec'ing this engine out. I'm going to need lots of help!
9.65 is a bit high for 87
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  #21 (permalink)  
Old 09-02-2010, 03:56 PM
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Quote:
Originally Posted by Duntov
We took down a similar engine a couple weeks ago. They guy who built it was very disappointed with it. He'd bought every bolt-on he could think of; hot ignition, big tube headers, big cam, nice intake, ported 461s, and more. First time I'd ever seen forged pistons sitting .043" in the hole. He had a machinist build the bottom end.

Our plan is to deck the block down to 9.004", leaving the piston at about .021" and use a coated Felpro steel head gasket.(That should give us a quench distance of about 039 or .040") Distance between top of piston and flat area of the head..
It's a small journal, and I'm sure came with steel gaskets anyway but the manifold is aftermarket and doesn't know that. So it will call for the intake surface of the heads to be cut about .022".
It will still take some window matching on that intake but that's just something we always do anyway.
With the right cam he should give him back a nice running engine. Total cost of machine work will be about 150 or 175 dollars.

Didn't consider pistons because it's a recently bored .040, but run long enough to warrant keeping the pistons where they are... Well we took them out and trued up the balance. IMO, a quench much over .040" is just asking for spark knock, and/or an engine that won't stay tuned.
One has got to watch for those 1.54 inch compression height pistons when rebuilding. They are designed for zero decked blocks that retain the original piston crown to head deck clearance, thus I guess it's safe to say some semblance of original compression ratios. If they are used in a stock height block the piston crown will around .040 inch below the head deck instead of .025. Or even zero if the intent of zero decking was to gain compression by having the head deck and piston crown the same distance from the crankshaft center line. Many builders, hobbyist and professional, get skunked by these pistons which between the pin and the crown are .020 inch shorter than stock production units.

The first thing is to carefully check the dimensions for the piston being ordered to be sure they are dimensionally what is needed for the desired results.

Bogie
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Old 09-02-2010, 05:09 PM
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Quote:
Originally Posted by oldbogie
One has got to watch for those 1.54 inch compression height pistons when rebuilding. They are designed for zero decked blocks that retain the original piston crown to head deck clearance, thus I guess it's safe to say some semblance of original compression ratios. If they are used in a stock height block the piston crown will around .040 inch below the head deck instead of .025. Or even zero if the intent of zero decking was to gain compression by having the head deck and piston crown the same distance from the crankshaft center line. Many builders, hobbyist and professional, get skunked by these pistons which between the pin and the crown are .020 inch shorter than stock production units.

The first thing is to carefully check the dimensions for the piston being ordered to be sure they are dimensionally what is needed for the desired results.

Bogie
Bogie, that's one of the things that has me confused about 327's. I always thought the factory standard deck clearance for SBC's was .025. But stock style 327 flat top pistons with 4cc valve reliefs have a compression height to 1.654, which puts the crown .046 in the hole (assuming standard 9.025 deck).

With 64cc heads and a standard .040 gasket that gives me a SCR of 8.8 put a quench of .084!
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  #23 (permalink)  
Old 09-02-2010, 05:17 PM
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Quote:
Originally Posted by Uncle Buck
Bogie, that's one of the things that has me confused about 327's. I always thought the factory standard deck clearance for SBC's was .025. But stock style 327 flat top pistons with 4cc valve reliefs have a compression height to 1.654, which puts the crown .046 in the hole (assuming standard 9.025 deck).

With 64cc heads and a standard .040 gasket that gives me a SCR of 8.8 put a quench of .084!
Actually, stock blueprint compression height for 327 pistons is 1.675", you have the typical .020" reduced height that many rebuild pistons are. Only way you are going to get it right is to deck the block or replace the pistons with real 1.675" height models.
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  #24 (permalink)  
Old 09-02-2010, 05:36 PM
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327 Ch

Quote:
Originally Posted by Uncle Buck
Bogie, that's one of the things that has me confused about 327's. I always thought the factory standard deck clearance for SBC's was .025. But stock style 327 flat top pistons with 4cc valve reliefs have a compression height to 1.654, which puts the crown .046 in the hole (assuming standard 9.025 deck).

With 64cc heads and a standard .040 gasket that gives me a SCR of 8.8 put a quench of .084!
I'm sure that 1.675" ch is more typical with 327. If memory serves it's factory, or maybe 1.671" was factory. I think you're dealing with what I call machine shop pistons. I'm not sure who they older machine shops order from, but they have access to .080" pistons for283, 307, 327, 350and lots of other stuff I assume. The availability of 080 makes me feel a little better about an 060 bore.
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Old 09-02-2010, 05:40 PM
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Quick Reference for CH

http://www.flatlanderracing.com/
This site is a good reference for minor specs.. select pistons//TRW/forged/ Chevrolet and go from there.
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Old 09-02-2010, 05:51 PM
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Quote:
Originally Posted by Duntov
http://www.flatlanderracing.com/
This site is a good reference for minor specs.. select pistons//TRW/forged/ Chevrolet and go from there.
Thanks, very useful site. Just curious, when you CR's listed like that, are they based on a .040 head gasket?

I now realize that I was looking at the specs for rebuild pistons and didn't realize they were .020 shorter than stock. Now I get what Bogie was warning about....
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Old 09-02-2010, 06:18 PM
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Quote:
Originally Posted by Uncle Buck
Thanks, very useful site. Just curious, when you CR's listed like that, are they based on a .040 head gasket?

I now realize that I was looking at the specs for rebuild pistons and didn't realize they were .020 shorter than stock. Now I get what Bogie was warning about....
Cr is measured from the center of the wrist pin to the top of the piston deck.
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Old 09-02-2010, 07:08 PM
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Quote:
Originally Posted by Uncle Buck
Bogie, that's one of the things that has me confused about 327's. I always thought the factory standard deck clearance for SBC's was .025. But stock style 327 flat top pistons with 4cc valve reliefs have a compression height to 1.654, which puts the crown .046 in the hole (assuming standard 9.025 deck).

With 64cc heads and a standard .040 gasket that gives me a SCR of 8.8 put a quench of .084!
OEM piston pin to crown is 1.678 for the 327, not 1.654. 1/2 the 3.25 stroke is 1.625. The rod is 5.7 inches center to center of the holes. This makes the OEM piston crown 9.003 inches above the crank centerline. That puts the crown .022 inch below the head deck. Pistons made for rebuilt engines often lower the pin to crown by .020 inch to restore the factory clearance after zero decking the block to 9 inches or thereabouts. These pistons in the case of the 327 would measure 1.658 inch from the pin center to the crown.

Your quench distance is much too great at .084 inch, it should be half that, and certainly not more than .060. As the quench distance becomes larger the tendency for detonation increases. This forces compromises in one or more of several other things which ends up sub-optimizing the engine for power and fuel efficiency. The compromises can be high octane fuel without taking thermal advantage of the fuel, this is lost power, lost fuel mileage often a large number, this hits you on the ET slip and in the wallet. Other compromises would be lowering the ignition advance, richening the mixture which again sub optimize the power and cost excessive fuel burn. Excessive fuel burn not only costs you at the pump, but reduces engine life because the excess fuel washes the upper cylinder wall lubrication away speeding the rate of wear on the piston, rings and cylinder wall.

Simple solution for you is to disassemble the engine marking everything as to where it came from so it goes back together the same way, assuming there is little wear on the pistons, rings and cylinder walls. Take the pistons out of the bottom will be better if there's any ridge on the top of the cylinder walls cylinder after smoothing and lower glaze from the cylinder walls with sand paper. Break any ridge at the top of the cylinder. Do a light hone on the walls to remove any glaze. Deck the block to zero. Clean everything super well and reassemble.

Bogie
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Old 09-02-2010, 08:21 PM
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Quote:
Originally Posted by oldbogie
Your quench distance is much too great at .084 inch, it should be half that, and certainly not more than .060. As the quench distance becomes larger the tendency for detonation increases. This forces compromises in one or more of several other things which ends up sub-optimizing the engine for power and fuel efficiency. The compromises can be high octane fuel without taking thermal advantage of the fuel, this is lost power, lost fuel mileage often a large number, this hits you on the ET slip and in the wallet. Other compromises would be lowering the ignition advance, richening the mixture which again sub optimize the power and cost excessive fuel burn. Excessive fuel burn not only costs you at the pump, but reduces engine life because the excess fuel washes the upper cylinder wall lubrication away speeding the rate of wear on the piston, rings and cylinder wall.

Simple solution for you is to disassemble the engine marking everything as to where it came from so it goes back together the same way, assuming there is little wear on the pistons, rings and cylinder walls. Take the pistons out of the bottom will be better if there's any ridge on the top of the cylinder walls cylinder after smoothing and lower glaze from the cylinder walls with sand paper. Break any ridge at the top of the cylinder. Do a light hone on the walls to remove any glaze. Deck the block to zero. Clean everything super well and reassemble.

Bogie
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?
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  #30 (permalink)  
Old 09-02-2010, 08:35 PM
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Quote:
Originally Posted by Duntov
Cr is measured from the center of the wrist pin to the top of the piston deck.
Don't know what Cr is, but the measurement you describe is compression height or CH. Also, piston deck is the distance from the piston crown to the block deck surface with the piston at TDC. Perhaps you meant to say:
CH is measured from the wrist pin centerline to the piston crown.
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