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Discussion Starter · #1 ·
I'm getting ready for final assembly on a very basic budget rebuild of a 350.
specs
350 with 0.020" oversized pistons/bore
flat tops with 4 valve reliefs
~0.045 in the hole based upon what I measured at teardown. I believe this may be due to something like 0.025 deck height and 0.020 jobber rebuilder pistons with the reduced compression height. But just a guess based on some reading.
Vortec heads
fairly basic 0.462/0.470 lift COMP CAM
Likely Felpro 7733PT2 PermaTorque head gaskets; 0.040 compressed thickness

this is approximately static 9.3:1 assuming 6cc for valve reliefs and 64cc for vortecs (I've heard 62cc may be actual)
could be ~9.5:1 if they are 5cc and 62cc values.

As you can see, I'm using the basic and forgiving 0.040" compressed PermaTorque head gaskets so my quench is going to far from the ideal 0.040". looks to be 0.085" or so. (I need to remeasure the deck height again to get exact numbers.)
A friend gave me the short block so I honed it myself, measured it, and threw in all new bearings, rings, and cam. Reused the whole rotating assembly. The engine ran well in its previous life with early camel hump heads and a 0.440/0.460" cam. So, with the better vortec heads I don't imagine I'll have much problem running it like I have it setup. I didn't have any machine work done myself hence why the pistons are so far down.

Does any one have any reservations to what I'll be putting together? I think machine work is out of the question at this point due to budget and progress so far, as well as the previous iteration working OK. My only other option seems to be the felpro thinner 0.015 head gaskets. However, from what I can tell I do not believe these will work out since I didn't have the bock decked and the surface prep'ed for these. re: typical budget rebuild head and deck surface.

also, often you see rebuilds reusing the 12cc dished factory type pistons. The only tight quench area on these is with the thin outer radius of the pistons that isn't dished. Does this thin outer ring really affect quench more beneficially vs what I'm running with the flat tops? Seems as if the dish shape doesn't result in much quench benefit.

Thanks!
 

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Hand honing isn’t any more like machine work that than sanding a board with a disk sander is a plainer.

Wrong, wrong pistons, two paths open to you being zero deck the block have a machine shop do it rather than you with a disk sander. Or replace the pistons with standard 1.56 factory compression height pieces. These low compression height pistons are used by rebuilders that zero deck the blocks for cosmetic and sealing reasons on engines to restore the original factory compression ratio. In this case the decking is not for increasing compression and or restoring best squish/quench clearance for aluminum head’s using thick composite gaskets.

You're not alone this is a common hobbiest and even small shop builder error.

Bogie
 

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It is not too late to have it 0-decked before you put it together,
we had a 350 (20+ years ago) similar to what you have, we had crappy 882 heads on it and a little too big cam and Torker 2 intake, you guess what it was like to drive:poop:
 

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If you've got a big quench 350, you can turn that low compression frown upside down with nitrous. Here's a link about one of my engines from back in 2014, have to admit I trolled some of the members here with that thread...
What do you guys think of this sbc street/strip combo?

Finally got over my obsession with the rebuilder hyper pistons, but still building sbc's with big quench. Here's a link with info about my current engine... http://grannys.tripod.com/20102.html

Grant
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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There is much to consider here. I will try to avoid tangents. While the .045 in the hole and .040 gasket thickness gives you an undesirable .085 quench area, it may have actually been what kept you out of trouble on the 1st build. Considering the 4 valve relief flat top pistons you have....If you were to zero deck your block, run the .040 gasket with the vortec heads (the 906 vortecs on my S10 did measure 62cc) you would end up with a nice quench area but likely too much compression for pump gas and the Comp cam. The larger quench area would serve as "lesser evil" perhaps.

As Bogie mentioned, wrong pistons. Something with a dish would serve you better, with the zero deck. You mentioned your budget and many of us are forced to consider that, too. Every person on this forum would likely agree that in hot rodding you pay for an education in this hobby one way or another...time, money and/or mistakes. Buying and building things twice adds up.

The basics on your question regarding the piston shape to the quench area. The broad strokes, as I understand them. Flat tops, with a good quench area do well. The "outer ring" on the dished piston you speak of works to usher the mixed fuel and air inward. In doing so, the turbulence creates a better mixture of the 2, resulting in a more efficient combustion. If you're looking at a dished piston, I think most would agree that the D shape is the way to go. That uses the quench pad on the cylinder head more effectively to enhance/shape the mix. The idea is that a more efficient mix burns faster/better. Less prone to detonation.

The Ricardo-esque combustion chambers on modern heads do a better job of enhancing and shaping the mixture too. The vortecs certainly help that far more than the camel humps on your first build.

In the end, have I run a large quench and driven the hell out of it? Sure have. Do I do that now? No.

I'm a shade tree guy and by no means a professional. This is the best of my understanding. Anyone can feel free to correct or comment on anything.
 

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Stop the bleeding now and get the block taken care of correctly and get the correct pistons for both quench and compression.

For a street driver, low HP build without any power added, .085” quench is terrible and both performance and fuel economy will suffer because of it. The next item that will arise is difficulty in keeping cool at low engine speed when cruising because the timing will have to be in the tank to keep it from detonating/pinging.
 

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Stop the bleeding now and get the block taken care of correctly and get the correct pistons for both quench and compression.

For a street driver, low HP build without any power added, .085” quench is terrible and both performance and fuel economy will suffer because of it. The next item that will arise is difficulty in keeping cool at low engine speed when cruising because the timing will have to be in the tank to keep it from detonating/pinging.
I'm at .084", no sign of detonation or pinging on pump gas @ 36 degrees with no curve. Likely a different story if my application was heavy car, poor gearing, stockish converter.

But for light car with proper gearing and manual trans, .084" works for me. 3133rpm @ 70mph, still gets almost 21mpg on the highway. Zero problems keeping it cool, runs pretty good NA. I could easily bolt on a turbo with little other changes, but I prefer not to carry around all that extra weight.

I got started down this path much like the OP, just making the best of what I had at the time. Would have missed out on a lot of first hand experience if I had listened to popular opinion, probably would not be where i'm at today. Everything you do is a learning experience regardless of the results, I found that it's far easier to think outside the box if you are never inside the box to begin with.

Grant
 

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The simple compromise is to ditch the .040 Perma Torque for a .015 or .019 shim. The only reason to use a Perma Torque gaskets is because the mating surfaces are so crappy that your unsure if you can get a tight seal. Using Perma Torques is like wearing rubber gloves because your pen leaks ink.

Bogie
 

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I'm at .084", no sign of detonation or pinging on pump gas @ 36 degrees with no curve. Likely a different story if my application was heavy car, poor gearing, stockish converter.

But for light car with proper gearing and manual trans, .084" works for me. 3133rpm @ 70mph, still gets almost 21mpg on the highway. Zero problems keeping it cool, runs pretty good NA.
so
You have a light car that gets 21 mpg at 3100rpms cruising. So if you have a mild cam at 2200rpms and pulling a trailer at 500-1000lbs - you wouldn’t be recommending this mess. And you’re leaving 5-10 mpg on the table with proper gearing and quench if you have compression in a reasonable range. And you’re damn near at the all in rpm for the timing. Of course you’re not overheating and pinging. But who runs a motor like that????
 

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so
You have a light car that gets 21 mpg at 3100rpms cruising. So if you have a mild cam at 2200rpms and pulling a trailer at 500-1000lbs - you wouldn’t be recommending this mess. And you’re leaving 5-10 mpg on the table with proper gearing and quench if you have compression in a reasonable range. And you’re damn near at the all in rpm for the timing. Of course you’re not overheating and pinging. But who runs a motor like that????
Me! OP didn't say what the application was, just giving him a "glass half full" perspective on how he may be able to enjoy what he already has. Real world experience from someone who has been there with the rebuilder pistons, found a way to make the best of it, and liked the results enough that i'm still refining the same general theme 20 some years later.

And you’re leaving 5-10 mpg on the table with proper gearing and quench if you have compression in a reasonable range.
If quench/compression is optimized to get that extra mileage on the highway, gets a lot harder to spray a 350 with rebuilder pistons to 700hp on pump gas. In my case it's all about the compromise.

Grant
 

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Discussion Starter · #16 ·
All,

I appreciate the myriad of responses to my questions.
The application will be in a 1985 firebird with 700R4, 3.42 rear gears, and either stock or 2200 or so stall converter. I would guess the car weighs 3400lbs max. My LT1 T56 4th gen weighed 3450 so this one should be a little less.

yes, this is a very budget build. party because that is the constraints I placed on it. the car cost $2400. Including rebuilding and fixing so many other things on it I'll have near $6K in it and I don't think the car is worth 5k and I may not keep it long term.

The reground rod journals is an interesting idea. Decking the block would probably be almost just as easy and more straight forward... other than I already installed the freeze plugs & cam bearings.

for the 0.015", 0.019", or 0.026" compressed shim head gaskets, are these OK to use with 'normal' block and head surfaces? I've never used a thin gasket and always understood that they needed a more refined surface finish. Maybe that is just certain MLS? My block surface is 'normal', for what ever thats worth, but not recently refinished. The heads I haven't gone through yet but can easily have them skimmed. They are in really good shape so that is likely all I would need to redo (other than springs, seals, cleanup, specs check etc)

with 62cc 906s and these gaskets this is approximate static CR
0.015: 10.1:1
0.019: 10.0:1
0.026: 9.85:1
 

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Discussion Starter · #18 ·
If your pistons have 20cc cups and are so deep in the bore, how are you getting these compression ratio?

I was using this basic calculator. one shortcoming is it doesn't account for specific gasket fire ring ID. Perhaps that is my mistake.
3.48 stroke
4.020" bore
0.045 in the hole
6cc pistons
62cc 602s (may vary)

If this one is a poor estimate then I should revisit with a different calculator.
I may have had bore a 4.030 in my previous post numbers.
 

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My computations are based on a 64 cc chamber which is more likely or even bigger on a 906 head. With a 4.020 bore, 3.48 stroke, 64 cc chamber, 4cc relief in flat top piston, .045 deck to crown clearance.

Formula is ((swept volume) plus (volume above the piston at TDC including crown volumes)) divided by (volume above the TDC piston including crown volumes).

With the following gasket thickness:

.040 nets 9.26 to 1 with .085 squish/quench
.026 nets 9.54 to 1 with .071 squish/quench
.019 nets 9.69 to 1 with .064 squish quench
.015 nets 9.78 to 1 with .060 squish/quench

My calc’s above are by long hand. But I compared them to the calculator you used,I think you may be putting a negative sign in front of your valve clearance volume where there should be plus sign or no sign.

Bogie
 

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Discussion Starter · #20 ·
.040 nets 9.26 to 1 with .085 squish/quench
.026 nets 9.54 to 1 with .071 squish/quench

.015 nets 9.78 to 1 with .060 squish/quench

My calc’s above are by long hand. But I compared them to the calculator you used,I think you may be putting a negative sign in front of your valve clearance volume where there should be plus sign or no sign.
thanks for the extra verification.
seems the summit calculator needs a revision. I was sure not to use (-) in any of my previous entries.
Rectangle Font Screenshot Software Parallel
Rectangle Font Parallel Screenshot Number

I had a little variation in my previous numbers with a 5cc instead of 6. I do not know the actual cc of my pistons, but a few I sampled online listed 5-6, so i used the smaller of the 2.
 

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