Keep it under 10

9.6 - 9.8

Additional compression ratio does not make significantly more power. Just makes it hard to keep it out of detonation and things like chamber design, piston dome, and quench become a lot more important so for a DIY build where you will be using L98 iron heads and likely not paying super close attention to these small detailed measurements..... keep it in the 9.5 neighborhood.

Stock sized Wiseco pistons. Clean the bores with lacquer thinner. Lube the piston skirts and rings with ATF. Motor oil is too thick.

GD

 

Compression depends on the heads.

Aluminum heads REQUIRE a higher CR than iron. Around 8 - 9% higher give or take. So at the 10:1 range with iron heads, you'd want 10¾ to 10.9:1 with aluminum, all else being equal.

Ceteris paribus... which it NEVER is.

EFI allows somewhat higher static CR than a carb, because the ECM can back off the timing to keep you out of the destruction zone. Which is why you hear all about LS motors running 11:1 or more on pump gas... NOT ONLY are the heads aluminum, BUT ALSO, their control system is GENERATIONS better than what came with TPI. The CR levels you can attain with the intelligence and data rate that TPI provides are the better part of 2 FULL POINTS less than what a properly tuned LSx ECM will tolerate.

I would aim, were I aiming, with an iron-headed TPI motor in CA, for a static (TRUE mechanical) CR in the high 9s with TPI, and the mid 10s with aluminum heads. The 9.6 - 9.8 range mentioned above is close enough to work with. That would require one of the larger compatible cams though; long intake duration bleeds off cyl pressure and allows for more static CR without causing the problems that come with that at low RPMs. Pinging, detonation, ring & piston destruction, plug gap anomalies, etc.

I would also advise against "zero decking" the block in your situation. Deck it enough to get all cyls EQUAL; but no more than that. Specify to your machinist that you want your clock's decks SQUARE and EQUAL, rather than a numerical spec for deck height. EQUAL being important in regard to tuning, especially with a CRUDE system such as TPI that is GLOBAL and doesn't provide for setting individual cyls to their individual need.

My personal preference for piston ring build lube, BTW, is 2-stroke oil. Think about it. Ashless, designed to be mixed with gasoline, no plug fouling, … perfect.

 

Found this CR calculator:

https://www.rbracing-rsr.com/compstaticcalc.html

Bore 4.0"
Stroke 3.48"
Head Gasket 0.026"
Deck Height 0.014"
Piston top volume +7cc
Combustion Chamber 64cc

CR = 10.04

 

All "calculators" are the same.

2 + 2 = 4, no matter what "calculator" you use. Unless of course, you have unusually large values of 2, in which case it could approach 5; but that's another matter entirely.

Any "compression calculator" will give you the same results for those same #s. The question then becomes, do those "numbers" ACTUALLY MATCH whatever you are building.

Any compression "calculator" is ONLY AS GOOD AS THE NUMBERS YOU PUT INTO IT. And in most cases, that means, MEASURE stuff, not "look up specs", "assume", "I always heard", etc. Specifically, the ONE NUMBER most frequently LEFT OUT ALTOGETHER in these things, is deck clearance. The next most frequently mangled piece of it, is chamber volume... heads vary DRAMATICALLY from "spec", especially if they've been worked, and the valves are sunk down into the seats. (to put this into perspective, if the valves are both sunk .025" into the head from seat grinding, this adds between 2 and 2.5cc to the chamber volume) Another thing usually left out is gasket volume... the gaskets are ALWAYS larger than the bore, and that extra volume adds up... the difference in gasket volume for a .026" gasket between a 4" bore (which is what that "calculator" "assumed") and the gasket you ACTUALLY BUY, with a 4.100" bore dia, is near .25cc. And so on. If you don't have the TRUE VALUES for those quantities, you can punch whatever GARBAGE you do have, or invent, or "assume", into ANY "calculator", and what the "calculator" spits out will be equally GARBAGE no matter how "accurate" the "calculator" itself is. Likewise, if you DO have the ACTUAL MEASURED values for all of the dimensions that affect compression, ANY "calculator" that allows you to enter them will be just as effective as any other.

"Nominal" stock deck clearance is .025", based on a "nominal" rotating assembly height of 9.000" ("nominal" block height is 9.025"). Virtually NO engines will have ANY cylinders with less than this, unless decked. Most cylinders in most blocks will MEASURE at anywhere from .005" to .015" MORE THAN 9.025"... IOW, between .030" and .040" of clearance, with a "nominal" height rot assy (which NEVER happens) out of the factory, with plenty of variation within any given motor. Rod work shortens those, adding clearance as well. Typical crank grinding, "on the wear", both shortens the stroke AND adds deck clearance. And so forth.

.014" of deck clearance is FANTASY LAND unless significant effort (money) is expended.

 

Sofa is absolutely correct again. You must measure everything. If you have a good machinist (I do), then they can set everything up to hit those magical "spec" numbers. And the deck height "spec" of 0.025" is always more than this as he mentioned - in fact my machinist was able to align hone my block (moves the crank up and shortens the deck height), resize my rods for ARP studs (moves the piston down and increases the deck height), and deck the block to make all cylinders even (shortens the deck)..... he did ALL these things and hit EXACTLY 0.025" deck height on every cylinder, and nearly exactly 0.002" on every main and rod bearing (within a couple tenths) using a single off-the-shelf set of Clevite bearings. If you can find a machinist that can do this you have found the person you need to do the work. If they aren't absolutely POSITIVE they can do this for you - find another shop.

BTW - the machine work (including block as mentioned above, crank polishing, rod resizing, and him supplying the bearings), hot tanking, installation of all new brass freeze plugs, and a coat of primer, was about $1650

That's the cost of quality work these days.

GD

 

I’m all in.

I will study this this thread and head into the shop to make sure all of these procedures and measurements are done. It’ll be my honor to drive around a properly built SBC 350. I really appreciate the tons of experience and knowledge imparted here. I will keep updating.

 

You'll have to define "significant effort (money)" to me. Unless it's defined by block machine work, aftermarket connecting rods and pistons plus expert assembly. I've 8 cylinders all at .014" below deck.

 

Ok. I just spent about an hour carefully reading through everything and taking notes. Many, many thanks. This kind of wisdom and experience gathered in one thread really illustrates the power of the internet and forums like this.

I’ve summarized my take-aways below and ended up with a few more questions. Please let me know how this looks. BTW, excellent "don't bore/hone" article, GD. Iron rings for me if that's the case! Fingers crossed.

Desired Build: Quality, budget, street, smog-check friendly, 150 shot NOS-capable

Assumptions: Short block is in unknown state, presumed good/rebuildable

Specs:

• Compression Height: 1.56”
• Target CR: 9.6-9.8
• Deck clearance target: 0.025”
• Rod length: 5.7”
• Square the deck to 9.025”
• Deck clearance: 0.025” is good, 0.014” is better but can be expensive.
• Iron head (083 casting) combustion chamber volume: 64cc

Notes:

• Try to minimize head gasket thickness
• Specs that affect CR: valve seats ground, thicker/thinner head gaskets, head gasket sealing diameter (larger than bore), deck height
• Use Clevite bearings and Hastings rings (If NOS, then gap rings)
• If replacing pistons, thinner rings are preferred
• Custom pistons might be the best option if replacement necessary
• If ring replacement only, 2-stroke oil or ATF for rings/skirts is good
• To do many of the machining checks and work adds extra cost but is worth it ($1,000-$1,650)

General Rules:

• · Measure everything, each cylinder
• · Don’t bore/hone unless needed
• · Always use a deck plate when boring/measuring
• · Don’t zero-deck unless required

Basic Steps:

1. Customary block inspection (magneflux, etc)
2. Determine if bores are good. Bore/hone only if necessary
   1. Inspect for factory honing marks around bore. If visible, bores are good. If not, or patchy, measure.
   2. Measure bore roundness, taper, etc. WITH DECK PLATE installed.
3. Align hone, clock’s square deck, cam line, polish crank, check/resize rods
4. Gap rings if NOS
5. Balance rotating assembly – no damper, no flywheel/flexplate
6. Install rotating assembly with deck plate and measure deck. Deck needs to equal for all cylinders at their TDC

Questions:

1. Using a deck plate distorts the block as if the heads were installed. What are all of the occasions a deck plate should be used and should not be used?
   1. Used when
      1. Measuring bores
      2. Measuring deck clearance
      3. Machining/Honing bores
      4. Decking?
      5. Other?
   2. Not to be used when?
      1. Measuring/machining main and cam journals?
2. What is the proper order of operations?
   1. Inspect bore surfaces and pistons, if OK then skip to 2-5.
   2. Clock deck
   3. square decks
   4. mount deck plate- align bore and hone
   5. Prep journals and install new bearings
   6. Assemble, install deck plate and measure deck clearance, machine as necessary to be equal across eight cylinders and aim for 0.025”
   7. Balance rotating assembly
   8. Buy proper thickness head gasket to get CR close. Thinner is better
3. What should I gap the rings to for NOS?

 

You may find this worth a read as well. It'll shed a little light on one of the criteria of camshaft selection. Having to pass smog will limit your options but a little education never hurts.

http://www.empirenet.com/pkelley2/DynamicCR.html

There's a nifty SCR/DCR calculator attached to that article. It offers a few more lines for data entry which cover a couple of the items discussed previously.

 

I can chime in here with what I've been through.

Your stock deck clearance, that is, what you have now. would be at a minimum of 9.025". One of my old 70's engine blocks had a measured deck that spanned from 9.026 to 9.036". The idea of the decking is to try and get all things equal and it's really not so much of hitting a specific number (as far as the machinist is concerned) but rather the minimum amount to make it all equal. That'll undoubtedly involve line honing too as mentioned above.

There is one advantage in trying to target a specific deck in that it aids in head gasket selection. I've had a couple of oddball scenarios where getting my desired quench meant an unusual and expensive head gasket. Not a deal breaker necessarily but the .014" deck I managed (and targeted) allowed a common and inexpensive gasket (at 026" x 4.100"). Similarly, if the decks come in at a perfect .025" down, a steel .015" shim style (common and inexpensive) could be used provided the deck and head surface finish is up to it. Or, a zero deck and a .040" gasket. Etc.

And I'm pretty sure you meant a 5.7" connecting rod and not 7". And speaking of connecting rods, don't forget about upgrading to quality rod bolts. That'll be part of the rod re-sizing also mentioned.

 

Thanks. So they deck it using measurements from the block and then the measurement is with the pistons in at TDC each to measure the deck clearance? This ensures that its the same deck clearance over all eight cylinders?

And do you have a particular recommendation on the rod bolts?

 

Ideally - a deck plate should be installed during ALL machining and measuring operations involving the cylinders. Some people even use them when doing the alignment hone of the mains - though this is going a little far IMO for a street engine - the clearances aren't enough different to worry about if the head and block are flat. We're talking tenths at that point, and I really don't bother making anything inside a street engine closer than about 0.0003"

Ring gap - I mostly use JE pistons and rings so I know their gap recommendations. For street/nitrous: Top Ring = 4 x .005" = 0.020", Second Ring = 4 x .0055" = 0.022", Oil Ring (rails) = Minimum of 0.015"

Rod bolts - ARP. They are cheap in the scheme of things.

GD

 

Yes, DON'T replace the rod bolts and recondition the rods. It will cost more to do this than buy new aftermarket rods.
Plus you really don't know the history of your stock rods. And it will affect (or is it effect?) your final deck clearance because this operation makes the rod a little shorter.
Edit: however, unless the new rods weigh exactly the same as your stock ones, then you would need to rebalance the rotating assembly (more money). So then, if your rods are good, then replace the bolts with ARP.

 

Are the L98 rods Powdered Metal? If they are I would use them with ARP bolts. If not..... probably go aftermarket. The PM rods are at least as strong as entry level aftermarket rods IMO.

GD

 

Ok, shop found a broken piston but otherwise things look not bad. The crank journals were a little scored so he recommended a grind on that. Clevite bearings, etc in progress. Heads have been cleaned up (valve job etc) and look great.

The shop likes to use Mahle pistons. He said he would order the stuff but I always like to check for the correct part number. Don, you had showed me and recommended a Mahle piston when I picked up the block. I looked up the Mahle number from the picture and this appears to be it. Can someone verify this is a good fit piston for my street power application?

 

Should be OK, IFF the ACTUAL MEASURED compression height is 1.56". (or even, 1.559" like the catalog says... just NOT the usual "rebuilder" 1.54")

 

Mahle does make pretty decent stuff. They own Cosworth.... FWIW. But no - the Mahle pistons are not rebranded Cosworth.... I asked

And they offer them in STD size. That's great.

The piston rings are a little on the large (thick) side. The Vortec rings are smaller and "modern" rings are even smaller than that. I would likely not use them because of that - I prefer the new thin rings personally. I would fill out a JE order form after I measured up the bores.

GD

 

So heres the bad piston. Bores look good and check out to < .003 out of round. Heads are done.

 

Nice. That was probably an injector failure that wrecked the piston. Multec went lean and cylinder temps went high - ring gap closed up and broke the ring land.

GD