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Question: Deck height on stock engine?

macx

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1981
I'm finally getting around to finishing my plans for a mild upgrade on my 81 I bought a few years ago in preparation for some trips after I retire.

I'm working on a mild upgrade that will yield better throttle response but also good mpg's preferably on 87 so I can afford those trips. The big items are a basic tuneable throttle body efi system that I already have and a vacuum style distributor to go along with it because it won't use the electric engine control, a mildly beefed 200-4R and 2200 converter which is pretty much a bolt in, aftermarket Vortec style heads and dual plane intake, and a mild cam somewhere in the 210@050 intake range with lift a dab under .500 along with roller rockers, a 3" main single exhaust pipe with a hi flow 3" cat, and 3.31 gears. Should give me 300+ torque under 2000 rpm and about 1750 rpm or so at around 75 mph.

In order to run 87 I'm working on gathering info on the stock pistons, which appear to be a nearly full recessed dish in the crown which is not good for detonation resistance. I had hoped to avoid tearing into the shortblock, other than for a cam, as it's in good serviceable condition, but if I can't achieve a decent quench situation might have to.

The one thing I can't pin down is the stock deck height dimension.
It calculates out to be about .045" depending on who's statistics I go by. I know the block height, piston compression height, etc, so can figure it out from there, but am somewhat surprised in all the existing statistics I've found there's no listing of the stock dimension of how far down the piston is in the cylinder.

From what I know so far, I'll likely go with 67cc heads which would give me around a 9-1 ratio or slightly less which would be fine if had some quench surface on top of the pistons which it doesn't appear that I do. So might have to replace the pistons, which means machine work, and parts that wouldn't make sense not to replace if I have the engine out and apart, which I had also hoped to avoid. That's why I want to make sure of what's in that engine stock for pistons and deck clearance. I only want to do this once!

I had figured on running a flat tappet hydraulic cam to save over the much higher costs of a roller setup, but if I have to replace the pistons it might pay to check to see what I can get for a good used roller shortblock that has quench style pistons stock, if there is such a thing.

Decisions, decisions!

Thanks for any info!
 

macx

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1981
After more research & calculating

Found out I'm probly better off leaving the stock dished pistons in the shortblock so I don't have to disassemble it and pay machining and misc small parts costs to switch pistons which would run at the bare minimum $300 to get flat top hypereutectic's, plus avoiding the work of removing/replacing & disassembling/reassembling the engine which I wouldn't do if I don't do more to the shortblock than change cam.

If I run with the stock pistons (1.54 compression height, 9.9cc dish, with actual deck height probly at least .05 with a calculated .045 at a blueprint block height of 9.025) and go with 64cc heads and an .020 x 4.1 coated head gasket (which should seal without decking the block) I'll end up with about 9.07 to 9.09 comp ratio and will have to use 91 octane as I won't have any effective quench area.

For a max expected total mileage on the car of 50k miles, and figuring 2 mpg increase with 91 octane due to a little higher compression ratio and advanced timing, and $.30 per gallon over 87 octane, that would only add up to about an extra $150.

So around $150 less total cost to leave shortblock alone and run 91 octane, with maybe a few lbs more low & mid range torque from more advanced timing.
 
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1981 HD Suspension; ZN1 Option
The average deck height is 9.03. Stock deck clearance is 0.025" (below).

GerryLP :cool
 

macx

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I've had a little problem coming up with that figure, altho it's about
what I expected when I started researching it.

If you add 1.74 (1/2 of stroke) plus 5.7 rod length and 1.54 piston
pin height / compression height (stock on my 81 L81 engine per all
the specs I can find) you come up with 8.98.

Subtracted from 9.03 gives .05". Or is my math fuzzy? :confused

Putting those figures along with the stock 76cc comb chamber
and the .016 (or .017 depending on who you read) compressed
head gskt into i.e. the KB Silvolite calculator gives close to the
advertised 8.2 comp ratio.
 
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1981 HD Suspension; ZN1 Option
Close, but you still have to subtract 0.025" from it, for the piston only comes that near the deck. Thus, the 0.025" Below deck figure I posted above.

It is like adding the CC's equivalent to 0.025" to the head chamber volume.

Another maintenance principle I have always encouraged everyone to practice is to measure the volume yourselves for the heads and piston's valve reliefs. Those advertised chamber and relief sizes are often off by an "X" quantity, and throws off your true CR (and your true quench as well ).


p.s. Remember that if the machine shop tries to sell you the squaring off of the deck or milling down the deck, that will erase your vin number of the block and kiss good bye your matching-numbers ride. :( you can have them check it, but demand a guarantee that they won't erase your numbers...not even by accident.
 

Peer81

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'81 Black
I also have this problem when calculating CR and DR for my new heads (someday). Problem is, there is no way you know the exact number before you remove the heads. Same thing with stock deck hight, is it at all 8 cilinders the same? If you're near detonation this is becomming more important.

Small tip, if you're looking for a CompCam extreme energy. The also have a subseries for computer controlled engines.

Greetings Peter
 

macx

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Absolutely. In my experience there can be a small variation from one end of the block to the other, besides just the other mfg tolerances in rod length, piston pin height, and comb chamber volume which can stack up.

I'm not going to change pistons or do any disassembly or machining to the block being it's in decent operating condition and no rear seal leaks, at least that I've seen. I'll probly not be putting over 50k miles on the engine all told after I retire and get it all going, and with the mild state of tune it shouldn't stress it much at all so it should last fine. Just wouldn't be worth all the work and even a $500 tab just for the basics.

On the Comp Cams, yes I searched thru all their whole lineup, and when I went into their CamQuest program I spec'd EFI so the list they came up with included only EFI compatible cams, altho the 2 I'm most interested in aren't in the XFI series but their duration, overlap and lobe center specs make them efi compatible.
 

macx

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Location
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1981
Close, but you still have to subtract 0.025" from it, for the piston only comes that near the deck. Thus, the 0.025" Below deck figure I posted above.

It is like adding the CC's equivalent to 0.025" to the head chamber volume.

Still, "adding CC's equivalent" ??? Sorry, but I never heard of that in over 40 years of building engines.

I ran the numbers on the KB Pistons deck height calculator here

United Engine & Machine Co. Incorporated

This is a copy of the info off of that page when I did the calculation.
I don't see anything like adding any CC equivalent or anything

(For some reason this forum wouldn't accept this Word doc as an attachment)

350 Deck Clearance.doc

That's seems a whole lot closer to my figure of approximately .05" than it is to .025".

And, as a check on the accuracy of that calculation and the .05" figure, I ran the
numbers on their compression ratio calculator, using the .05" deck height figure and
a nominal head gsket bore of 4.1 which may be just a tad off, and it came out to be
8.194 which also is "pretty close" to the advertised 8.2 compression ratio.

Use your figure of .025" deck height, it calculates out to a compression ratio of 8.586.

Just no other way to slice and dice it from what I have ever seen.
 
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1981 HD Suspension; ZN1 Option
Compression ratio is the rate of the volume of the cylinder with the piston at bottom dead center when. Compared with the volume of the cylinder with the piston at top dead center.

So the stroke information is used to give you the volume that the Piston swept on the way up to TDC. But to accurately calculate the CR, you have to calculate the volume of a cylinder 0.050" tall (0.025" high for the stock L81), because this volume must be added to the clearance volume. It's exactly the same reason why the thickness of the gasket must be added as well.

If you were to increase the CR, one way to do this is to reduce the deck height, and thus reducing the clearance volume. One other way is to either put smaller chamber heads or mill the heads to reduce the clearance volume.

GerryLP :cool

p.s. Also, it could be that the pin height or some other value (perhaps even the deck height), is not correct. To accurately calculate the deck height, bring piston to TDC and take a drop measure from deck to piston top. That's why measuring the volume directly pays off big time. Those valve reliefs on top of piston are adding to the total as well.
 

Tim81

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North Canton, Ohio
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81 Black on Black Corvette
I am dealing with this issue as we speak. The stock GM dish pistons have a compression height of 1.560 with give you a deck height of around .025. The KB Silv O Lite replacement flat top pistons have a compression height of 1.540 which means now the piston at TDC is a total of .045 in the hole. I'm going with a .015 compressed head gasket to end up with a pitiful .060 quench. Thats as good as its going to get with this set-up and not willing to deck the block. CR will be around 8.6:1. If I had to do it again I'd buy pistons with a compression height of 1.560 and end up with a nice quench around .040.

Good Luck Tim
 

macx

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Location
Cape Girardeau, MO
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1981
Tim

Are you sure on that stock piston pin height?

I did see both the 1.54 and the 1.56 figures, but in most places it
was the 1.54 figure. I thought I had seen specs in the forum but now can't find them.

If you're using the stock head with 76cc chambers, will that gasket
really give you that cr? The stock gasket is listed at .016 or .017
and is a nominal 8.2 with the stock head.

I'm definitely going to measure that once I get the heads off,
but that's going to be some time yet. I'm just trying to do
some pre-planning now regarding what heads I would hope to
put on there, and what chamber size.

If the stock pistons are indeed 1.56 pin height instead of 1.54,
that would change the calculation of course.

Would appreciate it if you'd post what you find when you measure.

Yes, either with 1.54 or 1.56 pin height, not much for quench.
I'd been considering changing pistons to flat tops that, with a
smaller heart shaped comb chamber head, would get me up to
a little over 9-1 and, with the right cam, maybe could at least
run mid grde instead of 91. But I'm going to use a short cam
that closes early on the intake, so I'd probly have to use 91
anyway.

But my short block I believe is in decent serviceable condition
and I'd never break even on the cost of installing new pistons,
everything considered, compared to just running 91. I'll probly
never put on more than 50k miles total on the car, during
retirement, while I'm still able to drive. So with about 9-1 and
91 gas, and more advance, I'm thinking I'd get a bit better
mileage and make up a little of the difference on the gas cost.
If not, I'd still be $ way ahead just going to smaller chambers
for compression and running 91 versus new pistons.

Decisions, decisions!
 

Tim81

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81 Black on Black Corvette
From the research that I have done the stock pin height is 1.560 and the piston deck height is around .025. The pistons with a pin height of 1.540 are refered to as rebuilder pistons to allow the owner room to deck the block surface. CR is 8.2:1 with a stock piston because of the dish style piston that GM used in the L81. I'm using flat top pistons with -5 valve reliefs cut in them so the CR is going to be a little higher.

Hope this helps Tim
 

macx

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Cape Girardeau, MO
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1981
Out of curiosity, where did you find the pin height specs?

I've looked and looked and the info isn't even in my oem shop manual.

OK - I looked again in KB and see where they say some pistons are not "de stroked" to allow deck milling, and the one piston that says "not de stroked" is listed at 1.56. I was working with 1.54 that I saw in most every parts house listing.

The strange thing is that going into the KB compression ratio calculator, if I plug in 1.56 piston pin height with a 9.9cc dish, and .016 compressed head gasket with a 4.2 bore (couldn't find that spec so guessed) and 87cc comb chamber, and .025 piston deck height (using spec'd block height of 9.025), it works out to over 8.5 ratio. I have to go to an .04 gasket to get down to about the spec'd 8.2 ratio.

So either the figure I've seen in several places for an .016 or .017 gasket, or the 9.9cc dish volume figure, has to be wrong. Either the stock gasket has to be closer to .04, or the piston dish has to be larger volume. I played with it and I had to plug in a 15cc dish to get it to list out to 8.2 comp ratio using the .025 deck height and .016 gasket. Or, if I go back to the 1.54 pin height (.045 deck height using the spec'd figure of 9.025 block height), it all works out to the spec'd 8.2 ratio with the 016 gasket and 9.9 dish.

Where is that calculation going wrong?
 

Tim81

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81 Black on Black Corvette
macx,

Do a internet search on SBC piston combustion or deck height. There is plenty of info there. I have actually measured my pistons after I saw my deck height increase when installing the KB pistons. My stock GM L81 dish piston measured 1.560 and the new KB 1436 measures 1.540. I use the Summit Racing CR calculator and this one. www.gtsparkplugs.com/compratiocalc.html I will try the one that you are using and see if there is a difference.......Tim
 

macx

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Cape Girardeau, MO
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1981
Oh, believe me, I have searched using every related term I could think of. Been working on this for better part of a week.

I have used several calculators, incl the Summit one, along with the KB one.

At least now, with someone actually having physically measured a piston, we know that much for sure.

You did not by any chance actually measure the cc volume of the dish and valve reliefs?

And did you check the compressed thickness of the stock head gasket?

Those are the only two variables left now that could have an effect on the ratio.

Thanks for that good bit of solid info, we might get to the bottom of this yet. :eyerole

I still can not find where I saw the 9.9cc figure for the dish volume.
I could have sworn it was on this forum, but now cant put my finger on it.
 

macx

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Cape Girardeau, MO
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I do know I've seen the .016 & .017 figures in a couple spots, so am fairly sure of that. I just use the .016 to be consistent.

That is a good calculator, even taking into account ring height and piston to wall spec. I know I've used a similar one in the past, but
would have to dig thru my volumes of bookmarks to find it. I didn't
see a gasket bore figure, tho. But that should make a minimal difference.

Anyway, I plugged in the info, using the .025 deck height, leaving the default ring height and piston-wall figures, and still get a similar 8.57 comp ratio. So, again, back to the same two variables that we haven't nailed down for totally sure - deck height and dish volume.

Leaving the other figures as is, I have to go clear down to .05 deck height to get 8.2 cr.
 

Tim81

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North Canton, Ohio
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81 Black on Black Corvette
macx

Remember I'm using flat top pistons with 4 valve reliefs equaling 5 cc of volume. All the calculators are telling me with a .015 head gasket I should have a CR of 8.8, Bore is 4.030 Stroke 3.48 Deck is .043 Compressed gasket .015 piston head volume 5 cc and combustion chamber 76cc. With a quench of .060 roughly I may have to run at least 91 octane fuel.
That has me upset because I really was hoping to get away with 87 or 89. I'm sure the motor will run strong, but to keep it from pinging I might need to run better gas and retard the timing a few degrees and that equals lost power and fuel economy.
 

macx

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1981
That sounds right in line with what I've found on at least 3 different CR calculators.

I also was hoping I could do something to enable me to run at least mid grade, but I couldn't even find any reasonable cost pistons that would give me a tad under 9-1 and good quench with quench type Vortec style heads at any chamber size. Besides I'm using a short cam that won't release some of the compression pressure which also doesn't help detonation. But low rpm torque is more important to me so I need the short cam to trap that pressure.

Among the many things I read while researching this was a pretty typical SBC build with Vortecs and pistons that ended up with about 9-1/4 cr that was able to run 87 but it was because, as they pointed out, they ran a Comp 268 cam which is a little low on low end torque because the intake closing event is relatively later than smaller cams, which releases some of that compress pressure and helps avoid detonation. Exactly the opposite of what I want to do regarding the torque curve.

Without quench clearance at or a dab under 040 you really don't have any effective quench.

We're in the same boat - rather than spend the $ to put in new pistons and still end up having to use at least 89 if not still 91, I'm going to leave the existing pistons in there, use heads probly with 64 chambers or whatever it takes to get to a low 9-1, and use 91 with as much advance as it'll tolerate.

Even without quench, using 91 should allow at least as much if not a little more advance than if you could attain decent quench and run 87 or 89. So hopefully that'll equal or maybe bettr your mpg a mile or 2, which will at least partly offset the extra cost of the 91.

Just for the sake of asking, our stock heads are really lo po and probly the biggest choke in the whole engine. Have you considered using low cost Vortec style heads? I've found some aftermarket Vortecs that are improvements on the oem Vortecs at about the same cost, and even only maybe $200 more for a pair of new ones assembled versus used rebuilt stock Vortecs. I first started looking at those ironically because of their quench area, then when I found inexpensive ones decided I'd stretch the budget enuf to include those if at all possible, probly adding fairly close to 100 hp and tq with even just a good but mild cam, and that's per Comp Cam's Camquest dyno program with my major engine specs fairly well represented.
 
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1981 HD Suspension; ZN1 Option
Here is where I got the info on the L81 block...



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