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Old 05-05-2008, 04:09 PM
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Piston to deck clearance, and compression

Hey, I need help!!!!! I have a chevy 350 bored .030, with .010 taken
off the deck. I have Dart 165cc with 67cc combustion cambers. I want
my compression to be about 8.7 to 8.9. What pistons in Speed Pro
Hyp. would work? Summit and Jeg's say that my pistons should be .045
in the hole,They say thats not counting head gasket thickness.... What
gasket do I need with this?
Thank's
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Old 05-05-2008, 04:27 PM
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Either they didn't explain it properly or you didn't understand what they said. You do not want the piston deck height at 0.045". If that were the case, when adding a 0.040" gasket, you'd be at 0.085" on the squish.

Keith Black KB142 pistons will put you where you want to be with the 67 cc heads. (8.79:1)
http://kb-silvolite.com/performance.php?action=comp

SBC blocks come from the factory with a 9.025" block deck height (give or take for manufacturing and machining tolerances). If yours was a virgin block and you cut it 0.010", then chances are that it's at 9.015". With half the stroke being 1.74", rod length being 5.700" and piston compression height being 1.561", the stack adds up to 9.001", leaving you with a piston deck height of 0.014". Using a GM 10105117 gasket (0.028" compressed) will achieve a squish of 0.042", just right for junk gas.

Last edited by techinspector1; 05-05-2008 at 04:50 PM.
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Old 05-05-2008, 04:56 PM
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Piston to deck clearance and compression

Hey, Thanks for the reply techinspector1, I know this seem crazy,
but thats what I was told. I have been reading threads for awhile now
and I thought that the highest part of the piston to the head needed
to be about .045. Is that right or am I way off the rocker? Say the
piston is .020 in the hole and a .028 Gm gasket makes .048 quench,
from top of piston to head.. Right P.S the pistons the machine shop
wanted to sell me was speed pro H423ncp30 which is a 1.540 compression
height which to me puts me farther down hole than the stock pistons .Flatlander racing chart sayes those pistons are 8.57 compression. Can you tell me if thats what I'll come out with??
Thank's Amillion
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Old 05-05-2008, 05:09 PM
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With your current situation (.014 in the hole, if that's correct) and a piston change reducing compression height by .021, that'd put you .035 in the hole. Add gasket thickness to this. Bottom line is, I don't think you'd be happy with anything more than a .015 stamped "shim" type gasket. I'd follow techinspector's recommendation. Just because the machinist wants to sell 'em to you, doesn't make them right.

Pat
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Old 05-05-2008, 05:34 PM
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Quote:
Originally Posted by BourgD
Hey, Thanks for the reply techinspector1, I know this seem crazy,
but thats what I was told. I have been reading threads for awhile now
and I thought that the highest part of the piston to the head needed
to be about .045. Is that right or am I way off the rocker? Say the
piston is .020 in the hole and a .028 Gm gasket makes .048 quench,
from top of piston to head.. Right P.S the pistons the machine shop
wanted to sell me was speed pro H423ncp30 which is a 1.540 compression
height which to me puts me farther down hole than the stock pistons .Flatlander racing chart sayes those pistons are 8.57 compression. Can you tell me if thats what I'll come out with??
Thank's Amillion
The H423's are "rebuilder" pistons. They have the shorter compression height to allow the rebuilder to cut the decks 0.020" and still have the same piston deck height as stock. My best advice is to use pistons with a 1.560" or 1.561" compression height and tell the guy at the machine shop to..........have a nice day
In my opinion, if you use the pistons and gaskets I mentioned above, you'll end up with a nice build. Call your favorite cam grinder and talk with one of the techs there. Tell him what you want the motor to do and give him all the specs on the parts and dimensions. It's a free service and you'll bypass all the hocus-pocus and end up with the correct camshaft the first time. Get all the associated parts to match up with the cam: springs, retainers, locks, shims, seals.

From your post:
"I thought that the highest part of the piston to the head needed
to be about .045."
Yep, the generally accepted squish is 0.035" to 0.045", including the head gasket thickness. (0.014" piston deck height plus 0.028" gasket = 0.042")

Last edited by techinspector1; 05-05-2008 at 05:51 PM.
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Old 05-05-2008, 06:19 PM
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Piston to deck clearance, and compression

Hey,Thank's for all the help. I got the cam picked out,I'm going with
crane power max 206/216 - 440/454 lift- 114 LS. It will have a good
idle plenty enough torgue and hp. It suppose to be good with 8.5-10.0
compression engines, and it will be used for daily driving and some
towing...
Thank's Again
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Old 05-06-2008, 09:32 AM
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compresion ratio calculator

Everyone should bookmark this page. http://kb-silvolite.com/calc.php?action=comp It answers most questions about CR.

thnx, jack vines
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Old 05-06-2008, 11:53 AM
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Quote:
Originally Posted by BourgD
Hey, I need help!!!!! I have a chevy 350 bored .030, with .010 taken
off the deck. I have Dart 165cc with 67cc combustion cambers. I want
my compression to be about 8.7 to 8.9. What pistons in Speed Pro
Hyp. would work? Summit and Jeg's say that my pistons should be .045
in the hole,They say thats not counting head gasket thickness.... What
gasket do I need with this?
Thank's
No; on the piston clearance, factory is usually about .025, that's production spec anyway. If .010 was cut then, at least without coming over and measuring it, the crown to deck clearance should be .015 inch. One needs to be cautious here as many replacement pistons have a lower pin to crown height to compensate for milled deck blocks, in an attempt to keep compression from going too high. .45 for the crown to be in the hole is about right for standard crown to deck clearance plus a thinnish gasket of .020.

You get into two problems if this distance is too great.

- The loss of squish and quench function and the loss of compression. Effective squish and quench allows you to run more compression against a given octane fuel. This mostly improves fuel economy, allows some additional power, and significantly reduces the tendency toward detonation. In this vein, you want a D dish piston which Speed Pro offers several selections as to dish volume. You want the piston crown's flat surface to close to around .040 to .060 inch with the similar surface of the head. That closure number is inclusive of crown to deck clearance and gasket thickness. The squish/quench effect begins to diminish beyond .060 inch with .080 being the upper limit beyond which the function becomes effectively lost. This why you should never use a circular dish piston.

- An uncalculated loss in compression if you just assume the pin to crown heigth is the production 1.56 inch. This greatly affects the enigne's efficiency and is reflected in greater fuel comsumption for the work performed. Greater fuel comsumption not only costs you more a fill up time but in absolute terms rather than the percentages the smog people check for, you're adding a larger amount of CO2 and other pollutants into the atmosphere. So there are a lot of reasons why you want to keep the compression as high as practical for the available fuel.

With 67 ccs in the head, assuming the piston is down the hole .015 which gives 3.1 ccs, a gasket of .025 thick for 5.23 ccs, this gives a .45 inch crown to squish/quench deck clearance. With a 30 overbore you've got a 728 cc swept cylinder.

- With a flat top piston having 5 cc valve pockets the compression would be 9.9 high test fuel engine.

- With a 12 cc D dish that would drop to 9.2 making a mid grade engine.

- With an 18 cc dish your down th 8.7 and with a 22 cc dish its at 8.38 either of these would be a regular grade fuel motor.

Assuming a cast iron head in all cases. The actual compression would be a bit less as I computed the gasket at the bore diameter of which the gasket is usually bigger.

You can control compression with the gasket, thinner making more and thicker less. Chev gaskets typically run from .020 for a metal shim type to about .055 for a laminated style. With .055 thick gasket the quench is still a safe and effective .070. In the above cases such a gasket would drop the CR in each case above by about .45 to 1; or nearly half a ratio.

Bogie
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Old 05-07-2008, 10:53 AM
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Piston to deck clearance, and compression

Hey, I read Oldbogie thread and I appreciate anything. About the pistons,
I had wanted to use speed pro H670cp30, which is 1.560 compression
height and the are the dish circular type, and I think my compression
would be around 9.3. I will use higher grade gas if I have to. I was
recommended KB 142's which gives a 8.7 compression and are the
D-dish shape pistons. I am kind of leary about KB's because of the way
the top's are made and the ring being so close to the top. How would
they (KB) hold up under extreme conditions like towing in the summer
months? ETC....,ETC... Cam, decided to go with comp cam 212/218,
447/452 lift,110-LS.Please Reply,
Thank's
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Old 05-07-2008, 11:47 AM
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Quote:
Originally Posted by BourgD
Hey, I read Oldbogie thread and I appreciate anything. About the pistons,
I had wanted to use speed pro H670cp30, which is 1.560 compression
height and the are the dish circular type, and I think my compression
would be around 9.3. I will use higher grade gas if I have to. I was
recommended KB 142's which gives a 8.7 compression and are the
D-dish shape pistons. I am kind of leary about KB's because of the way
the top's are made and the ring being so close to the top. How would
they (KB) hold up under extreme conditions like towing in the summer
months? ETC....,ETC... Cam, decided to go with comp cam 212/218,
447/452 lift,110-LS.Please Reply,
Thank's

OK where to start;
Speed Pro which sells the piston line from what had been TRW Power Forged piston which a generation ago was cutting edge forged pistons. They've been updated by Speed Pro with skirt coatings, but the ring placement is racer conservative intended to put some space between the ring and the heat of combustion. The Keith Black hypereutectic is not intended to be a racing piston, but rather an OEM replacement and therefore likely to end up in an engine that needs to be emissions compliant. As things have it, ring placement has a great effect upon emissions, the closer to the heat, the lower the emissions. Hypereutectic pistons were developed specifically to have the characteristics that allow the top ring to ride high without ring land failure. Ford actually tried raising the ring back in the 1960s as a performance enhancing trick, which it also is, but cast aluminum and forged aluminum alloys of the era didn't have the characteristics to let they do this. So after a raft of top ring land failures Ford had to go back to a conventional top land location. The idea didn't die with those problems it just had to wait for the materials people to catch-up with suitable alloys.

For a forged Speed Pro look at this, it has two part numbers but is the same piston, TRW-LW2603F30 or ZLW2603F30. It is has a 10-11 cc D dish, uses a 1.55 pin height which puts it .01 further into the bore which adds a couple ccs to the compression space. There are other choices with larger dishes which I'd dig out but that part of my browser is hung up.

The idea of high squish and quench, although though of as a "modern" development in combustion chamber design is really sourced in Sir Harry Ricardo’s work as far back as the 1920s and 30s.

Squish and quench is like getting two useful things for the price of one. With any shape combustion chamber, a close closing deck between the piston and combustion chamber's perimeter is used to squish the mixture toward the spark plug and to draw the extreme heat from the combusting mixture near the end of the burn cycle. Both of these features are useful towards the improvement in efficient power development and tend to minimize detonation. You can see this applied as a banded area around the outer diameter of hemi and pent chambered engines and as deck at the far distance from the spark plug of a wedge chamber. The squish pushes the mixture toward the spark plug with great force as the piston closes toward TDC. The resulting turbulence speeds the burn rate and improves total combustion by causing the fuel to have a greater exposure to the oxygen contained in the air. Hence the modern term, "Fast Burn" chamber. The second function of quench is that these closely located surfaces of the piston and head offer a great deal of surface area to the volume between them. This absorbs the high heat of the end burn, preventing the unburnt mixture ahead of the flame front from exploding. This is often referred to as mechanical octane.

These two features work best in the range of clearance between .040 to .060 with .080 about the effective upper limit. By the time you get to .1 clearance, as often happens with the cumulative distances of the piston to deck, plus the gasket, plus the circular dish the effect is nearly nulled. It probably works better with closures less than .040, but less than that you're getting in the range where the piston and head could collide. So .040 is simply a minimum safe mechanical distance.

The KB hypereutectic is a safe bet for a street engine with the cam youre looking at, this would be a good 350-375 horse engine. Used where the engine isn't expected to run for lengthy periods at full power, the KB should provide acceptable service. You need to remember that forged pistons need more skirt clearance because they expand a lot. So untill the engine gets warm the skirts tend to tick. Plus you can damage them by getting on the throttle before they've warmed up. What you can do with a race engine isn't always something you can get away with on a street engine. The demands on a street engine drive a whole different set of relationships and compromises to accommodate the widly varying conditions encountered on the street. For a race engine you can aim at point solutions that aren't good anywhere else. There are even different solutions for the same part running in different types of racing. For example, you can run a forged piston tighter in a drag engine and in a circle track enigne. This is because the drag engine is kept cooler overall, so to reduce skirt slap, the clearance can be kept tighter. A circle track engine will be running hot and needs to start with a wider clearance, such an engine needs to be warmed up before putting race power loads on the components to minimize the chance of damaging a piston skirt or cylinder wall.

Bogie
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Old 05-07-2008, 02:54 PM
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Bogie is correct about forged pistons, but the KB hypereutectics are not forged. They are cast pistons that are manufactured with an excessive amount of silicon in the mix to control expansion. With hypers, you run a very tight piston to wall clearance, while using a very wide piston ring gap to prevent butting the ring ends together. I like the "D" shape because it affords a maximum squish area when mated to the underside of the head. Because hyper pistons are a little more brittle and not as tough as a forged piston, the motor must be designed to prevent detonation. That calls for a tight squish, along with a static compression ratio and camshaft that are matched to each other to reach an optimum DYNAMIC compression ratio for the fuel being used.

Great articles from KB......
http://kb-silvolite.com/article.php?action=read&A_id=35
http://kb-silvolite.com/article.php?action=read&A_id=32

Last edited by techinspector1; 05-07-2008 at 03:42 PM.
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Old 05-08-2008, 06:03 AM
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Piston to deck clearance and compression

Hey, Oldbogie and techinspector1, Now I am convinced.. KB D-shaped
142's are the way to go. Oldbogie, thank's for taking the time to right
all what you did. The bore was bored .030 and honed, ?, when they
bored and honed .030 Speed pro bore-102.362mm and KB142-101.600mm,
Will the KB' fit properly or is there a problem here? One more ?, The stock
piston's were measured at 3.990 at top of piston and 4.000 at the
bottom of the skirt. Speed pro measures (.030) at 4.000 at top and
4.031 at bottom. That makes a tight fit at the skirt in a .030 bore!!
Why is that?? P.S. I am getting the KB142"s, But the reason I am asking
this was when I had got my block back from the shop, and they wanted
to sell me the H423 SP's, I miked them. Please Reply...
BourgD

Last edited by BourgD; 05-08-2008 at 01:03 PM.
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Old 05-08-2008, 01:02 PM
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Because I'm pretty much "metric ignorant", I have to convert to inches to understand what's going on. If you convert these mm to inches by multiplying times 0.03937, you'll find that you are comparing a 0.030"-overbore Speed-Pro with a standard bore KB.
102,362mm X 0.03937 = 4.0299" (0.030"-over 4.000")
101,600mm X 0.03937 = 3.999" (standard 4.000" bore)
Go to wherever you got the mm measurements and enter KB142+0.030" and you should come out with the same mm measurement for the KB as you did the Speed-Pro.
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Old 05-08-2008, 01:17 PM
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Piston to deck clearance and compression

Thanks for replying, I went to Summit ,I made a mistake, KB was a
standard bore.. Not .030

BourgD

Last edited by BourgD; 05-08-2008 at 01:32 PM.
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Old 05-08-2008, 01:31 PM
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Quote:
Originally Posted by BourgD
Hey, Oldbogie and techinspector1, Now I am convinced.. KB D-shaped
142's are the way to go. Oldbogie, thank's for taking the time to right
all what you did. The bore was bored .030 and honed, ?, when they
bored and honed .030 Speed pro bore-102.362mm and KB142-101.600mm,
Will the KB' fit properly or is there a problem here? One more ?, The stock
piston's were measured at 3.990 at top of piston and 4.000 at the
bottom of the skirt. Speed pro measures (.030) at 4.000 at top and
4.031 at bottom. That makes a tight fit at the skirt in a .030 bore!!
Why is that?? P.S. I am getting the KB142"s, But the reason I am asking
this was when I had got my block back from the shop, and they wanted
to sell me the H423 SP's, I miked them. Please Reply...
BourgD
No the H423 is back to a dished crown. You have three problems to solve on this; fit in the bore, proper combustion chamber shape, and pin to crown height.

The circular dish is a remnant of old technology when it saved a machining operation thus cost for the piston maker. The lack of detonating resistance and higher fuel consumption from lower efficiency was made up for by the end user with some combination of lesser performance, more expensive higher octane fuel, and more of it and higher emissions.

A flat top or D dish does a better job of improving resistance to detonation, improves power output, and does it all on better fuel economy and lower emissions.

Bogie
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