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allansmith859 11-14-2012 07:38 AM

Compression Calculator Help
Hey guys need some help figuring a few things out.

(1) First what is deck clearence. Is that how much clearence you have between the center of the piston and the actual height of the deck surface????

(2) How do I figure out what my compressed gasket thickness is??

(3) What is effective dome volume, and how would I figure that out

MouseFink 11-14-2012 11:38 AM

1. Deck clearance or "quench height" is the distance from top of piston to surface of block. Stock is usually .020" - .030". Not to be confused with deck height. SBC is 9.025" deck height. Rod length (5.700") + compression height (1.563") + quench height (.022") + 1/2 stroke length (1.740") = Deck height (9.025") is typical of a Chevrolet 350 CI and a 4.3L V6 engine .

2. The composition head gaskets packages will have the compressed thickness listed. My Fel Pro gaskets are .041" compress thickness.

3. Effective dome volume is listed on the piston specifications when you buy them. It is a negative amount such as -13 CC and is subtracted from the chamber volume. Example: 64 CC combustion chamber volume - 13 CC piston head volume = 51 CC chamber volume. Since the elimination of leaded high octane pump gasoline, the domed piston in most street driven engines are a thing of the past. A SBC combustion chamber volume less than 64 CC is all that 93 octane pump gas can take.

allansmith859 11-14-2012 11:56 AM

So when I bought my gaskets it says on the box/package what the compressed thickness is??? Also I have flat top pistons and around the outter edge of it its slightly higher than in the middle of it. So should I measure from the center of the piston or from the edge of the piston

Mr. P-Body 11-14-2012 12:05 PM


Deck clearance is the distance between the upper most portion of the flat part of the piston (usually at least the outer "edge") at TDC and the deck surface. Most manufacturers allow a deck clearance of greater than .010" to avoid incidental contact of the piston and head. Some are as high as .060". Measuring deck cleance requires a good magnetic-based dial indicator or a "bridge" (made specfically for the purpose). In our (machinists) lingo, TDC means EXACT top dead center, not "about". A degree wheel and indicator are required to determine that as well. Once TDC IS determined, one can use a straight-edge and "stack" feeler blades to get a rough idea of what the clearance is. Using the dial indicator is the proper way to determine exactly what the clearance is. Set the indicator to "zero" while on the deck. Move it to the edge of the pison closest to the pin. The difference in the readings is your clearance. In the vernacular, "in the hole" refers to negative deck clearance. The relationship of the center of the piston and the deck is of no concern. It's the shape of the dome and the chamber that must be compatible. Both a "dome" and "dish" will have different centers than deck clearance.

Most gasket makers supply that information, either in the catelog or with the product. A typical "stock" gasket for a 350 Chevy has a compressed thickness of .0394" (1 mm). We generally "round" that to .040". There are thinner and thicker gaskets, both "stock" and aftermarket for a variety of applications. For resistance to detonation, most builders agree .040" of "quench" is ideal, so using the stock gasket will usually get you as close as possible, depending on deck clearance.

Computing the dome volume from external dimensions is a tricky business unless it's a concentric shape. The engineers that made the piston have calculations that work, but "in the field", it's much easier just to measure it. At least one piston/ring assembly must be installed and attached to the crankshaft. Cam timing is not necesary at this point.

Find true TDC. Record deck clearance.

Using the dial indicator, lower the piston in the bore exactly 1". "CC" the cylinder in exactly the same manner as measuring the chamber volume in the head. Record the results.

Using the formula "Pi x R(squared) x H", find the "normal" volume of the 1" "cylinder". Where Pi = 3.14159, R = 1/2 bore size and H = stroke (in this case, 1"). Use the same formula to determine the amount "added" by the deck clearance where H = the clearance. Subtract the deck clearance volume from the actual volume. Subtract that from the "normal" volume of the 1" cylinder. To convert from "inch" to metric, multiply the results by 16.378 (how many CCs in one cubic inch). There is your "dome volume".

Doing this job isn't all that difficult. The tools aren't cheap. For a "once in a while" thing, I doubt it's worth the expense. A local machine shop can do the measuring for you at a reasonable charge if you do all the "prep" (subassembly, etc.). Call your local guys and ask. Of course, if you're up for it, buy or borrow the tools and do it yourself! You need a "CCing" kit, a dial indicator and base with at least a 1" stroke, a degree wheel and pointer and alcohol. Water will "do" in a pinch, but be sure to dry it up IMMEDIATELY when you're done. Nothing "thicker" than water.


MouseFink 11-14-2012 12:20 PM


Originally Posted by allansmith859 (Post 1611180)
So when I bought my gaskets it says on the box/package what the compressed thickness is??? Also I have flat top pistons and around the outter edge of it its slightly higher than in the middle of it. So should I measure from the center of the piston or from the edge of the piston

The Fel Pro gaskets I have used had the crushed thickness on the container or instruction sheet. The composition (sandwich) gaskets are usually from .039" to .041" crushed thickness. The razor or shim gaskets do not have a crushed thickness. The crushed thickness of a composition gasket cannot be measured and that is why it is provided on the container.

It appears that you have dished pistons. What are you trying to measure?

allansmith859 11-14-2012 12:26 PM

Well I am looking to buy all new engine parts for a 350 project from the bottom end to the top end and was just trying to figure out what my compression ratio would be with certain pistons and heads. But for now I was just trying to find out what my comp ratio is on my 305 for my truck. By the way what is the highest compression that can be ran on any type of pump gas???

MouseFink 11-14-2012 12:57 PM

I know for a fact that 10:1 can be used with 93 octane pump gas if the pump gas is Chevron/Texaco. The octane at the El Cheapo independent stations that buy gasoline on the spot market is questionable.

I was told buy an owner of a 1968 Corvette that if you have a older engine without computer controls, the engine can have as high as 11:1 CR and run well on E85 Ethanol 110 octane pump gas. My 1963 Pontiac 421 HO had 12:1 CR and it would not run on anything but 100% VP C12 108 octane racing fuel.

BogiesAnnex1 11-14-2012 02:16 PM


Originally Posted by allansmith859 (Post 1611180)
So when I bought my gaskets it says on the box/package what the compressed thickness is??? Also I have flat top pistons and around the outter edge of it its slightly higher than in the middle of it. So should I measure from the center of the piston or from the edge of the piston

These would not be flat top pistons; these are the awful circular dish that reduces the effectiveness of squish/quench by increasing the clearance dimension between the piston's crown surface and that of the squish/quench step of the head. The additional clearance while not being all the area since there is a higher rim surrounding the round dish does in fact substantially reduce the area of tight clearance resulting in a significant reduction in squish and quench activity. Some take the view that this is better for emissions than a highly active squish/quench, I haven't tested this but I rather think this is a specious argument the factory uses to deflect discussion as to why they insist on using these pistons because of cost advantages to themselves. Where compression ratios need to be contained to a suitable level with pump fuel octane ratings a D shape dish that is kept under the valve pocket and has a fully flat and close closing surface opposite the heads squish/quench deck is more effective at increasing efficiency and power this type piston needs to be made unique to left side or right side utilization. Where-as the round dish piston can be fitted to either side of the engine thus is produced in twice the quantity for the configuration than would be a D-dish piston thus allowing them greater quantity build for the design and tooling costs which lowers the unit part price. It may be only a few cents per part but across the quantity of parts they make it adds up to big buck savings for them which usually requires that the consumer either accept less power and efficiency or pay for a higher grade fuel when the engine is pushed to a greater output design.

When you are rebuilding you have an opportunity to correct these deficiencies if you realize they are there. This is where you see a true flat top piston is where the crown surface is the same surface level, except for valve reliefs, all the way across. Here again one encounters a cost savings measure taken on lower cost replacement pistons where there are 2 rows making 4 valve reliefs, again this is a means of using one set of tools to make left and right side pistons. For slightly more there are 2 relief pistons which are made in unique left and right side configurations.

Squish and quench are often referred to as "mechanical octane". This is to say that their function is such that for a given compression ratio they can add to the fuel's octane by the way they stir the mixture, concentrate the mixture before the spark plug this is the squish function as the piston closed toward Top Dead Center. The quench function is that the close closure of the piston and head on the far side of the chamber works as a heat sink to keep the mixture furthest from the spark plug from self-igniting from the temperature and pressure effects of combustion before the flame front gets to that side of the cylinder. As the bore becomes larger and where the spark plug is placed are huge players in this function's quality. To a large extent the bore is what the bore is so the place you can play is with where the cylinder head places the spark plug. Heads from the early emission days placed the spark plug way off on the exhaust side of the chamber, they were after long slow burns at low compression to keep nitrogen oxide formation low as this is hard to clean up where this also creates high levels of unburnt hydrocarbons these are easy to clean up from the exhaust. It once was very popular to put 305 heads on 350s to punch the compression up but these heads also suffer from the spark plug being a long way from the center of the cylinder so it's difficult to take full advantage of the compression improvement without hitting detonation and preignition well before all the potential power has been taken from the increased compression. Starting with the L98 and Swirl Port heads you see the spark plug being moved toward the valves to reduce the distance the flame from needs to travel which reduces the tendency to detonate or pre-ignite. This is continued even further and has the added advantage of improved porting with the LT1 and 4 heads, the Fastburn and L31 Vortecs. This idea has also become predominate with the aftermarket head industry which in many cases can also be seen in their large chamber 70 something cc heads where the spark plug is in a boss the protrudes up into the line of the valve diameter. These chamber designs that induce a lot of squish and quench while moving the spark plug as far into the cylinder as the valve location will allow are like adding 5 or more octane’s to the fuel you're using. This lets you push the compression which is where there is more power and efficiency to be had.

We've done a back to back dyno test of a 355 Chevy engine that had a common configuration except for piston crown shapes. At redline WOT and redline there isn't a lot of difference on power output between the round dish piston and the D-dish. Although the D-dish holds its top end power band a little stronger and higher. Where the big impact is seen between these dish configurations is off idle up almost to WOT, RPMs where especially the torque line is fatter sometimes as much as 30 foot pounds this of course translates into more horsepower in this region as well. The story here is where a street engine especially spends most of its time it will pull harder with less throttle which leads to less fuel consumption at cruise. It's a story that squish/quench is better with the D dish piston, the round dish starts to improve as the RPMs peak to where the torque and horsepower differences get down to only about 5 numbers torque or horsepower in favor of the D-dish. If I figure out how to get Windows 8 to make decent graphs I'll include that analysis.


cobalt327 11-14-2012 02:32 PM


Originally Posted by allansmith859 (Post 1611077)
(3) What is effective dome volume, and how would I figure that out

The dome or dish volume might be expressed as a positive OR a negative number, depending on the calculator you are using. Also catalog descriptions can vary the same way.

All this means is you need to pay attention to what the calculators or specs are calling for. Best to use one calculator and stick w/it. The one I use is here. There are others, obviously.

Below are two dynamic compression ratio (DCR) calculators. To use them you need the intake valve closing point from the cam card and the length of the connecting rod (stock SBC is 5.7 except for the 400 which is 5.565"):

Dynamic CR

United Engine & Machine Co. Incorporated

When you go to calculate the "quench" distance, be aware there are pistons for the SBC 350 that have a compression height (or CH, the distance from the wrist pin c/l to the top of the piston) of 1.56" (stock) or 1.54" (known a "rebuilder" pistons). Because of the difference in CH, these two types of pistons will have different quench figures, all else being equal.

1Gary 11-14-2012 05:30 PM

I use these formulas.

AutomotiveCalculators and Converters

allansmith859 11-14-2012 07:13 PM

The calculator I was using is on the summit racing site. It asks Bore, Stroke, Cylinder Head Volume, Effective Dome Volume, Deck Clearence, Compressed Gasket Thickness, and number of cylinders.

allansmith859 11-14-2012 07:14 PM

BOGIE are you saying that on swirl port heads they made the spark plug where it ignites on the exhaust side.

BogiesAnnex1 11-15-2012 10:33 AM


Originally Posted by allansmith859 (Post 1611323)
BOGIE are you saying that on swirl port heads they made the spark plug where it ignites on the exhaust side.

Moving in that direction but not quite there. The big thing is they move the plug well inboard toward the valves so it has a more centered position relative to the bore diameter. This results in less burn time as the flame front spreads out to the bore limits rather than starting on a distant side then having to travel the entire bore diameter. The Swirl Port head used on the trucks shares a lot of common design with the L98 Corvette head. The truck head gets messed up with the added swirl vane. There is a cast iron version of the L98 head or if you will the Swirl Port head without the vane which are casting numbers 14096217 and 14101083. Castings 14096217 and 14101083 make part number 10125377 for 1986 to 1995 intakes using the 72 degree center bolts. Casting 14096217 is also used to make part number 10159552 for 1955-1986 intakes using the 90 degree center bolts. If you're interested these can be had quite cheaply on the used and rebuilt market for a bolt on replacement to Swirl Port heads or other applications. These heads also show up on some crate motors.

The Vortec is another step forward it borrows pretty heavily from the combustion chambers and port designs of the mid 1990s LT1 and LT4 heads as does the aluminum GMPP Fast Burn head but both the Vortec and Fast Burn use conventional coolant routing and will fit any GEN I block that has enough bore diameter to clear the valves.


cobalt327 11-15-2012 10:45 AM

Several SBC chambers
From SBC head ID: cylinder head IDEarly 64 cc chamber, left vs. L31 5.7L Vortec chamber right'95 chamber, left vs. L31 5.7L Vortec chamber, right
305 Vortec L30 casting number 12558059 chamber

BogiesAnnex1 11-15-2012 11:24 AM

1 Attachment(s)
I might add the classic 72 cc chamber emission head:


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