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Those are the ones you want.Your head gasket size right now is .039.With the steel shim head gaskets you will up your compression some.You should definitley consider getting some larger(smaller) cc heads if you really want to raise your compression.Maybe something like 180/72cc or 180/64cc heads.bm
 

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Thanks DriveWFO!

Batman, i planned on upgraded heads but not any time soon as there are other stuff on the car tht i need to attend to. Mostly bodywork. I just thought that i could do this is maybe a day or two and have it back together with more compression...Does anyone know what my compression should be? Thanks
 

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I know GM says that engine has 8.5 CR but if you plug the numbers into a calculator, the actual comes in more like 7.90 to one. I believe a few of the car magazines confirmed this.

My "as is" assumptions
Bore: 4.000"
Stroke: 3.48"
Piston dish: 14cc
depth in the hole: .028"
compressed gasket thickness: .039"
Gasket Hole diameter: 4.03"
Squish: .067"
compression Ratio: 7.9 to 1

By modifying only the gasket, the following changes:
compressed gasket thickness: .015"
gasket hole diameeter: 4.100"
Squish: .043"
Compression ratio: 8.24 to 1

Paul Wright suggested in another post that 1 point in compression was worth about 25-30 HP, that would theoretically make this swap worth about 10 HP.
I'm not sure I have the piston dish size correct, or the "in-the-hole" number, so changing a variable changes the calculation.

In all honesty, this sounds like a lot of work for 10 HP. If you change the heads to ones with a smaller combustion chamber at the same time , like vortecs or iron eagles, you'd get the improvement of the compression ratio (to 9.24 to one) plus the better breathing. 50 HP would not be out of the question, and it would be the same amount of work. What happens if you postpone the change by 2 or three months while you get a line on a better set of heads?
 

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In all honesty, this sounds like a lot of work for 10 HP
You hit the nail on the head there. Save up for a head and / or cam up-grade and continue on with the body and paint work, is my thought. ;)
 

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You use different thickness of head gasket to control the quench distance. This is the distance between the top of the piston at TDC (Top Dead Center) of the stroke and the cylinder head.

"The quench distance (piston/head clearance) should always be set between .035" and .045" with the lower limit giving the best performance and detonation resistance." You can find this quote or variations of it on several web sites and in a some engine building books.

The pistons on a small block Chevy normally sit .025" down in the cylinder hole at TDC. So you would want around .015" of head gasket to give you a quench distance of .040" (.025 + .015). If the block has been decked then the pistons would not be as far down in the hole and you would use a thicker head gasket.

If you want to change the compression ratio, the correct method would be to change the cylinder head volume or go to a different shaped piston.

However, never take any of these measurements for granted and always measure them.

In general the easiest and cheapest performance mods you can do are a better intake, 4BBL carb if you don't have one, headers and free flowing exhaust system.
 

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Here's some actual numbers using Kev's engine:

4" bore 3.48" stroke 5.7" rod length.

Piston volume -10.2cc (4 eyebrows dished stock piston)
Deck clearance .038"
Stock Gasket .039"
Head volume 76cc
Stock cam

SCR= 8.01:1 DCR =6.44:1 cranking compression =150psi
A stock '78 L-48 4 barrel engine is rated at 180hp so VE is .6 at 5,800 rpm.

Changing the head gasket to a .015" thin shim gasket:

SCR =8.39:1 DCR =6.72 cranking compression= 160 psi. With the same VE the increase in horsepower is 188.5 or a modest 8.5Hp gain

Swapping on 67cc heads and same .015" gaskets.

SCR = 9.13 DCR = 7.30 cranking compression = 180 psi, too high for regular gas but 25-30 hp possible on premium with correct fuel and timing. Not taking into account any VE increases due to improved head flow, a stock L-48 will increase to 205 HP just from the compression bump (25hp).

Swap cam for 110 LSA, 106 Intake CL, [email protected] .050" duration.

SCR 9.13 DCR = 6.48 Cranking pressure 152 psi. Now the cranking pressure is back to 87 octane levels but due to VE gains from the cam the power increased by maybe 40-50+ hp over stock, assuming fuel mixture and timing are correct. This is with apples to apples port flow. A better flowing port will net additional gains.
With VE increased to a mild .8 the hp of the L-48 is 275Hp, a whopping 95 hp gain.

Removing the entire top end of an engine just to change gaskets in an attempt to gain noticeable power is too much work for the trouble. Remove the heads only when ready to install better ones. That's where you'll get noticeable power increases with a matching cam , intake and carb.
 

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Hi guys,

Jeeze thats a lot of numbers. Im not sure what head gasket is on the crate motor now, but the 78 engine should have came stock with a steel shim head gasket, a lot thinner than .039.

Its real important if you change to the shim type gasket (or any gasket for that matter) on the later Gm 350s (early 90s up) to carefully check the sealing bead on the bore of the gasket where it gets close to the spark plug cooling passages.

These heads have the passage cast very large and at an angle that makes it worse if you mill the surface. I try and find the smallest bore 4.00 gasket for theses heads. I have had to weld some of these heads to prevent them from leaking compression into the cooling system (I tig weld the edge of the passage with a nickle rod and then mill the head) this is a big problem with these heads and various gaskets.

Im not sure about the crate motor castings, but I would sure check that closely. The larger the bore on the gasket the worse the problem.

Just my opinions and observations.

Good luck
Jeff
 

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Good points Jeff! Some engines had thin shim gaskets from the factory. If you have one of these it will be a waste of time swap gaskets or if you use the thicker gaskets included in many gasket kits you'll actually lose compression and horsepower.
 

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I agree with max turbo, Re doing the entire top end will be a much better solution. To do the head gasket it all basically has to come apart anyways. You can still use the dished pistons, some good head work, performance cam and thinner head gasket. All this can be done at a very reasonable cost, and the gains will be way more than 10 hp. Good posts on this site, its my favorite. Good luck with the project.
 
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If the cam is big enough duration wise, then the ?DCR will be low enough to start the engine easily.

To test this, block vacuum advance, start engine, set to the desired initial, try to restart the engine. This is a temporary test to see if the engine will start with the high initial. If it will, then make a lockout plate that goes where the weights/springs go under the rotor and go racing.

If it won't, you will need to make/use some sort of defeat device or retard system for starting, just depends on the DCR.
 
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63II, good to hear.

If the duration is big enough, and the intake valve closes late enough, the DCR will be tolerable even with more static comp ratio. It all boils down to just how much compressible mixture is blown back up the inlet port to kill off compression resistance against the starter motor's torque ability to spin the engine over. Sounds like it is OK on your setup.
 

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According to the cam card mine closes at 66 deg after bdc, so it bleeds off a big chunk of swept volume. Guess i'll know for sure next summer, but it runs great right now. Hoping to get the car consistent enough to run (i mean win) a bracket event lol
 
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Where that intake valve closes starts the real world DCR, or dynamic compression ratio, the actual volume that gets compressed, discounting the volume that gets back past the inlet valve and back up the port before the valve closes. You may have 10.50:1 static c/r, but at idle, the DCR may well be right down there at 8.00:1.

Ain't engines fun!
 

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IgnitionMan said:
Where that intake valve closes starts the real world DCR, or dynamic compression ratio, the actual volume that gets compressed, discounting the volume that gets back past the inlet valve and back up the port before the valve closes. You may have 10.50:1 static c/r, but at idle, the DCR may well be right down there at 8.00:1.

Ain't engines fun!
I did a calc a while back and I think the DCR was 8.5 or 8.7, anyway it was close enough for pump gas! But right now all the top engine builders are calling DCR a myth, but I wont open that can of worms here lol
 
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That is because they don't understand engines well at all, and they didn't think of it first.

Think of DCR being active, ever changing. Ever see the vacuum actually rise as the rpms are raised? That is because the DCR changes as the engine is slow revved, allowing for less reversion because of the time between intake events speeding up. Less time for the mixture to travel up the inlet port, higher DCR.

DCR will actually rise as the rpms do, until the SCR is met when the cam "hits". Ever feel the cam come on at say, the cam's design 3,000 rpms in an engine? This is the time the DCR matches the SCR, and reversion is at its lowest, capturing the most mixture it will catch.

To really understand DCR, a person has to know two stroke engines and their theory as well as how all that interreacts to four stroke cams, mixture (NOT just air) flow works, and most of these four stroke Gurus just don't get it, so they berate it and say it isn't relevant.

DCR is vitally relevent, but some just don't want you to question them about it. Too bad for them misleading others.
 

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63IIPost said:
But right now all the top engine builders are calling DCR a myth, but I wont open that can of worms here lol
Sorry, you can't pass along a dumb *** internet rumor like that and not expect me to respond.

What "top" engine builders say DCR is a myth? You'd better have facts and references to back your statement up.

Intake valve closing has a definite impact on lower speed pumping ability and is described in engineering books going at least as far back as World War 2.

You have to agree that compression pressure will go up or down depending on the static ratio, right? Change the ratio and you will change the compression pressure. A 12:1 engine will squeeze a mixture to a higher pressure than an 8:1 engine would...IF all things are equal.

It's been scientifically observed that MEP (Mean Effective Pressure) is influenced by the Intake valve closing. Keep the static ratio fixed but vary the intake valve closing and the compression pressure will change which, in turn helps MEP.

Keep in mind that a ratio, even though it is expressed with numerals, is not a number. This is what goofs up many "experts" since a ratio means nothing without a reference. It's like you can't predict MPH solely from axle ratio. You have to know tire diameter and driveshaft rpm. A DCR value is only useful if you know volume and pressure. Even some of the DCR internet guru's don't do the calculations correctly or mistakenly explain how overlap "bleeds off" compression.

Builders that use big cams rely on the high rpm inertial ram effect to overcome the late closing so I can see how they could dismiss it as nonsense, however, this same kind of "common sense" once determined a round earth was nonsense too.

Among other things, DCR is useful to predict cranking pressure, which has importance to octane sensitivity. It's not a myth.

I have to agree with IgnitionMan's comment about 2 strokes.

It's late and my text books are at work. I may add/edit this tomorrow.
 

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Discussion Starter #60
DriveWFO said:
Paul, with my cam's valve timing event specs and compression test results, do I have the needed criteria for calculating my DCR?
There is a link in the weblink library to the silvolite/Keith Black site where there is a Calculator to figure out SCR and DCR. I know Paul says a lot of the sites do it wrong and maybe they do too but I would assume a company that sells pistons would have a decent calculator on their site

Hope this helps ya out:)

EDIT: here is the link;
http://kb-silvolite.com/calc.php?action=comp
 
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