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How thick of a head gasket, and what do you think!!

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Whoo Hoo! I will have my SBC 377 back soon. So I wanted to find out exactly what thickness of head gasket I should get and of course what you all think of my setup! Grumpy I would really appreciate you running this through your dyno software :D:hail:

As far as the head gasket goes, I want to know what thickness so I can run 93 octane gas. Thanks!!!


400 block bored 40 over

Scat 9000 crank, 400 mains, internally balanced, 3.5 stroke for 6" rods

Scat 6" I beam rods w 7/16" bolts

Keith Black 194 pistons 10.7:1 w/ 64 cc heads

Canfield heads - 59.2 cc chambers

Intake - 2.02 Exhaust - 1.60

.1 - 63.5 .1 - 50.5

.2 - 130 .2 - 94

.3 - 185 .3 - 128

.4 - 237 .4 - 151

.5 - 252 .5 - 165

.6 - 253.5 .6 - 175

.65 - 255 .65 - 179

.7 - 256.5 .7 - 182


CompCam Flat Tappet - 12-676-4


GRIND # CS XS268S-10






.015 TAPPET LIFT 268 274


AT .015 INT 28 60

EXH 71 23



AT 106 INT C/L


DUR AT .050 230 236

LOBE LIFT .3250 .3340




I also have a port matched VicJr intake and will probably go with a Predator carb.

and an aluminum flywheel. I also have a T-56 to bolt up!!


Yep kinda long, I think it will be a great engine for me, at least for the moment here in my delusional little world! :D:D



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Be kinda careful with that 10.7 to 1 at 64cc and your going 59 cc your going to be pushing the limit on the crappy gas they sell today. I would not go to thin with head gaskets as that will add up as well.


Thats not a big cam so it won't burn of too much compression.

Sounds killer to me!



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We'll need to know what the piston to deck height is to figure the compression and the quench distance.


The problem with the 400 is that all the gaskets I've been able to find are .039" thick or more. With the standard .025" piston to deck clearance (down in the block), that means a total quench distance to the head of .064" or more, which isn't good for detonation resistance, etc.


That's why I had my 406 zero decked (near zero deck height on all pistons). Then the standard .039" gasket gets you in the middle of the .035" to .050" desired quench height.

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Is it assembled yet? IF not, you may want to consider zero decking the block, or at least having it decked to about .010" piston deck height, so that a .039" gasket would give a .049" quench height.


Zero decking isn't cheap. It cost about $100 to zero deck my block. It's a good bit of metal to remove.

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I'm going to ask, but now that I think about it, if I do that, I will have to add thickness to the head gasket to keep my compression reasonable won't I? Isn't it the same thing? The area between the head and top of the piston will still have to be a specific size right? Whether it's the head gasket is providing the space or the block..... or am I missing something here?

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YOu have that right, but having the quench distance (top of piston to top of head gasket and therefore the flat part of the head above the piston) is important to keep between .035 and .050". So it's a balancing act between deck height, gasket thickness, cyl head volume and piston dish/dome/valve-notch volume to give a reasonable compression ratio (static) and deck height and gasket thickness to give good quench.

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7cc valve reliefs

59.2 cc heads

if your blocks not decked youll have close to a .025 below deck piston

figure on a .018-.022 head gasket to get the quench correct, that gives you a 11.0-11.2 static compression

the cam drops this to about 8.5-8.7 dcr

the computer makes its WILD GUESS AT



BTW YOULL MOST LIKELY NEED A TOUCH OF OCTANE BOOSTER, with that dcr, if you go thicker on the head gasket to drop compression slightly your very likely to INCREASE the ENGINES chances of DETONATING as the slight drop in compression will NOT offset the loss in effective quench.

in a light weight (Z) that should be good for close to 11.6 et @ 115mph




SCE Gaskets, Inc.

makes a .021 copper gaskets




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There's a real neat compression calculator on KB's site that let's you play with gasket thickness, deck height, head cc, etc, without doing all the math longhand. That looks to me like a little too much compression for pump gas, you might want to check on some different pistons.





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perhaps I failed to explain the answer well enough.



that will depend on the cam your useing, look at these diagrams and keep in mind that the piston compresses NOTHING untill both valves are closed

you don,t need to lower your STATIC COMPRESSION , what you need to do is lower your DYNAMIC COMPRESSION RATIO, a cam with a wider LSA or a cam with slightly greater duration or both will do that for you[/b]]valvetiming.giflca2.jpghere read this http://www.mercurycapri.com/technical/engine/cam/lca.html http://www.mercurycapri.com/technical/engine/cam/lca.html http://ourworld.compuserve.com/homepages/axelg/cams.htm'>http://ourworld.compuserve.com/homepages/axelg/cams.htm http://ourworld.compuserve.com/homepages/axelg/cams http://tru-442.tripod.com/camselect.htm


http://victorylibrary.com/mopar/cam-tech-c.htm http://victorylibrary.com/mopar/cam-tech-c.htm read this info http://www.newcovenant.com/speedcrafter/tech/camshaft/1.htm http://www.newcovenant.com/speedcrafter/tech/camshaft/1.htm >lessons 1-8

these are the valve timeing overlap ranges that are most likely to work correctly

trucks/good mileage towing 10-35 degs overlap

daily driven low rpm performance 30-55degs overlap

hot street performance 50-75 degs overlap

oval track racing 70-95degs overlap

dragster/comp eliminator engines 90-115 degs overlap

but all engines will need the correct matching dcr for those overlap figures to correctly scavage the cylinders in the rpm ranges that apply to each engines use range.<BR>


DCR and overlap are related but not directly related. However, both must be correct for the best performance.

Overlap, which is determined by the advertised duration and the lobe separation angle (LSA), has a profound effect on idle characteristics and high speed scavaging. I believe it was Ed Iskenderian that said we don't have a 4 stroke-4 cycle engine, we have a 4 stroke-5 cycle engine, with overlap being the 5th cycle. It is that important. Use the list posted to help select the proper overlap.


BTW, I include bracket cars in the "oval track racing 70-95degs overlap" catagory.


DCR is affected by the same advertised duration and LSA that overlap is. However, another cam factor enters into the equation, the installed "intake lobe centerline." This is how far the intake lobe centerline is offset from the LSA. If they are the same, then the cam is said to be installed "straight up" (say a 108º LSA cam installed with the intake lobe centerline at 108º ATDC). If they are different, then the cam is said to be advanced or retarded. Advancing or retarding the cam changes the cam timing in relation to the crankshaft. Nothing on the cam changes, just the relationship of the cam to the crankshaft. Getting back to DCR, advancing the cam causes the intake valve to close earlier than it would if the cam were straight up. When this happens, the piston is lower in the cylinder at intake valve closing increasing the sweep of the piston in the cylinder causing the DCR to be higher. Retarding the cam decreases the DCR for the same reason, namely the piston is now higher in the cylinder at intake closing since the intake valve closes later.


Changing overlap requires either changing the advertised duration or the LSA or both (both are ground into the cam and cannot be changed once the cam is made). Tightening up the LSA (say from 110 to 108º) without changing the adv dur increases the overlap. If the same amount of cam advance is maintained (say 4º), the DCR will increase. If the intake lobe center is maintained in the same location (say 106º, which is 4º advanced for a 110º LSA and 2º for a 108º cam), the DCR will not change (changing duration while maintaing the same LSA has a similar effect). So you could have a number of cams with a varity of different LSAs, durations, and overlap values yet all could have the same DCR. Overlap and DCR are related but, as I said above, not directly related.


first look at this simplifyied example<the V-8 is near the bottom of the page the A pushrod engine diagram http://www.howstuffworks.com/camshaft2.htm http://www.howstuffworks.com/camshaft2.htm

now in that example if you carefully watch the valves they have no overlap as one closes before the other opens but in the real world the exhaust valve is still open altho closeing when the intake valve starts to open, here read this http://www.newcovenant.com/speedcrafter/tech/camshaft/2.htm http://www.newcovenant.com/speedcrafter/tech/camshaft/2.htm http://www.newcovenant.com/speedcrafter/tech/camshaft/3.htm" http://www.newcovenant.com/speedcrafter/tech/camshaft/3.htm http://www.newcovenant.com/speedcrafter/tech/camshaft/4.htm http://www.newcovenant.com/speedcrafter/tech/camshaft/4.htm

now notice in the diagram below how the exhaust valve is still closeing as the intake is opening , this allows the fast moveing exhaust gases that have alread left the cylinder to DRAG the intake chager into the cylinder by negitive pressure (SUCKING THE INTAKE CHARGE INTO THE CYLINDER AS IT TRIES TO FOLLOW THE EXHAUST OUT THE EXHAUST PORT)<BR> valvetiming.gif now the closer or tighter spaced the valve angles are (LSA)like 104-108 the more the valves timeing that they are both open <B>OVERLAPS</B> and the greater the amount of suction from intake port caused bye that fast exiting exhaust gas can occure but keep in mind that the piston starts back up and as it compresses the fuel/air mix in the cylinder the mixture can,t compress untill both the valves are closed, if the intake remains open to long the intake port has a bunch of the cylinders volume pushed back into the intake port causeing a reversion pressure wave, if the exhaust stays open to long excess fuel air mix flows through the cylinder and follows the exhaust into the headers. not only does the cylinders displacement and cam timeing have an effect here but also the rpm level that the engine is spinning at, that one big reason that "hot racing style cams have that lope sound at idle" the low rpm cylinder mixture is poor because the valve timeing is very efficient at lets say 5000rpm- 7000rpm due to longer durration that allows the cylinders to effectively fill in the 1/42nd-1/58th of a second the cylinders have available to fill at those rpm levels but its allowing much of the fuel/air mix to push back into the intake ports at low rpms where the mixture has time to reveres direction as the piston starts back up at low rpms<p


in your case changing to a slightly higher duration cam will cure your high dynamic compression




drops the DYNAMIC COMPRESSION DOWN TO ABOUT 8.1:1 well under detenation range with 92 octane gas

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The Grumpster hit the nail on the head.


Dynamic Compression Ratio (DCR) has become my pet-peev. Most people will always brag about their Static Compression Ratio (SCR) and have also heard that the Cam is the Brain of the Engine.


But what most dont understand is exactly what Grumpy emphasized; and that is that the engine only sees DCR...which occurs only after both valves have closed.


If you wish to understand camshafts then you should begin w/DCR as the SCR should compliment your DCR and not the other way around. Your DCR not only determines the desired powerband but when complimented by the correct SCR it will also determine your engine's intensity within that powerband.


BTW: the 8.1:1 DCR will give your engine a choppy idle around the 800-850rpm range depending on your tuning skills. It is exactly what the old 60/70's muscle car engines did; they had a high SCR but their duration was so extensive in bleeding off extra cyl.preesure that their DCR's were brought down to respectable levels. You crane cam will work in the same manner.


A must for determining the DCR's is the Crank Angle Charts (Thanks PeteP. for turning me on to that one).



(Yea,Still an Inliner)

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heres that chart

look at a cam spec card





compare the piston position to the valve opening point, youll notice the piston is already on its way upward compressing at least in theory the cylinders volume, now think about this!...... if the piston does not start compressing ANYTHING on a 383 for example with its 3.75" stroke untill the piston is lets say 3.2" from TDC your effective stroke is 3.2" x 4.03" for an EFFECTIVE displacement of 326.5 cid, your trading MORE effective cylinder filling at higher rpms for slightly lower effective displacement knowing that because the formula for hp is

(tq x rpm/5252=hp)

that even though the tq drops the hp goes up!


look lets say you had 400 ft lbs at 1500rpm with a very mild cam and the hot cam had exactly the same tq per ci of displacement at 5000rpm

383/400 X326.5=340ft lbs

400 x 1500/5252=114hp

340 x 5000/5252=323hp

why does that matter?





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The answer why it matters is gearing - how well the car can use the torque at what rpm. I'm not sure where Grumpyvette was going with that, maybe it had to do with a long versus short stroke issue.


This particular engine build (377) has a 4.165" bore, 3.5" stroke, 6" rod, 7cc valve relief pistons, 59.2cc heads, and anywhere from .025" to 0" (if it get's zero decked) of deck height.


Quench and Static compression ratio:

Deck height and gasket thickness can be used to raise or lower compression, but there's little room to play with, as you want to keep the quench height below .050", preferably from .035" to .045", from my reading.


Gasket/Deck config. #1: So if the pistons are at the standard .025" down, with available gasket thicknesses from .021" on up, you can't do better than .046". Assuming a 4.200" gasket bore, the static compression ratio is 11.2:1.


Gasket/Deck config. #2: If you have the block decked to give zero piston to deck height, you could use a .039" gasket, giving you a .039" quench height. Assuming a 4.200" gasket bore, the static compression ratio is 11.4:1


Gasket/Deck config. #3: Gaming it a bit, you could have the block decked to give a 0.10" piston to deck height, and use the .039" thick gasket to get a .049" quench height and a static compression ratio of 11.1:1.


Let's use Gasket/Deck configs #1 and #3 in the following.


The 12-676-4 Comp cam if installed at the recommended Int lobe CL of 106 (IVC @ 60 ABDC) gives an effective stroke of 2.82".


The Crane 114681 cam, if installed at the recommended Int lobe CL of 111 (IVC@ 71 ABDC) gives an effective stroke of 2.55"


For Gasket/Deck config. #1:

(.025" deck height, .021", 4.200" bore SCE copper gasket, .046" quench height => 11.2:1 static compression ratio)


With the 12-676-4 cam installed at 106 Int lobe CL, (IVC@60 ABDC) => DCR= 9.22:1 WAY TOO HIGH FOR PUMP GAS


With the Crane 114681 cam installed at the recommended 111 Int lobe CL, (IVC@71 ABDC) => DCR= 8.44:1 - on the edge for 93 pump gas


For Gasket/Deck config. #3:

(.010" deck height, .039", 4.200" bore Fel-Pro gasket, .049" quench height => 11.1:1 static compression ratio)


With the 12-676-4 cam installed at 106 Int lobe CL, (IVC@60 ABDC) => DCR= 9.15:1 WAY TOO HIGH FOR PUMP GAS


With the Crane 114681 cam installed at the recommended 111 Int lobe CL, (IVC@71 ABDC) => DCR= 8.38:1 - on the edge for 93 pump gas


Not much difference between the two Gasket/Deck configs. But you can see how that Crane 114681 cam helps bleed some of that excessive (for pump gas) static compression. Surprisingly, these too cams aren't as far apart as you might think (in overlap), seeing how much more duration the Crane 114681 has (important if exhaust restrictions are present and a carb is used). The Comp Cam 12-676-4 has 51 degrees of overlap, and the Crane has 60 degrees. That wider LSA on the Crane helps that matter quite a bit.


Either Gasket/Deck config will be much better off on pump gas with the later Intake valve closing (IVC) of the Crane 114681.

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Ok Ok one more question well maybe 2... I understand that you should keep the quench area under .046. What I don't know is what are the negatives I will see if I go larger than that? How much of an impact will it have? Also I had someone at my local machine shop tell me that with a DCR of 8.44:1 and a static ratio of 11.2:1 I will experience alot of run on and will have to retard the timing is that true? I doubt it as they really didn't even want to understand quench area. Thanks!!!

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