318 Build.. Zero Deck or Stock Deck Clearance?

-
DJones942013

DJones942013

Active Member
Joined
May 5, 2010
Messages
26
Reaction score
5
My 318 out of my Scamp is currently at the machine shop getting checked and bored. I was wondering if anyone here has built a 318 and if they would go with zero deck or just leave the clearance stock? I will be using KB 167 .30 over pistons, stock length SCAT forged rods, file to fit rings, 360 Magnum heads with a mild port and polish and a Comp 280H cam. This will be a mostly street driven car but I want good reliable power. I thought about leaving the stock deck clearance and build compression up with a tighter ring gap and a thinner head gasket, but I think the zero deck with a standard .040 installed head gasket will produce better power. I am just looking for opinions and advice, thank you all in advance.
 
DJones942013 said:
My 318 out of my Scamp is currently at the machine shop getting checked and bored. I was wondering if anyone here has built a 318 and if they would go with zero deck or just leave the clearance stock? I will be using KB 167 .30 over pistons, stock length SCAT forged rods, file to fit rings, 360 Magnum heads with a mild port and polish and a Comp 280H cam. This will be a mostly street driven car but I want good reliable power. I thought about leaving the stock deck clearance and build compression up with a tighter ring gap and a thinner head gasket, but I think the zero deck with a standard .040 installed head gasket will produce better power. I am just looking for opinions and advice, thank you all in advance.
Click to expand...
I'd try to shoot for the zero deck. If you get close thats good enough. Cometics gaskets go as thin as .026 but I'd try for a close to zero on the deck height. Regardless of what you do you must absolutely cut your deck surface. DO NOT SKIP THIS! trust me.
 
Go with zero deck. Compression is power, and if you CR is a little to high a thicker head gasket will get you where you need to be.
 
I believe what from what I've read that with the kb167's and a stock deck they should be about .010 in the hole. Seems like with that and the magnum heads you'd have pretty good compression, but if you have the cash to spend you could have the decks cut too.

I currently have my teen out and apart and was thinking of a similar build, but sometimes I think I should just throw it away, return all my new parts, and do a 5.9 swap instead. Just cant make up my freakin mind.
 
Then I'll go against the norm. 3 things:
1. It's a street car, so being "street gas friendly" is a must.
2. Your magnum heads will automatically raise your compression
3. Your .030 pistons will also raise the compression
Now, how much compression are you trying to run on the street? I wouldn't go above a true 9 to1 compression with todays gas. I bet you'll be there or above with the components you have now.
 
whats the cc's on the heads? thay been cut any?? gonna cut them any??? then figure your cr like been said 9:1...9.5:1 high as a pump gas motor can stand without detonation and hypothermics hate detonation!!
 
The quench you'll gain by zero decking will eliminate any chance of pinging. Then, it becomes easy to measure your quench distance, because your quench distance is the head gasket thickness.

Why run the closed chamber if you don't take advantage of it? There's zero chance of detonation with the zero deck height and added quench.

....unless you do something stupid and run a stock cam. Even then I don't think it would detonate.
 
Somebody do the math for me.
318@.030 over + a zero deck slug (with how many cc valve relief?) a Magnum heads chamber + a thin gasket equals?

Then the cams timing bleeds off how much pressure to make the dynamic ratio what now?

All OK on pump 93?

AJ!!! Get your mathematic butt in here!
 
Last edited:
You probably need to cut the deck surface to square it regardless. When I had my 318 done for my dart the banks didn't even have the same deck heights from the factory. After the cut to square deck it my JE pistons, which have a 1.815" compression height, ended up .003" over the deck. My specs worked out like this- 318 .025" over, flat top pistons with a 6.8cc valve relief, 63 cc heads (RHS LA-X), .039" head gasket, lunati 60403 cam. Static compression ratio worked out to be 9.48:1, dynamic is 8.1. I haven't run that one yet but that's less static and dynamic compression than the 340 in my Duster and it does fine on 91 octane. The JE pistons were custom made, I bought them second hand never installed. Not sure why they went with .025" over, but it worked fine for me.

So, have your shop measure the decks first! Ideally before you buy pistons. I would shoot for zero deck even with the closed chambers, you can set your compression with the head gaskets thickness pretty easily that way. And standard thickness gaskets should put you fairly close to where you need to be for pump gas on the street.
 
MileHighDart said:
I believe what from what I've read that with the kb167's and a stock deck they should be about .010 in the hole. Seems like with that and the magnum heads you'd have pretty good compression.
Click to expand...
This ^^^^ is what I did, no decking and a thin head gasket puts you right about where you want to be. With closed chamber heads you should have good quench.
 
Quench is your friend...especially if its tight , like .030. .*Smoothing any sharp edges helps as well*
I run 10.1 static & 8.8 dynamic on 91 octane with .030 quench.
 
Exactly! Why anyone would argue against it escapes me......especially since the engine is already at the shop.

72bluNblu said:
You probably need to cut the deck surface to square it regardless. When I had my 318 done for my dart the banks didn't even have the same deck heights from the factory. After the cut to square deck it my JE pistons, which have a 1.815" compression height, ended up .003" over the deck. My specs worked out like this- 318 .025" over, flat top pistons with a 6.8cc valve relief, 63 cc heads (RHS LA-X), .039" head gasket, lunati 60403 cam. Static compression ratio worked out to be 9.48:1, dynamic is 8.1. I haven't run that one yet but that's less static and dynamic compression than the 340 in my Duster and it does fine on 91 octane. The JE pistons were custom made, I bought them second hand never installed. Not sure why they went with .025" over, but it worked fine for me.

So, have your shop measure the decks first! Ideally before you buy pistons. I would shoot for zero deck even with the closed chambers, you can set your compression with the head gaskets thickness pretty easily that way. And standard thickness gaskets should put you fairly close to where you need to be for pump gas on the street.
Click to expand...
 
He asked about compression, so I answered that he is at the compression already that will be challenged by todays gas. In other words, he doesn't need to "deck" the block to gain compression, IF all he is after is compression. His compression with those heads and pistons will be more than adequate for a street engine, no matter if his pistons end up .010 in the hole, or .040 in the hole.
AND...... AND..... the compression will be more than sufficient to perform.
 
318willrun said:
He asked about compression, so I answered that he is at the compression already that will be challenged by todays gas. In other words, he doesn't need to "deck" the block to gain compression, IF all he is after is compression. His compression with those heads and pistons will be more than adequate for a street engine, no matter if his pistons end up .010 in the hole, or .040 in the hole.
AND...... AND..... the compression will be more than sufficient to perform.
Click to expand...

The problem is that you're making a whole bunch of assumptions to get to the compression he's going to end up at. You're assuming the pistons will be .010" in the hole, but they may not be. The deck heights on these engines vary, the factory was not super accurate with them. My 340 was dead on factory specs for deck height and the same bank-to-bank even without a cut, my machinist couldn't have improved it. My 318 wasn't even the same from one bank to the other. So if I had built my 318 the way it was without decking it the compression wouldn't have even been the same on each bank. You're assuming the head volume on the magnum heads too. Are they really 59cc? Probably not, they're usually a couple cc's bigger. And of course that's all with a .026" gasket.

I mean, will it run? Sure. The 318 in my Dart went at least 80k miles the way it was and the couple times I drove it before I tore it down it was fine. But the OP's engine is at the machine shop. For a few hundred bucks more he can make sure that the compression is the same on all the holes. He can set the pistons up at zero deck so he can easily control the quench distance with the head gasket thickness using off the shelf gaskets. And if he does that now his engine will perform better, even if he sets the compression at 8.5:1. Now it would be better if he set it at 9.5:1, but regardless the engine will perform better. Maybe only a little bit, but better.

As far as "today's gas", the 340 in my Duster has a static compression ratio of 9.8:1 and a calculated dynamic compression of 8.2. It runs just fine on 91 octane, and that's basically at sea level with California gas and factory open chamber iron heads. It's piston's are .018" over the deck, so its quench is by no means ideal. It's pretty much at the limit, if I go much past 34* of mechanical timing I can get it to detonate, but it's not a "challenge". I've put over 12k miles of street driving on the engine just pumping 91 octane into it, using the cheapest gas I can find with no additives or anything special. The 318 is way easier to deal with than the early 340, set up for zero deck it's a piece of cake to control the compression ratio just by swapping the head gasket up or down for thickness.
 
I think true compression ratios on 318's without the 302 heads would average 8 to 1 up to 8:4 to 1, pending on head cc and how far down the hole the pistons are. Lets say his is an 8 to 1 motor. Ok, add the .030 over KB pistons he selected. Compression, without doubt, has risen. Now throw the closed chamber mag heads on there, and without doubt, compression has risen again. I don't doubt for a minute that a true 9 to 1 motor here lives, and that could be checked. It may be even higher.

Look, deck the block, I'm all for it, Just say'n to get a compression ratio for the street, I think he is already there... Check the compression now, he may be at 9:3 to 1 as it sits.
 
318willrun said:
I think true compression ratios on 318's without the 302 heads would average 8 to 1 up to 8:4 to 1, pending on head cc and how far down the hole the pistons are. Lets say his is an 8 to 1 motor. Ok, add the .030 over KB pistons he selected. Compression, without doubt, has risen. Now throw the closed chamber mag heads on there, and without doubt, compression has risen again. I don't doubt for a minute that a true 9 to 1 motor here lives, and that could be checked. It may be even higher.

Look, deck the block, I'm all for it, Just say'n to get a compression ratio for the street, I think he is already there... Check the compression now, he may be at 9:3 to 1 as it sits.
Click to expand...

My point is that you have no idea unless you at least measure it.

If he ends up with actual 59cc chambers, runs the KB167's and ends up at .010" down and uses a .026" gasket he could be at 10.2:1. Even with a .039" he could be at 9.8:1 with those specs. But without measuring the decks or the chambers he could just as easily end up at 9:1. Or even 9:1 on the left bank and 9.5:1 on the right, a .010" difference on the decks would be all it takes. And that's where it gets tricky, because if you calculate it for 9.5:1 and end up with one bank at 9.5:1 and the other at 10:1 because the decks aren't square with a mild cam it won't run on pump gas anymore.
 
Last edited:
72bluNblu said:
My point is that you have no idea unless you at least measure it.

If he ends up with actual 59cc chambers, runs the KB167's and ends up at .010" down and uses a .026" gasket he could be at 10.2:1. Even with a .039" he could be at 9.8:1 with those specs. But without measuring the decks or the chambers he could just as easily end up at 9:1. Or even 9:1 on the left bank and 9.5:1 on the right, a .010" difference would be all it takes. And that's where it gets tricky, because if you calculate it for 9.5:1 and end up with one bank at 9.5:1 and the other at 10:1 because the decks aren't square with a mild cam it won't run on pump gas anymore.
Click to expand...
100% agree. but to say "deck it to zero" just because it's at the machine shop without measuring isn't the way to go about it. this route may end up needing a .080 thick head gasket to be where you want to be. I agree, measure where it is now, then make an educated decision.
 
DJones942013 said:
My 318 out of my Scamp is currently at the machine shop getting checked and bored. I was wondering if anyone here has built a 318 and if they would go with zero deck or just leave the clearance stock? I will be using KB 167 .30 over pistons, stock length SCAT forged rods, file to fit rings, 360 Magnum heads with a mild port and polish and a Comp 280H cam. This will be a mostly street driven car but I want good reliable power. I thought about leaving the stock deck clearance and build compression up with a tighter ring gap and a thinner head gasket, but I think the zero deck with a standard .040 installed head gasket will produce better power. I am just looking for opinions and advice, thank you all in advance.
Click to expand...


If you are going to use KB 167's, DO NOT, REPEAT DO NOT Think or try to raise your compression by tightening up your ring end gap!!!! Thinner head gasket, OK; cut the deck, OK; cut the heads, OK; just DON'T close up ring end gap from KB's instructions.
 
JBurch said:
If you are going to use KB 167's, DO NOT, REPEAT DO NOT Think or try to raise your compression by tightening up your ring end gap!!!! Thinner head gasket, OK; cut the deck, OK; cut the heads, OK; just DON'T close up ring end gap from KB's instructions.
Click to expand...

Excellent point, thanks for bringing that up. The hypereutectic pistons require a specific ring end gap, and deviating from that spec can and has caused those pistons to fail. They're good pistons but you MUST use the specified ring gap because of the metallurgy of the piston.
 
JBurch said:
If you are going to use KB 167's, DO NOT, REPEAT DO NOT Think or try to raise your compression by tightening up your ring end gap!!!! Thinner head gasket, OK; cut the deck, OK; cut the heads, OK; just DON'T close up ring end gap from KB's instructions.
Click to expand...
I'm glad you and 72bluNblu mentioned this. I had no idea the KB 167 has a required ring gap. Didn't look for a spec or info sheet before I sent them to the machine shop for the the boring to be done. The The problem with decking the block and why I was asking, was the machine shop my block is at right now does not assemble anything and will not do the deck clearance measurements. I would have to get it back to my house, measure it and then send it back. I found another machine shop that will do the measurements and deck it, but that was just extra money and a day off work. Which I really did not want to do that to make sure I had days for The Nationals in August, and in case life happened and I needed days for whatever reason. I plan to measure the clearance when I get the block back to see where it is at then make a decision. If the clearances are close in both banks I may not mess with it being it will primarily be a street car and just use a thinner head gasket to make a little of the difference up.
 
rumblefish360 said:
Somebody do the math for me.
318@.030 over + a zero deck slug (with how many cc valve relief?) a Magnum heads chamber + a thin gasket equals?
Then the cams timing bleeds off how much pressure to make the dynamic ratio what now?
All OK on pump 93?
AJ!!! Get your mathematic butt in here!
Click to expand...

I can do the math, but that don't mean it'll run detonation free,lol
 
You must log in or register to reply here.
-

Back
Top