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Old 08-16-2010, 07:47 PM   #1
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Default How to check deck?

I bought a used head which had some work done to it. Porting, SS valves, springs etc. It was also decked. The P/O is a friend of mind and he dosen't have the papers anymore and the shop that did the work dosen't keep proper records. Is there any way to measure the head to see how much was decked? Without this info it's going to be impossible to figure out compression ratio.
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Old 08-16-2010, 08:04 PM   #2
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1.6L or 1.8L?
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Old 08-16-2010, 08:06 PM   #3
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1.6l
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Old 08-16-2010, 08:12 PM   #4
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Here is one that is decked .020" (n/a application)

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Pay attention to the flat nub area in the casting in the middle of the head on the exhaust side...visible here in between the two rightmost valves. With this amount of decking the surface was within .005" of being flush with that.

Either way compression ratio is hard to exactly calculate like this. I've heard it said that the "quoted" factory compression ratio is really only true at the max serviceable decking limit...~.010" is what's normally accepted.
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Old 08-16-2010, 08:22 PM   #5
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I just want to make sure that there will be enough clearance when assembling the engine.
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Old 08-16-2010, 08:25 PM   #6
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I'll grab a flat ruler and feeler gauges and see what the clearance is at that spot.
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Old 08-16-2010, 08:27 PM   #7
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Cool, glad to help. I didn't check beforehand, and am convinced the machine shop I took it to is incompetent.

Their hot tank job sucked...they lost my valve spring seats and didn't bother to tell me...and I'm pretty sure they took off way too much when they decked it.

It should give you an idea though...
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Old 08-17-2010, 12:20 AM   #8
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Quote:
Originally Posted by falcon View Post
I just want to make sure that there will be enough clearance when assembling the engine.
#1) If I recall correctly the FSM gives a total height spec for the head. This would tell you (with some measurement) whether and how much it's decked.

#2) What clearance are you trying to check? The only "clearance" decking the head changes is piston to valve clearance (assuming we're talking about flat top Miata pistons, domed pistons are another animal).

#3) If what you really want to know is compression ratio (which you discussed above) then CC'ing your combustion chambers is what you need to do.
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Old 08-17-2010, 01:42 AM   #9
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Old 08-17-2010, 02:38 AM   #10
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Quote:
Originally Posted by Rennkafer View Post
#1) If I recall correctly the FSM gives a total height spec for the head. This would tell you (with some measurement) whether and how much it's decked.

#2) What clearance are you trying to check? The only "clearance" decking the head changes is piston to valve clearance (assuming we're talking about flat top Miata pistons, domed pistons are another animal).

#3) If what you really want to know is compression ratio (which you discussed above) then CC'ing your combustion chambers is what you need to do.
All I really want to know is comp ratio. For now anyways. I know the head dosen't interfere since it was already run on the same block I will be running. The block had stock 9.5:1 pistons. I will be running supertech 8.8:1. I just want to make sure that my compression won't be too low. I have a feeling that when the head was done, the guy had the bowl work on a bit to drop his compression down a bit. Since he was running 18PSI making 280whp on a GT25...

I'm cc'ing tommorow. I have some lexan at work and just need to find a syringe.
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Old 08-17-2010, 08:54 AM   #11
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Bowl work won't affect compression. Chamber work will. This was the whole reason I decked this head, to make up for the nearly 1cc I took out of the chamber, and give it a little more compression bump to boot (.020 on the 1.6L stock bore is ~2.5cc).

When doing head work you can calculate the changes you're making to compression ratio ASSUMING the motor started at a stock compression ratio. (from what I can tell most motors roll off the factory line with 1cc extra in the chamber, and about 0.2:1 lower compression ratio than stated...decking your head to the stated limit (.008") brings you up to 9.0:1)


C.R. = swept volume / unswept volume

-Swept volume is of course one cylinder worth of displacement, or in this case: 1597/4 = 399.25c

-Unswept volume comprises two things: the combustion chamber inside the head, and the small volume left in the block and pistons that make up the "bottom" of the combustion chamber.

The chamber volume inside the head you can measure.

The second part of the unswept volume you have to determine using the stock CR and your initial chamber measurement unless you want to do a LOT of measuring (deck height, squashed head gasket, piston domes/dishes). This is why it's important to measure first, since this lower volume doesn't change, regardless of what you do to the chamber volume.

Piston changes affect the unswept volume...so making changes in both the head and the piston will make it hard to calculate afterwards, but you can get an idea. Decking the block, and stroke changes can also affect that volume.


So...stock CR is 9.0:1, meaning that you have 44.36cc of unswept and combustion chamber volume. 8.8:1 pistons will change this to 45.37cc...showing just how great of an effect on CR a single cubic centimeter can make. Deck height and head gaskets only make up about 6cc of this. Piston recesses of course vary.


Fun with math.
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Old 08-17-2010, 01:08 PM   #12
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Sorry, I meant the chamber.
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Old 08-17-2010, 01:22 PM   #13
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I hate math. Tell you what. I'm ccing it today and I'm just going to post the volume up here... someone who knows what's up can tell me what the comp ratio is.

Oh, and stock on the 1.6L is 9.5:1 is it not?
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Old 08-17-2010, 02:35 PM   #14
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Quote:
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Oh, and stock on the 1.6L is 9.5:1 is it not?
Yeah, good catch...I thought that chamber volume sounded kind of big. I wasn't worried about actual compression ratio as much as I was ensuring that the chambers were all the same volume after porting and cleaning up.
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Old 08-17-2010, 03:13 PM   #15
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Oh, hai.
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