Lower compression ratio (or not)

[spottedog]

I have a 1980 720 4WD with a LZ24 Frankenstein engine, pre ignition has been a problem for the 12k miles that I have driven it.

 

I have it apart now replacing a worn out cylinder head, and I am thinking about NOT using a Felpro 0.020" copper head saver shim along with a standard head gasket like I have been doing.

 

The two gaskets on top of each other never caused a problem, but I recently learned that a 0.020" spacer only drops the compression ratio by less than half a point.

I used it to try to reduce the high compression ratio of this LZ24 clusterfuck, around 10 to 1.

 

The "new" used head I bought, also a W58, is in GREAT shape and has NOT been milled,  it measures 4.255" high, machinist says it is  dead flat :thumbup:

The old head I took off was milled 0.033"

 

The method to my madness: If I put the engine back together with just a decent Felpro head gasket (yeah, I still have to drill the extra coolant holes) and NOT using the

0.020" copper shim, the new head being 0.033" taller (factory spec height) is going to drop the compression ratio by having more combustion chamber cc's (45.2CC total ?)

available to help this bitch NOT SPARK KNOCK!

 

I loosely bolted on the head with a old "pre compressed" composite gasket and set up the cam and chain with the dowel pin in the number 2 hole, set up the guides and tensioner and the dash marks on the sprocket lined up EXACTLY centered, not off to the right of left, frakking centered!  I have never had that happen before. :angel:

 

Am I nutz, or is not using the shim under the gasket remove a possible failure point and because of the thick OEM speck head get me from about 9.8 - 1 compression ratio

down to 8.(something) to one ?  (Even with removing the copper shim I will have 0.013" more of the combustion chamber open to the cylinder than I had before)

 

I await you comments, thanks guys, so much data here!  :w00t:

[Stoffregen Motorsports]

I've never had a head shim fail, but then I also use onyl Nissan factory head gaskets. I also coat the shim with copper coat to eliminate the possibility of a coolant leak.

 

I would also never assume anything on engine math. The only way to know for sure (especially with a franken-motor) is to CC it and do the math.

[datzenmike]

The two gaskets on top of each other never caused a problem, but I recently learned that a 0.020" spacer only drops the compression ratio by less than half a point.

Sort of ... yes. Depends on the compression you start with. The 0.020" shim represents 3.15ccs of extra combustion chamber volume. On a stock Z24 this will drop the compression from 8.25 down to 7.98 hardly 1/4 of a point. On a 12 to one engine the same 3.15cc added, drops the compression to 11.39 or about two and a half times as much.

 

 

 

The old head I took off was milled 0.033"

The W58 head milled 0.033" would make the open chamber volume 39.989cc. Stock Z24 engine and stock open chamber W58 head milled 0.033" with standard crushed gasket thickness of 1.2mm would have a compression of 10.14

 

The above with the 0.022" copper shim would be 9.47

 

 

The "new" used head I bought, also a W58, is in GREAT shape and has NOT been milled, it measures 4.255" high, machinist says it is dead flat :thumbup:

 

 

The method to my madness: If I put the engine back together with just a decent Felpro head gasket (yeah, I still have to drill the extra coolant holes) and NOT using the

0.020" copper shim, the new head being 0.033" taller (factory spec height) is going to drop the compression ratio by having more combustion chamber cc's (45.2CC total ?)

available to help this bitch NOT SPARK KNOCK!

Stock Z24 engine and stock open chamber W58 head with standard crushed gasket thickness of 1.2mm would have a compression of 9.47

 

The above plus the 0.20" shim would be 8.22

 

 

Am I nutz, or is not using the shim under the gasket remove a possible failure point and because of the thick OEM speck head get me from about 9.8 - 1 compression ratio

down to 8.(something) to one ? (Even with removing the copper shim I will have 0.013" more of the combustion chamber open to the cylinder than I had before)

 

The 9.47 compression you did have is up there, but high octane gas should tame it. Is any of the copper shim exposed into the combustion chamber? In other words is the shim for an L20B bore, which is 85mm while the Z24 cylinder is 89mm? The copper edge will get pretty damn hot if so.

[DanielC]

You need to get one of these, and some clear plastic plates.  Use ATF for fluid.

 

close up of the business end.

When you start to throw random parts together, that the Nissan factory never put together, in one engine, you have to check everything.  You cannot assume the volumes are correct.

 

Here is another thought.  If the combination of parts to make a LZ-23 ended up as such a good engine, why did Nissan not do it?

[datzenmike]

Tighter emission limits. The NAPS system was on the way. Dual plugs lots more EGR.

[DanielC]

EGR is used in modern engines sometimes to control detonation.  Eliminate EGR, and you could possibly end up withan engine that is hard to control detonation.  Even more reason to check everything, and CC the heads. 

[datzenmike]

EGR is used in modern engines sometimes to control detonation.  Eliminate EGR, and you could possibly end up withan engine that is gard to control detonation.  Even more reason to check everything, and CC the heads.

Yup and people remove them to make it look 'cleaner'.

[Stoffregen Motorsports]

I put an EGR system BACK ON a 22RE to get it to run well. Up to that point it was pinging like crazy. The truck was smog exempt so it fell under that same category Mike described (cleaner look), and the customer felt it wasn't required. It took some convincing, but after it was all re-installed and running properly, that's when I finally convinced the customer.

[datzenmike]

Yes. At part throttle the EGR is load dependent. More load... more EGR. There is a minor loss but you won't notice it as your foot just pushes down on the gas farther. This means the throttle is open further than the same carb without EGR so there is less pumping work for the engine to do pulling air through the more open throttle plate. At full throttle there is no EGR, so it will perform exactly like an engine with it disconnected. Still people think they are being ripped off somehow. You always fear what you don't know.

[620Turbo4X4]

What heat range spark plugs are you running? If you compression has been raised you should be using colder heat range plugs. Stock heat range could be glowing under load and pre-igniting the mixture before the plugs have a chance to fire. Changing ignition timing does nothing when this occurs.

 

Also, auto manufactures found when pumping in more and more EGR to reduce NOx emmisions that adding even more timing during part throttle when EGR is activated offsets the power loss from the exhaust re circulation. When a EGR is plugged or removed it it typically nessasary to reduce the total travel of the vacuum advance to stop knocking, or make the EGR operational again as previously mentioned.

[datzenmike]

High compression jams the fuel and air molecules closer together and the flame jump's from one molecule to the next faster over a shorter distance. Shorter burn time means you have to light the fire later or closer to TDC so that you get the maximum combustion pressure at the same point after TDC as a slower burning low compression engine.