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What will my compression ratio be?


wayno

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OK, so I have a Z20 block(it just says Z20) I got it out of a 1981 510 wagon as I recall, I had to take it to get the 2 shortshaft 5spd transmissions I wanted to buy(package deal), so I tore this engine down to get rid of it and when I pulled the head I could hardly believe what I was looking at, flat top pistons, so I started measuring and it has 152.5 rods(long rods), I listened to this engine run, it ran good in the shop.

 

So I have the Z20 tore down to a short block, the flat top pistons are less than a mm from coming flush with the top/deck, when I put a straight edge on the deck and run the piston to the top I can just get a .010 feeler gauge in there, that is .254mm, I cleaned the top of the piston and block to make the measurement.

 

I have a few heads around here, first is a closed chamber W53 with fairly large valves and the intake ports opened up to match the 1 1/2" SU manifolds, this one has a mild cam(RV), this head is a fresh rebuild, what would the compression ratio be for this head on that short block?

 

The second one I have is another W53 head that is open chamber(yes open chamber), it also has fairly large valves(same as the other one), it has the stock intake ports(1 1/4") and likely has brass seats and has not been rebuilt, it appears to have a stock L20b cam(SSS cam), what would the compression ratio be for this head on that short block?

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Z20s were all long rod even the Mileage Option in the 720.

 

Z20 with 1.2mm crushed gasket thickness and assuming an open chamber head is 45.2cc and a closed chamber head is 41cc, I ignored the tiny valve eyebrow relief cuts and considered the pistons to be true flattops.

 

baDrUG8.jpg

 

10.85 for the closed chamber and

10.08 for the open.

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Will it run without issues using premium non-ethanol pump gas using either head?

I ask because the guy back east had domed pistons and he says his runs properly, my W53 open chamber head has not been rebuilt or ported.

The closed chamber head has been rebuilt, ported, new hardened seats, and has the RV cam, since this block is a long rod and likely will rev better with the cam, it might be a good match as long as I don't have to go buy race fuel.

I had a small block 350 with at least 12-1 compression ratio when I was a kid, that thing was fast and it ran on pump gas.

Please correct me if I have said/figured something wrong, I don't want to put something together that is not going to work.

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My 70 Dart was 10.5 compression and it ran on Premium gas up here.

 

You have a closed chamber head AND flattop pistons ... the only combination that actually works with a closed chamber head. I don't see a problem. Maybe a thicker head gasket to drop it a bit?.

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We ran an L20b last summer on premium with a closed chamber head and flat tops.  No big deal, if the distributor is set up not to over advance, you run low Ohm plug wires, and 0 Ohm plugs.  Start adding resistance or extra timing and you'll get pinging.  Also be wary of old fuel at the pump, unless its non-oxy, it only keeps for a week before the octane is gone.  

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Longer spark duration, smaller weaker slower spark.  What you get is actually more of a rattle and potential late fire when the exhaust valve is starting to open, but it can sound like detonation.  Dynamically I'm not sure what its causing, but I've resolved this situation in plenty of cases with good plug wires.  Sometimes it just comes down to a basic tune-up with quality parts.  10K Ohms extra resistance per cylinder is common with typical Datsun tune-up parts.  Junk.  

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We ran an L20b last summer on premium with a closed chamber head and flat tops.  No big deal, if the distributor is set up not to over advance, you run low Ohm plug wires, and 0 Ohm plugs.  Start adding resistance or extra timing and you'll get pinging.  Also be wary of old fuel at the pump, unless its non-oxy, it only keeps for a week before the octane is gone.  

 

So this is new to me, I have never messed with the plugs or wires before, I just buy what is in the book at the auto parts store.

I also have a strange miss on my 1963 Datsun 320 #4 plug, I figured out that it misses if I have the spark plug wire all the way on the plug, but if I pull the wire and make it arc/jump the engine runs perfect, this only happens on #4 cylinder, I have been thinking that it might have something to do with the wires or the type of plug.

The engine is an E1(1200cc), but I have a MG electronic distributor/wires with a Crane ignition system and it is wired Positive Ground, it has the 1963 Datsun plugs in it that the parts store sold me(BPR6EY), the wires are "EnergyCore 8mm pure silicone", that is all they say, could these wires or plugs be causing this?

By the way it is a bad miss, it runs like crap until I pull that wire and then slowly move it close enough for spark to start jumping from the wire to the end of the plug, then it runs perfect, I even showed this to a guy and he was just as confused as I am.

 

I run non-ethanol(non-oxy) fuel in my trucks already if they sit a lot, as they all have metal tanks, so you are saying I want to buy low ohm wires and 0 ohm plugs, what is low ohm?

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I guess non resisitive spark plugs. an wires. most are for the radio interference I think. or  the coil goes to a higher voltage level before it arches the sparkplug(makes a spark)

 

I thought pipning was the gas ignites as its pressureing up like a diesel before the plug fires the gas off thus wanting to push the piston back down before its naural downward  firing stroke

 

 

In washington thatL20 w flattops, closed chamber head is a bit much here with only 91 octane as the highest rating gas we can buy. I only seen proplems with that setup unless avgas type octane levels

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By the way it is a bad miss, it runs like crap until I pull that wire and then slowly move it close enough for spark to start jumping from the wire to the end of the plug, then it runs perfect, I even showed this to a guy and he was just as confused as I am.

 

 

By lightly moving the wire away from the plug, you increase the resistance, get voltage rise, and have enough then to fire the plug.

Without this, the coil voltage bleeds off before firing the plug.

Could be several things, like slightly fouled plugs, or small oil film inside the distributor cap, or problem with coil.

Have seen this before, and normally, fresh ignition parts cures it.

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It has been an issue for a long time now that I think about it, I used to only start this truck/engine once every few months, then it went every 6 months, then I carb screwed up and I had it rebuilt, but that one wasn't done right, so I put a Weber on it and once I got that idle screw mixture right it ran good.

But then it started doing this miss thing again with a vengeance after that, it's likely loading up but if I take the newer plug out and clean it it don't fix it, that is why when wire ohm was mentioned I chimed in about that engine, I was wondering if the wires or plugs could be wrong for this ignition system on this engine.

I did not tuned up this distributor with a cap/rotor/wires when I installed it, I didn't even know if it would work as the distributor came off a MG Midget 1500 engine, I pulled the distributor out because I could not seem to sell the engine and I was going to scrap it, anyway I looked at that distributor and said to myself it looked like a 320 distributor, so I test fit it and it fit great so I wired it and started the engine, it had never started that easy since I owned it, so it is in there now and I finally sold that MG engine without a distributor.

The thing is that the other plugs don't miss, only #4 cylinder does this miss thing, and I switched wires and it was still #4 that missed, but I didn't change the plug yet, I used to be able to clean the plug and fix it, not since installing the Weber though.

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Duax is right - fresh ignition parts should cure it, but go for less resistance.  Non-resistor plugs, which are getting harder to find.  Ohm your plug wires.  Most new Datsun sets are 7K Ohms or more per wire.  Anything over 5K Ohms provides a misfire, or at least too slow of a spark, which results in a weak cylinder.  Since its on the longest plug wire that you're having an issue, its no surprise.  New NGK wires have been good, but it seems that for Datsuns they haven't made a new batch yet with lower resistance wire.  You could even run copper core wire if you're still running points.  DO NOT do that with electronics - it'll misfire worse than ever.  

 

It is also possible you have a weak coil, or if you have a Pertronix, the module's red wire is not getting full alternator voltage.   Check for a drop there.  Try rotating your trigger wheel one position.  Otherwise it could come down to cap-to-rotor clearance being very different on #4 from a worn shaft bushing.  Or points out of adjustment if you run points.  Or an old engine ground giving up the ghost.  

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Yeah, I'm cursed on my Toyota, with EFI, but still use the lowest resistance suppression wire I can find.

And it's hard to find good premade wire sets for a Datsun, so now I buy bulk wire, use more attainable universal V8 wire sets to cut down,

and invested in a good set of plug wire crimpers. I think the MSD crimpers with interchangeable jaws are running close to $75 these days.

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Duax is right - fresh ignition parts should cure it, but go for less resistance.  Non-resistor plugs, which are getting harder to find.  Ohm your plug wires.  Most new Datsun sets are 7K Ohms or more per wire.  Anything over 5K Ohms provides a misfire, or at least too slow of a spark, which results in a weak cylinder.  Since its on the longest plug wire that you're having an issue, its no surprise.  New NGK wires have been good, but it seems that for Datsuns they haven't made a new batch yet with lower resistance wire.  You could even run copper core wire if you're still running points.  DO NOT do that with electronics - it'll misfire worse than ever.  

 

It is also possible you have a weak coil, or if you have a Pertronix, the module's red wire is not getting full alternator voltage.   Check for a drop there.  Try rotating your trigger wheel one position.  Otherwise it could come down to cap-to-rotor clearance being very different on #4 from a worn shaft bushing.  Or points out of adjustment if you run points.  Or an old engine ground giving up the ghost.  

This is an E1 engine, the distributor is between #3 and #4 plugs on the passenger side, it is actually one of the short wires, #1 wire is the longest on this engine.

I also have electronic ignition, it is the optical type, that is why I can wire it positive ground with the Crane ignition module.

So non-resister plugs and 0 ohm wires is what I want, with a new cap and rotor, correct?

 

When it is not missing I have lots of power, when I put the disc brakes on the front and I screwed up and put them on the wrong sides(upside down), I kept bleeding the brakes and adjusting the pedal rod, one time the fronts seized up and the back wheels were spinning while I was trying to get it back into the garage, I am using Datsun E1(1200cc) spark plugs(new) but I am using a MG distributor(used), except for the miss it runs better than it ever has, and it starts way better/easier than it ever has, it may need a tune-up distributor wise, as I pulled the MG Midget engine out of a car that had been sitting for years.

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Yeah, I'm cursed on my Toyota, with EFI, but still use the lowest resistance wire I can find.

And it's hard to find good premade wire sets for a Datsun, so now I buy bulk wire, use more attainable universal V8 wire sets to cut down,

and invested in a good set of plug wire crimpers. I think the MSD crimpers with interchangeable jaws are running close to $75 these days.

The crimper from Magnecore is the good one!  The rest I've had marginal luck with.  

 

Wayno - you need wires with at least 800 Ohms resistance with an electronic ignition.  Less than 5000.  Preferably on the low end of that scale.  Go for something with quality RFI shielding.  

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